Curriculum Map 115 - BIOLOGY JUNCTION

Chapter 10 – Photosynthesis – Lecture Outline

Chapter 10 Photosynthesis Lecture Outline

Overview

· Life on Earth is solar powered.

· The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.

A. The Process That Feeds the Biosphere

1. Plants and other autotrophs are the producers of the biosphere.

· Photosynthesis nourishes almost all the living world directly or indirectly.

° All organisms use organic compounds for energy and for carbon skeletons.

° Organisms obtain organic compounds by one of two major modes: autotrophic nutrition or heterotrophic nutrition.

· Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment.

° Autotrophs are the ultimate sources of organic compounds for all heterotrophic organisms.

° Autotrophs are the producers of the biosphere.

· Autotrophs can be separated by the source of energy that drives their metabolism.

° Photoautotrophs use light as a source of energy to synthesize organic compounds.

§ Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes.

° Chemoautotrophs harvest energy from oxidizing inorganic substances, such as sulfur and ammonia.

§ Chemoautotrophy is unique to prokaryotes.

· Heterotrophs live on organic compounds produced by other organisms.

° These organisms are the consumers of the biosphere.

° The most obvious type of heterotrophs feeds on other organisms.

§ Animals feed this way.

° Other heterotrophs decompose and feed on dead organisms or on organic litter, like feces and fallen leaves.

§ Most fungi and many prokaryotes get their nourishment this way.

° Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen, a by-product of photosynthesis.

2. Photosynthesis converts light energy to the chemical energy of food.

· All green parts of a plant have chloroplasts.

· However, the leaves are the major site of photosynthesis for most plants.

° There are about half a million chloroplasts per square millimeter of leaf surface.

· The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.

° Chlorophyll plays an important role in the absorption of light energy during photosynthesis.

· Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf.

· O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf.

· Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant.

· A typical mesophyll cell has 30–40 chloroplasts, each about 2–4 microns by 4–7 microns long.

· Each chloroplast has two membranes around a central aqueous space, the stroma.

· In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids.

° The interior of the thylakoids forms another compartment, the thylakoid space.

° Thylakoids may be stacked into columns called grana.

· Chlorophyll is located in the thylakoids.

° Photosynthetic prokaryotes lack chloroplasts.

° Their photosynthetic membranes arise from infolded regions of the plasma membranes, folded in a manner similar to the thylakoid membranes of chloroplasts.

B. The Pathways of Photosynthesis

1. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis.

· Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O.

· The equation describing the process of photosynthesis is:

° 6CO2 + 12H2O + light energy à C6H12O6 + 6O2+ 6H2O

° C6H12O6 is glucose.

· Water appears on both sides of the equation because 12 molecules of water are consumed, and 6 molecules are newly formed during photosynthesis.

· We can simplify the equation by showing only the net consumption of water:

° 6CO2 + 6H2O + light energy à C6H12O6 + 6O2

· The overall chemical change during photosynthesis is the reverse of cellular respiration.

· In its simplest possible form: CO2 + H2O + light energy à [CH2O] + O2

° [CH2O] represents the general formula for a sugar.

· One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2.

° Before the 1930s, the prevailing hypothesis was that photosynthesis split carbon dioxide and then added water to the carbon:

§ Step 1: CO2 à C + O2

§ Step 2: C + H2O à CH2O

° C. B. van Niel challenged this hypothesis.

° In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis.

° These bacteria produce yellow globules of sulfur as a waste, rather than oxygen.

° Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria:

§ CO2 + 2H2S à [CH2O] + H2O + 2S

· He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis:

° CO2 + 2H2O à [CH2O] + H2O + O2

· Thus, van Niel hypothesized that plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a byproduct.

· Other scientists confirmed van Niel’s hypothesis twenty years later.

° They used 18O, a heavy isotope, as a tracer.

° They could label either C18O2 or H218O.

° They found that the 18O label only appeared in the oxygen produced in photosynthesis when water was the source of the tracer.

· Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the atmosphere (where it can be used in respiration).

· Photosynthesis is a redox reaction.

° It reverses the direction of electron flow in respiration.

· Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar.

° Because the electrons increase in potential energy as they move from water to sugar, the process requires energy.

° The energy boost is provided by light.

2. Here is a preview of the two stages of photosynthesis.

· Photosynthesis is two processes, each with multiple stages.

· The light reactions (photo) convert solar energy to chemical energy.

· The Calvin cycle (synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar.

· In the light reactions, light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH.

° NADPH, an electron acceptor, provides reducing power via energized electrons to the Calvin cycle.

° Water is split in the process, and O2 is released as a by-product.

· The light reaction also generates ATP using chemiosmosis, in a process called photophosphorylation.

· Thus light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP.

· The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s.

· The cycle begins with the incorporation of CO2 into organic molecules, a process known as carbon fixation.

· The fixed carbon is reduced with electrons provided by NADPH.

· ATP from the light reactions also powers parts of the Calvin cycle.

· Thus, it is the Calvin cycle that makes sugar, but only with the help of ATP and NADPH from the light reactions.

· The metabolic steps of the Calvin cycle are sometimes referred to as the light-independent reactions, because none of the steps requires light directly.

· Nevertheless, the Calvin cycle in most plants occurs during daylight, because that is when the light reactions can provide the NADPH and ATP the Calvin cycle requires.

· While the light reactions occur at the thylakoids, the Calvin cycle occurs in the stroma.

3. The light reactions convert solar energy to the chemical energy of ATP and NADPH.

· The thylakoids convert light energy into the chemical energy of ATP and NADPH.

· Light is a form of electromagnetic radiation.

· Like other forms of electromagnetic energy, light travels in rhythmic waves.

· The distance between crests of electromagnetic waves is called the wavelength.

° Wavelengths of electromagnetic radiation range from less than a nanometer (gamma rays) to more than a kilometer (radio waves).

· The entire range of electromagnetic radiation is the electromagnetic spectrum.

· The most important segment for life is a narrow band between 380 to 750 nm, the band of visible light.

· While light travels as a wave, many of its properties are those of a discrete particle, the photon.

° Photons are not tangible objects, but they do have fixed quantities of energy.

· The amount of energy packaged in a photon is inversely related to its wavelength.

° Photons with shorter wavelengths pack more energy.

· While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.

° Visible light is the radiation that drives photosynthesis.

· When light meets matter, it may be reflected, transmitted, or absorbed.

° Different pigments absorb photons of different wavelengths, and the wavelengths that are absorbed disappear.

° A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

· A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.

° It beams narrow wavelengths of light through a solution containing the pigment and measures the fraction of light transmitted at each wavelength.

° An absorption spectrum plots a pigment’s light absorption versus wavelength.

· The light reaction can perform work with those wavelengths of light that are absorbed.

· There are several pigments in the thylakoid that differ in their absorption spectra.

° Chlorophyll a, the dominant pigment, absorbs best in the red and violet-blue wavelengths and least in the green.

° Other pigments with different structures have different absorption spectra.

· Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis.

° An action spectrum measures changes in some measure of photosynthetic activity (for example, O2 release) as the wavelength is varied.

· The action spectrum of photosynthesis was first demonstrated in 1883 in an elegant experiment performed by Thomas Engelmann.

° In this experiment, different segments of a filamentous alga were exposed to different wavelengths of light.

° Areas receiving wavelengths favorable to photosynthesis produced excess O2.

° Engelmann used the abundance of aerobic bacteria that clustered along the alga at different segments as a measure of O2 production.

· The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a.

· Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.

° Chlorophyll b, with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

° Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.

° These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll.

° They also interact with oxygen to form reactive oxidative molecules that could damage the cell.

· When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy.

° The electron moves from its ground state to an excited state.

° The only photons that a molecule can absorb are those whose energy matches exactly the energy difference between the ground state and excited state of this electron.

° Because this energy difference varies among atoms and molecules, a particular compound absorbs only photons corresponding to specific wavelengths.

° Thus, each pigment has a unique absorption spectrum.

· Excited electrons are unstable.

· Generally, they drop to their ground state in a billionth of a second, releasing heat energy.

· Some pigments, including chlorophyll, can also release a photon of light in a process called fluorescence.

° If a solution of chlorophyll isolated from chloroplasts is illuminated, it will fluoresce and give off heat.

· Chlorophyll excited by absorption of light energy produces very different results in an intact chloroplast than it does in isolation.

· In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.

· A photosystem is composed of a reaction center surrounded by a light-harvesting complex.

· Each light-harvesting complex consists of pigment molecules (which may include chlorophyll a, chlorophyll b, and carotenoid molecules) bound to particular proteins.

· Together, these light-harvesting complexes act like light-gathering “antenna complexes” for the reaction center.

· When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center.

· At the reaction center is a primary electron acceptor, which accepts an excited electron from the reaction center chlorophyll a.

° The solar-powered transfer of an electron from a special chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions.

· Each photosystem—reaction-center chlorophyll and primary electron acceptor surrounded by an antenna complex—functions in the chloroplast as a light-harvesting unit.

· There are two types of photosystems in the thylakoid membrane.

° Photosystem I (PS I) has a reaction center chlorophyll a that has an absorption peak at 700 nm.

° Photosystem II (PS II) has a reaction center chlorophyll a that has an absorption peak at 680 nm.

° The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center.

° These two photosystems work together to use light energy to generate ATP and NADPH.

· During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic.

· Noncyclic electron flow, the predominant route, produces both ATP and NADPH.

1. Photosystem II absorbs a photon of light. One of the electrons of P680 is excited to a higher energy state.

2. This electron is captured by the primary electron acceptor, leaving the reaction center oxidized.

3. An enzyme extracts electrons from water and supplies them to the oxidized reaction center. This reaction splits water into two hydrogen ions and an oxygen atom that combines with another oxygen atom to form O2.

4. Photoexcited electrons pass along an electron transport chain before ending up at an oxidized photosystem I reaction center.

5. As these electrons “fall” to a lower energy level, their energy is harnessed to produce ATP.

6. Meanwhile, light energy has excited an electron of PS I’s P700 reaction center. The photoexcited electron was captured by PS I’s primary electron acceptor, creating an electron “hole” in P700. This hole is filled by an electron that reaches the bottom of the electron transport chain from PS II.

7. Photoexcited electrons are passed from PS I’s primary electron acceptor down a second electron transport chain through the protein ferredoxin (Fd).

8. The enzyme NADP+ reductase transfers electrons from Fd to NADP+. Two electrons are required for NADP+’s reduction to NADPH. NADPH will carry the reducing power of these high-energy electrons to the Calvin cycle.

· The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH.

· Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow.

° Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and return to the oxidized P700 chlorophyll.

° As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.

° There is no production of NADPH and no release of oxygen.

· What is the function of cyclic electron flow?

· Noncyclic electron flow produces ATP and NADPH in roughly equal quantities.

· However, the Calvin cycle consumes more ATP than NADPH.

· Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

· Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis.

° In both organelles, an electron transport chain pumps protons across a membrane as electrons are passed along a series of increasingly electronegative carriers.

° This transforms redox energy to a proton-motive force in the form of an H+ gradient across the membrane.

° ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.

· Some of the electron carriers, including the cytochromes, are very similar in chloroplasts and mitochondria.

· The ATP synthase complexes of the two organelles are also very similar.

· There are differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts.

· Mitochondria transfer chemical energy from food molecules to ATP; chloroplasts transform light energy into the chemical energy of ATP.

· The spatial organization of chemiosmosis also differs in the two organelles.

· The inner membrane of the mitochondrion pumps protons from the mitochondrial matrix out to the intermembrane space. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space inside the thylakoid.

· The thylakoid membrane makes ATP as the hydrogen ions diffuse down their concentration gradient from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane.

· The proton gradient, or pH gradient, across the thylakoid membrane is substantial.

° When chloroplasts are illuminated, the pH in the thylakoid space drops to about 5 and the pH in the stroma increases to about 8, a thousandfold different in H+ concentration.

· The light-reaction “machinery” produces ATP and NADPH on the stroma side of the thylakoid.

· Noncyclic electron flow pushes electrons from water, where they have low potential energy, to NADPH, where they have high potential energy.

° This process also produces ATP and oxygen as a by-product.

4. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar.

· The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.

· The Calvin cycle is anabolic, using energy to build sugar from smaller molecules.

· Carbon enters the cycle as CO2 and leaves as sugar.

· The cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make sugar.

· The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

· Each turn of the Calvin cycle fixes one carbon.

· For the net synthesis of one G3P molecule, the cycle must take place three times, fixing three molecules of CO2.

· To make one glucose molecule requires six cycles and the fixation of six CO2 molecules.

· The Calvin cycle has three phases.

Phase 1: Carbon fixation

· In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).

° This is catalyzed by RuBP carboxylase or rubisco.

° Rubisco is the most abundant protein in chloroplasts and probably the most abundant protein on Earth.

° The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2.

Phase 2: Reduction

· During reduction, each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3-bisphosphoglycerate.

· A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to G3P.

° The electrons reduce a carboxyl group to the aldehyde group of G3P, which stores more potential energy.

· If our goal was the net production of one G3P, we would start with 3CO2 (3C) and three RuBP (15C).

· After fixation and reduction, we would have six molecules of G3P (18C).

° One of these six G3P (3C) is a net gain of carbohydrate.

§ This molecule can exit the cycle and be used by the plant cell.

Phase 3: Regeneration

· The other five G3P (15C) remain in the cycle to regenerate three RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.

· For the net synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH.

· The light reactions regenerate ATP and NADPH.

· The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.

5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates.

· One of the major problems facing terrestrial plants is dehydration.

· At times, solutions to this problem require tradeoffs with other metabolic processes, especially photosynthesis.

· The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water.

· On hot, dry days, plants close their stomata to conserve water. This causes problems for photosynthesis.

· In most plants (C3 plants), initial fixation of CO2 occurs via rubisco, forming a three-carbon compound, 3-phosphoglycerate.

° C3 plants include rice, wheat, and soybeans.

· When their stomata partially close on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle.

· At the same time, O2 levels rise as the light reaction converts light to chemical energy.

· While rubisco normally accepts CO2, when the O2:CO2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O2 to RuBP.

· When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration.

° The two-carbon fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes.

° Unlike normal respiration, this process produces no ATP.

§ In fact, photorespiration consumes ATP.

° Unlike photosynthesis, photorespiration does not produce organic molecules.

§ In fact, photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle.

· A hypothesis for the existence of photorespiration is that it is evolutionary baggage.

· When rubisco first evolved, the atmosphere had far less O2 and more CO2 than it does today.

° The inability of the active site of rubisco to exclude O2 would have made little difference.

· Today it does make a difference.

° Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day.

· Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.

· C4 plants first fix CO2 in a four-carbon compound.

° Several thousand plants, including sugarcane and corn, use this pathway.

· A unique leaf anatomy is correlated with the mechanism of C4 photosynthesis.

· In C4 plants, there are two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells.

° Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf.

° Mesophyll cells are more loosely arranged cells located between the bundle sheath and the leaf surface.

· The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells.

· However, the cycle is preceded by the incorporation of CO2 into organic molecules in the mesophyll.

· The key enzyme, phosphoenolpyruvate carboxylase, adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate.

° PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot (i.e., on hot, dry days when the stomata are closed).

· The mesophyll cells pump these four-carbon compounds into bundle-sheath cells.

° The bundle-sheath cells strip a carbon from the four-carbon compound as CO2, and return the three-carbon remainder to the mesophyll cells.

° The bundle-sheath cells then use rubisco to start the Calvin cycle with an abundant supply of CO2.

· In effect, the mesophyll cells pump CO2 into the bundle-sheath cells, keeping CO2 levels high enough for rubisco to accept CO2 and not O2.

· C4 photosynthesis minimizes photorespiration and enhances sugar production.

· C4 plants thrive in hot regions with intense sunlight.

· A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families.

° These plants are known as CAM plants for crassulacean acid metabolism.

° They open their stomata during the night and close them during the day.

§ Temperatures are typically lower at night, and humidity is higher.

° During the night, these plants fix CO2 into a variety of organic acids in mesophyll cells.

° During the day, the light reactions supply ATP and NADPH to the Calvin cycle, and CO2 is released from the organic acids.

· Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle.

° In C4 plants, carbon fixation and the Calvin cycle are spatially separated.

° In CAM plants, carbon fixation and the Calvin cycle are temporally separated.

· Both eventually use the Calvin cycle to make sugar from carbon dioxide.

6. Here is a review of the importance of photosynthesis.

· In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds.

· Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells.

° About 50% of the organic material is consumed as fuel for cellular respiration in plant mitochondria.

° Carbohydrate in the form of the disaccharide sucrose travels via the veins to nonphotosynthetic cells.

§ There, it provides fuel for respiration and the raw materials for anabolic pathways, including synthesis of proteins and lipids and formation of the extracellular polysaccharide cellulose.

§ Cellulose, the main ingredient of cell walls, is the most abundant organic molecule in the plant, and probably on the surface of the planet.

· Plants also store excess sugar by synthesis of starch.

° Starch is stored in chloroplasts and in storage cells in roots, tubers, seeds, and fruits.

· Heterotrophs, including humans, may completely or partially consume plants for fuel and raw materials.

· On a global scale, photosynthesis is the most important process on Earth.

° It is responsible for the presence of oxygen in our atmosphere.

° Each year, photosynthesis synthesizes 160 billion metric tons of carbohydrate.

Chapter 1 AP Objectives

CHAPTER 1 INTRODUCTION: THEMES IN THE STUDY OF LIFE
OBJECTIVES
Exploring Life on Its Many Levels 1. Briefly describe the unifying themes that characterize the biological sciences. 2. Diagram the hierarchy of structural levels in biological organization. 3. Explain how the properties of life emerge from complex organization. 4. Describe the two major dynamic processes of any ecosystem. 5. Distinguish between prokaryotic and eukaryotic cells. 6. Describe the basic structure and function of DNA. 7. Describe the dilemma of reductionism. 8. Discuss the goals and activities of systems biology. List three research developments that have advanced systems biology. 9. Explain the importance of regulatory mechanisms in living things. Distinguish between positive and negative feedback. Evolution, Unity, and Diversity 10. Distinguish among the three domains of life. List and distinguish among the three kingdoms of multicellular, eukaryotic life. 11. Explain the phrase “life’s dual nature of unity and diversity.” 12. Describe the observations and inferences that led Charles Darwin to his theory of evolution by natural selection. 13. Explain why diagrams of evolutionary relationships have a treelike form. The Process of Science 14. Distinguish between discovery science and hypothesis-based science. Explain why both types of exploration contribute to our understanding of nature. 15. Distinguish between quantitative and qualitative data. 16. Distinguish between inductive and deductive reasoning. 17. Explain why hypotheses must be testable and falsifiable but are not provable. 18. Describe what is meant by a controlled experiment. 19. Distinguish between the everyday meaning of the term theory and its meaning to scientists. 20. Explain how science is influenced by social and cultural factors. 21. Distinguish between science and technology. Explain how science and technology are interdependent.
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Chapter 10 – Photosynthesis Objectives

Chapter 10 Photosynthesis
Objectives
The Process That Feeds the Biosphere 1. Distinguish between autotrophic and heterotrophic nutrition. 2. Distinguish between photoautotrophs and chemoautotrophs. 3. Describe the structure of a chloroplast, listing all membranes and compartments. The Pathways of Photosynthesis 4. Write a summary equation for photosynthesis. 5. Explain van Niel’s hypothesis and describe how it contributed to our current understanding of photosynthesis. Explain the evidence that supported his hypothesis. 6. In general terms, explain the role of redox reactions in photosynthesis. 7. Describe the two main stages of photosynthesis in general terms. 8. Describe the relationship between an action spectrum and an absorption spectrum. Explain why the action spectrum for photosynthesis differs from the absorption spectrum for chlorophyll a. 9. Explain how carotenoids protect the cell from damage by light. 10. List the wavelengths of light that are most effective for photosynthesis. 11. Explain what happens when a solution of chlorophyll a absorbs photons. Explain what happens when chlorophyll a in an intact chloroplast absorbs photons. 12. List the components of a photosystem and explain the function of each component. 13. Trace the movement of electrons in noncyclic electron flow. Trace the movement of electrons in cyclic electron flow. 14. Explain the functions of cyclic and noncyclic electron flow. 15. Describe the similarities and differences in chemiosmosis between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. 16. State the function of each of the three phases of the Calvin cycle. 17. Describe the role of ATP and NADPH in the Calvin cycle. 18. Describe what happens to rubisco when O2 concentration is much higher than CO2 concentration. 19. Describe the major consequences of photorespiration. Explain why it is thought to be an evolutionary relict. 20. Describe two important photosynthetic adaptations that minimize photorespiration. 21. List the possible fates of photosynthetic products.
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Chapter 1 – Exploring Life – Lecture Outline

Chapter 1 Exploring Life Lecture Outline

Overview: Biology’s Most Exciting Era

Biology is the scientific study of life.
You are starting your study of biology during its most exciting era.
The largest and best-equipped community of scientists in history is beginning to solve problems that once seemed unsolvable.
Biology is an ongoing inquiry about the nature of life.
Biologists are moving closer to understanding:
How a single cell develops into an adult animal or plant.
How plants convert solar energy into the chemical energy of food.
How the human mind works.
How living things interact in biological communities.
How the diversity of life evolved from the first microbes.
Research breakthroughs in genetics and cell biology are transforming medicine and agriculture.
Neuroscience and evolutionary biology are reshaping psychology and sociology.
Molecular biology is providing new tools for anthropology and criminology.
New models in ecology are helping society to evaluate environmental issues, such as the causes and biological consequences of global warming.
Unifying themes pervade all of biology.

Concept 1.1 Biologists explore life from the microscopic to the global scale

Life’s basic characteristic is a high degree of order.
Each level of biological organization has emergent properties.
Biological organization is based on a hierarchy of structural levels, each building on the levels below.
At the lowest level are atoms that are ordered into complex biological molecules.
Biological molecules are organized into structures called organelles, the components of cells.
Cells are the fundamental unit of structure and function of living things.
Some organisms consist of a single cell; others are multicellular aggregates of specialized cells.
Whether multicellular or unicellular, all organisms must accomplish the same functions: uptake and processing of nutrients, excretion of wastes, response to environmental stimuli, and reproduction.
Multicellular organisms exhibit three major structural levels above the cell: similar cells are grouped into tissues, several tissues coordinate to form organs, and several organs form an organ system.
For example, to coordinate locomotory movements, sensory information travels from sense organs to the brain, where nervous tissues composed of billions of interconnected neurons—supported by connective tissue—coordinate signals that travel via other neurons to the individual muscle cells.
Organisms belong to populations, localized groups of organisms belonging to the same species.
Populations of several species in the same area comprise a biological community.
Populations interact with their physical environment to form an ecosystem.
The biosphere consists of all the environments on Earth that are inhabited by life.
Organisms interact continuously with their environment.
Each organism interacts with its environment, which includes other organisms as well as nonliving factors.
Both organism and environment are affected by the interactions between them.
The dynamics of any ecosystem include two major processes: the cycling of nutrients and the flow of energy from sunlight to producers to consumers.
In most ecosystems, producers are plants and other photosynthetic organisms that convert light energy to chemical energy.
Consumers are organisms that feed on producers and other consumers.
All the activities of life require organisms to perform work, and work requires a source of energy.
The exchange of energy between an organism and its environment often involves the transformation of energy from one form to another.
In all energy transformations, some energy is lost to the surroundings as heat.
In contrast to chemical nutrients, which recycle within an ecosystem, energy flows through an ecosystem, usually entering as light and exiting as heat.
Cells are an organism’s basic unit of structure and function.
The cell is the lowest level of structure that is capable of performing all the activities of life.
For example, the ability of cells to divide is the basis of all reproduction and the basis of growth and repair of multicellular organisms.
Understanding how cells work is a major research focus of modern biology.
At some point, all cells contain deoxyribonucleic acid, or DNA, the heritable material that directs the cell’s activities.
DNA is the substance of genes, the units of inheritance that transmit information from parents to offspring.
Each of us began life as a single cell stocked with DNA inherited from our parents.
DNA in human cells is organized into chromosomes.
Each chromosome has one very long DNA molecule, with hundreds or thousands of genes arranged along its length.
The DNA of chromosomes replicates as a cell prepares to divide.
Each of the two cellular offspring inherits a complete set of genes.
In each cell, the genes along the length of DNA molecules encode the information for building the cell’s other molecules.
DNA thus directs the development and maintenance of the entire organism.
Most genes program the cell’s production of proteins.
Each DNA molecule is made up of two long chains arranged in a double helix.
Each link of a chain is one of four nucleotides, encoding the cell’s information in chemical letters.
The sequence of nucleotides along each gene codes for a specific protein with a unique shape and function.
Almost all cellular activities involve the action of one or more proteins.
DNA provides the heritable blueprints, but proteins are the tools that actually build and maintain the cell.
All forms of life employ essentially the same genetic code.
Because the genetic code is universal, it is possible to engineer cells to produce proteins normally found only in some other organism.
The library of genetic instructions that an organism inherits is called its genome.
The chromosomes of each human cell contain about 3 billion nucleotides, including genes coding for more than 70,000 kinds of proteins, each with a specific function.
Every cell is enclosed by a membrane that regulates the passage of material between a cell and its surroundings.
Every cell uses DNA as its genetic material.
There are two basic types of cells: prokaryotic cells and eukaryotic cells.
The cells of the microorganisms called bacteria and archaea are prokaryotic.
All other forms of life have more complex eukaryotic cells.
Eukaryotic cells are subdivided by internal membranes into various organelles.
In most eukaryotic cells, the largest organelle is the nucleus, which contains the cell’s DNA as chromosomes.
The other organelles are located in the cytoplasm, the entire region between the nucleus and outer membrane of the cell.
Prokaryotic cells are much simpler and smaller than eukaryotic cells.
In a prokaryotic cell, DNA is not separated from the cytoplasm in a nucleus.
There are no membrane-enclosed organelles in the cytoplasm.
All cells, regardless of size, shape, or structural complexity, are highly ordered structures that carry out complicated processes necessary for life.

Concept 1.2 Biological systems are much more than the sum of their parts

“The whole is greater than the sum of its parts.”
The combination of components can form a more complex organization called a system.
Examples of biological systems are cells, organisms, and ecosystems.
Consider the levels of life.
With each step upward in the hierarchy of biological order, novel properties emerge that are not present at lower levels.
These emergent properties result from the arrangements and interactions between components as complexity increases.
A cell is much more than a bag of molecules.
Our thoughts and memories are emergent properties of a complex network of neurons.
This theme of emergent properties accents the importance of structural arrangement.
The emergent properties of life are not supernatural or unique to life but simply reflect a hierarchy of structural organization.
The emergent properties of life are particularly challenging because of the unparalleled complexity of living systems.
The complex organization of life presents a dilemma to scientists seeking to understand biological processes.
We cannot fully explain a higher level of organization by breaking it down into its component parts.
At the same time, it is futile to try to analyze something as complex as an organism or cell without taking it apart.
Reductionism, reducing complex systems to simpler components, is a powerful strategy in biology.
The Human Genome Project—the sequencing of the genome of humans and many other species—is heralded as one of the greatest scientific achievements ever.
Research is now moving on to investigate the function of genes and the coordination of the activity of gene products.
Biologists are beginning to complement reductionism with new strategies for understanding the emergent properties of life—how all of the parts of biological systems are functionally integrated.
The ultimate goal of systems biology is to model the dynamic behavior of whole biological systems.
Accurate models allow biologists to predict how a change in one or more variables will impact other components and the whole system.
Scientists investigating ecosystems pioneered this approach in the 1960s with elaborate models diagramming the interactions of species and nonliving components in ecosystems.
Systems biology is now becoming increasingly important in cellular and molecular biology, driven in part by the deluge of data from the sequencing of genomes and our increased understanding of protein functions.
In 2003, a large research team published a network of protein interactions within a cell of a fruit fly.
Three key research developments have led to the increased importance of systems biology.

1.           High-throughput technology. Systems biology depends on methods that can analyze biological materials very quickly and produce enormous amounts of data. An example is the automatic DNA-sequencing machines used by the Human Genome Project.
2.           Bioinformatics. The huge databases from high-throughput methods require computing power, software, and mathematical models to process and integrate information.
3.           Interdisciplinary research teams. Systems biology teams may include engineers, medical scientists, physicists, chemists, mathematicians, and computer scientists as well as biologists.

Regulatory mechanisms ensure a dynamic balance in living systems.
Chemical processes within cells are accelerated, or catalyzed, by specialized protein molecules, called enzymes.
Each type of enzyme catalyzes a specific chemical reaction.
In many cases, reactions are linked into chemical pathways, each step with its own enzyme.
How does a cell coordinate its various chemical pathways?
Many biological processes are self-regulating: the output or product of a process regulates that very process.
In negative feedback, or feedback inhibition, accumulation of an end product of a process slows or stops that process.
Though less common, some biological processes are regulated by positive feedback, in which an end product speeds up its own production.
Feedback is common to life at all levels, from the molecular level to the biosphere.
Such regulation is an example of the integration that makes living systems much greater than the sum of their parts.

Concept 1.3 Biologists explore life across its great diversity of species

Biology can be viewed as having two dimensions: a “vertical” dimension covering the size scale from atoms to the biosphere and a “horizontal” dimension that stretches across the diversity of life.
The latter includes not only present-day organisms, but also those that have existed throughout life’s history.
Living things show diversity and unity.
Life is enormously diverse.
Biologists have identified and named about 1.8 million species.
This diversity includes 5,200 known species of prokaryotes, 100,000 fungi, 290,000 plants, 50,000 vertebrates, and 1,000,000 insects.
Thousands of newly identified species are added each year.
Estimates of the total species count range from 10 million to more than 200 million.
In the face of this complexity, humans are inclined to categorize diverse items into a smaller number of groups.
Taxonomy is the branch of biology that names and classifies species into a hierarchical order.
Until the past decade, biologists divided the diversity of life into five kingdoms.
New methods, including comparisons of DNA among organisms, have led to a reassessment of the number and boundaries of the kingdoms.
Various classification schemes now include six, eight, or even dozens of kingdoms.
Coming from this debate has been the recognition that there are three even higher levels of classifications, the domains.
the three domains are Bacteria, Archaea, and Eukarya.
The first two domains, domain Bacteria and domain Archaea, consist of prokaryotes.
All the eukaryotes are now grouped into various kingdoms of the domain Eukarya.
The recent taxonomic trend has been to split the single-celled eukaryotes and their close relatives into several kingdoms.
Domain Eukarya also includes the three kingdoms of multicellular eukaryotes: the kingdoms Plantae, Fungi, and Animalia.
These kingdoms are distinguished partly by their modes of nutrition.
Most plants produce their own sugars and food by photosynthesis.
Most fungi are decomposers that absorb nutrients by breaking down dead organisms and organic wastes.
Animals obtain food by ingesting other organisms.
Underlying the diversity of life is a striking unity, especially at the lower levels of organization.
The universal genetic language of DNA unites prokaryotes and eukaryotes.
Among eukaryotes, unity is evident in many details of cell structure.
Above the cellular level, organisms are variously adapted to their ways of life.
How do we account for life’s dual nature of unity and diversity?
The process of evolution explains both the similarities and differences among living things.

Concept 1.4 Evolution accounts for life’s unity and diversity

The history of life is a saga of a changing Earth billions of years old, inhabited by a changing cast of living forms.
Charles Darwin brought evolution into focus in 1859 when he presented two main concepts in one of the most important and controversial books ever written, On the Origin of Species by Natural Selection.
Darwin ’s first point was that contemporary species arose from a succession of ancestors through “descent with modification.”
This term captured the duality of life’s unity and diversity: unity in the kinship among species that descended from common ancestors and diversity in the modifications that evolved as species branched from their common ancestors.
Darwin ’s second point was his mechanism for descent with modification: natural selection.
Darwin inferred natural selection by connecting two observations:
Observation 1: Individual variation. Individuals in a population of any species vary in many heritable traits.
Observation 2: Overpopulation and competition. Any population can potentially produce far more offspring than the environment can support. This creates a struggle for existence among variant members of a population.
Inference: Unequal reproductive success. Darwin inferred that those individuals with traits best suited to the local environment would leave more healthy, fertile offspring.
Inference: Evolutionary adaptation. Unequal reproductive success can lead to adaptation of a population to its environment. Over generations, heritable traits that enhance survival and reproductive success will tend to increase in frequency among a population’s individuals. The population evolves.
Natural selection, by its cumulative effects over vast spans of time, can produce new species from ancestral species.
For example, a population fragmented into several isolated populations in different environments may gradually diversify into many species as each population adapts over many generations to different environmental problems.
Fourteen species of finches found on the Galápagos Islands diversified after an ancestral finch species reached the archipelago from the South American mainland.
Each species is adapted to exploit different food sources on different islands.
Biologists’ diagrams of evolutionary relationships generally take a treelike form.
Just as individuals have a family tree, each species is one twig of a branching tree of life.
Similar species like the Galápagos finches share a recent common ancestor.
Finches share a more distant ancestor with all other birds.
The common ancestor of all vertebrates is even more ancient.
Trace life back far enough, and there is a shared ancestor of all living things.
All of life is connected through its long evolutionary history.

Concept 1.5 Biologists use various forms of inquiry to explore life

The word science is derived from a Latin verb meaning “to know.”
At the heart of science is inquiry, people asking questions about nature and focusing on specific questions that can be answered.
The process of science blends two types of exploration: discovery science and hypothesis-based science.
Discovery science is mostly about discovering nature.
Hypothesis-based science is mostly about explaining nature.
Most scientific inquiry combines the two approaches.
Discovery science describes natural structures and processes as accurately as possible through careful observation and analysis of data.
Discovery science built our understanding of cell structure and is expanding our databases of genomes of diverse species.
Observation is the use of the senses to gather information, which is recorded as data.
Data can be qualitative or quantitative.
Quantitative data are numerical measurements.
Qualitative data may be in the form of recorded descriptions.
Jane Goodall has spent decades recording her observations of chimpanzee behavior during field research in Gambia .
She has also collected volumes of quantitative data over that time.
Discovery science can lead to important conclusions based on inductive reasoning.
Through induction, we derive generalizations based on a large number of specific observations.
In science, inquiry frequently involves the proposing and testing of hypotheses.
A hypothesis is a tentative answer to a well-framed question.
It is usually an educated postulate, based on past experience and the available data of discovery science.
A scientific hypothesis makes predictions that can be tested by recording additional observations or by designing experiments.
A type of logic called deduction is built into hypothesis-based science.
In deductive reasoning, reasoning flows from the general to the specific.
From general premises, we extrapolate to a specific result that we should expect if the premises are true.
In hypothesis-based science, deduction usually takes the form of predictions about what we should expect if a particular hypothesis is correct.
We test the hypothesis by performing the experiment to see whether or not the results are as predicted.
Deductive logic takes the form of “If . . . then” logic.
Scientific hypotheses must be testable.
There must be some way to check the validity of the idea.
Scientific hypotheses must be falsifiable.
There must be some observation or experiment that could reveal if a hypothesis is actually not true.
The ideal in hypothesis-based science is to frame two or more alternative hypotheses and design experiments to falsify them.
No amount of experimental testing can prove a hypothesis.
A hypothesis gains support by surviving various tests that could falsify it, while testing falsifies alternative hypotheses.
Facts, in the form of verifiable observations and repeatable experimental results, are the prerequisites of science.
We can explore the scientific method.
There is an idealized process of inquiry called the scientific method.
Very few scientific inquiries adhere rigidly to the sequence of steps prescribed by the textbook scientific method.
Discovery science has contributed a great deal to our understanding of nature without most of the steps of the so-called scientific method.
We will consider a case study of scientific research.
This case begins with a set of observations and generalizations from discovery science.
Many poisonous animals have warning coloration that signals danger to potential predators.
Imposter species mimic poisonous species, although they are harmless.
An example is the bee fly, a nonstinging insect that mimics a honeybee.
What is the function of such mimicry? What advantage does it give the mimic?
In 1862, Henry Bates proposed that mimics benefit when predators mistake them for harmful species.
This deception may lower the mimic’s risk of predation.
In 2001, David and Karin Pfennig and William Harcombe of the University of North Carolina designed a set of field experiments to test Bates’s mimicry hypothesis.
In North and South Carolina , a poisonous snake called the eastern coral snake has warning red, yellow, and black coloration.
Predators avoid these snakes. It is unlikely that predators learn to avoid coral snakes, as a strike is usually lethal.
Natural selection may have favored an instinctive recognition and avoidance of the warning coloration of the coral snake.
The nonpoisonous scarlet king snake mimics the ringed coloration of the coral snake.
Both king snakes and coral snake live in the Carolinas , but the king snake’s range also extends into areas without coral snakes.
The distribution of these two species allowed the Pfennigs and Harcombe to test a key prediction of the mimicry hypothesis.
Mimicry should protect the king snake from predators, but only in regions where coral snakes live.
Predators in non–coral snake areas should attack king snakes more frequently than predators that live in areas where coral snakes are present.
To test the mimicry hypothesis, Harcombe made hundreds of artificial snakes.
The experimental group had the red, black, and yellow ring pattern of king snakes.
The control group had plain, brown coloring.
Equal numbers of both types were placed at field sites, including areas where coral snakes are absent.
After four weeks, the scientists retrieved the fake snakes and counted bite or claw marks.
Foxes, coyotes, raccoons, and black bears attacked snake models.
The data fit the predictions of the mimicry hypothesis.
The ringed snakes were attacked by predators less frequently than the brown snakes only within the geographic range of the coral snakes.
The snake mimicry experiment provides an example of how scientists design experiments to test the effect of one variable by canceling out the effects of unwanted variables.
The design is called a controlled experiment.
An experimental group (artificial king snakes) is compared with a control group (artificial brown snakes).
The experimental and control groups differ only in the one factor the experiment is designed to test—the effect of the snake’s coloration on the behavior of predators.
The brown artificial snakes allowed the scientists to rule out such variables as predator density and temperature as possible determinants of number of predator attacks.
Scientists do not control the experimental environment by keeping all variables constant.
Researchers usually “control” unwanted variables, not by eliminating them but by canceling their effects using control groups.
Let’s look at the nature of science.
There are limitations to the kinds of questions that science can address.
These limits are set by science’s requirements that hypotheses are testable and falsifiable and that observations and experimental results be repeatable.
The limitations of science are set by its naturalism.
Science seeks natural causes for natural phenomena.
Science cannot support or falsify supernatural explanations, which are outside the bounds of science.
Everyday use of the term theory implies an untested speculation.
The term theory has a very different meaning in science.
A scientific theory is much broader in scope than a hypothesis.
This is a hypothesis: “Mimicking poisonous snakes is an adaptation that protects nonpoisonous snakes from predators.”
This is a theory: “Evolutionary adaptations evolve by natural selection.”
A theory is general enough to generate many new, specific hypotheses that can be tested.
Compared to any one hypothesis, a theory is generally supported by a much more massive body of evidence.
The theories that become widely adopted in science (such as the theory of adaptation by natural selection) explain many observations and are supported by a great deal of evidence.
In spite of the body of evidence supporting a widely accepted theory, scientists may have to modify or reject theories when new evidence is found.
As an example, the five-kingdom theory of biological diversity eroded as new molecular methods made it possible to test some of the hypotheses about the relationships between living organisms.
Scientists may construct models in the form of diagrams, graphs, computer programs, or mathematical equations.
Models may range from lifelike representations to symbolic schematics.
Science is an intensely social activity.
Most scientists work in teams, which often include graduate and undergraduate students.
Both cooperation and competition characterize scientific culture.
Scientists attempt to confirm each other’s observations and may repeat experiments.
They share information through publications, seminars, meetings, and personal communication.
Scientists may be very competitive when converging on the same research question.
Science as a whole is embedded in the culture of its times.
For example, recent increases in the proportion of women in biology have had an impact on the research being performed.
For instance, there has been a switch in focus in studies of the mating behavior of animals from competition among males for access to females to the role that females play in choosing mates.
Recent research has revealed that females prefer bright coloration that “advertises” a male’s vigorous health, a behavior that enhances a female’s probability of having healthy offspring.
Some philosophers of science argue that scientists are so influenced by cultural and political values that science is no more objective than other ways of “knowing nature.”
At the other extreme are those who view scientific theories as though they were natural laws.
The reality of science is somewhere in between.
The cultural milieu affects scientific fashion, but need for repeatability in observation and hypothesis testing distinguishes science from other fields.
If there is “truth” in science, it is based on a preponderance of the available evidence.
Science and technology are functions of society.
Although science and technology may employ similar inquiry patterns, their basic goals differ.
The goal of science is to understand natural phenomena.
Technology applies scientific knowledge for some specific purpose.
Technology results from scientific discoveries applied to the development of goods and services.
Scientists put new technology to work in their research.
Science and technology are interdependent.
The discovery of the structure of DNA by Watson and Crick sparked an explosion of scientific activity.
These discoveries made it possible to manipulate DNA, enabling genetic technologists to transplant foreign genes into microorganisms and mass-produce valuable products.
DNA technology and biotechnology have revolutionized the pharmaceutical industry.
They have had an important impact on agriculture and the legal profession.
The direction that technology takes depends less on science than it does on the needs of humans and the values of society.
Debates about technology center more on “should we do it” than “can we do it.”
With advances in technology come difficult choices, informed as much by politics, economics, and cultural values as by science.
Scientists should educate politicians, bureaucrats, corporate leaders, and voters about how science works and about the potential benefits and hazards of specific technologies.

Concept 1.6 A set of themes connects the concepts of biology

In some ways, biology is the most demanding of all sciences, partly because living systems are so complex and partly because biology is a multidisciplinary science that requires knowledge of chemistry, physics, and mathematics.
Biology is also the science most connected to the humanities and social sciences.

Chapter 26 Early Earth & the Origin of Life

Chapter 26 Early Earth & the Origin of Life

Objectives

The Origin of Life
1.	Describe the four stages of the hypothesis for the origin of life on Earth by chemical evolution.
2.	Describe the contributions that A. I. Oparin, J.B.S. Haldane, and Stanley Miller made toward developing a model for the abiotic synthesis of organic molecules. Describe the conditions and locations where most of these chemical reactions probably occurred on Earth.
3.	Describe the evidence that suggests that RNA was the first genetic material. Explain the significance of the discovery of ribozymes.
4.	Describe how natural selection may have worked in an early RNA world.
5.	Describe how natural selection may have favored the proliferation of stable protobionts with self-replicating, catalytic RNA.

	Introduction to the History of Life
6.	Explain how the histories of Earth and life are inseparable.
7.	Explain how index fossils can be used to determine the relative age of fossil-bearing rock strata. Explain how radiometric dating can be used to determine the absolute age of rock strata. Explain how magnetism can be used to date rock strata.
8.	Describe the major events in Earth’s history from its origin until 2 billion years ago. In particular, note when Earth first formed, when life first evolved, and what forms of life existed in each eon.
9.	Describe the mass extinctions of the Permian and Cretaceous periods. Discuss a hypothesis that accounts for each of these mass extinctions.

	The Major Lineages of Life
10.	Describe how chemiosmotic ATP production may have arisen.
11.	Describe the timing and significance of the evolution of oxygenic photosynthesis.
12.	Explain the endosymbiotic theory for the evolution of the eukaryotic cell. Describe the evidence that supports this theory.
13.	Explain how genetic annealing may have led to modern eukaryotic genomes.
14.	Describe the timing of key events in the evolution of the first eukaryotes and later multicellular eukaryotes.
15.	Explain how the snowball-Earth hypothesis explains why multicellular eukaryotes were so limited in size, diversity, and distribution until the late Proterozoic.
16.	Describe the key evolutionary adaptations that arose as life colonized land.
17.	Explain how continental drift explains Australia’s unique flora and fauna.
18.	Explain why R. H. Whittaker’s five-kingdom system has been replaced by a new system with three domains.

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