Biology Chapter 8 Notes

8/8/2019 Biology Chapter 8 Notes

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Objectives

y Describe the structure of a chloroplast.y Identify the overall reactants and products of photosynthesis.

Key Terms

y chloroplast

y chlorophyll

y stroma

y thylakoid

y light reactions

y Calvin cycle

As you read in Chapter 7, photosynthesis is the process by which plants and other producers convert the energy of sunlight into the energy stored

in organic molecules. Just as cellular respiration takes place largely within a cell's mitochondria, photosynthesis also occurs in a specificorganelle.

The Structure of Chloroplasts The cellular organelle where photosynthesis takes place is called a chloroplast. Chloroplasts contain chemical compounds called chlorophylls thatgive these organelles a green color. When you observe a plant, all the green parts you can see contain cells with chloroplasts and can carry out

photosynthesis. In most plants, the leaves contain the most chloroplasts and are the major sites of photosynthesis.

Within a leaf, the chloroplasts are concentrated in the cells of the mesophyll, the inner layer of ti ssue (Figure 8-2). Tiny pores called stomata(singular, stoma) are found on the surface of the leaf. Carbon dioxide enters the leaf and oxygen exits the leaf through the stomata. Veins carry

water and nutrients from the plant's roots to the leaves. The veins also deliver organic molecules produced in the leaves to other parts of the plant.

Figure 8-2

Photosynthesis takes place in cellular organelles called chloroplasts. In this sunflower, the greatestnumbers of chloroplasts are located in the leaves. Chlorophylls give the chloroplasts²and in turn the

leaves²their green color.

The chloroplast's structure is key to its function. Like a mitochondrion, a chloroplast has an inner and an outer membrane. The inner membrane

encloses a thick fluid called stroma. Suspended in the stroma are many disk-shaped sacs called thylakoids. Each thylakoid is enclosed by a

membrane. The thylakoids are arranged in stacks called grana (singular, gr anum). These various structures within the chloroplast organize thecomplex series of chemical reactions that make up the overall process of photosynthesis. Some of the steps take place in the thylakoid membranes, while others take place in the stroma.

Overview of Photosynthesis You have read that cellular respiration involves the process of electron transfer. The "fall" of electrons from glucose to oxygen releases energy,

which is then used to make ATP. The opposite occurs in photosynthesis. Electrons from water are boosted "uphill" by the energy from sunlight.



The chloroplast uses these "excited" electrons, along with carbon dioxide and hydrogen ions, to produce sugar molecules. The reaction steps add up to the overall chemical equation for photosynthesis shown in Figure 8-3.

Figure 8-3As in cellular respiration, the chemical equation for photosynthesissummarizes many reaction steps.

Photosynthesis occurs in two main stages, each with many steps: the light reactions and the Calvin cycle (Figure 8-4).

Figure 8-4This "road map" shows the two main stages of photosynthesis: the light

reactions, which occur in the thylakoids, and the Calvin cycle, which occursin the stroma.

The Light Reactions The light reactions convert the energy in sunlight to chemical energy. These reactions depend on molecules built into the

membranes of the thylakoids. First, chlorophyll molecules in the membranes capture light energy. Then the chloroplasts use the captured energy

to remove electrons from water. This splits the water into oxygen and hydrogen ions. The oxygen is a "waste product" of photosynthesis. It

escapes to the atmosphere through the stomata of leaves. What becomes of the water's electrons and hydrogen ions? Chloroplasts use them tomake an energy-rich molecule called NADPH. (NADPH is an electron carrier very similar to the NADH you read about in Chapter 7.) The

chloroplasts also use the captured light energy to generate ATP. The overall result of the light reactions is the conversion of light energy tochemical energy stored in two compounds: NADPH and ATP.

The Calvin Cycle The Calvin cycle makes sugar from the atoms in carbon dioxide plus the hydrogen ions and the high-energy electrons carried

by NADPH. The enzymes for the Calvin cycle are located outside the thylakoids and dissolved in the stroma. The ATP made by the lightreactions provides the energy to make sugar.

TheCalvin cycle is sometimes referred to as the "light-independent reactions" because, unlike the light reactions, it does not directly require light

to begin. However, this doesn't mean that the Calvin cycle can continue running in a plant kept in the dark. The Calvin cycle requires two inputs

supplied by the light reactions, ATP and NADPH. You'll explore both the light reactions and the Calvin cycle in more detail in Concept 8.2 and Concept 8.3.

Objectives

y Explain how light interacts with pigments.

y Describe how photosystems help harvest light energy.

y Identify the chemical products of the light reactions.



Key Terms

y wavelength

y electromagnetic spectrum

y pigment

y paper chromatography

y photosystem

Chloroplasts are like chemical factories inside plant cells. The energy to run these factories comes from the sun, an energy source more than 150million kilometers from Earth. In this section, you'll follow the chain of events that occurs when sunlight enters a chloroplast

Light Energy and Pigments Sunlight is a form of electromagnetic energy. Electromagnetic energy travels in waves that can be compared to ocean waves rolling onto a beach.The distance between two ad jacent waves is called a wavelength. The different forms of electromagnetic energy have characteristic wavelengths,as shown in Figure 8-5. The range of types of electromagnetic energy, from the very short wavelengths of gamma rays to the very long

wavelengths of radio waves, is called the electromagnetic spectrum.

Figure 8-5Different forms of electromagnetic energy have different wavelengths.

Shorter wavelengths have more energy than longer wavelengths.

Visible light²those wavelengths that your eyes see as different colors²makes up only a small fraction of the electromagnetic spectrum. Visible

light consists of wavelengths from about 400 nanometers (nm), violet , to about 700 nm, red. Shorter wavelengths have more energy than longer wavelengths. In fact, wavelengths that are shorter than those of visible light have enough energy to damage organic molecules such as proteinsand nucleic acids. This is why being exposed to the ultraviolet (UV) radiation in sunlight can cause sunburns and lead to skin cancer.

Pigments and Color A substance's color is due to chemical compounds called pigments. When light shines on a material that contains pigments, three things can happen to the different wavelengths: they can be absorbed, transmitted, or reflected. The pigments in the leaf's

chloroplasts absorb blue-violet and red-orange light very well. The chloroplasts convert some of this absorbed light energy into chemical energy.But the chloroplast pigments do not absorb green light well. As shown in Figure 8-6, most of the green light passes through the leaf (is

transmitted) or bounces back (is reflected). Leaves look green because the green light is not absorbed.

Figure 8-6

Of the visible light striking this chloroplast, the greenlight is reflected and transmitted more than other colors,which are absorbed. As a result, a leaf containing

chloroplasts appears green in color.



Identifying Chloroplast Pigments Using a laboratory technique called paper chromatography, you could observe the different pigments in agreen leaf. First you would press the leaf onto a strip of filter paper to deposit a "stain." Next you would seal the paper in a cylinder containing

solvents, working under a vented laboratory hood. (In Online Activity 8.2, you can carry out a virtual paper chromatography experiment.)

As the solvents move up the paper strip, the pigments dissolve in the solvents and are carried up the strip. Different pigments travel at different

rates, depending on how easily they dissolve and how strongly they are attracted to the paper. Figure 8-7 shows some chromatography results. Notice that several different pigments have separated out on the paper. Chlorophyll a, which absorbs mainly blue-violet and red light and reflectsmainly green light, plays a major role in the light reactions of photosynthesis. Chloroplasts also contain other "helper" pigments. These include

chlorophyll b, which absorbs mainly blue and orange light and reflects yellow-green; and several types of carotenoids, which absorb mainly blue-green light and reflect yellow-orange.

Figure 8-7The laboratory technique of paper chromatography can be used to analyzethe pigments in a leaf.

Harvesting Light Energy Suppose that you could observe what happens inside a chloroplast as sunlight strikes a leaf. Within the thylakoid membrane, chlorophyll and

other molecules are arranged in clusters called photosystems (Figure 8-8). Each photosystem contains a few hundred pigment molecules,including chlorophyll a, chlorophyll b, and carotenoids. This cluster of pigment molecules acts like a l ight-gathering panel, somewhat like a

miniature version of a solar collector.

Figure 8-8When light strikes the chloroplast, pigment molecules absorb the energy. This energy jumps frommolecule to molecule until it arrives at the reaction center.

Each time a pigment molecule absorbs light energy, one of the pigment's electrons gains energy²the electron is raised from a low-energy"ground state" to a high-energy "excited state." This excited state is very unstable. Almost immediately, the excited electron falls back to the

ground state and transfers the energy to a neighboring molecule. The energy transfer excites an electron in the receiving molecule. When this

electron drops back to the ground state, it excites an electron in the next pigment molecule, and so on. In this way, the energy "jumps" frommolecule to molecule until it arrives at what is called the reaction center of the photosystem.

The reaction center consists of a chlorophyll a molecule located next to another molecule called a primary electron acceptor. The primary

electron acceptor is a molecule that traps the excited electron from the chlorophyll a molecule. Other teams of molecules built into the thylakoid

membrane can now use that trapped energy to make ATP and NADPH.

Chemical Products of Light Reactions Two photosystems are involved in the light reactions, as shown in Figure 8-10. The first photosystem traps light energy and transfers the light-

excited electrons to an electron transport chain. This photosystem can be thought of as the "water-splitting photosystem" because the electrons are

replaced by splitting a molecule of water. This process releases oxygen as a waste product, and also releases hydrogen ions.



Figure 8-10

The light reactions involve two photosystems connected by an electron transport chain.

The electron transport chain connecting the two photosystems releases energy, which the chloroplast uses to make ATP. This mechanism of ATP

production is very similar to ATP production in cellular respiration. In both cases, an electron transport chain pumps hydrogen ions across amembrane²the inner mitochondrial membrane in respiration and the thylakoid membrane in photosynthesis. The main difference is that inrespiration food provides the electrons for the electron transport chain, while in photosynthesis light-excited electrons from chlorophyll travel

down the chain.

The second photosystem can be thought of as the "NADPH-producing photosystem." This photosystem produces NADPH by transferring excited

electrons and hydrogen ions to NADP+. Figure 8-11 shows a mechanical analogy for the light reactions. Note how the light energy "bumps up"

the electrons to their excited state in each photosystem.

Figure 8-11

In this "construction analogy" for the light reactions, the input of light energy is represented bythe large yellow mallets. The light energy boosts the electrons up to their excited states atop theplatform in each photosystem. The energy released as the electrons move down the electron

transport chain between the photosystems is used to pump hydrogen ions across a membraneand produce ATP.

The light reactions convert light energy to the chemical energy of ATP and NADPH. But recall that photosynthesis also produces sugar. So far no

sugar has been produced. That is the job of the Calvin cycle, which uses the ATP and NADPH produced by the light reactions.



Objectives

y Explain how the

Calvin cycle makes sugar.

y Summarize the overall process of photosynthesis.

It would be unfortunate for humans and many other living things if photosynthesis stopped after the light reactions. The process so far hasreleased one important final product, oxygen. But as you have read, organisms depend on the sugars and other organic compounds produced by

plants as fuel for cellular respiration and as building materials. The Calvin cycle is responsible for producing the raw materials for these

compounds.

A Trip Around the Calvin Cycle You can think of the Calvin cycle as being somewhat like a sugar factory within a chloroplast. It is called a cycle because, like the Krebs cycle in

cellular respiration, the starting material is regenerated each time the process occurs. In this case, the starting material that gets regenerated is acompound called RuBP, a sugar with five carbons.

With each turn of the Calvin cycle, there are chemical inputs and outputs. The inputs are carbon dioxide from the air and the ATP and NADPH

produced by the light reactions. The Calvin cycle uses carbon from the carbon dioxide, energy from the ATP, and high-energy electrons and

hydrogen ions from the NADPH. The cycle's output is an energy-rich sugar molecule. That sugar is not yet glucose, but a smaller sugar named G3P. The plant cell uses G3P as the raw material to make glucose and other organic molecules it needs. You can follow the process of the Calvincycle in Figure 8-13.

Figure 8-13Follow the fate of three carbon dioxide molecules through the Calvin cycle. This diagram showssimplified representations of some of the molecules formed during the reactions. Each gray ball

represents a carbon atom.

Summary of Photosynthesis

Now that you've read about the details of photosynthesis, take a step back and look at the overall process again. Recall that the overall equationfor photosynthesis is:

6 CO2 + 6 H2O C6H12O6 + 6 O2



The light reactions, which take place in the thylakoid membranes, convert light energy to the chemical energy of ATP and NADPH. The lightreactions use the reactant water from the equation and release the product oxygen. The Calvin cycle, which takes place in the stroma, uses ATP

and NADPH to convert carbon dioxide to sugar (Figure 8-14).

Figure 8-14

The light reactions and the Calvin cycle together convert light energy to thestored chemical energy of sugar. The plant can use the sugar to build other organic molecules.

By converting light energy to chemical energy, photosynthesis is the first step in the flow of energy through an ecosystem. Some of that chemicalenergy then passes from producers to consumers. Even when people eat meat, you can trace its stored energy back to photosynthesis. For

example, the beef in a hamburger came from cattle that ate plants. Photosynthesis is the ultimate source of all the food you eat and all the oxygen

you breathe.

Objectives

y Describe the path of carbon in the carbon cycle.

y Explain how photosynthesis is related to climate.

Key Terms

y carbon cycle

y greenhouse effect

For the past several chapters, you have been navigating the microscopic world of cells as though you were a miniature explorer. Now imagine

you could zoom out into space and look at planet Earth as a whole. Keep reading to see how these cellular processes fit into your new perspectiveof life on Earth.

The Carbon Cycle Some of the processes that occur on a global scale on Earth depend on the metabolism of tiny chloroplasts and mitochondria. An example is thecarbon cycle, the process by which carbon moves from inorganic to organic compounds and back. Through photosynthesis, producers such as

grass convert inorganic carbon dioxide to organic compounds. Consumers such as a Cape buffalo obtain the organic compounds by eating the producers. Cape buffalo may in turn be eaten by a lioness or another consumer. Ultimately, cellular respiration by both producers and consumersreturns carbon dioxide to the atmosphere.

No other chemical process on the planet matches the output of photosynthesis. Earth's plants and other photosynthetic organisms make about 160 billion metric tons of organic material per year. That's about equal to 80 trillion copies of this book²25 stacks of books reaching from Earth to

the sun!



Photosynthesis and Global Climate As you have just read, a key element of the carbon cycle is carbon dioxide. Plants use carbon dioxide to make sugars in photosynthesis, and most

organisms give off carbon dioxide as waste from cellular respiration. Though any one organism may use or produce relatively small amounts of carbon dioxide, the total effect of all the organisms on Earth has a very large effect on the amount of carbon dioxide in the atmosphere.

Before this century, carbon dioxide made up about 0.03 percent (300 parts per million) of Earth's atmosphere. This amount of carbon dioxide isenough to provide plants with plenty of carbon for photosynthesis.Carbon dioxide in the atmosphere also traps heat from the sun that would otherwise escape from Earth back into space (Figure 8-16). This important property, known as the greenhouse effect, keeps the world climate

warm enough for living things. The greenhouse effect keeps the average temperature on Earth some 10°C warmer than it would be otherwise.

Figure 8-16Some heat radiating from Earth's surface back out toward space is trapped bycarbon dioxide (along with some other types of gases) in the atmosphere. Thisgreenhouse effect keeps Earth warm enough for living things.

In the past century, the amount of atmospheric carbon dioxide has been rising, reaching more than 360 parts per million. In Chapter 36, you'llread more about this change and about its possible effects.

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Biology Chapter 8 Notes