Ch. 10 Photosynthesis Dynamic Study Module Flashcards

Through the stomata

Ex.

Carbon dioxide enter the leaf through the stomata.

Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores on the leaf surface called stomata. The CO₂ is required for photosynthesis. However, stomata are also the main avenues of transpiration, the evaporative loss of water from leaves. On a hot, dry day, most plants close their stomata, a response that conserves water. This response also reduces photosynthetic yield by limiting access to CO₂.

While it did have access to light, the plant stored energy in the form of sugars or starch, and it was able to derive energy from the stored molecules during your vacation.

Ex.

You have a large, healthy philodendron that you carelessly leave in total darkness while you are away on vacation. While it did have access to light, the plant stored energy in the form of sugars or starch, and it was able to derive energy from the stored molecules during your vacation.

Most plants manage to make more organic material each day than they need to use as respiratory fuel and precursors for biosynthesis. They stockpile the extra sugar by synthesizing starch, storing some in the chloroplasts themselves and some in storage cells of roots, tubers, seeds, and fruits. In accounting for the consumption of the food molecules produced by photosynthesis, let’s not forget that most plants lose leaves, roots, stems, fruits, and sometimes their entire bodies to heterotrophs, including humans. With this, it is apparent that a large, healthy plant that is neglected and left in total darkness can survive for a time on stores of food.

Thylakoid membrane

Ex.

The electrochemical gradient that drives ATP synthesis in the light reactions is formed across the thylakoid membrane.

Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis. An electron transport chain pumps protons (H⁺) across a membrane as electrons are passed through a series of carriers that are progressively more electronegative. Thus, electron transport chains transform redox energy to a proton-motive force, which is potential energy stored in the form of an H⁺ gradient across a membrane. An ATP synthase complex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming ATP. The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space (the interior of the thylakoid), which functions as the H⁺ reservoir. ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane. Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle.

The stroma is the fluid inside the chloroplast in which the Calvin cycle occurs.

Cristae are the inner membrane folds found in a mitochondrion.

None of the reactions of photosynthesis occurs on the outer membrane of the chloroplast.

Stomata are microscopic pores through which oxygen and carbon dioxide diffuse.

Photosystem II absorbs light energy, which drives the first steps in the light reactions, but it is not involved in chemiosmosis.

Autotrophs, but not heterotrophs, can nourish themselves beginning with CO₂ and other nutrients that are inorganic.

Ex.

The following statement is a correct distinction between autotrophs and heterotrophs: Autotrophs, but not heterotrophs, can nourish themselves beginning with CO₂ and other nutrients that are inorganic .

Almost all plants are autotrophs because the only nutrients they require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize organic substances. Photosynthesis also occurs in algae, certain other protists, and some prokaryotes.

Heterotrophs are unable to make their own food and they live on compounds produced by other organisms. Heterotrophs are the biosphere’s consumers. The most obvious form of this “other-feeding” occurs when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic litter such as carcasses, feces, and fallen leaves. These heterotrophs are known as decomposers. Most fungi and many types of prokaryotes get their nourishment this way.

Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on photoautotrophs for food—and also for oxygen, a by-product of photosynthesis.

The Calvin cycle incorporates each CO ₂ molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate.

Ex.

The Calvin cycle incorporates each CO₂ molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate.

The Calvin cycle is divided into three phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor. In carbon fixation, the Calvin cycle incorporates each CO₂ molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (abbreviated “RuBP”). The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or rubisco. (This is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.) The product of the reaction is a six-carbon intermediate that is short-lived because it is so energetically unstable that it immediately splits in half, forming two molecules of 3-phosphoglycerate (for each CO₂ fixed).

G3P is produced in the reduction phase of the Calvin cycle.

ATP is produced during the light reactions.

RuBP is regenerated in the third phase of the Calvin cycle, regeneration of the CO₂ acceptor.

NADPH is produced during the light reactions.

Molecular oxygen is produced during the light reactions.

ATP and NADPH produced in the light reactions provide the energy for the production of sugars in the Calvin cycle.

Ex.

ATP and NADPH produced in the light reactions provide the energy for the production of sugars in the Calvin cycle.

For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of nine molecules of ATP and six molecules of NADPH. The light reactions regenerate the ATP and NADPH, which are in turn used as energy sources in the Calvin cycle. Neither the light reactions nor the Calvin cycle alone can make sugar from CO₂.

ADP and NADP⁺ are the oxidized forms of ATP and NADPH and do not possess much energy.

Molecular oxygen is a by-product of the light reactions and does not provide energy for the Calvin cycle.

G3P is the main product of the Calvin cycle.

RuBP is a part of the Calvin cycle, but it is not produced in the light reactions and does not provide an energy source for the Calvin cycle.

Although photons of light drive the light reactions and, thus, the synthesis of ATP and NADPH, they are not directly involved in the Calvin cycle.

The light reactions by linear electron flow

Ex.

Oxygen is produced in the light reactions by linear electron flow.

Light drives the synthesis of ATP and NADPH by energizing the two photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called linear electron flow, and it occurs during the light reactions of photosynthesis. There are multiple steps in this process. In the step that produces oxygen, an enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H⁺), and an oxygen atom. The H⁺ are released into the thylakoid space. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O₂.

The excitation of electrons in chlorophyll is the first step in the light reactions, but it does not produce molecular oxygen.

The Calvin cycle produces G3P, not molecular oxygen.

Cyclic electron flow during the light reactions helps produce ATP and NADPH but not molecular oxygen.

Chemiosmosis in the light reactions produces ATP, not molecular oxygen.

CO₂ is reduced.

Ex.

CO₂ is reduced during the Calvin cycle.

Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis is a redox process: H₂O is oxidized, and CO₂ is reduced. The light reactions in the thylakoid membranes split water, releasing O₂, producing ATP, and forming NADPH. The Calvin cycle in the stroma forms sugar from CO₂, using ATP for energy and NADPH for reducing power.

their electrons become excited

Ex.

When chloroplast pigments absorb light, their electrons become excited.

When a molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital where it has more potential energy. When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption of a photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to the energy difference between the ground state and an excited state, and this energy difference varies from one kind of molecule to another. Thus, a particular compound absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum.

Stomata

Ex.

Carbon dioxide and oxygen diffuse through stomata on the underside of leaves.

All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants. There are about half a million chloroplasts in a chunk of leaf with a top surface area of 1 mm2. Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf. Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata (singular, stoma; from the Greek, meaning “mouth”).

The chloroplast is the organelle in a plant cell in which photosynthesis takes place.

The stem of a leaf is part of the vascular tissue of a plant.

The thylakoids are the internal membranes of a chloroplast.

Stroma is the fluid found inside a chloroplast.

A granum is a single unit of the membranous sacs that make up the thylakoid.

All of the listed structures are parts of a photosystem.

Ex.

All of the listed structures are parts of a photosystem.

A photosystem is composed of a reaction-center complex surrounded by several light-harvesting complexes. The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules. Each light-harvesting complex consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins. The reaction-center complex also contains a molecule capable of accepting electrons and becoming reduced; this is called the primary electron acceptor.

The Calvin cycle requires products only produced when the photosystems are illuminated.

Ex.

The reactions of the Calvin cycle are not directly dependent on light, but they usually do not occur at night because the Calvin cycle requires products only produced when the photosystems are illuminated .

The two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part).

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off O₂ as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+, where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH, a source of electrons as “reducing power” that can be passed along to an electron acceptor, reducing it, and ATP, the versatile energy currency of cells. The light reactions do not produce sugar; that happens in the second stage of photosynthesis, the Calvin cycle.

The Calvin cycle begins by incorporating CO₂ from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO₂ to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the light reactions provide the NADPH and ATP that the Calvin cycle requires.

So, in essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis.

NADPH; ATP; oxygen

Ex.

The light reactions of photosynthesis generate high-energy electrons, which end up in NADPH. The light reactions also produce ATP and oxygen.

The electrons are transferred to NADP+, forming NADPH. ATP is formed via photophosphorylation, and oxygen is produced when water molecules are split. The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H+) and giving off O₂ as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with an H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation.

Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH, a source of electrons as “reducing power” that can be passed along to an electron acceptor, reducing it, and ATP, the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.

Thylakoid membranes

Ex.

The chlorophyll molecules are embedded in the thylakoid membranes of the chloroplasts.

A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma. Suspended within the stroma is a third membrane system, made up of sacs called thylakoids, which segregates the stroma from the thylakoid space inside these sacs. In some places, thylakoid sacs are stacked in columns called grana (singular: granum). Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast.

water

Ex.

The source of the oxygen produced by photosynthesis has been identified through experiments using radioactive tracers. The oxygen comes from water.

C. B. van Niel hypothesized that plants split H₂O as a source of electrons from hydrogen atoms, releasing O₂ as a by-product. Nearly 20 years later, scientists confirmed van Niel’s hypothesis by using oxygen-18 (18O), a heavy isotope, as a tracer to follow the fate of oxygen atoms during photosynthesis. The experiments showed that the O₂ from plants was labeled with 18O only if water was the source of the tracer. If the 18O was introduced to the plant in the form of CO₂, the label did not turn up in the released O₂. Photosynthesis is a redox process where H₂O is oxidized and CO₂ is reduced. The light reactions in the thylakoid membranes split water, releasing O₂, producing ATP, and forming NADPH. The Calvin cycle in the stroma forms sugar from CO₂, using ATP for energy and NADPH for reducing power.

Stroma

Ex.

The Calvin cycle occurs in the stroma.

The Calvin cycle is named for Melvin Calvin, who, along with his colleagues James Bassham and Andrew Benson, began to elucidate its steps in the late 1940s. The cycle begins by incorporating CO₂ from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO₂ to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The entire process occurs in the stroma of the chloroplast.

The light reactions occur in the thylakoids.

A granum is one of the membrane stacks in the thylakoids. The light reactions occur in the thylakoids.

Stomata are microscopic pores through which oxygen and carbon dioxide diffuse.

Cellular respiration, not photosynthesis, occurs in the mitochondrion.

None of the reactions of photosynthesis occur on the outer membrane of the chloroplast.

carbon dioxide

Ex.

In photosynthesis, plants use carbon from carbon dioxide to make sugar and other organic molecules.

Less than one percent of the atmosphere is CO₂, but that is enough to support photosynthesis. As part of photosynthesis, the Calvin cycle incorporates CO₂ into organic molecules, which are converted to sugar. The cycle begins by incorporating CO₂ from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired its cargo of electrons in the light reactions. To convert CO₂ to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions.

Oxidative phosphorylation in cellular respiration

Ex.

The process that is most like photophosphorylation is oxidative phosphorylation in cellular respiration.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H⁺) and giving off O₂ as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP⁺ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. The electron acceptor NADP⁺ is a first cousin to NAD⁺, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP⁺ molecule. The light reactions use solar energy to reduce NADP⁺ to NADPH by adding a pair of electrons along with an H⁺. The light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds, NADPH and ATP. NADPH, a source of electrons, acts as a “reducing power” that can be passed along to an electron acceptor and reduce it, while ATP is the versatile energy currency of cells.

The Calvin cycle produces G3P, not ATP.

Glycolysis is not part of photosynthesis.

The citric acid cycle is not part of photosynthesis.

Photorespiration consumes ATP rather than produces it.

Carbon fixation is part of the Calvin cycle, which does not produce ATP.

use chemiosmosis to produce ATP

Ex.

Both mitochondria and chloroplasts use chemiosmosis to produce ATP.

Chloroplasts and mitochondria generate ATP by the same basic mechanism: chemiosmosis. An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. In this way, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a membrane. Built into the same membrane is an ATP synthase complex that couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. Some of the electron carriers, including the iron-containing proteins called cytochromes, are very similar in chloroplasts and mitochondria. The ATP synthase complexes of the two organelles are also very much alike.

But there are noteworthy differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. In mitochondria, the high-energy electrons dropped down the transport chain are extracted from organic molecules (which are thus oxidized), whereas in chloroplasts, the source of electrons is water. Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the transport chain.

In other words, mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy in ATP.

Water

Ex.

The electrons entering photosystem II come from the splitting of water molecules.

The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions of photosynthesis. They are called photosystem II (PS II) and photosystem I (PS I). (They were named in order of their discovery, but photosystem II functions first in the light reactions.) The two photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions. A photon of light strikes a pigment molecule in a light-harvesting complex of PS II, boosting one of its electrons to a higher energy level. As this electron falls back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state.

The process continues, with the energy being relayed to other pigment molecules until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. It excites an electron in this pair of chlorophylls to a higher energy state. This electron is transferred from the excited P680 to the primary electron acceptor. We can refer to the resulting form of P680, missing an electron, as P680+.

Next, an enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions (H⁺), and an oxygen atom. The electrons are supplied one by one to the P680+ pair, each electron replacing one transferred to the primary electron acceptor. The H+ are released into the thylakoid lumen. The oxygen atom immediately combines with an oxygen atom generated by the splitting of another water molecule, forming O₂.

thylakoids

Ex.

The light reactions occur in the thylakoids.

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Water is split, providing a source of electrons and protons (hydrogen ions, H⁺) and giving off O₂ as a by-product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP⁺ (nicotinamide adenine dinucleotide phosphate), where they are temporarily stored. The electron acceptor NADP⁺ is a first cousin to NAD⁺, which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP⁺ molecule. The light reactions use solar energy to reduce NADP⁺ to NADPH by adding a pair of electrons along with an H⁺. The light reactions also generate ATP using chemiosmosis to power the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds, NADPH and ATP. NADPH, a source of electrons, acts as a “reducing power” that can be passed along to an electron acceptor and reduce it, while ATP is the versatile energy currency of cells. This process occurs in the thylakoids.

The Calvin cycle occurs in the stroma of the chloroplast.

Stomata are microscopic pores through which oxygen and carbon dioxide diffuse.

Cellular respiration, not photosynthesis, occurs in the mitochondrion.

None of the reactions of photosynthesis occur on the inner membrane of the chloroplast.

G3P production

Ex.

G3P production occurs during the reduction phase of the Calvin cycle.

The Calvin cycle is divided into three phases: carbon fixation, reduction, and regeneration of the CO₂ acceptor. Each molecule of 3-phosphoglycerate produced in the carbon fixation phase receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate. Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming glyceraldehyde 3-phosphate (G3P). Specifically, the electrons from NADPH reduce a carboyxl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more potential energy. G3P is a sugar—the same three-carbon sugar formed in glycolysis by the splitting of glucose. Notice that for every three molecules of CO₂ that enter the cycle, there are six molecules of G3P formed. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate because the rest are required to complete the cycle.

RuBP is regenerated in the third phase of the Calvin cycle, regeneration of the CO₂ acceptor.

Molecular oxygen is produced in the light reactions.

Carbon fixation is the first phase of the Calvin cycle.

NADPH is produced in the light reactions.

ATP is produced in the light reactions.

They are the ultimate sources of organic compounds for all nonautotrophic organisms.

Ex.

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

Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are “self-feeders” (auto means “self,” and trophos means “feeder”); that is, they sustain themselves without eating anything derived from other living beings. Autotrophs produce their organic molecules from CO₂ and other inorganic raw materials obtained from the environment. They are the ultimate sources of organic compounds for all nonautotrophic organisms, and for this reason, biologists refer to autotrophs as the producers of the biosphere.

Autotrophs produce organic materials, not ATP, for other organisms.

Autotrophs produce their own organic molecules. They do not consume other organisms.

Autotrophs produce organic materials using carbon dioxide.

ATP and NADPH.

Ex.

The light reactions of photosynthesis supply the Calvin cycle with ATP and NADPH.

The light reactions of photosynthesis capture solar energy and use it to make ATP and transfer electrons from water to NADP+, forming NADPH. The Calvin cycle uses the ATP and NADPH to produce sugar from carbon dioxide. The energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds.

Chlorophyll absorbs all of the visible spectrum of light except green, which it reflects.

Ex.

Chlorophyll absorbs all of the visible spectrum of light except green, which it reflects.

When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light.

Why leaves are green: interaction of light with chloroplasts.

Chlorophyll reflects green light; it does not absorb it.

Chlorophyll absorbs all of the colors of the visible light spectrum except green.

Chlorophyll reflects only green light.

The first product of carbon fixation in C₄ plants is a four-carbon compound instead of a three-carbon compound.

Ex.

The first product of carbon fixation in C₄ plants is a four-carbon compound instead of a three-carbon compound.

The C₄ plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO₂ to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate.

C4 leaf anatomy and the C4 pathway.

There is a difference in carbon fixation between C₃ and C₄ plants. The first product of carbon fixation in C₄ plants is a four-carbon compound.

The first product of the C₄ pathway is a four-carbon compound, not a three-carbon compound.

Most plants fix carbon from carbon dioxide.

Plants use carbon from carbon dioxide, not glucose, for carbon fixation.

C₄ plants use the C₄ pathway, not the CAM pathway.

Water is oxidized and carbon dioxide is reduced.

Ex.

In photosynthesis, water is oxidized and carbon dioxide is reduced.

In comparing photosynthesis with cellular respiration, we can see that both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. The electrons lose potential energy as they “fall” down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP. Photosynthesis reverses the direction of electron flow. Water is split, and electrons are transferred along with hydrogen ions from the water to carbon dioxide, reducing it to sugar.

Tracking atoms through photosynthesis

Carbon dioxide is reduced and water is oxidized.

G3P is the product of the reduction of carbon dioxide, and ATP is already reduced.

ADP is the oxidized form of ATP, and NADPH is already in its reduced form.

During cellular respiration, glucose is oxidized and oxygen is reduced.

During the citric acid cycle, pyruvate is oxidized and NAD⁺ is reduced.

capture light energy

Ex.

The most important role of pigments in photosynthesis is to capture light energy.

The wavelengths most effectively absorbed by pigments are the colors most useful as energy for the light reactions. The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, because light can perform work in chloroplasts only if it is absorbed. When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments.

Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light whereas it transmits and reflects green light.

movement of H⁺ through a membrane

Ex.

The energy used to produce ATP in the light reactions of photosynthesis comes from movement of H⁺ through a membrane.

Termed chemiosmosis, the diffusion of hydrogen ions through ATP synthase provides the energy to produce ATP. Chloroplasts generate ATP by chemiosmosis. An electron transport chain assembled in a membrane pumps protons across the membrane as electrons are passed through a series of carriers that are progressively more electronegative. In this way, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H⁺ gradient across a membrane. Built into the same membrane is an ATP synthase complex that couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP. In chloroplasts, the high-energy electrons dropped down the transport chain are extracted from water.

All of the listed processes can use G3P.

Ex.

All of the listed molecules can be produced from G3P.

Enzymes in the chloroplast and cytosol convert the G3P made in the Calvin cycle to many other organic compounds. In fact, the sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons for the synthesis of all the major organic molecules of plant cells. About 50% of the organic material made by photosynthesis is consumed as fuel for cellular respiration in plant cell mitochondria. Technically, green cells are the only autotrophic parts of the plant. The rest of the plant depends on organic molecules exported from leaves via veins.

In most plants, carbohydrate is transported out of the leaves to the rest of the plant in the form of sucrose, a disaccharide. A considerable amount of sugar in the form of glucose is linked together to make the polysaccharide cellulose, especially in plant cells that are still growing and maturing. Cellulose, the main ingredient of cell walls, is the most abundant organic molecule in the plant—and probably on the surface of the Earth. Most plants and other photosynthesizers make more organic material each day than they need to use as respiratory fuel and precursors for biosynthesis. They stockpile the extra sugar by synthesizing starch, storing some in the chloroplasts themselves and some in storage cells of roots, tubers, seeds, and fruits.