8: Photosynthesis

Question 1

What is a chemoautotroph?

Answer 1

An organism that can build organic molecules using energy derived from inorganic chemicals instead of sunlight.

Question 2

What is a chloroplast?

Answer 2

An organelle in which photosynthesis takes place.

Question 3

What is a granum?

Answer 3

A stack of thylakoids located inside a chloroplast.

Question 4

What is a heterotroph?

Answer 4

An organism that consumes organic substances or other organisms for food.

Question 5

What is a light-dependent reaction?

Answer 5

The first stage of photosynthesis where certain wavelengths of the visible light are absorbed to form two energy-carrying molecules (ATP and NADPH).

Question 6

What is a light-independent reaction?

Answer 6

The second stage of photosynthesis, through which carbon dioxide is used to build carbohydrate molecules using energy from ATP and NADPH.

Question 7

What is the mesophyll?

Answer 7

The middle layer of chlorophyll-rich cells in a leaf.

Question 8

What is a photoautotroph?

Answer 8

An organism capable of producing its own organic compounds from sunlight.

Question 9

What is a pigment?

Answer 9

A molecule that is capable of absorbing certain wavelengths of light and reflecting others (which accounts for its color).

Question 10

What is a stoma?

Answer 10

An opening that regulates gas exchange and water evaporation between leaves and the environment, typically situated on the underside of leaves.

Question 11

What is the stroma?

Answer 11

The fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of photosynthesis take place.

Question 12

What is a thylakoid?

Answer 12

A disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of photosynthesis take place; stacks of thylakoids are called grana.

Question 13

What is the thylakoid lumen?

Answer 13

The aqueous space bound by a thylakoid membrane where protons accumulate during light-driven electron transport.

Question 14

What is the absorption spectrum?

Answer 14

A range of wavelengths of electromagnetic radiation absorbed by a given substance.

Question 15

What is an antenna protein?

Answer 15

A pigment molecule that directly absorbs light and transfers the energy absorbed to other pigment molecules.

Question 16

What is a carotenoid?

Answer 16

A photosynthetic pigment that functions to dispose of excess energy.

Question 17

What is chlorophyll a?

Answer 17

A form of chlorophyll that absorbs violet-blue and red light and consequently has a bluish-green color; the only pigment molecule that performs the photochemistry by getting excited and losing an electron to the electron transport chain.

Question 18

What is chlorophyll b?

Answer 18

An accessory pigment that absorbs blue and red-orange light and consequently has a yellowish-green tint.

Question 19

What is the cytochrome complex?

Answer 19

A group of reversibly oxidizable and reducible proteins that forms part of the electron transport chain between photosystem II and photosystem I.

Question 20

What is the electromagnetic spectrum?

Answer 20

The range of all possible frequencies of radiation.

Question 21

What is the photosynthetic electron transport chain?

Answer 21

A group of proteins between PSII and PSI that pass energized electrons and use the energy released by the electrons to move hydrogen ions against their concentration gradient into the thylakoid lumen.

Question 22

What is the light-harvesting complex?

Answer 22

The complex that passes energy from sunlight to the reaction center in each photosystem; it consists of multiple antenna proteins that contain a mixture of 300-400 chlorophyll a and b molecules as well as other pigments like carotenoids.

Question 23

What is P680?

Answer 23

The reaction center of photosystem II.

Question 24

What is P700?

Answer 24

The reaction center of photosystem I.

Question 25

What is a photoact?

Answer 25

The ejection of an electron from a reaction center using the energy of an absorbed photon.

Question 26

What is a photon?

Answer 26

A distinct quantity or “packet” of light energy.

Question 27

What is a photosystem?

Answer 27

A group of proteins, chlorophyll, and other pigments that are used in the light-dependent reactions of photosynthesis to absorb light energy and convert it into chemical energy.

Question 28

What is photosystem I?

Answer 28

The integral pigment and protein complex in thylakoid membranes that uses light energy to transport electrons from plastocyanin to NADP+ (which becomes reduced to NADPH in the process).

Question 29

What is photosystem II?

Answer 29

The integral protein and pigment complex in thylakoid membranes that transports electrons from water to the electron transport chain; oxygen is a product of PSII.

Question 30

What is the primary electron acceptor?

Answer 30

A pigment or other organic molecule in the reaction center that accepts an energized electron from the reaction center.

Question 31

What is a reaction center?

Answer 31

A complex of chlorophyll molecules and other organic molecules that is assembled around a special pair of chlorophyll molecules and a primary electron acceptor; capable of undergoing oxidation and reduction.

Question 32

What is a spectrophotometer?

Answer 32

An instrument that can measure transmitted light and compute the absorption.

Question 33

Why is visible light visible?

Answer 33

Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching.

Question 34

What is photosynthetically active radiation?

Answer 34

Plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm (visible light); plant physiologists refer to this range for plants as photosynthetically active radiation.

Question 35

Which end of the visible light spectrum has higher energy?

Answer 35

Violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy.

Question 36

What are the main classes of photosynthetic pigments?

Answer 36

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules.

Question 37

What are the types of chlorophylls?

Answer 37

There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts.

Question 38

What are some examples of carotenoids?

Answer 38

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers.

Question 39

What role do carotenoids play in photosynthesis?

Answer 39

In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Question 40

Why do many photosynthetic organisms have a mixture of pigments?

Answer 40

Using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation.

Question 41

How can the types of photosynthetic pigments be detected in an organism?

Answer 41

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

Question 42

What is the basic structure of a photosystem?

Answer 42

Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids.

Question 43

How is solar energy captured by chlorophyll molecules?

Answer 43

The absorption of a single photon by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Question 44

How is solar energy converted into high-energy electrons in a photosystem?

Answer 44

The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact. It is at this step in the reaction center, this step in photosynthesis, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate.

Question 45

What are the main components of the photosynthetic electron transport chain?

Answer 45

PSII and PSI are two major components of the photosynthetic electron transport chain, which also includes the cytochrome complex. The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

Question 46

How do high-energy electrons move from PSII to PSI?

Answer 46

The reaction center of PSII (called P680) delivers its high-energy electrons, one at the time, to the primary electron acceptor, and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI.

Question 47

How is the electron in PSII restored after it has been passed through the electron transport chain?

Answer 47

P680’s missing electron is replaced by extracting a low-energy electron from water; thus, water is split and PSII is re-reduced after every photoact.

Question 48

How is oxygen produced during photosynthesis?

Answer 48

Splitting one H₂O molecule in order to re-reduce P680 in PSII releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O₂ gas.

Question 49

How is the oxygen produced during photosynthesis used?

Answer 49

About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

Question 50

How is electronic energy used in the electron transport chain between PSII and PSI?

Answer 50

As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step.

Question 51

How are electrons re-energized by PSI?

Answer 51

Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700). P700 is oxidized and sends a high-energy electron to NADP+ to form NADPH.

Question 52

How do PSII and PSI work to produce ATP and NADPH?

Answer 52

PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP+ into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Question 53

How is ATP produced from the hydrogen ions generated by the electron transport chain?

Answer 53

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other. To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP. The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.

Question 54

What is the Calvin cycle?

Answer 54

The light-independent reactions of photosynthesis that convert carbon dioxide from the atmosphere into carbohydrates using the energy and reducing power of ATP and NADPH.

Question 55

What is carbon fixation?

Answer 55

The process of converting inorganic CO₂ gas into organic compounds.

Question 56

What is reduction?

Answer 56

The gain of one or more electrons by an atom or molecule.

Question 57

What are the lifespans of ATP and NADPH versus carbohydrates?

Answer 57

The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years.

Question 58

Where does the carbon used to produce carbohydrates come from?

Answer 58

It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

Question 59

How does CO₂ reach the stroma?

Answer 59

In plants, carbon dioxide (CO₂) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO₂ diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis.

Question 60

What is the name of the reactions that produce sugar from CO₂ and ATP?

Answer 60

These reactions actually have several names associated with them. Another term, the Calvin cycle, is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is dark reactions, because light is not directly required. However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.

Question 61

What are the stages of the Calvin cycle?

Answer 61

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

Question 62

What happens during the fixation stage of the Calvin cycle?

Answer 62

In the stroma, in addition to CO₂, two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP). RuBP has five atoms of carbon, flanked by two phosphates. RuBisCO catalyzes a reaction between CO₂ and RuBP. For each CO₂ molecule that reacts with one RuBP, two molecules of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO₂ + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation, because CO₂ is “fixed” from an inorganic form into organic molecules.

Question 63

What happens during the reduction stage of the Calvin cycle?

Answer 63

ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.

Question 64

What happens during the regeneration stage of the Calvin cycle?

Answer 64

At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO₂ to be fixed. Three more molecules of ATP are used in these regeneration reactions.

Question 65

How did photosystems evolve?

Answer 65

During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems.

Question 66

What are some adaptations used to conserve water in desert environments?

Answer 66

Because stomata must open to allow for the uptake of CO₂, water escapes from the leaf during active photosynthesis. A more efficient use of CO₂ allows plants to adapt to living with less water. Some plants such as cacti can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

Question 67

What is the energy cycle?

Answer 67

Photosynthesis absorbs light energy and uses carbon dioxide to build carbohydrates in chloroplasts and produce oxygen, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria and produce carbon dioxide. Photosynthesis and cellular respiration function cyclically to power both autotrophs and heterotrophs.