Photosynthesis Practice Questions

Correct answer: Chlorophyll a

Chlorophyll a is the primary pigment that participates directly in the light reactions by transferring electrons to the primary electron acceptor. Other pigments like chlorophyll b and carotenoids act as accessory pigments, funneling energy toward the chlorophyll a reaction center.

Correct answer: Water

Through a process called photolysis, water molecules are split into oxygen, protons, and electrons. The electrons generated from this split are used to replenish the electron-deficient P680 reaction center in Photosystem II.

Correct answer: Stroma

The stroma is the fluid-filled space surrounding the thylakoids. It contains the necessary enzymes, such as RuBisCO, required to catalyze the conversion of carbon dioxide into organic sugar molecules.

Correct answer: Fixing atmospheric carbon dioxide onto RuBP

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the first step of the Calvin Cycle, known as carbon fixation. It attaches CO2 to the five-carbon sugar RuBP, forming an unstable six-carbon intermediate.

Correct answer: ATP and NADPH

The light reactions convert solar energy into chemical energy stored in ATP and NADPH. These two molecules provide the chemical energy and reducing power needed to synthesize sugars in the light-independent phase.

Correct answer: Oxygen

Oxygen is produced during the photolysis of water in Photosystem II. While some is used for plant respiration, the majority diffuses out of the leaf as a byproduct, supporting aerobic life on Earth.

Correct answer: 700 nm

The reaction center of Photosystem I is referred to as P700 because it most effectively absorbs light at a wavelength of 700 nanometers. This wavelength corresponds to the far-red portion of the light spectrum.

Correct answer: Opening their stomata at night to fix CO2 into organic acids

CAM plants, typically found in arid environments, open stomata at night to minimize water loss. They store CO2 as organic acids overnight and then release it for the Calvin Cycle during the day when the stomata are closed.

Correct answer: Photophosphorylation

Photophosphorylation is the use of light energy to drive the synthesis of ATP. It occurs as protons flow back across the thylakoid membrane through the ATP synthase complex, a process known as chemiosmosis.

Correct answer: 3-phosphoglycerate (3-PGA)

In C3 plants, the first product of carbon fixation is a three-carbon compound called 3-phosphoglycerate. This occurs when RuBisCO fixes CO2 to RuBP and the resulting six-carbon molecule immediately splits.

Correct answer: Water to PSII to PSI to NADP+

Electrons are stripped from water at PSII, energized by light, passed down an electron transport chain to PSI, energized again, and finally used to reduce NADP+ to NADPH. This pathway is often illustrated as the 'Z-scheme'.

Correct answer: High O2 and temperature

RuBisCO can bind to oxygen instead of carbon dioxide, especially when stomata close on hot days and O2 levels rise relative to CO2. This wasteful process, photorespiration, consumes energy and releases CO2 without producing sugar.

Correct answer: Specialized Kranz anatomy

C4 plants possess Kranz anatomy, where the Calvin Cycle is confined to bundle-sheath cells that are shielded from high oxygen levels. This spatial separation allows them to maintain a high CO2 concentration around RuBisCO.

Correct answer: 3

To produce one net molecule of G3P that can exit the cycle to form glucose, the Calvin Cycle must turn three times, fixing three separate molecules of CO2. This requires the consumption of 9 ATP and 6 NADPH.

Correct answer: The thylakoid space

During the light reactions, protons are pumped from the stroma into the thylakoid lumen. This accumulation of H⁺ ions creates a high concentration (low pH), establishing the electrochemical gradient used to synthesize ATP.

Correct answer: Chlorophyll

Chlorophyll absorbs light primarily in the blue and red regions of the visible spectrum. Because it does not absorb green wavelengths effectively, it reflects or transmits them, making leaves appear green to the human eye.

Correct answer: ATP exclusively

Cyclic electron flow involves only Photosystem I and produces ATP but not NADPH or oxygen. This process helps the plant balance the ratio of ATP to NADPH required for the Calvin Cycle.

Correct answer: It is the terminal electron acceptor.

NADP+ acts as an electron carrier. At the end of the linear electron transport chain, it accepts two electrons and a proton to become NADPH, which then carries high-energy electrons to the Calvin Cycle.

Correct answer: Starch polymers

While sucrose is the primary form of sugar transported through the phloem, plants convert excess glucose into starch for storage. Starch is stored in chloroplasts and specialized amyloplasts.

Correct answer: Do not directly need light

Although the Calvin Cycle is often called the 'dark reactions,' it typically occurs during the day. It is 'light-independent' because it uses the chemical products of the light reactions (ATP and NADPH) rather than absorbing photons directly.