Photosynthesis

Is photosynthesis endothermic or exothermic?

Endothermic - overall reaction takes in energy.

What is the overall chemical equation of photosynthesis?

6CO2 + 6H2O --> C6H12O6 + 6O2

What is chemiosmosis?

Movement (diffusion) of protons across ATP synthase down the electrochemical gradient to generate ATP.

Where does the energy to pump protons in chemiosmosis come from?

Energy given off by an electron passing down the first electron transport chain in the light-dependent reaction.

What is the process called that converts ADP into ATP in photosynthesis?

Photophosphorylation.

What is pumped across the membrane into the thylakoid space during photosynthesis?

Hydrogen ions.

What pumps hydrogen ions into the thylakoid space?

A cytochrome complex (electron carrier).

Where does chemiosmosis occur in the chloroplast?

Along the membrane.

What are grana?

Stacks of thylakoids that contain photosynthetic pigments.

What is the stroma and its function?

The fluid enclosed in the chloroplast; Site of light-independent stage.

What connects grana together?

Intergranal lamella.

Describe the structure of a photosystem.

A light-harvesting complex that channels light towards a reaction centre containing chlorophyll a, with accessory pigments channeling photons.

Name one primary pigment and three accessory pigments.

Primary - chlorophyll A; accessory - xanthophylls, carotenoids, chlorophyll b.

How do photosystem I and photosystem II differ?

PSI absorbs light at a peak of 700nm (P700); PSII absorbs at 680nm; PSI excites electrons to a higher energy level than PSII.

What light do carotenoids, xanthophylls, and chlorophylls absorb?

Carotenoids absorb blue light; xanthophylls absorb blue and green light; chlorophylls absorb blue and red light.

Where does the light-dependent reaction take place in chloroplasts?

Thylakoid membrane.

Where does the light-independent reaction take place in chloroplasts?

Stroma.

Why do Rf values of some pigments show a range rather than a single figure?

Different types have slightly different molecular masses/sizes of molecules/solubilities.

Why do Rf values differ according to the solvent used?

Pigment molecules have different solubilities in different solvents.

What processes occur in the light-dependent reaction?

Light harvesting, photolysis of water, photophosphorylation, and formation of reduced NADP.

What role does water play in photosynthesis?

Electron donor, source of protons, source of by-product oxygen, keeps cells turgid.

Why might a lack of iron in soil reduce plant growth?

Iron is needed for electron carriers; reduced electron carriers could lower photosynthesis rate.

What is photolysis and where does it take place?

The enzyme-catalysed splitting of water molecules in PSII on thylakoid membranes.

What are the differences between non-cyclic and cyclic photophosphorylation?

Non-cyclic involves PSII and PSI, producing ATP, oxygen and reduced NADP; cyclic involves PSI only and recycles electrons with only ATP produced.

How does a proton gradient develop in chloroplasts?

Energy from electrons in the transport chain pumps protons into the thylakoid lumen.

What is the purpose of the proton gradient?

Protons diffuse down their gradient through ATP synthase to produce ATP.

What happens to protons after they pass through ATP synthase?

They are accepted by NADP along with electrons from the electron transport chain, forming NADPH.

Describe how an electron moves through the electron transport chain.

A photon excites an electron from PSII to a higher energy level, which moves through electron carrier proteins, losing energy to pump protons.

Why do guard cells contain only PSI?

PSI produces ATP to pump potassium ions, causing guard cells to become turgid and open stoma.

What is RuBisCo and its function?

An enzyme that catalyses carbon fixation (CO2 + RuBP --> GP).

Why are levels of RuBP in the stroma always low?

It is continually regenerated and combined with CO2.

What role does carbon dioxide play in the Calvin cycle?

Combines with RuBP to form a 6C intermediate that breaks into 2 x 3C-GP.

How is GP converted into TP in the Calvin cycle?

GP is converted into TP using ATP and hydrogen from reduced NADP.

How is RuBP regenerated in the Calvin cycle?

5/6 of TP is reshuffled back into RuBP using ATP.

What happens to triose phosphate after the Calvin cycle?

1/6 is converted into various substances; 5/6 is used to regenerate RuBP.

Name two products of the light-dependent reaction used in the Calvin cycle.

ATP and reduced NADP.

How do plants respond to water stress?

Stomata close to prevent water loss; reduced gaseous exchange limits photosynthesis.

What happens to the Calvin cycle if light intensity decreases?

Less ATP/NADPH produced, leading to more GP and reduced TP; RuBP cannot be regenerated.

What happens to the Calvin cycle if carbon dioxide levels decrease?

RuBP accumulates, GP and TP cannot be produced.

What are the three enzymes involved in photosynthesis and their functions?

ATP synthase (produces ATP), NADP reductase (produces reduced NADP), and RuBisCo (catalyzes carbon fixation).

What is meant by the term compensation point?

The point when the rate of photosynthesis equals the rate of respiration.

Define 'limiting factor'.

A factor that limits the rate of reaction when in short supply.

Name three factors that affect photosynthetic rate.

Temperature, carbon dioxide concentration, light intensity.

Why does increased light intensity raise photosynthetic rate?

More energy excites electrons, enhancing ATP and reduced NADP production.

Why does more carbon dioxide increase the photosynthetic rate?

Higher rate of carbon fixation, leading to more GP and TP production.

How does temperature affect the photosynthetic rate?

Lower temperatures reduce enzyme activity; optimal provides maximum rate, while too high leads to denaturation.