Photosynthesis

Is photosynthesis endothermic or exothermic? What does that mean?

Endothermic - overall reaction takes in energy

State 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

In chemiosmosis, where does the energy to pump protons come from?

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

Chemiosmosis allows the transformation of ADP into ATP. This process in photosynthesis is known as?

Photophosphorylation

During photosynthesis, what is pumped across the membrane and into the thylakoid space of the chloroplast?

Hydrogen ions

What pumps the hydrogen ions into the thylakoid space?

A cytochrome complex (electron carrier)

Where does chemiosmosis occur?

Along the membrane

What is 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 is the structure that connects grana together?

Intergranal lamella

Describe the structure of a photosystem

A light-harvesting complex that channels light towards a reaction centre (containing a primary pigment called chlorophyll a); accessory pigments channel photons towards the reaction centre

Name one primary pigment and 3 accessory pigments

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

Describe the differences between photosystem I and photosystem II

PSI contains chlorophyll a that absorbs light at a peak of 700nm (P700). PSII contains chlorophyll a that absorbs light at a peak of 680 nm. Electrons are excited to a higher energy level in PSI than PSII

Describe the light absorption of carotenoids, xanthophylls and chlorophylls

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

In which part of the chloroplast does the light-dependent reaction take place?

Thylakoid membrane

In which part of the chloroplast does the light-independent reaction take place

Stroma

Suggest why the Rf values of some pigments show a range rather than a single figure

There are different types of these, with slightly different molecular masses/sizes of molecules/solubilities in the solvent

Rf values for the different pigments differ according to the solvent used. Suggest why this is the case

The molecules of pigment will have different solubilities in different solvents.

What 4 processes occur in the light-dependent reaction?

Light harvesting at the photosystems; photolysis of water; photophosphorylation (production of ATP in the presence of light); formation of reduced NADP

Outline the role of water in photosynthesis

Electron donor; source of protons/hydrogen ions; source of by-product, oxygen; keeps cells turgid so they can function (all metabolic reactions need to be in solution).

Suggest why a lack of iron in soil may reduce growth in plants

Iron is needed for electron carriers in the chain (on thylakoid membranes) and for ferredoxin. A reduction of electron carriers could reduce rate of photosynthesis.

What is photolysis and where exactly does it take place?

The enzyme-catalysed splitting of water molecules, in the presence of light. It takes place in PSII on thylakoid membranes of chloroplasts.

Describe the differences between non-cyclic and cyclic photophosphorylation

NC - involves PSI and PSII; produces ATP, oxygen and reduced NADP (NADP is final electron acceptor). C - involves PSI only; produces ATP only as electrons are recycled back to PSI and NADP is not reduced

Describe how a proton gradient develops between the thylakoid lumen and the stroma

Energy from electrons moving through chain of electron carriers is used to actively pump protons across the thylakoid membrane into the thylakoid lumen

Describe the purpose of the proton gradient

Protons can diffuse down their gradient through ATP synthase channels (using proton motive force), causing ADP to combine with inorganic phosphate to form ATP

Describe what happens to protons that have moved through ATP synthase

They are accepted along with electrons from the electron transport chain, by NADP. This is facilitated by NADP reductase (NADP has been reduced to NADPH)

Describe how an electron moves through the electron transport chain

A photon excites an electron from PSII, the electron rising to a higher energy level and getting accepted by an electron acceptor. This energy level is unstable so the electron moves through a chain of electron carrier proteins, losing energy as it moves (this energy is transferred to pump protons). It is accepted by chlorophyll a in PSI and re-excited by another photon of light to a higher energy level than before.

Explain why chloroplasts in guard cells only contain PSI

Produces ATP which actively pumps potassium ions into the cell, lowering the water potential so water follows by osmosis. This causes the guard cell to become turgid and the stoma to open.

Describe the differences in the fate of electrons released from chlorophyll in both non-cyclic and cyclic photophosphorylation

Cyclic - pass via electron carriers back to PSI (little ATP generated); Non-cyclic - from PSII, pass via electron transport chain to PSI, PSI electrons accepted by NADP

What is RuBisCo and its function?

Ribulose bisphosphate carboxylase/oxidase; an enzyme found in the stroma that catalyses carbon fixation (CO2 + RuBP --> GP)

Suggest why there are always only low levels of RuBP in the stroma of chloroplasts

It is being continually regerated and then combined with CO2

Describe the role of carbon dioxide in the Calvin cycle

Combines with 5C-RuBP to form a 6C intermediate. This is unstable and so breaks into 2 x 3C-GP molecules. This reaction is catalysed by RuBisCO

Describe the conversion of GP into TP in the Calvin cycle

3C-GP is converted into 3C-TP using ATP and the H atom from reduced NADP

Describe how RuBP is regenerated in the Calvin cycle

5/6 of TP is converted back into RuBP through reshuffling of the arrangement of carbon atoms. This conversion also uses ATP (RuP --> RuBP)

Describe the fate of triose phosphate

1/6 leaves the Calvin cycle and is converted into sucrose, starch, cellulose, amino acids, glycerol, fatty acids, some is respired in glycolysis. The other 5/6 is used to regerate RuBP

State 2 products of the light dependent reaction used in the Calvin cycle

ATP and reduced NADP

Describe how plants respond to water stress. Explain how this affects their abiity to photosynthesise

Leaves lose more water by transpiration than can be replaced by uptake at roots. Cells lose water/become plasmolysed. Tissues become flaccid. Leaves wilt. Roots are unable to take in water and secrete abscisic acid, which travels in xylem to leaves causing stomata to close to prevent further water vapour loss. Closed stomata reduce gaseous exchange so not enough carbon dioxide for photosynthesis. Plasmolysed cells cannot function as enzymes cannot work. Reduced water availability for photosynthesis – as source of electrons and protons.

Describe the effects of reducing the light intensity on the Calvin cycle

Light dependent reaction cannot occur, so less ATP/NADPH; less GP --> TP; TP levels fall and GP accumulates; if TP levels fall then RuBP cannot be regenerated

Describe the effects of reducing carbon dioxide levels in the Calvin cycle

RuBP cannot accept it and RuBP accumulates; GP cannot be made; TP cannot be made

Name all three enzymes involved in photosynthesis and their functions.

ATP synthase (allows chemiosmosis to occur to join ADP and Pi together to make ATP); NADP reductase (joins NADP with H+ and e- to make reduced NADP); RuBisCo (catalyses carbon fixation - CO2 + RuBP --> GP)

Describe what is meant by the term compensation point

When the rate of photosynthesis is equal to the rate of respiration. This is when there is no net gain or loss of carbohydrate

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 conc; Light intensity

Why does increasing light intensity also increases photosynthetic rate?

More energy absorbed by photosystems to excite electrons, so more electrons undergo ETC to generate proton gradient for more ATP production and reduced NADP production

Why does more carbon dioxide increases photosynthetic rate?

Higher rate of carbon fixation at Calvin cycle for more GP and TP production

Describe and explain how temperature affects photosynthetic rate.

At lower temp, enzymes are inactive hence low PS rate. As temp increases, enzymes and substrates gain KE and hence higher rate of ESC formation, so higher PS rate. At higher temp, enzymes become denatured hence PS rate decreases.