Photosynthesis

Describe the structure of a chloroplast. [8] #

• 3 to 10 µm (diameter);
• double membrane;
• ground substance / stroma;
• contains enzymes / named enzyme, e.g. rubisco;
• also, sugars / lipids / starch;
• 70S / AW, ribosomes;
• circular DNA;
• internal membrane system / fluid-filled sacs / thylakoids;
• grana are stacks of thylakoids;
• (grana) membranes hold, photosynthetic pigments / ATP synthase

Relate the structure of the chloroplast to their roles in photosynthesis. #

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• Are large organelles containing their own DNA and have a double membrane.
• Chloroplasts have a folded inner membrane which gives a greater surface area for biochemical reactions to occur.
• Thylakoid membranes contains pigments/ electron carriers/ enzymes
• Used in cyclic and non – cyclic photophoshorilation/ light dependent reactions
• Stroma (contain enzymes) for the calvin cycle/ dark reaction
• Grana a network of proteins holding pigments into photosynthesis
• Light reactions on thylakoid which contain ATP/ stalked particles
• Membraine system separates the reactions of photosynthesis from other cell reactions
• Stroma fluid which surrounds grana so that products light dependent stage can easily pass into stroma.
• Contain DNA /ribosomes for manufacture of proteins needed for protein synthesis
• Granna are interconnected by membranes called lamella.

Explain how the palisade mesophyll cells of a leaf are adapted for photosynthesis. #

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• closely packed — to absorb more incident light ;
• palisade mesophyll near upper surface of leaf — to maximize light interception ;
• arranged at right angles to leaf surface — to reduce number of light absorbing walls;
• cylindrical cells — producing air spaces between cells ;
• air spaces — act as reservoir of carbon dioxide ;
• large surface area — for gas exchange ;
• cell walls thin — so short diffusion pathway ;
• large vacuole — pushes chloroplasts to edge of cell ;
• chloroplasts on periphery — to absorb light more efficiently ;
• large number of chloroplasts — to maximize light absorption ;
• chloroplasts can move within cells — towards light ;
• chloroplasts can move away from high light intensity — to avoid damage ;

Describe the arrangement and location of chloroplast pigments and discuss their effect on absorption spectra. [8] #

• chlorophyll a is primary pigment;
• carotenoids / chlorophyll b, is accessory pigment;
• arranged in, light harvesting clusters / photosystems;
• A antenna complex on, grana / thylakoids;
• ref. PI and PII ; A P700 and P680
• primary pigment / chlorophyll a, in reaction centre;
• accessory pigments / carotenoids / chlorophyll b, surround primary pigment;
• light energy absorbed by, accessory pigments / carotenoids / chlorophyll b;
• (energy) passed on to, primary pigment / chlorophyll a / reaction centre;
• chlorophyll a and b absorb light in red and blue/violet region;
• carotenoids absorb light in blue/violet region;
• ref. absorption spectrum peaks;
• diagram of absorption spectrum;
• different combinations of pigments (in different plants) give different spectra

Describe the structure of photosystems and explain how a photosystem functions in cyclic photophosphorylation. [8] #

• arranged in light harvesting, clusters/system ;
• primary pigments/chlorophyll a ;
• at reaction centre ;
• P700/P1, absorbs at 700(nm) ;
• P680/P11, absorbs at 680(nm) ;
• accessory pigments/chlorophyll b/carotenoids, surround, primary pigment/reaction centre/ chlorophyll a ;
• pass energy to, primary pigment/reaction centre/chlorophyll a ;
• P700 / PI, involved in cyclic photophosphorylation ;
• (light absorbed results in) electron excited ;
• emitted from, chlorophyll/photosystem ;
• flows along, chain of electron carriers/ETC ;
• ATP synthesis ;
• electron returns to, P700/P1 ;

Explain briefly how reduced NADP is formed in the light-dependent stage and how it is used in the light-independent stage [8] #

• photolysis (of water) ;
• releases H+ ;
• by, P680/PII ;
• e- released ;
• by, P700/PI ;
• both combine with NADP ;
• reduces, GP ; A PGA
• to TP ; A PGAL / GALP
• ATP used ;
• NADP, regenerated/oxidised ;

Describe how non-cyclic photophosphorylation produces ATP and reduced NADP. [8] #

• photosystem I (PI) and photosystem II (PII) involved ;
• light harvesting clusters ;
• light absorbed by accessory pigments ;
• primary pigment is chlorophyll a ;
• energy passed to, primary pigment / chlorophyll a ;
• electrons, excited / raised to higher energy level ;
• (electrons) taken up by electron acceptor ;
• (electrons) pass down electron carrier chain (to produce ATP) ;
• PII has (water splitting) enzyme ;
• water split into protons, electrons and oxygen ;
• photolysis ;
• electrons from PII pass to PI / electrons from water pass to PII ;
• to replace those lost ; give either in relation to PI or PII
• protons and electrons combine with NADP (to produce reduced NADP) ;

Describe the transfer of energy to ATP during photosynthesis [6] #

• light absorbed by chlorophyll ;
• An electron from P700 orP680 is boosted to a higher energy level when light strikes the photosystem;
• the electron which acquires excitation is is accepted by electron acceptor x or y ;
• The electron acceptor x or y become reduced and chlorophylls become oxidized with positive change;
• The electrons with the excess energy of oxidation are very unstable tend to fall down to their ground state ;
• The electrons then in terms of energy via a series of electrons acceptors;
• The excess energy lose when they fall back to the ground state is coupled is coupled in the production of ATP;
• The positive change left in the p680 contribute to the photochemical lysis of water which releases electrons which lost from electron acceptor X;
• Electrons flow X along a chain of electron carries loosing energy [used to phospholyate ADP to ATP ] filling the hole left in p700
• electrons also pass down from Y to NADP along a chain a chain of electron carries to and combine with hydrogen ions from water to form reduced NADP;
• This is called non cyclic photophosphorylation;
• In cycle photophosphorylation electrons from Y are recycled to P700 via a electron chain carries results in ATP being formed;

Outline the process of the photolysis of water and describe what happens to the products of photolysis. [8] #

• PII absorbs light;
• enzyme (in PII) involved ;
• to break down water ;
• 2H₂O 4H+ + 4e + O₂;
• oxygen is produced;
• used by cells for (aerobic) respiration;
• or released (out of plant) through stomata;
• protons used to reduce NADP ;
• with electrons from PI;
• reduced NADP used in, light independent stage / Calvin cycle;
• to convert GP to TP;
• electrons also used in ETC;
• to release energy for photophosphorylation;
• to produce ATP;
• electrons (from PII) go to PI;
• ref. re-stabilise PI

Explain briefly how reduced NADP is formed in the light-dependent stage and how it is used in the light-independent stage. [6] #

• photolysis of water ;
• releases H+ ; R H / hydrogen atoms
• by, P680 / PII ;
• electrons released from, P700 / PI ;
• electrons (from PI) and H+ combine with NADP ;
• used in Calvin cycle ;
• reduces, GP / PGA ;
• to TP ;
• ATP used (during reduction of GP) ;
• NADP, regenerated / oxidised ;

Outline/ describe the main features/reactions of the Calvin Cycle. #

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• RuBP 5C;
• combines with carbon dioxide;
• rubisco;
• to form an unstable 6C compound;
• which forms 2 X GP (PGA);
• ATP;
• energy source;
• and reduced NADP;
• forms TP (GALP);
• TP used to form glucose / carbohydrates 1 lipids / amino acids;
• TP used in regeneration of RuBP;
• requires ATP;
• as source of phosphate;
• light independent;
• Grana increase surface area for light absorption

Describe the photoactivation of chlorophyll and its role in cyclic photophosphorylation[8] #

• (photosynthetic pigments) arranged in light harvesting clusters ;
• primary pigments / chlorophyll a ;
• at reaction centre ;
• P700 / Pl, absorbs light at 700nm ;
• . accessory pigments / chlorophyll b / carotenoids ;
• surround, primary pigment / reaction centre / chlorophyll a ;
• absorb light ;
• pass energy to, primary pigment / reaction centre / chlorophyll a ;
• (light absorbed results in) electron excited ;
• emitted from, chlorophyll / primary pigment / reaction centre ;
• passes to electron, acceptor / carrier ;
• (electron) passes along, chain of electron carriers / ETC ;
• ATP (synthesis) ;
• electron returns to, P700 / Pl ;

Explain how the physiology of the leaves of a C4 plant, such as maize, is adapted for efficient carbon fixation at high temperatures. [7] #

• in C3 plants at high temperature rubisco combines with oxygen;
• less rubisco to combine with CO2;
• in C4 plant such as maize idea of spatial separation of light-dependent stage from carbon fixation;
• rubisco/RuBP, in bundle sheath cells;
• kept away from, oxygen/air;
• mesophyll cells, absorb CO2;
• CO2 released to combine with RuBP;
• avoid/reduce, photorespiration;
• high optimum temperatures of enzymes involved;
• Calvin cycle can continue;

Practice Questions #

1. Read the following passage.

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The living state requires a constant input of energy, and the most fundamental difference between animals and plants is the way they obtain their energy. Animals take in food – organic compounds – and release chemical energy during respiration; green plants absorb light energy from the sun, converting it to chemical energy in the process of photosynthesis.

adapted from RIDGE.I.(ED), Plant Physiology (Hodder and Stoughton, 1991)

(a) Describe how plants absorb light energy from the sun and use this energy to produce useful substances in the light-dependent stage of photosynthesis. (5)
(b) Describe how the products of the light-dependent stage of photosynthesis are used in the Calvin cycle and how carbohydrate is synthesized as a result of the cycle. (6)
(c) Describe the similarities between photosynthesis and respiration.

2. Read the following passage.

Photosynthesis takes place in the chloroplasts. These are disc-shaped organelles surrounded by an outer envelope consisting of two layers of membrane. Inside, there are further membranes which are arranged in stacks called grana. Surrounding these is the stroma. Chlorophyll and other light- capturing pigments are found on the membranes of the grana and it is here that the light-dependent reaction takes place. This generates the ATP and reduced NADP which are used in the light-independent reaction in the stroma.

1. Suggest how you could use chromatography to separate and identify the different light- capturing pigments present in leaf tissue. (5)
2. Describe the way in which ATP and reduced NADP are produced in the light-dependent reaction of photosynthesis. (5)
3. Explain how ATP and reduced NADP are used in the light-independent reaction of photosynthesis. (4)
4. Using the information in the passage, describe how the structure of a chloroplast is adapted to its function in photosynthesis (4)

3. A plant was allowed to photosynthesize normally. The light was then switched off. Explain why there was a rise in the amount of glycerate 3-phosphate present in the chloroplasts of this plant. [2]

4. The diagram represents some of the light-independent reactions of photosynthesis.

(a)  Describe the light-independent reactions of photosynthesis and explain how they allow the continued synthesis of hexose sugars [6]

(b) Describe the role of electron transport chains in the light-dependent reactions of photosynthesis. [6]

(c) Explain why the increase in the dry mass of a plant over twelve months is less than the mass of hexose produced over the same period. [3]

5. The diagram shows a summary of the light-independent reaction of photosynthesis.

(a) (i)  Complete the boxes to show the number of carbon atoms in the molecules. (2)

(ii) In which part of a chloroplast does the light-independent reaction occur?.(1)

(iii)  Which process is the source of the ATP used in the conversion of glycerate 3-phosphate (GP) to triose phosphate?(1)

(iv)  What proportion of triose phosphate molecules is converted to ribulose bisphosphate (RuBP)? (1)

(b)  Lowering the temperature has very little effect on the light-dependent reaction, but it slows down the light-independent reaction. Explain why the light-independent reaction slows down at low temperatures. (2)