photosynthesis

State 5 ways in which mitochondria and chloroplasts are similar in structure; [2]

1. Presence of a double membrane;; 2. Presence of electron transport chain (ETC);; 3. Presence of stalked particles / ATP synthase;; 4. Presence of double-stranded circular DNA;; 5. Presence of 70S ribosomes;;

with reference to the chloroplast structure, describe and explain how light energy is harnessed and converted into chemical energy during the light dependent reaction - Non-cyclic phosphorylation, of photosynthesis. (9 points)

1) a photon of light strikes a photosynthetic pigment molecule in PS II. Energy in the photon of light is passed on from one pigment molecule to the neighbouring pigment molecule via resonance until it reaches the special chlorophyll a (P680) in the reaction centre of PS II. 2) an electron in P680 is excited and boosted to a higher energy level. This excited electron is then accepted by a primary electron acceptor. 3) The primary electron acceptor of PS II passes the excited electron to PS I, down the electron transport chain between PS II and PS I, comprising of a series of electron carriers of progressively lower energy levels. A series of redox reactions takes place as electrons are transferred down the ETC from one electron carrier to the next. As the electrons are passed down the ETC from PS II to PS I, energy is released, and used to pump H+ via active transport, from the stroma, across the thylakoid membrane, into the thylakoid lumen. 4) in addition, PS II includes a water-splitting enzyme that catalyses the photolysis of water in the presence of light. A water molecule is split into 2H+, 2 electrons, 1/2 oxygen. The electrons from photolysis of water replace the electrons lost by P680 to the primary electron acceptor. The H+ remains in the thylakoid lumen. O2 is released as a by-product. The accumulation of H+ in the thylakoid lumen sets up an electrochemical and proton gradient between the thylakoid lumen and the stroma of the chloroplast for ATP synthesis. 5) Potential energy of this proton gradient allows ATP synthesis as H+ diffuses down their concentration gradient through stalked particles from the thylakoid lumen back into the stroma. The flow of H+ releases electrical potential energy which drives the phosphorylation of ADP to form ATP, catalysed by ATP synthase. This way of synthesising ATP is phosphorylation. ATP synthesised is released into the stroma. 6) meanwhile at PS I, energy from another photon of light is relayed from pigment molecules to the special chlorophyl a (P700) in the reaction centre of PS I. An electron in P700 is excited and boosted to a higher energy level. 7) The electrons lost from P700 will be replaced by the electrons released from P680 of PS II. 8) In a second series of reactions, the excited electrons are accepted by another primary electron acceptor, passed down a second ETC and passed to another electron carrier. 9) These electrons eventually combine with H+ from water to form hydrogen atoms, which are then used to reduce NADP+ to NADPH, catalysed by the enzyme NADP+ reductase. The flow of electrons in non-cyclic photophosphorylation adopts a 'Z-scheme'

Explain the photoactivation of chlorophyll in photosystem II. [3]

1. A photon of light strikes a photosynthetic pigment in photosystem II (PS II);; 2. Energy in the photon of light is passed on from one pigment molecule to the neighbouring pigment molecule via resonance until it reaches the special chlorophyll a (P680) in the reaction centre of PS II;; 3. An electron in special chlorophyll a / P680 is excited and boosted to a higher energy level, and this excited electron is accepted by a primary electron acceptor;;

Herbicides act on the chloroplast thylakoids. They interfere with electron transport by accepting electrons. Suggest how this prevents the light-independent stage of photosynthesis from taking place and causes plants to die. [3]

1. ATP and NADPH are not produced;; 2. ATP is not produced so glycerate phosphate (GP) cannot be phosphorylated to form glycerate bisphosphate and RuBP cannot be regenerated;; 3. NADPH is not produced so glycerate bisphosphate cannot be reduced to triose phosphate (TP);; 4. There is no production of hexose sugars / glucose to act as respiratory substrates. Aerobic respiration does not take place and synthesis of ATP for essential metabolic processes does not occur;; 5. and plants die.

Explain the importance of ATP and NADPH to photosynthesis

1. ATP serves as the energy source for the light-independent reaction;; 2. ATP is needed to phosphorylate glycerate phosphate (GP) to form glycerate bisphosphate;; 3. ATP is needed for the regeneration of RuBP;; 4. NADPH provides the reducing power for the light-independent reaction;; 5. NADPH is needed to reduce glycerate bisphosphate to form triose phosphate (TP) which is required for the synthesis of sugars and regeneration of RuBP;;

Outline the role of the ATP synthase that is held in the thylakoid membrane and granum [2]

1. Acts as H+ channels that allow diffusion of H+ from the thylakoid lumen back into the stroma;; 2. Catalyses the phosphorylation of ADP to ATP;;

Explain why the CO2 assimilation rate is negative at the lowest light intensity. [3]

1. At the lowest light intensity, there would be a low rate / no photosynthesis and carbon dioxide is not used during carbon/CO2 fixation in the Calvin cycle;; 2. However, aerobic respiration would still occur and carbon dioxide would be given off;; 3. Rate of respiration is greater than rate of photosynthesis below the compensation point;;

Explain how the flow of electrons results in the high concentration of protons in the mitochondrial matrix. [3]

1. Electrons transferred down the electron transport chain (ETC) comprising of a series of electron carriers of progressively lower energy levels via redox reactions to oxygen. 2. Energy is released and used to pump protons via active transport from the mitochondrial matrix, across inner mitochondrial membrane, into the intermembrane space. 3. The inner mitochondrial membrane is impermeable to protons and protons accumulates in the intermembrane space;; 4. resulting in the high concentration of protons in the intermembrane space

What is the role of high concentration of protons at the mitochondrial matrix?

1. High concentration of protons at C set up an electrochemical and a proton gradient (between the intermembrane space and the mitochondrial matrix) for ATP synthesis;; 2. Protons diffuse down their concentration gradient through stalked particles from intermembrane space back to the mitochondrial matrix and releases electrical potential energy which drives the phosphorylation of ADP to form ATP catalysed by ATP synthase;;

Explain how water molecules are utilised in the process of photosynthesis. [3]

1. In PS II, photolysis of water is catalysed by the water-splitting enzyme which splits water into 2 H+, 2 electrons and ½ oxygen;; 2. The electrons from the photolysis of water replace the electrons lost from special chlorophyll a (P680) in the reaction centre of PS II;; 3. H+ combine with electrons from special chlorophyll a (P700) in the reaction centre of PS I to form hydrogen atoms which reduce NADP+ to NADPH;;

Suggest why plant cells with chloroplasts also contain mitochondria. [3

1. Mitochondria are the site of aerobic respiration to synthesise ATP;; 2. ATP synthesised in the mitochondria is released into the cytosol of plant cells and used for sucrose loading / active transport / vesicle transport / mitosis etc.;; 3. However, ATP synthesised (as a result of photophosphorylation) in the chloroplasts is not released into the cytosol and is used only in the Calvin cycle for the phosphorylation of glycerate phosphate to glycerate bisphosphate / for the regeneration of ribulose bisphosphate;;

Describe the source of electrons at photosystem II [2]

1. NADH formed during glycolysis, link reaction and Krebs cycle and FADH2 produced during Krebs cycle are the source of electrons at A;; 2. Hydrogen atoms dissociate from NADH and FADH2. and are split to form protons and electrons at A;;

Describe the role of NADP in linking the light dependent reactions to the Calvin cycle. [2]

1. NADP acts as a coenzyme (for dehydrogenase) and hydrogen atom / proton and electron carrier;; 2. NADP is reduced to NADPH in non-cyclic photophosphorylation of the light dependent reactions;; 3. NADPH carries hydrogen atom and electron from the light dependent reaction to the Calvin cycle to provide reducing power for the reduction of glycerate bisphosphate to triose phosphate;;

State 5 ways in which mitochondria and chloroplasts are different in structure; [2]

1. Presence of stacks of thylakoids/grana in chloroplasts but absence of these grana in mitochondria;; 2. Presence of cristae in mitochondria but absence of cristae in chloroplasts;; 3. Presence of chlorophyll/other photosynthetic pigments in chloroplasts but absence of chlorophyll/other photosynthetic pigments in mitochondria;; 4. Presence of starch grains in chloroplasts but absence of starch grains in mitochondria;;

Explain why the CO2 assimilation rate levels off at higher light intensity. [3]

1. Rate of photosynthesis is at maximum;; 2. because light saturation had occurred;; 3. Light intensity is no longer a limiting factor and other factors, e.g. temperature is limiting;;

Describe the fate of electrons at the last step of the ETC in respiration

1. The final electron acceptor, oxygen, is present at the last step of the ETC;; 2. Electrons, H+, and 1/2 O2 will rejoin at the last step of the ETC when cytochrome oxidase catalyses the transfer of 2H+ to 1/2O2 and 2 electrons to form water.

List two possible fates of the hexose sugars in the krebs cycle

1. The hexose sugars may be used as respiratory substrate for the production of ATP;; 2. α-glucose polymerises to form starch for energy storage;; 3. β-glucose polymerises to form cellulose for the formation of new cell walls;;

State 3 advantages of the photosystems being held in the thylakoid membrane

1. The photosynthetic pigments of photosystems I and II are held in a suitable position for absorbing the maximum amount of light energy;; 2. Allows for compartmentalisation within the chloroplast so that specialised metabolic pathways can take place in different areas e.g. ATP synthesis at the thylakoid membranes where ATP synthase are found;; 3. Site of ATP synthesis by chemiosmosis when H+ diffuse from thylakoid lumen to stroma down their concentration gradient through ATP synthases / stalked particles;;

Suggest two advantages of photosystems I and II being held in the thylakoid membrane and granum [2]

1. The photosynthetic pigments of photosystems I and II are held in a suitable position for absorbing the maximum amount of light energy;; 2. Site of ATP synthesis by chemiosmosis when H+ diffuse from thylakoid lumen down their concentration gradient through stalked particles back to stroma;; 3. Allows for compartmentalisation within the chloroplast so that specialised metabolic pathways can take place in different regions;; e.g. ATP synthesis at the thylakoid membranes where ATP synthase are found.

with reference to the chloroplast structure, describe and explain how light energy is harnessed and converted into chemical energy during the light dependent reaction - cyclic phosphorylation, of photosynthesis.

A photon of light strikes a photosynthetic pigment molecule in PS I. Energy in the photon of light is passed on from one pigment molecule to the neighbouring pigment molecule via resonance, until it reaches special chlorophyll a (P700) in the reaction centre of PS I. An electron in P700 is excited and boosted to a higher energy level. This excited electron is then accepted by a primary electron acceptor in PS I. The excited electrons are passed to the ETC between PS II and PS I before returning to the same special chlorophyll a (P700) of PS I, completing the cyclic electron flow. During this process, the energy released is coupled to ATP synthesis. The ATP synthesised is channelled to the light-independent reactions.

Explain the effect of carbon dioxide concentration on the rate of photosynthesis

A. As carbon dioxide concentration increases, the rate of photosynthesis increases proportionally. Therefore carbon dioxide concentration is the limiting factor. B. As carbon dioxide concentration increases, the rate of photosynthesis remains constant. Further increase in carbon dioxide concentration causes no further increase in the rate of photosynthesis. Carbon dioxide concentration is no longer a limiting factor, other factors like light intensity and temperature, are limiting.

Explain the effect of light intensity on the rate of photosynthesis

A. As light intensity increases, the rate of photosynthesis increases proportionally. Therefore, the light intensity is the main limiting factor. Light is needed for photoactivation of special chlorophyll A in the reaction centre in the light-dependent stage of photosynthesis. There is increased movement of electrons down both ETCs, resulting in a higher ATP and NADPH synthesis for the light independent reaction. B. As light intensity increases, the rate of photosynthesis increases gradually. light intensity is becoming less of a limiting factor. Some factor other than light, like CO2 concentration or temperature, is becoming limiting. C. As light intensity increases, the rate of photosynthesis remains constant. The rate of photosynthesis is at maximum. Light saturation has occurred. Light intensity is no longer the limiting factor, other factors like CO2 concentration or temperature are limiting. This is because even though sufficient ATP and NADPH are synthesised, the enzymes in the light independent reactions are temperature dependent and carbon dioxide is required for carbon fixation for synthesis of carbohydrates.

Explain the effect of temperature on the rate of photosynthesis

A. As the temperature increases, the rate of photosynthesis increases. The rate of reaction doubles for every 10 degrees rise up to the optimum temperature. B. As temperature increases beyond the optimum temperature, the rate of photosynthesis decreases. At higher temperatures, the decreased rate of photosynthesis can be due to enzymes denaturation, or closure of stomata in turn leading to a decrease of CO2 concentration

State a summary of non-cyclic photophosphorylation.

ATP and NADPH produced during non-cyclic photophosphorylation are used for the synthesis of carbohydrates in the light-dependent reactions. ATP serves as the energy source in light-independent reactions. NADPH provides the reducing power for the light-independent reactions.

What is the significance of ATP and NADPH

ATP serves as the energy source in light-independent reactions. NADPH provides the reducing power for the light-independent reactions.

What is the role of ATP and reduced NADP (Same as NADPH) in Calvin cycle?

ATP serves as the energy source while NADPH provides the reducing power required for Calvin cycle.

Roles of stalked particle/ ATP synthase

Acts as H+ channels that allow diffusion of H+ from thylakoid lumen back into the stroma. Consists of ATP synthase which catalyses the phosphorylation of ADP to ATP

What is the formula for the rate of photosynthesis?

Collected volume of oxygen/time(min) = ___mm3 Oxygen evolved per minute

What happens after the saturation point for light intensity?

Further increase in light intensity will cause no increase in the rate of photosynthesis, as other factors such as temperature or carbon dioxide concentration are limiting.

What is the role of Rubisco?

It catalyses the reaction of CO2 fixation. Carbon dioxide is fixed by combining with a 5C sugar and ribulose bisphosphate to form a 6C unstable intermediate

List 10 ways in which photophosphorylation differs from oxidative phosphorylation.

Photophosphorylation occurs at the thylakoid membrane of chloroplast, while OP occurs at the inner mitochondrial membrane. Accumulation of protons in photophosphorylation occurs in the thylakoid lume, but for OP, occurs, in the intermembrane space. Light energy is required to energize the electrons in special chlorophyll a in photophosphorylation, but light energy is not required in OP. Light energy is converted to chemical energy in photophosphorylation, while chemical energy is converted to chemical energy in OP Oxygen is released as a by-product in photophosphorylation, while oxygen is used as a final electron acceptor in OP Electron donor in non cyclic photophosphorylation is water, and in cyclic photophosphorylation is special chlorophyll a in PS I, while in OP, it is NADH and FADH2 Electron acceptor in non cyclic photophosphorylation is NADP+ and in cyclic photophosphorylation it is special chlorophyll a in PS I, but in OP, it is oxygen. Source of energy for ATP synthesis in photophosphorylation is light, while in OP, it is from glucose oxidation processes Proton gradient in photophosphorylation is established when H+ is pumped from the stroma to the thylakoid lumen, while in OP, H+ is pumped from the mitochondrial matrix to the intermembrane space.

outline the 3 phases of the Calvin cycle in C3 plants: i) CO2 fixation, ii) PGA reduction, and iii) ribulose bisphosphate (RuBP) regeneration

Stage 1: CO2 fixation Carbon dioxide is fixed by combining with a 5C sugar, ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco to form a 6C unstable intermediate. The unstable 6C intermediate breaks down immediately to form 2 molecules of glycerate phosphate (GP), which is the first stable product of photosynthesis. Stage 2: PGA reduction This step involves the reduction of glycerate phosphate. The products of light-dependent reactions, ATP and NADPH, are required. Reduction takes place in two steps: 1) Each molecule of GP is phosphorylated by 1 molecule of ATP, forming glycerate bisphosphate. 2) Each molecule of glycerate bisphosphate is reduced by 1 molecule of NADPH to form triose phosphate. NADPH is oxidised to form NADP+ In summary, 2 NADPH and 2 ATP produced in the light-dependent reactions are used to reduce 2 molecules of glycerate phosphate into 2 molecules of triose phosphate. Stage 3: RuBP regeneration Some of the triose phosphate has to be used to regenerate the RuBP used during CO2 fixation. This is to ensure that there is RuBP available to combine with more carbon dioxide. 1 molecule of ATP is needed to regenerate 1 molecule of RuBP

Role of stroma in photosynthesis

Stroma contains enzymes that catalyse reactions in the Calvin cycle;; CO2 is fixed by combining with ribulose bisphosphate (RuBP) and a 5c sugar, catalysed by rubisco, to form two molecules of glycerate phosphate (GP). GP is phosphorylated and reduced to triose phosphate (TP) for hexose sugar formation;;

What happens to triose phosphate after it is produced in Calvin cycle?

Triose phosphate does not accumulate in large quantities. Pairs of triose phosphate molecules are combined to produce an intermediate hexose sugar, sugar or fructose, which may be used as respiratory substrate for the production of ATP. A-glucose polymerises to form starch for energy storage while B-glucose polymerises to form cellulose for the formation of new cell walls.

What occurs at the light compensation point?

rate of respiration = rate of photosynthesis. CO2 produced in respiration is used in photosynthesis. there is no net loss or gain in CO2.

explain the absorption spectrum of photosynthetic pigments

the absorption spectrum is a graph showing the degree of absorption at each wavelength of light by a particular photosynthetic pigment. it shows the absorption qualities of pigments.

explain the action spectrum of photosynthetic pigments

the action spectrum is a graph showing the effectiveness of different wavelengths of light on a photochemical process, eg photosynthesis.