Photosynthesis and Respiration

Question 1

Describe the structure of a chloroplast.

Answer 1

Chloroplasts consist of the chloroplast envelope (outer an inner membrane) and the inter-membrane space. Inside this is the stroma, starch grains, lipid droplets, DNA, ribosomes and a system of thylakoid membranes, which stack up to give grana. Membranes between grana are called lamellae.

Question 2

Where does the light-dependent reaction occur?

Answer 2

This happens on the thylakoid membranes and in the thylakoid space.

Question 3

How are thylakoids adapted for the light-dependent stage of photosynthesis?

Answer 3

The thylakoids contain pigments, enzymes and electron carriers required for the light dependent reaction. The grana create a large surface area to increase the number of light-dependent reactions that can occur. There are many different photosynthetic pigments (e.g. chlorophyll a, chlorophyll b, carotene, xanthophyll etc.), to ensure as much light as possible is absorbed.

Question 4

Describe the light-dependent reaction of photosynthesis

Answer 4

• light is absorbed by chlorophyll in photosystem 2, exciting electrons in chlorophyll to a higher energy level (photoionisation)
• the excited electrons move down a collection of membrane proteins called the electron transport chain
• light energy is used to break down water (photolysis), into hydrogen ions, oxygen and electrons (catalysed by the enzyme oxygen-evolving complex)
• the electrons removed from water replace those lost from photosystem 2
• as electrons move down the electron transport chain, they lose energy. This energy can be used to actively transport H+ from the stroma to the thylakoid space, across the thylakoid membrane (chemiosmosis), creating an electrochemical gradient
• H+ move down a concentration gradient form the thylakoid space to the stroma, via the transmembrane protein ATP synthase. This allows the production of ATP from ADP and Pi (photophosphorylation)
• electrons in photosystem 1’s chlorophyll are excited and move down the ETC. the electrons lost from PS2 replace those lost from PS1. The electrons combine with H+ and NADP to produce NADPH (reduced NADP).

Question 5

Describe cyclic photophosphorylation

Answer 5

Light hits chlorophyll in PS1, exciting electrons. These electrons move down the electron transport chain, providing just enough energy to synthesise ATP and NADPH, but they then return to PS1. This occurs when there is very little light available.

Question 6

Describe the light-independent stage of photosynthesis (Calvin cycle).

Answer 6

The cycle begins with ribulose biphosphate (RuBP), which has 5 carbon atoms. CO2 enters the leaf through the stomata then diffuses across the chloroplast envelope into the stroma. The CO2 combines with RuBP in a process called carbon fixation, catalysed by the enzyme rubisco (large quantities of rubisco are needed as this process is slow). This produces two molecules of glycerate 3-phosphate (GP). 2 molecules of reduced NADP and 2 ATP molecules are hydrolysed, this reduces GP to two molecules of triose phosphate (TP).
- one molecule of ATP is used to regenerate RuBP from TP (5/6 of all TP made is converted back to RuBP)
- 1/6 of TP made is converted into organic products, like glucose, lipids, amino acids etc.

Question 7

How many Calvin cycles produce one glucose molecule?

Answer 7

6

Question 8

Where does the light-independent reaction take place?

Answer 8

In the stroma of chloroplasts

Question 9

What is the compensation point?

Answer 9

The point at which O2 used up in respiration is equal to O2 produced in photosynthesis, so there is no net gas exchange.

Question 10

What is a limiting factor?

Answer 10

A limiting factor is a factor which affects the rate of a reaction, whereby the rate is proportional to the value of this factor (so increasing the value (making the conditions more favourable) increases the reaction rate).

Question 11

Why does light intensity affect the rate of photosynthesis?

Answer 11

Low light intensity means a shortage of the products of the light-dependent reaction (ATP and reduced NADP), so the rate-limiting step of the Calvin cycle is the reduction of glycerate 3-phosphate to triose phosphate as this requires the hydrolysis of ATP and NADPH.

Question 12

Why does temperature affect the rate of photosynthesis?

Answer 12

Low temperature means the enzymes catalysing the Calvin cycle are slowed (particularly the activity of rubisco). This means that the carbon fixation step (RuBP to GP) is the rate-limiting step.

Question 13

Why does carbon dioxide concentration affect the rate of photosynthesis?

Answer 13

Low CO2 concentration means that RuBP can’t be converted into GP, so this is the rate-limiting step. RuBP, NADPH and ATP accumulate, levels of GP reduce

Question 14

What is the method for separating photosynthetic pigments?

Answer 14

• grind a leaf using a pestle and mortar with acetone, obtaining a leaf solution
• draw a line on TLC (thin layer chromatography) paper 1cm from the bottom, using a pencil
• draw another line 1cm from the top (solvent front)
• use a paintbrush to spot some leaf solution onto the TLC paper (on the origin line)
• add running solvent to a beaker and place the TLC paper in the beaker, ensuring that the solvent line does not start above the origin line.
• leave the reaction to progress. The pigments will move different distances up the paper depending on their solubility in the solvent.
• calculate the Rf values of each spot and determine which pigment caused which one

Question 15

What is the purpose of the acetone used in the chromatography practical?

Answer 15

The acetone dissolves the phospholipid bilayer that makes up the thylakoid membranes, the cell membranes, and breaks down the cell wall, allowing the pigments out of their cells and suspending them in solution.

Question 16

What are some colours of common photosynthetic pigments?

Answer 16

Chlorophyll a - blue-green
Chlorophyll b - yellow-green
Beta carotene - orange
Xanthophyll - yellow

Question 17

Describe the method for the Hill reaction

Answer 17

Grind leaves using a pestle and mortar and place in a chilled isolation solution. Transfer to a centrifuge tube and centrifuge at high speed for 10 minutes. This will produce a pellet containing nuclei and chloroplasts. Remove the supernatant and add the pellet to fresh isolation medium. Add DCPIP to three test tubes. Add dehydrogenase and chloroplast solution to 2 test tubes but tinfoil should be wrapped around one to ensure light cannot get to the solution. Add chloroplast solution and boiled dehydrogenase to another test tube. Add chloroplast solution and dehydrogenase to another test tube (but no DCPIP). Place all test tubes under a lamp and wait. If the DCPIP decolourises, this shows that electrons are being excited from chlorophyll but they cannot move down the ETC as the DCPIP accepts the electrons instead, becoming reduced. This provides evidence for the light-dependent stage.

Question 18

What is the difference between aerobic and anaerobic respiration?

Answer 18

Aerobic respiration uses oxygen and includes glycolysis, the Krebs cycle and oxidative phosphorylation. Anaerobic respiration does not use oxygen and involves glycolysis and further stages to convert the pyruvate into other products (which products these are depends on the organism).

Question 19

What is the equation for aerobic respiration?

Answer 19

C6H12O6 + 6O2 —> 6CO2 + 6H2O

Question 20

What is the equation for anaerobic respiration in animals?

Answer 20

C6H12O6 —> 2C3H6O3

Question 21

What is the equation for anaerobic respiration in microorganisms (e.g. yeast)?

Answer 21

C6H12O6 —> 2C2H5OH + 2CO2

Question 22

What are the definitions of aerobic and anaerobic respiration?

Answer 22

The breakdown of organic compounds like carbohydrate, in a series of enzyme-catalysed reactions without/using oxygen to produce ATP. ATP breakdown releases energy.

Question 23

One important product of respiration is ATP, what is ATP used for?

Answer 23

• muscle contraction
• protein synthesis
• cell division
• maintaining constant body temperature
• active transport
• passage of nerve impulses
• growth
• activation of molecules (phosphorylation)

Question 24

Where does glycolysis occur?

Answer 24

In the cytoplasm

Question 25

What are coenzymes?

Answer 25

Molecules which aid the function of an enzyme by transferring a chemical group from one molecule to another. In respiration, NAD and FAD are coenzymes which transfer hydrogen, coenzyme A transfers acetate.

Question 26

What are the four stages of aerobic respiration?

Answer 26

• glycolysis
• the link reaction
• Krebs cycle
• oxidative phosphorylation

Question 27

Is glucose the only respiratory substrate?

Answer 27

No, some products resulting from the breakdown of other molecules (e.g. fatty acids from lipids or amino acids from proteins), can be converted into molecules which are able to enter the Krebs cycle (usually acetyl CoA). This only happens if there is not enough glucose to respire.

Question 28

Describe the process of glycolysis.

Answer 28

Glucose is phosphorylated by 2 ATP molecules, forming glucose phosphate and then hexose biphosphate (though hexose biphosphate is usually skipped in mark schemes). The glucose phosphate is split into two triose phosphate molecules. An extra phosphate group is added to each triose phosphate molecule. These molcecules are then oxidised by the formation of 2 NADH, forming 2 glycerate-3-phosphate molecules. The glycerate-3-phosphate molecules are then dephosphorylated by the formation of 2 ATP, producing 2 pyruvate molecules.

Question 29

What is the net gain in products of glycolysis?

Answer 29

2x ATP
2x NADH
2x pyruvate

Question 30

What is substrate-level phosphorylation?

Answer 30

When a phosphate group is transferred from a substrate molecule to ADP in order to produce ATP. This is the method of producing ATP used in glycolysis and the Krebs cycle, which is unusual in that ATP synthase is not required (a different enzyme is used).

Question 31

What happens to the pyruvate produced on glycolysis in aerobic respiration?

Answer 31

The pyruvate produced is actively transported from the cytoplasm to the matrix of the mitochondria for the next stage of respiration.

Question 32

What happens to the pyruvate produced in glycolysis in anaerobic respiration?

Answer 32

In plants and yeast, pyruvate is converted to ethanal with the loss of CO2 (decarboxylation), which is converted to ethanol by the oxidation of NADH.
In animals and some bacteria, pyruvate is reduced to lactic acid by the oxidation of NADH.

Question 33

Why is it important that pyruvate in converted into other products in anaerobic respiration?

Answer 33

This allows NAD to be regenerated, allowing glycolysis to continue. This allows ATP production to continue, which keeps the organism alive (ATP is needed for metabolic reactions, muscle contraction, active transport etc.).

Question 34

What does each part of the mitochondria do during respiration?

Answer 34

Inter membrane space - H+ (protons) are pumped into this space to set up a concentration gradient for oxidative phosphorylation
Matrix - fluid inside the matrix contains enzymes needed to catalyse the link reaction and Krebs cycle
Cristae - projections of the inner mitochondrial membrane, designed to optimise surface area for oxidative phosphorylation
70S ribsosome - smallest type of ribosomes, site of protein synthesis (produces respiratory enzymes)
DNA - naked loops of DNA found in the matrix which contain genes for respiratory enzymes and other proteins

Question 35

Describe the link reaction

Answer 35

Pyruvate is decarboxylated (CO2 is removed) and NAD is reduced to NADH, oxidising the pyruvate, forming acetate. Acetate combines with Coenzyme A (CoA) to form acetyl CoA. This process can be described as oxidative decarboxylation.

Question 36

What is the net gain in products from the link reaction?

Answer 36

2x NADH (goes to oxidative phosphorylation)
2x CO2 (released as a waste product)
2x acetyl CoA (to the Krebs cycle)
This is the gain in products from 2 link reactions as one glucose produces 2 pyruvate.

Question 37

Describe the Krebs cycle (citric acid cycle).

Answer 37

Oxaloacetate (4C) combines with acetyl CoA (2C) to give citrate (6C). NAD is reduced to NADH, oxidising citrate. CO2 is released (oxidative decarboxylation), forming a 5-carbon intermediate compound). Oxidative decarboxylation occurs (CO2 released, NADH produced) and GTP is released (which helps convert ADP and Pi to ATP by substrate-level phosphorylation). This forms a 4-carbon intermediate compound. This compound is oxidised by the reduction of NAD to NADH and FAD to FADH2, forming oxaloacetate again. The cycle can then continue, assuming there is more acetyl CoA.

Question 38

Where do the link reaction and Krebs cycle take place?

Answer 38

In the matrix of the mitochondria.

Question 39

What is the net gain of respiratory products from the Krebs cycle?

Answer 39

Citric acid cycle alone:
2x ATP, 4x CO2, 6x NADH, 2x FADH2
(This is the products from 2 cycles as we started with 2 acetyl CoAs)
The Krebs cycle in its entirety also includes the link reaction, so in total:
2ATP, 6CO2, 8NADH, 2FADH2

Question 40

Describe oxidative phosphorylation.

Answer 40

If oxygen is present, NADH and FADH2 move from the cytoplasm into the matrix, where they are oxidised, releasing protons and high energy electrons. Electrons are passed to the electron transport chain(ETC). The passage of electrons from one carrier to the next releases energy, which is used to actively pump H+ ions into the inter membrane space from the matrix (against their electrochemical gradient). 5 membrane proteins are involved in this process, the first accepts the electrons, then the electrons move down the chain. At the 4th protein, the electrons are accepted by water (the terminal/final electron acceptor), forming water by combining with H+ in the matrix. There is a much higher concentration of H+ in the intermembrane space than in the matrix, so electrons move down their electrochemical gradient through ATP synthase (5th protein) (chemiosmosis), which activates the ATP synthase, allowing ATP to be produced from ADP and Pi.

Question 41

Approximately how many ATP are generated by oxidative phosphorylation?

Answer 41

28

Question 42

What role does oxygen play in aerobic respiration?

Answer 42

Oxygen is the terminal/final electron acceptor of the electron transport Cham of oxidative phosphorylation. This allows electrons to combine with protons and oxygen to form water.

Question 43

Why is oxidative phosphorylation important?

Answer 43

Produces lots of (28) ATP molecules, which are needed to release energy required for many metabolic/other processes in the body. Also regenerates NAD and FAD. NAD needs to be regenerated or glycolysis, the link reaction and Krebs cycle can’t occur without it. FAD is needed in the Krebs cycle as well.

Question 44

In total, how many ATP molecules are produced in respiration?

Answer 44

32 (2 from glycolysis, 2 from Krebs cycle, 28 from oxidative phosphorylation)

Question 45

How can proteins be respired?

Answer 45

• hydrolyse to amino acids
• remove amino groups (deamination)
• products enter respiratory pathway based on number of carbon atoms (3C converted to pyruvate, 4C/5C enter in Krebs)

Question 46

How are lipids respired?

Answer 46

• hydrolyse triglycerides into glycerol and fatty acids
• phosphorylate the glycerol to triose phosphate, enters glycolysis
• break down fatty acids to acetate, which is converted to acetyl CoA, which enters Krebs

Question 47

What is the method of core practical 9 (investigating factors affecting respiration in yeast)?

Answer 47

Set up water baths at 5 different temperatures (e.g. 20 degrees, 30 degrees, 40, 50, 60). Use a graduated pipette to add equal volumes of yeast suspension to 5 boiling tubes. Place the boiling tubes in the water bath for 5 minutes to equilibrate to the temperature. Add equal volumes of glucose solution to each one. Add a few drops of methylene blue solution to each boiling tube and shake to get the methylene blue to the bottom. Start a timer and wait for the methylene blue to change colour from blue to colourless. Note the time taken for this to happen. Repeat twice for each temperature (total of 3 trials for each).

Question 48

Why does the methylene blue change colour in the respiration practical?

Answer 48

The methylene blue is blue when oxidised and colourless when reduced. The methylene blue accepts electrons when substances are dehydrogenated (e.g. NADH—> NAD + H+ +e-). As the methylene blue accepts electrons, it becomes reduced, turning colourless. The less time it takes for the methylene blue to change colour, the faster the rate of dehydrogenase enzymes (and respiration), so the temperature of that trial is closer to the dehydrogenase’s optimum temperature.

Question 49

What does a respirometer measure?

Answer 49

The rate of respiration

Question 50

How do respirometers work?

Answer 50

There is a coloured liquid in a capillary tube and a ruler/scale. A respiring organism is placed in a sealed boiling tube on gauze on top of a CO2 absorbent (e.g. soda lime, KOH (aq)). The position of the coloured liquid is noted at the start and the experiment is left to run. At the end, the position of the coloured liquid is measured again. The liquid moves closer to the boiling tube containing the organisms as they are taking up oxygen but the CO2 produced gets absorbed, so the pressure in the boiling tube decreases. The liquid moves down a pressure gradient.

Question 51

What would the control experiment for respirometers be?

Answer 51

Exactly the same set up as for organisms but use glass beads/rocks of an equal mass to the organisms instead of the organisms.

Question 52

What are some issues with respirometer experiments?

Answer 52

There may be leaks in the apparatus, meaning the measured oxygen consumption is inaccurate. May cause distress to organisms/organisms may die.

Question 53

What is the control experiment for the respiration in yeast practical?

Answer 53

Exactly the same set up as normal, but use boiled yeast suspension as the dehydrogenase (and other enzymes) will be denatured by the high temperatures, so respiration will not occur. Therefore, we would not expect the methylene blue to change colour.