Photosynthesis & Respiration

Question 1

What are thylakoids, and where are they found?

Answer 1

Thylakoids are stacked structures called grana found in the chloroplast, and they contain photosynthetic pigments.

Question 2

What are chlorophyll a and chlorophyll b?

Answer 2

Chlorophyll a is the primary pigment that absorbs red and blue light. Chlorophyll b absorbs light at 500–640 nm and acts as an accessory pigment.

Question 3

What is the role of carotenoids in photosynthesis?

Answer 3

Carotenoids absorb blue light and pass energy to chlorophyll for use in photosynthesis.

Question 4

What happens when chlorophyll absorbs light?

Answer 4

When chlorophyll absorbs light, it loses an electron, which is used in the light-dependent reaction.

Question 5

What are the two types of photosystems, and what wavelengths do they absorb?

Answer 5

PS1 absorbs light at a maximum of 700 nm, and PS2 absorbs light at a maximum of 680 nm.

Question 6

Where does the light-dependent reaction take place?

Answer 6

The light-dependent reaction occurs in the thylakoid membranes of chloroplasts.

Question 7

How does light energy excite electrons in chlorophyll?

Answer 7

Light energy is absorbed by PS2, exciting electrons in the magnesium atom of chlorophyll, which are released into the electron transport chain and passed to PS1.

Question 8

How is oxygen produced in the light-dependent reaction?

Answer 8

Light energy splits water into protons (H⁺), electrons, and oxygen during photolysis.

Question 9

How is ATP produced in the light-dependent reaction?

Answer 9

Energy from excited electrons is used to transport protons into the thylakoid, creating a concentration gradient. Protons diffuse back through ATP synthase, driving the formation of ATP from ADP and Pi (chemiosmosis).

Question 10

What is the role of NADP in the light-dependent reaction?

Answer 10

Electrons from PS1 are transferred to NADP, along with H⁺ from the stroma, to form reduced NADP (NADPH).

Question 11

What is the Calvin cycle, and where does it occur?

Answer 11

The Calvin cycle is the light-independent stage of photosynthesis and occurs in the stroma of the chloroplast.

Question 12

What happens during carbon fixation in the Calvin cycle?

Answer 12

CO₂ combines with RuBP (5C) to form an unstable 6C compound, which breaks down into 2 molecules of GP (3C). This reaction is catalysed by rubisco.

Question 13

How is GP converted into TP in the Calvin cycle?

Answer 13

GP is converted into triose phosphate (TP) using energy from ATP and reduced NADP (NADPH).

Question 14

What happens to the TP molecules produced in the Calvin cycle?

Answer 14

1/6 of TP is used to synthesise glucose and other molecules. 5/6 of TP is used to regenerate RuBP using ATP.

Question 15

How many cycles of the Calvin cycle are needed to produce one hexose sugar?

Answer 15

The Calvin cycle must occur 6 times to produce 1 hexose sugar, requiring 18 ATP and 12 NADPH.

Question 16

How does light intensity affect photosynthesis?

Answer 16

As light intensity increases, the rate of photosynthesis increases up to a point where another factor becomes limiting.

Question 17

How does CO₂ concentration affect photosynthesis?

Answer 17

As CO₂ concentration increases, the rate of photosynthesis increases until a limiting factor is reached.

Question 18

What is the effect of temperature on the rate of photosynthesis?

Answer 18

At low temperatures, enzyme activity is limited. The rate increases to an optimum point, after which enzymes denature, decreasing the rate.

Question 19

What are the two types of respiration?

Answer 19

Aerobic respiration, which requires oxygen, and anaerobic respiration, which does not require oxygen.

Question 20

What are the key steps in glycolysis?

Answer 20

1. Glucose is phosphorylated to form fructose 1,6-bisphosphate using 2 ATP. 2. Fructose 1,6-bisphosphate splits into 2 triose phosphates. 3. Triose phosphate is oxidised to pyruvate, producing 2 ATP and 1 NADH per triose phosphate.

Question 21

What happens in the link reaction?

Answer 21

Pyruvate is decarboxylated (CO₂ removed), oxidised to form acetate, and combined with coenzyme A to form acetyl CoA. No ATP is produced.

Question 22

How many times does the link reaction occur per glucose molecule?

Answer 22

The link reaction occurs twice per glucose molecule.

Question 23

What are the key steps of the Krebs cycle?

Answer 23

1. Acetyl CoA combines with oxaloacetate to form citrate (6C). 2. Citrate is decarboxylated and oxidised to a 5C compound, producing NADH. 3. The 5C compound is further decarboxylated and oxidised to produce ATP, NADH, and FADH₂.

Question 24

What are the products of the Krebs cycle per molecule of glucose?

Answer 24

2 CoA, 2 oxaloacetate, 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂.

Question 25

What happens during oxidative phosphorylation?

Answer 25

1. NADH and FADH₂ release H atoms, which split into H⁺ and e⁻. 2. Electrons move down the electron transport chain, releasing energy to pump protons into the intermembrane space. 3. Protons diffuse back through ATP synthase, producing ATP. 4. At the end, protons, electrons, and O₂ combine to form water.

Question 26

Why is oxygen important in oxidative phosphorylation?

Answer 26

Oxygen acts as the final electron acceptor, combining with H⁺ and e⁻ to form water.

Question 27

What happens during anaerobic respiration in animals?

Answer 27

Pyruvate is converted to lactate, regenerating NAD for glycolysis to continue.

Question 28

What happens during anaerobic respiration in plants and yeast?

Answer 28

Pyruvate is converted to ethanol and CO₂, regenerating NAD for glycolysis.

Question 29

Why is NAD regeneration important in anaerobic respiration?

Answer 29

NAD is required for glycolysis to continue producing ATP in the absence of oxygen.