Photosynthesis and Respiration

Question 1

give three biological processes in plants that use energy

Answer 1

photosynthesis,active transport, DNA replication

Question 2

explain the difference between reduction and oxidation in terms of electrons,hydrogen and oxygen lost or gained

Answer 2

Reduction
-gains electron
-gain hydrogen
-lose oxygen
oxidation
-lose electrons
-lose hydrogen
-gain oxygen

Question 3

explain how the plant leaf is adapted to carry out photosynthesis

Answer 3

-large SA-> absorb as much sunlight as possible
-thin-> short diffusion pathway
-airspaces in lower mesophyll-> rapid diffusion of gases
-xylem and phloem-> transport water and sugar efficiently
-many chloroplasts-> absorb maximum sunlight

Question 4

what are the 2 stages of photosynthesis

Answer 4

1.light dependent reaction
2.light independent reaction

Question 5

where do the light-dependent and light independent reactions occur in plants

Answer 5

-light dependent= in the thylakoids of chloroplasts
-light independent= stroma of chloroplasts

Question 6

steps of light dependent reaction

Answer 6

1.chlorophyll molecule absorbs light energy and a pair of electrons is raised up energy levels (excited). Electrons leave the chlorophyll . the chlorophyll is ionised (photoionisation)
2. excited electrons passed from electron carrier in a series of redox reactions which are located in the thylakoid membrane. after each electron carrier, electrons lose more energy
3. energy is used to pump hydrogen ions into the space inside the thylakoid
4.hydrogen ion move from high conc inside thylakoid to lower conc in stroma, through ATP synthase which combines ADP+Pi to form ATP
5.hydrogen ions pass through ATP synthase and are taken up along with electrons by NADP to produce reduced NADP
6. photolysis ( splitting water with light) provides electrons to replace those lost from chlorophyll during photoionisation

Question 7

what happens to the products of the photolysis of water ?

Answer 7

-H+ ions : move out of thylakoid space via ATP synthase and are used to reduce the coenzyme NADP
-e- : replace electrons from chlorophyll
-O2 : used for respiration or diffuses out of leaf as waste gas

Question 8

what are coenzymes

Answer 8

molecules which transfer chemical groups in reactions and are continually recycled

Question 9

products of light dependent reaction

Answer 9

-ATP light ->independent reaction
-Reduced NADP-> light independent reaction
-Oxygen -> leaves cell as a by-product or used in respiration

Question 10

explain the role of light in photoionisation

Answer 10

-chlorophyll molecules absorb energy from light
-excites 2 electrons causing them to be released from the chlorophyll

Question 11

how does chemiosmosis produce ATP in the light dependent reaction stage ?

Answer 11

-H+ ions move down their conc gradient from the thylakoid space into the stroma via the channel protein ATP synphase

Question 12

how and where is reduced NADP produced in the light dependent reaction

Answer 12

-NADP + 2H+ + 2e- —> reduced NADP
-catalysed by dehydrogenase enzymes
-stroma of chloroplasts

Question 13

which chemicals are needed for the light-dependent
reaction?

Answer 13

NADP, ADP, Pi and water

Question 14

Atrazine binds to proteins in the electron transfer chain in chloroplasts of
weeds, reducing the transfer of electrons down the chain.
Explain how this reduces the rate of photosynthesis in weeds.

Answer 14

1. Reduced transfer of protons across thylakoid membrane
2. (So) less ATP produced;
3. (So) less reduced NADP produced;
4. (So) light-independent reaction slows / stops;

Question 15

steps of the light independent reaction

Answer 15

1. CO2 reacts with RuBP (ribulose bisphosphate) (5C), catalysed by the enzyme rubisco
2. Produces 2 molecules of glycerate 3-phosphate (GP) (3C)
3. GP reduced to triose phosphate (TP) using products from light dependent reaction: energy from the hydrolysis of ATP and H+ from reduced NADP
4. Some TP converted into useful organic substances
   eg. glucose.
5. (5/6) TP used to regenerate RuBP (using rest of ATP).

Question 16

Heat stress decreases the light-dependent reaction of photosynthesis.
Explain why this leads to a decrease in the light-independent reaction.

Answer 16

1.(Less/no) ATP;
2. (Less/no) reduced NADP;

Question 17

A decrease in the activity of the enzyme rubisco would limit the rate of
photosynthesis.
Explain why.

Answer 17

1. (Less/no) carbon dioxide (reacts) with RuBP;
2. (Less/no) GP;

Question 18

Where precisely is rubisco found in a cell?

Answer 18

Stroma

Question 19

Describe and explain how temperature affects rate of photosynthesis

Answer 19

1. As temperature increases, rate increases
   -Enzymes eg. rubisco gain kinetic energy
   -So more enzyme-substrate (E-S) complexes form
2. Above an optimum temperature, rate decreases
   -Enzymes denature as H bonds in tertiary structure break
   -So fewer enzyme-substrate (E-S) complexes form

Question 20

Describe and explain how light intensity affects rate of photosynthesis

Answer 20

1. As light intensity increases, rate increases
   -Light-dependent reaction increases (eg. more photoionisation of chlorophyll) so more ATP and reduced NADP produced
   - So light-independent reaction increases as more GP reduced to TP and more TP regenerates RuBP
2. Above a certain light intensity, rate stops increasing
   - Another factor is limiting eg. temperature / CO2 concentration

Question 21

Describe and explain how CO2 concentration affects rate of photosynthesis

Answer 21

1. As CO2 concentration increases, rate increases
   - Light-independent reaction increases
   - As more CO2 combines with RuBP to form GP
   - So more GP reduced to TP
   - So more TP converted to organic substances and more
   RuBP regenerated
2. Above a certain CO2 concentration, rate stops increasing
   - Another factor is limiting eg. temperature / light intensity

Question 22

In natural ecosystems, most of the light falling on producers is not used in
photosynthesis.
Suggest two reasons why.

Answer 22

1. (Light is) reflected;
2. (Light is) wrong wavelength;

Question 23

Why is respiration important?

Answer 23

● Respiration produces ATP (to release energy)
● For active transport, protein synthesis etc.

Question 24

Summarise the stages of anaerobic respiration

Answer 24

1. Glycolysis - cytoplasm
2. NAD regeneration - cytoplasm

Question 25

Summarise the stages of aerobic respiration

Answer 25

1. Glycolysis - cytoplasm (anaerobic)
2. Link reaction - mitochondrial matrix
3. Krebs cycle - mitochondrial matrix
4. Oxidative phosphorylation - inner
   mitochondrial membrane

Question 26

Describe the process of glycolysis

Answer 26

1. Glucose phosphorylated to glucose phosphate
   ○ Using inorganic phosphates from 2 ATP
2. Hydrolysed to 2 x triose phosphate
3. Oxidised to 2 pyruvate
   ○ 2 NAD reduced
   ○ 4 ATP regenerated (net gain of 2)

Question 27

Explain what happens after glycolysis if respiration is anaerobic - (NAD regeneration)

Answer 27

1. Pyruvate converted to lactate (animals &
   some bacteria) or ethanol (plants & yeast)
2. Oxidising reduced NAD → NAD regenerated
3. So glycolysis can continue (which needs NAD) allowing continued production of ATP

Question 28

Suggest why anaerobic respiration produces less ATP per molecule of glucose than aerobic respiration

Answer 28

● Only glycolysis involved which produces little ATP (2 molecules)
● No oxidative phosphorylation which forms majority of ATP (around 34 molecules)

Question 29

Describe the link reaction

Answer 29

1. Pyruvate oxidised (and decarboxylated) to acetate
   ○ CO2 produced
   ○ Reduced NAD produced (picks up H)
2. Acetate combines with coenzyme A, forming acetyl coenzyme A

Question 30

Describe the Krebs cycle

Answer 30

1. Acetyl coenzyme A (2C) reacts with a 4C molecule
   ○ Releasing coenzyme A
   ○ Producing a 6C molecule that enters the Krebs cycle
2. In a series of oxidation-reduction reactions, the 4C molecule is regenerated and:
   ○ 2 x CO2 lost
   ○ Coenzymes NAD & FAD reduced
   ○ Substrate level phosphorylation (direct transfer of Pi to ADP) → ATP produced

Question 31

Describe the process of oxidative phosphorylation

Answer 31

1. Reduced NAD/FAD oxidised to release H atoms → split into protons (H+) and electrons (e-)
2. Electrons transferred down electron transfer chain (chain of carriers at decreasing energy levels)
   ○ By redox reactions
3. Energy released by electrons used in the production of ATP from ADP + Pi (chemiosmotic theory):
   ○ Energy used by electron carriers to actively pump protons from matrix → intermembrane space
   ○ Protons diffuse into matrix down an electrochemical gradient, via ATP synthase (embedded)
   ○ Releasing energy to synthesise ATP from ADP + Pi
4. In matrix at end of ETC, oxygen is final electron acceptor (electrons can’t pass along otherwise)
   ○ So protons, electrons and oxygen combine to form water

Question 32

Give examples of other respiratory substrates

Answer 32

● Fatty acids from hydrolysis of lipids → converted to Acetyl Coenzyme A
● Amino acids from hydrolysis of proteins → converted to intermediates in Krebs cycle

Question 33

Malonate inhibits a reaction in the Krebs cycle.
Explain why malonate would decrease the uptake of oxygen in a respiring
cell.

Answer 33

1. Less/no reduced NAD/coenzymes
2. Oxygen is the final/terminal (electron) acceptor;

Question 34

In muscles, pyruvate is converted to lactate during prolonged exercise.
Explain why converting pyruvate to lactate allows the continued production
of ATP by anaerobic respiration.

Answer 34

1. Regenerates/produces NAD
2. (So) glycolysis continues;

Question 35

Describe the advantage of the Bohr effect during intense exercise.

Answer 35

1. Increases dissociation of oxygen;
2. For aerobic respiration at the tissues/muscles/cells

Question 36

EPO is another performance-enhancing drug. It can increase the haematocrit
(the percentage of red blood cells in blood).
A heart attack is caused by a lack of glucose and oxygen being delivered to
cardiac muscle via the coronary arteries. The overuse of EPO can increase
the risk of a heart attack.
Suggest how.

Answer 36

1. (EPO) causes blood to thicken;
2. (The thickened blood) could block the coronary arteries