Mitochondria and Chloroplasts 2

Question 1

Name agents that interfere with oxidative phosphorylation

Answer 1

1. Cyanide and carbon monoxide inhibit cytochrome oxidase
2. Block the passage of electrons to O2
3. ATP synthesis grinds to a halt

Question 2

What happens when mitochondria become uncoupled

Answer 2

1. Generate heat
2. In most newborn mammals including humans a type of adipose tissue (brown fat) uses fuel oxidation to produce heat and not ATP
3. This is achieved by a protein called thermogenin which provides a path for protons to return to the matrix without passing through the F0F1 complex
4. The energy is dissipated as heat
5. DNP is a mitochondrial uncoupler. Extremely dangerous.

Question 3

Describe the structure of chloroplasts

Answer 3

1. 3 membrane system
2. Thylakoid: Photosystems I and II, ATP synthase, NADP reductase
3. Stroma: ATP synthesised, NADPH Synthesised, carbon fixation, DNA- encodes for some proteins but majority of proteins are imported, Calvin cycle
4. Harvest energy from sunlight, convert it to chemical energy and use this chemical energy to fix carbon from the atmosphere (CO2) into sugars.

Question 4

What are the main features of photophosphorylation

Answer 4

1. The electrons that get passed along the ETC in chloroplasts come from water.
2. Unlike NADH, H2O is a poor donor of electrons
3. Requires energy input in the form of light to create a good electron donor

Question 5

Where does the proton gradient occur in chloroplasts

Answer 5

Thylakoid membrane

Question 6

Describe the proton gradient in chloroplasts

Answer 6

1. Electron transfer coupled to proton pumping
2. Protons released upon water oxidation contribute to the electrochemical proton gradient across the thylakoid membrane.

Question 7

How are high energy electrons produced

Answer 7

1. Sunlight is absorbed by chlorophyll molecules and electrons interact with photons of light raising them to a higher energy level
2. The energy from hundreds of chlorophyll molecules (in the antenna complex) is channelled into a special pair of chlorophyll molecules in the reaction centre

Question 8

Describe the basic structure of chlorophyll

Answer 8

1. Looks a bit like Haem.

2. The metal ion at the centre is Magnesium (not Iron).

Question 9

Describe the photosystems

Answer 9

1. Chlorophylls are held in large multi subunit protein complexes called photosystems.
2. Photosystems consist of an antenna/light harvesting complex and a reaction centre.
3. The antenna complex collects the energy from sunlight and channels the energy (energy transfers) to a pair of electrons in the reaction centre.
4. The reaction centre excites the chlorophyll molecule with high-energy electrons

Question 10

How are electrons passed on from the photosystems

Answer 10

1. The excited high energy electrons are passed on
2. Leaves positive charge behind on chlorophyll
3. Electrons are replaced by electron donor- leaving another positive charge
4. This allows electrons to be pulled away from water
5. Also contain mobile electron carriers

Question 11

How do the two different photosystems work together

Answer 11

1. In higher plants there are 2 photosystems that work together in series. PS I actually acts after PS II.
   2) The nomenclature is due to the fact that PSI was discovered first.

Question 12

Describe the flow of electrons through the transport chain

Answer 12

1. Passed from PSII to quinine to cytochrome b6-f
2. Then to plastocyanin – mobile electron carrier
3. Then to PS I- lose energy but boosted by light energy
4. Electrons passed to Fd
5. high energy electrons go to NADP reductase
6. NADP+ is reduced to NADPH

Question 13

What are the three mobile electron carriers in chloroplasts

Answer 13

1. Plastoquinone (closely resembles ubiquinone of mitochondria)
2. Plastocyanin (a small copper containing protein)
3. Ferredoxin (a small protein containing an iron-sulphur centre)

Question 14

Describe the redox potentials in photosynthesis

Answer 14

1. The electrons move from high-low energy with ‘boosts’ of energy given in PS II and PS I.

Question 15

How is light energy converted to chemical energy

Answer 15

1. Light energy used to generate proton gradient
2. ATP is generated by the proton gradient across the thylakoid membrane in the same way as in mitochondria
3. H+ generated by the splitting of H2O also contributes to the proton gradient
4. The high energy electrons are ultimately passed on to form the high energy compound NADPH
5. NADPH has high energy electrons and is a strong reducing agent (readily donates it’s electrons).
6. Water is a poor electron donor- needs boost from light energy

Question 16

What are the uses of ATP and NADPH

Answer 16

1. Carbon fixation (by the Calvin cycle)

2. For every 3 carbon sugar produced 9 molecules of ATP and 6 of NADPH are required

Question 17

Describe the structure and function of Rubisco

Answer 17

1. Ribulose bis phosphate carboxylase
2. Catalyses the initial reaction in carbon fixation
3. It is a sluggish enzyme, processing only about 3 molecules of substrate per second
4. Therefore in order to fix a lot of carbon a lot of enzyme is required
5. Can make up to 50% of total chloroplast protein
6. Claimed to be the most abundant enzyme on earth
7. Catalyses the addition of one carbon onto a five carbon molecule to make a six carbon intermediate that is then cleaved to produce 2 three carbon molecules.

Question 18

What is produced from the Calvin Cycle

Answer 18

1. For every 3 Carbons that are fixed a 3 carbon containing molecule can be removed from the cycle to be used in the synthesis of sugars, fatty acids, amino acids.
2. There are 3 carbons in 3 carbons out without loss of carbon in the cycle.

Question 19

Similarities between chloroplast and mitochondria processes

Answer 19

1. Both use proton gradients across membranes to produce ATP using ATP synthase
2. Electron transport along an electron transport chain drives proton pump
3. Similarities between some of the components of electron transport chain (cyochrome bc and b6f show sequence similarity and ubiquinone and plastoquinone resemble one another)

Question 20

Differences between chloroplast and mitochondria processes

Answer 20

Chloroplast
1. Low energy electrons come from H2O but are excited to higher energy by light
2. Ultimate electron acceptor is NADP+
3. Chemical bond energy and reducing power utilised in carbon fixation
Mitochondria
1. High energy electrons come from NADH
2. Ultimate electron acceptor is O2
3. Chemical bond energy used in cellular processes