Lecture 1: Chloroplasts

Question 1

Which organisms have chloroplasts in their cells?

Answer 1

Plants and algae

Question 2

How is it thought that chloroplasts became part of cells?

Answer 2

It is thought that chloroplasts were originally bacterial which gained a symbiotic relationship with cells. Eventually, the chloroplasts transferred most of its genes into the nuclear DNA of the cell and its own DNA now only contains a few of the genes.

Question 3

What is a plastid organelle?

Answer 3

A small organelle with two membranes and its own small genome commonly found in plants and algae.

Question 4

Give the permeability of the inner and outer membranes of the chloroplast?

Answer 4

The outer membrane is impermeable and the inner membrane is permeable.

Question 5

What is the consequence of having a large surface area of folded membrane?

Answer 5

There are many enzymes present on/in the membrane.

Question 6

Which is smaller: a chloroplast or a mitochondria?

Answer 6

A mitochondria is smaller than a chloroplast

Question 7

Give 3 characteristics about chloroplasts and mitochondria which are different from each other, e.g. organism the organelle is in.

Answer 7

• Bacterial origin
• ATPase orientation
• Enzyme series

Question 8

Why are starch granules in chloroplasts an important area of research?

Answer 8

If more starch can be packed into chloroplasts, plants would have more calories, which would help to prevent starvation.
Packing lots of calories into a small amount of food is very useful for space travel.

Question 9

Describe the main function of chloroplasts.

Answer 9

The chloroplast’s main function is to use energy from the sun to fix carbon from CO2 during photosynthesis to build up complex macromolecules.

Question 10

Give 3 enzymes/enzyme sets required on the thylakoid membranes for photosynthesis to occur.

Answer 10

• ETC
• Photosynthetic complexes (PSI, PSII)
• ATPsynthase

Question 11

Compare the functions of chloroplasts and mitochondria and how they carry these out.

Answer 11

Both produce high energy electrons and use the energy from these to produce a transmembrane proton motive force. This in turn leads to ATP synthesis.

Chloroplasts get these high energy electrons when a normal electron is excited by a photon of light.
Mitochondria get these high energy electrons from oxidation of glucose.

Question 12

Describe the process of photosynthesis.

Answer 12

1) Photosystem II (PSII) absorbs light and splits water to generate high energy electrons.
2) An ETC is used to generate a proton motive force using the energy from the high energy electrons. This proton motive force is used to synthesise ATP by ATPase.
3) More light hits Photosystem I (PSI), raising more electrons to an excited (high-energy) state, which reduces NADP+ to NADPH.
4) The NADPH and ATP are used to convert CO2 into carbohydrates. This is carbon fixation.
Steps 1 to 3 occur in the thylakoid membranes and are light-dependent, whereas step 4 occurs in the stroma/cytosol and is light-independent.

Question 13

What is special about Photosystem II?

Answer 13

It is the only enzyme which splits water.

Question 14

How energetically favourable is carbon fixation?

Answer 14

Very energetically unfavourable

Question 15

Where does carbon fixation occur?

Answer 15

In the stroma

Question 16

Which enzymes is responsible for fixing carbon?

Answer 16

Ribulose biphosphate carboxylase (RuBisCo)

It is the most abundant protein on earth.

Question 17

How many molecules of ATP and NADPH does each molecule of CO2 use in the Calvin cycle?

Answer 17

3 ATP and 2 NADPH (reducing power)

Question 18

Give the overall reaction of one turn of the Calvin cycle, i.e. what goes in and what comes out.

Answer 18

3 CO2 + 9 ATP + 6 NADPH + water —> glyceraldehyde-3-phosphate + 8 Pi + 9 ADP + 6 NADP+. The last three are recycled.

Question 19

Describe the Calvin cycle, naming the all the substrates.

Answer 19

CO2 combines with ribulose-1,5-bisphosphate (5C) to give two molecules of 3-phosphoglycerate (3C). This reaction is catalysed by ribulose bisphosphate carboxylase (RuBisCo). Then each 3-phosphoglycerate is phosphorylated to 1,3-bisphosphoglycerate, with ATP donating the Pi, releasing ADP. Then each of the 1,3-bisphosphoglycerate molecules is converted to glyceraldehyde-3-phosphate, with the release of Pi and the oxidation of NADPH to NADP+. For every 6 molecules of glyceraldehyde-3-phosphate (requires the fixation of 3 CO2 molecules), 1 is used in the synthesis of sugars, fatty acids and amino acids, and 5 are used to regenerate ribulose-1,5-bisphosphate. This occurs in two steps: first Pi is lost, giving ribulose-5-phosphate, which is then converted to ribulose-1,5-bisphosphate with ATP becoming ADP.

Question 20

What happens to the glyceraldehyde-3-phosphate produced by the Calvin cycle?

Answer 20

Most is exported to the cytosol and converted to sucrose. Some remains in the chloroplast, where it is made into a starch store.

Question 21

What are the two main components of a photosystem?

Answer 21

An antenna complex and a photochemical reaction centre

Question 22

Where is a chloroplast are the photosystems?

Answer 22

In the thylakoid membranes

Question 23

Describe what happens in the antenna complex.

Answer 23

When a photon of light hits a chlorophyll molecule in the antenna complex, an electron in the chlorophyll moves from one molecular orbital to another of higher energy. This energy is passed onto a neighbouring chlorophyll molecule, raising one of its electrons to a higher energy orbital as the electron in the first chlorophyll moves back to its original lower energy orbital. This process is called ‘resonance energy transfer’. The energy is transferred to the photochemical reaction centre, where it moves an electron (from the splitting of water) to a higher-energy orbital.

Question 24

What is the function of carotenoids in the antenna complex?

Answer 24

They protect chlorophyll from oxidation and collect light from other wavelengths.

Question 25

What is the function of the photochemical reaction centre?

Answer 25

It traps the energy from the antenna complex in the form of a high-energy electron, which it transfers it to the e- acceptor.

Question 26

During photosynthesis, where is ATP generated?

Answer 26

In the stroma of the chloroplast

Question 27

What is the function of water in photosynthesis?

Answer 27

Water provides the electrons which are raised to a higher energy levels using the energy in a photon.

Question 28

What is another name for the Calvin cycle?

Answer 28

The carbon-fixation cycle

Question 29

What is a) the first electron donor and b) last electron acceptor in the light-dependent phase of photosynthesis?

Answer 29

The first electron donor is water and the last electron acceptor is NADP+.

Question 30

What happens after the electron in PSII has been excited?

Answer 30

The high-energy electron is passed from the photochemical reaction centre to plastoquinone (in lipid bilayer). Plastoquinone passes the electron to an H+ pump called cytochrome b6-f complex, which pumps H+ into the thylakoid space. The proton motive force allows ATPsythase to synthesise ATP.
Photosystem I accepts the energy depleted electron which is transferred from the cytochrome b6-f complex to PSI by plastocyanin, replacing the electron lost when a photon struck it and raised it to a higher energy level. The high energy electron then passes to ferredoxin, which contains an FeS centre. Then the electron is passed to FNR, where it is accepted by NADP+, generating NADPH.

Question 31

Which complex in the mitochondrial ETC does cytochrome b6-f resemble?

Answer 31

Cytochrome bc-1

Question 32

Which mitochondrial electron carrier resembles plastoquinone?

Answer 32

Coenzyme Q (aka ubiquinone)

Question 33

Describe the relative redox potentials of the complexes in the photosynthesis ETC and explain why they are like this.

Answer 33

Each complex must have a higher redox potential than the previous complex, so must accept electrons more readily. This is necessary to give the ETC directionality.

Question 34

What is produced in non-cyclic photophosphorylation?

Answer 34

ATP, NADPH and O2

Question 35

What is produced in cyclic photophosphorylation?

Answer 35

ATP

Question 36

Describe cyclic photophosphorylation.

Answer 36

A high energy electron is passed from PSI to cytochrome b6-f. The energy is used by cytochrome b6-f to pump H+ into the thylakoid space, increasing the proton motive force available for ATP synthesis. The electron is then passed back to PSI at a low energy. No NADPH or O2 is produced.

Question 37

Why is cyclic photophosphorylation necessary?

Answer 37

The chloroplast cannot synthesise enough ATP for every NADPH it synthesises using non-cyclic photophosphorylation, so it uses cyclic photophosphorylation to make extra ATP without making NADPH.

Question 38

Is protein import into chloroplasts post or co-translational?

Answer 38

Post-translational

Question 39

Why must proteins be imported into the chloroplast?

Answer 39

Chloroplasts only have a small genome so do not synthesise all of their own proteins, so the rest must be imported.

Question 40

Discuss the amino acid signal for chloroplasts.

Answer 40

It is an N-terminal sequence, is amphiphilic and is cleaved once the protein is inside the chloroplast.

Question 41

What produces the energy for protein import into the chloroplast?

Answer 41

Hydrolysis of GTP and ATP

Question 42

Name 4 possible methods/families for protein import into the chloroplast.

Answer 42

• Sec protein family (requires ATP electrochemical gradient)
• SRP-like pathway (requires ATP electrochemical gradient)
• TAT (twin arginine) pathway (requires H+ gradient)
• spontaneous insertion, unknown mechanism (no energy requirements)

Question 43

Give 2 functions of chloroplasts other than photosynthesis.

Answer 43

• stroma enzymes make all of the cell’s fatty acids and some amino acids
• reducing power converts nitrite (NO2-) to ammonia (NH3), providing N for amino acid and nucleotide synthesis

Question 44

What is the Artificial Leaf Project?

Answer 44

A research project trying to make artificial leaves that can photosynthesis much more efficiently than real plant leaves, in order to meet the energy consumption of the world. These artificial leaves would be placed in the arid desert areas of the world.