6. Photosynthesis

Question 1

What are organisms that photosynthesise called?

Answer 1

‘photoautotrophs’ as they use autotrophic nutrition

Question 2

What is Autotrophic nutrition?

Answer 2

When inorganic chemicals are converted into organic molecules (in photosynthesis this is glucose)

Question 3

Where does photosynthesis occur?

Answer 3

The chloroplasts

Question 4

What is the main product of photosynthesis?

Answer 4

Monosaccharide sugar.

It can be converted to disaccharides for transport and starch for storage.

Question 5

What is carbon fixation?

Answer 5

When Carbon dioxide is converted into sugars.
It is endothermic so requires energy.
Requires electrons for a reduction reaction
It helps to regulate the concentration of carbon dioxide in the atmosphere.

Question 6

What are the 3 types of photosynthetic organisms?

Answer 6

Plants, Algae, Cyanobacteria

Question 7

What are the two main stages of photosynthesis?

Answer 7

Light dependent stage- requires light to carry out.

Light independent stage- does not need light but depends on the products of the first stage.

Question 8

How do plants use respiration?

Answer 8

During respiration, they oxidise the organic molecules that are synthesised by photosynthesis to produce carbon dioxide.
Respiration releases chemical energy that can drive the organism’s metabolism.

Question 9

How does respiration and photosynthesis interrelate in plants?

Answer 9

The products of one process are the raw materials for the other process.

Question 10

What is compensation point?

Answer 10

The compensation point is when photosynthesis and respiration proceed at the same rate so that there is no net gain or loss of carbohydrate.

Question 11

What is Compensation period?

Answer 11

The time a plant takes to reach its compensation point, it is different for different plant species.
Shade plants reach the compensation point sooner.
Sun plants require a higher light intensity to achieve their optimum rate of photosynthesis

Question 12

What is the structure of chloroplasts?

Answer 12

They are surrounded by a double membrane (the envelope), with an intermembrane space.
The outer membrane is highly permeable.
They contain a fluid-filled matrix called the stroma.
The stroma contains thylakoid membranes arranged in grana.

Question 13

What is the structure of grana?

Answer 13

The thylakoid membrane is folded into thylakoids.
Different thylakoids form stacks.
Each stack is called a granum.
They are surrounded by the stroma, so the products of the light-dependent stage pass into it to be used in the light-independent stage.

Question 14

How are grana adapted to their function?

Answer 14

They have a large surface area for…
Photosystems containing photosynthetic pigments that trap sunlight energy
Electron carriers and ATP synthase enzymes to convert light energy to ATP.

Question 15

What is the structure of photosystems?

Answer 15

They are embedded with an array of photosynthetic pigments.

Each pigment absorbs light (photons) of a particular wavelength.

Question 16

What are the different photosynthetic pigments?

Answer 16

Chlorophyll a, Chlorophyll b and the accessory pigments

Question 17

What is the role of chlorophyll a?

Answer 17

It is the main photosynthetic pigment and sits in the primary reaction centre.

Question 18

What is the role of the accessory pigments?

Answer 18

The light energy absorbed by the accessory pigments gets passed into the primary pigment reaction centre.

Question 19

What are the two forms of chlorophyll a?

Answer 19

P680 is found in photosystem II and its peak wavelength is 680nm.
P700 is found in photosystem I and its peak wavelength is 700nm.

Question 20

What are the different accessory pigments?

Answer 20

Carotenoids- absorb blue light of wavelengths 400-500nm.

Xanthophylls- absorb blue green light of wavelengths 375-550 nm.

Question 21

What is the role of the light dependent stage?

Answer 21

To make ATP and reduced NADP to be used in the light independent stage.

Question 22

What are the two stages of the light dependent stage?

Answer 22

cyclic photophosphorylation,

non-cyclic photophosphorylation.

Question 23

What are the stages of the first part of non-cyclic photophosphorylation?

Answer 23

1. Photosystem II absorbs light.
2. Electrons, which are in pairs, absorb this energy and become excited.
3. Electrons pass along the electron transport chain in the thylakoid membranes.
4. Energy is released which is used to pump the protons to the intermembrane space.
5. Protons flow down their concentration gradient through ATP synthase, causing a proton motive force.
6. The ATP synthase turns and combines ADP and P to form ATP.

Question 24

What happens to the electrons in the second part of non-cyclic photophosphorylation?

Answer 24

1. They get passed to photosystem I and get excited again
2. They pass to the electron acceptor ferredoxin and carried to NADP.
3. At the same time photolysis is occurring where light energy, in the presence of an enzyme, splits water, releasing hydrogen ions.
4. Hydrogen and electrons both reduce NAD and the electrons travel to PS II to replace those excited.

Question 25

What is cyclic photophosphorylation?

Answer 25

Only involves PS I Rather than being accepted by ferredoxin, the electrons excited by PS I return to the electron transport chain. This allows for the further formation of ATP.

Question 26

What is the light independent stage of photosynthesis?

Answer 26

Occurs in the stroma and consists of the Calvin cycle.

Question 27

What are the stages of the Calvin cycle?

Answer 27

1. Begins with ribulose bisphosphate (5c) 2. Carbon fixation of ribulose bisphosphate into two molecules of glycerate-3-phosphate. 3. Conversion of glycerate-3-phosphate into triose phosphate (3c) 4. 1/6th of triose phosphate is converted into glucose. 5. 5/6th of triose phosphate is recycled into ribulose bisphosphate.

Question 28

How does carbon fixation of ribulose bisphosphate into glycerate-3-phosphate occur?

Answer 28

Combines with Carbon Dioxide Catalysed by the enzyme RuBisCO Some glycerate phosphate can be converted into amino acids with nitrogen which comes from nitrates in the soil.

Question 29

How is glycerate-3-phosphate converted into triose phosphate?

Answer 29

This step is called reduction. ATP is broken into ADP+P. Reduced NADP is oxidised into NADP and donates its hydrogen to the glycerate-3-phosphate.

Question 30

Why is only 1/6th of triose phosphate converted into glucose?

Answer 30

It is important to keep the cycle moving

Question 31

How is triosephosphate converted back into ribulose bisphosphate.

Answer 31

``` This step is called phosphorylation ATP is hydrolysed into ADP+P The phosphate (P) is used to phosphorylate triosephosphate to ribulose bisphosphate. ```

Question 32

How many turns of the Calvin cycle are needed to complete the photosynthesis equation?

Answer 32

6

Question 33

What are the limiting factors of photosynthesis?

Answer 33

Light intensity, temperature, carbon dioxide concentration

Question 34

What impact does changing light intensity have on the Calvin cycle?

Answer 34

Glycerate-3-Phosphate cannot be reduced to Triose Phosphate as it requires ATP and reduced NADP which is produced in the light dependent stage. Triose Phosphate levels fall and Glycerate-3-Phoshphate accumulates.

Question 35

How does temperature affect the rate of photosynthesis?

Answer 35

At low temperatures, all of the enzymes that catalyse photosynthesis work slowly. Some other factor is limiting. An increase in temperature increases the rate of the enzyme-catalysed reactions and therefore photosynthesis. At very high temperatures, enzymes are damaged reducing the rate of photosynthesis, it is the limiting factor.