5 Energy transfers- Photosynthesis

Question 1

In which chloroplast feature does the Light-dependent reaction occur + describe it?

Answer 1

Thylakoid membranes

folded membranes, have photosynthetic proteins (chlorophyll)
electron carrier proteins embedded here
^ both involved in LDR

Question 2

In which chloroplast feature does the Light-independent reaction occur + describe it?

Answer 2

Stroma

Fluid centre, has enzymes involved in LIR

Question 3

What is the first stage of photosynthesis and what does it require & produce?

Answer 3

Light-dependent reaction

Requires light energy & water, makes ATP & reduced NADP

Question 4

What are the 4 key stages within the light-dependent reaction?

Answer 4

1. Photolysis
2. Photoionisation of chlorophyll
3. Chemiosmosis
4. Production of ATP & reduced NADP

Question 5

What happens during photolysis?

Answer 5

Light energy is absorbed by chlorophyll, splits water into oxygen, H+ (protons) and e- (electrons)
H20 –> 1/2 O2 + 2e- + 2H+
H+ is picked up by NADP to form NADPH, used in LIR
e- are passed along chain of electron carrier proteins
oxygen is used either for respiration/ diffuses out of leaf via stomata

Question 6

What happens during the photoionisation of chlorophyll?

Answer 6

Light energy absorbed by chlorophyll, electrons become excited and raise up an energy level to leave chlorophyll
So, the chlorophyll has been ionised by light
Some energy from released electrons -> used to make ATP & reduced NADP in chemiosmosis

Question 7

What happens during chemiosmosis and how are ATP & reduced NADP made?

Answer 7

Electrons which gained energy + left chlorophyll move along series of proteins embedded within thylakoid membrane. As this happens, they release energy, some of which is used to pump the protons across the chloroplast membranes. An electrochemical gradient is therefore created so protons move back to stroma through facilitated diffusion. The protons pass through enzyme ATP synthase; results in ATP production as enzyme phosphorylates ADP.
The protons combine with co-enzyme NADP to make reduced NADP
B/c the protons move from a high to low concentration gradient this is known as chemiosmosis.

Question 8

What are the products from the light dependent reaction and what do they do?

Answer 8

ATP, reduced NADP- necessary for the light-independent reaction to occur
Energy from ATP, hydrogens of reduced NADP are passed from the light-dependent to independent reaction to produce complex organic molecules like sucrose, cellulose and starch

Question 9

What is the first stage of the light-independent reaction and what occurs?

Answer 9

Carbon fixation

Carbon dioxide reacts with a five-carbon sugar (5C) called ribulose biphosphate (RuBP) , catalysed by the enzyme rubisco. The resulting 6-carbon compound is unstable and splits into two 3C compounds, glycerate 3-phosphate (GP). The carbon has been fixed; removed from the environment and becomes part of the plant cell. The GP is not a carbohydrate, but will convert into one later in the cycle.

Question 10

What is the second stage of the light-independent reaction and what occurs?

Answer 10

The reduction of glycerate 3-phosphate

Energy from the ATP + hydrogen from the r NADP is used to reduce GP into a phosphorylated 3C sugar, triose phosphate (TP).
1/6 of the TP molecules are used to produce organic molecules needed by the plant;
- can be used to make amino acids for protein synthesis
-can be converted to glycerol -> fatty acids -> lipids
- can condense to hexose which can make starch/cellulose

Question 11

What is the third stage of the light-independent reaction and what occurs?

Answer 11

Regeneration of ribulose biphosphate

5/6 of TP molecules are used to regenerate RuBP, which requires ATP

Question 12

What are the limiting factors of photosynthesis?

Answer 12

-Temperature
-Carbon dioxide concentration
-Light intensity

Question 13

What are photosystems?

Answer 13

Funnel-like structures found in the thylakoid membranes. Made from accessory pigments hat absorb light energy, transfer to primary pigment reaction centre

Question 14

Why is water not one of the limiting factors?

Answer 14

A lack of water can reduce the rate of photosynthesis, but it usually affects other processes in the plant before photosynthesis, so isn’t one of the main limiting factors

Question 15

Why is light intensity a limiting factor?

Answer 15

When temperature and carbon dioxide concentration remains constant, changes in light intensity affect rate of photosynthesis.
The greater the light intensity, the more energy supplied to the plant, so light-dependent stage can occur faster. It makes more ATP and reduced NADP so then the Calvin Cycle can also occur at a greater rate
At some point, when the light intensity continues to increase, the rate of photosynthesis will plateau

Question 16

How is carbon dioxide a limiting factor?

Answer 16

CO2 is one of the raw materials required for photosynthesis, it’s needed for the light-independent stage when CO2 combines with RuBP during carbon fixation. This step is faster when there’s more CO2, so the overall reaction of photosynthesis is quicker. CO2 is in short supply so the rate of photosynthesis will continue until another factor prevents it. The natural level of CO2 in the atmosphere is 0.04%, so it’s not advisable to increase it further or it may become toxic

Question 17

How is temperature a limiting factor?

Answer 17

As temperature increases, so does the rate of photosynthesis as it is controlled by enzymes. After a certain point, the enzymes will begin to denature and the rate will decrease
Has no control on light dependent stage, as it uses energy from light rather than kinetic energy from reacting molecules
The Calvin Cycle is affected as it is enzyme-controlled (e.g. rubisco)
As long as there’s enough ATP and reduced NADP in the LDR, optimum temp increases LIR rate
Temperature can also cause stomata on the leaf to close to reduce water loss. When the stomata is closed, CO2 cannot enter leaves; balance must be met
The light-dependent reaction relies on a proton gradient forming across the thylakoid membrane so
it is important that a too high/low temperature does not affect the permeability of the
membrane which may lead to a dissipation of the proton gradient