Key Knowledge 4

Question 1

photosynthesis equation

Answer 1

6 CO2 + 12 H2O = C6H12O6 + 6 O2 + 6 H20

carbon dioxide + water = glucose + oxygen

Question 2

The inputs of the light-dependent stage

Answer 2

• 12 water
• 12 NADP+
• 18 ADP + Pi

Question 3

The outputs of the light-dependent stage

Answer 3

• 6 oxygen (O2) molecules
• 12 NADPH
• 18 ATP

Question 4

The light-dependent stage

Answer 4

In the first stage of photosynthesis, plants are dependent on light to split water into oxygen and hydrogen. This light-dependent stage occurs on the thylakoid membranes of chloroplasts.

Question 5

The steps in the light-dependent stage

Answer 5

1. Sunlight excites an electron within chlorophyll.
2. Water absorbed by a plant’s root hairs is split into O2 and H+ as it donates one electron to the chlorophyll.
3. The excited electron and H+ ion from water lead to the production of the coenzymes NADPH and ATP.
4. The oxygen is released out of the chloroplast, and the coenzymes are ready for the second stage of photosynthesis.

Question 5

The light-independent stage

Answer 5

During the second stage of photosynthesis, glucose is produced from carbon dioxide, NADPH, and ATP through a cycle of reactions occurring in the stroma of chloroplasts.

Question 6

The inputs of the light-independent stage

Answer 6

• 6 carbon dioxide (CO2) molecules
• 12 NADPH
• 18 ATP.

Question 7

The outputs of the light-independent stage

Answer 7

• glucose (C6H12O6)
• 6 water (H2O) molecules
• 12 NADP+
• 18 ADP + Pi

Question 8

The steps in the light-independent stage

Answer 8

1. Carbon dioxide molecules enter the Calvin cycle and undergo initial reactions. During these changes, the carbon from CO2 combines with a five-carbon molecule, then splits into 2 x three-carbon molecules, which continue along the cycle.
2. NADPH molecules formed in the light-dependent reactions donate their hydrogen ions and electrons, and ATP molecules break into ADP and Pi to release energy to facilitate further changes to the carbon molecules.
3. Carbon molecules continue to change and rearrange as they move around the cycle. Eventually, one specific three-carbon molecule is created and leaves the cycle, going on to contribute to the formation of glucose. Overall, six CO2 molecules must enter the cycle to produce glucose (carbon dioxide = one carbon; glucose = six carbons).
4. Some of the oxygen molecules leftover from the breaking of CO2 at the beginning of the cycle combine with hydrogen ions from NADPH to create the output water.

Question 9

Rubisco process

Answer 9

1 Carbon fixation – which refers to the conversion of CO2 and RuBP into 3-PGA. Here, we say that the carbon from the inorganic CO2 is ‘fixed’ into an organic compound. Rubisco is responsible for taking carbon from an inorganic, gaseous form (CO2) and incorporating it into an organic compound (3-PGA)
2 Reduction – NADPH donates electrons to (aka ‘reduces’) an intermediate three-carbon molecule in the cycle to produce G3P
3 Regeneration – the RuBP molecules needed to start the cycle again are reproduced.

Question 10

Rubisco

Answer 10

Binding CO2 and fixing the carbon into the organic 3-PGA, thus initiating the Calvin cycle

Question 11

The problem with Rubisco

Answer 11

Rather than using CO2 as a substrate, it uses O2 instead. When Rubisco binds to O2 instead, a different reaction called photorespiration occurs.

Question 12

photorespiration

Answer 12

a wasteful process in plants initiated by Rubisco that limits photosynthesis

Question 13

The two key factors that influence whether Rubisco binds CO2

or O2

Answer 13

Substrate concentration – the more substrate is present, the greater chance it can bind to an enzyme and undergo a reaction. However, when a plant needs to conserve water it will close its stomata, causing the O2 produced during the light-dependent stage of photosynthesis to build up inside its cells. A greater concentration of oxygen in the cells leads to increased photorespiration.
• Temperature – at regular temperatures, Rubisco’s affinity for CO2 is far greater than that for O2. At higher temperatures, the affinity for O2 is higher, leading to Rubisco binding oxygen more often.

Question 14

C3 plants 

Answer 14

plants with no evolved adaptation to minimise photorespiration

Question 15

mesophyll cell 

Answer 15

a plant cell type found in leaves that contain large amounts of chloroplasts

Question 16

C4 plants 

Answer 16

plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over space

Question 17

CAM plants 

Answer 17

plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over time

Question 18

Light

Answer 18

Light is required for the light-dependent stage of photosynthesis to occur. Without it, the reaction rate is limited. As light increases, the photosynthesis rate increases – until a certain point.

Question 19

limiting factor 

Answer 19

a factor that prevents the rate of reaction from increasing

Question 20

pH

Answer 20

Enzymes function best at their optimal pH and photosynthesis occurs fastest under these conditions. Above and below the optimal pH, enzymes denature.

Question 21

temperature

Answer 21

The rate of photosynthesis is greatest when the temperature matches the enzyme’s optimal temperature. Every enzyme is unique, but the optimal temperatures of enzymes within a plant is likely to be similar as they have evolved to be suited to the plant’s environment

Question 22

carbon dioxide

Answer 22

As carbon dioxide concentration increases, the rate of photosynthesis increases, up until a certain point. Due to their photosynthetic adaptations, C4 and CAM plants are less affected by carbon dioxide concentration reductions than C3 plants are.

Question 23

water

Answer 23

Water can also influence the rate of photosynthesis given it is an input in the lightdependent stage of photosynthesis and influences the opening and closing of stomata

Question 24

Enzyme inhibition

Answer 24

Competitive and non-competitive inhibitors can act on enzymes to reduce the rate of photosynthesis

Question 25

enzyme inhibitor 

Answer 25

a molecule that binds to and prevents an enzyme from functioning

Question 26

competitive inhibitor 

Answer 26

a molecule that hinders an enzyme by blocking the active site and preventing the substrate from binding

Question 27

non-competitive inhibitor 

Answer 27

a molecule that hinders an enzyme by binding to an allosteric site and changing the shape of the active site to prevent the substrate from binding

Question 28

CRISPR-Cas9 in agriculture

Answer 28

CRISPR-Cas9 technologies can be used to edit the genome of agricultural crops, potentially improving photosynthesis and crop yields in a variety of ways.