Photosynthesis

Question 1

Photosynthesis Equation

Answer 1

Carbon dioxide + water → glucose + oxygen + water

Question 2

Where does photosynthesis occur?

Answer 2

• It occurs in the chloroplasts.
• Principally in the leaves.

Question 3

Chloroplasts

Answer 3

• Contains chlorophyll - a pigment protein which is embedded in the thylakoid membranes of the grana.
• The chlorophyll traps light.

Question 4

Photosynthesis

Answer 4

• The process by which light energy is trapped and used to build organic molecules, such as glucose, from inorganic substances such as carbon dioxide and water.
• During this process light energy is transformed into chemical energy.

Question 5

Stage 1: Splitting Water

Answer 5

• Occurs in the chloroplasts, specifically in the thylakoid membrane in the granum.
• Stage 1 needs light. The chlorophyll molecules absorb the light and transform this into chemical energy. This is called the light dependent stage.

Question 6

What happens in stage 1: Splitting water?

Answer 6

• The water molecule (H2O) splits into oxygen (O2 byproduct) and high energy hydrogen ions (H2+) (protons) and electrons.
• H+ ions are picked up by a coenzyme called NADP+ and becomes loaded to NADPH and carried to the next stage.
• Excited electrons are used to make 18 ATP, which are also moved to the next stage.
• 12H2O + 12NADP+ + 18ADP +18Pi → 6O2 + 12NADPH + 18ATP + 12H+

Question 7

Stage 1: Light Dependent stage: Inputs and outputs

Answer 7

Inputs: Light, water, NADP+, ADP
Outputs: Oxygen, Hydrogen ions, NADPH, ATP

Question 8

Stage 2: Light Independent stage (fixing carbon)

Answer 8

• ATP from the first stage is used.
• Fixing of the carbon, using carbon dioxide and H+ ions to produce glucose.
• Provides the chemical energy source for most living things.

Question 9

Stage 2: Light independent stage (where it occurs)

Answer 9

• Stage 2 occurs in the stroma (the liquid in the chloroplasts, it’s own version of cytoplasm).
• Stage 2 does not need light to function.

Question 10

Stage 2: Light Independent stage
Inputs and Outputs

Answer 10

Inputs: Carbon dioxide, Hydrogen ions (H+), ATP, NADPH.
Outputs: ADP, NADP, Glucose, and water.

Question 11

Rubisco

Answer 11

• Rubisco is responsible for taking carbon from an inorganic, gaseous form (CO2) and incorporating it into an organic compound (3-PGA).
• Sometimes instead of binding CO2 it binds O2 which causes a process called photorespiration. This is due to competitive inhibition.
• This process wastes energy and resources for the plant
  Hot and dry conditions increase the chance that photorespiration will occur

Question 12

C4 plants photosynthesis

Answer 12

• C4 plants differ in their light-independent stage. The carbon fixation is separate from the rest of the stage as the rest occurs in cells called bundle-sheath cells. (C3 have these, but they don’t use them like this)
• The CO2 is temporarily stored as malate, and this alone is moved to the bundle-sheath cell
• This allows Rubisco to work in a low O2, and high CO2 environment, ensuring that there is a lot less photorespiration

Question 13

CAM plants photosynthesis

Answer 13

• CAM plants are very similar to C4 plants except happens over time (day (light dependent) and night (light independent)).
• A CAM plant achieves this by doing the steps in different stages
  It does the first step of getting CO2 into the cell and converting it into malate at night, and stores it in a vacuole. (It also does this at night to prevent water loss)
• During the day the large storage of malate can be converted back to CO2 and released near Rubisco to ensure there is a large amount of CO2.
• The plant has its stomata closed during the day, so there is little O2 coming from the air. This ensures minimal photorespiration occurs.

Question 14

Factors affecting photosynthesis

Answer 14

• In various situations any of the factors that influence photosynthesis may limit the rate of photosynthesis.
  E.g.
• Carbon dioxide levels
• Water levels
• Light intensity
• Wavelength of light
• Temperature