Photosynthesis (the process in which light energy is harnessed to produce glucose)

Plant structures involved in photosynthesis

leaves are the main site of photosynthesis in plants and typically have a large surface area to maximise the amount of light hitting the surface

the main cells in leaves that photosynthesis are mesophyll cells

• inside mesophyll cells are large population of chloroplasts, the organelle that is the site of photosynthesis

  • inside chloroplasts is the photosynthetic pigment called chlorophyll, which is responsible for initiating photosynthesis by capturing and being energised by light energy

there are tiny spores on the surface of leaves called stomata

• they open to allow carbon dioxide to diffuse into the leaf

• they can close to prevent water loss from the leaf in dry conditions

water is absorbed by the root hair cells of plants from soil and transported through the xylem to photosynthesising cells

Light-Dependent stage (the purpose of this stage is to generate the high energy coenzymes NADPH and ATP to power the second stage)

• occurs in the thylakoid/grana
  Inputs

• water

• NADP+

• ADP + Pi 

Outputs

• oxygen

• NADPH

• ATP

Light-Independent stage

• occurs in the stroma in the chloroplast

Inputs

• CO2

• NADPH

• ATP

Outputs

• glucose

• water

• NADP+

• ADP + Pi

Rubisco

a key enzyme of the light-independent stage of photosynthesis

• binds to CO2 (substrate) and fixes the carbon into the organic 3-PGA, initiating the Calvin cycle

Photorespiration

Rubisco binds to O2 instead of CO2

• less photosynthesis means less glucose is produced, combined with wasted energy used in the photorespiration pathway, negatively affecting  a plant’s ability to grow, survive, and reproduce

Factors that influence whether Rubisco binds to CO2 or O2

• Substrate concentration (CO2 or O2)

• Temperature (low temps = more likely to bind to CO2)

  
  

  
  

C3 plants

have no adaptations to reduce photorespiration and undertake ‘normal’/C3 photosynthesis

In the calvin cycle in C3 photosynthesis, Rubisco is responsible for fixing CO2 into the 3-carbon 3-PGA that the initial carbon fixation produces in a single mesophyll cell

• eg. all trees, cereals, majority of nuts fruits and veggies

C4 plants (hot environments)

the initial carbon fixation and the remainder of the Calvin cycle are separated into two different cells (rather than in a single mesophyll cell)

• the initial carbon fixation occurs in a mesophyll cell, but the remaining Calvin cycle occurs in bundle-sheath cells

• there is an energy cost to undertaking C4 photosynthesis, as ATP is required to prepare the enzyme for the initial carbon fixation

• eg. corn, sugarcane, switchgrass, 

CAM plants (hot, dry, environments)

the initial carbon fixation and the remainder of the Calvin cycle is separated over time

• at night, CAM plants open their stomata to bring in CO2

• during the daytime, the plants undergo the light-dependent stage and produce the loaded coenzymes needed to complete the light-independent stage

the controlled release of CO2 out of vacuoles ensures a high concentration of CO2 is maintained near Rubisco, maximising photosynthesis and minimising photorespiration

• require more ATP

• eg. pineapples, vanilla, orchids

Factors affecting the rate of photosynthesis

1. light intensity

2. light colour

3. temperature

4. pH

5. CO2 concentration

6. water

7. enzyme inhibition