Photosynthesis

Overview Photosynthesis

What is photosynthesis?

Photosynthesis is an essential process to all living organisms. The energy from light is converted into chemical energy (glucose), which is used to fuel the organism’s metabolic reactions.

Summary: Photosynthesis has 2 main phases

(There are more details on these below, this a high level overview)

• Light dependent
  • Takes place on thylakoid membrane
  • Requires light
  • Chlorophyll absorbs this light energy and converts it into ATP and NADPH
  • Water is converted (oxidised) to oxygen
• Light independent (also called Calvin Cycle)
  • Takes place in the stroma
  • Doesn’t require light
  • Uses the ATP and NADPH from the light dependent reaction to fix carbon dioxide into GP, and then reduce it to triose phosphate.
  • Some of this triose phosphate is used to regenerate RuBP (keep the Calvin cycle going)
  • Some of triose phosphate is used to create glucose and other organic substances

Structure of Chloroplasts

Photosynthesis generally takes place within the leaf, within chloroplast structures. Many of these structures within the chloroplasts are vital to photosynthesis

Light Dependent Reaction

Photosystems - PSII & PS1

• Are large complexes made of proteins and pigments
• Pigments:
  • PSII has a specialised pair of chlorophyll a molecules - P680, they absorb light best at wavelength of 680nm
  • PS1 also has chlorophyll a molecules - P700, they absorb light best at wavelength of 700nm
  • Note: It’s a little confusing due to the order they present

1 - Photoionisation

• Chlorophyll, located in PSII, absorbs light energy
• This light energy causes an electron in the chlorophyll (P680) to become ‘excited’
• These high energy electrons leave PSII and are passed along the electron transport chain, via electron carrier proteins to PS1

2 - Photolysis

• The electrons which have left the chlorophyll and passed to the electron carriers, need to be replaced
• Light energy is used to split water molecules into:
  • Protons (H+)
  • Electrons
  • Oxygen $$ H_2O \xrightarrow{\text{light}} 2H^+ + 2e^- + \tfrac{1}{2}O_2 $$

3 - ATP Synthesis

• As these high energy electrons travel down the electron transport chain, they lose energy
• This energy is used to pump H+ ions (protons) from the stroma into the thylakoid
• Now, the thylakoid has a higher concentration of H+ ions (protons) than the stroma
• H+ ions will now move down their concentration gradient, from the thylakoid into the stroma
• H+ ions move through an enzyme ATP synthase embedded in the thylakoid membrane, and this movement drives the production of ATP.
• This process is known as chemiosmosis.

$$ ADP + P_i \xrightarrow{\text{ATP synthase}} ATP $$

4 - NADPH Formation

• Light is also absorbed by PS1 (by p700 chlorophyll)
• This light energy causes an electron in the chlorophyll (P700) to become ‘excited’
• These high energy electrons leave PS1 and are passed along the electron transport chain.
• At the end of the chain is NADP+, which is joined by an electron travelling down the electron transport chain. This reduces NADP+ to NADPH
• The electrons from P700 in PS1 are replaced by electrons coming from PSII

$$ NADP^+ + 2H^+ + 2e^- \rightarrow NADPH + H^+ $$

Cyclic Photophosphorylation

• The process described above can be called non-cyclic photophosphorylation, where the electrons flow from PSII to PS1 - in one direction. There is another less common form of photophosphorylation known as, cyclic photophosphorylation
• As the name suggests it goes in a circle, where electrons only pass through PS1 but not PSII (see image)
• No NADPH is produced
• Only a small amount of ATP is produced

Light Independent Reaction

Overview

• As the name suggests, light is not required for this reaction.
• The 2 products from the light dependent reaction are essential to this reaction
  • ATP: Hydrolysis of ATP releases energy to reduce GP to Triose Phosphate
  • Reduced NADP+ (NADPH): Provides electrons to reduce GP to Triose Phosphate
• Carbon dioxide is also required for this reaction. It enters through the stomata and diffuses into the chloroplast (stroma)

Carbon Fixation

• Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form an unstable 6C compound, which splits into two molecules of glycerate-3-phosphate (GP)
• The enzyme rubisco catalyses this reaction

Reduction

• NADPH from the light dependent reaction is used to reduce GP into triose phosphate. It does this by donating a hydrogen (electron) to the GP
• ATP hydrolysis releases energy required for GP to be reduced to triose phosphate

Regeneration

• Regenerating RuBP (5/6):
  • Some of the triose phosphate is used to regenerate RuBP in the Calvin cycle.
  • This is 5 out of every 6 molecules of triose phosphate
  • Requires ATP
• Converted to useful organic substances (1/6):
  • This is 1 out of every 6 molecules of triose phosphate
  • Useful substances include: Glucose, Amino acids, Lipids
    • Triose phosphate can be converted into glycerol
    • Glycerol combines with fatty acids to form lipids

Limiting Factors

What is a limiting factor?

A limiting factor (with regards to photosynthesis) is a condition that can stop or slow down the rate of photosynthesis, even if all other required resources are at optimal supply.

3 factors which affect the rate of photosynthesis

• Light intensity
  • Needed for light dependent reaction
  • Photosynthetic pigments (e.g. chlorophyll) can absorb different wavelengths of light
• Carbon Dioxide Concentration
  • Needed for the light independent reaction (Calvin cycle)
• Temperature
  • As photosynthesis involves many enzymes (e.g ATP synthase from light dependent, Rubisco from light independent), the temperature can affect these enzymes. Too high and it can denature them, too low and there isn’t enough kinetic energy for the reaction occur (less collisions between enzymes and substrates, less substrate enzyme complexes formed)

Light Intensity Example Graphs

• Common agricultural practices aim to ensure they can optimise the rate of photosynthesis for their crops, controlling limiting factors such as carbon dioxide concentration, light and temperature.
• Example of this includes:
  • Light intensity: Greenhouses (glass) to allow more light to enter or artificial lighting
  • Carbon dioxide: Increase carbon dioxide in greenhouse air, using propane or natural gas burners
  • Temperature: Greenhouses allow sunlight to enter but heat cannot easily escape, so increases the temperature