11 Photosynthesis

Biology_Photosynthesis_A_level

Site of photosynthesis

Photosyntheis takes place in chloroplast.

Light dependent reactions: take place in the thylakoid membrane

Light independent reactions: takes place in the stroma

Adaptations of leaf

• Large surface area, increase amount of light energy

• leaf arrangement to minimise overlapping so each leaf has light

• thin so diffusion of gasses is short

• transparent cuticle and upper epidermis to allow light through

• network of xylem to bring water

• palisade cells on upper epidermis to maximise photosynthesis

• lots of stomata for gas exchange

Structure of chloroplasts, and adaptations:

• Grana: stacks of (100) thylakoids where light dependent reactions take place (contains chlorophyll)

  • Thylakoid membrane has high SA for maximising ATP and NADPH formed

  • Proteins in grana hold the chlorophyll in a way to maximise the absorption of light

  • ATP synthase is selectively permeable to establish a proton gradient

• Stroma: where the light independent reactions take place, and contains starch grains.

  • Contains DNA and ribosomes, allowing proteins required for light dependent/ independent reactions to be formed quickly (e.g. rubisco)

Light dependent reaction (process)

Takes place on thylakoid membrane, the stages of photosynthesis include: Photoionisation, Photolysis, electron transport chain and photophosphorylation

Photoionisation: Light energy is absorbed by chlorophyll, and this excites the electrons which leave the chlorophyll forming an electron chain. The lost electrons are replaced by the photolysis of water (2H20 → 4H+ + 4e- + O2) => oxygen used in respiration or leaves the leaf, protons for maintaining a proton gradient.

Electron transfer chain: the chain of electrons move through the electron carriers, undergoing redox reactions that releases energy allowing energy for the proton pump, for protons to actively transport from the stroma into the thylakoid space. This maintains a proton concentration gradient. When the electrons make their way at the end, the NADP + protons (with electrons) → reduced NADP (NADPH)

Photophosphorylation: When a high concentration of protons inside the thylakoid space, moves by facilliacted diffusion through ATP synthase (chemiosmosis) this generates energy for ADP + Pi → ATP

The products:

• Oxygen (used in respiration)

• NADPH (used in light independent reaction)

• ATP (used in light independent reaction)

Describe the process of light independent reaction:

This is called the calvin cycle

• The CO2 reacts with the ribulose biphosphate (RuBP) [5C], forming 2 molecules of Glycerate 3-phosphate (GP) [3C]. With the enzyme rubisco catalysing reaction.

• Then each glycerate is reduced into triose phosphate (TP) using the hydrogen (NADPH → NADP + H+) supplied from energy from ATP to ADP and Pi

• Then the TP gets converted into organic substance such as amino acids, glucose, and the other gets converted back into RuBP requiring ATP

  • The only one you need full form is Triose phosphate

Limiting factors

• When there is a high light intensity, more light dependent reactions so more ATP and NADPH is formed

• When there is more CO2 concentration more light independent reaction takes place so more glycerate 3-phosphate is formed (GP)

• When the temperature increases too much then Rubisco is an enzyme it will denature and the rate of reaction in light independent reaction decreases, additionally for light dependent reactions the thylakoid membrane proteins denature and the rate of products (ATP and NADPH formed decreases)