Photosynthesis

Photosynthesis is a metabolic process that converts light energy into chemical energy through synthesising organic molecules.

Chloroplasts are found in mesophyll tissues, predominantly in the palisade mesophyll cells.

• Chloroplasts orientate themselves to receive the maximum exposure to light such as evenly distributing themselves across the cytoplasm in low light conditions. In high light they line up in vertical columns against the cell wall to protect from sun damage. • Thylakoids have a large surface area • Stroma directly surrounds the grana allowing products of photosynthesis to diffuse easier into the stroma.

The wavelengths of light used for photosynthesis are absorbed by photosynthetic pigments such as chlorophyll a and b, carotene and xanthophylls.

A graph showing how much light it absorbed by a pigment at different wavelengths.

A graph to show the rate of photosynthesis at different wavelengths of light.

• Spirogyra, a algae was placed in aerobic bacteria where white light was shone through a prism rafracting it into a rainbow. • The wavelengths that passed through the spirogyra refracted where oxygen was produced the most as it is a product of photosynthesis. This is because oxygen is needed for the aerobic bacteria to respire so they migrate towards it. Thus he concluded the blue and red regions has the most photosynthetic activity. Green light is reflected by the plant and doesnt contribute to photosynthesis.

In the thylakoid membranes of chloroplasts, antenna complexes form with a reaction centre at the base. Many accessory pigments are found in the antenna complex which absorb energy and pass it to the base where 2 molecules of chorophyll a is found. These become excited as they gain energy and jump an electron level. *Proteins in the antenna complex prevent the light escaping.

They absorb different wavelengths of light.

• Light dependent stage • Light independent stage

6CO2 (g) + 6H2O (l) → C6H12O6 (aq) + 6O2 (g)

• Double membrane • Grana- Stacks of flattened disks (thylakoids) that are joined by intergranal lamellae. • Stroma- Fluid filled matrix containing lipids, starch granules and enzymes.

• Light harvesting occurs. • The high energy electrons are passed used in either cyclic phosphorylation or non cyclic phosphorylation. • In cyclic phosphorylation, the electron is donated from photosystem I to the electron transport chain, generating a proton gradient for chemiosmosis and passed back to photosystem I. • In noncyclic phosphorylation, 2 high energy electrons from photosystem I reduce NADP to NADPH. This gives photosystem I a positive charge where it can steal an electron from photosystem II. Photolysis occurs providing a high energy electron in photosystem II which is passed to the electron transport chain to trigger chemiosis. The electron produced from this is passed to photosystem I, not photosystem II

To convert light energy to chemical energy as by synthesising ATP or NADPH.

To convert carbon dioxide into carbohydrates for plant growth.

• In the stroma of the chloroplast, carbon dioxide reacts with 5C ribulose biphosphate (RUBP) to form an unstable 6C compound. • This splits into two 3C glycerate-3-phosphate (GP) molecules by the enzyme rubisco. • ATP and NADPH from the light dependent stage reduce GP to 3C triose phosphate (TP), which converted into 2 glucose and then starch in a condensation reaction. • Most of the TP is converted to RUBP using ATP to continue the light independent stage.

A photographic image or X-ray film used to detect the location of radioactive substances (like 14CO2) in a cell or tissue.

2 ATP 1 NADPH

Cyclic and non-cyclic photophosphorylation

The addition of a phosphate group to ADP to produce ATP.

Cyclic phosphorylation uses only photosystem I to form ATP where as non- cyclic used both photosystem I and II.

Within a thylakoid, water molecules absorb light energy and split to form protons, electrons and oxygen. The electrons are passed to photosystem II to replace any lost during non cyclic phosphorylation. The protons reduce NADP. the oxygen is a waste product and diffuses out of the chloroplast.

NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺

• Sugars such as glucose and fructose • Glycerol which can form triglycerides.