Photosynthesis | Plant Physiology - Biology IB23

Photosynthesis

The production of organic molecules (Glucose) from inorganic molecules (CO₂, H₂O) through the help of light energy.

The light energy is converted into chemical energy which is found in the bonds of glucose.

Photosynthesis Formula

Water + Carbon Dioxide —> Glucose + Oxygen

6H₂O + 6CO₂ —> C₆H₁₂O₆ + 6O₂

Glucose

Can be turned into other types of carbohydrates such as fructose, sucrose, starch or cellulose and fats or amino acids with the addition of N- and S-atoms or DNA with N- and P-atoms.

Light Energy

Light is composed of several different colours which corresponds to different wavelengths of light and follows the order ROYGBIV.

Chlorophyll

The main pigment in plants which absorbs blue light best, then red second and reflects green light which is why most plants look green.

Light Dependent Reaction Steps

1. Happens in the stroma of the chloroplast on the thylakoid membranes.

2. Light is absorbed by PSII which excites an electron from the chlorophyll and leaves for a higher energy level.

3. The chlorophyll will take an electron from water to replace the lost electron by photolysis, splitting the water into 2H⁺, 2e- and ½ O₂. The oxygen will later be released as waste product.

4. Redox Reaction - The electron is passed down a series of electron carriers in an electron transport chain on the thylakoid membrane of the chloroplast while losing energy but also creates a H⁺ gradient in the thylakoid interior.

5. Chemiosmosis - The H⁺ ions only return to the stroma through the enzyme ATP-synthase located on the chloroplast membrane which makes ATP.

6. Photophosphorylation - ATP is made using light energy.

7. When chlorophyll within PSI loses an electron when it absorbs light the electron will replace it.

8. The lost electron from PSI will be passed to the H carrier NADP reducing it to NADPH to carry it to the light independent stage.

The Calvin Cycle | Light Independent Reaction

1. Happens in the stroma of the chloroplast.

2. Carboxylation - CO₂ will be added to a 5C-compound, RUBP, which the enzyme rubisco makes possible. The 6-C molecule will be unstable and break down into two 3-C molecules called G3P.

3. Reduction - G3P is reduced by H from water which has been carried by NADPH. This process requires energy so ATP from the light dependent reaction is broken down into ADP + P_i. This results in triose phosphate and some will leave the cycle to become glucose and the rest continue on in the cycle.

4. Regeneration - Triose phosphate molecules will change to become RUBP which also requires ATP.