IB Biology 8: Photosynthesis

The purpose of photosynthesis is to transform light CO2, water and light energy into simple sugars that the plant uses as food. Plants are autotrophs (meaning they produce their own food in this way)

Overall photosynthesis word equation: carbon dioxide + water + (light energy) → glucose + oxygen

Overall photosynthesis symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis occurs in the chloroplast

Light dependant and independent reactions:

• Light dependant reactions- photolysis, photoactivation, electron transport, chemiosmosis, ATP synthesis, reduction of NADP. Light energy is converted into chemical energy (ATP and reduced NADP)

• Light independent reactions- carbon fixation, carboxylation of RuBP, production of triose phosphate, ATP and NADPH as energy sources, ATP used to generate RuBP, ATP used to produce carbohydrates

• Light dependant reactions take place in the intermembrane space of the thylakoids

• Light independent reactions take place in the stroma

Parts of the chloroplast:

• Thylakoids – flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation

• Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane

• Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption

• Stroma – a thick, protein-rich central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur

• Lamellae – connects and separates thylakoid stacks (grana), maximising photosynthetic efficiency

Stages of light-dependant reactions

Photolysis-

Photoactivation- protons excite electrons in chlorophyll molecules in the photosystems, causing them to move to a higher energy level

Electron transport-

Chemiosmosis-

ATP synthesis-

Reduction of NADP-

Stages of light independent reactions/ Calvin cycle

Carbon fixation- CO2 in the stroma reacts with the 5C sugar ribulose biphosphate to create a 6C compound. Catalysed by enzyme rubisco*. Immediately forms 2 molecules of glycerate-3 phosphate.

Carboxylation of RuBP*-

Production of triose phosphate-

Six turns of the Calvin cycle produce 1 hexose sugar. This is because three turns of the cycle produce 6 molecules of triose phosphate (2 molecules of triose phosphate per molecule of carbon dioxide) but five out of the six are needed to regenerate RuBP.

TP and GP* are used are used to make carbohydrates, lipids and proteins.

• Hexose sugars: 2x TP

• Starch: many hexose sugars

• Lipids: fatty acids from GP and glycerol from TP

• Proteins- some amino acids from GP

*RuBP= Ribulose biphosphate

Rubisco= RuBP carboxylase

GP=glycerate-3 phosphate (3C)

TP= triose phosphate (also 3C)

Sources: bioninja, Oxford IB Diploma course companion textbook (2014), image from toppr