Light Independent Reaction

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• Happens in the stroma
• Does not require light
• ATP and NADPH from light dependent reaction is used to reduce carbon dioxide to triose phosphate and regenerate ribulose biphosphate (RuBP)
• Carbon dioxide reacts with RuBP to form an unstable 6-carbon intermediate which breaks down into two molecules of 3 phosphoglycerate (PGA) catalysed by ribulose biphosphate carboxylase/oxygenase (Rubisco)
• ATP and NADPH reduces PGA to form triose phosphate (G3P), forming ADP and NADP in the process
• Most of the triose phosphate is used to regenerate RuBP
• Some triose phosphate exit the cycle and is used to synthesise other products like starch and glucose
• ADP and NADP are channeled back into the light dependent reaction to produce more ATP and NADPH

Light Dependent Reaction

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• There are two photosystems at the thylakoid membranes of the chloroplast.
• Photosynthetic pigments (eg. chlorophyll and carotenoids) are organized into the two photosystems, PSII and PSI.
• Photosynthetic pigments absorb light energy which cause the excitation of their electrons (photoactivation).
• Excited electrons are ejected from PSI and PSII due to their high energy.
• The excited electrons produce adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using ATP synthase.
• Nicotinamide adenine dinucleotide phosphate (NADPH) is also produced from nicotinamide adenine dinucleotide phosphate ion (NADP+).
• The electron transport chain and the ATP synthase complex at the thylakoid membrane are involved in the production of ATP and NADPH.
• As light is involved in the production of ATP, it is known as photophosphorylation.
• At PSII, the electrons used to produced ATP and NADPH are replenished by electrons from water molecules. 
• Splitting of water (photolysis) results in the release of oxygen gas.

In simpler terms, when light falls on the thylakoid membrane,

• Light causes photoactivation, which is the excitement of electrons by absorption of light energy, at the photosynthetic pigments at PSII and PSI.
• Excited electrons are ejected from the reaction centers of PSII and PSI.
• Ejected electrons are used to produce ATP and NADPH.
• Electrons lost are replenished from photolysis of water.
• Oxygen gas is released in the process.

Chloroplasts

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Chloroplasts

• found in mesophyll cells
• site for photosynthesis
• surrounded by double membrane called chloroplast envelope
• thylakoids are fluid-filled sacs
• a stack of thylakoids is called a granum
• stroma is a gel-like fluid that surrounds the grana
• stroma contains soluble enzymes of the Calvin cycle, sugars, organic acids, etc.
• excess carbohydrates from photosynthesis is stored as starch grains in the chloroplast
• the two main photosynthetic pigments, chlorophyll and carotenoids, are embedded at the surface of thylakoid membranes

Chlorophyll

• converts light energy to chemical energy
• absorb mainly red and blue-violet light 
• reflects green light which gives leaves their green colour

Carotenoids

• yellow, orange, red or brown pigments
• absorb strongly in blue-violet light

Leaf Structure

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Cuticle 

• thin waxy layer of lipids
• prevents water loss through evaporation
• allows light to pass through

Epidermis

• transparent layer of cells
• allows light to pass through
• protects the cells underneath from damages

Palisade Mesophyll Cells

• contains a large number of chloroplasts
• tightly packed together vertically to allow as many cells as possible to absorb light

Spongy Mesophyll Cells

• contains few chloroplasts
• loosely packed with intercellular air spaces for fast diffusion of carbon dioxide

Stoma

• surrounded by two guard cells
• allows exchange of gases 

Vascular Bundle (Vein of leaf)

• made up of xylem and phloem vessels
• xylem transports water and mineral salts
• phloem transports nutrients