Photosynthesis

Photosynthesis

Structures of Photosynthesis

Leaves

Flattened leaf shape exposes large surface area to catch sunlight

Upper and lower leaf surfaces of a leaf comprise the epidermis

Waxy, waterproof cuticle on outer surfaces reduces water evaporation

Adjustable pores called stomata allow for entry of air with CO2

Inner mesophyll cell layers contain majority of chloroplasts (40- 200 each mesophyll cell)

Vascular bundles (veins) supply water and minerals to the leaf while carrying sugars away from the leaf

Chloroplasts- double-membrane (inner and outer)

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Chloroplast Structure

Pigments

Pigment absorbs visible light Chlorophyll a and b absorb violet, blue, and red

light but reflect green light (hence they appear green) Carotenoids absorb blue and green light but

reflect yellow, orange, or red (what color would they appear?)

Photophosphorylation

Process that creates ATP using a proton gradient created by the energy of sunlight

Similar to electron transport chain in respiration Is light dependent, therefore called

photophosphorylation 2 types-non-cyclic and cyclic

Non-cyclic photophosphorylation

Light Dependent Reaction

Path of Electron

Path of electron

Non-Cyclic Photophosphorylation

ATP is generated by the protons moving across the thylakoid membranes to create a proton gradient

Proton gradient is used to generate ATP during chemiosmosis

NADPH2 is formed Oxygen released due to photolysis of water PSII and PSI working together Needs external source of electrons Performs best under optimum light, with CO2 present

and aerobic conditions Continues to light independent reactions to fix

carbon

Chemiosmosis

Process that uses the movement of a proton (H+) to join ADP and Pi to form ATP

ATP synthase is needed H+ ions create a proton-motive force

Cyclic Photophosphorylation

Electrons are recycled, return back to PS I Proton gradient is formed, therefore ATP

formation happens No reduction of NADP+ occurs, only ATP is made Requires only PS I Typical of low light situations, limited CO2 and/or

anaerobic conditions Not very common, except with photosynthetic

bacteria

Light Independent Reactionsa.k.a. the Calvin Cycle

Light Independent Reactions

CO2 diffuses into the stroma of the cloroplasts CO2 is fixed to a 5-carbon molecule (ribulose

biphosphate) to form a 6 carbon molecule Rubisco, an enzyme, catalyzes this reaction 6-carbon molecule is not stable, and splits to

form 3PGA Energy from ATP and NADPH is used to remove a

phosphate group from 3PGA to form G3P RuBP is regenerated from G3P

Occurs in the stroma Depleted carriers (ADP and NADP+) return to

light-dependent reactions for recharging 6 CO2 used to synthesize 1 glucose (C6H12O6)

Light Independent Reactions

Quick Review

1. How is the structure of a chloroplast related to its function?

2. Why do plant cells need mitochondria if they can make ATP in chloroplasts?

3. Explain the role of water in photosynthesis.4. Explain why light-independent reactions of

photosynthesis can only continue for a short time in darkness.