LECTURE 13- photosynthesis light reactions

where does photosynthesis take place

in the leaves below the epidermis of plants

how many chloroplasts can cells have

up to 50

how are thylakoids arranged

arranged in stacks called stroma and absorb light

what are the 2 reactions in photosynthesis

light reaction and carbon fixation

what occurs during light reaction

light energy is captured to create energy (NADPH) and ATP to fix CO2 and make sugars

• light reaction creates high energy molecules by removing electrons from water

what occurs during carbon fixation

occurs in stroma and doesn’t require light. Drives calvin cycle to produce sugars from CO2

why are chloroplasts green

because chloroplasts mainly absorb red/blue light, and reflect green light

what are carotenoids

part of the chloroplasts. Help capture some of the light

what is light energy captured by in plants

chlorophyll

how does energy transfer occur in photosynthesis

1. light captured by atoms in chloroplasts (antenna complex) using chlorophyll molecules

2. molecules become energised and emit energy through photons

3. other chlorophyll molecules can capture and pass around energy

4. photons jump between antenna complex

5. energy eventually reaches special chlorophyll in the centre that transforms photons into electrons

how does photosystem 2 work

1. light energy captured and bounces around until it hits chlorophyll A molecule

2. molecule can then capture electrons that come from water (using water splitting complex Mn)

3. chlorophyll moves electron to pheophytin

4. electron moves from pheophytin to plastoquinone → plastoquinone B → cytochrome Bf6 complex

5. hydrogen ions are then moved to the inside of the membrane to create ATP

how does photosystem 1 work

1. electron moves to from photosystem 2 to plastocyanin in photosystem 1

2. light energy bounces until it hits special chlorophyll molecules

3. electron is taken from plastocyanin and moved to electron accepters

4. electron loss energy and creates NADPH in process

what does the photosystem process create

ATP and NADPH: high energy molecules

what is energy used for

used in calvin cycle to produce sugars

what occurs during Chemiosmotic coupling of photophosphorylation

occurs inside stroma membrane

1. light energy is captured by photosystem 2

2. water in lumen splits to make hydrogen ion and oxygen

3. electrons are moved around to take its hydrogen ions and release it outside of membrane to increase hydrogen gradient

4. electron moves → cytochrome b6F compress → plastocyanin → photosystem 1 with light energy present

5. electrons are moved to other side. hydrogen is removed and bound to NADP+ to increase proton gradient

6. hydrogen moved from one side of membrane to the other

7. ATP synthase complex produces ATP for creating sugars

what does cyclic electron flow produce

only ATP. Can occur in photosystem 1 without photosystem 2

why does cyclic electron flow produce ATP

production of sugars in carbon fixation process uses more ATP than NADH, which causes additional requirement of ATP

what is the cyclic electron flow process

1. light energy capture electrons from plastocyanin and moves it up in energy

2. energy activates cytochrome complex → pumps protons from stroma to the lumen

3. proton gradient increases ATP

4. extra ATP is used in calvin cycle