8.3 Photosynthesis

Understanding:- Light-dependent reactions take place in the

intermembrane space of the thylakoids- Reduced NADP and ATP are produced in the

light-dependent reactions- Light-independent reactions take place in

the stroma- Absorption of light by photosystems

generates excited electrons- Photolysis of water generates electrons for

use in the light-dependent reactions- Transfer of excited electrons occurs

between carriers in thylakoid membranes- Excited electrons from Photosystem II are

used to generate a proton gradient- In the light-independent reactions a

carboxylation of ribulose bisphosphate- Glycerate 3-phoshpate is reduced to triose

phosphate using reduced NADP and ATP- Triose phosphate is used to regenerate

RuBP and produce carbohydrates

Applications:- Calvin’s experiment to elucidate the

carboxylation of RuBP

Skills:- Annotation of a diagram to indicate

the adaptations of a chloroplast to its function

Nature of science:- Developments in scientific research follow improvemnts in apparatus: sources of 14C and autoradiography enabled Calvin to elucidate the pathways of carbon fixation

Understanding:- Ribulose bisphosphate is reformed

using ATP- The structure of the chloroplast is

adapted to its function in photosynthesis

Chloroplast structure

Chloroplasts are adapted to their function

Draw a chloroplast and label the following onto it:- Thylakoid- Granum- Inner membrane- Outer membrane- Stroma

Chloroplast structure

Thylakoids – flattened membrane sacs with components for the absorption of light (first step of photosynthesis)

Grana – Stacked up thylakoids = more photosynthesis. Contains chlorophyll.

Stroma – fluid outside of the grana. Contains many enzymes and chemicals for photosynthesis.

Chloroplast structure

Chloroplast structure

Chloroplast structure Function Allowed

Extensive membrane surface area of the thylakoids

Small space (lumen) within the thylakoidsStroma region similar to the cytosol of the cell

Double membrane on the outside

Chloroplast structure

Chloroplast structure Function Allowed

Extensive membrane surface area of the thylakoids

Greater absorption of light by photosystems

Small space (lumen) within the thylakoids

Faster accumulation of protons to create a concentration gradient

Stroma region similar to the cytosol of the cell

Area for the enzymes necessary for the Calvin cycle to work

Double membrane on the outside

Isolates the working parts and enzymes of the chloroplast from the surrounding cytosol

Photosynthesis

Photosynthesis vs respiration

Essentially the opposite

Respiration = catabolism (breaks down moleculesinto smaller units to release energy)

Photosynthesis = anabolism (construct large molecules from smaller units using energy)

Photosynthesis

Light dependent reactions

Light independent reactions

Light dependent reactions

- Thylakoid membrane- Uses light directly - Use pigments to absorb the light (chlorophyll/carotenoids…)- Pigments organised into photosystems in thylakoids

Photosystems include:1. Chlorophyll a molecules2. Accessory pigments3. Reaction centre

A reaction centre in these photosystems contains:4. Pair of chlorophyll a molecules5. A Primary electron acceptor

Light dependent reactions

Modern day plants have 2 photosystems

Each absorbs light efficiently at different wavelengths

Photosystem I: 700nm (P700)Photosystem II: 680nm (P680)

Work together to to transfer electrons to create energy

Photosystem II

1. A photon of light is absorbed by a pigment in Photosystem II and is transferred to other pigment molecules until it reaches chlorophyll a. (Photoactivation)

2. Water split by enzymes to produce e-, H+ and oxygen – driven by light energy (photolysis)

3. Electrons supplied one by one to chlorophyll a

4. The photon energy excites a chlorophyll a electron to a higher energy state

5. Electron captured by primary acceptor of the reaction centre

Electron transport chain

6. Excited electrons pass from primary acceptor down electron transport chain, losing energy at each step- Involves Plastoquinone (electron carrier) and plastocyanin

(electron acceptor)- Involves cytochrome complex where the e- transport chain occurs)

7. Protons pumped into thylakoid space against concentration gradient

This stored energy drives chemiosmosis – phosphorylation of ADP to ATP using ATP synthase

(ATP is the first product)

Photosystem I

8. Another photon of light is absorbed by a pigment in Photosystem I.

9. Energy is transferred until it reaches chlorophyll a

10. Electron is energised and transferred to primary electron acceptor

11. De-energised electron from Photosystem II fills the void left by the newly energised electron

Electron transport chain

12. High energy electron passed down a second electron transport chain that involves ferredoxin (electron carrier)

13. Enzyme NADP reductase catalyses the transfer of electron form ferredoxin to energy carrier NADP+

14. 2 electrons are needed to fully reduce NADP+ to NADPH

(NADPH is the second product)

Light dependent reactions

NADPH and ATP are the final products

Supply energy for the light independent reactions to occur

Also where oxygen is released (water splitting in the first few steps)

ATP

ATP production in photosynthesis and respiration are similar

Both use chemiosmosis to phosphorylate ADP to ATP (ATP synthase enzyme)

When it is produced using light = photophosphorylation

ChemiosmosisRespiration chemiosmosis Photosynthesis chemiosmosis

Electron transport chain in cristae membranes

Energy released when electrons are exchanged from one carrier to another

Energy used to pump H+ into intermembrane space

H+ ions come from matrix

H+ ions diffuse back into matrix through channels of ATP synthase

ATP synthase catalyses the phosphorylation of ADP to ATP

Electron transport chain in thylakoid membranes

Energy released when electrons are exchanged from one carrier to another

Energy used to pump H+ into thylakoid space

H+ ions come from the stroma

H+ ions diffuse back into stroma through channels of ATP synthase

ATP synthase catalyses the photophosphorylation of ADP to ATP

- In the stroma (contains enzymes)

- Uses ATP and NADPH from light dependent reaction

- Involves the Calvin cycle (begins and ends with the same substance)

Light Independent Reactions

1. 5-carbon compound Ribulose bisphosphate (RuBP) binds to an incoming carbon dioxide molecule

2. Catalysed by enzyme RuBP carboxylase (rubisco) – results in an unstable 6-carbon compound

3. Breaks down into two 3-carbon compounds called glycerate 3-phosphate (GP)

Carbon Fixation

1. GP uses ATP and NADPH to form triose phosphate (TP) – REDUCTION

2. TP can go in 2 directions

Reduction

1. Some leave cycle to become sugar phosphates that become more complex carbohydrates (STARCH)

2. Most continue in the cycle to reproduce the originating compound of RuBP (uses ATP) REGENERATION

As much RuBP must be produced as is consumed.

Example:3 RuBP used = 6 TP molecules produced5 used to regenerate RuBP – 1 used to produce carbohydrates

Regeneration

To form one 6 carbon sugar molecule:

12 TP molecules6 RuBP molecules12 NADPH18 ATP

Annotate onto your diagrams

Calvin and his team worked out carbon fixation in plants.

Research the lollipop experiment- What is it?- What is the process?- What did he find out?

Calvin’s experiment

Involves light dependent and light independent reactions

Light dependent reactions produce ATP and NADPH for the light independent reactions

Light is needed for the light independent reactions to occur – but not directly.

Photosynthesis summary

Photosynthesis summaryLight dependent Light independent

Occurs in thylakoids

Uses light energy to form ATP and NADPH

Splits water in photolysis to proved replacement e- and H+ and to release oxygen to atmosphere

Includes 2 electron transport chains and photosystems I and II

Occurs in the stroma

Uses ATP and NADPH to form triose phosphate

Returns ADP, inorganic phosphate and NADP to the light dependent reactions

Involves the Calvin cycle