Biochemistry of Photosynthesis

An introduction…

Photosynthesis – What can you remember?

• What is the word equation for photosynthesis?

• What is the balanced symbol equation for photosynthesis?

• Where do all of the raw materials come from and what happens to the products

Photosynthesis

Energy Transfers & Photosynthesis __________ energy (from sunlight)

__________ energy (glucose)

respiration

_________ energy in glucose is transferred to another molecule ________

Light

Chemical potential

Chemical potential

ATP

What provides the energy within cells?

• ATP…Adenosine Tri Phosphate• Common to ALL living things

•Chemical energy is stored in the phosphate bonds

3 phosphates

Ribose sugar

Adenine base

What Does ATP Do for You?What Does ATP Do for You?

It supplies YOU withIt supplies YOU with ENERGY!ENERGY!

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How Do We Get Energy From How Do We Get Energy From ATP?ATP?

Hydrolysis Hydrolysis reaction - reaction - breaking the breaking the high- energy high- energy bonds between bonds between the the last two last two phosphates in phosphates in ATPATP

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How does ATP provide the energy?

• Chemical energy is stored in the phosphate bonds, particularly the last one

• To release the energy, a HYDROLYSIS reaction takes place to break the bond between the last two phosphate molecules

• Catalysed by ATP-ase• ATP is broken down into ADP and Pi

• For each mole of ATP hydrolysed, about 34kJ of energy is released

• Some is lost, but the rest is useful and is used in cell reactions

Synthesising ATP

• ADP + Pi + energy ATP

ATP is created via respiration in both animals and plants

Where does the energy to synthesise ATP come from?

• Catabolic (breakdown) reactions• Redox (reduction/oxidation) reactions

• Reduction • The main way in which ATP is synthesised is by the removal of

hydrogen atoms from intermediate compounds in a metabolic pathway

• We say the hydrogen carrier is reduced• Electrons from the hydrogen atoms are passed along carriers

(Electron Transfer Chain)• When a component of the chain receives one of the hydrogen

atoms, we say it is REDUCED• When a component passes an electron on, we say it is OXIDISED• Each of these redox reactions releases a small amount of energy

and this energy is used to synthesise ATP

Phosphorylation

What does this have to do with photosynthesis?

• ATP is both synthesised and broken down during photosynthesis!

6CO2 + 6H2O = C6H12O6 + 6O2• Light energy is required• Chlorophyll• Stored within chloroplasts• 10-50 chloroplasts per plant cell

Why is Photosynthesis important?Only by photosynthesis that light energy is converted into chemical potential energy and simple in organic molecules eg CO2 and H2O can be built up into organic ones.

Ultimately all animals rely on photosynthesis

Where in plant cells does photosynthesis happen?

Structure of the chloroplast

Label you diagram to show the structure of the chloroplast:

• Granum• Outer membrane• Inner membrane• Stroma • Thylakoids

Where in the plant cell does photosynthesis take place?

The membranes from the grana provide a large surface area for chlorophyll molecules and other light absorbing pigments

The pigments form clusters called photosystems.

Photosystems act as light-collecting systems. Light is captured and passed from one pigment molecule to another before it finally reaches a chlorophyll molecule

Photosynthesis involves lots of separate reactions

2 basic steps:

•Light-dependant reactions

•Light independent reactions

Structure of the chloroplast

Summarise the structure of the chloroplast as discussed last lesson.

Try and include key words that we used last lesson

Structure of the chloroplast

The light dependant reaction

• Learning objectives:

• Explain the role of photosystems in the light dependant reactions

• Describe the light-dependant reactions to explain how sunlight is used to synthesise ATP and reduced NADP

Photosystems• Pigments arranged in

funnel shaped photosystems that sit in the thylakoid membrane

• Each photosystem = hundreds of pigment molecules

• Various pigments absorb light of different wavelengths and pass the energy down the photosystem.

Photosystems

• 2 types:– Photosystem 1 – Photosystem 2

Each photosystem is sensitive to light if different wavelengths

Step 1 – Photolysis

Water (H2O)

2H+ + 2e- + 1/2O2

The electrons pass to photosystem 2

Water is broken down to protons, electrons and oxygen

2e-PS 2

Step 2 – Light energy excites electrons in chlorophyll

Light strikes chlorophyll molecule in photosystem II

Energy levels of 2 electrons are raised. The electrons leave the chlorophyll molecule

2e-PS 2

Light energy 2e-

Step 2– Energy from these excited electrons generate ATP

Electrons are passed to electron carrier Electrons passed along

series of electron acceptors which form an electron transfer chain in chloroplast membrane

2e-PS 2

Light energy

2e-

Electron Carrier

Step 2– Energy from these excited electrons generate ATP

2e-PS 2

Light energy

2e-

Electron Carrier ADP+ Pi

ATP

e- loose energy as they pass along. This energy is used to generate ATP

Step 3– Light also strikes PS1

2e-PS 2

Light energy

2e-

Electron Carrier ADP+ Pi

ATP

2e-PS 1

Light energy

2e-

Light strikes chlorophyll molecule in photosystem I

Electron Carrier

Step 3– Light also strikes PS1

2e-PS 2

Light energy

2e-

Electron Carrier ADP+ Pi

ATP

2e-PS 1

Light energy

2e-

Electron Carrier

Electrons pass down another series of electron carriers.

Step 3– Energy from these excited electrons generate NADP

2e-PS 2

Light energy

2e-

Electron Carrier ADP+ Pi

ATP

2e-PS 1

Light energy

2e-

Electron Carrier

NADP + 2H+ 2e-

The e- are used with the H+ (produced from photolysis of H2O) to produce reduced NADPH

NADP

• NADP Nicotinamide Adenine dinucleotide phosphate

• Coenzyme found in plant cells

• Aids electron transfer during photosynthesis

The light dependant reaction

• In the light dependant reactions of photosynthesis what happens to the electrons that come from:

a. A water moleculeb. A chlorophyll molecule in photosystem Ic. A chlorophyll molecule in photosystem II

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The Light Dependant reaction

1. Where in the chloroplast do these take place?

2. What are the 2 main products of the light dependant reactions?

3. What waste product is made during the light dependant reactions?

Light-independent reactions –The Calvin Cycle

• Take place in the liquid stroma

• Makes hexose sugars eg glucose from carbon dioxide and RuBP (ribulose bisphosphate)

Light-independent reactions –The Calvin Cycle

• Needs energy and H+

• Provided by products of light dependant reaction – ATP and reduced NADPH

The Calvin Cycle

CO2RuBisCO

1C

6C

Ribulose bisphosphate

5C

CO2 enters the leaf through the stomata & diffuses into the stroma of the chloroplast

CO2 reacts with ribulose bisphosphate (5-C) The reaction is catalysed by the enzyme RuBisCO

This produces an unstable 6-C compound which quickly breaks down

The Calvin Cycle

CO2RuBisCO

1C

6C

Ribulose bisphosphate

5C

2x Glycerate 3-phosphate

3C 3C

2 molecules of Glycerate 3 –phosphate are formed

Energy from ATP and hydrogen from reduced NADP is used to convert each glycerate 3-phosphate into triose phosphate

ATP

ADP + Pi

Reduced NADPH

NADP

3C 3C

Triose

Phosphate

The Calvin Cycle

CO2RuBisCO

1C

6C

Ribulose bisphosphate

5C

2x Glycerate 3-phosphate

3C 3C

2 molecules of Glycerate 3 –phosphate are formed

Energy from ATP and hydrogen from reduced NADP is used to convert each glycerate 3-phosphate into triose phosphate

ATP

ADP + Pi

Reduced NADP

NADP

3C 3C

Triose

Phosphate

The Calvin Cycle

CO2RuBisCO

1C

6C

Ribulose bisphosphate

5C

2x Glycerate 3-phosphate

3C 3C

Most of the triose phosphate is used to make more ribulose bisphospate

ATP

ADP + Pi

Reduced NADP

NADP

3C 3C

Triose

Phosphate

ATP

ADP + Pi

The Calvin Cycle Some is used to form carbohydrates

CO2RuBisCO

1C

6C

Ribulose bisphosphate

5C

2x Glycerate 3-phosphate

3C 3C

ATP

ADP + Pi

Reduced NADP

NADP

3C 3C

Triose

Phosphate

1C

The Calvin cycle needs to turn 6 times to make 1 glucose molecule

6CO2 (1C)

6RuBP (5C)

12 glycerate 3-phosphate (3C)

12 triose phosphate (3C)

10 triose phosphate (3C)

6 RuBP(5C)

2 triose phosphates (3C)

1 glucose (6C)

If there was a build up of RuBP – what does this suggest?

If there was a build up of GP what does this suggest?