Biochemistry of Photosynthesis

An introduction…

Why is energy needed within cells?

• Allows chemical reactions to take place

• BUILD UP (synthesis) or BREAKDOWN of molecules

• In order to do this, energy is required to make and break bonds

Where does the energy come from?

• The SUN is the ultimate source of energy for nearly all living organisms (the exceptions being a few deep sea chemosynthetic bacteria)

• Autotrophs make their own food (organic compounds) using carbon dioxide

• Heterotrophs assimilate energy by consuming plants or other animals

http://www.youtube.com/watch?v=O7eQKSf0LmY

What provides the energy within cells?

• ATP…Adenosine Tri Phosphate• Common to ALL living things• Any chemical that interferes with the

production or breakdown of ATP is fatal to the cell and therefore the organism

•Chemical energy is stored in the phosphate bonds

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

Where does the energy to synthesise ATP come from?

• Catabolic (breakdown) reactions• Redox (reduction/oxidation) reactions• The main way in which ATP is synthesised is by the removal

of hydrogen atoms from intermediate compounds in a metabolic pathway

• When two hydrogen atoms are removed from a compound, they are picked up by a HYDROGEN CARRIER or ACCEPTOR

• 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

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• In order to understand the complex processes of photosynthesis and respiration, it is essential to understand reduction reactions and oxidation reactions.

• Oxidation: A chemical reaction involving the loss of electrons. It can also involve the loss of hydrogen or the gain of oxygen. Most oxidation reactions are exothermic- energy is released.

• Reduction: A chemical reaction involving the gain of electrons. It may also involve the gain of hydrogen or the loss of oxygen. Most reduction reactions are endothermic- they require an input of energy.

What does this have to do with photosynthesis?

• Photosynthesis makes use of ATP.• 2 stages:

– Light dependent stage (thylakoids)– Light independent stage (stroma)

Photosynthesis both produces and uses ATP!

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Adenosine triphosphate

Adenosine triphosphate, or ATP, is an important molecule found in all living cells. It readily diffuses around the cell and provides energy for cellular processes.

ATP releases energy in the light-independent reaction when a bond between inorganic phosphate groups is broken, producing ADP and an inorganic phosphate group.

adenine

ribose

3 phosphate groups

ATP is made in the light-dependent reaction in photosynthesis from adenosine diphosphate (ADP) and an inorganic phosphate group (Pi). This requires energy.

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Adding a phosphate molecule is phosphorylation. In this case, light is used as an energy source so the process is called photophosphorylation.

In the light-dependent reaction:

+ + energy

In the light-independent reaction:

The use of water makes this a hydrolysis reaction.

Photophosphorylation and hydrolysis

ADP ATP

+ energy+H2O ADPATP

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Leaves and photosynthesis

Chloroplasts and Photosynthesis

• Light energy is required for the light dependent reactions

• Chlorophyll absorbs light• Chlorophyll is stored within chloroplasts• 10-50 chloroplasts per plant cell

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Label both diagrams in as much detail as possible

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Structure of the chloroplast

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Chloroplast Outer membrane

Inner membrane

Thylakoid (containing chlorophyll)- Where light dependent stage of photosynthesis takes place

Granum (singular)Grana (plural)

Stroma- Where light independent stage of photosynthesis takes place

Intergranal lamella

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Thylakoid (containing chlorophyll)

Granum (singular)Grana (plural)

Intergranal lamella

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Plant photosynthetic tissues

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Functions of photosynthetic structures

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In plants there are two main types of pigments: chlorophylls and carotenoids. They are coloured because they absorb particular wavelengths of light and reflect others.

A photosynthetic pigment is a coloured biological compound that is present in chloroplasts and photosynthetic bacteria, and which captures light energy for photosynthesis.

Chlorophyll is the pigment that gives plants their green colour by reflecting green light. Carotenoids reflect red, orange or yellow light.

Photosynthetic pigments

Chlorophyll

• Found within chloroplasts• Absorb and capture light• Made up of a group of five pigments • Chlorophyll a• Chlorophyll b• Carotenoids; xanthophyll and carotene• Phaetophytin• Chlorophyll a is the most abundant• Proportions of other pigments accounts for

varying shades of green found between species of plants

Other notes…

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NADP and NADPH

Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme involved in the photosynthesis reactions.

The compound is a dinucleotide, containing an adenine base and a nicotinamide base. The nucleotides are joined through their phosphate groups. In addition there is an extra phosphate on the ribose of the adenine-containing nucleotide.

NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons.

nicotinamide

adenine

ribose

Photosystem I and Photosystem II

• These are distinct chlorophyll complexes

• Each contains a different combination of chlorophyll pigments

• PSI absorbs light at 700nm and PSII at 680nm

• PSI particles are found on the intergranal lamellae

• PSII particles are found on the grana