Photosynthesis

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PHOTOSYNTHESIS

Photosynthesis is the process by which plants and some other organisms convert light energy into chemical energy. The chemical equation for photosynthesis is:

6CO2 + 6H2O + energy from photons--*chlorophyll*----> C6H12O6 + 6O2

Photosynthesis is a 2 stage process which takes place in chloroplasts.

Chloroplasts

Surrounded by an envelope

Outer membrane is permeable to small ions

Inner membrane is less permeable

Folded into lamellae

One granum consists of several stacked thylakoids

Between grana is the intergranal lamellae

The stroma is the fluid filled matrix, which contains the necessary enzyme to carry out the light dependent reactions

Chloroplast Adaptations

Many grana provide a large surface area for photosynthetic pigments

Photosynthetic pigments are arranged into photosystems

Proteins embedded in grana to hold the photosystems in place

Light Dependent Stage

Photophosphorylation

When a photon hits a chlorophyll molecule, the energy is transferred to 2 electrons exciting them

“Photoionisation”

Electrons captured by electron carriers and passed along a series of electron carriers

Energy is used to pump protons across the thylakoid membranes

Proton gradient is formed and protons flow down the concentration gradients through ATP channels

This is chemiosmosis

This produces a force which joins ADP and Pi to form ATP

ATP is used in the light independent stage of the reaction

The making of ATP using light energy is called photophosphorylation

Cyclic Photophosphorylation

Only uses PS1

Electrons pass to an electron acceptor, then return to the chlorophyll molecule from which they were lost

No photolysis of water

No generation of reduced NADP

Small amounts of ATP made

This may be used in the light independent reaction to actively transport potassium into the cell, lowering the water potential and causing ions to flow in by diffusion

This causes the guard cells to swell and the stomata to open

Non-cyclic Photophosphorylation

Involves both PS1 and PS2

Light strikes PS2, exciting a pair of electrons

Electrons leave the chlorophyll molecule from the primary pigment reaction centre

Electrons pass along the chain of electron carriers

Energy is released, which is used to synthesize ATP

Light strikes tPS1, causing the release of 2 electrons

Electrons from oxidized PS2 replace those lost from PS1

Electrons from photolysed water replace those lost from PS2

Protons fro photolysed water take part in chemiosmosis, to make ATP

They are captured in NADP in the stoma and used in the light-independent stage

Non light dependent stage

This takes place in the stroma

It is called the Calvin Cycle

Although light is not used, the products of the light-dependent stage are used, so the light independent stage will soon cease if light supply ceases

The Calvin Cycle

Carbon dioxide from the air diffuses into the leaf

Diffuses through air spaces and reaches the spongy palisade layer

Diffuses through the cellulose cell walls and into the chloroplast envelope

In the stroma, carbon dioxide combines with the 5C ribulose bisphosphate

Reaction is catalysed by the rubisco enzyme

Two 3C compounds, called glycerate phosphate (GP) are produced

Some GP is used to make amino acids and fatty acids

GP is reduced and photophosphorylated to another 3C compound, triose phosphate (TP)

ATP and reduced NADP are used in this reaction

ATP and redNADP are also used in converting GP into glyceraldehyde phosphate (GALP)

5/6 TPs are recycled to ribulose bisphosphate

Pairs of TP combine to form hexose sugars

Conversion of Energy

Light converted through cyclic phosphorylation and non-cyclic phosphorylation

Both of which are light –dependent

In Cyclic phosphorylation:

Light energy absorbed by PS1

Energy excites electrons, raising their energy level

This causes electrons to be transferred to carriers

As electrons drop down energy levels, they release energy

Energy is used to pump H+ ions through ATP synthase from thylakoid space to the stroma

This is the process of chemiosmosis

In this process, ATP is produced

This process is used to make glucose

In non-cyclic phosphorylation:

Light energy causes hydrolysis of water

This is photolysis

2 electrons are produced, which pass into the PS2

They are excited and transferred to the electron carriers

As they pass along the carriers, they release energy,

This is used to pump H+ ions into the stroma at the same time

Role of Water in the Light Dependent Stage

Photosystem 2 contain an enzyme that can split water into protons, electrons and oxygen

This is photolysis

Some of the oxygen is used in aerobic respiration

Water is a source of hydrogen ions, which are used in chemiosmosis

Protons are accepted by a coenzyme NADP, which is reduced to reduced NADP and used in the light independent stage

Role of Carbon Dioxide in Light Independent Stage

Carbon dioxide is the source of carbon and oxygen for the production of all large organic molecules

These molecules are used as energy stores for all life forms

​EXAM TIP

Yes - it’s a lot - but you need to know the specifics listed above about the process of photosynthesis in order to reach top marks

Factors Affecting the Rate of Photosynthesis

The rate of photosynthesis is highly sensitive to 3 main factors:

Light Intensity

If light intensity was dramatically reduced:

Levels of ATP and NADP would fall, as the light dependent reaction cannot occur

Conversion of GP to TP requires ATP and reduced NADP, so this process will slow down

As ATP is used to convert TP to ribulose bisphosphate, levels of this will also fall

Reduction in ribulose bisphosphate means less carbon dioxide can react

Eventually the whole cycle stops

Carbon Dioxide

If carbon dioxide concentration was dramatically increased:

More carbon dioxide fixation allows for more GP, TP and ribulose bisphosphate to be created

Stomata will be open more, causing an increased loss of water and wilting

Stress response will close the stomata

This will reduced carbon dioxide supply

Photosynthesis rate will slow

Temperature

If temperature were raised:

Below 25 degrees activity in light dependent stages is minimally affected

Activity of molecules in the Calvin cycle are dramatically affected as they gain more kinetic energy

Higher temperatures will increase transpiration and cause the plant to wilt

Stress reaction will cause the stomata to close, reducing carbon dioxide availability

High temperatures may denature rubisco

At higher temperatures, rubisco is more likely to bind to oxygen than carbon dioxide

Photosynthesis rate will start to decrease