Photosynthesis

Mrs Martin

Plants and animals are dependent upon the products of photosynthesis.

They use the products of photosynthesis to for respiration

6CO2 + 6H2O (+ light energy) - C6H12O6 + 6O2

C6H12O6 + 6O2 6CO2 + 6H2O

Photoautotrophs and heterotrophs can release the chemical potential energy in complex organic molecules during photosynthesis. They can also use the waste product Oxygen produced by photosynthesis for aerobic respiration.

Autotroph Organisms that can produce their own complex

organic compounds, using light energy or chemical energy, from inorganic sources.

Chemoautotroph Prokaryotes that use the energy from exergonic

reactions (chemical reactions that release heat) to synthesise complex organic compounds

Nitrifying bacteria that oxidise ammonia to nitrate or oxidising nitrite to nitrate.

Bacteria using sulphur rich substances bubbling from the vents of volcanoes. These bacteria help to support food chains living in those areas.

Photoautotroph Use the sun as their energy source and the

inorganic molecules carbon dioxide and water. Most of the world’s food chains have photoautotrophs as their producers

Heterotrophs These organisms cannot make their own food and

are reliant on digesting other organisms to obtain their complex organic molecules into more simple soluble molecules. These simple molecules can then be rebuilt to produce proteins, lipids and nucleic acids.

Structure of the chloroplast

Chloroplasts- main regions

Stroma Fluid filled matrix contains starch grains, oil droplets,

DNA and and prokaryote-type ribosomes Chloroplast DNA and ribosomes can make some proteins

needed for photosynthesis

Site of light independent reactions Enzymes needed for these reactions are found here Surrounds Grana so products of light dependent

reactions can easily pass into the stroma

Chloroplasts- main regions

Grana Stacks of flattened membrane compartments –

thylakoids Up to 100 stacks Provides large surface area for photosynthetic pigments,

electron carriers and ATP synthase (required for light dependent reactions)

Can only be seen using an electron microscope Site of light dependent reactions

Light is absorbed and ATP synthesised

Contain Photosynthetic pigments arranged into photosystems

Photosystem

Photosystem, a funnel shaped light harvesting cluster of photosynthetic pigments held in place by proteins in the thylakoid membrane.

The primary pigment reaction centre is a molecule of Chlorophyll a. The accessory pigments consist of molecules of chlorophyll b and carotenoids.

Structure of chlorophyll a

Essential molecule in the process of photosynthesis.

Gives plants their intense green colour

When vegetation is cooked the central magnesium is replaced with a hydrogen ion. This changes the colour of the leaves

Chlorophyll a Active part of the chlorophyll molecule. Central Mg bonded to 4 N atoms. Changes in the electrons in this part of the molecule enable it to absorb light very strongly

Hydrocarbon tail is non-polar. It is hydrophilic so sticks into the thylakoid membrane

Photosynthetic pigmentsSubstances that absorb some

wavelengths of light and reflect others.The colour they appear is the colour they

are reflecting

This light is reflected

PigmentsTwo forms of chlorophyll a

P680 absorbs at 680nm- found in photosystem II

P700 absorbs at 700nm- found in photosystem I

Both also absorb blue light at around 450nm

Chlorophyll b absorbs around 500nm & 640nm

Carotenoids absorb blue light and reflect orange/yellow light

Knowledge check

1. What is a chemoautotroph? Can you name an example2. What type of trophic organism are the majority of food chains

reliant upon?3. Name the membrane that makes up the grana4. Suggest what this membrane is composed of5. Suggest why scientists have proposed the idea of

endosymbiosis6. State where the light independent reactions of photosynthesis

take place7. Explain why the majority of plants are green8. Suggest why in the autumn leaves may turn red9. Explain how a chlorophyll molecule is adapted for its function10. Describe how a photosystem is able to maximise the amount

of energy it can obtain from light

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(a) The distribution of photosystems on granal and intergranal lamellae; (b) Formation of ATP during non-cyclic photophosphorylation

(b)

© Pearson Education Ltd 2009This document may have been altered from the original

The Z-scheme

Light dependent reactions- non-cyclic photophosphorylation

Light energy strikes photosystem II and cascades through to the primary pigment reaction centre

If enough energy strikes the primary pigment molecule the energy levels of a pair of electrons are raised. The electrons leave the chlorophyll molecule to an electron transport chain

As the electrons go down the electron transport chain they lose energy which is used to convert ADP + Pi ATP

New electrons are needed to replace the electrons lost from photosystem II

Photolysis of water

HH220 0 2H 2H++ + 2e + 2e- - + ½ O + ½ O22

enzyme

ProtonsUsed for NADP Electrons used as a

Replacement for PS II

Waste product

Light energy also hits photosystem I. If enough light hits the photosystem II electrons are lost from the primary pigment centre.

The electrons are replaced by the electrons from photosystem II

The electrons again leave to the electron transport chain

The electrons lose energy as they go down the chain. This energy is used to make reduced NADP (rNADP)

OILRIGOXIDATION Is Loss of electrons REDUCTION Is Gain of electrons

OILRIGOXIDATION Is Loss of electrons REDUCTION Is Gain of electrons

NADP + 2H+ + 2e- Reduced NADP

Protons from photolysisof water

From Photosystem II

Energy source

OILRIGOXIDATION Is Loss of electrons REDUCTION Is Gain of electrons

REMEMBER!!!

Cyclic Phosphorylation

Only photosystem I is used Excited electrons are passed to an electron

receptor and back to the chlorophyll molecule from which they were lost No photolysis of water Small amounts of ATP are produced which maybe

used in photosynthesis or by guard cells to bring in K ions lowering water potential and causing water to enter by osmosis. This causes the guard cells to swell and open, allowing exchange of gas.

© Pearson Education Ltd 2009This document may have been altered from the original

The Z-scheme

Light dependent photosynthesis

© Pearson Education Ltd 2009This document may have been altered from the original

(a) The distribution of photosystems on granal and intergranal lamellae; (b) Formation of ATP during non-cyclic photophosphorylation

(b)

© Pearson Education Ltd 2009This document may have been altered from the original

Week 10

• Outline how the products of the light-dependent stage are used in the light-independent stage (Calvin cycle) to produce triose phosphate (TP), referring also to ribulose bisphosphate (RuBP), ribulose bisphosphate carboxylase (rubisco) and glycerate 3-phosphate (GP).

• Explain the role of carbon dioxide in the light-independent stage.

• State that TP (and GP) can be used to make carbohydrates, lipids and amino acids.

• State that most TP is recycled to RuBP.

Light independent photosynthesis

The reactions are not truly independent of light as the ATP and rNADP could not be produced without light.

The light independent reactions occur within the stroma.

Carbon Dioxide is needed to produce all large organic molecules. These molecules are used as structures or act as energy stores or sources for all the carbon based life forms on this planet

Calvin Cycle

Inorganic formOf Carbon3CO2

5C acceptorRibulose bisphosphate(RuBP) 5C Sugar

Rubisco

2x Glycerate 3 phosphate GP(2x 3C, not sugar)

This reaction is catalysed by Ribulose Bisphosphate Carboxylase Oxygenase Rubisco. Described as the most important enzyme on earth.

Read Stretch and challenge on page 65!

Now has carboxy group

Calvin Cycle

GP Reduced and phosphorylated

ATPADP + Pi

rNADP

NADP

2x triose phosphate (3C)

Ose= sugar

5 of every 6 molecules of TP are recycled by phosphorylation (using ATP from light dependent reaction, to 3 molecules of RuBP (5C)

© Pearson Education Ltd 2009This document may have been altered from the original

Calvin cycle

Limiting Factors

Lesson ObjectiveTo discuss the factors that limit

photosynthesisDesign an experiment to test one factor that

affects photosynthesis

Limiting Factors

Law of limiting Factors statesAt any given moment, the rate of

metabolic process is limited by the factor that is present at its least favourable (lowest) value

Limiting factors

At any given moment, the rate of metabolic process is limited by the factor that is present at its least favourable (lowest) value

Light Intensity

When light is the limiting factor the rate of photosynthesis is directly proportional to the light intensity.

What does this mean?

Light causes the stomata to open so that Carbon dioxide can enter the leaves

Light is trapped by chlorophyll where it excites electrons

It splits water molecules to produce protons

Electrons and protons are involved are involved in photophosphorylation producing ATP for the fixation of carbon dioxide

Questions

1. Which stage of photosynthesis will affected by temperature? And why?

2. Why does light intensity affect the rate of photosynthesis?

3. Why would burning an oil-fired stove in a greenhouse increase the growth of plants in a greenhouse?

Questions

1. Which stage of photosynthesis will affected by temperature? And why?

The light independent Calvin cycle. As many of the reactions are catalysed by

enzymes, the rate rises between 0-25oC. At the rate plateaus and fall as enzymes work less efficiently and as oxygen competes more successfully than carbon dioxide for the active site of rubisco

Questions

1. Which stage of photosynthesis will affected by temperature? And why?

2. Why does light intensity affect the rate of photosynthesis?

Light is required for the light dependent phase, as photons hit the photosystem electrons are excited which then leave the photosystem via electron carriers and passed along a series of protein electron carriers which eventually produce ATP and rNADP. A reduced light intensity will reduce these levels. G3P will accumulate as it is not reduced to triose phosphate (calvin cycle). Ribulose bisphosphate levels will fall as it is not being regenerated

Questions

1. Which stage of photosynthesis will affected by temperature? And why?

2. Why does light intensity affect the rate of photosynthesis?

3. Why would burning an oil-fired stove in a greenhouse increase the growth of plants in a greenhouse

Burning hydrocarbons (oil, coal, natural gas) will produce Carbon Dioxide

Burning fuel will also increase the temperature of the greenhouse

Use your knowledge to explain the graph

Use your knowledge to explain the graph