PhotosynthesisUnit 10 BTEC triple

Applied science

Photosynthesis is the process that uses light energy to synthesise organic molecules such as glucose from inorganic carbon dioxide. This occurs in a series of chemical steps arranged in two stages.

Learning aim C: Explore the factors that can affect the pathways and the rate of photosynthesis in plantsC1 Pathways in photosynthesis

• Light-dependent reaction:

o stages in and location of photophosphorylation, including role of coenzymes, and photolysis

o light energy converted to chemical energy held in ATP.

• Light-independent reaction:

o stages in and location of the Calvin cycle

o role of ribulose bisphosphate (RuBP) and ribulose bisphosphate carboxylase (RuBisCO)

o production of glucose.

C2 Factors that can affect the pathways in photosynthesis

• Requirements for photosynthetic organisms, including sources and control of limiting factors,

e.g.

light intensity,

CO2 concentration,

temperature,

water.

• Role of photosynthetic pigments (chlorophylls and carotenoids) in absorbing different

wavelengths of light.

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Chloroplasts are found within plant cells and are where photosynthesis takes place. Most chloroplasts are found in the palisade mesophyll tissue of the leaf, this is the main site of photosynthesis. They can move around within each palisade cell to be nearer the light source. There are also chloroplasts in the spongy mesophyll and the guard cells of the lower epidermis.

Plants use light energy in photosynthesis to form organic molecules (glucose) from inorganic carbon dioxide and water. This takes place in the chloroplasts.

The role of chloroplasts is to convert the light energy into chemical energy (ATP)

Angiosperm adaptations

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Chloroplasts

Angiosperms (flowering plants) have several adaptations which increase efficiency of light absorption. These include:

Flat leaves – increases surface area for light absorption

Thin leaves - Allows light to penetrate to lower cell layers.

Transparent upper and lower cuticle – allows light to penetrate to the palisade layer

Palisade mesophyll cells are elongated and arranged at the top of the leaf - Maximise light absorption

Large number of chloroplasts that are able to move - chloroplasts can gain the best position which increases the photosynthetic capacity

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Palisade cells in a leaf

The job of palisade cells is to perform photosynthesis and provide sugars for respiration.

Photosynthesis needs water and carbon dioxide

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Chloroplast structure

Electron microscope pictures of a plant cell and a chloroplast

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The chloroplast:

Has a double membrane Is filled with the stroma Has thylakoids, which are like discs; stacked together are called the grana

o The spaces inside all the thylakoids are connected together to form the thylakoid space

o The thylakoid membranes contain the photosynthetic pigment chlorophyll Acts as a transducer – converting light energy into chemical energy, in the form of

ATP

Chloroplasts are the site of photosynthesis in plants. Photosynthesis involves two stages: the light dependent stage and the light independent stage (the Calvin Cycle).

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The light dependent stage converts light energy to chemical energy. The reactions include photolysis (splitting) of water into protons and high energy electrons. The protons are used to reduce the molecule NADP and the energy from the electrons is used to make ATP by photophosphorylation.

The reduced NADP and the ATP from the light dependent stage are used in the light independent to reduce carbon dioxide to produce energy-containing glucose.

Chloroplasts have compartments within them; each of which is where a stage of photosynthesis takes place.

Stroma – Light independent stage

Thylakoids – Light dependant stage

Thylakoid space – Photolysis of water

There are different types of photosynthetic pigments found in plants’ chloroplasts.

a) Chlorophyll a – the primary pigmentb) Chlorophyll bc) Carotenoids – ( beta carotene and Xanthophyll)

Chlorophyll a and b absorb light at the red and blue parts of the spectrum, whereas the carotenoids absorb the light energy from the blue-violet end of the spectrum. The carotenoids act as ‘accessory pigments’. Accessory pigments are important as they absorb wavelengths that are not absorbed by the primary pigments. This ensures that a large range of wavelengths are absorbed and increases the efficiency of photosynthesis.

CHROMATOGRAPHY

Can be used to separate the pigments. A means of separating one type of molecule from another Can identify pigments by their position on a chromatogram and

their Rf value can then be calculated

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Light Harvesting

These pigments are arranged in a cluster in the thylakoid membrane. Each cluster is called an Antenna complex. Their arrangement helps them to trap as much light as possible and funnel that light energy to one pigment, chlorophyll a, found in the reaction centre.

Light photons hit the pigments and the energy is transferred between the accessory pigments to the chlorophyll a in the reaction centre. This passes the excited electron to the primary acceptor molecule also within the reaction centre

Reaction centre + other pigments = photosystem

There are two photosystems which harvest light energy in chloroplasts: Photosystem I and II.

They are both found in the thylakoid membrane and are both a cluster of pigments.

Photosystem I

reaction centre is p700 Its chlorophyll a has a maximum absorption at 700nm (red light)

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Photosystem II

Reaction centre is P680 Its chlorophyll a has a maximum absorption at 680nm (orange-red light)

Chlorophyll and wavelength of light

Sunlight travels in waves. Wavelength is measured in nanometres (nm). The visible light spectrum is made up of many different wavelengths which combine to form white light. When each wavelength is seen on its own, we see a colour.

When light hits the leaf of a plant, only some of these wavelengths are absorbed. Some of the wavelengths are reflected (green) and some transmitted (go all the way through the leaf).

Grass is green the chlorophyll reflects green wavelengths of light and absorbs the other wavelengths

Absorption Spectrum

Remember: - each photosynthetic pigment absorbs a different wavelength of light

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Chlorophyll a absorbs red-orange and blue-violet wavelengths Chlorophyll b absorbs blue and orange wavelengths Carotenoids absorb blue-violet wavelengths

NB – none of the pigments absorb the green-yellow wavelengths, these are reflected

If a plant can absorb the light energy at a particular wavelength, then it can use it for photosynthesis.

This is shown in the action spectrum

There is a correlation between the two spectra, which is that, where most light is absorbed by the pigments, there is the most photosynthesis occurring; the wavelengths absorbed are the wavelengths used in photosynthesis.

Photosynthesis can be measured by oxygen production. The more photosynthesis that occurs the more oxygen is produced.

Having accessory pigments enables plants to harvest more wavelengths of light therefore more efficient photosynthesis.

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Research methods

In 1887 a botanist called Englemann devised an experiment to determine which wavelengths for light were the most effective in carrying out photosynthesis. He used green alga which contained a ribbon shaped chloroplast and placed the alga into a suspension of motile aerobic bacteria (bacteria which respire using oxygen and can move). He exposed the algae to a range of wavelengths. After a while he noticed the bacteria had clustered near to the chloroplasts at the red and blue wavelengths.

Englemann concluded that this was because there was more photosynthesis at the blue and red wavelengths as there must have been more oxygen produced to attract the bacteria.

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Photosynthesis is a series of reactions within the chloroplast, arranged in two main stages

Light dependent stage – on the thylakoid membraneo Cyclic and non-cyclic photophosphorylation

Light independent stage – in the stroma

The light dependent Stage – on the thylakoid membrane

The light dependent stage produces ATP and reduced NADP, both of which are needed in the light independent stage to form carbohydrates from CO2.

There are two pathways in the light dependent stage:

Non-cyclic photophosphorylation uses both photosystems – produces reduced NADP and ATP

Cyclic photophosphorylation uses only Photosystem I - only produces ATP

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Phosphorylation – the addition of a phosphate, i.e. ADP + Pi ATP

Photo – using light energy

Photophosphorylation – using light energy to produce ATP

Non-Cyclic Phosphorylation

1. Light energy hits Photosystem II and an electron in chlorophyll a is excited and emitted. a. Chlorophyll a loses an electron (e-)b. This electron is replaced by an electron from the photolysis of water

2H2O 4H+ + 4e- + O2. This occurs in the thylakoid space, raising the proton, H+, concentration there.

c. Oxygen given off as a waste gas2. The electron is then accepted by an electron acceptor3. e- then passed along a series of electron carriers, called an electron transport chain4. As the electron is passed along, it loses energy.

a. Energy from the electrons used to ‘power’ a proton pump (labelled cytochrome) which then pumps protons (H+) into the thylakoid space

b. This generates an electrochemical or proton gradient which causes…c. Chemiosmosis to form ATP, as H+ flow through ATP synthetase into the

stroma5. The electron from photosystem II is accepted by chlorophyll a in photosystem I6. Light energy hits photosystem I and an electron in chlorophyll a is excited and emitted

a. Chlorophyll a loses an electronb. This electron will be replaced by an electron from photosystem II

7. The electron is accepted by an electron acceptor8. It then passes down a series of electron carriers, again an electron transport chain9. Electron finally accepted by NADP+ (Non-cyclic photophosphorylation)

a. NADP+ + H+ + e- NADPH2 (reduced NADP)

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NADP accepting the H+ and e- maintains a low concentration of H+ in the stroma maintaining the electrochemical gradient for chemiosmosis.

The reduced NADP and the ATP are used in the next stage of reactions – light independent stage.

This diagram shows non-cyclic photophosphorylation and the energy changes that occur.

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Cyclic photophosphorylation

When there is plenty of reduced NADP then non cyclic photophosphorylation does not take place. Instead cyclic photophosphorylation happens instead.

Light hits photosystem I and excites n electron.

This electron instead of going the second set of electron carries and reducing NADP instead goes to the first set of electron carriers.

The high energy electron provides energy to the proton pump, which pumps H+ into the thylakoid space (just as in non-cyclic). Therefore, ATP is made.

This makes PS II redundant as no electrons are needed from there to fill the space in PS I. Only PS I is active. This is called cyclic photophosphorylation.

To summarise, in cyclic photophosphorylation there is:

No PSII involved No reduced NADP made ATP still made

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(h) The Light Independent Stage (Calvin Cycle) – in the stroma

1. The enzyme RUBISCO catalyses the combination of CO2 with RuBP (ribulose bisphosphate)

2. Forms 2 x Glycerate 3-phosphate (GP) – each has 3 carbons

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Summary

The Light Dependant Stage produces reduced NADP and ATP which is subsequently used in the Light Independent Stage.

NADP is reduced using H+ and electrons. The H+ are formed from the photolysis of water and the electrons from the excitation of electrons in the reaction centre of the photosystems.

ATP is formed by chemiosmosis due to a build-up in the thylakoid space of H + due to photolysis of water and the action of the proton pumps. The concentration of H+ in the stroma is kept low due to the formation of reduced NADP.

There are two distinct pathways – cyclic and non-cyclic.

Non-cyclic uses both photosystem and produce ATP and NADPH. This occurs when NADPH is low.

Cyclic uses only PSI and produces only ATP. This occurs when there is plenty of NADPH.

O2 is the waste product produced.

NADPH and ATP will be used in the next stage: the light independent reactions (Calvin Cycle).

3. GP is reduced to Triose phosphate (TP) – this needs energy from ATP and hydrogen from NADPH (both supplied by the light dependent stage)

4. TP can then be used to form carbohydrates e.g. glucose, sucrose; lipids and amino acids

5. Some Triose phosphate is regenerated back into RuBP (5 carbons) to continue the cycle

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You may see GALP on the mark schemes as an alternative to TP

This reaction is not dependent on light being present if there is sufficient ATP and NADPH. BUT it does rely on these being supplied by the light dependent stage.

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Research of Photosynthesis

Another researcher into the pathways of photosynthesis was Calvin.

Calvin used radioactive tracers to identify the stages of the light independent stage (Calvin cycle). The radioactive tracers where introduces as part of the CO2 used by plants in photosynthesis. This radio labelled Carbon atom could then be ‘followed’ to see which molecules it was used within. The longer the time period, the further on in the cycle the carbon atom had reached. This technique of using radioactive atoms and tracking them by using photographic film is called autoradiography.

Calvin used a ‘lollipop’ containing algae. He stopped the experiment at different times and viewed how far along the sequence the radiolabelled carbon could be found. In this way he worked out the steps of the Calvin

cycle.

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Calvin’s lollipop and the rest of his equipment set up.