PHOTOSYNTHESIS

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What is photosynthesis?

What is photosynthesis?

A physiological process used by plants, algae and some types of bacteria to convert light energy into chemical energy.

What is an autotroph?

They can make organic molecules from inorganic molecules

Why do plants need energy?

• Photosynthesis

• Active transport

• DNA replication

• Cell division

• Protein synthesis

What are organisms that photosynthesise called?

Photoautotrophs

What is the general equation of photosynthesis?

6CO₂ + 6H₂O + energy from photons → C₆H₁₂O₆ + 6O₂

What is the main product of photosynthesis?

1) A monosaccharide sugar

2) It can be converted to disaccharides for transport

3) It can then be converted to starch for storage.

What is carbon fixation?

• The process by which carbon dioxide is converted into sugars.

• The carbon for synthesising all types of organic molecules is provided by carbon fixation.

• Helps regulate the concentration of carbon dioxide in the atmosphere and oceans.

What type of reaction is carbon fixation?

• Endothermic- needs energy

• It also needs electrons

What is the reduction reaction?

The addition of electrons

What are heterotrophs?

• Organisms that cannot make their own organic molecules using simpler molecules found in their environment

• They rely on feeding on those that can.

Give examples of heterotrophs

Fungi, animals, many types of bacteria

What happens to glucose and other organic compounds during respiration?

They are oxidised to produce CO2 and water

How is photosynthesis and respiration interrelated?

1) Both photo and aerobic respiration are important in cycling CO2 and O2 in the atmosphere

2) The products of 1 process are the raw materials for the other process

3) Aerobic respiration removes O2 from the atmosphere and adds CO2 while photosynthesis does the opposite

What is a plant's compensation point?

When photosynthesis and respiration proceed at the same rate, so that there is no net gain or loss of carbohydrate

What is the compensation period?

The amount of time a plant takes to reach its compensation point

Which plants have a shorter compensation period?

• Shade plants can utilise light of lower intensity better than sun plants can.

• When exposed to light after being in darkness, shade plants reach their compensation point sooner than sun plants

• This is because sun plants require a higher light intensity to achieve their optimum rate of photosynthesis.

What are chloroplasts surrounded by?

• A double membrane called the chloroplast envelope

• An intermembrane space separates the outer and inner membrane (They make up the chloroplast envelope)

What is the fluid-filled matrix in a chloroplast called?

The stroma

What does the grana consist of?

Stacks of thylakoid membranes

Where does the light-dependent stage of photosynthesis take place?

In the grana.

What are the 3 distinct membranes of chloroplasts?

• Outer membrane

• Inner membrane

• Thylakoid membrane

What are the thylakoids within a granum connected to other grana by?

Intergranal lamellae.

What is the thylakoid membrane folded into?

Thylakoids that form stacks called granum

Function of the many grana in chloroplasts

Has a huge surface area for:

1. The distribution of the photosystems.

2. The electron carriers and ATP synthase enzymes needed to convert that light energy into ATP.

What holds the photosystems in place?

Proteins embedded in the thylakoid membranes

What are the grana surrounded by?

Stroma

Why is it useful that the grana are surrounded by the stroma?

The products of the light-dependent stage can easily pass to the stroma to be used in the light-independent stage.

What does the stroma contain?

• The enzymes needed to catalyse the reactions of the light-independent stage.

• Starch grains.

• Oil droplets.

• Small ribosomes.

• DNA that contains genes to code for some proteins needed for photosynthesis.

Where is the chloroplasts DNA found?

In the stroma

What is a photosystem?

• A system of photosynthetic pigments found in thylakoids of chloroplasts

• The molecules of chlorophyll trap photons and pass their energy to a primary pigment reaction centre

What do photosystems contain?

Photosynthetic pigments.

• Each pigment absorbs light of a particular wavelength and reflects other wavelengths of light

How big are chloroplasts?

They're around 2-10 micrometres and disc shaped

What happens to the energy associated with the wavelengths of light captured?

It is funnelled down to the primary pigment reaction centre at the base of the photosystem.

What are the two forms of chlorophyll a?

• P₆₈₀

• P₇₀₀

• Chlorophyll a also absorbs some blue light, of wavelength around 440nm.

About both forms of chlorophyll a

• Both appear green

• Both situated at the centre of photosystems

• Both absorb red and blue light but have different absorption peaks

P₇₀₀

• is found in photosystem I

• its peak of absorption is light of wavelength 700nm.

P₆₈₀

• is found in photosystem II

• its peak of absorption is light of wavelength 680nm.

What is chlorophyll b?

• Absorbs light of wavelengths 400-500nm and around 640nm.

• It appears yellow green.

Name some accessory pigments.

• Carotenoids

• Xanthophylls

About Carotenoids

• Absorb blue light of wavelengths 400-500nm.

• They reflect orange and yellow light.

About Xanthophylls

• Absorb blue and green light of wavelengths 375-550nm.

• They reflect yellow light.

What does the light-dependent stage consist of?

1. Light is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy

2. Photolysis of water

3. Photophosphorylation occurs - the production of ATP in the presence of light.

4. The formation of reduced NADP

5. The ATP transfers energy and the reduced NADP transfers hydrogen to the light-independent reaction

5. Also during this process H20 is oxidised to O2

Where does the light-dependent reaction happen?

The thylakoid membranes

Name the 2 types of photosystems

• Photosystem 1

• Photosystem 2

About Photosystem 1

• The pigment at the primary reaction centre is a type of chlorophyll a

• This chlorophyll a has a peak absorption of red light of wavelength 700nm (P700)

About photosystem 2

• The pigment at the primary reaction centre is also a type of chlorophyll a

• However this chlorophyll a has a peak absorption of red light of wavelength 680nm (P680)

Describe photolysis in photosystem 2

• There is an enzyme that, in the presence of light, splits water molecules into protons, electrons and oxygen.

• The splitting of water in this way is called photolysis.

2H₂O → 4H⁺ + 4e⁻ + O₂

What happens to the by-product, oxygen, produced in photolysis?

• Some is used by plant cells for aerobic respiration

• During periods of high light intensity when the rate of photosynthesis is greater than the rate of respiration in the plant, most of the oxygen will diffuse out of the leaves

• They diffuse through stomata, into the surrounding atmosphere.

What is the role of water in photosynthesis?

• Source of protons that will be used in photophosphorylation.

• Donates electrons to chlorophyll to replace those lost when light strikes chlorophyll.

• Source of the by-product, oxygen

• Keeps plant cells turgid, enabling them to function.

What is photophosphorylation?

• Light energy is used to add a phosphate group to ADP to form ATP

• It also reduces NADP to form reduced NADP

What are the two types of photophosphorylation that occur in the light dependent reaction?

• Non-cyclic

• Cyclic

About cyclic photophosphorylation

• Involves only Photosystem 1

• Produces ATP but in smaller quantities than are made by non-cyclic.

About non-cyclic photophosphorylation

• Involves Photosystem 1 and Photosystem 2.

• Produces ATP, oxygen and reduced NADP.

What do both types of photophosphorylation involve?

• Iron-containing proteins embedded in the thylakoid membranes

• These proteins accept and donate electrons and form an electron transport system.

What are the photosystems in the thylakoid membranes linked by?

They are linked by electron carriers

(Proteins that transfer electrons)

What do the photosystems and electron carriers form?

An electron transport chain

Explain non-cyclic photophosphorylation.

1. Light energy is absorbed by PS2

2. The light energy excites electrons inside the chlorophyll molecule.

3. The electrons then move to a higher energy level (become excited)

4. This releases the electrons from the chlorophyll

5.The high energy electrons then move along the electron transport chain towards PS1

6. As excited electrons leave PS2 to move along the ETC they must be replaced

7. The electrons from photolysis replace the high-energy electrons lost by PSII

8. The excited electrons lose energy as they move along the ETC

9. This energy is used to pump protons across the thylakoid membrane into the thylakoid space via proton pumps

10. The thylakoid now has a higher concentration of protons compared to the stroma so it forms a proton gradient across the membrane

11. Protons move down the conc grad into the stroma, via ATP synthase (chemiosmosis)

12. The energy from this movement combines ADP and an inorganic phosphate to form ATP

13. Light energy is then absorbed by PS1

14. This excites another electron which is passed along the rest of the electron transport chain.

15. These electrons are then transferred to NADP along with a proton from the stroma to form NADH

16. The ATP and reduced NADP move into the stroma for the light independent reaction.

What is the light-independent stage also called?

The Calvin cycle

Why is cyclic phosphorylation called cyclic?

• Because the electrons from the chlorophyll molecules aren’t passed onto NADP

• They are passed back to PS1 via electron carriers

Explain cyclic photophosphorylation.

1. Uses only Photosystem 1.

2. As light strikes Photosystem 1, a pair of electrons in the chlorophyll molecule at the reaction centre gain energy and become excited.

3. They escape from the chlorophyll and pass to an electron carrier system

4. They then pass back to Photosystem 1.

4. During the passage of electrons along the electron carriers, a small amount of ATP is generated.

5. No photolysis of water occurs, so no protons or oxygen are produced.

6. Therefore no reduced NADP is generated.

Why do chloroplasts in guard cells only contain Photosystem 1?

1) They produce only ATP which actively brings potassium ions into the cell

2) This lowers the water potential so that water follows by osmosis.

3) This causes the guard cells to swell and opens the stoma.

What photosystem does the chloroplasts in guard cells contain?

Only photosystem 1

Where does the light-independent stage(calvin cycle) of photosynthesis take place?

The stroma of chloroplasts

Does the light independent stage(calvin cycle) directly use light energy?

No it uses the products of the light-dependent stage

What would happen to the light-independent stage (Calvin cycle) if the plant is not illuminated?

It will soon cease

Why does the light-independent stage (Calvin cycle) cease if the plant is not illuminated?

Because ATP and hydrogen are not available to reduce the CO2 and synthesise large complex organic molecules

How does carbon dioxide reach the stroma for the Calvin cycle?

1) Enters the leaf through the stomata

2) It then diffuses through the spongy mesophyll layer to the palisade layer, into the palisade cells

3) It then diffuses through the chloroplast envelope into the stroma.

What aids the diffusion of CO2 from the leaf to the stroma?

The fixation of carbon dioxide in the stroma which maintains a concentration gradient.

Define the Calvin cycle?

The series of reactions whereby carbon dioxide is converted to organic molecules

What enzyme catalyses the combining of CO2 with Ribulose bisphosphate?

RuBisCO

Ribulose biphosphate carboxylase-oxygenase

Explain the Calvin cycle ( Light independent reaction)

1. CO₂ combines with a five-carbon compound ribulose bisphosphate (RuBP)- RuBisCO catalyses this reaction

2. By accepting the carboxyl group, RuBP becomes carboxylated

3. This forms an unstable six-carbon compound that immediately breaks down.

3. The product of this reaction is two molecules of GP (glycerate-3-phosphate).

5. GP is then reduced, using hydrogens from the reduced NADP and ATP, to triose phosphate (TP).

7. Reduced NADP is recycled to NADP (for use in light-dependent reaction again)

8. TP is then converted into many useful organic compounds such as glucose.

9. 5 out of every 6 molecules of TP aren’t used to make hexose sugars, but to regenerate RuBP

10. This process requires phosphate groups.

11. Regenerating RuBP uses the rest of the ATP produced by the light dependent reactions.

Why does the Calvin cycle only run during the daylight?

1) During the light-dependent stage, hydrogen ions are pumped from the stroma into the thylakoid space

2) Therefore the concentration of free protons in the stroma falls, raising the pH to around 8

3) This pH is optimum for the enzyme RuBisCO.

3) RuBisCO is also activated by the presence of extra ATP in the stroma.

4) In the daylight the concentration of magnesium ions increases in the stroma.

5) These ions attach to the active site of RuBisCO, acting as cofactors to activate it.

6) The ferredoxin that is reduced by electrons from Photosystem 1 activates enzymes involved in the reactions of the Calvin cycle

What organic compounds can Triose phosphate (TP) molecules be used to synthesise?

• Some glucose is converted to sucrose, some to starch and some to cellulose.

• Some TP is used to synthesise amino acids, fatty acids and glycerol.

• The rest of the TP is recycled to regenerate the supply of RuBP

Name some limiting factors of photosynthesis

• Carbon dioxide concentration

• Amount of water

• Light intensity

• Availability of chlorophyll, electron carriers and the relevant enzymes

• Temperature

• Turgidity of the cells

How many turns does the carbon cycle need to do to produce 2 molecules of TP?

6 times

How much ATP and reduced NADP does the six turn of the calvin cycle require?

• 18 ATP

• 12 reduced NADP

What is light intensity's role in photosynthesis?

• Provides the energy to power the light dep stage of photosynthesis- produces ATP and NADPH needed for the light indep stage of photosynthesis.

• Light causes the stomata to open so that gaseous exchange can occur.

• When stomata are open transpiration also occurs

• This transpiration leads to the uptake of water and its delivery to the leaves.

How does light intensity affect the rate of photosynthesis?

1) When it's low, rate is low.

2) As light intensity increases, the rate of photosynthesis increases.

3) At a certain point, even when light intensity increases, the rate of photosynthesis does not increase.- saturation point

4) This means a factor other than light intensity is limiting factor

What is the effect of low light intensity on the Calvin cycle?

1. GP cannot be reduced to TP.

2. TP levels fall and GP accumulates.

3. If TP levels fall, RuBP cannot be regenerated.

What are the effects of low carbon dioxide concentration on the Calvin cycle?

1. RuBP cannot accept CO₂, and accumulates.

2. Conversion of RuBP to GP is slow

3. Levels of RuBP rise

3. Levels of GP and TP will fall

What is the effect on the rate of photosynthesis at temperatures between 25-30°C?

If plants have enough water and carbon dioxide and a sufficient light intensity, the rate of photosynthesis will increase as temperature increases.

What is the effect on the rate of photosynthesis at temperatures between 30-45°C?

• Stoma will close to avoid losing too much water- slows down photosynthesis due to lack of CO2

• Thylakoid membranes may become damaged- Reduces the rate of the light dependent stages by reducing the number of sites available for electron transfer

• The membrane around the chloroplast may also get damaged

• This causes enzymes important to the Calvin cycle to be released out of the cell-reduces rate of light independent stage

• Chlorophyll could get damaged- reduces amount of pigment that can absorb light energy- reduces the rate of the light-dependent stage reactions

What is photorespiration?

Its where oxygen competes with carbon dioxide for the enzyme RuBisCO's active site.

What is the effect on the rate of photosynthesis at temperatures above 45°C?

1) Enzymes involved in photosynthesis may be denatured.

2) This would reduce the concentrations of GP and TP, and eventually RuBP as it could not be regenerated due to lack of TP.

Why is access to sufficient water important for plants?

• The transpiration stream has a cooling effect on the plant.

• The water passing up the xylem to the leaves also keeps plant cells turgid so they can function.

• Turgid guard cells keep the stomata open for gaseous exchange.

What are the effects of water stress?

1. The roots are unable to take up enough water to replace that lost via transpiration.

2. Cells lose water and become plasmolysed.

3. Plant roots produce abscisic acid that, when translocated to leaves, causes stomata to close, reducing gaseous exchange.

4. Tissues become flaccid and leaves wilt.

5. The rate of photosynthesis greatly reduces.

How do you calculate the rate of photosynthesis?

Calculate the quantity of raw materials taken up or products produced per unit time.

What are the limitations of measuring volume of oxygen produced per minute to measure rate of photosynthesis of an aquatic plant?

• Some of the oxygen produced by the plant will be used for respiration.

• There maybe some dissolved nitrogen in the gas collected.

• Some oxygen will dissolve in water

How do you set up a photosynthometer?

Set up so it is air tight and there are no air bubbles in the capillary tubing.

How does a photosynthometer work?

1) Gas given off by the plant, over a known period of time, collects in the flared end of the capillary tube.

2) As the experimenter manipulates the syringe, the gas bubble can be moved into the part of the capillary tube against the scale and its length measured.

3) If the radius of the capillary tube bore is known, then this length can be converted to volume

Volume of gas= Length of bubble x Pir^2

What is decarboxylation?

The removal of CO2 from a molecule

What is dehydrogenation?

The removal of hydrogen from a molecule

What is the coenzyme in photosynthesis?

NADP

• It transfers hydrogen from 1 molecule to another

• This means it can reduce or oxidise

What happens to electrons as the move down the ETC?

• They lose energy

• This energy pumps protons from the stroma into the thylakoids.

What happens as protons build up in the thylakoids?

• A proton gradient forms across the thylakoid membrane - allows chemiosmosis to occur

• The concentration of protons inside the thylakoids is greater than in the stroma.

What is the first stage of non-cyclic photophosphorylation?

Photoionisation of chlorophyll

Describe the photoionisation of chlorophyll

• Light energy is absorbed by PSII.

• Light energy excites electrons in the chlorophyll of PSII

• Excited electrons are moved to a higher energy level (they are high-energy electrons).

What colour will the redox indicator DCPIP be in its reduced state?

Colourless

Outline the importance of photosynthetic pigments in photosynthesis.

• Pigments absorb light energy

• This excites electrons and they move to higher energy level.

• Accessory pigments pass energy to the primary pigments

• Primary pigments pass the electrons to the ETC for light dependent reaction.

State a material that can be used as the stationary phase in thin layer chromatography.

Silica (gel)