Module 4.5

In photosynthesis light energy is transferred to ATP in the light dependent reactions and used to make carbohydrate in the light independent reactions.

Know photosynthesis has both light dependent and light independent reactions.Know that in the light dependent reactions light energy excites electrons, the energy they contain is used to generate ATP and reduced NADP as they pass along electron transport chains in the chloroplast membranes, photolysis of water produces protons, electrons and oxygen.Know that in the light independent reactions ribulose bisphosphate (RuBP) and CO2 join and form two molecules of glycerate phosphate (GP); ATP and reduced NADP are required (obtained from LDR); RuBP is regenerated in the Calvin cycle. Triose phosphate (TP) can be converted to carbohydrates, fats and proteins as needed by the plant.Appreciate how temperature, light intensity, CO2 concentration affect the rate of photosynthesis and can be limiting factors.

Thylakoid: series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependent reaction of photosynthesis.

Grana: a stack of thylakoids in chloroplasts that resembles a pile of coins. This is the site of the light-dependent reaction of photosynthesis.

Stroma: matrix of a chloroplast where the light-independent reaction of photosynthesis takes place.

Light Dependent reactions: stage of photosynthesis in which light energy is required to produce ATP and reduced NADP.

Light independent reactions: stage of photosynthesis which does not require light energy directly but does need the products of the light dependent reaction to reduce carbon dioxide and so form carbohydrate.

Chlorophyll:

NADP: (nicotinamide adenine dinucleotide phosophate) – a molecule that carries electrons produced in the light-dependent reaction of photosynthesis.

Photolysis: splitting of water molecule by light such as occurs in the light dependent reaction of photosynthesis.

RuBP: Ribulose Bisphosophate

GP: Glycerate phosophate

TP: Triose phosphate

Calvin Cycle: a biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate.

Photophosphorylation:

Limiting factor: a variable that limits the rate of a chemical reaction.

Light dependent reaction:

Purpose:o To add an inorganic phosphate molecule to ADP to make ATPo To split water into H+ ions (protons) and OH- ions. As the splitting is caused by light –

photolysis. The making of ATP

o Chlorophyll absorbs light energy.o The electrons gain energy and rise to a higher energy state.o Electrons are said to be excited – so they leave the chlorophyll.o The electrons are taken up by an electron carrier.

Chlorophyll is oxidised Electron carrier is reduced.

o Electrons are now passed along a number of electron carriers in a series of oxidation and reduction reactions.

o This forms a transfer chain that is located in the membrane of the thylakoid.o Each new carrier is at a slightly lower energy level than the previous one in the

chain, and so the electrons lose energy at each stage.o The energy is used to combine ADP + Pi to make ATP

Photolysis of watero The loss of electrons when light strikes a chlorophyll molecule leaves it short of

electrons.

o If the chlorophyll molecule is to continue absorbing light energy it needs a replacement of electrons.

o They are provided from water molecules.o Photolysis also yields H+ ions.o 2H2O 4H+ + 4e- + O2

o The hydrogen ions are taken up by an electron carrier called NADP.o This make reduced NADPo This then enters the light independent reaction along with the electrons from the

chlorophyll molecules.o NAPD is important:

Because it is a further potential source of chemical energy to the plant. o The oxygen by-product from the photolysis of water is either used in respiration or

diffuses out of the plant as a waste product. Site of the light-dependent reaction

o Takes place in the thylakoido Thylakoids are disc-like structures that are stacked together in groups called granao This is in the chloroplast.o The thylakoid membranes provide a large surface area for the attachment of

chlorophyll, electron carriers and enzymes.o Networks of proteins in the grana hold the chlorophyll in a very precise manner that

allows maximum absorption of light.o The granal membranes have enzymes attached to them, which help manufacture

ATP.o Chloroplasts contain both DNA and ribosomes so they can quickly and easily

manufacture some of the proteins needed for the light-dependent reaction.

The light-independent reaction

o Products from the light dependent reaction – ATP + reduced NADP are used to reduce carbon dioxide.

o Takes place in the stroma of chloroplasts.

o The Calvin cycle.

1. Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and dissolves in water around the walls of the mesophyll cells. It then diffuses through the

plasma membrane into the stroma of the chloroplasts.2. In the stroma carbon dioxide fixates with the 5-carbon compound ribulose bisphosphate

using an enzyme.3. The combination of carbon dioxide and RuBP produces a 6-carbon compound which is

unstable so breaks down into 2 x 3-carbon compound glycerated 3-phosphate (GP).4. ATP and the reduced NADP from the light-dependent reaction are used to redce the

activated GP to triose phosphate TP.5. The NADP is re-formed and goes back to the light-dependent reaction to be reduced again.6. Some TP molecules are converted to useful organic substances such as glucose.7. Most TP molecules are used to regenerate ribulose bisphosphate using ATP from the light

dependent reaction.

ATP and reduced NADP from light dependent reaction.

ADP + Pi

NADP

Site of the light-dependent reaction

Takes place in the stroma of chloroplasts. The fluid of the stroma contains all the enzymes needed to carry out the light-independent

reaction (reduction of carbon dioxide) The stroma fluid surrounds the grana and so the products of the light-dependent reaction in

the grana can readily diffuse into the stroma. It contains both DNA and ribosomes so it can quickly and easily manufacture some of the

proteins needed for the light-independent reaction.

Factors affecting photosynthesis

Rate of photosynthesis can be measured by:o The volume of oxygen released by a planto The volume of carbon dioxide taken up by a plant.

Light intensityo When light is the limiting factor; the rate of photosynthesis is directly proportionate

to the light intensity.o Compensation point – when there is no net exchange of gases as the uptake of

carbon dioxide is equal to the amount of oxygen being produced. Carbon dioxide concentration

o 0.04% carbon dioxide in our atmospheres.o Plants optimum carbon dioxide concentration is 0.1%o Carbon dioxide affects enzyme activity, in particular the enzyme that catalyses the

combination of ribulose bisphosphate with carbon dioxide in the light independent reaction.

Temperatureo Rate of photosynthesis is directly proportionate to temperature.o Optimum is normally 25Co Higher than this normally denatures the enzymes.

Measuring photosynthesis:

In the light-dependent reaction of photosynthesis, light energy generates ATP. Describe how:

Light energy excites/raises energy level of electrons in chlorophyll. Electrons pass down electron transfer chain; Electrons reduce carriers/passage involves redox reactions Electron transfer chain/role of chain associated with chloroplast membranes/in

thylakoids/grana; Energy released/ carriers at decreasing energy levels; ATP generated from ADP and Pi/ phosphorylation of ATP Example such as chemiosmosis

In the light-independent reaction of photosynthesis, the carbon in carbon dioxide becomes carbon in triose phosphate. Describe how:

Carbon dioxide is fixated with ribulose bisphosphate To produce two molecules of glycerated 3-phosphate/GP Reduced to TP Required reduced NADP And energy from ATP.