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Photosynthesis Cheat Sheet (DRAFT) by

photosynthesis uses light energy to synthesise organic molecules

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Key Terms

Photop­hos­pho­ryl­ation
Synthesis of ATP from ADP and and Pi (inorganic phosphate) using light energy.
Transd­ucers
Change energy from one form into another
Absorption Spectrum
A graph that shows how much light energy is absorbed at different wavele­ngths.
Action Spectrum
A graph that shows the rate of photos­ynt­hesis at different wavele­ngth.
Antenna Complex
An array of protein and pigment molecules in the thylakoid membranes with chloro­phyll a at the reaction centre. It transfers energy from light of a range of wavele­ngths to chloro­phyll a.
Limiting Factor
A factor that limits the rate of a physical process by being in short supply.

Overview of Photos­ynt­hesis

The overall equation for photos­ynt­hesis is:
6CO2 + 6H2) -> C6H12O6 + 6O2
Photos­ynt­hesis involves two stages:
1. Light-­dep­endent stage where light energy is converted into chemical energy as the photolysis of water releases protons and electrons which produce ATP via photop­hos­pho­ryl­ation and reduce the co-enzyme NADP.
2. Light-­ind­epe­ndent stage or Calvin cycle where ATP and NADPH from the light-­dep­endent reaction reduce carbon dioxide to produce glucose.

Photos­ynt­hesis Summary

Structure of the leaf

The leaf is adapted for gas exchange and photos­ynt­hesis by having a large surface area allowing the leaf to capture light, and having pores called stomata through which gases diffuse.
Air spaces between cells allow for carbon dioxide to diffuse to the photos­ynt­hes­ising cells.
The highest concen­tration of chloro­plasts is found in the palisade mesophyll on the leaf's upper surface.
The palisade cells are arranged vertically, which allows more light to be absorbed by chloro­plasts than if they were stacked horizo­ntally, as light only has to pass through the cuticle, epidermal cells and one palisade cell wall.

Leaf structure

Chloro­plasts as transd­ucers

The site of photos­ynt­hesis was detected by Engelmann in 1887. In his experi­ment, he shone a light through a prism to separate the different wavele­ngths of light, and exposed this to a suspension of algae with evenly distri­buted, motile, aerobic bacter­ia.**
After a period of time, he noticed that the bacteria congre­gated around the algae exposed to blue and red wavele­ngths.
This was because this algae photos­ynt­hesised more and so produced more oxygen, attracting the motile bacteria.

Transd­ucers

Photos­ynt­hetic pigments

In flowering plants there are 2 main types of pigments:
1. Chloro­phylls which absorb red and blue-v­iolet regions of the spectrum, e.g. chloro­phyll a and chloro­phyll b.
2. Carote­noids which absorb light energy from the blue-v­iolet region of the spectrum, e.g. B-carotene and xantho­phylls, and act as accessory pigments.
The presence of several pigments allows the plant to absorb a wider range of wavele­ngths of light than a single pigment.

Absorption and action spectra

The absorption spectrum shows how much light energy a particular pigment absorbs at different wavele­ngths, for example chloro­phyll a which absorbs red and blue-v­iolet regions of the spectrum.
It does not indicate whether the particular wavelength is used in photos­ynt­hesis.
An action spectrum shows the rate of photos­ynt­hesis at different wavele­ngths, by measuring the mass of carboh­ydrate synthe­sised by plants.
There is a close correl­ation between the 2.

Absorption and action spectra

Light Harvesting

The chloro­phylls and accessory pigments are found lying in the thylakoid membranes, grouped into structures called antenna complexes.
With the aid of special proteins associated with these pigments, light energy (photons) is funnelled towards the reaction centre at the base, containing chloro­phyll a.
There are 2 types of reaction centre:
1. Photos­ystem 1 (PS1) chloro­phyll a, with an absorption peak of 700nm, also called P700.
2. Photos­ystem 11 (PS11) chloro­phyll a, with an absorption peak of 680nm also called P680.
 

Antenna Complex

Identi­fying different photo pigments from chloro

Pigments can be extracted by grinding plant material in a suitable solvent, e.g. propanone, and separated by paper chroma­tog­raphy.
By dividing the distance travelled by the pigment by the distance travelled by the solvent front, the Rf value can be calcul­ated.

Calcul­ating the Rf value

The light-­dep­endent stage of photos­ynt­hesis

Occurs on the thylakoid membranes.
Photop­hos­pho­ryl­ation occurs via 2 pathways:
1. Non-cyclic photop­hos­pho­ryl­ation, which involves both photos­ystems 1 and 11, generating 2 ATP molecules and reduced NADP.
Photolysis generates oxygen.
The electrons take a linear pathway which is referred to as the 'Z scheme'.

Non-cyclic photop­hos­pho­ryl­ation

Non-cyclic photop­hos­pho­ryl­ation

1. Light energy (photons) strikes chloro­phyll (PS11) exciting its electrons, boosting them to a higher energy level.
2. Electrons are accepted by an electron carrier in the thylakoid membrane.
3. The oxidised chloro­phyll removes electrons from water, producing protons and oxygen (photo­lysis). This occurs in the thylakoid space.
4. As electrons pass from carrier to carrier, electron energy is lost, which pumps protons from the stroma into the thylakoid space. As protons flow back through the stalked particle, ADP is phosph­ory­lated; 2 ATP are made in total.
5. Electrons enter photos­ystem 1 where light excites them, boosting them to an even higher energy level.
6. Electrons enter a final electron carrier.
7. Electrons and protons reduce NADP to reduced NADP which pass to the Calvin Cycle with the 2 ATP made.

ATP production in the chloro­plast

Cyclic Phosph­ory­lation

Involves only photos­ystem 1, producing 1 ATP molecule only.
As photolysis does not occur, no oxygen is released.
Electrons take a cyclical pathway.
If there is no NADP available, then electrons fall back into the electron transport chain (at an interm­ediate energy level) and generate 1 ATP.
This cycle continues until NADP is available.
The ATP produced can be used in the Calvin Cycle, in the stomatal opening mechanism, or for other cellular processes.
ATP is produced in the chloro­plast when protons are pumped across the thylakoid membrane using energy from the electrons and accumulate with protons generated from photolysis of water in the thylakoid space generating an electr­och­emical (proton) gradient.
The H+ ions diffuse back into the stroma through stalked particles generating ATP.
The protons and electrons reduce NADP, which removes H+ ions from the stroma, further contri­buting to the H+ gradient.
The movement of protons is referred to as chemio­smosis.

Cyclic Phosph­ory­lation

Calvin Cycle - light indep stage of photos­ynt­hesis

This stage occurs in the stroma.
ATP and reduced NADP from the light-­dep­endent reaction are used to fix carbon from carbon dioxide with the help of the enzyme RuBisCO.
The sequence was first worked out by Calvin and his team using a radioa­ctive isotope of carbon (14C) present in hydrogen carbonate.
At regular intervals, Calvin removed samples into hot methanol to kill the chlorella algae used and to stop all enzyme reactions.
He then performed chroma­tog­raphy to identify the products.
He exposed his chroma­togram to piece of x-ray film which would detect radiation emitted from 14C used.
This identified products containing 14C in the order they were produced: first was hydrogen carbonate ions, then glycerate 3-phos­phate (GP), triose phosphate (TP), ribulose bispho­sphate (RuBP) and finally glucose.

Calvin's lollipop apparatus

Calvin Cycle

  

Stages in the Calvin Cycle

1. CO2 diffuses into leaf via stomata, dissolving in the water surrou­nding palisade mesophyll cells before diffusing into the cells.
2. CO2 combines with the 5 carbon compound ribulose bispho­sphate (RuBP) using the enzyme RuBisCO to form an unstable 6C compound.
3. Unstable 6C compound immedi­ately breaks down into 2 molecules of glycerate 3-phos­phate (GP).
4. Using one ATP molecule from the light reaction, GP is reduced to triose phosphate (TP) using hydrogen atoms from reduced HADP.
5. Triose phosphate molecules combine in pairs to form hexose sugars.
6. 5 out of every 6 triose phosphate molecules produced are used to regenerate RuBP (via the interm­ediate ribulose phosphate) using ATP from the light-­dep­endent reaction to supply energy and phosphate. This allows the cycle to continue.

Stages in the Calvin Cycle

1. CO2 diffuses into leaf via stomata, dissolving in the water surrou­nding palisade mesophyll cells before diffusing into the cells.
2. CO2 combines with the 5 carbon compound ribulose bispho­sphate (RuBP) using the enzyme RuBisCO to form an unstable 6C compound.
3. Unstable 6C compound immedi­ately breaks down into 2 molecules of glycerate 3-phos­phate (GP).
4. Using one ATP molecule from the light reaction, GP is reduced to triose phosphate (TP) using hydrogen atoms from reduced HADP.
5. Triose phosphate molecules combine in pairs to form hexose sugars.
6. 5 out of every 6 triose phosphate molecules produced are used to regenerate RuBP (via the interm­ediate ribulose phosphate) using ATP from the light-­dep­endent reaction to supply energy and phosphate. This allows the cycle to continue.

Product Synthesis

Plants must produce all the carboh­ydr­ates, fats and proteins they need from the products of the Calvin Cycle.
Fructose phosphate formed from the 2 molecules of triose phosphate can be converted to glucose, or combined with glucose to produce sucrose.
Sucrose is then transl­ocated in the phloem to the growing regions of the plant.
Some a glucose is stored as starch, B glucose forms cellulose in cell walls.
Fatty acids can be formed from glycerate 3-phos­phate, and glycerol from triose phosphate, the building blocks of trigly­cer­ides.
Proteins can be formed from glycerate 3-phos­phate, but the amino group requires nitrogen from nitrate ions.
Other compounds, e.g. chloro­phyll, require additional ions e.g. Mg2+, and the middle lamella of cell walls needs Ca2+.
A lack of nitrogen results in stunted growth in plants, as plants cannot synthesise proteins due to lack of nitrogen, whereas a lack of magnesium causes chlorosis, the yellowing of the leaves, as chloro­phyll cannot be synthe­sised.
This can be shown experi­men­tally by placing plants in soils with different nutrient contents and observing growth.

Limiting factors in photos­ynt­hesis

The rate of photos­ynt­hesis is controlled by a number of factors including the concen­tration of CO2, light intensity, and temper­ature.
The limiting factor is the one which is in shortest supply which controls the rate-l­imiting step, and therefore an increase in it increases the rate of photos­ynt­hesis.

Limiting Factors Table

Factor
Explan­ation
Carbon Dioxide
At low concen­tra­tions, CO2 concen­tration is limiting, but above 0.5%, the rate plateaus, showing that something else must be limiting. Above 1% the stomata close, preventing the uptake of carbon dioxide.
Light intensity
As light intensity increases the rate of photos­ynt­hesis increases up to about 10,000 lux (SI unit if illumi­nance) when sone other factor becomes limiting. At very high light intens­ities the rate decreases as chloro­plast pigments become bleached. Different plants have evolved to be most efficient at light intens­ities found in their enviro­nment, e.g. sun and shade plants.
Temper­ature
Temper­ature increases the kinetic energy of the reactants and enzymes involved in photos­ynt­hesis. Unlike other factors, a plateau is not reached as enzymes, e.g. RuBisCO, begin to denature so the rate of photos­ynt­hesis decreases above the optimum temper­ature. This will be higher in species adapted to hot, dry enviro­nments.

Limiting Factor Graphs

Measuring the rate of photos­ynt­hesis

Aquatic plants are a good subject to use when invest­igating how different factors affect photos­ynt­hesis.
Temper­ature and CO2 concen­tration are more easily controlled than with terres­trial plants, by using a water bath and contro­lling hydrogen carbonate concen­tra­tion.
It is also easy to collect and accurately measure the oxygen produced in a capillary tube.
The volume of the bubble collected is calculated by the formula:
Volume = pi r2 x length of bubble
Where pi = 3.14 and r = radius or diameter/2