Photosynthesis:

The importance of photosynthesis:

What is photosynthesis?

Photosynthesis is the process whereby light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances.

It is perhaps the most important biochemical process, since nearly all life on Earth depends on it. It transforms light energy into chemical potential energy that is then available to consumers and

decomposers. It also releases oxygen, from water, into the atmosphere, so all aerobes depend on it for their respiration.

Autotrophs and heterotrophs:

AUTOTOPHS: organisms that use light energy or chemical energy and inorganic molecules (CO2 and water to synthesise complex organic molecules

HETEROTROPHS: organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them.

Autotrophs are organisms that can synthesise complex organic molecules, such as carbohydrates, lipids, proteins, nucleic acids and vitamins, from inorganic molecules and an energy source

- The first life forms on Earth where chemoautotrophs – prokaryotes that synthesised complex organic molecules, using energy derived from exergonic chemical reactions.

- Many bacteria are chemoautotrophs, for example the nitrifying bacteria that are so important in the recycling of nitrogen.

- They obtain their energy from oxidising ammonia to nitrite or oxidising nitrite to nitrates. - There are also chemoautotrophs living in darkness, near thermal oceanic vents, supporting very specific food

chains in those areas.- Organisms that can photosynthesise are described as photoautotrophs. - Their source of energy is sunlight and the raw materials are inorganic molecules – carbon Dioxide, and

Water. Plants, some bacteria and some protoctsists (such as algae) are photoautotrophs.- The majority of food chains on Earth have producers that are photoautotrophs. Animals, fungi and some bacteria are heterotrophs- Cannot make their own food but digest complex organic molecules into simpler soluble ones from which

they can synthesise complex molecules such as lipids, proteins and nucleic acids

Why does respiration in autotrophs and heterotrophs depend on photosynthesis?

Both photoautotrophs and heterotrophs can release the chemical potential energy in complex organic molecules (made during photosynthesis).

- This is respiration.- They can also use the oxygen for aerobic respiration.

The equation below summarises the process of photosynthesis:

6CO2+6H2O (+Light Energy )C 6H 12O6+6O2

Organisms evolved to use oxygen for aerobic respiration. This releases carbon dioxide back into the atmosphere and produces water.

C6H12O6+6O2 6CO2+6H 2O (+energy , someas ATP)

Where does photosynthesis take place?

Photoautotroph’s have special organelles within their cells called chloroplasts It happens in two main stages. Chloroplasts acts as compartments, isolating the photosynthetic reactions from other cellular activities Each chloroplast consist of two membranes enclosing a matrix called the stroma The stroma contain ribosomes, circular DNA and enzymes Suspended in the stroma are thylakoids

Endosymbiont theory:

Evidence that mitochondria and chloroplasts were once primitive bacterial cells.

This evidence is described in the endosymbiotic theory

Symbiosis occurs when two different species benefit from living and working together.

When one organism actually lives inside the other it's called endosymbiosis.

The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship.

Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell

Suggests that photosynthetic bacteria were acquired, possibly by endocytosis, by early eukaryotic cells to form the first algae or plant cells

How the structure of chloroplasts enables them to carry out their functions

The structure of chloroplasts:

Photosynthesis takes place within organelles called chloroplasts.- They vary in shape and size but most are disc-shaped and between 2 -10 micrometers long.

- Each chloroplast is surrounded by a double membrane – an envelope.- There is an intermembrane space, about 10-20nm wide, between the inner and outer membrane.- The outer membrane is permeable to many small ions- The inner one is less permeable and has transport proteins embedded in it. It is folded into lamellae (thin

plates), which are stacked. - Each stack of lamellae is called a granum (pl Grana)- Between the grana are intergranal lamellae.

How chloroplasts are adapted for their role:

The inner membrane, with its transport proteins can control entry and exit of substances between the cytoplasm and the stroma inside the chloroplasts.

The many grana, consisting of stacks of up to 100 thylakoid membranes- Provide a large surface area for the photosynthetic pigments, electron carriers and ATP synthase enzymes, - All involved in the light-dependent reaction. The photosynthetic pigments are arranged into special structures called photosystems,- Allows maximum absorption of light energy. Proteins embedded in the grana hold them photosystems in place. The fluid filled stroma contains the enzymes needed to catalyse the reactions of the light-independent stage

of photosynthesis. The grana are surrounded by the stroma so the products of the light-dependent reaction, which are needed

for the light independent reaction, can readily pass into the stroma.

Chloroplasts can make some of the proteins they need for photosynthesis, using genetic instructions in the chloroplast DNA, and the chloroplast ribosomes to assemble the proteins.

Photosynthetic pigments:

PHOTOSYNTHETIC PIGMENTS: molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected.

Substances that absorb certain wavelengths of light and reflect others. They appear to us as the colour of the light wavelengths that they are reflecting. There are many different pigments that act together, to capture as much light energy as possible. They are in the thylakoid membranes, arranged in funnel-shaped structures called photosystems, held in

place by proteins. Arranged in clusters called antenna complexes The molecules in each complex are arranged so as to channel light energy to just one molecule of

chlorophyll a - This is the only molecule that actually donates an electron to the primary acceptor molecule Chlorophyll a and the primary acceptor make up what is called the reaction centre

The reaction centre and all the other light gathering molecules combine to form a photosystem

Chlorophylls:

Chlorophyll is a mixture of pigments. All have a similar molecular structure, consisting of a long phytol (hydrocarbon) chain and a porphyrin group. The porphyrin group is similar to haem, found in haemoglobin, but contains a magnesium atom instead of an iron atom.

Light hitting chlorophyll causes a pair of electrons associated with the magnesium to become excited. There are two forms of chlorophyll a – p680 and P700.

- Both appear yellow/green. Each absorbs red light at a slightly different wavelength (absorption peak) Both are found at the centre of photosystems and are known as primary pigment reaction centres P 680 is found in photosystems 2 and its peak of absorption of light is at wavelength of 680nm. P 700 is found at photosystems 1 and its peak of absorption is light at wavelength of 700nm. Chlorophyll a also absorbs blue light of Wavelength around 450nm Chlorophyll b absorbs light of wavelength around 500nm and 640nm. It appears blue green.

Accessory pigments:

Do not participate directly in the light-dependent reactions, but they broaden the range of light that a plant can use

Carotenoids reflect yellow and orange light and absorb blue light They do not contain a porphyrin group and are not directly involved in the light dependent reaction. They absorb light wavelengths that are not well absorbed by chlorophylls and pass the energy associated

with that light to the chlorophyll at the base of the photosystems. Carotene (orange) and xanthophylls (yellow) are the main carotenoids pigments.

How light arrives at a plant:

Energy that arrives at the light dependent reactions come from sunlight, a form of electromagnetic radiation

The particulate model of how light travels is used when explaining the events in photosynthesis As sunlight falls on a plant, some wavelengths are absorbed, while others are reflected or transmitted The pattern of absorption and reflection depends on coloured substances called pigments

Diagrams:

Wavelengths of light that a specific pigment absorb can be displayed in an absorption spectrum

Wavelengths of light that bring about photosynthesis in a particular plant can be displayed in an action spectrum

The absorption spectrum of combined photosynthetic pigments in a plant coincides very closely with the action spectrum of photosynthesis for that plant, supporting the theory that these pigments harvest light for photosynthesis

The light-dependent stage:

Occurs on the thylakoid membranes of the chloroplasts. The photosystems with the photosynthetic pigments are embedded in these membranes. Photosystems 1 (PS1) occurs mainly on the intergranal membrane PSII occurs almost exclusively on the granal lamellae. These pigments trap light energy so that it can be converted to chemical energy in the form of ATP.

The role of water:

Photosystem II also contains an enzyme that, in the presence of light, can split water H ions (protons) and electrons and oxygen.

The splitting of water is called photolysis:

2H20 4H+ + 4e- + O2

Some of the water is a source of:

Hydrogen ions, which is used in chemiosmosis to produce ATP. - These protons are then accepted by a coenzyme NADP (nicotinamide adenine dinuclotide phosphate), which

become reduced to NADPH, to be used during the light independent stage to reduce carbon dioxide and produce organic molecules.

Electrons to replace those lost by the oxidised chlorophyll.

Water is one of the raw materials used in photosynthesis. It also keeps plant cells turgid enabling them to function. The by-product of photosynthesis (oxygen) comes from water.

Photophosphorylation:

PHOTOPHOSPHORYLATION: the making of ATP from ADP and Pi in the presence of light

ELECTRON CARRIERS: molecules that transfer electrons

ELECTRON ACCEPTORS: chemicals that accept electrons from another compound. They are reduced while acting as oxidising agents.

Light has a dual nature- We usually think of it as

travelling in waves - Travels in particles called

photons. - Each photon of light contains a

fixed amount of energy called a quantum

- The size of the quantum varies with the wavelength of light: the shorter the wavelength, the larger the quantum

When a photon hits a chlorophyll molecule the energy of the photon is transferred to two electrons and they become excited.

These electrons are captured by the electron acceptors and passed along a series of electron carriers embedded in the thylakoid membranes.

The electron carriers are proteins that contain iron atoms. Energy is released as electrons pass along the chain of electron carriers- This pumps protons across the thylakoid membranes into the thylakoid space where they accumulate. - A proton gradient is formed across the thylakoid membrane and the protons flow down their gradient,

through channels associated with ATP synthase enzymes. - This flow of protons is called chemiosmosis.- It produces a force that joins ADP and Pi to make ATP. - The kinetic energy from the proton flow is converted to chemical energy in the ATP molecules, which is

used in the light-independent stage of photosynthesis. The making of ATP uses light energy is called photophosphorylation There are two types - Cyclic- Non-cyclic.

Cyclic photophosphorylation:

This uses only photosystem 1 (P700). The excited electrons pass to an electron

acceptor and back to the chlorophyll molecule from which they were lost.

There is no photolysis of water and no generation of NADPH

Small amounts of ATP are made- This may be used in the light-

independent reaction of photosynthesis or it may be used in guard cells (their chloroplasts contain only Photosystem 1) to bring in potassium ions, lowering the water potential and causing water to follow by osmosis.

- This causes the guard cells to swell and opens the stomata.

Non-cyclic photophosphorylation:

This involves both photosystems – PSI and PSII

1) Light strikes Photosystem II, exciting a pair of electrons that leave the chlorophyll molecule from the primary pigment reaction centre.

2) The electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP.

3) Light has also struck Photosystem I and a pair of electrons has been lost.

4) These electrons along with the protons (produced at photosystem II by photolysis of water), join NADP, which becomes NADPH.

5) The electrons from the oxidised photosystems II replace the electrons lost from PS1.

6) Electrons from photolysed water replace those lost by the oxidised chlorophyll in PS11.7) Protons from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP, in

the stroma. - They will be used in the light-independent stage of photosynthesis.

The Light-independent stage:

LIGHT INDEPENDENT STAGE: stage in photosynthesis where carbon dioxide is fixed and used to build complex organic molecules.

The light independent stage of photosynthesis takes place in the stroma of chloroplasts. It is also called the Calvin cycle. Although light is not directly used, the products of the light dependent stage are used, and the light

independent stage soon ceases if light is not available. Can take place in the dark if sufficient ATP and NADPH is present Under most conditions, sufficient ATP and NADPH are only available when they are being generated by the

light dependent reactions

The role of carbon dioxide:

Source of carbon (and oxygen) for the production of all 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.

The Calvin cycle:

1) Carbon dioxide from the air diffuses through open stomata, most of which are on the underside of the leaf,- It then diffuses through the air spaces in the spongy mesophyll and reaches the palisade mesophyll.- Here it diffuses through the thin cellulose walls, the cell surface membrane, the cytoplasm and the

chloroplast membrane into the stroma.2) In the stroma, carbon dioxide combines with a 5 carbon compound Ribulose Bisphosphate (RuBP) (A carbon

dioxide acceptor). - The reaction is catalysed by the enzyme, Ribulose Bisphosphate carboxylase-oxygenase, usually called

Rubisco- RuBP becomes carboxylated (combines with carbon dioxide so it now has a carboxyl group)3) The product of this reaction is two molecules of a 3 carbon compound, Glycerate-3-Phosphate (GP). - The carbon has now been fixed.

4) GP is reduced and phosphorylated to another 3 carbon compound, triose phosphate (TP)- ATP and NADPH from the light-dependent

reaction are used in this process.5) Five out of 6 molecules of TP (3C) are

recycled by phosphorylation, using ATP from the light – dependent reaction, to 3 molecules of RuBP (5C)

How the products of the Calvin cycle are used:

6) Some GP can be used to make amino acids and fatty acids.7) Pairs of TP molecules combine to make hexose (6C) sugars, such as glucose.8) Some glucose molecules may be isomerised to form another hexose sugar, fructose.9) Glucose and fructose molecules may be combined to form the disaccharide sucrose – the sugar translocated

in phloem tubes.10) Hexose sugars can be polymerised into other carbohydrates (polysaccharides) such as cellulose and starch.11) TP can be used to convert glycerol and this may be combined with fatty acids formed from GP, to make

lipids.

Limiting factors:

6CO2+6H2O (+Light Energy )C 6H 12O6+6O2

The chlorophyll is present in plant’s chloroplasts. The other factors, light and supplies of Carbon Dioxide and Water, are present in the environment. - They may influence the rate at which photosynthesis proceeds and therefore the rate at which food for

nearly all organisms on the planet becomes available.

Limiting factors:

LIMITING FACTOR: in metabolic processes, it is the factor that is present at the lowest or least favourable value

Photosynthesis is a complex process and the factors that affect its rate are all operating simultaneously. However, the factor that is present at the least favourable level is the one that limits the process. The factor present in the least favourable amount is called the limiting factor.

In the early 1900s Blackman investigated the effect of varying light intensity and temperature on the rate of photosynthesis.

At constant temperature, the rate of photosynthesis varies with light intensity- At 0 light intensity there will be no photosynthesis. At low light intensities, as light intensity increases so does the rate of photosynthesis, so light intensity is the

limiting factor. However at higher light intensities, the rate of photosynthesis plateaus. Light intensity is no longer the

limiting factor because changing the light intensity does not affect the rate of photosynthesis. Some other factor, such as carbon dioxide availability, must

now be limiting the process. Increasing the carbon dioxide concentration will increase the

rate of photosynthesis but not indefinitely. - At some point the rate will plateau again as the temperature is

now limiting the process. Increasing the temperature can increase the rate of

photosynthesis, although it will reach a plateau, where some other factor is limiting the process.

At too high a temperature, proteins such as enzymes involved in the Calvin Cycle may be denatured.

The law of limiting factors states that at any given moment, the rate of a metabolic process is limited by the factor that is present at its least favourable value.

The effect of carbon dioxide concentration on the rate of photosynthesis:

Carbon dioxide constitutes about 0.03 – 0.06% of the Earth’s atmosphere 500 million years ago there was 20 times this amount This fell during the carboniferous period, when fossil fuels were made. It rose again in the Triassic and Jurassic then continues to gradually decrease until the beginning of

industrialisation, where it began to slowly rise again, because of the burning of fossil fuels by humans. This releases carbon dioxide that was fixed during the Carboniferous period. The oceans act as carbon sinks and have absorbed about 1/3 of the carbon dioxide produced by human

activities. Growing forests can absorb carbon dioxide from the atmosphere but mature forests produce carbon dioxide

by respiration and decomposition of dead leaves and wood, as much as they take in for photosynthesis. In greenhouses, where many plants are grown in close proximity, even with ventilation the levels of carbon

dioxide drop to around 0.02%. Growers can introduce more carbon dioxide by burning methane or oil fired heaters. Enhanced levels of

carbon dioxide will increase the rate of photosynthesis; provided that no other factor’s is limiting the process.

The effect of light intensity on the rate of photosynthesis:

When light is the limiting factor then the rate of photosynthesis is directly proportional to the light intensity. As light intensity increases, the rate of photosynthesis increases,

Light has three main effects:

It causes stomata to open so that carbon dioxide can enter the leaves It is trapped by chlorophyll where it excites electrons It splits water molecules to produce protons

The electrons and protons are involved in photophosphorylation, producing ATP for the fixation of CO2

The rate of photosynthesis will vary throughout the day as the light intensity increases and then decreases.

The effect of temperature on the rate of photosynthesis:

The photochemical reactions of the light dependent stage of photosynthesis are not very much influenced by temperature.

However, the enzyme-catalysed reactions of the Calvin cycle are. Between 0 - 25 degrees, the rate of photosynthesis approximately doubles for each 10 degree rise 25 degrees + the rate levels off and then falls as the enzymes work less efficiently and as oxygen more

successfully competes for the active site of Rubisco and prevents it from accepting Carbon Dioxide. High temperatures will also cause more water loss from stomata, leading to a stress response in which the

stomata close, limiting the availability of carbon dioxide.

Limiting factors and the Calvin cycle:

Light intensity:

Light intensity gives a measure of how much energy is associated with the light. Light from a source spreads out and so if the distance from the source is doubled the light intensity is quartered. It follows the inverse square law. (l=1/d²)

An increase in light intensity will alter the rate of the light dependent reaction:

More light energy is available to excite more electrons. The electrons take part in photophosphorylation, so increased light intensity means that more ATP and

more NADPH will be produced.

These are both used in the light-independent stage (Calvin cycle), as sources of hydrogen and energy, to reduce the Glycerate phosphate (GP) to triose phosphate (TP)

- ATP is also used to phosphorylate five out of every 6 molecules of TP to regenerate RuBP.

If there is no or very little light available, the light dependent stage will cease. This will stop the light independent stage as it needs the products of the light dependent stage

GP cannot be changed to TP, so GP will accumulate and levels of TP will fall. This will lower the amount of RuBP, reducing the fixation of Carbon Dioxide and the formation of GP.

Carbon dioxide concentration:

An increase in carbon dioxide concentration may lead to an increase in carbon dioxide fixation in the Calvin cycle, provided that light intensity is not the limiting factor.

More carbon dioxide fixation leads to more molecules of GP (some of which may be converted to amino acids or fatty acids) and hence more molecules of TP (to be converted into hexose sugars, disaccharides and polysaccharides, and glycerol) and regeneration of RuBP.

However, the number of stomata that open to allow gaseous exchange leads to increased transpiration and may lead to the plant wilting, if its water uptake from the soil cannot exceed water loss by transpiration.

- This, in turn, leads to a stress response and, following release of a plant growth regulator, stomata close. This will reduce carbon dioxide uptake and reduce the rate of photosynthesis. If carbon dioxide concentration is reduced below 0.01% then RuBP, the carbon dioxide acceptor, will

accumulate- As a result, levels of GP, and subsequently of TP, will fall.

Temperature:

Increasing the temperature will have little effect upon the rate of the light-dependent reaction as, apart from photolysis of water, it is not dependent upon enzymes.

However it will alter the rate of the light independent reaction as each step is catalysed by a specific enzyme

- Increasing temperature will, at first, increase the rate of photosynthesis. As temperature rises above 25 degrees, the oxygenase activity of Rubisco increases more than its

carboxylase activity increases. This means that photorespiration exceeds photosynthesis. As a result ATP and NADPH, from the light dependent reaction, are dissipated and wasted. This reduces the overall rate of photosynthesis. Very high temperatures may also damage proteins involved in photosynthesis. Increased temperatures cause an increase in water loss from leaves by transpiration- This may lead to closure of stomata and subsequent reduction in the rate of photosynthesis.