Photosynthesis Notes - A2 Biology OCR

Overview of photosynthesis

Plants use light energy to build organic molecules from inorganic molecules

o   6CO₂ + 6H₂O à C₆H₁₂O₆ + 6O₂

Organisms that feed this way – self sufficient, not needing organic molecules – are called autotrophs.

Not just plants, but phytoplankton and other bacteria.

Complex metabolic pathway – a series of reactions linked by steps, catalysed by enzymes. Split into two stages taking place in the chloroplast:

o   Light-dependent stage in the thylakoid membrane

o   Light-independent stage in the stroma of the chloroplast

Chloroplast structure

Found especially in the palisade mesophyll and spongy mesophyll tissue of the leaf. (Each plant cell contains 10x chloroplasts). Around 2-10μm long, consisting of double membranes forming an envelope around the organelle. Outside membrane is permeable to small ions. The inside membrane is less permeable but has transport proteins embedded in it, this membrane is folded into:

o   Membranes which form thin plates are called lamellae,

o   Fluid filled sacs called thylakoids

o   Thylakoids are packed into stacks called grana – site of light-dependent stage, and of light adsorption and ATP synthesis.

This is surrounded by ‘background material’ called stroma, which contains prokaryote type ribosomes, starch grains, their own small circular strand of DNA and lipid droplets. (Starch grains are a form of carbohydrate storage in plants). The stroma is also called a ‘fluid-filled matrix’ and the light-independent stage takes place here.

How the Chloroplast adapted to its role

• Inner membrane – controls the movement of substances with transport proteins between the cytoplasm (in cells)ßàThe stroma (in chloroplasts) 

o   Grana – has a large surface area for the light-dependent stage as it consists of hundreds of thylakoids. This means lots of photosynthetic pigments, electron carriers and ATP synthase. The grana is surrounded by the stroma therefore PRODUCTS from the light dependent stage can pass to the light independent stage when needed easily.

o   Stroma – catalyses reactions in the light-independent stage with enzymes.

o   Photosynthetic pigments – arranged as photosystems as it allows maximum adsorption of light energy.

o   Protein synthesis – The chloroplast can make some proteins used in photosynthesis with its chloroplast DNA and chloroplast ribosomes.

Photosynthetic pigments

Pigment – substances whose molecules absorb certain wavelengths (colours) of light. The try to capture as much light as possible.

The other lengths of light are either reflected or transmitted through the substance (the actual colour that hits our eyes/that we can see). Embedded in the membranes are several different kinds of synthetic pigments – coloured substances that absorb energy at certain wavelengths of light. These photosynthetic pigments are found in thylakoid membranes arranged in a funnel shape determined by proteins.

Chlorophyll

Pigments include the most abundant form chlorophyll; its two forms are chlorophyll a and chlorophyll b. They both absorb similar wavelengths of light.

 Chlorophyll a  slightly longer lengths > slightly shorter lengths Chlorophyll b

Consisting of a porphyrin group that contains a magnesium atom (like a haem group), and a long phytol (hydrocarbon) chain.

o   Lights hits chlorophyll, exciting a pair of electrons found in the magnesium head.

o   Two forms of chlorophyll;  P₆₈₀  and P₇₀₀ (both a yellow/green)

o   Both have a red adsorption peak, absorbing red light at slightly different wavelengths.

o   P₆₈₀ is found in photosystem II, absorbing light at wavelength 680nm

o   P₇₀₀ is found in photosystem I, absorbing light at wavelength 700nm

Accessory Pigments

Other pigments include carotenoids= such as carotene and xanthophyll

o   Absorbing blue light, reflecting yellow and orange light.

o   No porphyrin group, not directly involved in light-dependent stage

Light-dependent Stage

Takes place on the thylakoid membrane with

o   Embedded photosystems (PSI and PSII) containing photosynthetic pigments

o   PSI is found in the intergranal lamellae

o   PSII is only found the granal lamellae

 

Pigments trap light energy à to be converted into chemical energy.

Making ATP through Photophosphorylation

When photons (light energy) hits chlorophyll molecule, it excites a pair of electrons in the magnesium by transferring energy from the photon to the electrons.

• These electrons are captured by electron acceptors (ferredoxin) which contain iron atoms

• And passed along a series of electron carriers, which are embedded in the thylakoid membrane.

Energy is released as the electrons pass along the chain of electron carriers. This energy pumps protons across the thylakoid membranes into the thylakoid space from (the stroma) where they accumulate;

• This produces a proton gradient across the thylakoid membrane
• Protons flow down this gradient into the stroma through ATP synthase.

This is called chemiosmosis.

The kinetic energy from the proton flow à is converted into chemical energy in ATP. This is because the force joins ADP + P_i  = ATP, the ATP is then used in the light-independent stage of photosynthesis.

Non-Cyclic Photophosphorylation

This involves both Photosystem I (P₇₀₀) and Photosystem II (P₆₈₀).

1. Light strikes PSII, exciting a pair of electrons that leave the chlorophyll from the primary pigment reaction center.
2. The electrons pass along the electron carriers and the energy released is used to synthesis ATP.
3. Light strikes PSI, and it loses a pair of electrons.
4. These electrons + protons + NADP join to produce reduced NADP.
5. The electrons from PSII replaced the electrons lost from PSI.
6. The electrons from photolysis replace the electrons lost from PSII.
7. Protons from photolysis take part in chemiosmosis to make ATP and are then captured by NADP, in the stroma. They will then take part in the light-independent stage.

Cyclic Photophosphorylation

In Photosystem I (P₇₀₀ ), the excited electrons pass from an electron acceptor back to the chlorophyll.

• No photolysis of water
• No generation of reduced NADP
• But small amounts of ATP

This ATP is transferred to the light-independent stage. Or used in guard cells to bring in K⁺ potassium ions, which in turns lowers water potential, making the guard cells swell up and opening the stomata.

The Role of Water

Photolysis: the splitting of water with light. This occurs in PSII as it contains a special enzyme that will split water in the presence of light into protons, electrons and oxygen.

2H₂O à 4H⁺ + 4e^- + O₂  

o   Hydrogen ions, used in chemiosmosis to produce ATP. They are then accepted by a coenzyme called NADP (nicotinamide adenine dinucleotide phosphate) à turns into Reduced NADP. This reduced NADP is used in the light-independent stage.

o   Electrons, replace those lost by PSII

o   Oxygen is used by the plant for aerobic respiration, but a lot of it diffuses through the stomata out of the leaf.

Light-Independent Stage

Takes place in in the stroma of chloroplast = also known as the Calvin Cycle. Light is not directly used, although the products of the light-dependent stage are. No light? No Calvin cycle.

Carbon dioxide CO₂ is needed for the production or large organic molecules, which are used as energy stored or sources, or for structural means.

The Calvin Cycle

1.     Carbon dioxide, CO₂, from the air diffuses into the leaf through open stomata on the underside of the leaf. CO₂ travels by diffusion through the air spaces through the; spongy mesophyll layer and palisade mesophyll layer. It then diffuses through the cellulose walls, then the cell membrane, through the cytoplasm and then past the chloroplast envelope into the stroma.

airàstomataàair spacesà palisadeàcellulose wallà cytoplasmàchloroplast envelopeàstroma

2.     In the stroma, the CO₂ combines with the 5-carbon compound ribulose biphosphate (RuBP), the commonest enzymes in organisms. RuBP is a carbon acceptor, and the reaction is usually catalyzed by the enzyme rubisco. The RuBP is therefore carboxylated (now having a carboxyl group) –COOH.

3.     When RuBP is carboxylated, it splits into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP). This fixes the carbon dioxide.

4.     The GP is then reduced and phosphorylated into another 3-carbon compound called triose phosphate (TP).       

Reduced= Reduced NADP into NADP Phosphorylated= ATP into ADP and P_i

5.     5 out of 6 molecules of TP (3C) are recycled in phosphorylation to make 3 molecules of RuBP (5C).

Products of the Calvin Cycle

1. GP is used in making amino acids and fatty acids.
2. Pairs of TP molecules combine to form hexose (6C), such as glucose.
3. TP can also be converted into glycerol. They can combined with the fatty acids made from the GP to form lipids.
4. Glucose can then be isomerized into another hexose sugar, fructose.
5. Glucose and Fructose can then combined to form a disaccharide sucrose (translocated in the phloem tubes).
6. The hexose (6C) sugars can be polymerized into other polysaccharides such as cellulose or starch.

Limiting Factors – Photosynthesis

Light Intensity

When light is a limiting factor, the rate of photosynthesis (RoP) is directionally proportional to the light intensity.

• It causes the stomata to open, allowing the carbon dioxide to enter the leaf for the light-independent stage of photosynthesis
• Light is trapped by the chlorophyll where it excites electrons.
• It splits water molecules, through photolysis to produce protons, electrons and oxygen. e^-  and H⁺ is used in photophosphorylation to then produce CO₂ and ATP.

Meaning that the RoP varies throughout the day as light intensity decreases and increases – directionally proportional.

Temperature

The light dependent stage is not very limited by temperature. But the enzyme controlled Calvin cycle is.

• Between 0^oC and 25^oC the RoP doubles for every 10^oC increase in temperature
• At 25^oC and above the RoP levels off and then it falls. This is because the enzymes work less efficiently and O₂ successfully competes for the active site of rubisco, preventing it from binding to CO₂
• Above 25^oC there is more water loss from the stomata, which produces and stress response in which the stomata close limiting the CO₂ availability.

But increasing the temperature increases the enzyme activity and therefore the rate of photosynthesis.

Carbon Dioxide Concentration

Very often THE LIMITING FACTOR in photosynthesis. So, as CO₂ concentration increases, so does the rate of photosynthesis, therefore CO₂ is a limiting factor.

Limiting Factors – Calvin Cycle

Light Intensity

Light intensity gives a measure of the energy associated with light. Light from a source spreads out, so as the distance doubles, the light intensity is quartered. Inverse square law l = 1/d²

An increase in light intensity will alter the rate of the light-independent reaction.

• More light energy available to excited electrons
• These electrons take part in photophosphorylation; therefore an increase in light intensity will result in more ATP and more reduced NADP.
• Both ATP and reduced NADP are used as sources of hydrogen and energy in the light-independent stage (Calvin cycle), to reduced glycerate phosphate (GP) into triose phosphate (TP). ATP also helps phosphorylate 5 out of 6 TP regenerate into RuBP

No or very little light intensity, the light-independent stage will cease. It stops because the light-independent stage needs products of the light-dependent stage.

o   GP cannot be changed into TP, so GP will accumulate and TP levels will fall.

o   Less TP will lower the amount of RuBP, reducing the fixation of carbon dioxide and formation of GP.

Carbon Dioxide concentration

If light is not a limiting factor, then an increase in CO₂ concentration will lead to an increase of CO₂ fixation.

• More carbon dioxide fixation leads to more molecules of GP (which may be converted into amino acids or fatty acids) and hence more molecules of TP (to be converted into hexose sugars, polysaccharides and glycerol) or more regeneration of RuBP.
• The number of stomata that open will allow gaseous exchange leading to increased transpiration and plant wilting if the water uptake does not exceed the water lost from transpiration. This leads to a stress response whereby the plant will release plant growth regulators (abscisic acid) to close the stomata.
• The stomata closing will result into a reduced uptake carbon dioxide, therefore reduced rate of photosynthesis.

Temperature

Although increasing the temperature can increase the rate of photosynthesis from the range of 0^oC to 25^oC, the rate can be reduced if the temperature exceeds 25^oC.

• Temperature has very little effect on the light-dependent reaction, as it does not depend on enzymes.
• An increase in temperature, however, will increase the light-independent reaction and overall photosynthesis, as the Calvin cycle is a series of biochemical steps, each catalyzed by specific enzymes.
• When temperatures rise about 25^oC, the oxygenase activity of rubisco increases more than its carboxylase activity.
• This means that photorespiration exceeds more than photosynthesis.
• As a result; ATP and reduced NADP are wasted and dissipated.

Reducing the rate of photosynthesis

Very high temperatures also;

 damage proteins in photosynthesis, increase water loss from leaves by transpiration, reduce rate of photosynthesis.

 

 

 

Questions and Answers

1.     Q: What evolved first, aerobic respiration or photosynthesis?

A: Photosynthesis. Before photosynthesis there was no oxygen in the air, but photosynthesis released oxygen in the atmosphere, from the photolysis of water. Over a long amount of time the O₂ concentration was large enough to support aerobic respiration.

2.     Q: What wavelength does chlorophyll reflect?

A: Around 550nm, the green part of visible light.

3.     Q: How does the structure of grana enable it to carry out its function?

A: The stacks of thylakoid membranes give a large surface area of for the photosystems to capture light energy, and for the ATP synthase enzymes and the electron transport proteins. The space between the membrane stacks allows the movement of protons in chemiosmosis to generate ATP. There are also proteins in grana which hold the photosystems in place.

4.     Q: How does the structure of the stroma allow the chloroplast to carry out their function?

A: The fluid filled stroma contains enzymes that allow the Calvin cycle to take place. The stroma surround the grana, allowing the products of the light-dependent stage to easily travel to the light-independent reaction. The stroma also contains chloroplast DNA which is used in making proteins the chloroplast needs for photosynthesis, as well as ribosomes to assemble the proteins.

5.     Q: Suggest how the lack of iron in soil can reduce plant growth.

A: Lack of iron may mean fewer electron carriers, called ferredoxin as they contain a iron. This reduces the light-dependent reaction, which in turn leads to less reduced NADP and ATP, both needed in the light-independent stage therefore also reducing this reaction. Overall, less amino acids are synthesized which are needed in making proteins used for plant growth.

6.     Q: Why are there only small amounts of RuBP in the stroma of chloroplasts?

A: Because it is continuously being used and generated in the Calvin cycle.

7.     Q: What is the role of carbon dioxide in photosynthesis?

A: It is the source of carbon that will, when fixed, be used to make larger more complex organic molecules which are carbon based. For example hexose sugars, disaccharides, polysaccharides, amino acids, proteins, fatty acids, lipids etc.

8.     Q: How can the lack of nitrates in the soil effect the growth of a plant?

A: This reduces the production of amino acids, and therefore fewer proteins which are needed for growth. This reduces growth. It also means fewer enzymes, such as rubisco so there is also a reduced rate of photosynthesis.

9.     Q: How can; lack of magnesium, lack of water alter the rate of photosynthesis?

A: Less magnesium means fewer chlorophyll molecules can be made,  a photosynthetic pigment need to absorb light energy needed for the light-dependent reaction. The magnesium in chlorophyll provides the pair of electrons that passes along the photosystem, therefore photophosphorylation decreases, also hindering the light-independent reaction which relies on products from the light-dependent reaction. Photosynthesis would decrease.

Water is a source of hydrogen ions, used to reduce GP into TP, and also acts as an electron donor that provides and pair of electrons to PSII when light excites the chlorophyll. If there is a lack of water in the soil, light-dependent reaction is slowed down, providing less products for the Calvin cycle and overall the reduction in the rate of photosynthesis.

Light-independent stage: is a stage of photosynthesis where carbon dioxide is fixed and used to build complex organic molecules

 

Photophosphorylation: is the making of ATP from ADP and P_i in the presence of light

 

Electron Carriers: are molecules that transfer electrons

 

Electron Acceptors: are chemicals that except electrons from another compound. They are reduced as they are oxsidising agents.

 

Photosynthetic Pigments: are molecules that absorb light energy, each pigment absorbs light at a different wavelength in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected.

 

Autotrophs: are organisms that used light energy or chemical energy and inorganic moles to synthesise complex organic molecules.

 

Heterotroph: are organisms that ingest and digest molecules releasing the chemical potential energy stored in them.

 

Limiting Factor: for a metabolic process is the factor that is present at the lowest or least favourable value.