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Storing and Releasing Energy

Why is energy important?

Plant and animal cells need energy for biological processes to occur. Plants need energy for photosynthesis, active transport, DNA replication and cell division. Animals need energy for muscle contration, maintaining body temperature, active transport, DNA replication and cell division.

Plants are autotrophs- they can make their own food. Photosynthesis is the process where energy from light is used to make glucose from water and carbon dioxide.

6CO2 + 6H2Reaction (light energy) ( C6H12O6 + 6O2

Energy is stored in the glucose until the plant releases it by respiration. 

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Storing and Releasing Energy

Animals are heterotrophs- they can't make their own food. So, they obtain glucose by eating plants and other animals, the repire the glucose to release energy.

Respiration and energy

Plant and animal cells release energy from glucose. This energy is used to power all the biological processes.

Aerobic respiration is using oxygen and anaerobic respiration is without oxygen

Aerobic respiration produces carbon dioxide and water:

C6H12O6 + 6O2    →    6CO2 + 6H2O + Energy

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ATP (Adenosine triphosphate) is the immediate source of energy in a cell. A cell can't get its energy directly from glucose. So, in respiration, the energy released is used to make ATP. ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups. 

ATP is synthesised for ADP and inorganic phosphate using energy from the breakdown of glucose. The energy is stored as chemical energy in the phosphate bond. The enzyme ATP synthase catalyses this reaction (ATP + Pi --> ATP).

This process is known as phosphorylation- adding phosphate to a molecule. ADP is phosphorlyated to ATP.

ATP then diffuses to the part of the cell that needs energy. Here, it is broken down into ADP and Pi. Chemical energy is released and used by the cell.

ATPase catalyses this reation (ATP + H2O --> ADP + Pi).

This is known as hydrolysis. 

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Properties of ATP

ATP has specific properties that make it a good energy source.

  • ATP stores or releases only a small, manageable amount of energy at a time, so no energy is wasted. 
  • It is a small, soluble molecule so it can be easily transported around the cell. 
  • It is easily broken down, so energy can be quickly released.
  • It can transfer energy to another molecule by transferring one of its phosphate groups.
  • ATP can't pass out of the cell, so the cell always has an immediate supply of energy. 

ATP doesn't make energy- it is a store of energy.

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Light-dependent Reaction (Chloroplasts)


Photosynthesis takes place in the chloroplasts of the plant cells. Chloroplasts are small, flattened organelles found in plant cells. They have a double membrane called the chloroplast envelope. Thylakoids are stacked up in the chloroplast into structures called grana. The grana are linked together by lamellae. 

Chloroplasts contain photosynthetic pigments (e.g. Chlorophyll a, chlorophyll b and carotene). These are coloured substances that absorb light energy. The pigments are found in the thylakoid membranes and they are attached to proteins. The protein and pigment is called photosystem. 

A photosystem contains two photosynthetic pigments- primary and secondary pigments. Primary pigemts are reaction centres where electrons are excited during the light-dependent stage. Accessory pigments transfer light energy. Photosystem 1 (PS1) absorbs light at 700 nm and photosystem 2 (PS2) absorbs light at 680nm. 

The stroma surround the thylakoids. It contains enzymes, sugars and organic acids. Carbohydrates produced by photosynthesis are stored as starch in the stroma. 

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Chloroplast adaptations

The structure of a chloroplast is adapted for photosynthesis in many ways:

  • The chloroplast envelope keep the reactants for photosynthesis close to their reaction sites.
  • The thylakoids have a large surface area to allow as much light energy to be absorbed as possible. 
  • Lots of ATP synthase molecules are present in the thylakoid membranes to produce ATP in the light-dependent reaction. 
  • The stroma contains all the enzymes, sugars and organic acids for the light-independent reaction to take place
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Redox reaction

Redox reactions

These are reactions that involve oxidation and reduction and occur in photosynthesis.

  • If something is reduced it has gained electrons, and may have gained hydrogen or lost oxygen.
  • If something is oxidised it has lost electrons, and may have lsot hydrogen or gained oxygen.
  • Oxidation of one molecule always involves reduction of another molecule.


A coenzyme is a molecule that aids the function of an enzyme. They work by transferring a chemical group from one molecule to another. A coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to another- this means it can reduce or oxidise a molecule. 

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The Stages of Photosynthesis

1) The light-dependent reaction 

This reaction takes place in the thylakoid membrane and requires light energy. The light energy is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy. The light energy is used to add a phosphate group to ADP to form ATP, and to reduce NADP to form NADPH2. ATP transfers energy and NADPH2 transfers hydrogen to the light-independent reaction. During the process water is oxidised to oxygen. 

2) The light-independent reaction ( The Calvin Cycle)

This reaction does not use light energy directly. It takes place in the stroma of the chloroplast. Here, the ATP and NADPH2 from the light-dependent reaction supply thr energy and hydrogen to make glucose from carbon dioxide. 

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The light-dependent reaction

In the light-dependent reaction, the light energy absorbed by thr photosystem is used for three things:

1) Making ATP from ADP and Pi. This is called photophosphorylation.

2) Making NADPH2 from NADP.

3) Splitting water into protons, electrons and oxygen. This is called photolysis- it is the splitting of a molecule using light energy.

The light-dependent reaction actually includes two types of photophosphorylation- non cyclic and cyclic. Each of these processes has differnet products. 

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Non-cyclic Photophosphorylation

Non-cyclic photophosphorylation produces ATP, NADPH2 and oxygen. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain.

1) Light energy excites electrons in chlorophyll
   Light energy is absorbed by PS2. The light energy excites electrons in chlorophyll. The electrons move to a higher energy level. These hight-energy electrons move along the electron transport chain to PS1.

2) Photolysis of water produces protons, electrongs and oxygen
   As the excited electrons from chlorophyll leave PS2 to move along the electron transport chain, they must be replaced. Light energy splits water into protons, electrons and oxygen.

3) Energy from the excited electrons makes ATP
   The excited electrons lose energy as they move along the electron transport chain as the energy is used to transport protons into the thylakoid. The creates a proton gradient so they move into the stroma, via ATP synthase. 

4) Light energy is absorbed by PS1, electrons are excited and transfers to NADP to form NADPH2

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Cyclic Photophosphorylation

Cyclic photophosphorylation produces ATP and only uses PS1. The chlorophyll aren't passed onto NADP, but are passed back to PS1 via electron carriers. This means that the electrons are recycled and can repeatedly flow through PS1.

ATP is formed by the movement of protons across the thylakoid membrane. This happens by chemiosmosis, where the movement of protons generates ATP.

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Light-independent reaction (The Calvin Cycle)

The Calvin cycle takes place in the stroma of the chloroplasts. It makes a molecule called triose phosphate from carbon dioxide and ribulose bisphosphate. Triose phosphate can be used to make glucose and other useful substances.

1) Formation of glycerate 3-phosphate
   Carbon dioxide enters the leaf through the stomata and diffuses intot the stroma of the chloroplats. It then combines with ribulose bisphosphate (RuBP), a 5- carbon compound. This creates an unstable 6-carbon compouns, which quickly breaks down into two molecules of a 3-carbon compound glycerate 3-phosphate (GP). Ribulose bisphosphate carboxylase (rubisco) catalyses the reaction.

2) Fromation of triose phosphate
   GP is then reduced to a differnet 3-carbon compound called triose phosphate (TP). ATP provides the energy for this reaction. The hydrogen ions come from NADPH2 and then NADP is recycled to NADP. Triose phosphate is then converted into many useful organic compounds, e.g. glucose.

3) Regeneration of ribulose bisphosphate (RuBP)
   5 out of every 6 molecules aren't used to make useful organic compounds, but to regenerate RuBP. This uses the rest of the ATP.

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Hexose Sugars

A hexose sugar is a monosaccharide that 6 carbon atoms, e.g. glucose. One hexose sugar is made by joining two molecules of TP together. Hexose sugars can be used to make larger carbohydrates.

The Calvin cycle needs to turn 6 times to make one hexose sugar. This reason for this is that three turns of the cycle produce 6 molecules of triose phosphate because two molecules of TP are made for every one CO2 molecule used. 5 out of 6 of these TP molecules are used to regenerate RuBP. This means that for 3 turns of the cycle only one TP is produced that's it used to make a hexose sugar.

As a hexose sugar has 6 carbons, two TP molecules are needed to form one hexose sugar. This means the cycle must turn six times to make one hexose sugar. Six turns of the cycle needs 18 ATP and 12 NADPH2 from the light-dependent stage.

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Carbohydrates, Lipids and Amino Acids

The Calvin is the starting points for making all the organic substances a plant needs. TP and GP molecules are used to make carbohydrates, lipids and amino acids.

  • Carbohydrates- hexose sugars are made from two TP molecules and larger carbohydrates (e.g. sucrose, starch, cellulose) are made by joining hexose sugars together in differnet ways.
  • Lipids- these are made using glycerol, which is synthesised from TP, and fatty acids, which are synthesised from GP.
  • Amino acids- some amino acids are made from glycerate 3-phosphate.
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Limiting Factors in Photosynthesis

Optimum conditions for photosynthesis

1) High light intensity of a certain wavelength
   Light is needed to provide the energy for the light-dependent reaction- the higher the intensity of the light, the more energy it provides. Only certain wavelenghts of light are used for photosynthesis. The photosynthetic pigments chlorophyll a, chlorophyll b and carotene only abosrb the red and blue light in sunlight.

2) Temperature around 25°c 
   Photosynthesis involves enzymes. If the temperature falls below 10°c the enzymes become inactive, but if the temperature is more than 45°c they may start to denature. Also, at high temperature stomata close to avoid losing too much water.

3) Carbon dioxide 0.4% and water
   Carbon dioxide makes up 0.04% of the gases in the atomsphere. Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher adn the stomata start to close. Plants also need a constant supply of water- too little and photosynthesis has to stop but too much and the soil becomes waterlogged.

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