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  • Created by: Zoe
  • Created on: 13-05-13 20:16



  • Plants and animals need energy required for biological processes.
  • Plants need energy for photosynthesis, active transport via the roots, DNA replication, Cell division and protein synthesis.
  • Animals need energy for things like muscle contraction, maintenance of body temperature, active transport, DNA replication, cell division and protein synthesis


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  • ATP is the immediate source of energy in a cell.
  • It carries energy around the cell to where it is needed. It does this by diffusion.
  • ATP is synthesised from ADP and inorganic phosphate using energy from an energy releasing reaction. The energy is stored in the phosphate bond, and when the ATP reaches the area of the cell where it is needed, it is broken down into ADP and inorganic phosphate which releases the energy from the phosphate bond.
  • ADP and inorganic phosphate are then re-used.


  • ATP stores or releases only small, managable amounts of energy at a time, therefore no energy is wasted
  • Its a small, soluble molecule so can be easily transported around the cell
  • Easily broken down, so energy is released easily
  • Transfer energy to other molecules by transferring one of its phosphate groups
  • It can't pass out of the cell, so is always an immediate source
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Key Words

  • Metabolic pathway- a series of small reactions controlled by enzymes
  • Phosphorylation- adding phosphate to a molecule
  • Photophosphorylation- adding a phosphate group using light
  • Photolysis- the splitting of a molecule using light energy
  • Hydrolysis- the splitting of a molecule using water
  • Decarboxylation- the removal of carbon dioxide from a molecule
  • Dehydrogenation- the removal of hydrogen from a molecule
  • Redox reaction- reaction that involves oxidation and reduction
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  • Chloroplasts are the site of photsynthesis.
  • Each chloroplast is surronded by two plasma membranes
  • A system of membranes is found inside the chlorplast. These membranes form flattened sacs called Thylakoids and in some places these thylakoids form stacks called grana.
  • Membranes join the grana together and produce a large surface area for chlorophyll molecules and other light absorbing pigments.
  • The pigments form clusters called photosystems which catch the light energy and pass it from one pigment to the next until it finally reaches a chlorophyll molecule.
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  • Animals need a supply of complex orgainc molecules provided by food (other animals but mainly plants) to provide the chemical potential energy needed to build new cells and tissues.
  • It is only by photosynthesis that light energy can be converted into chemical potential energy and only by this process can simple inorganic molecules such as Carbon Dioxide gas and water be built up into organic ones.
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The Light Dependent reaction

  • Light Strikes a chlorphyll molecule in photosystem II and the energy levels of 2 electrons rise and leave the chlorphyll molecule where they are passed on to an electron carrier
  • The electrons then pass to an electron acceptor which in turn passes down a series of electron acceptors making up the electron transfer chain (ETC).
  • As the electrons pass along the ETC they lose there energy, and it is this energy that is used with inorganic phosphate and ADP to produce a molecule of ATP.
  • Photolysis of water then takes place, as light enegy causes the water molecule to split

    H2O à 2H+ + 2e- + ½ O2

  • The electrons replace those that are lost in photosystem I, and the oxygen is given off as a waste product.
  • Light then strikes a chlorphyll molecule in photosystem I and the electrons leave the chlorophyll molecule to pass along another ETC, they are then used along with the protons produced by the splitting of water and a molecule of NADP to produce reduced NADP which is used in the next process of photosynthesis.
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The Light Independent reaction (Calvin Cycle)

  • Carbon dioxide is accepted by RuBP to form two molecules of GP
  • GP is then converted to TP in a reduction reaction, and requires the two substances, ATP and reduced NADP produced in the cycle before.
  • Triose Phosphate (TP) is a carbohydrate, some of it is built up into other carbohydrates such as glucose and starch, also amino acids and lipids. The rest is used to make Ribulose Phosphate.
  • Ribulose Phosphate is finally converted to RuBP by taking a phosphate group from another molecule of ATP.
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Limiting Factors

Environmental factors that affect the rate of photosynthesis are:

  • Light Intensity
  • Carbon dioxide concentration
  • Temperature
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Efficiency of photosynthesis

  • Only a small part of light energy is converted into chemical potential energy during photosynthesis.
  • 15% of the visible- light energy is reflected
  • 30% is transmitted which means it passes directly through the plant without striking a chlorophyll molecule.
  • Around 50% is lost as heat
  • The rate at which a plant is able to produce organic substances as a result of photosynthesis is called gross productivity.
  • Some of the substances formed during photosynthesis are not however used to form new cells and tissue, they are used in respiration and the difference between the gross productivity and respiration is net productivity, and it is net productivity that can tell us how much food is avaliable to the next organism in the next trophic level.

Net Productivity= gross productivity - respiration

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