nutrition

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• Edexcel IGCSE Biology: Double Science
• Contents

  • 2.18 The Process of Photosynthesis
  • 2.19 Photosynthesis Equations
  • 2.20 Factors Affecting the Rate of Photosynthesis
  • 2.21 Leaf: Structure & Adaptations
  • 2.22 Plants & Mineral Ions
  • 2.23 Practical: Investigating Photosynthesis
  • 2.24 Balanced Diet
  • 2.25 Sources & Functions of Dietary Elements
  • 2.26 Variation in Energy Requirements
  • 2.27 Human Alimentary Canal: Structure & Function
  • 2.28 Peristalsis
  • 2.29 Role of Digestive Enzymes
  • 2.30 Bile: Production & Storage
  • 2.31 Bile: Function
  • 2.32 Small Intestine: Structure & Adaptations

  
• Photosynthesis

  An endothermic reaction in which energy from sunlight is transferred to the chloroplasts in green plants

  
• Chlorophyll

  A green pigment found inside chloroplasts that absorbs energy from sunlight
• Photosynthesis

  1. Energy from sunlight is absorbed by chlorophyll

  2. Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water

  3. Oxygen is made and released as a waste product

  
• Photosynthesis

  The process by which plants manufacture carbohydrates from raw materials using energy from light

  
• Plants

  • Autotrophs - they can make complex molecules (glucose) from simple molecules (carbon dioxide and water)
  • Producers - they can make their own food and so are the first organism at the start of all food chains
• Products of photosynthesis

  • Glucose for respiration
  • Starch for storage
  • Lipids for energy in seeds
  • Cellulose for cell walls
  • Amino acids for proteins

  
• If asked for the raw materials required for photosynthesis, the answer is carbon dioxide and water. Although required for the reaction to take place, light energy is not a substance and therefore cannot be a raw material.

  
• Photosynthesis equation


  Carbon dioxide + Water → Glucose + Oxygen

  
• The photosynthesis equation is the exact reverse of the aerobic respiration equation so if you have learned one you also know the other one! You will usually get more marks for providing the balanced chemical equation than the word equation.

  
• Limiting factor


  Something present in the environment in such short supply that it restricts life processes
• Factors limiting the rate of photosynthesis


  • Temperature
  • Light intensity
  • Carbon dioxide concentration

  
• Increasing temperature


  Increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products
• Higher temperatures


  Enzymes that control the processes of photosynthesis can be denatured, reducing the overall rate of photosynthesis

  
• Increasing light intensity


  Increases the rate of photosynthesis until another factor becomes limiting
• Increasing carbon dioxide concentration


  Increases the rate of photosynthesis until another factor becomes limiting

  
• Chloroplasts


  Contain the pigment chlorophyll which absorbs light energy for photosynthesis

  
• The number of chloroplasts or the amount of chlorophyll in the chloroplasts can also affect the rate of photosynthesis

  
• Interpreting graphs of limiting factors can be confusing for many students, but it's quite simple. In the section of the graph where the rate is increasing (the line is going up), the limiting factor is whatever the label on the x axis (the bottom axis) of the graph is. In the section of the graph where the rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on the x axis - choose from temperature, light intensity or carbon dioxide concentration.
• Leaf structures and functions


  • Epidermis - protects the leaf
  • Palisade mesophyll - contains many chloroplasts for photosynthesis
  • Spongy mesophyll - allows gas exchange
  • Vascular bundles - transport water, minerals and food
  • Guard cells - control stomata for gas exchange

  
• Adaptations of plant leaves for photosynthesis


  • Thin, flattened shape to maximise surface area
  • Transparent epidermis to allow light penetration
  • Palisade cells packed with chloroplasts
  • Spongy mesophyll with air spaces for gas exchange
  • Stomata to allow carbon dioxide and oxygen to diffuse in and out

  
• Mineral ions required by plants


  • Nitrogen - for proteins and chlorophyll
  • Magnesium - for chlorophyll
  • Phosphorus - for nucleic acids and energy transfer
  • Potassium - for enzyme activation and water regulation
  • Iron - for chlorophyll synthesis
• Mineral deficiencies


  Affect plant growth and photosynthesis
• Practical: Evolution of oxygen


  1. Use a water plant like Elodea or Cabomba

  2. Oxygen gas produced during photosynthesis is released as bubbles from the cut end of the plant

  3. Collect the gas in a boiling tube and test it by relighting a glowing splint

  
• Although plants synthesise glucose during photosynthesis, their leaves cannot be tested directly for the presence of glucose.

  
• Experiment to show the evolution of oxygen from a water plant


  1. Take a bundle of shoots of a water plant

  2. Submerge them in a beaker of water underneath an upturned funnel

  3. Fill a boiling tube with water and place it over the end of the funnel

  4. As oxygen is produced, the bubbles of gas will collect in the boiling tube and displace the water
• Testing the leaf for starch


  1. Drop the leaf in boiling water

  2. Transfer the leaf into hot ethanol in a boiling tube for 5−10 minutes

  3. Rinse the leaf in cold water

  4. Spread the leaf out on a white tile and cover it with iodine solution
• Leaf covered with aluminium foil


  Does not receive any sunlight and could not photosynthesise
• Leaf exposed to sunlight


  Turns blue-black as photosynthesis is occurring
• Photosynthesis cannot occur in sections of the leaf where light cannot reach the chloroplasts

  
• Care must be taken when carrying out this practical as ethanol is extremely flammable
• The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame
• CORMS evaluation


  • C - Changing variable
  • O - Organism/object
  • R - Repeating
  • M - Measurement
  • S - Standardising
• Investigating the requirement for carbon dioxide in photosynthesis


  1. Destarch the plant by placing it in a dark cupboard for 24 hours

  2. Enclose 1 leaf with a conical flask containing potassium hydroxide

  3. Enclose another leaf with a conical flask containing no potassium hydroxide (control experiment)

  4. Place the plant in bright light for several hours

  5. Test both leaves for starch using iodine solution

  
• Leaf from conical flask containing potassium hydroxide


  Remains orange-brown as it could not photosynthesise due to lack of carbon dioxide
• Leaf from conical flask not containing potassium hydroxide


  Turns blue-black as it had all necessary requirements for photosynthesis