B8 - Photosynthesis

• Photosynthesis


  The process by which plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen
• Photosynthesis


  • It is an endothermic reaction
  • It needs an input of energy from the environment
• Photosynthesis


  Carbon dioxide + water light, glucose + oxygen
• Plants and algae need food for respiration, growth, and reproduction
• Plants don't need to eat - they can make their own food by photosynthesis
• Photosynthesis takes place in the green parts of plants (especially the leaves) when it is light
• Algae can also carry out photosynthesis
• Chloroplasts


  Small green parts in the cells of algae and plant leaves that contain chlorophyll
• Photosynthesis


  1. Energy is transferred from the environment to the chloroplasts by light

  2. This energy is then transferred to convert carbon dioxide and water into glucose

  3. Oxygen is also produced as a by-product
• Plants produce about 368,000,000,000 tonnes of oxygen every year
• Leaves


  • Broad to give a large surface area for light
  • Thin so diffusion distances for gases are short
  • Contain chlorophyll in chloroplasts to absorb light
  • Have veins to transport water and remove products
  • Have air spaces to allow gas exchange
  • Have guard cells to regulate gas exchange
• Algae are adapted to photosynthesising in water by having a large surface area and absorbing carbon dioxide dissolved in the water
• Path of carbon atom


  From carbon dioxide in the air to being part of a starch molecule in a plant
• Leaf kept in light for 24 hours

  Turns iodine solution blue-black
• Leaf kept in light for 24 hours then dark for 24 hours


  Has no effect on iodine solution
• Endothermic reaction


  Energy is transferred from the environment to the chloroplast by light
• Limiting factors for photosynthesis

  • Light intensity
  • Temperature
  • Carbon dioxide concentration
• Light intensity increases


  Rate of photosynthesis increases
• Temperature increases


  Rate of photosynthesis increases up to 40-50°C, then decreases as enzymes are denatured
• Carbon dioxide concentration increases

  Rate of photosynthesis increases
• In a controlled environment, carbon dioxide is no longer the limiting factor
• In an outdoor environment, light intensity, temperature, and carbon dioxide concentration interact and any one may be the limiting factor
• Chlorophyll levels


  If limited, less photosynthesis will take place
• Plants use some of the glucose produced in photosynthesis for respiration
• Plants convert some of the glucose into cellulose to strengthen cell walls
• Plants convert some of the glucose into starch for storage
• Starch is insoluble and doesn't affect the plant's water balance
• Respiration


  1. Glucose is broken down using oxygen

  2. Provides energy for cells

  3. Carbon dioxide and water are waste products

  4. Chemically, respiration is the reverse of photosynthesis
• Cellulose


  For strength
• Starch


  For storage
• Energy transfer in respiration

  1. Used to build smaller molecules into bigger molecules

  2. Plants build up glucose into complex carbohydrates such as cellulose

  3. Used to strengthen cell walls
• Plants convert glucose to starch

  1. Glucose is soluble in water

  2. Storing glucose could affect water balance

  3. Starch is insoluble in water

  4. Starch is the main energy store in plants
• Where starch is stored in plants

  • In cells of leaves
  • In special storage areas like tubers and bulbs
• Testing for starch


  1. Boil leaf in ethanol to remove cuticle and chlorophyll

  2. Add iodine solution

  3. Blue-black colour indicates presence of starch
• The presence of starch in a leaf is evidence that photosynthesis has taken place
• Making amino acids and proteins

  1. Plants combine sugars with nitrate ions and other minerals

  2. Algae take nitrate ions and other materials from water
• Very few plants can survive well if soil is low in minerals
• Carnivorous plants


  Obtain nutrients from animals they catch and digest
• Making lipids


  1. Plants and algae use glucose and energy from respiration

  2. Fats and oils used as energy store or to strengthen cell walls

  3. Stored in seeds as energy for germination
• Some algal cells are very rich in oils and are being considered as a possible source of biofuels