Biology - B8: Photosynthesis


B8.1 - Photosynthesis

  • carbon dioxide + water —> sunlight energy (chlorophyll) —> glucose + oxygen
  • 6CO2 + 6H2O —> C6H12O6 + 6O2
  • carbon dioxide is taken in from the air through stomata
  • water is taken up by the roots and travels through the plant to leaves, via the xylem
  • chlorophyll is a green pigment which absorbs light and is found in chloroplasts
  • glucose is converted to starch and stored or respired to release the chemical potential energy
  • oxygen is released into the air through the stomata
  • photosynthesis is an endothermic reaction
  • this means that it takes in energy from its surroundings, in this case in the form of light energy, which is the extra energy required for the reaction to take place
  • in photosynthesis, light energy is converted into chemical energy (glucose)
  • plants are well adapted for maximum photosynthesis (see next card)
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B8.1 - Photosynthesis (adaptations)

  • the whole leaf : most leaves are thin and very flat —> big surface area for absorption of light and short diffusion distance for gases
  • waxy cuticle : in evergreens, the upper surface is covered with a layer of wax —> acts as a waterproofing layer to reduce water loss through stomata on the top surface
  • upper epidermis : a single layer of transparent cells near the surface —> enables light to enter the leaf easily
  • palisade cells : long, thin and packed with chloroplasts —> can absorb lots of light as cells are tightly packed and full of chloroplasts
  • spongy mesophyll cells : irregular in shape with lots of air spaces between them —> speeds up the diffusion of gases into the cells by providing space for them
  • veins/vascular bundle : contain xylem vessels and phloem tubes which are continuous with those in the stem and root —> xylem brings plenty of water to the cells and phloem removes the products of photosynthesis
  • lower epidermis : have lots of tiny pores called stomata which are opened and closed by guard cells —> opens and closes to control entry of carbon dioxide and loss of water from the leaf
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B8.2 - The rate of photosynthesis

  • the rate of photosynthesis is affected by TEMPERATURE (enzymes), LIGHT INTENSITY, CARBON DIOXIDE and CHLOROPHYLL
  • the factor in the shortest supply will stop the rate of photosynthesis increasing, this is called the limiting factor
  • only one factor can limit the rate at any one time
  • to find out if a factor is limiting the rate of photosynthesis, add more of it, and if the rate increases, it was limiting, but if the it has no effect on the rate, it was not limiting
  • the limiting factor at the first upwards slope of a line graph is always the x-axis title
  • LIGHT INTESITY = 1/d2 (one over distance squared)
  • the light intensity is dependent upon the distance from the source of the light
  • as the distance of the light from the plant increases, the light intensity decreases
  • by doubling the distance, light intensity is quartered (this is the inverse square law)
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B8.3 - How plants use glucose

  • Plant and algal cells use the glucose produced during photosynthesis for...
  • RESPIRATION : all living cells respire constantly, some of the glucose is broken down using oxygen to provide energy for the cells
  • TO CONVERT INTO INSOLUBLE STARCH FOR STORAGE : glucose is soluble in water so if it were stored in plant cells it could affect osmosis, and lots of it might affect the water balance of the whole plant. Starch is insoluble so large amounts of it can be stored in plant cells with no effect on the water balance of the plant. Starch is also the main energy store in plants and is found in the cells of the leaves, stems and roots and in special storage areas (such as tubers and bulbs, to help the plant survive the winter)
  • TO PRODUCE FATS OR OILS FOR STORAGE : plants and algae use some of the glucose to build up fats and oils which may be used in the cells as an energy store, sometimes used to strengthen cell walls and often used as an energy store in seeds
  • TO PRODUCE CELLULOSE TO STRENGTHEN CELL WALLS : plants build up glucose into complex carbohydrates such as cellulose, used to strengthen their cell walls
  • TO PRODUCE AMINO ACIDS FOR PROTEIN SYNTHESIS : sugars (glucose) are combined with nitrate ions and other mineral ions from the soil to make amino acids which are built up into proteins to be used in the plant cells in many ways, including as enzymes. Some carnivorous plants get the nutrients they need from the insects they eat instead of from the soil, so can survive in soil with few or no minerals (e.g. bogs)
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B8.4 - Making the most of photosynthesis

  • In modern greenhouses, growers use their knowledge of photosynthesis to maximise plant growth. They artificially control the factors that limit photosynthesis, meaning certain plants (including tropical ones) can be grown out of season.
  • Plants need sunlight, carbon dioxide and water for photosynthesis, and a source of oxygen for respiration. Both reactions happen faster in warmer temperatures.
  • The factors that limit the rate of photosynthesis interact and any one of them may limit the rate of photosynthesis
  • Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis, while still maintaining profit
  • Features of a greenhouse for maximum photosynthesis...
  • made of glass : lets sunlight in for photosynthesis
  • artificial lights : provide light energy for photosynthesis when there is not enough from the sun
  • windows/fan : cool the greenhouse in summer, so optimum temperature for photosynthesis
  • heater : heat the greenhouse in winter, so optimum temperature for photosynthesis
  • watering can/sprinkler system : provides water for the plants which they need for photosynthesis, and of course, to survive
  • CO2 cylinder : provide plenty of carbon dioxide for plants to photosynthesise
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B8.3 - How plants use glucose (testing for starch)

  • The presence of starch in a leaf is evidence that photosynthesis has taken place
  • You can show that light is vital for photosynthesis to take place, by using iodine solution to test for starch
  • Take a leaf from a plant kept in the light and a plant kept in the dark for at least 24 hours
  • Just adding iodine solution to a leaf does not work, as the waterproof cuticle keeps it out, and the green chlorophyll would mask any colour changes even if it did react with the starch
  • The leaves must be specially prepared by boiling them in ethanol to destroy the waxy cuticle and then to remove the colour
  • The leaves are then rinsed in hot water to soften them
  • After treating the leaves, add iodine solution to them both
  • The leaf that has been kept in the light will turn blue-black, as starch is present
  • The iodine solution on the leaf kept in the dark remains orange-red
  • ethanol is highly flammable so heat it in a hot water bath - never use a Bunsen burner as naked flames would be very dangerous (but be careful with hot water)
  • always wear eye protection when using ethanol or iodine solution
  • wash hands thoroughly at the end of the experiment
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