Biology

• Chloroplasts


  • Small objects inside plant cells that contain chlorophyll and are where photosynthesis takes place
• Chlorophyll

  A green pigment that traps the light energy needed to make photosynthesis happen
• Plants and algae can only carry out photosynthesis in the light
• Parts of a plant cell

  • Cell wall
  • Cell membrane
  • Nucleus
  • Cytoplasm
  • Chloroplasts
  • Vacuole
• Cell wall

  The outside of the cell which provides support and prevents the cell from bursting from the uptake of water
• Cell membrane


  A selectively permeable membrane surrounding the cell that controls the entry and exit of materials
• Nucleus


  The control centre of the cell that controls what happens inside the cell and holds instructions needed to make new cells
• Cytoplasm


  The living substance inside a cell where cell reactions happen
• Chloroplasts


  Contains chlorophyll, a green pigment that traps light energy to make food during photosynthesis
• Vacuole


  A space within the cytoplasm of plant cells that contains cell sap
• Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots. Light energy comes from the Sun. The oxygen produced is released into the air from the leaves.
• The glucose produced can be turned into other substances, such as starch and plant oils, which are used as an energy store. The glucose is also used to release energy through the process of respiration.
• Photosynthesis is really important for animals, including humans because without photosynthesis we wouldn't have food because it converts energy from the sun into chemical energy for the food chains.
• Photosynthesis keeps the levels of oxygen and carbon dioxide in the atmosphere in balance – without it we would very quickly run out of oxygen.
• Photosynthesis is really important for the plant because it provides the plant with food: some of the glucose is used immediately, to give the plant energy in the process of respiration, and some of the glucose is changed into starch and stored in all parts of the plant.
• Adaptations of the leaf

  • Thin
  • Large surface area
  • Contains chlorophyll
  • Stomata
  • Guard cells
  • Network of tubes
• Thin


  Provides a short distance for carbon dioxide to move by diffusion into the leaf
• Large surface area


  It can absorb a lot of light
• Stomata


  Small holes in the underside of the leaf that allow carbon dioxide and oxygen to move in and out of the leaf by diffusion
• Guard cells


  To open and close the stomata depending on the conditions
• Network of tubes


  To transport water and food
• The upper part of the leaf is where the light falls, and it contains many cells called palisade cells. This has many chloroplasts, with lots of chlorophyll to trap as much light as possible. It is shaped like a tall box which helps pack them closely together.
• Plants get the carbon dioxide they need from the air through their leaves. It moves by diffusion through small holes in the underside of the leaf called stomata. Guard cells control the size of the stomata so that the leaf does not lose too much water in hot, windy or dry conditions.
• The lower part of the leaf is a spongy layer with loose-fitting cells. Between the cells in this layer there are 'air spaces' - a bit like a sponge. These allow the gases to diffuse through the leaf.
• Stomata let carbon dioxide enter the leaf, and let the oxygen produced in photosynthesis leave the leaf easily. In many plants, stomata are open during the day and closed at night.
• The water needed for photosynthesis is absorbed through the roots and transported through tubes to the leaf. The roots have a type of cell called a root hair cell. These project out from the root into the soil, and have a big surface area and thin walls. This lets water pass into them easily.
• Root cells do not contain chloroplasts, as they are normally in the dark and cannot carry out photosynthesis.
• Aerobic respiration


  Glucose + Oxygen → Carbon dioxide + Water + Energy
• Plants respire all the time, whether it is dark or light. They photosynthesise only when they are in the light.
• Hydrogencarbonate indicator


  Can detect increases and decreases in carbon dioxide concentration
• Hydrogencarbonate indicator colour changes

  • Red (normal)
  • Yellow (increase in CO2)
  • Purple (decrease in CO2)
• When photosynthesis and respiration happen

  • Bright light (photosynthesis rate > respiration rate, CO2 taken in, O2 given out, indicator turns purple)
  • Dim light (photosynthesis rate = respiration rate, no net gas exchange, indicator stays red)
  • Dark (only respiration, O2 taken in, CO2 given out, indicator turns yellow)
• When the rate of photosynthesis is greater than the rate of respiration, then there is a gain in glucose. Plants that lose their leaves in winter store food produced during the summer by photosynthesis. They store enough food to last them over winter, and to provide energy reserves for new growth in the spring. They are called deciduous plants.
• Testing a leaf for starch

  1. Destarch plant

  2. Stand plant in bright light

  3. Remove leaf and boil in water

  4. Boil in ethanol to remove chlorophyll

  5. Dip in water to soften

  6. Add iodine to test for starch
• If starch is present the iodine will change from yellow-brown to blue-black. If starch is absent the iodine will remain yellow-brown.
• Wear goggles throughout the experiment, Ensure that the ethanol is not exposed to a naked flame during step 5, as it is highly flammable. It should be heated using a hot water bath instead of using a Bunsen burner.
• Showing the need for light in photosynthesis

  1. Destarch plant

  2. Cover part of leaf with lightproof paper/foil

  3. Place plant in bright light

  4. Test leaf for starch
• Only the areas exposed to light tested positive for starch, showing that light is necessary for photosynthesis.
• Showing the need for chlorophyll in photosynthesis

  1. Destarch plant with variegated leaves

  2. Place plant in light

  3. Test leaf for starch
• Only the green areas of the leaf that contained chlorophyll tested positive for starch, proving that chlorophyll is needed for photosynthesis.