Photosynthesis

• Photosynthesis is the process that produces glucose in plants.
• Photosynthesis happens in the leaves of all green plants.
• Photosynthesis happens inside the chloroplasts, which are found in leaf cells and in other green parts of a plant.
• Chloroplasts contain a green pigment called chlorophyll, which absorbs sunlight and uses its energy to convert carbon dioxide and water into glucose. Oxygen is also produced.
• Word equation for photosynthesis:

  carbon dioxide + water ---> glucose + oxygen
• Symbol equation for photosynthesis:

  6CO2 + 6H2O → C6H12O6 + 6O2
• Photosynthesis converts light energy to chemical energy, which is stored in the glucose. This chemical energy is released when glucose is broken down during respiration.
• Structure of a leaf:

  A) Upper epidermis

  B) Waxy cuticle

  C) Mesophyll

  D) Palisade mesophyll

  E) Spongy mesophyll

  F) Waxy cuticle

  G) Stoma

  H) Air space

  I) Lower epidermis

  J) Guard cell

  
• Leaves are broad, so there's a large surface area exposed to light.
• Most of the chloroplasts are found in the palisade layer. This is so that they're near the top of the leaf where they can get the most light.
• The upper epidermis is transparent so that light can pass through it to the palisade layer.
• Leaves have a network of vascular bundles - these are the transport vessels xylem and phloem. They deliver water and other nutrients to every part of the leaf and take away the glucose produced by photosynthesis. They also help support the leaf structure.
• The waxy cuticle helps to reduce water loss by evaporation.
• Leaves have adaptations for efficient gas exchange. For example, the lower surface is full of little holes called stomata, which let carbon dioxide diffuse directly into the leaf.
• A plant's rate of photosynthesis is affected by the amount of light, the amount of carbon dioxide, and the temperature of its surroundings.
• The limiting factor is something which stops photosynthesis from happening any faster. Light intensity, CO2 concentration and temperature can all be the limiting factor.
• The limiting factor depends on the environmental conditions.
• Light intensity in photosynthesis:

  1. If the light intensity is increased, the rate of photosynthesis will increase steadily, but only up to a certain point.
  2. Beyond that, it won't make any difference because then it'll either be the temperature or the CO2 level which is now the limiting factor.

  
• Carbon dioxide in photosynthesis:

  1. Increasing the concentration of CO2 will increase the rate of photosynthesis up to a point.
  2. After this the graph flattens out, showing that CO2 is no longer the limiting factor.

  
• Temperature in photosynthesis:

  1. As the temperature increases, so does the rate of photosynthesis - up to a point.
  2. If the temperature is too high (over about 45ºC), the plant's enzymes will be denatured, so the rate of photosynthesis rapidly decreases.
  3. Usually, if the temperature is the limiting factor it's because it's too low, and the surroundings need to warm up.

  
• How to test a leaf for starch:

  1. Dunk the leaf in boiling water (hold it with tweezers or forceps). This stops any chemical reactions happening inside the leaf.
  2. Put the leaf in a boiling tube with some ethanol and heat it in an electric water bath until it boils - this gets rid of any chlorophyll and makes the leaf a white color.
  3. Finally, rinse the leaf in cold water and add a few drops of iodine solution: if starch is present the leaf will turn blue-black.
• If a plant can't photosynthesise, it can't make starch.
• Experiment that shows chlorophyll is needed for photosynthesis:

  1. Take a variegated (green and white) leaf from a plant that's been exposed to light for a bit. Make sure you record which parts are green and which parts aren't.
  2. Test the leaf for starch. Only the parts that were green turn blue-black.
  3. This suggests that only the parts of the leaf that contained chlorophyll are able to photosynthesise and produce starch.
• Experiment that shows CO2 is needed for photosynthesis:

  1. The soda lime will absorb CO2 out of the air in the jar.
  2. If you leave the plant in the jar for a while and then test a leaf for starch, it won't turn blue-black.
  3. This shows that no starch has been made in the leaf, which means that CO2 is needed for photosynthesis.

  A) soda lime

  
• Experiment that shows light is needed for photosynthesis:

  1. Take a plant that's been grown without any light. This will mean that it has used up its starch stores.
  2. Cut a leaf from the plant and test it for starch using iodine solution. The leaf won't turn blue-black.
  3. This shows that light is needed for photosynthesis, as no starch has been made.
• Oxygen production shows the rate of photosynthesis. The faster the rate of oxygen production, the faster the rate of photosynthesis.
• Experiment that shows the rate of photosynthesis with Canadian pondweed:

  1. The gas syringe should be empty to start with. Sodium hydrogencarbonate may be added to the water.
  2. A source of white light is placed at a specific distance from the pondweed.
  3. The pondweed is left to photosynthesise for a set amount of time.
  4. At the end of the experiment, the syringe is used to draw the gas bubble in the tube alongside a ruler and the length of the gas bubble is measured.
  5. The experiment is then repeated with the light source placed at different distances from the pondweed.
• Experiment that shows the rate of photosynthesis with Canadian pondweed:

  A) Small O2 bubbles

  B) Syringe

  C) Capillary tube

  D) water (+ sodium hydrogencarbonate)

  
• Plants need three main mineral ions for growth:

  1. Nitrates.
  2. Phosphates.
  3. Potassium.
• Plants get mineral ions from the soil. If there aren't enough mineral ions in the soil, plants suffer deficiency symptoms.
• Nitrates contain nitrogen for making amino acids and proteins. These are needed for cell growth. If a plant can't get enough nitrates it will be stunted and older leaves will turn yellow.
• Phosphates contain phosphorus for making DNA and cell membranes and they're needed for respiration and growth. Plants without enough phosphate have poor root growth and their older leaves are purple.
• Potassium helps the enzymes needed for photosynthesis and respiration. If there's not enough potassium in the soil, plants have poor flower and fruit growth and discolored leaves.
• Plants need magnesium in small amounts. Magnesium is significant as it's required for making chlorophyll (needed for photosynthesis). Plants without enough magnesium have yellow leaves.
• The cells in all living organisms need a variety of substances to live. They also need to get rid of waste substances.
• In unicellular organisms, substances can diffuse directly into and out of the cell across the cell membrane. The diffusion rate is quick because of the short distances substances have to travel.
• In multicellular organisms, direct diffusion from the outer surface would be too slow - that's because substances would have to travel large distances to reach every single cell.
• Multicellular organisms need transport systems to move substances to and from individual cells quickly.
• Plants have two main transport systems:

  1. Xylem vessels.
  2. Phloem vessels.
• Xylem tubes carry water and mineral salts from the roots up the shoot to the leaves in the transpiration stream.