Plant nutrition and transport

• photosynthesis is the process by which plants convert light energy into chemical energy in the form of glucose.
• Photosynthesis happens in the leaves of all green plants and happens inside the chloroplast. Chloroplasts contain a pigment called chlorophyll which absorbs sunlight and converts CO2 and H2O into glucose.
• carbon dioxide + water > glucose + oxygen
• 6CO2 + 6H2O → C6H12O6 + 6O2
• Leaves are adapted for efficient photosynthesis.

  
• • leaves are broad so there are a large surface area exposed to light
  • most of the chloroplasts are found in the palisade layer, that's so they are near the top of the leaf where they can get the most light.
  • the upper epidermis is transparent so that light can pass through the palisade layer.
• • leaves have a network of vascular bundles (xylem and phloem). they deliver water and other nutrients to the plant and take away the glucose produced by photosynthesis. They also help to support leaf structure.
  • the waxy cuticle helps to reduce water loss by evaporation
  • stomata allows CO2 to diffuse directly into the leaf.
• The rate of photosynthesis depends on the light intensity, temperature, and carbon dioxide concentration.
• • chlorophyll uses light energy to preform photosynthesis, if the light intensity is increased, the rate of photosynthesis will increase steadily but only up to a certain point
  • beyond that it won't make a difference because either the temperature or the CO2 will now become the limiting factor.
• • CO2 will also increase the rate of photosynthesis
• the temperature will increase the rate of photosynthesis until 45 degrees as the enzymes will be denatured so the rate of photosynthesis rapidly decreases.
• • A test for starch in a leaf - dunk the leaf in boiling water, to stop any chemical reactions happening inside the leaf.
  • Now 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 chlorophyll and makes the leaf a whitish colour)
  • 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.
• The starch test shows whether photosynthesis is taking place. If a plant can't photosynthesis, it can't make starch. Use a variegated leaf (white and green) as the white with no chlorophyll will not photosynthesise thus not able to produce starch.
• You can also show that CO2 is needed for photosynthesis by using a bell jar and a soda lime that will absorb CO2 in the jar, if you leave the plant in the jar and the test it, it won't turn blue/black showing the no starch has been made and that CO2 is needed for photosynthesis.
• Plants need three main mineral ions for growth: nitrates, phosphates and potassium. they get these elements from mineral ions in the soil. If there aren't enough mineral ions, 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 phosphates have poor root growth and their older leaves are purple.
• Potassium helps the enzymes that are needed for photosynthesis and respiration. If there's not enough potassium in the soil, plants have poor flower and fruit production and discoloured leaves.
• Magnesium is needed in small amounts.

  magnesium is needed for making chlorophyll which is needed for photosynthesis. Plants without magnesium have yellow leaves.
• unicellular organisms, these substances can diffuse directly in and out of the cell, the diffusion is quick because of the short distances substances have to travel.
• Multicellular organisms direct diffusion would be slower as substance have to travel a large distance. So they need transport systems to move substances to and from individual cells quickly.
• Plants have two main transport systems: xylem tubes and phloem tubes.
• Xylem tubes transport water and mineral ions from the roots to the leaves (in the transpiration).
• Phloem tubes transport food. They transport sugars like sucrose and amino acids from where they are made in the leaves to other parts of the plant. This is called translocation.
• This is a vascular bundle inside the stem.

  
• Transpiration is the loss of water from the leaves of a plant through the stomata. Its caused by evaporation and diffusion from a plant's surface.
• Evaporation creates a slight shortage of water in the leaf so more water is drawn up from the rest of the plant through the xylem vessels to replace it. this means more water is drawn up from the roots and so there's a constant transpiration of water through the plant.
• There are 4 factors that affect the rate of transpiration: light intensity, temperature, wind speed and humidity
• Light intensity- the brighter the light, the greater the rate of transpiration. Stomata close as it gets darker, photosynthesis can't happen in the dark so they don't need to be open to let CO2 in. When the stomata is closed, very little water can escape.
• Temperature - the warmer it is, the faster transpiration happens. When it's warmer the water particles have more energy to evaporate and diffuse out of the stomata.
• Wind speed - the higher the wind speed around a leaf, the greater the transpiration rate. If wind speed is low, water vapour just surround the leaf and doesn't evaporate.
• Humidity - the drier the air around a leaf, the faster transpiration happens. If the air is humid there's a lot of water in the air already so there's not much difference between the inside and outside of the leaf. Diffusion happens fastest if there's a large difference in the concentration gradient.
• A potometer can be used in estimating transpiration rate.