plants and photosynthesis

• Osmosis - the diffusion of water through a selectively permeable membrane from a region of high water concentration to an area of low water concentration.
• Transpiration -

  1. Water is lost from leaf by evaporation
  2. This process is called transpiration
  3. The cycle of water moving through the plant
  4. It is important to keep the water flowing through the plant to get it where it is needed
  5. Guard cells can close the stomata if too much water is being lost
• Active transport- movement of particles against the concentration gradient using ATP
• Nitrates - needed for healthy growth, defficiency leads to poor growth
• Potassium - needed for photosynthesis, deficiency leads to yellowing around the edge of leaf due to lack of chlorophyll
• Phosphates - needed for DNA and cell membranes, deficiency displays poor root growth
• Photosynthesis is the way green plants and other photosynthetic organisms use chlorophyll to absorb light energy and convert carbon dioxide and water into glucose, producing oxygen as a bi-product
• Importance of photosynthesis


  • Plant life depends on it as a source of food
  • All animals rely on plants for food, either directly or indirectly, and photosynthesis makes that food
  • Photosynthesis produces oxygen which is necessary for respiration
  • Early plant life added oxygen to the atmosphere, which allowed it to sustain life as we know it
• What plants need to survive

  • Carbon dioxide needed for photosynthesis
  • Oxygen in the day, plants make more oxygen in photosynthesis than they need for respiration. At night, photosynthess stops and plants need to obtain oxygen from the air
  • Light energy for photosynthesis
  • Water-needed for photosynthesis and other living processes
  • Minerals needed for a range of living processes; nitrates are needed to make proteins from the glucose made in photosynthesis
• The process of photosynthesis

  Carbon dioxide + water -> glucose + oxygen
• Requirements for photosynthesis


  • Carbon dioxide-Glucose is made of carbon, hydrogen and oxygen. The carbon dioxide provides the carbon and oxygen
  • Water-This provides the hydrogen needed to make glucose. The oxygen from the water molecules is not needed and is given off as a waste product
  • Light-This provides the energy for the chemical reactions in photosynthesis
  • Chlorophyll - The green pigment in chloroplasts is chlorophyll, which absorbs the light to provide the energy for photosynthesis
• All the chemical reactions involved in photosynthesis are controlled by enzymes, which are available in the chloroplasts of the photosynthesising cells. For photosynthesis to work, the temperature must be suitable for those enzymes to work.
• Experimental techniques related to photosynthesis


  • Experiments that show the need for light or carbon dioxide in photosynthesis
  • Experiments that investigate the effect of various factors on the rate of photosynthesis
• Testing for starch


  1. The leaf to be tested is dipped in boiling water to kill the cells and melt the waxy covering (the cuticle) so that liquids can soak into the leaf

  2. The leaf is then placed in boiling alcohol. This removes the green colouring so that the test's colour change can be seen. This is done in a boiling water bath, not by direct heat, because alcohol is flammable

  3. The leaf is dipped in water briefly (the alcohol makes the leaf brittle and the water softens it)

  4. Brown iodine solution is dripped onto the leaf. A blue-black colour indicates starch
• When doing experiments about the formation of starch in photosynthesis, it is important that we know that the starch was formed during the experiment, not before. To ensure this the plant is de-starched before the experiment by putting it in a dark cupboard for 24 houn. Any stored starch will then be used up to feed the plant as it will not be able to photosynthesise.
• Testing the need for light


  Part of a leaf is covered up with foil or black paper and then the leaf is tested for starch after about 24 hours. The part which was covered will not contain starch, but the rest of the leaf will.
• Removing carbon dioxide and control experiments


  1. Carbon dioxide is removed from one leaf by adding sodium hydroxide, which absorbs it, to the flask. The other leaf is a control experiment with just water instead of sodium hydroxide.

  2. If the leaf without carbon dioxide does not contain starch, and the control leaf does, we can be certain it is due to the absence of carbon dioxide.
• Using sensors and data loggers


  • Electronic sensors can detect and measure oxygen and carbon dioxide. The sensor needs to be connected to a data logger, which records and stores information about the level of the gas over time.
• Testing with iodine simply shows that photosynthesis has taken place: it does not measure the rate of photosynthesis.
• Factors affecting the rate of photosynthesis


  • Light intensity
  • Level of carbon dioxide
  • Temperature
  • Water
• Limiting factor


  The factor that is more important than the others in setting the rate of photosynthesis
• Limiting factors


  • Light (at night and dawn/dusk)
  • Temperature (on cold winter days)
  • Carbon dioxide (whenever light and temperature are not limiting)
• Use of glucose by the plant


  • It may be used directly in respiration to provide energy for the plant
  • It may be transported to other parts of the plant (particularly the growing points in the stem and the roots). To do this, it is changed into sucrose
  • It may be stored by conversion to starch or oils
  • It can be transformed into cellulose for cell walls, or into proteins for growth. To make proteins, the plant will need a supply of nitrogen from nitrates in the soil
• Adaptations of leaf structure


  • Adaptations to absorb light and get the water and carbon dioxide needs for photosynthesis
  • Ways of avoiding too much water loss
• Parts of the leaf structure


  • Cuticle
  • Upper epidermis
  • Palisade layer
  • Spongy layer
  • Lower epidermis
  • Stomata
  • Guard cells
  • Veins
• Functions of leaf structures


  • Cuticle - Waxy, waterproof layer that reduces water loss, transparent to allow light through
  • Palisade layer - Cells packed with chloroplasts for photosynthesis
  • Spongy layer - Contains large air spaces, allowing carbon dioxide to reach the palisade cells
• Plant


  • Adaptations to absorb light and get the water and carbon dioxide needed for photosynthesis
  • Ways of avoiding too much water loss
• The leaf needs to be open to the atmosphere so it can take in carbon dioxide, but that means it is bound to lose some water as well
• Cuticle


  Waxy, waterproof layer that reduces water loss
• Upper epidermis


  Transparent, allowing light through to the lower layers of cells, which contain chloroplasts
• Palisade layer


  Cells packed with chloroplasts for photosynthesis
• Spongy layer


  Contains large air spaces, allowing carbon dioxide to reach palisade cell for photosynthesis, but the cells here also contain chloroplasts for photosynthesis
• Vein


  Contains xylem and phloem (transports sugar away)
• Guard cells


  Open and close the stomata, allowing carbon dioxide to enter or preventing water loss
• Water is lost through the leaf pores (stomata) when they are open
• Leaf adaptations to avoid water loss


  • Stomata close at night when no photosynthesis occurs, so no carbon dioxide is needed
  • Leaf has a waxy outer covering, the cuticle, which is waterproof and stops water being lost from any part of the leaf apart from the stomata
• Cells


  Mostly water, and the chemicals in them need to be dissolved in water in order to carry out the reactions that make up life
• Minerals


  Enter the plant via the roots, and water provides the medium that transports them up the plant
• Water


  Transports dissolved sugar away from the leaves
• Turgid cells


  Full of water, rigid and keep the plant upright