Plants and photosynthesis

• Photosynthesis:

  Carbon dioxide + water -> glucose + oxygen
• Chloroplasts contain green pigment, chlorophyll, which traps light energy. Chlorophyll absorbs light energy for the reaction.

  Within the cells, the chemical reactions are controlled/catalysed by enzymes.
• Use of glucose:

  • Starch - storage
  • Fats & oils - storage
  • Cellulose - cell walls
  • Proteins - growth and repair
  • Respiration - release energy
• Testing leaf for starch:

  1. Boiling water to kill the leaf to stop photosynthesis- breaking down the cellulose cell wall.
  2. Boil leaf in ethanol to extract chlorophyll.
  3. Rinse leaf in hot water to soften as alcohol dehydrates.
  4. Spread leaf on white tile and blot dry - able to examine leaf and stops iodine diluting.
  5. Add a few drops of iodine to test for starch - brown to blue/black.
• Sodium hydroxide removes carbon dioxide.

  Vaseline is used to air tight - gas can’t enter or escape.
• Only parts of the leaf exposed to light will carry out photosynthesis and produce glucose, stored as starch.
• Factors affecting the rate of photosynthesis:

  • Light intensity
  • Carbon dioxide concentration
  • Temperature
• If there isn‘t enough of one factor, it limits the rate of photosynthesis.

  If you increase a limiting factor, the rate increases - after a certain point, something else becomes the limiting factor.

  If temperature is too high, the enzymes denature - optimum temperature is fastest rate.
• Water is used in photosynthesis and travels to leaves, but most evaporates and diffuses out of leaves via pores on underside of leaf, called stomata.

  The loss of water vapour from stomata of leaves is transpiration.
• Water is needed to transport minerals for healthy growth and support - water fills vacuoles, keeping cells turgid, pushing against cell walls and prevents cells becoming flaccid and wilting.
• Water

  Soil water is more dilute than root hair cell sap - water moves into root hair cell by osmosis.
• Minerals

  They are at a lower concentration in the soil - move from low concentration inti root hair cells where minerals are in higher concentration, against concentration gradient - taken up by active transport.
• Nutrient deficiencies:

  • Nitrates - poor growth
  • Potassium - yellowing of leaf
  • Phosphate - poor root growth

  NPK fertilisers (used by farmers) provide these nutrient.
• Stomata: Small holes in the leaf that allow gases to pass through the leaf.

  Two guard cells with a hole in the middle - stomata.

  Inner cell wall is thicker than the outer to allow opening and closing.

  OPEN - allows O2 and CO2 in, allows water to escape via transpiration.

  CLOSED - no gas exchange, reduces water loss by transpiratio.
• Xylem: A specialised tissue that transports water and mineral ions from the roots to the leaves (up the plant).
• Phloem: transports sucrose (sugar) from the leaves (photosynthetic parts) to the rest of the plant where it is needed for respiration or to be stored.
• Transpiration: The loss of water vapour from a plant by diffusion from the stomata of leaves.

  Water evaporating through the stomata causes more water to be pulled up the xylem vessels.
• Factors affecting the rate of transpiration:

  • Light intensity - increases - stomata opens wider allows more CO2 into leaf for photosynthesis.
  • Temperature - increases - evaporation and diffusion are faster at higher temperature.
  • Wind - increases - moving air removes water vapour, increasing rate of diffusion of water vapour from leaf.
  • Humidity - decreases - decreases concentration gradient between inside and outside leaf, reduces transpiration.
• Potometer

  Can be used to measure rate of transpiration- measures how far water moves in a certain time - uptake of water.

  Rate of transpiration = Distance water moves / Time
• Plant shoot in a simple potometer:

  1. Single air bubble is introduced into the capillary tubing.
  2. Tap on reservoir is opened to add water to push the air bubble back to zero on the scale.
  3. The distance the air bubble had travelled along the scale is recorded - measuring millimetre.
  4. This measures how long it takes the bubble to move to a certain point.
  5. Repeat within time limit, but with different condition.
  6. Once all results have been recorded, compare them.
• The faster the air bubbles move, the greater the rate of water uptake, and so the greater the rate of transpiratio.
• Pondweed - Experiment to show the effect of light intensity on photosynthesis:

  1. Place lamp at set distance from pondweed.
  2. Record number of bubbles of oxygen produced over a set time.
  3. Repeat three times.
  4. Repeat this experiment with different distances (at least five), repeating each three times.
  5. Use results to calculate of photosynthesis.