nutrition

• Photosynthesis


  The process by which plants use light energy to produce carbon compounds, such as carbohydrates
• Energy conversion in photosynthesis


  Conversion of light energy into chemical energy
• Carbon dioxide is a reactant in photosynthesis; it combines with water to produce glucose and oxygen
• Chloroplasts


  • Absorb light energy during photosynthesis
• Chlorophyll, a green pigment found inside chloroplasts, absorbs energy from sunlight for photosynthesis
• Plants are known as producers because they produce their own food
• Glucose use in plant cells

  • Respiration
  • Storage as starch
  • Production of cellulose cell walls
  • Conversion into lipids and amino acids
• Photosynthesis is needed for the production of proteins because it produces glucose, which can be combined with soil minerals to produce amino acids. Amino acids are used to build proteins
• Photosynthesis


  Carbon dioxide + water → glucose + oxygen
• Role of water in photosynthesis


  Water is a reactant in photosynthesis. It reacts with carbon dioxide to produce glucose and oxygen
• Light energy is required for photosynthesis, but it is not a chemical substance and so is not considered to be a reactant in photosynthesis
• The products of photosynthesis are glucose and oxygen
• Oxygen is produced as a waste product and released from leaves via the stomata. Note that some of this oxygen may be used by the plant for respiration
• Chlorophyll is not a reactant, and so is not used up during the photosynthesis reaction. The role of chlorophyll is to absorb light energy
• 6 molecules of water and 6 molecules of carbon dioxide are needed to produce a single glucose molecule
• The photosynthesis equation is the reverse of the equation for aerobic respiration
• Photosynthesis chemical equation


  6CO2 + 6H2O → C6H12O6 + 6O2
• Limiting factor


  A factor that limits the rate of photosynthesis
• Main limiting factors for photosynthesis


  • Temperature
  • Light intensity
  • Carbon dioxide concentration
• Water is not considered a limiting factor in photosynthesis; this is because plants need relatively little water for photosynthesis, and a lack of water also influences a plant's ability to absorb light and carbon dioxide
• Photosynthesis occurs slowly at low temperatures


  Molecules (such as enzymes involved in photosynthesis) have little kinetic energy, meaning fewer successful collisions take place
• Rate of photosynthesis only increases with temperature up to a certain point, after which higher temperatures lead to a decrease in the rate of photosynthesis
• At very high temperatures the enzymes that control photosynthesis denature, meaning that the reaction can no longer take place
• In a graph of light intensity against rate of photosynthesis, the level part of the graph shows a point at which a factor other than light intensity is limiting, e.g. temperature or carbon dioxide concentration
• The higher the light intensity


  The faster the rate of photosynthesis
• The higher the carbon dioxide concentration


  The faster the rate of photosynthesis
• In a graph of carbon dioxide concentration against rate of photosynthesis, the part of the graph where the line is horizontal is a point at which a factor other than carbon dioxide concentration is limiting. This factor could be temperature or light intensity
• Gas exchange in plants

  Carbon dioxide diffuses into leaves while oxygen diffuses out via the stomata
• The waxy cuticle is a waterproof layer that reduces water loss by evaporation from the surface of leaves
• Palisade mesophyll layer


  • Contains tall, thin cells that pack together closely and contain many chloroplasts to maximise light absorption
• The air spaces in the spongy mesophyll allow the outer surfaces of many cells to come into direct contact with the air, maximising the available surface area for gas exchange
• Role of guard cells


  Can change shape to open or close the stomata, allowing plants to control the rate of water loss from leaves
• Vascular bundle


  Contains xylem and phloem for transport of useful substances, such as water, around the plant
• Phloem vessels carry sucrose and amino acids around inside plants. It is the xylem vessels that carry water and dissolved minerals
• Leaf structure


  • Broad to maximise surface area for light absorption, and thin to reduce diffusion distance for gas exchange
• The palisade mesophyll layer is located just beneath the waxy cuticle and upper epidermis, ideally located for light absorption
• Plants make carbohydrates during photosynthesis. They can convert carbohydrates into other substances, but may need additional mineral ions from the soil to produce certain molecules
• Difference between proteins and carbohydrates

  Proteins contain nitrogen, carbohydrates do not
• Essential mineral ions required by plants


  • Nitrates
  • Magnesium ions
• How plants obtain mineral ions

  Active transport from the soil into root hair cells