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
• Role of chloroplasts in photosynthesis

  To absorb light energy
• Chlorophyll, a green pigment found inside chloroplasts, absorbs energy from sunlight for photosynthesis
• Plants are known as producers because they produce their own food
• Ways plant cells can use glucose


  • Respiration
  • Storage, in the form of 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
• Word equation for photosynthesis

  carbon dioxide + water → glucose + oxygen
• Role of water 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
• Products of photosynthesis


  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
• Molecules needed to produce one glucose molecule


  6 molecules of water and 6 molecules of carbon dioxide
• The photosynthesis equation is the reverse of the equation for aerobic respiration
• Balanced chemical symbol equation for photosynthesis


  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
• Why rate of photosynthesis slows at low temperatures

  Molecules (such as enzymes involved in photosynthesis) have little kinetic energy, so 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
• Why very high temperatures stop photosynthesis


  The enzymes that control photosynthesis denature, meaning 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
• Effect of increasing light intensity on photosynthesis rate


  The higher the light intensity, the faster the rate of photosynthesis. This continues until another factor becomes limiting
• Effect of increasing carbon dioxide concentration on photosynthesis rate


  The higher the carbon dioxide concentration, the faster the rate of photosynthesis. This continues until another factor becomes limiting
• 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
• How gas exchange occurs 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
• Adaptation of palisade mesophyll layer


  Contains tall, thin cells that pack together closely, with many chloroplasts to maximise light absorption
• The air spaces allow the outer surfaces of many cells in the spongy mesophyll to come into direct contact with the air. This maximises 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
• Benefits of broad, thin leaves


  Being broad maximises surface area for light absorption, and being thin reduces diffusion distance for gas exchange
• The palisade mesophyll layer is located just beneath the waxy cuticle and upper epidermis. This means that it is 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
• Essential mineral ions required by plants


  • Nitrates
  • Magnesium ions
• Nitrates are needed for the production of amino acids, which are joined together to build proteins
• How plants obtain mineral ions

  By active transport from the soil into root hair cells