Biology

• Autotrophs


  Plants that make their own food by photosynthesis
• Photosynthesis


  1. Light energy trapped by chlorophyll

  2. Converted into chemical energy

  3. Used in synthesis of carbohydrates from carbon dioxide and water

  4. Oxygen released as by-product
• Word equation for photosynthesis

  Carbon dioxide + water → Glucose + oxygen
• Chemical equation for photosynthesis

  6CO2 + 6H2O → C6H12O6 + 6O2
• Starch formation and use

  1. Excess glucose is changed to starch for storage

  2. In darkness, starch is changed back to glucose for use as respiratory substrate
• Testing a leaf for starch


  1. Leaf removed from plant exposed to sunlight

  2. Boiled in water to kill cells and burst starch grains

  3. Chlorophyll extracted with alcohol

  4. Leaf tested with iodine solution - blue-black colour indicates presence of starch
• Experiments are done to find out whether each factor (carbon dioxide, light and chlorophyll) is necessary for photosynthesis
• Destarching a plant


  Plant placed in dark for 2 days to break down starch to glucose
• Control experiment contains all the conditions necessary for photosynthesis
• Experiment to show importance of light for photosynthesis

  1. Destarched plant with leaf sandwiched between black paper strips

  2. Exposed to sunlight for 2 hours

  3. Starch test shows only exposed part of leaf turns blue-black
• Experiment to show importance of CO2 for photosynthesis

  1. Two destarched plants placed in bell jar

  2. One with CO2 absorbed, one with normal air

  3. Leaf from CO2-free plant remains yellow, control plant leaf turns blue-black
• Experiment to show importance of chlorophyll for photosynthesis


  1. Destarched variegated leaf exposed to sunlight

  2. Only green parts containing chlorophyll turn blue-black in starch test
• Experiment to show release of oxygen during photosynthesis

  1. Aquatic plant placed under inverted funnel in water with sodium hydrogen carbonate

  2. Gas bubbles formed in sunlight tested and found to be oxygen
• Conditions needed for photosynthesis are carbon dioxide, light, chlorophyll and water
• Factors affecting rate of photosynthesis

  • Temperature
  • Water availability
  • Light intensity
  • Carbon dioxide concentration
  • Chlorophyll
• Limiting factor


  Any factor that directly affects the rate of a process if its quantity is changed
• As light intensity increases

  Rate of photosynthesis increases until another factor becomes limiting
• Increasing temperature from 20°C to 30°C


  Increases rate of photosynthesis but not greatly
• Increasing CO2 concentration from 0.03% to 0.13%

  Causes a large increase in photosynthetic rate, indicating CO2 is the true limiting factor
• Experiment to show relationship between photosynthesis and respiration

  1. Hydrogen carbonate indicator used to detect changes in CO2 concentration

  2. In bright light, indicator turns purple as photosynthesis rate exceeds respiration rate

  3. In dark, indicator turns yellow as only respiration is occurring
• Experiment to show effect of different light intensities on rate of photosynthesis

  1. Aquatic plant in dilute sodium hydrogen carbonate solution

  2. Number of oxygen bubbles released counted at different distances from light source

  3. Graph plotted of rate of bubbling against distance from light
• Glucose is used by cells during respiration to provide energy, and is also used to make cellulose
• Photosynthesis


  Release of CO2 from leaf into hydrogen carbonate indicator changes red to yellow
• Effect of different light intensities

  • On rate of photosynthesis
• Water bath (keep temperature constant)
• Apparatus set up

  1. Dilute NaHCO3 solution

  2. Aquatic plants

  3. Lamp (low bulb)

  4. Light intensity changed by bringing light source closer to beaker (e.g. 40 cm, 30 cm, 20 cm)

  5. Nearer the light source, higher light intensity
• Light constant
• Procedure


  1. Lamp placed 50 cm from plant

  2. Air bubbles given out from cut end of plant

  3. Plant allowed to adapt to new conditions before taking readings

  4. Number of air bubbles counted over 5 minutes

  5. Repeat and obtain mean value

  6. Plot graph of rate of bubbling against distance of lamp
• Glucose


  Used by cells during respiration to provide energy<|>Used to make cellulose cell walls<|>Converted to sucrose which is transported to storage organs like seeds, stem and root tubers and stored as starch<|>Reconverted by enzymes in leaves to simple sugars at night
• Glucose use

  1. Reacts with nitrates to form amino acids

  2. Amino acids combine to form proteins used to make new cells
• Most forms of life depend on photosynthesis as it converts light energy to chemical energy stored in carbohydrates, and produces oxygen for respiration
• Leaf structure


  • Leaf lamina - broad, flat surface to absorb maximum light
  • Midrib and lateral veins provide mechanical support and contain xylem and phloem
  • Xylem transports water and minerals, phloem transports sucrose and amino acids
• Internal leaf structure

  • Cuticle - transparent, protects from water loss
  • Upper epidermis - protects from mechanical damage, no chloroplasts
  • Palisade mesophyll - main site of photosynthesis, contains many chloroplasts
  • Spongy mesophyll - contains chloroplasts but fewer, has large intercellular air spaces
  • Lower epidermis - has more stomata than upper epidermis, no chloroplasts
  • Guard cells - regulate gas exchange, contain chloroplasts
• Stomatal opening

  1. In sunlight, K+ ions increase in guard cells, lowering water potential, causing guard cells to swell and open stomata

  2. At night, K+ ions diffuse out, increasing water potential, causing guard cells to become flaccid and close stomata
• CO2 entry into leaf

  CO2 diffuses from external environment into intercellular air spaces, dissolves in moisture on mesophyll cell surfaces, then diffuses into cells
• Water and mineral entry into leaf

  Xylem transports water and minerals from roots, water diffuses from xylem into mesophyll cells
• Mineral nutrition

  • Nitrogen - essential for protein, protoplasm, enzymes and nucleic acids, lack causes poor growth
  • Magnesium - essential component of chlorophyll, lack causes chlorosis and poor growth
• Chlorophyll


  Green pigment that traps and absorbs light energy, converts it to chemical energy
• Chloroplast


  Organelle in plant cells that contains chlorophyll and enzymes for photosynthesis
• Effect of temperature on photosynthesis

  1. Enzymes work faster at higher temperatures, increasing rate of photosynthesis

  2. But higher temperatures can also cause stomatal closure and enzyme denaturation, decreasing rate of photosynthesis