plant biology

• Root Hair Cell


  Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis. The absorbed water is transported through the roots to the rest of the plant.
• Xylem


  Xylem vessels are involved in the movement of water through a plant from its roots to its leaves (in one direction only). Water is absorbed from the soil through root hair cells, transported through the xylem vessels up the stem to the leaves, and evaporates from the leaves (transpiration). Xylem contains lignin.
• Phloem


  Phloem vessels are involved in translocation. This is the movement of food substances (in both directions) from the stems to growing tissues and storage tissues.
• Transport in xylem is a physical process. Xylem is composed of hollow tubes. Water flows in one direction. Transport requires energy (comes from companion cell). Xylem transports glucose and amino acids in both directions.
• Photosynthetic Cells


  Cells with lots of chloroplasts are involved in photosynthesis, e.g. palisade cells. Palisade cells are positioned to absorb the maximum amount of sunlight, contain a lot of chloroplasts, and have a large permanent vacuole to support the stem and leaf.
• How plants grow


  1. Zone of cell division

  2. Zone of cell elongation

  3. Zone of cell differentiation
• Meristem


  Growing area of a plant, found in the root and shoot tips.
• Elongate


  In the growing areas, plant cells become longer and grow before they differentiate.
• Redifferentiate


  Plant cells can become a different specialised cell even after they have first differentiated.
• Clone


  A genetically identical copy.
• How the leaf structure is adapted to maximise photosynthesis

  • Large surface area
  • Thin blades
  • Large air spaces between cells
  • Vascular tissue in the midrib and veins
• Stomata


  Plants have stomata to obtain carbon dioxide from the atmosphere and to remove oxygen and water vapour produced in photosynthesis. They are where gas exchange takes place in the plant. The gases move in and out of the stomata via diffusion.
• Stoma


  Each stoma is surrounded by two guard cells.
• How stomata open and close


  1. During photosynthesis, low carbon dioxide levels inside the plant cause guard cells to gain water and become turgid. They curve out, opening the stoma and allowing gases in and out. Water also evaporates through the stomata.

  2. High carbon dioxide levels cause the guard cells to lose water, closing the stoma and preventing further water loss.
• If the stomata were permanently open, the plant would continue to lose water until it dried up.
• Waxy cuticle


  An outer layer on leaves that prevents evaporation from cells.
• Most stomata are located on the underside of leaves, keeping them in the shade. This prevents excess evaporation when the stomata are open.
• When a plant does not have enough water to spare, guard cells cannot become turgid and open the stomata.
• Transpiration


  Xylem tissue transports water and mineral ions from the roots to the stems and leaves. It is composed of hollow tubes strengthened by lignin, adapted for the transport of water in the transpiration stream. The rate of transpiration is increased as the temperature, air movement and light intensity increase.
• Translocation


  Phloem tissue transports dissolved sugars from the leaves to the rest of the plant for immediate use or storage. Phloem is composed of tubes of elongated cells with pores in the end walls. Cell sap can move from one phloem cell to the next through pores in the end walls.
• If the rate of transpiration increases


  The rate of absorption of water by the root increases too
• Factors that affect the rate of transpiration

  • Temperature
  • Humidity
  • Wind speed
  • Light intensity
• Measuring Transpiration Rates


  1. Independent: Time

  2. Dependent: Distance moved by air bubble

  3. Control: draughts, water source, temperature
• Adaptations of plants to reduce transpiration

  • Leaves reduced to spines
  • Reduced number of stomata
  • Waxy leaf cuticle
  • Rolled leaves, leaf hairs, and stomata sunk in pits
• Marram grass grows on sand dunes. Its leaves have adaptations to reduce transpiration in dry, windy conditions.
• Factors that increase the rate of transpiration

  • Temperature
  • Humidity
  • Air flow
  • Light intensity
• Transpiration is more rapid in hot, dry, windy, or bright conditions.
• Investigating Transpiration


  1. Independent: Time

  2. Dependent: Distance moved by air bubble

  3. Control: draughts, water source, temperature
• Contact transmission

  Disease is spread by contact between diseased and healthy plants and insects that act as a vector.
• Airborne transmission


  Fungal spores are spread by the wind to the plants.
• Plant diseases

  • TMV Tobacco Mosaic Virus
  • Rose Black Spot
• Signs of plant disease

  • Unusual growths
  • Spots or discoloured leaves
  • Malformed leaves or stems
  • Presence of pests
  • Stunted growth
  • Areas of decay (rot)
• Ways to identify plant disease

  • Gardening manuals
  • Gardening websites
  • Test kits containing monoclonal antibodies
  • Taking infected plants to a laboratory to identify the pathogen
• Nitrate ions


  Needed in protein synthesis and hence growth.
• Lack of nitrate ions


  Causes stunted growth
• Lack of magnesium


  Causes chlorosis (yellowing of leaves due to lack of chlorophyll)
• Plant defences against pathogens and herbivores

  • Thick waxy layers
  • Tough cellulose cell walls
  • Bark
  • Antibacterial and poisonous chemicals
  • Touch can make Mimosa leaves suddenly curl
  • Thorns
• Photosynthesis


  1. Carbon dioxide + water -> glucose + oxygen (light)

  2. Carbon dioxide from the air, absorbed through the stomata, moved through plant by diffusion

  3. Water from the soil, absorbed into the roots by osmosis, moved via xylem up the stem to the leaves

  4. Oxygen released through the stomata by diffusion

  5. Some used for respiration

  6. Glucose converted to starch for storage, cellulose for structure, fats/oils, proteins with nitrogen
• Chlorophyll

  Green pigment that absorbs light energy
• Factors that may limit the rate of photosynthesis

  • CO2 concentration
  • H2O
  • Light intensity
  • Temperature
  • Chlorophyll