Chapter 9: Transport in Flowering Plants

Transport/ Vascular bundle (tissue)

• Xylem Vessel:

  • Role

    • Transporting water and mineral salts, from roots to the stems and leaves.

    • Providing mechanical support within the plant.

  • Is a long hollow space or lumen in the vessel is continuous.

  • Vessel is a dead cell structure - its wall is strengthened by a substance called lignin.

  • Adaptations:

    • A continuous lumen without partition wall or protoplasm - continuous flow without hindrance.

    • Walls lignified to prevent collapse of the vessel. - vessels provide mechanical support.

• Phloem

  • Role:

    • Conducts manufactured food (sucrose and amino acid) from leaves to other parts.

  • Adaptations:

    • Consists of sieve tubes and companion cells.

    • Through sieve tubes transport of manufactured food occurs by diffusion and active transport.

  Internal structure of Plant Organs:

• stem

  • Vascular bundles - arranged in a ring round a central region.

  • Stem is covered by a layer of cells called epidermis - protected by cuticle.

• roots

  • Cortex of the root is the storage tissue.

  • Each root hair is a tubular outgrowth of an epidermal cell.

Translocation of Organic Food Substances:

Translocation occurs when phloem vessel transport amino acids and sucrose.

• Test:

• Using aphids:

  • leave aphids on leaf of the plant and give it anaesthesia to make unconscious.

  • after some time, dissect the body of aphid a liquid will exude from the cut. Analyse the liquid.

  • analysis will show that it contains sucrose and amino acids.

• Using isotopes

  • a leaf to be fed with CO2 containing the radioactive carbon.

  • after photosynthesis, sugars will contain radioactive carbon.

  • later expose the stem on a X-ray photographic film. It is found that the radioactive substances are present in the phloem.

Entry of Water into the plant:

• Absorption starts to take place in roots.

• Each root hair grows b/w soil and with close contact of surrounding water.

• Water enters the root through osmosis; due to concentration difference in root sap and soil.

Absorption of dissolved mineral salts in roots:

• Through diffusion and active transport; membrane in the root hair cell does not allow mineral salts to pass son diffusion and active transport works.

• Mineral intake by diffusion occurs when there is more mineral concentration in soil water then in roots (concentration difference).

• Mineral intake by active transport occurs when there is higher mineral concentration in roots then is soil solution. It requires energy which is given by photosynthesis in plant.

Adaptation of root hair cell for absorption:

• Long and narrow - increase in surface area: volume ratio.

• Cell sap contains sugars, amino acids and salts. It is more concentrated than soil solution; this results in water entering through osmosis.

• Since root hair cell is living so it provides energy for active transport.

Moving water against the gravity:

• Water is transported up by root pressure and transpiration pull.

• Root pressure is when the water from the living cells enter into the xylem vessels by osmosis and its starts flowing upwards.

• Transpiration is the loss of water vapour from stomata of plant leaf to the environment.

  NOTE: water is lost from inside the leaf not from the leaf surfaces, since cuticle is covering is covering the leaf surfaces.

• The mesophyll of the leaf has numerous intercellular spaces among the mesophyll cells. Water continuously moves out of mesophyll cells to form a thin film of moisture over their surfaces.

• From the wet surfaces, water evaporates.

• Importance of transpiration:

  • As water evaporates from the leaves it removes latent heat, thereby cooling the leaves and preventing them from being scorched by the hot sun.

  • The suction force due to transpiration is the main factor in lifting water and dissolved mineral salts up the plant from the roots to the leaves. In this way, the leaves are supplied with sufficient water and salts for photosynthesis.

• Factors affecting rate of transpiration:

  • Humidity of air - more humid the air, slower the rate of transpiration, assuming all other factors are constant.

  • Temperature of air - higher the temperature, high rate of transpiration.

  • Wind - stronger the wind, higher rate of transpiration.

  • Light - high light intensity, higher rate of transpiration.