Transport of Water and Minerals

• Water is needed for photosynthesis.
• Minerals are soluble in water and are vital for the health of the plant.
  • Magnesium (Mg): Needed for chlorophyll formation.
  • Calcium (Ca): Needed for the formation of the middle lamella.
• Other soluble substances like phosphates and nitrates (needed for DNA replication) are absorbed by roots.

Mineral Uptake:

• Minerals enter the root hairs of plants by active transport, which requires energy.
• Active transport is the movement of substances from an area of low concentration to an area of high concentration against a concentration gradient.
• Root hairs, cortex, and xylem are all involved in the uptake of water and minerals.

Water Uptake:

• The solute concentration in root cells is higher than in the surrounding soil water.
• Water from the soil enters root hairs by osmosis.
• The root hairs are adapted for absorbing water as they have:
  • a large surface area
  • thin walls
  • no cuticle
• Water moves from cell to cell through ground tissue in the root until it reaches the xylem.
• Xylem forms a hollow pipeline that carries water from the roots through the stem and into the leaves.
• Water passes up through the xylem by two mechanisms: root pressure and the cohesion-tension model of water movement.

Root Pressure

• Water is moved by osmosis from the soil into the root hairs.
• This causes a build-up of pressure called root pressure.
• This pressure forces water upwards into the xylem vessels and tracheids.
• However, root pressure alone is not enough to push a column of water to the tops of very high plants.

The Cohesion-Tension Model

• This model, first proposed in 1894 by Henry Dixon and John Joly of Trinity College Dublin, explains the upward movement of water in the xylem.
• Although the exact mechanism isn't fully understood, this is the most widely accepted theory.
• Cohesion is the force of attraction between similar molecules (due to hydrogen bonding).
• Water is highly cohesive, meaning water molecules tend to stick together.
• Adhesion is the force of attraction between different molecules (e.g., water and xylem walls).
• Cohesive forces between water molecules are stronger than adhesive forces between water and xylem walls.
• This balance ensures that water molecules stick together in a continuous column, allowing them to be pulled upward in the xylem rather than sticking to the walls.
• Transpiration is the loss of water vapour from the aerial parts of a plant.
• Transpiration occurs through the air spaces in leaves.
• Each water molecule pulls on the one behind it, creating a force that helps pull water up through the plant.
• This upward movement of water is called the transpiration stream.
• To prevent the xylem from collapsing under pressure, its walls are strengthened by lignin.
• Transpiration is the most important factor in moving water to the tops of tall plants.

Control of Transpiration

• To reduce the rate of transpiration (and prevent the plant from wilting and dying), leaves have a waxy cuticle and stomata.

• The waxy cuticle prevents excessive water loss, indirectly controlling transpiration.

  

• The stomata directly control transpiration.

• Stomata are tiny openings found on the undersurface of leaves.

• Each stoma is surrounded by two guard cells that can change shape to open or close the stoma.

• Stomata open to allow gas exchange and close to prevent excess water loss.

Opening and Closing of Stomata

• Stomata normally open by day and close at night, but light does not control the opening and closing of the stomata.
• Guard cells control the opening and closing:
  • When guard cells become turgid, they curve away from each other because of the uneven thickness of their cell walls.
  • This creates a gap between them (stoma opens).
  • When guard cells lose water, they collapse, and the stoma closes.
• CO₂ levels regulate stomatal activity:
  • Low CO₂ levels (daytime) stimulate the stomata to open.
  • High CO₂ levels (nighttime) stimulate the stomata to close.
• During the day, photosynthesis lowers CO₂ levels as the gas is used up.
• At night, there is no photosynthesis occurring. The carbon dioxide levels increase, due to respiration. These high levels of CO₂ cause the stomata to close.

Excretion in Plants

• Plants produce less waste than animals.
• Oxygen is a by-product of photosynthesis that is excreted through the stomata of leaves.