Root Hairs

Extensions of the root epidermis.
They are specially adapted for absorption by:

• Thins walls
  - Easier for osmosis

• Not covered by a cuticle
  - Increased absorption of H₂O

• Numerous (large surface area)
  - Increased absorption of H₂O

Water Uptake by Roots

- Soil is surrounded in a layer of pure water (capillary water).
- The cytoplasm of the root hairs contains dissolved solutes which allow the water to move in by osmosis.
- Water continues to diffuse until it reaches the xylem.

Upward Movement of Water

Upward movement of water depends on two mechanisms, root pressure and transpiration.

Root Pressure

• Water enters the xylem through osmosis.

• Continuous movement of water creates pressure.

• Pressure pushes water up through xylem.

Another process must be involved as it is a weak force.

Transpiration

The loss of water vapour (by evaporation) from the leaves and other aerial parts of the plant.

• Most transpiration occurs through the openings called stomata, water moves from the cells in the leaves into the atmosphere.

• As each water molecule evaporates it pulls another water molecule to replace it. This flow of water is called the transpiration system.

Control of Transpiration

At certain times, it is necessary for plants to reduce their water loss by transpiration.
This can be done by:

• Presence or absence of a cuticle.
  - A cuticle is a waxy layer through which water cannot pass through. It does not cover the stomata on the underside of the leaf.

• Stomata are on the underside of the leaf, away from direct sunlight which reduces transporation as it is cooler.

• Each stoma is surrounded by two guard cells. These guard cells open and close. During the day the stomata are open, which allows for gas exchange during photosynthesis, and during the night they close to reduce water loss.

• During adverse weather, e.g. high temperatures, lack of water, high winds, the stomata will remain closed.

Cohesion - Tension Model of Water Transport in Xylem

- Dixon and Joly

• Cohesion: the sticking together of similar molecule, e.g. water.

• Adhesion: occurs when different molecules stick together, e.g. water and xylem tissue.

  1. Water evaporates from xylem tissue into air spaces in the leaf.
     - As transpiration pulls water from xylem, another water molecule is pulled along with it.
     - This is caused by water’s high cohesion.
     - This process extends down to roots.
     - Transpiration and cohesion creates a pulling force.

  2. Transpiration puts the column of water under tension.
     - Tension is strong enough to pull water to a height of 150m.

  3. Stomata open during the day and transpiration occurs.
     - Constant tension in the water column causes the xylem to narrow by day.
     - Xylem has thick lignified walls.

  4. Transpiration stops at night which creates a lack of tension and xylem returns to original shape.

Mineral Uptake and Transport

Minerals needed for synthesizing plant components:

• Calcium

• Magnesium

• Nitrates

• Phosphates

• Potassium Ions

They enter root hairs, are dissolved in water, enter by diffusion or active transport.
They are transported by xylem.

Active Transport:
Movement of a substance from an area of high concentration to an area of low concentration through a membrance facilitated by a protein. It is an active process, so it requires energy.

Carbon Dioxide Uptake and Transport

Carbon dioxide is needed for photosynthesis.

Two sources of carbon dioxide are:

• External: Carbon dioxide diffuses into air spaces, and then ground tissue where photosynthesis occurs.

• Internal: Carbon dioxide is produced during respiration, and can be used in photosynthesis. Higher temperature increases carbon dioxide production, as there is increased respiration.

Products of Photosynthesis

Oxygen

• Oxygen produced can diffuse into air spaces in the leaf.

• From there it diffuses through stomata into the atmosphere.

• Some oxygen is used in the leaves for respiration.

Glucose

• C₆H₁₂O₆ is a carbohydrate made during photosynthesis.

• Used immediately for respiration.

• Converted to starch for storage.

• Converted to sucrose and transported in the phloem.

Modified Food Storage in Plants

Modified Root

• In dicot plants (carrots, beetroot, turnips) one main tap root is produced. Tap root anchors and absorbs water.

• Root becomes swollen with starch.

Modified Stem

• Potatoes form underground stem systems, where tips of stems became swollen with starch. Swollen tips are called stem tubers.

Modified Leaves

• Garlic and onions produce bulbs - swollen leaves.

• Modified to store food.

• Leaves surround central apical bud, e.g. lettuce, cabbage.

Gas Exchange

Sites of Gas Exchange

Branch and stems → Lenticels

Leaf → Stomata

Gas Exchange - Leaf

• Carbon dioxide diffuses through stomata.

• Millions of stomata.

• Oxygen is diffusing outwards as well.

• Excess water lost through stomata.

• Loss of water vapour is called transpiration.

  

Gas Exchange - The Stem

• Bark does not normally allow gas exchange, however lenticels in the bark allow gas exchange. They appear as tiny blisters on the bark’s surface.

Stomatal Opening and Closing

• Each stomata is surrounded by two guard cells.

• Kidney shaped, inner wall is thicker than the outside.

• Gap between buckled cells after absorbing water through osmosis is called stomata.

High Carbon Dioxide - At night when no photosynthesis is occurring, stomata close as build up of carbon dioxide in the leaf spaces occurs.