Transport In Plants - Form 2 High School Biology

Absorption of Water and Mineral Salts

Absorption of Water

- Root hair cell has solutes in the vacuole and hence a higher osmotic pressure than the surrounding soil water solution.
- Water moves into the root hair cells by osmosis along a concentration gradient.
- This makes the sap in the root hair cell to have a lower osmotic pressure than the surrounding cells.
- Therefore water moves from root hair cells into the surrounding cortex cells by osmosis.
- The process continues until the water gets into the xylem vessels.

Absorption of Mineral Salts

- If the concentration of mineral salts in solution is greater than its concentration in root hair cell, the mineral salts enter the root hair cell by diffusion.
- If the concentration of mineral salts in the root hair cells is greater than in the soil water, the mineral salts enter the root hairs by active transport.
- Most minerals are absorbed in this way.
- Mineral salts move from cell to cell by active transport until they reach the xylem vessel.
- Once inside the xylem vessels, mineral salts are transported in solution as the water moves up due to root pressure, capillary attraction and cohesion and adhesion forces.

Transpiration

Definition of Transpiration

- Transpiration is the process by which plants lose water in the form of water vapour into the atmosphere.
- Water is lost through stomata, cuticle and lenticels.

Types of Transpiration

a) Stomatal transpiration

- This accounts for 80-90% of the total transpiration in plants.
- Stomata are found on the leaves.

b) Cuticular transpiration

- The cuticle is found on the leaves, and a little water is lost through it.
- Plants with thick cuticles do not lose water through the cuticle.

c) Lenticular transpiration

- Is loss' of water through lenticels.
- These are found on stems of woody plants.
- Water lost through the stomata and cuticle by evaporation leads to evaporation of water from surfaces of mesophyll cells.
- The mesophyll cells draw water from the xylem vessels by osmosis.
- The xylem in the leaf is continuous with xylem in the stem and root.

Structure and Functions of Xylem

- Movement of water is through the xylem.
- Xylem tissue is made up of vessels and tracheids.

a) Xylem Vessels

- Xylem vessels are formed from cells that are elongated along the vertical axis and arranged end to end.
- During development, the cross walls and organelles disappear and a continuous tube is formed.
- The cells are dead and their walls are strengthened by deposition of lignin.
- The lignin has been deposited in various ways.
- This results in different types of thickening:

1. Annular.
2. Simple spiral.
3. Double spiral.
4. Reticulate.

- The bordered pits are areas without lignin on xylem vessels and allow passage of water in and out of the lumen to neighbouring cells.

b) Tracheids

- Tracheids have cross-walls that are perforated.
- Their walls are deposited with lignin.
- Unlike the xylem vessels, their end walls are tapering or chisel-shaped.
- Their lumen is narrower.
- Besides transport of water, xylem has another function of strengthening the plant which is provided by xylem fibres and xylem parenchyma.

c) Xylem fibres

- Are cells that are strengthened with lignin.
- They form wood.

d) Xylem parenchyma

- These are cells found between vessels.
- They form the packing tissue.

Forces involved in Transportation of Water and Mineral Salts

1. Transpiration pull

- As water vaporises from spongy mesophyll cells into sub-stomatal air spaces, the cell sap of mesophyll cells develop a higher osmotic pressure than adjacent cells.
- Water is then drawn into mesophyll cells by osmosis from adjacent cells and finally from xylem vessels.
- A force is created in the leaves which pulls water from xylem vessels in the stem and root.
- This force is called transpiration pull.

2. Cohesion and Adhesion

- The attraction between water molecules is called cohesion.
- The attraction between water molecules and the walls of xylem vessels is called adhesion.
- The forces of cohesion and adhesion maintain a continuous flow of water in the xylem from the root to the leaves.

3. Capillarity

- Is the ability of water to rise in fine capillary tubes due to surface tension.
- Xylem vessels are narrow, so water moves through them by capillarity.

4. Root Pressure

- If the stem of a plant is cut above the ground level, it is observed that cell sap continues to come out of the cut surface.
- This shows that there is a force in the roots that pushes water up to the stem.
- This force is known as root pressure.

Importance of Transpiration

- Transpiration leads to excessive loss of water if unchecked.
- Some beneficial effects are:

• Replacement of water lost during the process.
• Movement of water up the plant is by continuous absorption of water from the soil.
• Mineral salts are transported up the plant.
• Transpiration ensures cooling of the plant in hot weather.
• Excessive loss of water leads to wilting' and eventually death if water is not available in the soil.

Factors Affecting Transpiration

- The factors that affect transpiration are grouped into two. i.e. environmental and structural.

a) Environmental factors

Temperature
- High temperature increases the internal temperature of the leaf.
- which in turn increases kinetic energy of water molecules which increases evaporation.
- High temperatures dry the air around the leaf surface maintaining a high concentration gradient.
- More water vapour is therefore lost from the leaf to the air.
Humidity
- The higher the humidity of the air around the leaf, the lower the rate of transpiration.
- The humidity difference between the inside of the leaf and the outside is called the saturation deficit.
- In dry atmosphere, the saturation deficit is high.
- At such times, transpiration rate is high.
Wind
- Wind carries away water vapour as fast as it diffuses out of the leaves.
- This prevents the air around the leaves from becoming saturated with vapour.
- On a windy day, the rate of transpiration is high.
Light Intensity
- When light intensity is high; more stomata open hence high rate of transpiration.
Atmospheric Pressure
- The lower the atmospheric pressure the higher the kinetic energy of water molecules hence more evaporation.
- Most of the plants at higher altitudes where atmospheric pressure is very low have adaptations to prevent excessive water-loss.
Availability of Water
- The more water there is in the soil, the more is absorbed by the plant and hence a lot of water is lost by transpiration.

b) Structural Factors

Cuticle
- Plants growing in arid or semi-arid areas have leaves covered with a thick waxy cuticle.
Stomata
- The more the stomata, the higher the rate of transpiration.
- Xerophytes have few stomata which reduce water-loss.
- Some have sunken stomata which reduces the rate of transpiration as the water vapour accumulates in the pits.
- Others have stomata on the lower leaf surface hence reducing the rate of water-loss.
- Some plants have reversed stomatal rhythm whereby stomata close during the day and open at night.
- This helps to reduce water-loss.
Leaf size and shape
- Plants in wet areas have large surface area for transpiration.
- Xerophytes have small narrow leaves to reduce water-loss.
- The photometer can be used to determine transpiration in different environmental conditions.

Translocation of organic compounds

- Translocation of soluble organic products of photosynthesis within a plant is called translocation.
- It occurs in phloem in sieve tubes.
- Substances translocated include glucose, amino acids, vitamins.
- These are translocated to the growing regions like stem, root apex, storage organs e.g. corms, bulbs and secretory organs such as nectar glands.

Functions of Phloem

- Phloem is made up of:

1. sieve tubes,
2. companion cells.
3. parenchyma, a packing tissue.
4. sclerenchyma, a strengthening tissue.

Sieve Tubes

- These are elongated cells arranged end to end along the vertical axis.
- The cross walls are perforated by many pores to make a sieve plate.
- Most organelles disappear and those that remain are pushed to the sides of the sieve tube.
- Cytoplasmic strands pass through the pores in the plate into adjacent cells.
- Food substances are translocated through cytoplasmic strands.

Companion Cells

- Companion cells are small cells with large nuclei and many mitochondria.
- They are found alongside each sieve element.
- The companion cell is connected to the tube through plasmodesmata.
- The mitochondria generate energy required for translocation.

Phloem Parenchyma

- These are parenchyma cells between sieve elements.
- They act as packing tissue.