Transport System in Plants

As I discussed in an earlier post about means of transport system, the medium or means of transport in plant is latex or cell sap and the materials transported are water, carbon dioxide, oxygen, amino acids, e.t.c. Plants require sufficient quantities of several minerals and other substances which are transported in them.

[Photo credit: Smartse]


While gases enter and leave the cells of aquatic, unicellular and simple multicellular plants by diffusion, water enters by osmosis, manufactured food and waste products are transported by diffusion.

In multicellular plants like flowering plants, mineral salts and water are absorbed through the root system, and the gases are mainly absorbed through the stomata in the leaves and lenticles in the stem. While the gases are transported by diffusion, mineral salts and soluble foods are transported in the vascular bundles. 

A vascular bundle is made up of several vascular tissues, and it consists of mainly xylem and phloem tissues. Vascular bundles are found in the root, stems and leaves of flowering plants.

Some Plant Tissues and Their Functions
1. The cambium tissues: These are made up of narrow living cells with thin walls and dense cytoplasm.

Function of cambium tissues 
Cambium tissues can divide and multiply. Because of this ability, plants are able to produce secondary xylem and phloem, thus resulting in the growth of secondary thickening - growth in width of stems. Therefore, they are responsible for the growth of secondary thickening in plants.

2. The xylem tissues: These are made up mainly of dead cells with lignified cell walls.

Functions of xylem tissues

1. Xylem tissues are responsible for the transportation of water and dissolved mineral slts from the roots to other parts of the plants.
2. They also give support and rigidity to plants.

3. The phloem tissues: These consist of thin-walled living cells with dense cytoplasm, which have perforated cross walls.

Function of phloem tissues

The phloem tissues transport manufactured food from the leaves to other parts of the plant. The transported manufactured food can be stored or used immediately.

PROCESSES NECESSARY FOR TRANSPORTATION IN PLANTS

The following processes aid and are necessary for transportation in plants:

1. transpiration
2. translocation
3. transportation of water in the xylem tissue
4. absorption of water and mineral salts

1. Transpiration

Transpiration is the removal of water from aerial parts of plants into the atmosphere in form of water vapor. Plants can loose water through:

1. the lenticles in the stem and is referred to as lenticular transpiration
2. the stomata in the leaves and is referred to as stomata transpiration 
3. the cuticle of the leaf surface and is referred to as cuticular transpiration

Conditions/Factors Affecting the Rate of Transpiration

The following are the factors/conditions which affect the rate of water loss from the aerial parts of plants:

1. Humidity: The higher the humidity of the atmosphere, the slower the rate of transpiration, and the lower the humidity, the faster the rate of transpiration.
2. Light: High light intensity leads to high rate of photosynthesis, leading to rise in temperature, thus higher rate of transpiration, and vice versa.
3. Temperature: Increase in temperature leads to higher rate of transpiration while decrease in temperature leads to lower rate of transpiration.
4. The size of the stomata pores: When stomata opens due to turgidity of the guard cells, transpiration occurs, while flaccidity of the cells causes the guard cells to close and prevent transpiration from taking place.
5. Soil water:  The higher the level of soil water, the higher the rate of absorption, consequently the higher the rate of transpiration and vice versa.
6. Wind: The higher the speed of wind, the higher the rate of transpiration and vice versa.

Advantages/Benefits/Importance of Transpiration in Plants

1. It brings about the movement of soil water.
2. It helps to remove excess water from plants.
3. It enables plants to absorb water and mineral salts from the soil.
4. It cools the plants.

2. Translocation

This is the process by which materials or manufactured food substances are transported from where they are produced to where they are needed. It begins from the leaves to other parts of the plants. It occurs through the tissue called phloem. The materials translocated in plants include: sugar, glucose, oil, hormones, e.t.c.

3. Transportation of water in xylem tissue

 Transport of water in xylem tissue is due to the following processes:

• Transpiration pull: The continuous flow of water from the roots to the leaves forms transpiration stream. As the water evaporates from the leaf cells and as photosynthesis produces more sugar, the osmotic pressure in the leaf cells increases with respect to that in the xylem cells. This causes more water to flow into the leaf cells from the xylem vessels. So, there is a pull on the water columns in the xylem vessels and water is drawn up in the plant
• Capillary action: The upward movement of water through the xylem is mainly achieved through capillary action. It is due to the attraction between the water molecules and the walls of the xylem vessels. The xylem vessels that extend from the roots to the leaves form very fine capillary tubes. Water the rises up such tubes.
• Root pressure and suction pressure: Root pressure is usually created as a result of differences in osmotic pressure between the cell sap and the concentration of soil nutrients. The cell sap, being more concentrated, tends to draw up the nutrients. On the other hand, suction pressure is the total force by which the cell absorbs water from its surroundings. The suction pressure is normally created when water is lost in form of transpiration through the stomata of the leaves. By these exposure, the movement of water from the soil to the xylem tissues through the root hair is achieved. 

4. Absorption of water and minerals by roots of plants

 The young root hairs of flowering plants have direct contact with water in the soil. The cell sap in the root hairs is more concentrated than the soil water, hence water is able to pass from the soil into the root hairs by osmosis. The water passes through the thin layer of cytoplasm or cell membrane which is selectively permeable. into the vacuole of root hairs. The extra water raises the tugor pressure of the vacuole or reduces the osmotic pressure and forces water out in the cell walls towards the cortex. 

The cell next to the root hair cell on the inside has a lower tugor or higher osmotic pressure, hence water will pass into it by osmosis. In this way, the water absorbed will get to the xylem vessels.

Similarities Between Transport System in Plants and Animals

These include:

1. Materials or food nutrients and hormones are transported in fluid form in both.
2. Diffusion plays a major role in both.
3. Tubular or Cylindrical vessels are necessary in both.
4. Liquid medium is required in both.

Differences Between Transport System in Plants and Animals

Plants	Animals
Materials are transported through different vessels (xylem and phloem)	Materials are transported in the same vessels
Cell sap is the medium of transport	Blood is the medium of transport
Root pressure or transpiration generates force for pull for transport	Heat generates forces for transport
The transport medium is not tissue	The transport medium is made up of cells of different types of tissues

Categories: Biology, Transport System, Transportation