Mechanisms of Food Transport

Food is needed in all parts of the plant therefore it is translocated from the leaves to all parts of the plants. Translocation in the phloem takes place by using energy.

The areas where the nutrients are stored is known as sinks, examples are the roots, flowers, fruits and stems. While where they originate is a source, the leaves are the only source in the plant.

The movement of food is always from the source to the sink. In the source, the food from the leaf is prepared in the form of glucose from carbon dioxide and amino acids. Glucose is converted into sucrose as it is more chemically stable, makimg it easier to transport. Later it enters into the phloem at the expense of energy.

The osmotic concentration of phloem will increase. Water will then enter into the sieve tubes by the process of osmosis due to which the hydrostatic pressure in the phloem tissue rises. This high pressure produced in the phloem tissues allow translocation to all parts of the plants to have low pressure in their tissues.

At sinks, sucrose move from the phloem into the storage site or growing parts of the plants. Water will also move out from the phloem tube. Hydrostatic pressure will decrease in the phloem at sink. A pressure gradient is set up in the phloem with high pressure at source and low pressure at sink so that the phloem sap with food will move from source to the sink. The phloem transports food according to the need of the plant.

Acknowledgements:

http://manravbioeducation.blogspot.sg/2007/08/transport-of-materials-in-plants.html

http://en.wikipedia.org/wiki/Leaf#Mesophyll

Transport of Food in Plants

Food is manufactured in the leaves and green stems of the plant during a process called photosynthesis. The food made by the leaves is in the form of a simple sugar named glucose. Other types of substances are called plant hormones, they are released from the tips of roots and shoots.  The movement of food from leaves to other parts of the plant is called translocation. In plants, the phloem vessel translocates the food and other substances.

Phloem consists of sieve tubes and companion cells. Sieve tubes are living cells which contain cytoplasm but do not have a nucleus. So its function is supported by a companion cell. The phloem vessel in the vascular bundle will transport sucrose and amino acids away from the leaf. As the vascular bundles are connected with similar structures in the roots, each sieve tube is continuous with those in the roots. Sucrose and amino acids enters the phloem by active transport. Later, it is translocated from the sources to the sinks.

Food is prepared in the mesophyll cells of the leaf. The palisade mesophyll cells is just below the upper epidermis layer. It that contains more chloroplats than the spongy mesophyll layer. The spongy mesophyll layer is below the palisade mesophyll layer that have large air sacs. There is a slight separation between the cells to  provide absorption of carbon dioxide, this separation must be very little to support capillary action for water distribution. Food is translocated in the form of sucrose. The movement of water and dissolved minerals in xylem is always upward, from the soil to the leaves. The movement of food can be upward as well as down ward depending upon the needs of the plants.

Acknowledgements:

http://manravbioeducation.blogspot.sg/2007/08/transport-of-materials-in-plants.html

http://en.wikipedia.org/wiki/Leaf#Mesophyll

An interesting video-podcast on Plant Transport! :]

*All credits go to the owner of this video. We used it only for learning purposes.

Transpiration Pull

Transpiration is the loss of water from the exposed surfaces of the plants, especially through the stomata.

Water is constantly being lost from the aerial parts of the plant. This occurs mainly through the stomata of the leaves during daylight hours.

Transpiration reduces the concentration of water in the Mesophyll cells. The water potential of the Mesophyll cell sap becomes lower than the water potential in the neighbouring xylem vessels, which creates a pulling force which draws water out from the xylem vessels in the Mesophyll cells. As the leaf veins are continuous with the xylem vessels in the stem and roots, the force is transmitted through the water column all the way down the roots.

This force called transpiration pull, pulls water up the plant. It requires no energy input from the plant, yet it is the strongest force that causes water to rise up to the leaves of tall trees.

The most important cause of xylem sap flow is the evaporation of water from the surfaces of mesophyll cells to the atmosphere. This transpiration causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil.

 

Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. The high surface tension of water pulls theconcavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches.

Transpirational pull requires the vessels transporting the water to be very small in diameter, otherwise cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it.

When the water pressure within the xylem reaches extreme levels due to low water input from the roots, the gases then come out of the solution and form a bubble. An embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present. These have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading.

Source:

http://en.m.wikipedia.org/wiki/Xylem#section_3

Capillary Action

There is a great importance of a transport system in plants. It helps to ensure efficient and uninterrupted movement of substances from one part of the plant to another.

In the plants there are two types of conducting vessels. They are the xylem, which conducts water, and mineral salts from the roots to the stem and leaves and the phloem, which transports sucrose and amino acids from the leaves to all parts of the plant.

Water is absorbed by the roots of the plant.

The root hair cells protect the outside of the plant and have specialised features to favour water absorption. There are two paths that water moves through.

First is the Symplast pathway that consist of the living cytoplasm of the cells in the root (10%). Water is absorbed into the root hair cells by osmosis, since the cells have a lower water potential that the water in the soil. Water then diffuses from the epidermis through the root to the xylem down a water potential gradient. The cytoplasm of all the cells in the root are connected by plasmodesmata through holes in the cell walls, so there are no further membranes to cross until the water reaches the xylem, and so no further osmosis.

Secondly, the Apoplast pathway consists of the cell walls between cells (90%). The cell walls are quite thick and very open, so water can easily diffuse through cell walls without having to cross any cell membranes by osmosis. However the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem.

Water moves up the xylem by two modes of transport. They are transpiration pull and capillary action.

In the narrow tubes such as the xylem vessel, two processes occur. When water molecules are attracted to each other due to electrical attractions, this attraction is called cohesion. When water molecules are attracted to other substances, like soil or the xylem wall, it is called adhesion.

Acknowledgements:

http://www.biologymad.com/master.html

http://www.biologymad.com/planttransport/planttransport.htm