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OCR AS Biology - Revision - Part 1 - Plants

Updated on December 3, 2012
the Xylem (left) and the Phloem (right).
the Xylem (left) and the Phloem (right).

Transport

Xylem - A hollow tube made of aligned dead cells that transport water and mineral ions up the plant to where it is needed.

Phloem - Transport tissue that carries sugars, and other products of photosynthesis, to the rest of the plant. The phloem contains sieve tube elements and companion cells.

Xylem:

  • The root hair cells actively transport water and minerals from the soil into the cell using ATP produced by mitochondria within the cell.
  • The minerals that are absorbed reduce the water potential of the cell's cytoplasm.
  • A gradient is then formed as the water potential in the cell is lower than that in the soil.
  • Water is then absorbed and transported across the cell's membrane by osmosis.
  • The xylem is reinforced with lignin which waterproofs the xylem, provides support and allows flexibility.

Phloem:

  • Sieve tubes and sieve plates make up the phloem. Sieve tubes are lined up end to end with cross-walls at intervals.
  • These cross-walls have many pores to allow substances to pass through and are called 'sieve plates'.
  • In between the sieve tube cells there are companion cells.
  • These companion cells have a large nucleus, dense cytoplasm and lots of mitochondria for ATP production.
  • These companion cells carry out all the metabolic processes that the sieve tubes needs to do, but can't do for itself.
  • Joining the sieve tube cells to one and other are small strips

A diagram of water potential.
A diagram of water potential.

Water Potential

The definition of water potential is:

The total potential of energy of water molecules in a system.

The water potential of pure water is 0. This is because it has no dissolved solutes in it, so there are lots of free water molecules. The water potential of a region with dissolved solutes in it is always negative.

Water always moves from a place of high water potential (for example, distilled water) to a place of low water potential (for example, a concentrated sugar solution).

If you place a cell into pure water, it will take up water molecules via osmosis, this is because all of the dissolved solutes in the plant's cytoplasm will cause the cell to have a lower water potential than the water outside the cell (which has no dissolved solutes in it).

Plant cells have a strong cellulose wall and because of this will not burst if they absorb too much water. If a plant cell absorbs too much water the cell will become turgid and create a pressure against the cell wall, called the pressure potential. When the pressure potential builds up the plant cell will begin to reduce it's water absorption.

If you place a plant cell into a solution with lots of dissolved solutes, for example a concentrated sugar solution, then it will lose water molecules via osmosis because the cell's cytoplasm will have a higher water potential than the solution outside the cell.

If water keeps on moving out then the cell will lose it's turgidity and the vacuole and cytoplasm will shrink. Eventually the cell's cytoplasm will pull away from the cell wall, this is called incipent plasmolysis. If water continues to leave the cell then the plasma membrane will also lose contact with the cellulose wall, this is simply known as plasmolysis.


Pathways

The Apoplast Pathway:

  • The cell wall is made of cellulose.
  • The cellulose molecules that make up the wall have lots of water-filled spaces in between them.
  • The apoplast pathway carries dissolved mineral ions and salts with the water because it does not cross any plasma membranes.

The Symplast Pathway:

  • Water enters the cell cytoplasm via the plasma membrane.
  • The plamodesma is a small strip of cytoplasm that creates gaps in the cell wall connecting the cells.
  • The symplast pathway crosses the plasmodemata in order to get to the next cell.

The Vacuolar Pathway:

  • This is very similar to the symplast pathway in that it transports water through the plasmodesmata.
  • However the water is not restricted to travelling just through the cytoplasm, it can travel through the vacuoles of the cells too.

Water Movement Up The Stem

There are 3 processes that help water move up the stem, they are:

Root Pressure:

When minerals are actively transported from the endodermal cells and into the xylem a water potential gradient is created.

Water then moves down the concentration gradient from the endodermis and into the xylem.

This flow of water into the xylem pushes the water up a few meters up the stem, but cannot push the water all the way to the top of the plant.

Transpiration Pull:

The loss of water from the leaves by evaporation is called transpiration. This water must be replaced by more water coming up from the xylem.

Water molecules are attracted to one and other (this is called cohesion). These cohesion forces are strong enough to hold the water molecules in a chain and as the water molecules in the chain are lost due to transpiration, it pulls the rest of the molecules up. This creates a transpiration stream.

The transpiration stream creates tension in the xylem (called cohesion-tension theory), which is why it's important for the xylem to be supported by lignin so that it doesn't collapse.

Capillary Action:

The same forces that hold the water molecules to one and other attract the water molecules to the side of the xylem. This is known as adhesion.

The xylem vessels are very narrow which makes the forces of adhesion to pull the water molecules up the xylem more efficient.

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