Transport Systems in Plants (HSC SSCE Biology): Revision Notes

Transport Systems in Plants

Introduction to transport systems

Multicellular organisms need specialised transport systems to move substances around their bodies efficiently. Simple organisms like unicellular organisms can rely on diffusion, osmosis and active transport directly across their surface. However, larger multicellular organisms require more complex systems to ensure all cells receive the nutrients and gases they need, and to remove waste products from cellular metabolism.

Plant transport systems use vascular tissue arranged in vascular bundles. These bundles contain two types of tissue: xylem and phloem. Xylem carries water and mineral ions upward from the roots to the leaves. Phloem transports the products of photosynthesis to all parts of the plant where they are needed.

Xylem tissue

Structure of xylem

Xylem is specialised tissue that transports water and dissolved mineral ions from the roots to the leaves. This movement only occurs in one direction: upward from the roots.

Xylem tissue consists of two main types of conducting elements:

Xylem vessels form continuous tubes for water transport. These develop when immature cells specialise:

• The cell walls at the ends break down completely
• Cells stack on top of each other to form long tubes
• The cell contents die, leaving hollow vessels
• This allows easy flow of water and dissolved minerals

Xylem tracheids are long structures with tapered ends:

• End walls come into contact and overlap
• Water and ions pass between tracheids through small holes called pits
• They are less efficient than vessels but still functional

Both vessels and tracheids have walls strengthened with lignin. This woody substance is deposited in rings, spirals or other patterns.

Xylem tissue also contains:

• Fibres: provide structural support
• Parenchyma cells: conduct materials between xylem regions and may store substances

The transpiration-cohesion-tension theory

The upward movement of water and minerals through xylem is explained by the transpiration-cohesion-tension theory.

This theory centres on the evaporation of water from leaves (transpiration) creating a pulling force that draws water up from the roots. The continuous column of water moving up the stem is called the transpiration stream.

The seven-step process

1. Transpiration occurs: Water vapour diffuses out of the stomata (leaf pores) because the concentration of water vapour outside the leaf is lower than inside.

2. Water evaporates from mesophyll cells: When water is lost from the air spaces inside the leaf, it is replaced by water evaporating from the surface of mesophyll cells surrounding these spaces.

3. Surface tension increases: Evaporation increases the surface tension of water on the outside of mesophyll cells.

4. Water is drawn from veins: Water moves from the xylem tissue in leaf veins to replace water lost from mesophyll cells.

5. Tension increases in the water column: This pulling action increases the tension throughout the entire column of water in the xylem.

6. Water column moves upward: The tension draws more water up from the roots through the stem.

7. Water enters roots by osmosis: Water moves into the xylem in the roots by osmosis, continuing the cycle.

Supporting factors

Several other factors assist water movement up the xylem:

Cohesion of water molecules

Water molecules stick together because they are polar. This means one end has a slight positive charge and the other end has a slight negative charge. Opposite charges attract, creating cohesive forces between molecules. This forms a continuous stream of water, so when some molecules are drawn up, others follow.

Adhesion to xylem walls

Adhesive forces cause water molecules to stick to the walls of xylem vessels. The narrower the vessel, the higher water can rise by adhesion. Combined with cohesion, this ensures the continuous water column moves smoothly through the xylem.

Structural properties of xylem

The narrow, thickened, lignified walls can withstand the tension in the water column while offering little resistance to water flow.

Root pressure

Once water is absorbed into roots (by osmosis) along with mineral ions (by diffusion and active transport), these substances move across the root into the xylem. The continual influx creates a small amount of root pressure, forcing the solution already in the xylem to move upwards. However, this pressure alone cannot lift water very high and is not the main driving force.

Investigation: Movement of materials in xylem

Students can investigate water transport in xylem using celery stalks placed in coloured dye solution (eosin).

This practical demonstration confirms that:

• Water moves through specific vascular tissue (xylem)
• The movement is upward through the plant
• Xylem forms a continuous transport system from roots to leaves

Phloem tissue

Structure of phloem

Phloem is specialised tissue that transports sugars and other products of photosynthesis from the leaves (where they are made) to all other parts of the plant where they are needed or stored.

Phloem tissue contains two types of cells:

Sieve tube cells (sieve tube elements)

• Long, thin cells arranged end-to-end
• Have large pores through the cell walls at each end
• These perforated end walls are called sieve plates
• Contain some mitochondria and endoplasmic reticulum
• Lack nuclei and most other organelles
• Share cytoplasm with neighbouring cells
• Form channels through which sugars and plant products flow

Companion cells

• Found alongside sieve tubes
• Contain a nucleus and all normal cell organelles
• These organelles are absent in sieve tube cells
• Assist the effectiveness of sieve tube elements
• Provide ATP and nutrients
• Help load and unload sugars into and out of sieve tubes

The source-sink theory

After glucose is produced during photosynthesis in leaves, it is either:

• Stored as starch in the leaf
• Converted to sucrose and distributed around the plant

The distribution of sucrose through phloem is called translocation. Unlike the one-way movement in xylem, substances in phloem can move in any direction depending on where they are needed.

How the source-sink theory works

Movement in phloem is driven by pressure differences:

• High-pressure regions form where sucrose is produced (source)
• Low-pressure regions form where sucrose is needed (sink)
• Phloem sap flows from high pressure to low pressure

At the source (typically leaves):

1. Energy (ATP) is used to actively pump sugars into the phloem tissue
2. This creates a highly concentrated solution
3. Water moves into the phloem by osmosis from nearby xylem
4. This creates a high-pressure region

At the sink (roots, flowers, fruits, storage organs):

1. Energy is used to actively remove sugars from the phloem
2. This creates a dilute solution
3. Water leaves the phloem by osmosis and returns to the xylem
4. This creates a low-pressure region

Investigation: Microscopic structure of xylem and phloem

Examining prepared microscope slides reveals the structural differences between xylem and phloem tissues.

Key differences between xylem and phloem

Feature	Xylem	Phloem
Substances transported	Water and dissolved mineral ions	Sugars (mainly sucrose), amino acids, some minerals
Direction of movement	One direction only: upward from roots to leaves	Any direction as required by the plant
Cell structure	Dead hollow tubes with lignified walls	Living cells with sieve plates and companion cells
Cell components	No cell contents, just empty space	Contain some organelles, shared cytoplasm
Driving mechanism	Transpiration pull, cohesion, adhesion, root pressure	Pressure differences between source and sink
Wall strengthening	Lignin thickenings in various patterns	No lignin, flexible cell walls