AS Biology: Plant cells and water

Water potential

• Pure water has a water potential of 0 however cells have negative water potential when there are more solute particles than water molecules therefore cells have higher water potential when there are less solute particles and more water molecules
• The plant cells becomes TURGID when water enters by osmosis, vacuole sweels and pushes against the cell wall. Plant cell is FLACCID when the water is lost from the cell and the vacuole shrinks hence the cell loses its shape

Water can travel via three different pathways:

1. Apoplast pathway - water moves through the cell wall

• The cellulose cell wall has many water filled spaces between the cellulose molecule, water can move through these spaces and between cells. The water does not pass through any plasma membranes.

2. Symplast pathway - water moves through the plasma membrane into the cytoplasm

• Water enters the cytoplasm via the plasma membrane. The plasmodesmata allows the movement of water from one cell to the next.

3. Vacuolar pathway - water moves through the vacuole

• This is similar to symplast pathway but the water can now enter and pas through the vacuoles as well.

Water uptake from soil

• The roots have root hair cells, to increase the surface area to absorb minerals by active transport.
• This lowers the water potential in the roots so water moves into the root hair cells by osmosis. 
• Water enter root hair cell by osmosis - minerals are actively transported into xylem. (water moves into xylem my osmosis.) 

Solute can enter the xylem by going through cell membrances - the Casparian strip blocks apoplastic route (outside cells) - water cannot pass between the cell or through cell walls - it must pass into cytoplasm or into symplast pathway

Water moves up the stem due to root pressure, transpirational pull, capillary action

• Transpiration pull - as water molecules are removed from the xylem, more water molecule are pulled to replaced them aka transpirational pull.

Mass flow of water also relies on the properties of water

• Cohesion - the water molecules tend to stick together
• Adhesion - the water molecules also stick the the inside of the xylem vessel
• the drawing of continuous column of water up to xylem vessel is aka cohesion-tension theory

AS Biology: Plants' transport system: Xylem and Phloem

Plants need a transport system as every cell of a multicellular plant needs a regular supply of water and nutrients. Cells inside the plant would not be able to receive enough nutrients and water to survive simply by diffusion.

Plants require:

• Carbon dioxide for photosynthesis
• Oxygen for aerobic respiration
• Organic nutrients for growth

PHLOEM transports sugars from the leaves  - it's also for amino acids. - they can move upwards or downwards.

XYLEM transports water, minerals up from roots. 

Vascular tissue is distributed throughout the plant and it helps with the plant transport. Xylem and phloem are found together in vascular bundles which also contains other tissues. It helps transport water from toots to leaves via the stem. The xylem and phloem run the entire length of the plant from the roots to the midrib and veins of the leaf.

Xylem vessels are empty tube shaped cells/ Their cytoplasm has been removed by the plant and their walls are strengthened and thickened with lignin. The lignin strengthens the tubes and help support the plant by giving rigidity to the xylem. Minerals from the soil are also carried in the xylem, they are needed by the plants in many of its chemical reactions.

Features:

• Wall thickened by lignin prevents collapse under tension and adhesion to lignin
• Hollow tubes means that there is less resistance to flow
• No end walls so there's a continuous columns so there is less resistance to flow
• Pits inside the walls allows lateral movement
• Narrower the lumen the higher water will rise by capillarity
• Stacked end to end develops as a continuous water filled column; allows tension to pull water up

Phloem (sieve) tubes carry sugar around the plant. Phloem cells are alive and have a cytoplasm unlike hollow xylem vessels. It's is made up of two types of cells: sieve tubs and companion cells.

Sieve tubes: the ends walls of the tube cells have pores which dissolved sucrose is transported from cell to cell. They have sieve plates at the end with pores so sugar can get through. they have no nucleus, the cytoplasm is controlled by companion cell nucleus. The vacuoles of the tube are joined and sugary sap flow along them.

Companion cells - proves the energy for the sieve tube cells. The nuclei tends to be large to compensate for the lack of nucleus in the sieve tube.

Features:

• Both cells are living which allows active processes
• Plasmodesmata (connections between sieve tube and companion cell) allows exchange between cells.
• Companion cell have many mitochondria to make energy and a nucleus to control functions both cells.
•  Sieve tubes have little cytoplasm and elongated cells so there's less resistance of fluid flow
• Sieve plates allow material through, it also joins end to end to provide continuous tubes.
• Sieve tubes are bi-directional which allows sugar to go to sink or it can travel either direction.