specialised cells in the stem
there are 3 main types of tissue found in plants: dermal tissue (epidermis), vascular tissue and ground tissue.
the vascular tissue is found at the centre of the stem. each vascular bundle contains xylem vessels and phloem sieve tubes. on the outside of the bundle are sclerenchyma fibres.
the ground tissue is found surrounding the vascular tissue in the middle section of a cross-section of a stem.
and the epidermis is on the outer layer of the stem.
xylem vessels for transport
the xylem vessels are made up of large cells with thick cell walls.
they form a column of cells acting as tubes for the transport of water and mineral ions.
the plant produces a polymer called ligin which allows it to be waterproof so it can transport the water. the polymer ligin impregnates the cellulose wall and as the cells become lignified the entry of water and solutes into them is restricted.
at the same time the tonoplast breaks down and there is autolysis of the cell contents. during autolysis the cell organelles,cytoplasm and cell surface membrane are broken down by the action of enzymes and are lost, leaving an empty tube.
how is water transported through xylem vessels?
water moves upwards from the roots to the shoots through xylem vessels.water evapourates mainly through the large surface area of the leaves.
water diffuses out through the stomata down a diffusion gradient. water evapourating from the plant in this way is known as transpiration.
the water that exits the leaves by transpiration is replaces by water absorbed through the roots.it is the evapouration of water from the cells in the substomatal cavities of the leaves that provides the force needed to draw water up a plant.
the channels between the cellulose microfibrils in the cell walls act as capilaries which can draw water up them by capillary action caused by surface tension. the stream of water passing through the plant is known as the transpiration stream.
there are cohesive forces between the water molecules from hydrogen bonding. meaning water in narrow tubes sticks together. this is called the cohesion-tension theory.
cohesion and surface tension and water solvent pro
hydrogen bonding between water molecules results in strong cohesive forces between water molecules that keep water together as a continuous column in xylem vesses.
this partly causes surface tension at water surfaces causing the surface layer to contract.
the solvent properties of water allow substances to disolve in water and be transported through the xylem and phloem.
waters thermal properties and ensity and freezing
with water a large input of energy cuases only a small increase in temperature, so water warms up and cools down very slowly.
this is very useful for organsims preventing rapid changes in internal temperature when external temperature changes.
unlike most liquids, water expands when it freeze. as liquid water cools the molecules slow down enabling the maximum number of hydrogen bonds to form holding the water further apart than in liquid water. meaning ice is less dense than liquid water.