Transport in Plants

Xylem and phloem

transport vessels that help move substances around the plant.

Xylem

transports water and mineral ions, from the roots to the stem and leaves, provides structural support to the plant

Structure of Xylem

1. xylem tissues have no cell organelles.
2. Cells joined end to end with no cross walls to form a long continuous tube.
3. Thick walls for lignin to strengthen and support xylem vessels; stops from collapsing inwards.
4. structural adaptations allow the xylem to form an uninterrupted tube for water to pass easily.

Phloem

transports sucrose and amino acids made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem.

Vascular bundles

xylem and phloem are collectively called vascular bundles, found throughout the root, stem, and leaves.

Position of the xylem and phloem in sections of roots

Xylem always on inside, Phloem always on outside.

Position of the xylem and phloem in sections of stems

Xylem always on inside, Phloem always on outside.

Position of the xylem and phloem in leaves

Xylem always on inside, Phloem always on outside.

Cross section of roots

1. Xylem
2. Phloem
3. Cortex
4. Root hairs

Root hair cell

specialised cell found in roots of plants that is specially adapted to absorb water and minerals from soil, cells have root hairs which are extensions or outgrowths of epidermal cells.
The large surface area of root hairs increases the uptake of water and mineral ions by increasing rate of absorption of water by osmosis and mineral ions by active transport.

Pathway of water

1. Water first enters root hair cells from soil thru osmosis because water potential higher in soil than cytoplasm of root hair cells
2. Water from RHC to root cortex cells
3. Water travels into xylem, up the stem, upto leaves then finally mesophyll cells
4. Diffusion through stomata

Investigation of pathway of water

1. Place plant like celery into water (control)
2. In two other beakers, place celery in water mixed with food dyes, red in one and blue in one
3. Leave all 3 plants in sunlight for a few hours
4. Cut cross-sections of the celery stalks and observe
5. The one in plain water will leave no stain, the other 2 stalks will show specific areas highlighted by the dye; showing that water is moving in specific vessels (xylem vessels)
6. We will also observe color changes in leaves showing that water is being taken up by the plant

Transpiration

loss of water vapour from leaves.
Water evaporates from surfaces of mesophyll cells into the air spaces and diffuses out the leaves through stomata as water vapour, the water from soil travels through the roots to xylem and back to leaves to replace the lost water.

Water vapour loss

connected to 1. The large internal surface area provided by interconnecting air spaces between mesophyll cells 2. The number and size of stomata

Transpiration pull

water moves upwards in the xylem because transpiration pull draws up a column of water molecules, up the xylem vessels because water molecules are held together by forces of attraction between them called cohesion.
As water evaporates at the leaf and diffuses out of stomata, more water is drawn up the plant from roots.

Factors affecting transpiration rate investigation

potometer investigation underwater to prevent air bubbles in xylem vessels: water beaker, capillary tube, ruler, reservoir, cut shoot
1. A single air bubble is introduced into the capillary tubing
2.the tap on the reservoir is opened to add water and push the air bubble back to 0 on the ruler.
3. A timer is started and a set time is measured.
4. The distance the air bubble travels is measured.
5. The experiment can be repeated with different environmental conditions.
The faster the air bubble moves, the greater the rate of water uptake so greater the transpiration. Temperature (room temp can be changed by a heater) and wind speed (can be tested using fan at different speeds) are the factors that are tested.

Factors affecting transpiration rate

temperature (increases): kinetic energy of water molecules increases so they evaporate and diffuse faster from mesophyll cells,
wind speed (increases): the wind removes away the water vapour surrounding the leaf quickly, so water from soil must replace lost water faster,
humidity (decreases): if air surrounding has more water vapour there will be weak concentration gradient for diffusion.

Wilting

the water in a plant keeps it turgid and supports it.
If the amount of water a plant loses from its leaves > the amount of water that is present in soil to travel into plant thru roots then wilting will occur. Plant becomes soft and droops because the cell wall becomes flaccid and can’t support the plant

Translocation

the movement of sucrose and amino acids in the phloem (made from living cells) from sources to sinks.
Sucrose and amino acids may be transported in different directions based on growth stage of plant or time of year
Sources: parts of plant that release sucrose or amino acids
Sinks: parts of plant that use or store sucrose or amino acids.

source/sink

Some parts of plant may at as source/sink at different times eg. during winter and spring when less photosynthesis takes place, the phloem tubes will transport dissolved sucrose/amino acids from storage organs such as roots (source) to other parts of plant (sinks).
In summer the leaves photosynthesise and produce large quantities of sugar so leaves become source and roots become sinks, storing sucrose as starch until it is needed again.