Transport systems

Why needed?

Organisms need to be able to exchange nutrients/gases/waste with the environment
In multi-cellular organisms
- Most of the cells are too far away from the external environment
- Complex transport system needed for this exchange
(In single-celled/few-celled organisms complex transport systems not needed: diffusion is enough)

Xylem [tissue]
- For carrying water and dissolved mineral ions absorbed from the roots to all parts of the plant
- Long tubes formed by hollow remains of dead xylem cells
Phloem [tissue]
- For carrying dissolved food material to different parts of the plant
  - Particularly sucrose and amino acids formed in leaf
- (Phloem cells are living)
Xylem and phloem can be in separate groups

Translocation = The transport of sucrose and other materials

Root hair cells at the end of the root absorb water from the soil
- Have fine extensions ("hair-like") to increase surface area
Water is transported from root cells to neighbouring cells through osmosis
- until they reach xylem tubes in vascular bundle
  - xylem transports the water to the leaves

(Water molecules are small; easily cross cell membranes)

Water cross the cell of spongy mesophyll cells into surrounding air spaces
- Turn from liquid to vapour (evaporation)
- Diffuse out of open stomata into the environment
Water from cells surrounding mesophyll move into the mesophyll
Water from cells further from the mesophyll move into cells closer to mesophyll ...
- This is all happens due to osmosis (concentration gradient)
- Called the transpiration stream

(Remember: Water is used up in photosynthesis, but excess water may still need to be removed)

Humidity: higher the humidity, slower the transpiration
Wind speed: the greater the wind speed, faster the transpiration
Temperature: The higher the temperature, faster the transpiration
Light intensity: Higher the light intensity, faster the transpiration
- Why? Higher light intensity => Greater rate of photosynthesis => Stomata open widest for faster oxygen and CO2 exchange => Water also transpires faster

Consists of circulatory system
Blood is the main "medium" for transport
- Carries digested food molecules, water, oxygen, carbon dioxide, urea (waste), hormones ...
  - (See table on page 129 of book)
- Also help heat energy transfer
  - Helps cool overheated parts, eg.
    - transfer heat from muscles after intense exercise
  - brain (where respiration is slower)
Blood is made from plasma (=water + dissolved substances), red blood cells, white blood cells and platelets

(*) Urea is formed by liver from excess amino acids

Red blood cells
- Most common type of cell in blood
- Transports oxygen (for respiration)
Adaptations:
- Biconcave disc shape(*)
  - Increases their S.A.-to-volume ratio for faster diffusion
- Are small and flexible, so they can get through even the smallest capillaries
- Containing haemoglobin, that carries oxygen
  - In region of high O2 concentration (where?), it binds with oxygen (to form oxyhaemoglobin)
  - In regions of low oxygen concentration, it releases oxygen
- Have no nucleus (to maximise volume available for haemoglobin)
  - But this means they cannot divide to make new cells
    - old/defectives RBCs are cleaned out by liver
    - ... and new ones created by bone marrow