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)

Transport in flowering plants

• 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

or

• can be together (as vascular bundles or veins)

 

 

Translocation = The transport of sucrose and other materials

Water absorption in plants

• 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)

Losing water = Transpiration

• 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)

Factors affecting transpiration

• 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

Transport in humans

• 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

Blood - plasma

• Plasma = Straw-coloured, liquid part of blood
• Mainly consists of water
• Glucose and amino acids dissolved in it
• Urea(*) dissolved in it
• Many hormones dissolved in it
• CO2 dissolves in it (to form carbonic acid)

 

 

 

 

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

Blood - RBCs

• 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

Blood - WBCs

• Part of the immune system
  • They try to kill pathogens
• Several types ("specialised")