Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
Biology Spec Led Revision
Chapter 7 - Exchange and Transport
Specialised Exchange Surfaces
(a) the need for specialised exchange surfaces. To include surface area to volume ra o (SA:V),
metabolic activity, single-celled and multicellular organisms.
Organisms need to exchange substances with their environment
1) Cells need to take in oxygen and glucose for aerobic respiration and other metabolic reactions
2) They also need to excrete waste products from these reactions i.e. CO2 & Urea
How easy the exchange of substances occurs depends on the organisms SA:V ratio
Smaller animal = higher SA:V ratios
Multicellular organisms need exchange surfaces
An organism needs to supply each of its cells with substances, it also needs to remove waste
products to avoid damaging cells
1) In single celled organisms - these substances can diffuse directly into or out of the cell
across the cell surface membrane. The diffusion rate is quick because of the small distances
needed to travel.
2) In multicellular organisms - the diffusion across the outer membrane is too slow, for several
reasons:
- Some cells are deep within the body, very large distance from outside environment to the cell
- Large animals have low SA:V ratios so it is difficult to exchange enough substances to supply
the late volume of there animal through the relatively small surface area
- Multicellular organisms have a higher metabolic rate, which means demand outruns the
supply
RATHER THAN USING DIFFUSION MULTICELLULAR ORGANISMS REQUIRE SPECIALISED
EXCHANGE SURFACES - LIKE AVEOLI AND LUNGS
(b) the features of an efficient exchange surface
Exchange surfaces have special features to improve their efficiency
Most exchange surfaces have a large surface area:
Root Hair Cells
1) The cells on plant roots grow into long hairs with stick
out into the soil. Each branch is covered in millions of
these microscopic hairs
2) This gives the roots a large SA which helps increase
the rate of absorption of water (VIA OSMOSIS) and
mineral ions ( VIA Active TRANS)
They're also Thin:
Alveoli:
1) The alveoli are the gas exchange surfaces in the lungs
2) Each alveolus is made from a single layer of thin, flat cells called the alveolar epithelium
3) O2 diffuses out of the alveolar space into the blood. CO2 diffuses in the opposite direction
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