B3.1.2: Transport in animals - OCR A Biology A level A* student notes
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Biology A Level
Year 1
Module 3: Exchange and transport
3.1.2: Transport in animals
The need for transport systems in multicellular animals
● In single celled organisms, processes eg. diffusion, osmosis, active transport,
endocytosis and exocytosis supply everything the cells needs to import or export - they
can get everything they need through their body surface. Have favourable diffusion
gradients and are small with relatively low levels of activity so their demand for
nutrients and oxygen is small. However, as organisms get bigger/have developed
(multicellular organisms) the distances between the cells and the outside of the body get
greater and therefore diffusion could not supply the body at a rate fast enough to sustain
life. Specialised transport mechanisms are therefore needed because:
- Increased size - as an organism increases in size, the furthest distance that
oxygen has to reach (if absorbed at the surface) gets larger and as diffusion is a
slow process, the time taken for oxygen to reach the innermost cells would be so
long that the cells would die before they received it (wouldn’t occur at a rate fast
enough to meet the cells demands and therefore sustain life)
- Larger animals tend to be more active - even if not that physically active species,
would have higher level of activity than smaller counterparts because of the
increase no. of cells in the body (this increases metabolic demands)
- Metabolic demands of most multicellular animals are high/higher metabolic rate
(TMT they need lots of oxygen and food and produce lots of waste products) -
diffusion over long distances is not sufficient to supply the quantities needed
- Waste products of metabolism need to be removed from the cells and transported
to excretory organs
- SA:V ratio gets smaller as multicellular organisms get bigger - not only do
diffusion distances get bigger but the amount of surface area available to absorb
or remove substances becomes relatively smaller
, - Molecules eg. hormones, enzymes may be made in one place but needed in
another - must be transported (larger → increased distances between body parts)
- Food will be digested in one organ system but needs to be transported to every
cell for use in respiration and other aspects of cell metabolism
The different types of circulatory systems
● Mass transport system: when substances transported in a mass of fluid with a
mechanism for moving the fluid around the body
● Key features of transport system:
1. Liquid transport medium that circulates around the system (blood)
2. Vessels that carry transport medium
3. Pumping mechanism to move liquid transport mechanism around the system
● Large multicellular animals usually have either an open or closed circulatory system:
- Open circulatory system:
- Found mainly in invertebrate animals - most insects and some molluscs
- Insect blood (transport medium) is called haemolymph - it carries food,
nitrogenous waste products and cells involved in defence against disease
- Insects have one main blood vessel to contain the transport medium - the dorsal
vessel, which delivers haemolymph straight into the haemocoel (body cavity) due
to the pumping of a tubular heart in the abdomen
- In the transport medium the haemocoel is under low pressure, and comes into
direct contact with the tissues and cells (tissues/organs are directly bathed in it) -
exchange takes place between the transport medium and the cells
- Haemolymph re-enters the heart through openings with one way valves called
ostia when the heart relaxes
- Haemolymph circulates through the body cavity but steep diffusion gradients
cannot be maintained for efficient diffusion
- Amount of haemolymph flowing to a particular
tissue cannot be varied to meet changing demands
- Body cavity of insect is split by a membrane and the
heart extends along the length of the thorax and
abdomen
, - In insects, oxygen is delivered directly to the tissues by a system of tubes called
tracheae that connect to the outside, reducing the importance of having a very
efficient circulatory system
- Closed circulatory system:
- Found in many different animal phyla - echinoderms (eg. starfish), cephalopod
molluscs (eg. squid), annelid worms (eg. common earthworm) and all of the
vertebrate groups (includes mammals!) - vertebrates
- Blood is enclosed in blood vessels (circulates only within blood vessels) - doesn’t
come into direct contact with the cells of the body. Substances leave and enter
the blood by diffusion through walls of blood vessels
- Heart pumps blood around the body under high pressure and relatively quickly,
and the blood returns to the heart
- Volume of blood flowing to a particular tissue can be adjusted by widening or
narrowing blood vessels
- Most closed circulatory systems contain a blood pigment (eg. haemoglobin)that
carries the respiratory gases
● Closed circulatory systems can be single or double:
- Single closed circulatory system:
- Found in eg. fish, annelid worms
- Blood flows through the heart and is pumped out to travel all around the body before
returning to the heart - for each complete circulation of the body, blood travels only
once through the heart (so the heart has a single atrium and a single ventricle)
- In a single closed circulation, blood passes through 2 sets of capillaries (1st -
deoxygenated blood is pumped by the heart to the gills, where it absorbs oxygen and
carbon dioxide and is excreted, 2nd - oxygenated blood then travels to other areas of the
body, passing through organs in capillaries from which it delivers oxygen to cells. Then
returns to the heart). As it passes through these 2 very narrow blood vessels, the blood
pressure in the system drops considerably so blood returns to the heart slowly - limits
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