Exchange and Transport

Including plants, insects, humans and fish

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  • Created by: Emilie
  • Created on: 25-01-15 20:04
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Exchange and Transport
For exchange to be effective:
The surface area of an organism must be large compared with its volume.
Thin exchange surface to give a short diffusion pathway
Partially permeable to allow selected materials to cross
Movement of the environmental medium (e.g. air)
Movement of the internal medium (e.g. blood)
Fick's law ­ diffusion surface area x difference in concentration
length of diffusion path
Gas exchange in single-celled organisms
Single-celled organisms have a large surface area to volume ratio so oxygen can be absorbed by
diffusion across their body surface, which is covered by only a cell-surface membrane. Carbon
dioxide from respiration diffuses out across their body surface in the same way.
Gas exchange in insects
Insects must balance the opposing needs of exchanging respiratory gases with reducing water loss
(which occurs easily for terrestrial organisms).
To reduce water loss, terrestrial organisms have waterproof coverings and a small surface area to
volume ratio to minimise the area over which water is lost. These features mean that insects
cannot use their body surface to diffuse respiratory gases in the same way a single-celled
organism does. Insread, they have developed an internal
network of tubes called trachae, which are supported by
strengthened rings to prevent them from collapsing. These
then divide into smaller tubes called tracheoles which extend
throughout all the body tissues of the insect. In this way,
oxygen can be brought directly to respiring tissues.
Respiratory gases move in and out of the tracheal system via a
diffusion gradient (oxygen is used up in respiration so its
concentration towards the end of the tracheoles falls) and
ventilation (movement of muscles in insects can create mass
movements of air in and out of the trachae).
Gases enter and leave trachae through tiny pores called spiracles, on the body's surface. The
spiracles may be opened and closed by a valve. When they are open, water can evaporate from
the insect.
The tracheal system relies mostly on diffusion to exchange gases between the environment and
cells. For diffusion to be effective the pathway must be short, as a result, this limits the size that
insects can attain.

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Gas exchange in fish
Fish have developed a specialised internal gas exchange surface (gills) because their waterproof
covering and small surface area to volume ratio means their body surface is not adequate to
supply and remove their respiratory gases via diffusion.
Gills are
located behind
the heads of
fish and are
made up of gill
which are
stacked up in a
pile. At right
angles to te
filaments are
gill lamellae
which increase
the surface
area of the
gills.…read more

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Circulatory system of a mammal
Features of transport systems:
A suitable medium in which to carry materials (e.g. blood)
A form of mass transport in which the transport medium is moved around in bulk
A cosed system of tubular vessels that contains the transport medium and forms a
branching network to distribute it to all
parts of the organism
A mechanism for moving the transport
medium within vessels (e.g. muscular
A mechanism to maintain the mass flow
movement in one direction (e.g.…read more

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It provides a mostly constant environment for the cells it
Blood is pumped along arteries, into narrower arterioles and then narrower capillaries, creating
hydrostatic pressure at the arterial end of the capillaries.…read more

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Epidermis ­ a single layer of cells often with long extentions called root hairs which
increase the surface area. A single plant may have 1010 root
Cortex ­ a thick layer of packing cells often containing stored
Endodermis ­ a single layer of cells that surround the vascular
tissue containing a waterproof layer called the casparian strip
which allows the plant to control the movement of ions into
the xylem.
Pericycle ­ a layer of undifferentiated meristematic (growing) cells.…read more

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The uptake of water by osmosis actually produces a force that pushes water up the xylem. This
force is called root pressure which can be measured by placing a manometer over a cut stem. This
force helps push water up short stems i.e. a few centimetres however longer distances like up
trees would require a much greater pressure.
Movement through the stem (mass flow):
The xylem vessels form continuous pipes from the roots to the leaves.…read more

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Factors affecting transpiration
Temperature ­ high temperature increases the rate of evaporation of water from the surface of
the spongy mesophyll cells because it increases the kinetic energy of the water molecules. This
raises the in the sub-stomatal air space and means that the molecules are moving faster so
tanspiration increases.
Humidity ­ high humidity means a higher in the air surrounding the stomata, so a lower
gradient between the sub-stomatal air space and the eair outside,so less evaporation.…read more

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Hydrophytes ­ adapted to freshwater habitat
Mesophytes ­ adapted to habitat with adequate water
Adaptations of xerophytes:
Adaptation How it works Example
Thick waxy cuticle Stops uncontrolled Conifer needles
evaporation through palisade
Small leaf surface area Less area for evaporation Conifer needles
Low stomata density Fewer gaps for water loss Marram grass
Sunken stomata Maintains humid air around Marram grass
Stomatal hairs stomata
Folded leaves
Succulent leaves and stem Store water Cacti
Extensive roots Maximise water uptake Cacti…read more


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