Reminder 1: Other than for active
transport, diffusion, naturally occurs in gases and liquids,
because all the particles
(molecules or ions) have sufficient kinetic energy to move around freely
at random from a region of higher concentration to a region of lower
Active transport is the movement of particles (molecules or
ions) through a cell membrane from a region of lower concentration to a
region of higher concentration using energy from respiration.
Within cell membranes there are carrier proteins
use energy from respiration to transport molecules or ions across
the membrane, against the concentration gradient, therefore cells
that use active transport usually have more mitochondria for
respiration compared to other cells.
In complex multicellular organisms
(eukaryotes) the smaller surface
area to volume ratio means surfaces and organ
systems are specialised for exchanging
materials which include, as well as water, small dissolved molecules (e.g.
glucose, amino acids), ions (e.g. Na+, K+, Ca2+,
Fe2+, Cl-) and gases (O2, CO2).
This is to efficiently allow sufficient molecules to
be transported into and out of cells for the
The effectiveness of an
exchange surface is increased by:
having a large surface area
a membrane that is thin, to provide a short
in animals, having an efficient blood supply
in animals, for gaseous exchange being
Living organisms must be to exchange
substances with their surroundings in order to survive - grow, mature and
The size (volume) of an organism or a
specific organ, and its surface area, greatly affects how efficient this
exchange process is.
The rate of transfer is often governed by the
surface area : volume ratio.
Diffusion is used by cells to take in
useful substances and remove waste products.
Why do we need exchange surfaces?
Exchange or transfer of substance usually involves diffusion through a
membrane (permeable, partially permeable), water movement by osmosis and also active transport e.g.
the transfer needs of organisms include:
(i) Useful nutrient substances e.g. food molecules
from digestion like amino acids and sugars, mineral ions, water taken up by
cells by osmosis,
(ii) Removal of waste products e.g. carbon dioxide from respiration, urea
(poisonous) from breakdown of proteins in animals - diffuses
from cells into blood plasma and transferred to be absorbed by the kidneys prior to
(iii) Gas exchange usually involves taking
oxygen into cells for aerobic respiration and passing out carbon dioxide to the environment.
Reminder: Osmosis as the net movement of
water molecules from a region of higher water potential (from a more
dilute solution) to a region of lower water potential (a more
concentrated solution), through a partially permeable cell membrane
(concentrated refers to dissolved molecules or ions).
Know and understand that many organ systems are specialised for exchanging
The ease with which an organism
can exchange substances with the environment depends on the organisms
surface area to volume ratio AND you can extend this idea to an organ
itself e.g. the lungs.
In single-celled microorganisms
gases and dissolved substances can often diffuse directly into and out of the cell
through the cell membrane.
This is very efficient because a
single cell has a large surface area to volume ratio membrane -
large surface area relative to the volume of the cell.
Therefore the single-celled organism
has no trouble in exchanging sufficient materials with its environment.
Multicellular organisms have a
smaller surface area to volume ratios compared to a single celled
This surface area is NOT sufficient to provide efficient rates
of diffusion of substances in and out of the organism without
significant adaptation through evolution - some examples are
described and explained on this page.
It is essential that the
transfer processes of moving sugars, amino acids, oxygen etc. into cells and
the removal of waste products, can happen as efficiently as possible.
Therefore exchange surfaces have
evolved to maximise the rate of transfer of wanted substances into, and unwanted chemicals out of, multicellular organisms.
To increase and maximise the efficiency of
transfer the exchange system needs to have/be ...
large surface area to increase
diffusion rate eg alveoli in lungs, villi in intestine,
permeable cell membranes are usually quite
thin to provide a short diffusion distance (part of thin layers of
cell tissue, so diffusion
distance and times
are short over a wide area),
exchange surface - gases can dissolve into and diffuse through.
Animals have lots of thin blood vessels to
bring in essential nutrient molecules and ions for life and carry waste molecules away e.g. the
thin bronchiole tubes in the lungs,
Thin capillaries which
have a particularly large surface to volume ratio - this allows fast
diffusion in either direction,
Animals need an efficient gaseous exchange
ventilation system to take in air for oxygen and give out air
including waste carbon dioxide,
in the lungs the tiny pockets
called alveoli greatly increase the gas exchange surface area :
The larger a multicellular
organism, the more difficult it is to exchange substances.
Cells deep in the body are some
distance to the surrounding environment - air or water.
Larger organisms have low
surface to volume ratio reducing exchange efficiency.
Therefore, through evolution,
instead of exchange through an outer membrane ('skin') multicellular
organisms have developed specialised exchange organs
including an equally specialised exchange surface.
BUT, specialised organs are not
enough on their own to serve a relatively large body, you also need
specialised transport systems to convey substances to and
from the body cells e.g. to provide nutrients or remove waste
In animals the
transport system is the circulatory system - blood
and also gaseous exchange
in lungs, the lengthy digestive system and the excretory
system - and all systems must work in harmony with each