Transport and Exchange
Mechanisms

cells need to receive metabolites
from surrounding metabolic processes

  • e.g. glucose (respiration)
  • nitrates for plants + synthesis amino acids

waste products need to be
removed from cells

  • e.g. CO2 from respiration
  • urea - waste product

Types of
Organisms

unicellular

use cell membrane as
exchange surface

multicellular

specialised exchange surfaces
to exchange substances

complex
organisms

need transport system to
link exchange surface to all cells

Surface Area

D: can be represented as the total
number of cells in direct contact with
surrounding environment

absorptive surface area - measure of
rate of supply of metabolites to tissue

Volume

D: the total 3D space occupied by
metabolically active tissues

a measure of an organisms
demand for metabolites

Surface Area to
Volume Ratio

as surface area
INCREASES,
SA:V DECREASES

CUBE

as side of cube
increase, SA:V
ratio decrease

Small
Organisms

can gain ALL metabolites +
remove all waste via body surface

large SA:V ratio

Large
Organisms

small SA:V ratio

greater metabolic needs so
requirement of metabolites
higher

multicellular animals so majority
of cells not in direct contact
with surrounding environment

have SPECIALISED
EXCHANGE SURFACES

Features of Exchange
Surfaces

Increased Surface
Area

Thin Separating
Surface

Large
Conc.
Gradients

will INCREASE
overall SA:V ratio

FLATTENED
SHAPE

SPECIAL EXCHANGE
SURFACES

e.g. flatworm

increases SA:V ratio

decreases diffusion
distance

need to be MOIST

EXTERNAL

INTERNAL

e.g. gills of
young tadpole

e.g. fish gills +
alveoli in mammals

need to be thin so RATE
of DIFFUSION is MAXIMISED

conc. gradient necessary for
diffusion to occur

Small organism - constantly
use oxygen for respiration

Large organisms - ventilation systems
e.g. breathing in mammals

Mass Flow

internal
transport
system

D: allows transport of substances in
larger organisms

  • All molecules swept along
    in SAME direction.
  • Brought about by pressure difference within organism

SOURCE - Pressure
generated

SINK - Pressure
much less

Examples

Xylem tissue

transports water + mineral ions
from the root to the leaves

water evaporation from leaf
creates tension (negative pressure)
that pulls water up through xylem
via transpiration system

Phloem
tissue

transports sucrose (translocation)
to roots (for storage of carbohydrates)
and to growing regions (energy for growth)

  • energy expenditure involved
    moving sucrose into phloem.
  • ATP used move sucrose from
    companion cell to phloem sieve tube

Circulatory system

High pressure generated
by pumping of heart

transports substances - O2, CO2, glucose,
amino acids, lipids, urea in blood system
around body

Ventilation (breathing)
in mammals

  • reduction of pressure in thorax
    causes air to enter lungs (inhalation)
  • increased pressure in thorax
    causes air to be expelled from lungs
    (exhalation)

brings in O2 rich air into lungs and removes
air with raised CO2 levels.
Ensures diffusion of respiratory gases
can take place between alveoli and capillaries