Transport in
living things
25.1 Diffusion
• How are you able to smell various aromas from
food?
o
o
o
o
o
The aromas come from vapour from the food.
Molecules in the vapour move about randomly.
They bump into one another and spread out quickly.
As a result, some molecules enter your nostrils.
The spreading out of molecules is brought about by
diffusion.
• Diffusion in the overall movement of molecules
from a region of higher concentration to a region
of lower concentration.
Diffusion in non-living systems
permeable
membrane
A
B
permeable
membrane
water
molecules
10 % sucrose
solution
2 % sucrose solution
Hence,
there
is
net
of
Solutions
Aofand
Bhas
are
separated
bysugar
a
Both types
Since
Also,
solution
solution
molecules
BAahas
aamovement
higher
higher
are free
concentration
concentration
to
move
of
molecules
from
solution
Athe
to solution
Bare
and
permeable
membrane,
which
allows
both
across
of
water
sugar,
molecules.
the
more
membrane
sugar
Thus,
molecules
as
more
particles
water
will diffuse
in a
net
movement
ofto
water
molecules
small
andrandom
large
molecules
to
pass from
through.
constant
from
molecules
solution
willAdiffuse
motion.
solution
from
B.
solution
B to
solution
Bsolvent
to solution
A. Eventually,
both (the
Both
theA.
(water)
and the solutes
solution
solutions
will have the
sucrose molecules)
cansame
passconcentration
through it. of
sugar and water molecules.
sucrose
molecules
Movement of sucrose
molecules from A to B
Movement of water
molecules from A to B
Diffusion in living organisms
• Diffusion in living organisms occurs
continuously and it does not always take
place across a membrane (e.g. cell
membranes).
• Diffusion is important in moving materials
in and out of the cells.
• Diffusion in the
absence of
membranes
Similarly,dioxide
Diffusion
Carbon
occurs
oxygenenters
during
giventhe
off
photosynthesis
leaves
during
photosynthesis
through the
in the
absenceout
stomata,
diffuses
of
moving
membranes.
of leafinto
airair
spaces as
in the
there
leaves.
is a lower
concentration of oxygen
This is because
outside
the leaves.
the
carbon dioxide
concentration is higher
outside the leaves than in
the air spaces inside the
leaves.
cross-section of part of leaf
Diffusion in living organisms
carbon dioxide diffuses
in through stomata
air spaces
high
concentration
of oxygen
low
concentration
of carbon
dioxide
oxygen diffuses out
through stomata
Diffusion in living organisms
• Diffusion through a partially permeable membrane
o A partially permeable membrane is a membrane that
allows some molecules to pass through but not others.
o The cell surface membrane in plants is an example of a
partially permeable membrane.
• Gases (e.g. oxygen) and smaller molecules (e.g. sugar) diffuse into
the plants cells through the membrane.
o In humans, gaseous exchange occurs in the lungs by diffusion.
• When we breathe in, oxygen enters the air sacs in our lungs.
• The oxygen concentration is higher in the air sacs than in the blood
surrounding them.
• Thus, oxygen diffuses out of the air sacs into the bloodstream.
Diffusion in living organisms
• Diffusion through a partially permeable
membrane
o In humans, gaseous exchange occurs in the
lungs by diffusion.
• During respiration in the cells, carbon dioxide is
produced as waste.
• Carbon dioxide is carried in the bloodstream leading
to the lungs.
• It then diffuses out into the air sacs, and leaves the
body when we breathe out.
Diffusion in living organisms
• Diffusion through a partially permeable membrane
lungs
Carbon dioxide diffuses Blood leaving blood vessel has
Blood
entering
blood vessel
out of the
bloodstream
high concentration of oxygen
has
concentration
of Oxygen
diffuses from of
and high
into the
air sac.
and low concentration
carbon dioxide and low carbon
surrounding
air into the air
dioxide.
concentration of oxygen. sac and bloodstream.
tube
cluster of
air sacs
air sac
25.2 Osmosis
• Osmosis is the net movement of water
molecules from a region of higher
concentration of water molecules (higher
water potential) to a region of lower
concentration of water molecules (lower
water potential) through a partially
permeable membrane.
Osmosis in non-living systems
Water molecules
can pass freely
through the
membrane
A
B
Sucrose molecules
are too big to pass
through its pores.
10% sucrose solution
(lower concentration of
water molecules; lower
water potential)
5% sucrose solution
(higher concentration of
water molecules; higher
water potential)
Partially
permeable
membrane
As water
During
osmosis,
molecules
the flow,
waterthe
molecules
volume of
willsolution
flow from
A rises
solution
and B
the
with
volume
higherof
Solutions
and
are separated
by
a partially
permeable
membrane.
concentration
solution
BAdrops.
of B
water
When
molecules
the concentrations
(higher
water
of both
potential)
solutions
to
solution
A and B A
are
The
membrane
allows
water
molecules
to molecules.
pass
but not
withsame,
the
lower there
concentration
will be smaller
noofnet
water
movement
molecules
of water
(lower
waterthrough
potential).
the larger sugar molecules.
Osmosis in living organisms
• Recall some properties of cells:
o Cell membrane of both plant and animal cells
is partially permeable.
o The plant cell wall is made of cellulose. It is
permeable.
o The plant cell has a relatively rigid (firm) cell
wall.
Osmosis in living organisms
• What happens to animal cells when they are
placed in distilled water?
• Animal cells
o Animal cells have elastic membranes. When the
water molecules flow in, the animal cells will swell and
eventually burst.
water
moves in
Water potential outside the cell is
higher than that in the cytoplasm.
Cell expands and eventually
bursts.
Osmosis in living organisms
• What happens to plant cells when they are placed in
distilled water?
• Plant cells
o Plant cells have strong, rigid cells walls which prevent the cells
from expanding too much.
o When water molecules flow in, the contents in the cell press the
cell wall.
o The water creates a pressure on the cell wall of the plant cell.
This is called turgor pressure and keeps the plant tissues
turgid.
o Soft tissues in plants depend on turgor for support. If plants lose
too much water, they will wilt.
Osmosis in living organisms
• Plant cells
water
moves in
Water potential outside the cell is
higher than that in the cell sap.
Cell expands and becomes turgid.
The rigid cellulose cell wall
expands slightly only. This
prevents the cell from bursting.
Osmosis in living organisms
• What happens to animal cells when they are
placed in concentrated solution?
• Animal cells
o Animal cells will shrivel up as they lose water.
Water leaves
the cell by
osmosis
Concentration of water molecules
outside the cell is lower than that
in the cytoplasm.
Cell shrinks and becomes soft.
It is dehydrated.
Osmosis in living organisms
• What happens to plant cells when they are
placed in concentrated solution?
• Plant cells
o When the concentration of water molecules of the
cytoplasm and cell sap is higher than that of the
surrounding solution, water leaves the plant cells by
osmosis.
o The vacuoles shrink and the cell contents pulls away
from the cellulose cell walls. The plant cells lose
turgor pressure and become flaccid.
Osmosis in living organisms
• Plant cells
Water leaves
the cell by
osmosis.
Concentration of the water
molecules outside the cell is lower
than that in the cell sap.
Cell contents pull away from the
cell walls and the cell becomes
flaccid.
25.3 The Need for a Transport
System
• A multicellular organism like a plant or a human
being is made up of billions of cells.
• Most of these cells lie deep inside the body of
the organism.
• Nutrients and oxygen need to reach each cell in
the body to support the life of the cells.
• A transport system ensures that substances are
transported fast enough throughout the
organism.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• Water and mineral salts are absorbed by
the root hairs from the surrounding soil.
• Water enters the roots by osmosis.
• Mineral salts enter by active transport.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• How is water absorbed by the roots?
o There are numerous root hairs at the ends of
young roots.
o A root hair is an outgrowth of a root hair cell.
o It has a cell membrane, cytoplasm and a large
central vacuole.
o The vacuole contains cell sap, which contains
mainly water and dissolved substances such
as mineral salts.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• How is water absorbed by the roots?
soil particles
film of
water
water
entering
root hair
root
hair
cell
lower
concentration
of water
molecules
higher
concentration
of water
molecules
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• How is water absorbed by the roots?
o The soil solution consists of water with
dissolved mineral salts.
o It is less concentrated (higher concentration of
water molecules) than the cell sap and the
cytoplasm.
o As the cell membrane is partially permeable,
water enters the root hairs by osmosis.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• How are dissolved mineral salts absorbed
by the roots?
o The concentration of mineral salts in the root
hairs is often higher than in the soil solution.
o Thus, mineral salts should diffuse out of the
root hairs.
o However, this does not occur as the root hairs
not only prevent the outflow of the salts, but
absorb salts from the soil solution.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• How are dissolved mineral salts absorbed
by the roots?
Flow of mineral salts upwards
xylem
soil particles
root hair
absorbing mineral
salts from the soil
The
of substances
against
a concentration
The movement
root hairs absorbed
dissolved
mineral
salts from agradient
region of
using
is called
transport.
lower energy
concentration
to active
a region
of higher salt concentration. This
occurs because the root hairs are using energy during the
absorption process.
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• Why is energy required in
active transport?
o The higher concentration of
molecules is at the top of
the slope.
o The lower concentration of
molecules is at the bottom
of the slope.
o In diffusion, molecules can
roll down the slope without
using much energy.
Diffusion
25.4 Absorption of Water and
Mineral Salts by Roots of Plants
• Why is energy required
in active transport?
o If the molecules were to
move up the slope,
energy is needed.
o This is what happens in
active transport.
Active transport