Movement in and out of cells

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What is diffusion?

Diffusion and concentration
Why can the student smell the sock from a distance?
The sock can be smelt because sweat and other
molecules are moving away from it and spreading out in
the air. This is called diffusion.
Where is the smell strongest?
The smell is strongest at the sock. The smell becomes
weaker further away from the sock.

Moving molecules
In which states are molecules able to diffuse?
Molecules in liquids and gases are constantly moving and
bumping into each other. This means that they tend to
spread out.
gas (e.g. steam)liquid (e.g. water)solid (e.g. ice)

Diffusion in action: one gas

Diffusion in action: two gases

Changing concentrations
The rate of diffusion depends on several factors, such as
the distance the particles have to travel and the
difference in concentration.
Molecules diffuse until they
are evenly spaced apart
and equilibrium is reached.
During diffusion molecules move from an area of high
concentration to an area of low concentration. They are
said to move down a concentration gradient.
Diffusion is a passive
process which means that
no energy is needed.
high
concentration
low
concentration

Diffusion: true or false?

Why is diffusion important to life?
Diffusion is the main way in which substances move over
short distances in organisms. What substances need to
move?
Oxygen, food and waste products are some of the
substances that move by diffusion.
In animals, how do these vital substances get to where
they are needed?
The substances are
transported in the
bloodstream, from
where they can diffuse
in and out of cells.

Diffusion and breathing
Breathing involves the exchange of gases in the lungs; a
process that occurs by diffusion. What happens when
you breathe in?
Oxygen in inhaled air diffuses
through the lungs and into the
bloodstream. The oxygen is then
transported throughout the body.
Carbon dioxide is the waste gas produced by respiration.
Carbon dioxide diffuses from body tissues into the
bloodstream and is exhaled via the lungs.
Where does gas exchange take place in the lungs?

Structures of the respiratory system

Cross-section through an alveolus
deoxygenated blood
(from body tissues)
oxygenated blood
(to body tissues)
air in/out
Alveoli are the tiny air sacs at the end of the bronchioles,
in which gas exchange occurs.
alveolus
capillary
red blood cell

How are alveoli adapted?
Alveoli have several adaptations that help to make gas
exchange very efficient:
 They are very thin – only one cell thick.
 They are covered by a network of fine capillaries,
enabling gases to pass almost directly between the
lungs and bloodstream.
 They are moist, encouraging gas molecules to easily
dissolve.
 They have a large combined surface area, allowing
large amounts of gases to be exchanged with each
breath.

What happens in the alveoli?

Diffusion and digestion
Digestion breaks down large food molecules into smaller
molecules such as glucose, amino acids and fatty acids
that can be easily absorbed.
Carbohydrates, proteins and fats are made up of large
molecules that cannot be readily absorbed by the body.
In which part of the digestive system is most food absorbed?
Small food molecules are usually absorbed in the small
intestine, diffusing across the intestine wall and into the
bloodstream.

How is the small intestine adapted?
The wall of the small intestine is lined with many tiny finger-
like projections called villi. These are very thin and increase
the surface area of the small intestine, both of which increase
the speed of diffusion.
blood vessels
capillary network villus
small intestine

Diffusion and the small intestine
How does the initial concentration of dissolved food
molecules in the small intestine compare with the
concentration of the blood in the villi?
The concentration of dissolved food molecules is higher
in the small intestine than in the blood entering the villus.
This means that the
dissolved food
molecules diffuse from
the small intestine into
the blood, moving from
higher to lower
concentration.
glucose

Diffusion and the placenta
The placenta is an organ that develops in the uterus of
female mammals during pregnancy. The umbilical cord
connects the placenta to the fetus.
The placenta enables
nutrients and oxygen to
pass from the mother to the
fetus by diffusion, and waste
substances to diffuse from
the fetus back to the mother.
The placenta can filter out certain molecules and bacteria,
but is unable to stop many harmful substances such as
alcohol, chemicals and some types of virus from reaching
the fetus.

Diffusion and the placenta
How does the placenta work?
umbilical
cord
blood to mother
low in O2/nutrients,
high in CO2/waste
umbilical
artery
umbilical
vein
placental villi
increase surface
area for diffusion
blood from mother
high in O2/nutrients,
low in CO2/waste

Diffusion and nerves impulses
nerve impulseneurotransmitter
receptor
synaptic cleft
A synapse is a junction between two neurones across
which electrical signals must pass.
Neurotransmitter molecules diffuse from vesicles towards
the neurotransmitter receptors, moving from an area of
high concentration to low concentration.
neurotransmitter

What is osmosis?
water
glucose
partially-
permeable
membrane
(visking tubing)
Osmosis is the diffusion of water molecules from a low
concentration solution to high concentration solution,
across a partially-permeable membrane.
A partially-permeable membrane has holes in it that permit
water molecules through but are too small to allow larger
molecules through. Osmosis can be demonstrated using
visking tubing filled with a solution and placed in a beaker
of pure water.

Dilute vs. concentrated
 Concentrated solutions have a low concentration
of water molecules.
 Dilute solutions have a high concentration of
water molecules.
During osmosis, water molecules diffuse from pure water or
dilute solution to more concentrated solutions.
pure water dilute solution
concentrated
solution

Predicting osmosis

Osmosis in action

Osmosis and cells
Plant and animal cells are surrounded by a partially-
permeable plasma membrane. This allows water and other
small molecules to diffuse across.
plant cell
red blood cell
cell wall
Plant cells additionally have a strong cell wall surrounding
the membrane which offers support and protection.
plasma
membrane

Osmosis and plant cells

Osmosis and animal cells
Animal cells do not have a cell wall. This means they
respond differently to plant cells to the gain and loss of
water.
In concentrated solutions, water
loss causes the cells to shrink.
When this happens to red blood
cells, it is called crenation.
In dilute solutions, osmosis can
cause animals cells, such as red
blood cells, to swell up and burst.
This is called lysis.

In order to remain healthy, animal cells need to maintain
an isotonic water balance. This means that the water
concentration both inside and outside the cell are equal.
The concentration of water
and salt in the blood are
controlled by the kidneys.
The kidneys are controlled
by the portion of the brain
called the hypothalamus.
hypothalamus

Movement in and out of cells
Substances move in and out of cell by passive and active
transport. Which methods of transport are illustrated?

What is active transport?
Substances can move passively in and out of cells by
diffusion until the concentration on both sides of the cell
membrane reaches an equilibrium.
Substances can continue to move in and out of a cell using
a process called active transport.
As the name suggests, active transport
requires energy from the cell, which is
made available by respiration.
During active transport, protein
carriers in the cell membrane ‘pick
up’ particles and move them against
the concentration gradient.

What is active transport?

Active transport in plants
Plants need to absorb mineral elements such as nitrogen,
phosphorus and potassium from the soil for healthy growth.
When the concentration of minerals in soil is lower than
inside the plant, active transport is used to absorb the
minerals against the concentration gradient.
What would happen if the
plant relied on diffusion to
absorb minerals?
The cells would become
drained of minerals because
they would travel down the
concentration gradient.
minerals

Active transport in humans
During digestion, the villi in the small intestine absorb the
soluble nutrients. Over time, the concentration of nutrients in
the villi reach an equilibrium with the concentration in the gut.
Active transport is
used to continue the
transport of the small
amounts of remaining
nutrients against the
concentration gradient.
glucose

Glossary (1/2)
active transport – The movement of molecules against
a concentration gradient, and which requires energy.
alveoli – The tiny air sacs at the end of the bronchioles in
which gas exchange takes place in mammals.
concentration gradient – The difference in
concentration across a given area.
crenation – The shrinking of animal cells in response to
water loss by osmosis.
diffusion – The movement of molecules from an area of
high concentration to low concentration.
osmosis – The diffusion of water molecules from a dilute
solution to a more concentrated solution.

Glossary (2/2)
partially-permeable membrane – A membrane that
allows only certain sized molecules to pass through it.
placenta – The organ that transports oxygen, nutrients
and waste products to and from the developing fetus.
turgid – The state caused by high water uptake in plant
cells.
villi – Tiny finger-like projections on the inner surface of the
small intestine, across which nutrients diffuse.

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Which process?

Multiple-choice quiz

Movement in and out of cells

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