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KS4 Biology
Movement
In and Out of Cells

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Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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Why does the colour of
concentrated fruit drink change
when it is added to water?
Why can you smell cooked
food from a distance?
Smells and liquids spreading out
Why can you smell
perfume or aftershave
when someone walks
past you?

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These processes are all examples of diffusion.
Smelling cooked food or perfume and diluting juice in water
all involve things moving around and spreading out.
Smells and liquids spreading out
Diffusion involves the movement of molecules.
What type of molecules are moving in these examples?

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The molecules in every substance are always moving.
Moving molecules
In which state are molecules able to spread out?
water (liquid) water vapour (gas)
Diffusion involves the movement of gas and dissolved
molecules. Why don’t solid molecules diffuse?
ice (solid)

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When smelling body spray where is the smell strongest
and where is the smell weakest?
Diffusion and changing concentrations
Smell is strongest
at source.
Diffusion means the smell
spreads out and gets weaker
further away from the source.
How does the concentration of smell molecules change
during diffusion?
Smell molecules move from an area of high concentration
to an area of lower concentration. This is called a
concentration gradient.

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Where will the molecules be after diffusion?
Diffusion can be represented on a simple diagram.
Changing concentrations
before diffusion
Diffusion is the movement of gas or dissolved molecules
from higher to lower concentration.
Molecules diffuse down a concentration gradient.
What does this mean?
after diffusion
What has happened to the concentration of the molecules?

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Diffusion animation 1

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Diffusion animation 2

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Diffusion animation 3

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Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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Why is diffusion so important?
Diffusion occurs in the organ systems that control your
breathing, circulation, digestion and other life
processes.
Diffusion explains why you can smell smelly smells and
what happens when you dilute a concentrated fruit drink.
More importantly, diffusion is an essential process
that is going on inside your body right now and
keeping you alive!

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Diffusion is the movement of gas or dissolved molecules.
What gas and dissolved
molecules do body cells need?
Your body’s survival depends
on oxygen and dissolved food
molecules getting into your cells.
Diffusion and life processes
How do these vital substances cross boundaries
within the body to get to where they are needed?
Oxygen and dissolved food molecules
must diffuse into and out of the blood
for transportation around the body.
Where does diffusion take place within the body?

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Breathing involves the exchange of
gases in the lungs, a process which
occurs by diffusion.
What is the vital gas that
you breathe in?
Diffusion and breathing
The lungs have a huge surface area to maximize the
exchange of oxygen and carbon dioxide with each breath.
Where in the lungs does gas exchange take place?
oxygen
carbon dioxide
What is the waste gas that
you breathe out?

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Gas exchange takes place in the alveoli, the tiny air sacs
at the end of the bronchioles.
Diffusion and breathing
What are these blood vessels around each alveolus called?

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Inhaling increases the concentration of oxygen molecules
in an alveolus.
At this point, how does this compare with the concentration
of oxygen in deoxygenated blood in the capillary?
Diffusion and breathing – oxygen
deoxygenated blood
from the body
oxygenated blood
to the body
When you inhale,
the concentration
of oxygen inside
each alveolus is
higher than in
deoxygenated
blood.
oxygen
into alveoli

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How is the concentration of oxygen in the blood increased?
Oxygen molecules diffuse across the lining of the alveolus
into the blood in the capillary, moving from higher to lower
concentration.
Diffusion and breathing – oxygen
oxygenated blood
to the body
deoxygenated blood
from the body
oxygen
into alveoli
diffusion
When you inhale,
the concentration
of oxygen inside
each alveolus is
higher than in
deoxygenated
blood.

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Deoxygenated blood from the body is low in oxygen
but has high levels of carbon dioxide.
How does this compare with concentration of carbon dioxide
in the alveolus?
Diffusion and breathing – carbon dioxide
deoxygenated blood
from the body
oxygenated blood
to the body
The concentration
of carbon dioxide
in deoxygenated
blood is higher
than in
deoxygenated
blood.

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How is the concentration of carbon dioxide in the blood
reduced?
Carbon dioxide molecules diffuse from the blood in the
capillary into the alveolus, moving from higher to lower
concentration.
Diffusion and breathing – carbon dioxide
deoxygenated blood
from the body
oxygenated blood
to the body
The concentration
of carbon dioxide
in deoxygenated
blood is higher
than in
deoxygenated
blood.
carbon dioxide
out of alveoli
diffusion

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Gas exchange animation

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Diffusion and digestion
Digestion breaks large food
molecules into smaller molecules
that can be used by the body.
Dissolved food molecules
then need to be transported
from the small intestine into
the bloodstream.
Where in the small intestine does
this diffusion take place?
Carbohydrates, proteins and fat are made up of large
molecules that cannot be used directly by the body.

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Inside the small intestine

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Cross-section of a villus
Each villus separates the digested food in the small intestine
from a network of capillaries.

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Diffusion from the small intestine
Each villus has a surface layer that is one cell thick.
How does the concentration of dissolved food molecules in
the small intestine compare with the blood entering a villus?
The concentration of
dissolved food molecules
is higher in the small
intestine than in the
blood entering the villus.
wall of small
intestine
blood entering
villus
blood leaving
villus
inside the
small intestine

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Diffusion from the small intestine
Dissolved food molecules diffuse from the small intestine
into the blood, moving from higher to lower concentration.
How is the concentration of dissolved food molecules in the
blood increased?
The concentration of
dissolved food molecules
is higher in the small
intestine than in the
blood entering the villus.
wall of small
intestine
blood entering
villus
blood leaving
villus
inside the
small intestine
diffusion

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Diffusion in and out of cells
Oxygen and dissolved food molecules are transported to
the body’s cells in the bloodstream.
How does the concentration of these useful substances in
the blood compare with the concentration inside the cells?
The concentration of
oxygen and dissolved
food molecules
is higher in the blood
arriving at the cells
than inside the cells.

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Diffusion in and out of cells
How do oxygen and dissolved food molecules pass
from the blood into the cells?
Oxygen and dissolved food molecules diffuse into the
body cells, moving from higher to lower concentration.
The concentration of
oxygen and dissolved
food molecules
is higher in the blood
arriving at the cells
than inside the cells.
What process involving
these useful substances
takes place inside the cell?
diffusion

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Diffusion in and out of cells
The cells use the food and oxygen for respiration.
Carbon dioxide is the waste product of respiration and
will poison a cell if it is not removed. How is it removed?
Carbon dioxide molecules
diffuse from the cells into
the bloodstream, moving
from higher to lower
concentration.
How is this waste carbon
dioxide removed from the
body?
diffusion

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Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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What is active transport?
Movement of substances in and out of cells by diffusion
involves molecules moving down a concentration gradient
from high to low concentration.
Sometimes substances move into cells from low to high
concentration. This is called active transport.
Active transport needs energy to make it happen.
How do molecules move along the concentration gradient
during active transport?
high concentration
low concentration
diffusion active transport

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Active transport and plants
Plants need mineral elements from the soil for healthy
growth. Minerals enter a plant though its roots.
The concentration of minerals in the soil is lower than that
inside a root hair cell, so how do minerals enter the root cell?
soil
mineral
element
root hair
cell

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Active transport and plants
Minerals enter a root cell by active transport.
The plant uses energy to move minerals up the
concentration gradient from the soil into its root cells.
Why is it important for plants to use energy in this way?
soil
mineral
element
root hair
cell
active transport

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Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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What is osmosis?
Osmosis is a special type of diffusion.
Diffusion involves gas or dissolved molecules, but osmosis
only involves the movement of water molecules.
Osmosis occurs across a semipermeable membrane
which has tiny holes in it.
These holes are small enough for water molecules to pass
water
molecule
through but larger molecules cannot pass through.

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What happens during osmosis?
Osmosis is the movement of water molecules from a
region of high water concentration to a region of low
water concentration across a semipermeable membrane.
Osmosis is a type of diffusion, so what do you think
happens to water molecules during this process?
water
molecule
semipermeable membrane
osmosis

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Osmosis experiment
A bag made from a semipermeable membrane is tied
to a glass tube and filled with a strong sugar solution.
What happens to the water molecules in the weak solution?
What happens to the liquid inside the semipermeable bag?
This bag is placed in a weak sugar solution.
strong sugar
solution
weak sugar
solution

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strong sugar
solution
weak sugar
solution
Osmosis experiment
Water molecules diffuse across the membrane from the
weak sugar solution into the strong sugar solution.
This continues until the concentration is the same on both
The volume of liquid in the semipermeable bag increases.
The liquid rises up the glass tube and then stops.
Why does this happen?
sides of the membrane.

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Osmosis animation 1

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Osmosis animation 2

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Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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Experiment to demonstrate diffusion

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Experiment to investigate osmosis

43. © Boardworks Ltd 200443 of 44
Osmosis
Experiments: diffusion and osmosis
Movement In and Out of Cells
Diffusion and life processes
Active transport
Contents
What is diffusion?
Multiple-choice quiz

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Multiple-choice quiz