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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 networkvillus

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 solutionconcentrated

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 cellred 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.


Anagrams


Which process?


Multiple-choice quiz

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