© Boardworks Ltd 20071 of 44. © Boardworks Ltd 20072 of 44

© Boardworks Ltd 20071 of 44



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.

highconcentration

lowconcentration


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?

umbilicalcord

blood to motherlow in O2/nutrients,high in CO2/waste

umbilicalartery

umbilicalvein

placental villiincrease surface area for diffusion

blood from motherhigh 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-permeablemembrane

(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.

plasmamembrane


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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