Starter Activity:

Answer the following question in your classwork jotter

1. What type of fibres are the cell walls of plants made of?

2. Name the small rings of DNA found in bacterial cells.

3. Name the process by which yeast cells reproduce.

4. What is the function of a) ribosomes? b) mitochondria?

Answers

1. Cellulose2. Plasmids3. Budding4. a) protein synthesis b) respiration

Key Area 2

Transport across cell membranes

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Be able to describe the structure of the cell

membrane• Understand the terms passive and active transport • Understand the term diffusion and give examples

of diffusion in cells• Understand the term osmosis and its effect on

cells• Understand the idea of concentration gradient

The Function of the Cell Membrane

Twig Video Clip: The Cell Membrane

Whilst you are watching the video clip write down key words on a show me board.

Structure of the cell membrane

• The cell membrane is very thin.

• It is able to control the movement of materials into and out of cells.

• When seen under an electron microscope, it can be seen to consist of a double layer.

Cytoplasm of cell

Cell membrane

Structure of the cell membrane

• It has a double layer made of molecules called phospholipids.

• Phospholipid molecules have a head and a tail end.

• All the tails point inward and the heads point outwards.

Head end

Head end

Tails

Structure of the cell membrane• There are also proteins embedded in the

membrane.• These proteins have various functions.• Some are involved in the transport of

materials across the membrane.

proteins

Phospholipid double layer

Fluid mosaic model• The membrane is a very flexible structure

which can move and change shape easily.• It is often described as a fluid mosaic model. • Fluid refers to the moving phospholipids and

mosaic refers to the patchy arrangement of the proteins.

Homework task - Make a 3D cell membrane model

• Your task is to make a 3D model of the cell membrane.

• You can use any materials you can find at home, especially recycling materials.

• Your model should show the proteins and the phospholipids with their head and tail ends.

• The component parts should be labelled or there should be a key to identify them.

• Examples of materials you could use include paper, cardboard, plastic bottle lids, plasticene, playdoh, straws, cotton buds, cocktail sticks, buttons, ribbons, fabric, lolly sticks, dry pasta, lego, stickle bricks, k’nex.

• THE POSSIBILITIES ARE ENDLESS SO HAVE FUN!

Selectively permeable membrane

• Permeability means the ability of a surface to allow molecules to pass through it.

• The cell membrane is usually described as selectively permeable.

• This means that it allows some molecules to move through it easily, but others move more slowly, and some cannot move across it at all.

Visking tubing

• Visking tubing is a selectively permeable material that acts like a cell membrane.

• We can use it to demonstrate which molecules are able to pass through the membrane, and which are not.

• Your teacher will show you how to use the visking tubing in an experiment.

Visking tubing experiment

Water

Visking tubing bag

Starch and glucose solution

Test water for starch and glucose after 20 minutes

Visking tubing experiment1. Soften a length of visking tubing under running

water until you can open it up.2. Tie a firm knot in one end.3. Add equal volumes of starch and glucose

solutions using droppers.4. Tie a firm knot in the other end. Cut off any

excess tubing to leave 1cm tails.5. Wash the bag thoroughly under running water.6. Place bag in a test tube, cover with water and

leave for 20 minutes. Remove the bag.7. Transfer a drop of the water to a spotting tile

and test for starch using iodine.8. Test the remaining water for glucose by adding

Benedicts solution and heating to 80°C for 5 minutes.

Visking tubing experimentResults:

Colour change observed

Result of test – positive or negative

Starch test (iodine)

Glucose test(Benedicts)

Conclusion : The ______was able to pass through the visking tubing but the _______was not. This is because starch has ________ molecules but glucose has ________molecules.

Selectively Permeable Membranes

• Cell membranes allow small molecules like glucose, oxygen and water to pass through them freely.

• This is because the membrane has tiny holes in it called pores that make it permeable. Large molecules like starch are unable to pass through.

Selectively Permeable Membranes

• Pores in the membrane are small, so only small molecules such as glucose, water, oxygen and carbon dioxide can get through.

• Large molecules such as starch cannot pass through.

• Selectively permeable membranes allow certain molecules to pass through but not others.

nucleus

selectively permeable membrane

cytoplasm

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Be able to describe the structure of the cell

membrane• Understand the terms passive and active transport • Understand the term diffusion and give examples

of diffusion in cells• Understand the term osmosis and its effect on

cells• Understand the idea of concentration gradient

Starter

Starter

C

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the terms passive and

active transport • Understand the term diffusion and

give examples of diffusion in cells

Passive transport

• passive transport is the movement of a substance across a cell membrane down a concentration gradient

• It does not require energy. • The two types of passive transport

are:1. Diffusion2. Osmosis

Diffusion

•Diffusion is the movement of

molecules in a liquid or gas from high

to low concentration until they are

evenly spread out.

•Diffusion moves down a

concentration gradient. Diffusion clip

Diffusion in liquids

• Diffusion in liquids can be seen by adding dye to a beaker of water.

Add dye to one side only

Water

Do not shake or stir. Leave for 20 minutes.

Diffusion in liquids

Red dye in water

Red dye molecules and water molecules have moved until they are evenly spread – this is DIFFUSION.

Direction of diffusion

Diffusion always occurs from high concentration of a molecule to low concentration of that molecule.

Concentration Gradient

The difference in concentration of two solutions is called a concentration gradient.

In diffusion, molecules will always move down the concentration gradient from high concentration to low concentration.

Concentration Gradient

high ground

gradient (slope)

low ground

ball ball rolls down gradient

ball stops

•Like a ball on a slope, molecules diffuse down a concentration gradient from high to low.

Concentration Gradient

• A concentration gradient exists when there

is a difference in concentration from one

area to another.

• Molecules move down a concentration

gradient from high to lower concentration.

The molecules will stop moving when the

two concentrations are equal.

Cells and diffusion

• Many substances can enter or leave cells by diffusion.

• This happens across the cell membrane.

• Animal cells take in glucose, oxygen and amino acids by diffusion.

• Carbon dioxide and waste materials leave animal cells by diffusion.

Diffusion in an animal cell

Glucose

Oxygen

Carbon dioxide

Waste

Amino acids

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the terms passive and

active transport • Understand the term diffusion and

give examples of diffusion in cells

Starter

Starter

Diffusion

Waste/carbon dioxide

Cell membrane

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis and its

effect on cells• Understand the idea of concentration

gradient

Osmosis• Water can also diffuse into and out of cells across the cell membrane.

• Osmosis is the special diffusion of water from an area of high concentration to an area of low concentration through a selectively permeable membrane.

High water concentration

Low water concentration

Twig Video Clip - Osmosis

Visking tubing

• Visking tubing is a selectively permeable material that can be used to show the effect of osmosis on cells.

• The visking tubing behaves like a cell membrane, so we can use it to make model cells.

Osmosis experiment

A B

Visking tubing bag

10% sugar solution

water

Boiling tube

1. Wash and dry bags when filled.2. Weigh both bags.3. Place in test tubes for 20 minutes then dry and reweigh.

TEACHER DEMONSTRATION

Results

A B

Mass of bag and contents at start (g)

Mass of bag and contents after 20 minutes (g)

Difference in mass (g)

Conclusion

• Bag A increased/decreased in mass. This was because water moved in/out by osmosis.

• Bag B increased/decreased in mass. This was because water moved in/out by osmosis.

• Water always moves from ________ water concentration to ______ water concentration.

Answer the following questions in sentences:

1. Why was the visking tubing bag dried in a paper towel before being weighed?

2. Why was visking tubing used in this experiment? What property does it have that makes it a good model cell?

3. What would happen to an onion cell placed in pure water?

4. What would happen to a cheek cell placed in 10% sucrose solution?

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis and its

effect on cells• Understand the idea of concentration

gradient

Starter

Starter

C

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis

Understand the effect of osmosis on plant cells

Osmotic Effect On Cells

Water concentrations • If we think about solutions in terms of

their water concentrations, it is easier to recognise which direction water molecules will flow in.

• A dilute sugar solution will have a high concentration of water, whereas a concentrated sugar solution will have a lower water concentration.

Dilute Sugar Solution

Concentrated Sugar

Solution

Low water concentration

High sugar concentration

High water concentration

Low sugar concentration

Water Concentrations

H2O concentration

>

The surrounding solution has a higher water concentration than the water concentration within the cell, so water enters by osmosis.

cell

Direction of water

movement

Cell in pure water

Water ConcentrationsCell in concentrated salt solution

cellH2O

concentration <

The surrounding solution has a lower water concentration than the water concentration within the cell, so water leaves the cell by

osmosis.

Water Concentrations

cellH2O concentration =

Cell in solution with equal concentration to cell contents.

The surrounding solution has a water concentration that is equal to the water concentration within the cell, so there is

no gain or loss of water by osmosis.

Investigating osmosis in potato tissue

1. Copy the title and the aim from the instruction card.

2. Prepare a results table.3. Set up the experiment by

following the instructions on the card.

Results

Conclusion

Evaluation

Bathing solution

Initial mass (g)

Final mass (g)

% Change in mass

Water

Strong sucrose solution

Plant Cells and OsmosisPure water: there is more water outside of the cell than inside, therefore water will move into the cell by osmosis. This causes the cell to swell and become turgid.

Strong salt solution: there is more water inside the cell than outside, therefore water will move out of the cell by osmosis. This causes the cell to become softer or flaccid.

Cell in pure water

Cell in strong salt solution

Plant Cells and Osmosis

Plant Cells and Osmosis

Cell in pure water - turgid

Normal cell Cell in strong salt solution - plasmolysed

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis

Understand the effect of osmosis on plant cells

Starter

Starter

A

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis

Understand the effect of osmosis on animal cells

Viewing plasmolysed cells

• In this practical, you will expose onion cells to a solution of salt to observe plasmolysis. You will also prepare cells in water for comparison (as a control experiment).

• You will view the cells under the microscope and make drawings of what you see.

• You will need a pencil and a circular object (eg beaker) to draw round.

• Follow the instructions on the card provided.

Plasmolysis in onion cells• The photo shows

onion cells which have been bathed in strong salt solution and have become plasmolysed.

• Notice how the cytoplasm and cell membrane have shrunk inwards away from the cell wall.

cell wall

cell membrane

nucleus

cytoplasm

Osmosis in red cabbage cellsYour task in this experiment is to identify liquids A and B by looking at the results.

1. Label two boiling tubes A and B.2. Half fill tube A with liquid A and tube B

with liquid B.3. Cut 10 discs from red cabbage using a

cork borer.4. Wash the leaf discs in a small beaker of

water and dry them gently.5. Add 5 discs to each test tube.6. Leave for 20 minutes and observe any

colour change in the liquids.

Osmosis in red cabbage cells

Results:

Test tube A Test tube B

Appearance of liquid

Conclusion:Which test tube contained strong sucrose solution? How do you know this?Which test tube contained water?How do you know this?

Animal Cells & Osmosis

• The effects of osmosis on animal cells are totally different to plant cells because animal cell structures are different.

• Animals cells do not have:Cell wallsVacuoles

Animal Cells & Osmosis

• Red blood cells (RBCs) float in a liquid called plasma which is the same water concentration as the cells.

• RBCs in plasma do not change size because the water has no concentration gradient to follow.

• RBCs will change if there is a concentration gradient for water to follow.

Normal red blood cell

RBC in strong salt

solution

RBC in equal concentration solution

RBC in pure water

Cell loses water by osmosis

and shrinks.

No net osmosis, cell stays the same.

Cell takes in water

by osmosis,

swells and eventually

bursts.

Animal Cells & Osmosis

Osmosis in eggs

• Your teacher will show you a demonstration experiment involving eggs placed in different solutions.

• Write a description of this experiment in your notebook. Include the following:

TitleDiagramMethod ResultConclusion

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term osmosis

Understand the effect of osmosis on animal cells

Starter

Starter

Turgid

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term active transport

and its effect on cells• Understand the idea of concentration

gradient

Passive transport

• Diffusion and osmosis are examples of passive transport.

• This always involves movement of molecules down a concentration gradient.

• Passive transport does not need any energy input.

Active transport• Some molecules and ions are moved

across the membrane by active transport. • This involves movement of molecules and

ions against a concentration gradient. • Active transport does need energy input.• Membrane proteins are involved in this

process.

Transport across cell membranes (Glow)

Active transport

The sodium potassium pump

Sodium pumped out

Potassium pumped in

OUTSIDE THE CELL

INSIDE THE CELL

CELL MEMBRANE

•This is an example of active transport which occurs in nerve cells.•Sodium ions are pumped out of cells against the concentration gradient.•Potassium ions are pumped into cells against the concentration gradient.

Membrane proteins

Active transport in cells

• When a cell is alive, the membrane can use active transport to make sure that suitable molecules are allowed to build up in the cell, and unsuitable or harmful molecules are kept out of the cell.

Practical application of this idea

• You will probably have had antibiotics at some time for an infection.

• Many antibiotics work by destroying the membranes of live bacteria in your body.

• The damaged membranes allow vital nutrients to escape and poisonous substances to enter and kill the bacterial cells.

Transport across cell membranes

Learning Intention:Learning Intention:Investigate the cell membrane and the movement of molecules across it

Success Criteria:Success Criteria:• Understand the term active transport

and its effect on cells• Understand the idea of concentration

gradient