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. Cellulose

2. Plasmids

3. Budding

4. a) protein synthesis

b) respiration

Key Area 2

Transport across cell membranes

The Cell Membrane

Learning Intention: • Describe the structure of the cell membrane Success Criteria: • Be able to describe the structure of the cell

membrane. • Explain the terms ‘selectively permeable’ and ‘fluid

mosaic’

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.

• The cell membrane is known to contain protein and phospholipid molecules.

• These molecules are arranged in a fluid mosaic structure

Fluid Mosaic Model

Fluid Mosaic Model

The cell membrane consists of:

• phospholipid molecules that are constantly moving which makes the structure fluid.

• Patches of protein molecules which form a mosaic.

Phospholipid bilayer

• The water-soluble hydrophilic heads make up the two outer surfaces of the bilayer.

• One layer of heads forms the outside of the cell membrane and the other layer forms the inside of the cell membrane in contact with the cytoplasm.

Water insoluble hydrophobic tails point inwards to the centre of the bilayer since they are attracted to those in the opposite layer.

An effective boundary.

• This arrangement of phospholipids is fluid but also stable.

• It forms an effective boundary around the cell.

• It allows tiny molecules such as water to pass through it rapidly.

• Larger molecules such as glucose rely on the membrane’s protein molecules to enter or leave the cell.

Cell membrane proteins. These are found: • attached to surface of the membrane. • embedded within the membrane. • as channels (pores) which span the membrane.

Functions of Proteins

• Structural support

• Channels

• Carriers

• Enzymes

• Receptors

• Antigenic markers

The cell membrane is SELECTIVELY PERMEABLE, and controls which substances can enter and leave the cell.

Specificity

• Proteins have a unique shape and only allow certain molecules to pass through the membrane.

Mobile

The fluid nature of the membrane allows proteins to move about within the bilayer.

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!

Quick Questions

1. What does the word Hydrophobic mean?

2. What does Hydrophillic mean?

3. Which molecules in the membrane have hydrophillic heads and hydrophobic tails?

4. Give 3 functions of a protein in the cell membrane.

5. Why is the membrane described as ‘fluid’?

6. Name a molecule that can move freely through the cell membrane.

7. Name a molecule that has to pass through a protein channel to get through the cell membrane.

8. Explain why the membrane is ‘selectively permeable’

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 experiment

1. 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 experiment Results:

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

The Cell Membrane

Learning Intention: • Describe the structure of

the cell membrane Success Criteria: • Be able to describe the

structure of the cell membrane.

• Explain the terms ‘selectively permeable’ and ‘fluid mosaic’

Starter

Starter

C

Diffusion

Learning Intention: • Understand the process of diffusion • Identify which molecules will pass across

membranes via diffusion. Success Criteria: • Explain the terms passive and active

transport • Define 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. Diffusion

2. 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

Diffusion

Learning Intention: • Understand the process of diffusion • Identify which molecules will pass

across membranes via diffusion. Success Criteria: • Explain the terms passive and active

transport • Define the term diffusion and give

examples of diffusion in cells

Starter

Starter

Diffusion

Waste/carbon dioxide

Cell membrane

Transport across cell membranes: Osmosis

Learning Intention: Describe the term ‘Osmosis’ Success Criteria: • Define the term osmosis • Investigate the effect of osmosis on

cells.

Osmosis • 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

Osmosis in action

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: Pure water B: Concentrated Sugar solution

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

Osmosis Experiment: 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?

Predicting osmosis

Transport across cell membranes: Osmosis

Learning Intention: Describe the term ‘Osmosis’ Success Criteria: • Define the term osmosis • Investigate the effect of

osmosis on cells.

Starter

Starter

C

Osmosis Experiment

Learning Intention:

Investigate the cell membrane and the movement of molecules across it

Success Criteria:

• Understand the term osmosis Understand the effect of osmosis on plant cells

Investigation

Aim: To investigate osmosis in potato cells

Effect of water & concentrated sugar solution on potato cylinders

Diagram

Method 1. Label 2 test tubes A and B.

2. Collect 2 pieces of potato which have been cut with a cork borer.

3. Wash them and roll them on a paper towel to dry them.

4. Weigh one piece, note its mass in the table on your worksheet.

5. Put it in test tube A with water.

6.Note the mass of the other piece and put it in test tube B with concentrated sugar solution.

7.Leave for about 24 hours.

Method

• Remove potato cylinders and pat dry with a paper towel.

• Weigh and measure cylinder A (water) and record on worksheet.

• Weigh and measure cylinder B (sucrose) and record on worksheet.

• Record Softness: + = hard

++ = normal

+++ = very soft

Results

Mass of potato (g)

Change in mass

Length of potato (mm)

Change in length

Softness (+ ++ +++)

Before After (g) % Before After (mm) % Before After

Potato in water (A)

Potato in sugar solution (B)

Calculate % change in mass & length CHANGE IN MASS = mass after expt – original mass

% CHANGE IN MASS = Change in mass x 100

original mass

CHANGE IN LENGTH = length after expt – original length

% CHANGE IN LENGTH = Change in length x 100

original length

What happened to your potato cylinders? Comment on the mass, length and softness of both before and after. Why do you think this happened? Use your knowledge of osmosis to explain your results. Was water moving into or out of the potato cells? Why?

Conclusion

Osmosis Experiment

Learning Intention:

Investigate the cell membrane and the movement of molecules across it

Success Criteria:

• Understand the term osmosis Understand the effect of osmosis on plant cells

Predict the results

• Look at the following examples.

• Which side has the higher water concentration?

• Which way will the water molecules move?

50%

sucrose

10%

sucrose

LWC HWC water

100%

water

10%

sucrose

HWC LWC water

25%

salt

30%

salt

HWC LWC water

10%

sucrose

0.5%

sucrose

LWC HWC water

10%

salt

2%

salt

LWC HWC water

1.0M

NaCl

0.5M

NaCl

LWC HWC water

2.0M

HCl

1.0M

HCl

LWC HWC water

6%

NaOH

10%

NaOH

HWC LWC water

100%

water

100%

water

HWC HWC water

1%

salt

10%

sucrose

HWC LWC water

The effect of osmosis on plant and animal cells

Learning Intention:

Investigate the cell membrane and the movement of molecules across it

Success Criteria:

• Understand the term osmosis Understand the effect of osmosis on plant cells

Which direction?

The direction of the movement of water molecules into or out of a cell depends on the water concentration of the liquid the cell is in compared with the water concentration of the cell contents.

What happens in living cells?

Osmosis and plant cells

Plant Cells

Vacuole

Cell wall membrane

Concentrated salt/sugar solution

Dilute sugar/salt solution

Pure Water

Shrinks as high water concentration inside cell- water lost by osmosis Plasmolysis (cell is Plasmoysed)

Cell Swells-high water concentration outside the cell. Water enters the cell by osmosis Full turgor- cells is turgid

No change

Osmosis and animal cells

Watch cells in water

ANIMAL CELLS Shrinks as high

water concentration

inside cell- water lost

by osmosis

Remains the same

as the movement of

water is equal

across the

membrane

Bursts as high water

concentration

outside the cell.

Water enters the cell

by osmosis

Placed in pure

water

Placed in 0.85% salt

solution (weak salt

solution

Placed in strong salt

solution

Task

• Complete the worksheet

Osmosis Rap!

The effect of osmosis on plant and animal cells

Learning Intention:

Investigate the cell membrane and the movement of molecules across it

Success Criteria:

• Understand the term osmosis Understand the effect of osmosis on plant cells

Starter

Starter

A

Starter

Starter

Turgid

Active Transport

Learning Intention: Describe the process of active transport 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.

Task

• Complete the card sort to compare the 3 processes.

Quiz

1. What is osmosis?

A. a cafe on Grassyards road

B. a water hole in the desert

C. a ‘special case’ of the diffusion of water

D. how cells get energy

2. Which solution has the highest water concentration?

A. 20% sucrose

B. 5% sucrose

C. 40% sucrose

D. 2% sucrose

3. Which solution has the highest water concentration?

A. 5M NaOH

B. 0.1M NaOH

C. 2M NaOH

D. 3M NaOH

4. Which solution has the lowest water concentration?

A. pure water

B. 50% salt solution

C. 10% sugar solution

D. 6% acid solution

5. In which direction will the water flow?

A. left to right

B. right to left

C. stay the same

D. upwards

1.0M

NaCl

0.5M

NaCl

6. Osmosis only occurs if a membrane is.......

A. selectively permeable

B. blue

C. in a cell

D. waterproof

7. Osmosis and diffusion occur if there is a.......

A. cell wall

B. concentration span

C. concentration gradient

D. group of cells

8. Water will enter a cell by osmosis if....

A. the temperature is high

B. the water concentration inside the cell is lower than outside the cell

C. the water concentration inside the cell is higher than outside the cell

D. the cell is alive

9. Water will leave a cell by osmosis if....

A. the temperature is high

B. the water concentration inside the cell is lower than outside the cell

C. the water concentration inside the cell is higher than outside the cell

D. the cell is alive

10. What happens when red blood cells are placed in water?

A. they burst

B. they become plasmolysed

C. they become turgid

D. they lose water

11. What happens when plant cells are placed in water?

A. they burst

B. they become plasmolysed

C. they become turgid

D. they lose water

12. Which of these is an example of active transport?

A. plant cells taking in water by osmosis

B. salt molecules diffusing through a semi-permeable membrane

C. red blood cells bursting in water

D. sodium ions moving out of cells against the concentration gradient

Active Transport

Learning Intention: Describe the process of

active transport Success Criteria: • Understand the term active

transport and its effect on cells

• Understand the idea of concentration gradient