National 5 Biology – Cell Biology – Plasma Membrane

1 | P a g e

Transport across cell membranes

We have already learnt the basic function of the plasma membrane however, it

is much more complex than a line around the outside of a cell. The membrane

carries out a number of different functions. You will learn how substances move

across the membrane and why this is important to life.

Some of the key words you will need to use during this section are: plasma

membrane, protein, phospholipid, fluid-mosaic, selectively permeable,

antigen, receptor, enzyme, diffusion, osmosis, active transport,

concentration gradient, hypertonic, hypotonic, isotonic, plasmolysis, turgid.

Learning Objectives

You will be able to:

label and name the two main components of the plasma membrane

explain the term selectively permeable

state the functions of proteins in the plasma membrane

explain the term concentration gradient

describe the process of diffusion

describe the process of osmosis

explain how osmosis affects different cells

explain how isotonic, hypertonic and hypotonic solutions affect cells

describe the process of active transport

Structure and Function of the Plasma Membrane

The plasma membrane or cell membrane acts as a barrier between the inside

and outside of the cell. It is important for the cell to maintain its internal

environment at optimum conditions (optimum means ideal or best).The cell

membrane controls what substances can enter and exit a cell. The structure of

the cell membrane is described as the ‘fluid mosaic model’. This is because the

membrane is made of two main parts: lipids and proteins. The lipids make the

membrane fluid and able to move. The protein parts are found within the

National 5 Biology – Cell Biology – Plasma Membrane

2 | P a g e

membrane and on the surface.

Learning Activity 1

Collect handout 1 ‘plasma membrane’. Stick into your jotter and then label the

different parts.

The plasma membrane is described as being ‘selectively permeable’. This means

that the membrane controls what substances enter and leave the cell. Some

substances move through the membrane quickly whilst other substances are

unable to pass through.

Substances can move across the membrane by a number of different methods

including diffusion, osmosis and active transport.

Learning Activity 2 – Chemical Nature of the Plasma Membrane Practical

Collect practical activity ‘Chemical Nature of the Plasma Membrane’.

Focus on the following skills whilst carrying out this experiment:

1. Experimental Procedures – think carefully about what you are doing and

why you are doing it. How are you going to make the experiment reliable?

How are you going to make the experiment reliable? How will you make

the experiment fair?

2. Drawing Conclusions – Use the information you have collected from your

experiment to decide upon your own conclusion. Make sure the conclusion

links in with the original aim of the experiment.

3. Group Work – get everyone in your group involved without being bossy.

National 5 Biology – Cell Biology – Plasma Membrane

3 | P a g e

The proteins that help make up the plasma membrane can carry out a number of

functions including:

Provide structural support

Contain channels allowing the transport of small molecules through the

membrane

Act as carriers which actively pump molecules across the membrane (see

active transport)

Serve as enzymes that help carry out chemical reactions in and on the

membrane

Act as receptors for hormones (chemical messengers) arriving at the

outer surface of the cell

Serve as antigenic markers which identify the cell.

Learning Activity 3 – Plasma Membrane

Complete the following questions in full sentences:

1. Name the two components (things that make up) of the plasma membrane.

2. State the name given to this model of the plasma membrane.

3. Explain why it is given this name.

4. List 5 functions of proteins found within the plasma membrane.

Using materials from the classroom and from home, make a 3-D model showing

the fluid mosaic model of the plasma membrane.

Success Criteria

1. Show and label the 2 main components of the membrane

2. Show the bi-layer

3. Try and make your model ‘fluid’ and ‘mosaic’.

4. Show the structure of the phospholipids

5. Show variety in the proteins found within the membrane

6. Show the functions of some of the proteins in the membrane

7. Explain the similarities and differences between a real plasma

membrane and your model. What are the limitations of your model?

National 5 Biology – Cell Biology – Plasma Membrane

4 | P a g e

Movement across the membrane

Diffusion

Diffusion is the movement of molecules from an area of high concentration to an

area of low concentration, down a concentration gradient. It is a passive

process, in other words, it does not require any energy.

A concentration gradient is the difference in concentration between two areas,

in this case, the inside and the outside of a cell. The greater the concentration

gradient, the faster the molecules will move.

This net movement of molecules will continue until there is an equal

concentration on either side of the membrane.

Diffusion is important to cells because

They gain the raw materials for respiration

They remove harmful waste products

Oxygen

Glucose

Carbon

Dioxide

National 5 Biology – Cell Biology – Plasma Membrane

5 | P a g e

Oxygen and glucose are needed for respiration to take place. Respiration is

essential to life because it is a chemical reaction that takes place in all cells to

provide the cell with energy. Carbon dioxide is a waste product from the

process of respiration and need s to be removed from the cell.

Glucose + Oxygen Carbon Dioxide + Energy

The diagram below shows why diffusion is essential in the delivery of oxygen to

the cells of the body. Diffusion of oxygen takes place between the alveoli of the

lungs and the blood. The blood then carries the oxygen to the cells that require

it where it diffuses from the blood into the cells.

Carbon Dioxide moves in the opposite direction so that it can be expelled from

the body.

Learning Activity 4 - Diffusion

Collect handouts 2 and 3 and cut and stick the diagrams into your jotter.

Complete the diagrams by labelling them accurately.

Complete the following questions in full sentences:

1. Describe what is meant by the term a concentration gradient.

National 5 Biology – Cell Biology – Plasma Membrane

6 | P a g e

2. What is diffusion?

3. Using 2 named examples, explain why diffusion is essential for life.

4. Predict what you think will happen to the speed of diffusion over time.

Explain (CLUE: think about the concentration gradient).

Learning Activity 5 – Visking Tubing Experiment

Collect practical activity ‘visking tubing experiment’.

Focus on the following skills whilst carrying out this experiment:

1. Careful and Disciplined – Read the instructions carefully first. Ask if you

are not sure BEFORE you start the investigation. Follow the instructions

step by step.

2. Making Predictions – before you carry out the experiment, think

carefully about what we have been learning. Make a ‘best guess’ based on

theory of what you think will happen in the experiment.

3. Command Words – what is the difference between Describe and Explain

(other than how you spell them!).

Once you have completed the experiment, carry out the tasks below:

1. Draw a labelled diagram to show the experiment you carried out.

2. Write a method, in the past passive tense, to explain what you did.

3. Suggest which part of the equipment is acting as the plasma membrane.

4. Explain the term selectively permeable and explain how your experiment

is a demonstration of selective permeability.

5. Describe your results.

6. Explain your results.

Let your teacher see your work. Use their feedback to make any

improvements.

National 5 Biology – Cell Biology – Plasma Membrane

7 | P a g e

Osmosis

Osmosis is the movement of water molecules from an area of high water

concentration to an area of low water concentration through a selectively

permeable membrane. Osmosis, like diffusion, is a passive process.

You need to understand the effect osmosis can have on plant and animal cells.

National 5 Biology – Cell Biology – Plasma Membrane

8 | P a g e

You need to understand the meaning of the terms isotonic, hypotonic and

hypertonic.

A solution that is isotonic will have equal concentrations of water inside and

outside the cell. This means that there is no overall movement of water, the

mass will stay the same and the cell will not change.

A solution that is hypotonic is a solution that has a higher water concentration.

If a cell is placed in a hypotonic solution, water will move into the cell and the

mass of the cell will increase.

A solution that is hypertonic is a solution that has a lower water concentration.

If a cell is placed in a hypertonic solution, water will move out of the cell and

the mass of the cell will decrease.

Learning Activity 6 – Osmosis

Collect practical activity ‘Osmosis’ and follow the instructions carefully.

Focus on the following skills whilst carrying out this experiment:

Imagination – the best scientists have a great imagination. As we carry out this

experiment and you see your results, I want you to imagine what is happening

and why it is happening. You will need to use your imagination because you will

not see anything happening.

Draw a diagram and write an explanation of what you imagine has happened in

the 2 Visking tubes. This should not be a complete guess, link it in with what you

have been learning.

National 5 Biology – Cell Biology – Plasma Membrane

9 | P a g e

The diagrams below show potato cylinders immersed in solutions of different

salt concentrations. The concentration of the potato cell sap is 2%.

The mass of the potato placed in the 0% salt solution would increase as water

would move into the potato from high water concentration to low water

concentration (0% salt is the same as 100% water).

The mass of the potatoes placed in 4% and 6% salt solution (96% and 94%

water) would decrease as the water would move out of the potato from high

water concentration to low water concentration.

For the potato placed in the 2% salt solution, the water concentration outside

the potato is the same as the water concentration inside the potato. Therefore,

there would be no change in mass as there is no concentration gradient.

Learning Activity 7 – Osmosis

1. Rewrite the following paragraph choosing the correct answer at each

underlined choice.

Osmosis is a form of diffusion/ion uptake. It is an active/passive process that

requires/does not require energy. During osmosis oxygen/water molecules move

across a freely/selectively permeable membrane from a higher/lower water

concentration to a higher/lower water concentration along/against a

concentration gradient.

2. What do the terms isotonic, hypotonic and hypertonic mean?

3. What happens to an animal cell when placed in hypotonic solution?

National 5 Biology – Cell Biology – Plasma Membrane

10 | P a g e

4. Describe what happens to a plant cell when placed in a hypotonic solution?

Explain the difference.

5. What happens to a plant cell when it is placed in a hypertonic solution?

6. Explain what happens to the mass of an animal cell placed in an isotonic

solution.

Focus on the following skills whilst carrying out this experiment:

1. Calculating Percentage Change – REMEMBER the following equation:

(Change/Original) x 100% = percentage change

2. Drawing Line Graphs – as a group, write down the rules of a good line

graph. As a class, come up with a mark scheme for line graphs.

3. Experimental Procedure – when carrying out experiments you have to

make sure the experiment is fair and reliable. To ensure it is fair, you can

only change one thing at a time. To make sure it is reliable, you must

repeat the experiment and calculate an average. This helps to take

account of atypical (unusual) results.

4. Calculating Averages – total / how many numbers there are = average

1. Collect handout 4, cut out, stick in and complete the diagrams of osmosis.

2. Define the term ‘osmosis’.

3. Describe the osmotic effects of the following:

Transferring animal cells from a weak salt solution to a strong salt

solution

Transferring animal cells from a weak salt solution to a solution of

pure water

Transferring plant cells from a weak sugar solution to a strong

sugar solution

Transferring plant cells from a weak sugar solution to a solution of

pure water.

4. Read the information below then complete the task.

The experiment described below was set up to investigate the effects of

osmosis on plant tissue.

National 5 Biology – Cell Biology – Plasma Membrane

11 | P a g e

Three boiling tubes were set up; 1 with 10ml of pure water, 1 with 10ml of

5% sugar solution and 1 with 10% sugar solution.

Three cylinders of potato were cut and blotted to remove excess water.

The mass of each cylinder was recorded before being placed into the

different solutions.

The experiment was left set up for 2 hours.

The cylinders of potato were removed from the boiling tubes, blotted and

re-weighed.

The mass of the potato cylinders was recorded.

The change in mass was calculated (difference between the initial and

final mass) and using the following calculation, the percentage change in

mass was obtained.

Change in mass x 100 = percentage change in mass

Initial mass

The entire experiment was repeated a further two times and the results

averaged to improve their reliability.

The results are shown in the table below:

National 5 Biology – Cell Biology – Plasma Membrane

12 | P a g e

Concentration

of sugar

solution (%)

Initial

mass (g)

Final

Mass (g)

Change in

Mass (g)

% change

in mass

Average

change in

mass (%)

0 2.0 2.3 0.3 15 +9.1

2.5 2.6 0.1 4

2.4 2.6 0.2 8.3

5 2.3 2.2

2.2 2.1

2.5 2.2

10 2.2 1.9

2.4 1.9

2.5 2.2

(a) Collect the handout 5 ‘Results Table’. Cut out and stick into your jotter.

Complete the table.

(b) Use the data to draw a line graph showing the average percentage change

in mass against concentration of sugar solution (use graph paper).

(c) The independent variable is the variable altered by the experimenter.

Name the independent variable.

(d) The dependent variable is the variable that changes as a result of

altering the independent variable. Name the dependent variable.

(e) Make a list of the variables that would need to be controlled in the

experiment.

(f) What was done in the experiment to ensure that the results obtained

were reliable?

Learning Activity 8 – Investigation

You are going to use all the skills and knowledge you have gained from the above

example to carry out your own investigation into osmosis.

Collect the practical activity sheet ‘Osmosis Investigation’.

National 5 Biology – Cell Biology – Plasma Membrane

13 | P a g e

Active Transport

Active transport is the movement of molecules from an area of low

concentration to an area of high concentration against the concentration

gradient. Active transport works in the opposite direction to the passive

process of diffusion and requires energy.

This process requires energy to carry the substance from a low concentration

to a high concentration. Substances cross the membrane through a protein

molecule.

Each carrier protein is specific to one substance.

Sodium and potassium move in and out of a nerve cell by active transport.

An example of active transport - Sodium/Potassium Pump

The Sodium/Potassium pumps are located in nerve cells.

Nerve cells must maintain a higher concentration of Sodium (Na+) outside the

cell and Potassium (K+) inside the cell.

Special transport proteins in the membrane pump sodium ions out of the cell

and potassium ions into the cell. This requires energy.

National 5 Biology – Cell Biology – Plasma Membrane

14 | P a g e

Factors affecting the rate of active transport

Oxygen concentration

Glucose concentration

Temperature

The above factors have an effect on active transport because they all influence

the rate of respiration. If there is a lower concentration of oxygen or glucose,

respiration will not be as effective and not as much energy will be produced.

Energy is required for active transport.

If the temperature is too high or too low, the enzymes that control the process

of respiration will not be able to function properly. Again, this will reduce the

amount of energy produced and so active transport will not be as effective.

Learning Activity 9 – Active Transport

Answer the following questions in full sentences:

1. What is the definition of active transport?

2. Give 2 ways in which active transport is different from diffusion.

3. What is the difference between an active process and a passive process?

4. Explain why a decrease in temperature, oxygen or glucose would reduce

the rate of active transport in a cell.

Research Task

How do nerve cells work and what role does active transport play in this.

Research this as a group and present your work as a poster.

Check your Understanding

1. Sometimes the plasma membrane is compared to a fence around a school

because it contols what comes in and out of as cell like a membrane

controls what goes in and out of a cell. Why is this not necessarily a good

comparison?

2. Complete the Venn Diagram to show the similarities and differences

between diffusion, osmosis and active transport.

3. Design a 5-10 minute starter activity to check the understanding of

others in the class on this topic.

National 5 Biology – Cell Biology – Plasma Membrane

15 | P a g e

Success Criteria

Read over the following success criteria and evaluate your own learning. Your

teacher may want to have a conversation with you about your progress.

Skills

I can make sure that my investigation is reliable

I can make sure that my investigation is fair

I can draw a conclusion that links to my aim and is backed up by evidence

I can make a prediction before an investigation

I can explain the difference between the command words ‘describe’ and

‘explain’.

I can carefully follow written instructions when carrying out practical

work.

I can calculate percentage change

I can draw an accurate line graph

I can calculate an average

Knowledge and Understanding

I can label and name the two main components of the plasma membrane

I can explain the term selectively permeable

I can state the functions of proteins in the plasma membrane

I can explain the term concentration gradient

I can describe the process of diffusion

I can describe the process of osmosis

I can explain how osmosis affects different cells

I can explain how isotonic, hypertonic and hypotonic solutions affect cells

In can describe the process of active transport

Discuss the meaning of the following key words with your group and be prepared

to say a sentence using the words: plasma membrane, protein, phospholipid,

fluid-mosaic, selectively permeable, antigen, receptor, enzyme, diffusion,

osmosis, active transport, concentration gradient, hypertonic, hypotonic,

isotonic, plasmolysis, turgid.

National 5 Biology – Cell Biology – Plasma Membrane

16 | P a g e

Didbook

Copy and complete the following sentences and then enter them into Didbook to

add to your learning profile.

1. One thing I need to know more about is…..because….

2. I think the way I can build on what I have learnt in this topic is by….

3. The most important thing I learnt in this topic was….because…

Think about the skills you have worked on during this topic:

Which skills have you improved? How do you know?

Choice Extension Task

Task Choose at least 2 of the following activities to complete.

1 Build a 3-D model of the plasma membrane.

2 Create an animation/flickbook demonstrating diffusion, osmosis and/or

active transport.

3 Carry out the egg experiment, write it up and take photographs of the

results - http://ilovebacteria.com/eggosmosis.htm

4 Research examples of where diffusion, osmosis and active transport take

place in nature. Show your findings.

5 Create a poster on 'movement of molecules across membranes'.

6 Design an experiment that would demonstrate osmosis or diffusion.

7 Find 10 interesting facts about diffusion, osmosis and active transport

that are not in your textbook and present your findings to the class.

8 Imagine you are a glucose molecule. Describe your journey from the

stomach to a muscle cell.

9

Find out about other membranes that are found within a cell. Explain the

difference between them and the plasma membrane. Explain their

purpose.