Lesson 1: Functions of Cell Membranes

Every living cell is surrounded by a membrane. This is called the plasma membrane or the cell surface membrane.

The plasma membrane defines the limits of the cell. It seperates the cell’s contents from its external environment, and it controls what can pass from this environment into the cell, and from the cell into the external; environment.

It is partially permeable.

Membranes are also found inside cells. Some organelles are surrounded by a single membrane – for example, lysosomes. The nucleus, mitochondria and chloroplasts each have two membranes around them making up an envelope. Most eukaryotic cells also have an extensive network of membranes within their cytoplasm, forming the RER, SER and golgi apparatus.

Like the plasma membrane, these membranes inside the cell are partially permeable, and therefore able to control what can pass through them. They seperate what happens inside the organelle from what is happening in the rest of the cell.

Figure 1: SEM of a Cell

Function Example

Membranes are partially permeable, controlling what passes through them.

The plasma membrane allows small or uncharged perticles to pass trhough it; protein channels and transporters control the passage of larger or charged particles

Membranes separate the cell components from the cytoplasm and the outside.

Mitochondria are surrounded by two membranes, which isolate the reactions taking place inside from the reactions taking place in the cytoplasm.

Membranes are important in cell signalling.

A substance produced by one cell docks into a receptor in the plasma membrane of another, causing something to happen in the second cell.

Membranes allow electrical signals to pass along them.

The membrane of the axon of a motor neurone transmits action potentials from the central nervous system to a muscle.

Membranes provide attachment sites for enzymes and other molecules involved in metabolism.

The inner membrane of a mitochondrian contains molecules needed for the production of ATP. The inner membrane of a chlororplast contains chlorophyll needed for photosynthesis.

Table 1: Functions of Membranes

Homework

Answer the following questions:

1) Why do we describe the plasma membrane as being partially permeable?

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2) Name 3 membrane-bound organelles and explain their function in the cell.

A ___________________________________________________________________________________________________________________________________________________________________________________________________

B___________________________________________________________________________________________________________________________________________________________________________________________________

C

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Outline the roles of membranes within cells and at the surface of cells.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Section 2: Structure of Cell Membranes

All cell membranes have a similar structure.

They are normally between 7nm and 10nm thick (*Remember this fact).- This makes them invisible with a light microscope but they can be

seen with an electron microscope.

Hey are formed from a double layer of molecules called phospholipids in which many proteins are situated.

Phospholipid BilayerPhospholipid molecules have an unusual property. Their heads have a tiny charge, and this attracts them to water molecules. But their tails don’t have a charge, and they are repelled from water molecules.

We say that the heads of the phospholipids are hydrophilic (‘water-loving’) and the tails are hydrophobic (‘water-hating’).

The cytoplasm inside a cell contains a lot of water, and so does the fluid outside cells. The hydrophilic heads of phospholipid molecules are therefore drawn to these fluids, while they repel the hydrophobic tails. This causes the phospholipids to rearrange themselves in a double layer,

Figure 2: Structure of a Phospholipid

with heads facing outwards and tails facing inwards. This is called a phospholipid bilayer.

Text book ref: p16-19

Questions: phospholipids and bilayers1) Distinguish between the terms hydrophilic and hydrophobic. [2]

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2) With the aid of a diagram if required describe the properties and structure of a phospholipid molecule.

[6]

3) What is the thickness of a cell surface membrane? [1]

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4) Explain why phospholipids align themselves into a bilayer at the interface between two watery areas such as the cell cytoplasm and the extracellular fluid. [4]

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The Fluid Mosaic Model

The different substances that make up the cell membrane are not fixed in place.They are arranged in a ‘mosaic’ but are also free to move around within the mosaic, so we describe the membrane as fluid.

The different components of cell membranes have different roles:

Phospholipids form a barrier to dissolved substances.Cholesterol gives the membrane stability.Proteins control what enters and leaves the cell.Glycolipids and glycoproteins act as receptors for messenger molecules (hormones, drugs, antibodies) and for cell signalling.

Membrane proteins are two kinds:1. Extrinsic – they are found on one surface of the membrane only.2. Intrinsic – they span the entire membrane.

Membrane proteins have a variety of functions:

1. They may act as enzymes eg carbohydrase enzymes are found in the cell membranes of intestinal epithelial cells.

2. They act as receptors for hormones.3. They act as carrier proteins, controlling what enters and leaves the

cell.4. Carbohydrates and lipids attached to proteins (forming glycolipids

and glycoproteins) , allow cells to recognise one another.

Substances enter and leave the cell by diffusion (including facitlitated diffusion), osmosis and active transport.

Other componentsMembranes also contain another type of lipid. This is cholesterol. Cholesterol molecules lie alongside the phospholipids, helping to make up the bilayer.

There are also many different protein molecules in cell membranes. They are much larger than phospholipid molecules. Some of the proteins lie in the membrane, protruding from both sides. Others float in just the outer layer or inner layer.

Figure 3: The fluid mosaic model

Many of the lipid molecules and protein molecules have short strings of sugar molecules attached to them, forming glycolipids and glycoproteins.

Figure 3 shows the structure of a plasma membrane, including all of these components. This is called the fluid mosaic model of membrane structure.

The term ‘fluid’ refers to the fact that the molecules in the membrane are in constant motion, moving around within their own layer (they don’t normally swap sides).

The term ‘mosaic’ refers to the way the membrane would look if viewed from above, with a mosaic pattern formed by the protein molecules that are scattered throughout.Complete the following table.

Component RolesPhospholipid Forms the bilayer which is fundamental basis of the

membrane in which all components are embedded Provides a barrier to water-soluble (hydrophilic)

substances, such as ions and molecules that carry a charge

Cholesterol Helps to maintain the fluidity of the membrane, preventing it from becoming too stiff when temperatures are low, or too fluid when temperatures are high

Protein and Glycoprotein

Glycolipids Provide energy Serve as markers for cellular recognition.

Basic Membrane Structure

Questions: fluid mosaic model

The diagram below shows parts of a plasma membrane.

1) Identify structures A–E. [5]a)

Table 2: Roles of the components of cell membranes

b) c) d) e)

2) Explain why the model of cell membrane structure shown in the diagram is called the fluid mosaic model. [2]

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3) Is this cell membrane likely to be from an animal or plant cell? Give a reason for your answer. [2]

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4) Match the cell membrane components given below with their correct functions. [6]

Components FunctionsIntrinsic proteins

Extrinsic proteins

Cholesterol

Phospholipids

Glycolipids

Glycoproteins

Recognition sites for substances such as insulin and acetylcholine and act as antigens

Waterproofs membrane and makes it more rigid

Make membrane fluid, allow fat soluble molecules but not water soluble substances, to cross

Form channels or carriers for water soluble molecules

Act as enzymes

Make membrane stable and help cells attach to one another

5) What is the difference between the structure of glycoproteins and glycolipids? [2]..................................................................................................................................................................................................................................................................................

The diagram below shows some of the components of the plasma membrane

6) Using the information shown and your own knowledge, draw a diagram to show the structure of the plasma membrane. [3]

7) On your diagram label the components drawn and indicate the outer surface of the membrane. [4]

8) State two functions of the proteins in the plasma membrane. [2]........................................................................................................................................................................................................................................................................................................

The diagram below shows the fluid mosaic model of the plasma membrane.

9) Identify,

A. [1]

B. [1]

C. [1]

10) How do the properties of phospholipids ensure that the membrane remains fluid? [2]........................................................................................................................................................................................................................................................................................................

11) Describe the purpose of cholesterol in plasma membranes [2]........................................................................................................................................................................................................................................................................................................

12) Suggest why marine organisms living in polar regions have a very high proportion of cholesterol in their membranes [1]

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