Download Lesson 1: Functions of Cell Membranes

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
Example
Function
Membranes are partially permeable,
The plasma membrane allows small or
controlling what passes through them.
uncharged perticles to pass trhough it;
protein channels and transporters control
the passage of larger or charged particles
Membranes separate the cell
Mitochondria are surrounded by two
components from the cytoplasm and
membranes, which isolate the reactions
the outside.
taking place inside from the reactions
Membranes are important in cell
A substance produced by one cell docks
signalling.
into a receptor in the plasma membrane of
taking place in the cytoplasm.
another, causing something to happen in
the second cell.
Membranes allow electrical signals to
The membrane of the axon of a motor
pass along them.
neurone transmits action potentials from
the central nervous system to a muscle.
Membranes provide attachment sites
The inner membrane of a mitochondrian
for enzymes and other molecules
contains molecules needed for the
involved in metabolism.
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
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B
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C
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 Outline the roles of membranes within cells and at the surface of
cells.
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Section 2: Structure of Cell Membranes
 All cell membranes have a similar structure.
 They are normally between 7nm and 10nm thick (*Remember this fact).
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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 Bilayer
Phospholipid 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’).
Figure 2: Structure of a Phospholipid
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, with heads facing
outwards and tails facing inwards. This is called a
phospholipid bilayer.
Text book ref: p16-19
Questions: phospholipids and bilayers
1) 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 components
Membranes 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).
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 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
Phospholipid
Roles
 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.
Table 2: Roles of the components of cell membranes
Basic Membrane Structure
Questions: fluid mosaic model
The diagram below shows parts of a plasma membrane.
1) Identify structures A–E.
[5]
a)
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
Functions
Intrinsic proteins
Recognition sites for substances such as insulin and acetylcholine and act as antigens
Extrinsic proteins
Waterproofs membrane and makes it more rigid
Cholesterol
Make membrane fluid, allow fat soluble molecules but not water soluble substances, to cross
Phospholipids
Form channels or carriers for water soluble molecules
Glycolipids
Act as enzymes
Glycoproteins
Make membrane stable and help cells attach to one another
5) What is the difference between the structure of glycoproteins and glycolipids?
[2]
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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]
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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]
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11) Describe the purpose of cholesterol in plasma membranes
[2]
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12) Suggest why marine organisms living in polar regions have a very high proportion of
cholesterol in their membranes
[1]
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