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Cell Membranes
Phospholipids in Membranes
The role of phospholipids in membranes is to act as a barrier to most substances (semip___________), helping control what enters/exits the cell.
Generally, the smaller and less ______ a molecule, the easier and faster it will d_____
across a cell membrane.
 Small, non-polar molecules such as ________ and carbon dioxide rapidly diffuse
across a membrane.
 Small, polar molecules, such as w______ and urea, also diffuse across, but much more
slowly.
 C_________ particles (ions) are unlikely to diffuse across a membrane, even if they
are very small.
The Cell Membrane
The cell membrane forms the boundary between the cell cytoplasm and the environment.
This separates the contents of the cells from their external environment. Seen using a light
microscope, the cell membrane appears as a thin line, but with an electron microscope, it
appears as a double line.
Membranes cover the surface of every cell, and also surround most o________ within cells.
They have a number of functions, such as:
 keeping all cellular components inside the cell
 allowing selected molecules to move in and out of the cell
 isolating organelles from the rest of the cytoplasm, allowing cellular processes to
occur separately.
 a site for biochemical reactions
 allowing a cell to change shape.
The Fluid Mosaic Model of Membranes
Chemical analysis has shown that cell membranes are made almost entirely of ________ and
__________. The phospholipids form a double layer known as a __________
______________.
It is suggested that the proteins and lipids are arranged as a 'fluid _________'. The cell
membrane is not static.
FLUID:
MOSAIC:
It is important that a cell membrane maintains its fluidity otherwise the cell would not be
able to function.
A fluid membrane is needed for many processes, such as for:
 the diffusion of substances across the membrane
 membranes to fuse, e.g. a vesicle fusing with the cell membrane during exocytosis
 cells to move and change shape, e.g. macrophages during phagocytosis.
Functions of the lipids
The p________ bilayer allows movement of small, lipid soluble molecules into and out of the
cell. (Water is small enough to get through the gaps between the phospholipids). Larger,
p__________ substances such as ions, sugars and amino acids, cannot diffuse across the
phospholipid membrane
Proteins in Cell Membranes
Proteins help transport substances, such as ions, sugars and amino acids, that cannot diffuse
across the membrane but are still vital to a cell’s functioning.
Integral (or i________, or transmembrane) proteins span the whole width of the membrane.
Peripheral (or e________) proteins are confined to the inner or outer surface of the
membrane. Many proteins are glycoproteins – proteins with attached carbohydrate chains.
Many integral proteins are carrier molecules or channel proteins.
Other integral proteins are receptors for h_______ and neurotransmitters, or enzymes for
catalyzing reactions. Peripheral proteins can act as receptors for hormones or
neurotransmitters, or are involved in cell r_________ , cell signalling or chemical reactions.
Cholesterol
Cholesterol is a type of l_____ that is very important in controlling membrane f______.
The more cholesterol, the less fluid – and the less p_______ – the membrane.
Cholesterol is also important in keeping membranes stable at normal body temperature –
without it, cells would burst open.
Questions
1. Even though too much cholesterol is linked to heart disease, our cells would not be
able to survive without a supply of cholesterol. Referring to the fluid mosaic model,
explain why cholesterol is so important in animal plasma membranes. (3 marks)
2. Three factors affect the permeability of a cell membrane:
 heat
 ethanol
 pH
Explain how changing temperature and pH can affect the membrane, by referring
to the fluid mosaic model. (4 marks)
3. Beetroot contains betalin pigments which give the tissue a dark red colour. This
pigment is contained in the cell vacuole (membrane bound organelle containing water
and enzymes). A sample of beetroot was placed in a boiling tube with a set volume of
water. The boiling tube was placed in a water bath set to a temperature of 5oC. After
a set amount of time, a sample of the solution surrounding the beetroot was taken and
using a colorimeter the % light absorption of the solution was recorded. This method
was repeated at increasing temperatures. The results are displayed below.
a) Describe the effect of temperature on % light absorption through the solution.
(2)
b) Explain this pattern by linking cell membrane permeability with % light
absorption. (4)
c) Suggest why the % light absorption remained relatively stable between 60OC
and 68OC.
Graph to show change in membrane permeability
with an increase in temperature
0.8
0.7
Absorption / %
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
Tem perature/°C
60
70
80