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Cell Structure Revision
Cell Membrane
Introduction

The cell membrane:
– surrounds the outside of the cell.
– is semi-permeable (allows certain molecules in and
out)
– Is composed of a double layer (bilayer) of molecules
called phospholipids
 These lipids have a hydrophilic head part and a hydrophobic
tail

The cell membrane can be explained using the
Fluid Mosaic model.
– This model says that membrane is a lipid bilayer that
has various proteins bobbing in it.
Fluid Mosaic Model
Diffusion and Osmosis
Diffusion

Diffusion is the net movement of
molecules from an area of high
concentration to an area of low
concentration.
The difference in concentration between 2
different areas is called the concentration
gradient
 The larger the concentration difference
between the 2 areas the steeper the
gradient.

– Therefore a smaller concentration difference
will have a less steep gradient.

Diffusion is also called passive transport
– The cell membrane doesn’t use up any energy
to make diffusion happen.
Rate of Diffusion

The rate of diffusion depends on:
– The size of the molecules
– Smaller molecules move faster
– The temperature of the liquid or gas
– Warmer matter moves faster
– The state of the matter that is diffusing
– Gases move faster than liquids
– The concentration of the chemicals
– More concentrated chemicals move faster
Concentration Gradient
Membrane
95%
65%
A
95%
B
90%
Facilitated Diffusion

Facilitated diffusion is a form of passive
transport that uses transport proteins to
help certain molecules across the cell
membrane faster.
Summary
Osmosis
Osmosis is the net movement of WATER
molecules from an area of high water
concentration to an area of low WATER
concentration through a semi-permeable
membrane.
 Osmosis is also passive transport

Osmosis Definitions
Osmosis always refers to the water
concentration.
 Therefore:

– A solution with a high water concentration is
called hypotonic
– A solution with a low water concentration is
called hypertonic
– If both solutions involved in osmosis have the
same water concentration, they are called
isotonic
Osmosis in Action

The process of osmosis works in cells
because:
– Cell membranes are semi-permeable
– The cytoplasm consists of about 90% water,
but the remaining 10% is large molecules that
cannot pass through by diffusion

Water will diffuse into a cell when the
water outside is at a higher concentration.
– This will make the cell turgid

Water will diffuse out of the cell when the
outside concentration is less than inside
the cell.
– This loss of water in animal cells makes the
cell shrink and become flaccid
– If this happens to plant cells then the
cytoplasm will shrink away from the cell wall
 This is called plasmolysis
Osmoregulation
The movement of water into a cell can build up
pressure. If cells continue to take on water by
osmosis they keep swelling till the burst.
 In plants the cell wall prevents this from
happening.
 This is a problem for unicellular organisms such
as amoeba and paramecium who live in fresh
water.

– These organisms have a special contractile vacuole
the actively pumps out excess water.
– This process of osmoregulation keeps the water
content of the organism relatively constant.
Textbook
Copy the diagram at the bottom of page
89.
 Using the paragraph beside it – name 3
body systems that need to have the
correct osmotic balance

Revision Question
A
B
Give the name and function of both organelle A
and organelle B.
2. Cell membranes are semi-permeable. Explain
what this means.
3. Describe how the structure of a cell membrane
is related to its selective permeability
1.
Active Transport

In certain circumstances molecules can
pass in and out of cells by moving from a
low to high concentration.
– That is it moves against the concentration
gradient.

This process is called active transport and
uses energy to actively move the
molecules.

There are several ways to tell if active
transport is occuring:
– Heat is given off
– Oxygen is used up
– Carbon dioxide is produced
– Glucose is used up

Another way to tell whether a lot of active
transport is taking place is the number of
mitochondria present in the cells.
– Why?
High
Conc
Low
Conc
Passive Transport
High
Conc
Low
Conc
Active Transport
Revision Question
ion
plasma
Red blood cell
K+
9
152
140
18
Na+
Concentrations of sodium and potassium ions in human red blood cells and in
the surrounding plasma.
What method of transport is responsible for
moving sodium ions out of, and potassium ions
into, the red blood cell? Explain your answer.
2. When red blood cells are separated from blood
plasma and placed into distilled water they
burst open. Explain why the blood cells burst
open.
1.
Surface Area to Volume