Cell Membranes
What is the cell membranes structure
and functions ?
Starter:
In biological terms, what do the following words mean?
Semi permeable
Cell surface
Envelope
Signalling
Phospholipid
Bilayer
hydrophobic
hydrophillic
Cell membranes – success criteria.
• Outline the roles of the membranes within
cells and at the surface of cells.
• State that plasma membranes are partially
permeable.
• describe the fluid mosaic model of the
membrane.
Membrane Vs envelope
Membrane Vs envelope
A cellular membrane is made up of a
phospholipid bilayer.
A cellular envelope is made up of 2 membranes
which form the envelope. These are found
around the chloroplast, mitochondria and
nucleus.
Roles of Membranes
Read the information and make a bullet point
list of the roles.
Explain in your own words what this means.
The major roles of the cell membrane
1) membranes are partially permeable.
2) Membranes separate components from each
other and the outside.
3) cell signalling
4) Membranes provide attachment sites for
enzymes and other molecules involved in
metabolism.
5) Regulate the transport of materials into and
out of cells.
The phospholipid bilayer
Phosphate head – hydrophillic
Fatty acid tails
Hydrophobic
The phospholipid bilayer
Phosphate head – hydrophillic. These are polar
or charged, in this case negatively. This enables
them to interact with water molecules which are
also polar but slightly +ve.
Fatty acid tails – Hydrophobic
These have no charge and so do
not interact with water.
-ve
-ve
-ve
-ve
-ve
-ve
What would happen if this was mixed with
water?
The polar hydrophilic heads are water soluble
and the hydrophobic heads are water insoluble
Hydrophobic (water-hating) tail
air
aqueous solution
Hydrophilic (water-loving) head
Phospholipids form
micelles when
submerged in water
In 1925 Gorter and Grendel proposed that the unit
membrane is formed from a phospholipid bilayer
Extracellular space (aqueous)
Phosphate heads
face aqueous
solution
phospholipid
bilayer
Cytosoplasm (aqueous)
Hydrophobic tails
face inwards
Question: why does a phospholipid
bilayer form?(3).
• The phospholipid molecules have a hydrophobic tail
and hydrophilic head.
• The inside and outside of a cell are both aqueous.
• This means the phospholipids
align as a bilayer so that
Click to reveal the answers
the tails are not touching water.
• Click here to hide answers
The Bilayer
• When completely surrounded by water, a
bilayer can form where the tails are held away
from the water.
• The molecules are not bonded together in
anyway so they are free to move about like a
fluid.
• The membranes are said to be fluid mosaic.
That’s not all folks …
But that is next lesson .
• Complete the questions on the phospholipid
bilayer.
• The phospholipid bilayer has to have other
components in order to do its job as a barrier etc.
e.g.
Presence of receptors on growing shoots to allow
regulation of growth.
Muscle cell membranes have extra channels for glucose
uptake – why?
Internal chloroplast membranes contain chlorophyll.
White blood cell membranes contain proteins to
recognise other cells.
Task
• Read the information and complete the
questions.