1. Cell surface
(Plasma)
Membranes
Transport across a partially-
permeable surface

2. Learning
Objectives
 Describe a
phospholipid bilayer
 Describe what is meant by ‘fluid mosaic’.
 Understand the role of intrinsic proteins
 Explain what selective permeability means

3. The Phospholipid Bilayer
 Cell membranes are made of phospholipid, like a layer of
oil.
 The phospholipid membrane has two (bi) layers of
phospholipid molecules.
 The phospholipid molecule has two ends (a hydrophobic
(water hating) and a hydrophilic (water loving end).
 This is the principal of how all cell membranes are formed.

4. Fluid Mosaic model (Singer
and Nicholson 1972)

5. Intrinsic and Extrinsic Proteins
 Intrinsic proteins go all the way through the phospholipid bilayer,
some have holes (channels) to let molecules through.
 Extrinsic proteins are only in the upper or lower layer only. They
often have sugar chains on their surface which often act as ‘markers’
to identify the cell as ‘friendly’ (cell recognition), for cell adhesion, or
hormone binding sites.

6. Membrane Transport – getting
substances across the membrane
 The intrinsic proteins are specialized, each
one carries specific molecules across the
bilayer
 They help with selective permeability, only
allowing certain molecules through
 Sometimes this happens by active transport
and sometimes by facilitated diffusion
 Some substances can pass directly through
the phospholipid bilayer (small and fat
soluble – non polar) water, although polar is
small enough to pass

7. Facilitated diffusion
 Some proteins have
‘holes’ through them.
These simple protein
channels let through
molecules of the right
shape only
 This happens by
diffusion: molecules
move from an area of
high concentration to
an area of low
concentration

8. Carrier Proteins
 The binding of the
molecule to be
transerred causes
deformation of the
protein
 This mechanism helps
deliver the molecule
but does not require
energy
 It will not occur against
a concentration
gradient

9. Active Transport
 Is made possible
by carrier
proteins using
energy to actively
shift molecules
across a
membrane.
 This can happen
against a
concentration
gradient

10. Summary of transport
across the membrane

11. Effect of heat
 Heat will affect permeability of the
phospholipid bilayer, the
phospholipids become more fluid.
 Small ‘holes’ may appear
 Extreme heat may cause big holes or
destroy the bilayer completely