Download Membranes - gcate.org

Cell membrane
(plasma membrane)

The plasma membrane can be
thought of as a gatekeeper,
allowing only specific
substances in or out and passing
messages from the external
environment.
1
The cell membrane must
perform several specific
functions:
Isolate the cell cytoplasm from
the environment.
 Regulate the exchange of
essential substances between the
cytoplasm and the environment.


Communicate with other cells.

Identify the cell as belonging to
a particular species and a
particular member of that
species.
2
Fluid mosaic model

A membrane when viewed from
above, looks something like a
lumpy, constantly shifting
mosaic of tiles.

A bilayer of phospholipids
forms a viscous fluid for the
mosaic, while an assortment of
proteins are the tiles, often
sliding about within the
phospholipid bilayer.
3
Structure of the
cell membrane

Phospholipid bilayer
»
»
the polar end of the phospholipid
interfaces with the watery
environment surrounding the
membranes
the nonpolar fatty acids are found in
the interior of the bilayer sheet.
4
Structure of the
cell membrane

Proteins function as:
»
»
»
receptors, channels, transporters and
markers
some are integral; span entire
membrane - f. ex. transport protein.
some are peripheral; at the borders f. ex. receptor proteins for hormones.
5
Structure of the
cell membrane

Composition of a typical
membrane:
~ 50% lipid
~ 50% protein

Regulation of interactions
1. Passage of water.

Freely permeable to water.
2. Bulk passage into the cell.

Phagocytosis. Big gulps.
3. Selective transport of molecules.

Transports only some molecules.
6
Structure of the
cell membrane

Regulation of interactions cont.
4. Reception of information.

Identify chemical messages.
5. Expression of cell identity.

Molecular name tags.
6. Physical connections with other
cells.

In forming tissues, make special
connections.
7
Solutions

Solvent
–

Solute
–

Water is the solvent, the most common
substance
The molecules dissolved in the water,
f.ex. sugars, amino acids and ions.
Solution
–
Solvent and solutes mixed together.
Both the solvent and the solutes seek
to move from the area of greater
concentration to the area of less
concentration.
8
Transport across
membranes
With reference to water and solutes a
membrane is called:
 permeable
 non-permeable
 partially-permeable
 Semi-permeable
 differentially permeable
 Selectively permeable membrane
Partially permeable membrane
9
Transport across
membranes
Passive transport - no energy used

Diffusion (simple diffusion)
–
–
Diffusion is the random movement of
molecules from an area of high
concentration to an area of low
concentration.
Water, gasses like O2, CO2 and lipidsoluble molecules like ethyl alcohol
and vitamin A easily diffuse across.
10
Transport across
membranes

Osmosis (a special kind of
diffusion)
–
Osmosis is the net movement of
water molecules across a partially
permeable membrane from a region
of high water concentration to a
region of low water concentration.
–
Osmosis will occur whenever two
solutions containing different
concentration of water molecules
are separated by a partially
permeable membrane.
11
Transport across
membranes

The movement of water in
osmosis can also be seen as the
movement of water from a
region of low solute
concentration to a region of
high solute concentration.
12
Transport across
membranes
In relation to cells the concentration
of surrounding solutions can be:
 Hypertonic
if the concentration of
solutes in the solution is higher than
in the cell.
–
Water will move in or out?
 Isotonic
if the concentration of
solutes in the solution is equal to
that in the cell.
–
Water will move in or out?
 Hypotonic
if the concentration of
solutes in the solution is lower than
in the cell.
–
Water will move in or out?
13
Transport across
membranes

Facilitated diffusion
–
–
Facilitated diffusion is the transport
of molecules across a membrane by
a carrier protein in the direction of
lowest concentration.
Its is a boat with no oars, sail or
engine - it can only work when the
tide is in the right direction.
14
Transport across
membranes
Active transport - energy used
–
–
–
Active transport is the transport of a
solute across a membrane to a
region of higher concentration by
the expenditure of energy.
It moves molecules against a
concentration gradient.
The energy is provided by ATP or
adenosine triphosphate
ATP
ADP + energy
ATP ase
15
Transport across
membranes

There are three main types
–
The sodium potassium pump (Na, K)
 Most
animal cells maintain a higher level
of K+ in side the cell and lower Na+
level than on the outside.
 The pump transports 3 Na+ out for every
2 K+ it moves in.
–
The proton pump (H+)
 Most
likely to be found in the
membranes of mitochondria and
chloroplasts.
 Energy from metabolism or
photosynthesis is used to produce ATP.
16
Transport across
membranes
–
Coupled channels
 In
one channel a molecule is linked
to another that is moving down a
concentration gradient.
 The other channel pumps out the
carrier molecule and so keeps up the
concentration gradient.
17
Transport across
membranes

Endocytosis is when materials are
surrounded by and taken into
membrane lined vesicles.
–
Phagocytosis - cell eating
 big
–
parts or whole cells are taken in.
Pinocytosis - cell drinking
 minute

vacuoles (drops) are taken in
Exocytosis
–
The emptying of a membrane lined
vesicle at the surface of the cell.
18
The relationship between the nucleus,
rough endoplasmic reticulum (rER),
Golgi complex and the cell surface
–
–
–
–
–
–
–
information about a protein leaves the
nucleus through a nuclear pore
the protein is synthesised in the
ribosomes on the rEr
after travelling through the rEr it is
encapsulated in a vesicle
the vesicle fuses with a Golgi complex
where the enzyme is further modified
at the ends or the cisternae it goes to a
secretory vesicle
which carries it to the cell membrane,
where it fuses with the membrane and the
enzyme is released outside the cell
19
Membrane proteins

Transport across membranes
–

Antibody recognition sites
–
–

act as a triggers that sets of a cellular
response when hormones bind to them
Catalysis for biochemical reactions
–

identification tags f.ex. immune cells
recognise bacteria and mark them for
destruction
blood groups, tissue groups
Hormone binding sites
–

channels, carriers, pumps
act as enzymes, particularly on the inside
of the cell membrane
Sites of electron carriers
–
the proton pump
20