Function of Membranes
Compartmentalisation/ Dynamic boundary
• Isolation of cell from external world
• Alter cell form as necessary
• Division cell contents
Functions of Membranes
• Selective permeability
– Control entry/exit in cells
– Control over organelle contents
• .Lysosome pH 5
• Electrical isolation
– Electrochemical gradient maintained
• Neuronal function
Functions of Membranes
• Localisation of Chemical reactions e.g.
• Mitochondrial cristae - development of H+ ion
gradients; required for ATP generation
• Contain electron transport chain proteins
• Chloroplast membranes - light gathering proteins
• Lysosomes contain digestive enzymes
Functions of Membranes
• Transport
– Contain proteins for transport
– Active transport processes
• ATP dependent transport
• co transporters
• Pinocytosis, endocytosis
– Including generation of gradients
– Use of these gradients to provide energy for
active transport
Functions of Membranes
• Signal transduction
– Receptors e.g.
• Insulin receptor
– Transduction mechanism
• e.g. cAMP cascade requires enzymes adenylate
cyclase
Functions of Membranes
• Cell-cell recognition
– glycoproteins
• Cell-cell adhesion
– Stick cells together
STRUCTURE
• Phospholipid bilayer
• FLUID MOSAIC MODEL
–
–
–
–
Proteins embedded in a sea of lipid
Cholseterol reduces fluidity
Proteins can be anchored by cytoskeleton
Proteins can span the membrane
(transmembrane),
– Extra/intracellular surface
REGULATION OF FLUIDITY
Fatty acids are crucial regulators of fluidity – determined
by chain length and degree of saturation
Short chain fatty acids reduce the tendency of
hydrocarbon chains to interact and hence increase
fluidity
The kinks in unsaturated fatty acids result in less
stable van der Waals interactions with other lipids
and hence increase fluidity
High cholesterol content restricts the random
movement of polar heads and decreases fluidity.
IMPAIRED FLUIDITY CAN DAMAGE CELLS
Increased cholesterol content of red blood cell membranes
is associated with severe liver disease eg. Cirrhosis
Cholesterol content of red blood cell membranes is
increased by 20-60%, leading to decreased fluidity
Alters cell shape, impairs O2 transport, destruction of
red blood cells and anaemia
PROTEINS
• Key to (most) membrane functions are the
proteins embedded/ attached in the
membrane
Pore forming
proteins
Cell-cell
adhesion
Carrier
protein –
active
transport
Membrane
bound
enzyme
Cell-cell
recognition
Receptor
protein
Cytoskeleton
Poison Dart Frogs-
Channel Proteins
Batrachatoxin opens Na+
channels
• Pore forming proteins
• Transmembrane
• Membrane spanning regions contain
hydrophobic amino acids
• Allow diffusion of polar molecules across
cell membrane e.g. sodium ions
– Na+ channel in cell membrane voltage gated
(action potential)
–Tend to show some selectivity
–Water will also flow through a channel
Carrier Proteins
• 3 methods
• Facilitated diffusion – solute assisted by carrier
protein to diffuse down concentration gradient,
no additional energy supply needed.
• Active transport - ATP hydrolysis provides the
energy. e.g. Na+ K+ pump
• Co transport of e.g. Na+ or H+ down their
electrochemical gradient provides energy for a
second solute to be moved against its
concentration gradient e.g. Na+ are co
transported with sugar molecules in the gut
Membrane bound enzyme
• Enzymes attached to the membrane
• Can be external e.g.
• cell wall synthesis in plants
• Acetylcholinesterase (neuromuscular junctions)
– Breaks down acetylcholine released by nerve cells to cause
muscular contraction.
• Poisoned by
– Organophosphorous
insecticides (OPs),
• malathion (specific for insects • Galantamine (snowdrop),
insecticide)
•Poisoning causes muscle spasms,
•spasticity
sarin (nerve gas),
–
•Internally bound enzymes are often
involved in cell signalling cascades
(see cell signalling section for
details)
Receptor Protein generating an
intracellular response
• Receptor protein specifically binds a ligand
e.g histamine binds to a receptor on blood
vessels.
• Binding induces conformational change, on the
receptor’s intracellular surface.
• Variety of intracellular proteins activated
• Production of second messengers or opening of
an ion channel.
• The second messengers (or ions) change cell
function
• e.g. Causes vasodilation/ increases permeability
of blood vessel
Cell adhesion molecules
• Responsible for connecting cells to cells
– Often glycoproteins
– Bind to proteins on neighbouring cells or
extracellular matrix
• Maintain tissue structure
Cell:cell recognition via a
glycoprotein
• Cell surface glycoproteins specific for each
species
• Determine e.g. blood group (causes
difficulty for blood transplants/ xenografts)
Membrane protein attached to the
cytoskeleton
• Membranes are fluid and proteins can move
around in them
• But – some proteins may need to be localised e.g.
– Underneath a synaptic cleft or neuromuscular junction
– Specific localisation on cells to give sidedness
• e.g. intestinal lumen
• Cytoskeletal proteins can anchor membrane
proteins in a specific location
Cytoskeleton
• Intricate network of thread-like filaments
– Microfilaments (or Actin filaments)
– Intermediate filaments
– Microtubules
• Support the interior, produce movements,
shape changes
(a) Light micrograph of the cytoskeleton. The microtubles
are green and the microfilaments are blue. Intermediate
filaments form most of the rest of the network. (b)
Scanning electron micrograph of a portion of a cell's
Microfilaments
• Actin filaments (microfilaments)
–
–
–
–
–
Two actin strands twisted together
“rope” approx. 7nm diameter
Concentrated under the cell membrane
Important for cell movements
Dynamic
Intermediate filaments
• Provide mechanical strength (important for
animal cells)
• Act like a scaffold
Microtubules
• Hollow tubes made from tubulin
• Heterodimers (one  and one ) arrange to
form 13 protofilaments
• Important in cell division forming spindle
fibre
• Also involved in intracellular transport
Centrosome (Microtubule
organising Centre)
• Area in the cell which controls
polymerisation, depolymerisation of
microtubules
– Centrioles are found there (animal cells only)
• Plant cells have MOC, but no centrioles
The cytoskeleton
•
CytoskeletonCytoskeleton and its major functionsProvides internal support and
streanghtDifferent types of fibers all supply the same need The life a cytoskeleon is
never complete. Its chief functions include movement for the cell, movement of material
through the cell, maintaining the shape of the cell. But keeping the cell from getting
smashed by other cells and moving the cell to where it need to be are its main roles.
Early on, when not much was known about the structure of the cell, scientist believed
that the cytoplasm that surrounds the organelle was completely random. As the years
went on, better microscopes were invented, and better techniques were improved upon
for making a slide, "a lacy network of fibers was revealed." These fibers look similar to
girders that hold up a bridge, so it was hypothesized that they would do the same for the
cell, hold its shape. These fibers can be broken down into three main groups:
Microfilaments, microtubules, and intermediate filaments, all of which can be
recognized by there structure and their protein makeup. Regardless of their differences
all three of them serve the same goal in the cell, to make the cell more ridged.
MicrofilamentsThey, along with other proteins and ions, are responsible for every
muscle contraction The microtubules and the microfilaments play a role in whole cell
activities, including cell division, contraction of cell, and the crawling of the cell to a
new location. Also they help in movement of vesicles, small sacs used for holding
molecules, within the cell. The microfilaments resemble a string of beads of the protein
actin, thus earning them the name of filaments. These filaments are the smallest
cytoskeletal component, ranging in at 6 nanometers. These actin proteins play a large
role in muscle contraction. A model was made by Hugh Huxley that shows that the actin
proteins are in alternating rows that alternate with myosin. The contraction of the muscle
is caused by a calcium atom that excites the myosin, thus grabbing the actin and causing
the muscle to shorten. After more studying of this event, it was learned that ATP was
• The Cytoskeleton
• Every cell contains specialized cytoplasmic
proteins which serve as a support and contractile
system, maintaining or changing cell shape. The
cytoskeletal structures include the microtubules,
microfilaments, and intermediate filaments.
• Microtubules
• Cytoplasmic microtubules appear as 25 nanometer
tubular structures and are readily assembled and
disassembled from cytoplasmic pools of the
protein tubulin. Microtubules are fairly rigid and
play a role in the maintenance of cell shape
(Microtubules 1). They are associated with cilia