CELL MEMBRANES

WHAT ARE MEMBRANES?

* selectively permeable barriers
* maintains constant internal environment
* enclose cell contents
* internal organelles often have different environments to the cytosol
* very thin

FLUID MOSAIC MODEL

* fluid: = the different components are free to move around
* mosaic = the proteins are embedded into the membrane

composed of:


1. lipid bilayer= 2 layers; head on the exterior; tails on the interior
2. phospholipids
3. carbohydrates
4. cholesterol

PHOSPHOLIPIDS

* ampiphilic - both hydrophilic and hydrophobic regions
* polar head - -vely charged, phosphate group
* non-polar tail - no charge
* phospholipids self assemble to create the bilayer
* tails can be either saturated or unsaturated:

SATURATED

single bonds between the carbon atoms on the hydrocarbon chain; many saturated fatty acids means neatly packed together so fewer gaps

UNSATURATED

a double bond between the carbon atoms on the hydrocarbon chain; forms a kink due to the presence of the double bond; many unsaturated fatty acids means larger gaps in between

higher conc. of unsaturated acids promote

membrane FLUIDITY

higher conc. of saturated fatty acids REDUCE

membrane FLUIDITY

CARBOHYDRATES

2 main types:


1. glycolipids
2. glycoproteins

* maintains membrane stability and cell protection
* aid cell-cell adhesion (cell adhesion molecules)
* facilitate cell recognition

CHOLESTEROL

* a lipid (20% of the membrane composed of cholesterol)
* has polar and non-polar regions
* wedges itself between phospholipid tails
* can migrate and flip between membrane layers

PROTEINS

1. serve different functions: enzymes, carrier proteins, channel proteins, receptors, recognition. cell adhesion
2. two main types:

* integral
* peripheral

INTEGRAL PROTEINS

large protein inserted all the way through the lipid bilayer


1. Firmly inserted into the membrane
2. Span the bilayer of the membrane
3. Transmembrane portion
4. hydrophobic
5. Extracellular and cytosolic portions are hydrophilic
6. Carrier proteins and channels are integral proteins

PERIPHERAL PROTEINS

proteins found on the outer or inner surface, attached to other components of the cell membrane


1. loosely attached to membrane
2. may be removed easily form the membrane with minimal disruption

Membrane Fluidity Affected by...

1. Extreme temperatures (hot or cold)
2. Concentration of unsaturated fatty acids
3. Cholesterol levels

MEMBRANE FLUIDITY

1. Low temp

* Less kinetic energy
* Phospholipids pack together
* Membrane less fluid

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1. High temp

* More kinetic energy
* Phospholipids tend to move further away
* Membrane more fluid

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1. Cholesterol

* Maintains fluidity by preventing lipids from getting too close or too far apart from each other

are the properties of the individual components the same in membranes of different cell types?

1. Red Blood cell – equal amounts of lipid and protein and a small amount of carbohydrate
2. 2. Nerve cells – higher amounts of lipid in cell membranes (80% lipid)

transport of small molecules across the membrane:

1. passive:

* down a conc. (electrochemical) gradient

3 TYPES OF TRANSPORT ACROSS A MEMBRANE

* Simple diffusion
* Facilitated diffusion
* Osmosis Uses inherent kinetic energy

ACTIVE TRANSPORT

Moves against a concentration (electrochemical) gradient

* Has directionality
* Requires a specific carrier protein

5 types:


1. Primary
2. Secondary
3. Symport
4. Antiport
5. Needs an external energy source

SIMPLE DIFFUSION

1. Small, uncharged, non polar molecules
2. O2, CO2, NO, urea
3. No metabolic energy required
4. Uses kinetic energy (natural motion)
5. No specificity
6. Rate of diffusion proportional to concentration gradient

FACILITATED DIFFUSION

1. Small, polar molecules
2. H2O, Glucose, Na+
3. Uses specific integral proteins (carrier proteins)
4. Rate of diffusion proportional to concentration gradient BUT also on one other factor...

Simple and facilitated diffusion kinetics

The rate of diffusion reaches a plateau (transport maximum; Tm) in facilitated diffusion as carrier proteins become fully occupied.

* Osmosis

1. Net movement of water down a concentration gradient
2. Water highly polar, but small amounts can ‘sneak’ through the membrane unaided
3. Water moves freely and reversibly through specific protein channels called AQUAPORINS

PRIMARY ACTIVE TRANSPORT

* movement of sodium and potassium ions uses the Na+/K+ ATPase pump
* maintains ion conc. differences inside and outside the cells
* hydrolysis of ATP results in the phosphyrylation of the pump
* 3Na+'s out and 2K+'s into the cell (against the gradient)

SECONDARY ACTIVE TRANSPORT (CO-TRANSPORT)

* movemnt of a substance (against the conc. gradient) is coupled to ion movement (down the conc. gradient) i.e. the Na+/glucose movement chain
* the energy used for the secondary active transport comes from the electrochemical gradient across the membrane

SYMPORT:

* transported substances move in the same direction
* Na+/glucose transporter

ANTIPORT

* transported in the opposite direction

vesicular transport of large molecules

ENDOCYTOSIS

1. moves large molecules into the cells (ingestion/uptake)
2. Three types:

* Phagocytosis
* Pinocytosis
* Receptor-mediated

EXOCYTOSIS:

1. Moves large molecules out of the cell (excretion / secretion)
2. Used for:

* Hormone secretion
* Neurotransmitter release
* Mucous secretion

VESICLES

1. Bubble-like, membranous sacs
2. Made of a phospholipid bilayer containing fluid
3. Transport bubble
4. Protects substances being transported

ENDOCYTOSIS:

1. cell eating’
2. Ingestion of large particles by specialised cells (phagocytes; in the immune system):

* Macrophages
* Certain white blood cells


1. Used for removing bacteria and debris
2. When a particle (e.g. bacteria) binds to a phagocyte, the cell membrane wraps itself around the particle
3. Resulting in a phagosome containing the particle

PINOCYTOSIS

1. cell drinking
2. process used to take in extracellular fluid with dissolved substances
3. droplet enters the cell and fuses with an endosome (sorting vesicle)

ENDOCYTOSIS - receptor mediated

1. main mechanism for specific uptake of macromolecules
2. very selective method
3. Uptakes substances (hormones, cholesterol) found in small amounts
4. Receptors for this are specific membrane proteins
5. Some viruses can hijack this route and enter our cells

EXOCYTOSIS

1. Substances to be moved out of the cell

* Hormones
* Waste products
* Neurotransmitters


1. Process is stimulated by a cell-surface signal that results in a cascade of events:

* Substance is enclosed in a vesicle
* Transported to plasma membrane
* Vesicle attaches to ‘docking’ sites on the membrane
* Vesicle


1. Transported to plasma membrane

* Fuses with membrane and ejects substance

a balance in cell membrane content

while exocytosis adds membrane material to the cell membrane, endocytosis removes it from the cell membrane

how do oral rehydration therapies work?

* mixture of sodium chloride and glucose
* exploits the physiology of ion and water transport to treat dehydration (from diarrhoea, extreme sports etc)
* normal conditions:


1. glucose is transported across the enterocyte (intestinal cell), whilst the Na/K ATPase maintains a concentration gradient
2. water is also reabsorbed from the intestine following the water potential created by movement of Na and glucose.