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Cell membrane structure & function:
- 7nm thick
- Made up phospholipid bilayer (hydrophilic head and hydrophobic tails)
- They are selectively permeable and controls what goes in and out of the cell
Fluid mosaic model:
‘Fluid’ - The phospholipid and protein molecules move in a fluid like motion and diffuse sideways
within its monolayer
‘Mosaic’ - The proteins scattered within the membrane creates a mosaic pattern
Components of plasma membrane:
- Phospholipids
- Cholesterol
- Glycolipids (carbohydrate chains attached to phospholipid molecule)
- Glycoproteins (carbohydrate chains attached to protein molecule)
- Proteins (ex. Transmembrane proteins)
Phospholipids:
They’re dual in nature as they have a hydrophilic head (polar), and hydrophobic tails (non-polar)
so that they can come in contact with the aqueous medium but still control what goes in and out
of the cell
The hydrophilic head forms H-bonds with water molecules to stabilize the membrane
The hydrophobic tails repel from water molecules, that’s why these properties allow the
phospholipid bilayer to reassemble themselves to a certain structure
- Barrier to water soluble substances + H2O
- Allow lipid molecules / lipid soluble molecules to pass through
- Polar molecules can’t pass through & large molecules can’t pass through too
- Maintain the fluidity of the membrane
Factors affecting the fluidity of the membrane:
- Ratio of saturated to unsaturated FA (if it’s unsaturated then the x2 bonds cause kinks to
the tails so it becomes looser, which makes the membrane more fluid)
- Temp (the higher the temp, the higher the fluidity of the membrane as there is higher KE
so the molecules move around)
- Length of FA chains (the shorter it is, the more fluid it is)
- Cholesterol (if the temp is high, then cholesterol decreases fluidity. But at low temps, the
cholesterol increases fluidity)
, Cholesterol:
- It has a similar structure to phospholipids so it can fit between the phospholipid
molecules but it can’t form a bilayer
- It regulates the fluidity of the membrane, stabilized the membrane, and blocks the entry
of very small ions
If the temp is low, then cholesterol increases the fluidity because since it’s fit in between the
phospholipid molecules, it prevents close packing of the phospholipid tails
If the temp is high, then cholesterol decreases the fluidity because it reduces the mobility of
phospholipids (since at high temp there is high KE and the molecules move around, these
cholesterol sort of stays between the molecules and prevent them from moving)
Membrane proteins:
- Extrinsic / peripheral (outer surface of the membrane)
- Intrinsic / integral (embedded within the membrane)
- Some are transmembrane proteins
- All transport proteins are transmembrane proteins but not all transmembrane proteins
are transport proteins
Integral proteins:
They have both hydrophobic and hydrophilic regions, and the hydrophilic R group will form a
bond with the hydrophilic heads while the hydrophobic R groups will form a bond with the
hydrophobic FA tails
Ex. Transport proteins
Types of membrane proteins:
1) Transport proteins (channel + carrier): transport ions & polar molecules to pass through
(this is because since polar molecules and ions can’t pass through the hydrophobic
region of the membrane, it requires a transport protein to help it enter the cell
2) Channel proteins[gated or non gated]: help in facilitated diffusion (diffusion of molecules
across the cell membrane with the help of channel proteins)
{they’re water filled in the inside and has hydrophilic R groups on the inside of the channel so
water molecules can pass through} —> aquaporins
3) Carrier proteins [carriers or pumps]: carrier proteins help in facilitated diffusion of
molecule across the membrane, while pumps help in active transport
{It happens because the carrier proteins change their shape to release and receive molecules
as they constantly flip between 2 shapes so the binding sites alternatively open from one side to
the other} —> ex. Na+, K+ pumps
Roles of membrane proteins:
1) Transport proteins - channel & carrier proteins
2) Enzymes
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