Fluid-mosaic model

Plasma membrane consists of a phospholipid bilayer studded with proteins, polysaccharides, lipids
The lipid bilayer is semipermeable
- Regulates passage of substances into and out of the cell
- H2O and some small, uncharged, molecules (O2, CO2) can pass through
Phospholipids have two parts
- "Head": hydrophilic → attracts and mixes with H2O
- Two "fatty acid tails": hydrophobic

Function of proteins

Carrier (change shape for different molecules) for water-soluble molecules such as glucose
Channels for ions (sodium and chloride ions)
Pumps use energy to move water-soluble molecules and ions
Adhesion molecules for holding cells to extracellular matrix
Receptors enable hormones and nerve transmitters to bind to specific cells
Recognition sites, which identify a cell as being of a particular type
Enzymes, which speed up chemical reactions at the edge of the membrane
Adhesion sites, which help some cells to stick together
E.g. glycoprotein acts as a receptor and recognition site

Substances move down their conc. gradient until the conc. are in equilibrium
Microvilli are extensions of the plasma membrane
- They increase the surface area of the membrane, therefore
- They accelerate the rate of diffusion
Fick's law → rate of diffusion across an exchange surfaces (e.g. membrane, epithelium) depends on
- surface area across within diffusion occurs (larger)
- thickness of surface (thinner)
- difference in concentration gradient (larger)
- Fick’s law = (surface area x difference in conc gradient) / thickness of surface
Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Transmembrane proteins form a water-filled ion channel
- Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. gradient //passive - no ATP required
- Some channels use a gate to regulate the flow of ions
- Selective permeability - Not all molecules can pass through selective channels
How do molecules move across the membrane?
- Substrate (molecule to move across the membrane) binds to carrier protein
- Molecule changes shape
- Release of the molecule (product) at the other side of the membrane
Example
- If you want to move a muscle a nerve impulse is sent to this muscle
- The nerve impulse triggers the release of a neurotransmitter
- Binding of the neurotransmitter to specific transmembrane proteins
- Opens channels that allow the passage of Na+ across the membrane
- In this specific case, the result is muscle contraction
- These Na+ channels can also be opened by a change in voltage

Special term used for the diffusion of water through a differentially permeable cell membrane
Water is polar and able to pass through the lipid bilayer
Transmembrane proteins that form hydrophilic channels accelerate osmosis, but water is still able to get through membrane without them
Osmosis generates pressure called osmotic pressure
- Water moves down its concentration gradient
- When pressure is equal on both sites net flow ceases (equilibrium)
- The pressure is said to be hydrostatic (water-stopping)

Measurement of the ability or tendency of water molecules to move
Water potential of distilled water is 0, other solutions have a negative water potential
Measured in kPa - pressure
Hypotonic
- Solution is more dilute / has a lower conc. of solute / gains water by osmosis
- Cells placed in a hypotonic solution will increase in size as water moves in
- For example, red blood cells would swell and burst
- Plant cells are unable to burst as they have a strong cellulose cell wall
Hypertonic
- Solution with a higher conc. of solutes / loses water by osmosis
- Cells will shrink in hypertonic solutions
Isotonic
- Solutions being compared have equal conc. of solutes
- Cells which are in an isotonic solution will not change their shape
- The extracellular fluid of the body is isotonic
Molecules collide with membrane / creates pressure, water potential
More free water molecules, greater water potential, less negative
Solute molecules attract water molecules which form a "shell" around them
- water molecules can no longer move freely
- less "free water" which lowers water potential, more negative

Movement of solute against the conc. gradient, from low to high conc.
Involves materials which will not move directly through the bilayer
Molecules bind to specific carrier proteins / intrinsic proteins
Involves ATP by cells (mitochondria) / respiration
- Direct Active Transport - transporters use hydrolysis to drive active transport
- Indirect Active Transport - transporters use energy already stored in gradient of a directly-pumped ion
Bilayer protein transports a solute molecule by undergoing a change in shape (induced fit)
Occurs in ion uptake by a plant root; glucose uptake by gut cells

Substances are transported across plasma membrane in bulk via small vesicles
Endocytosis
- Part of the plasma membrane sinks into the cell
- Forms a vesicle with substances from outside
- Seals back onto the plasma membrane again
- Phagocytosis: endocytosis brings solid material into the cell
- Pinocytosis: endocytosis brings fluid materials into the cell
Exocytosis
- Vesicle is formed in the cytoplasm //May form from an edge of the Golgi apparatus
- Moves towards plasma membrane and fuses with plasma membrane
- Contents are pushed outside cell
- Insulin is secreted from cells in this way