BYA1 SECTION 10.3

Properties of plasma membranes are related to the passage of substances through them

"recognise" chemical messengers, such as insulin
Regulates passage of substances into and out of cell
- The membrane has got different permeability for different substances
- Some substances can cross more easily than others
- Glucose, for example, is only selectively permeable

Fluid-mosaic model

Membranes consist of a phospholipid bilayer studded with proteins, polysaccharides, lipids
The lipid bilayer is semipermeable - H₂O and some small, uncharged, molecules (O_2, 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

Passive transport uses energy from moving particles (Kinetic Energy)

Diffusion

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 conc. gradient (larger)
- (Surface Area * Difference in Conc.)/Thickness of Surface
Temperature increases rate of diffusion due to increasing K.E. (kinetic energy)

Facilitate diffusion (proteins)

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
[MECHANISM] How do molecules move across the membrane?
- Carrier protein binds to substrate (the specific molecule)
- Molecule changes shape
- Release of the diffusing 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

Osmosis

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)

Water potential (measured in kPa - pressure)

Measurement of ability or tendency of water molecules to move
Water potential of distilled water is 0, other solutions have a negative water potential
Hypotonic: Solution with a lower conc. of solute / gains water by osmosis
- Solution is more dilute
- Cells placed in a solution which is hypotonic will grow as water moves in
- Red blood cells will swell and burst if it is in a hypotonic solution
- Plant cells are unable to burst / they posses a strong cellulose wall
Hypertonic: Solution with a higher conc. of solutes / loses water by osmosis
- Cells will shrink in hypertonic solutions (eg red blood cells)
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 an isotonic solution
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

Active Transport

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

Endocytosis and Exocytosis

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

References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]
BAKER, M, INDGE, B, & ROWLAND, M (2001) Further Studies in Human Biology. Hodder Arnold H&S

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