Monday 30 January 2017

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
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 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)
Facilitate diffusion
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
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
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
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




Cells & Molecules

Cell Division
Cell Types
Cell Ultrastructure
Enzymes
Gene Technology
Genes, DNA, RNA
Large Molecules
Plasma Membrane
Respiration
Content

Fluid-mosaic model
Function of proteins
Passive transport
Diffusion
Facilitate diffusion
Osmosis
Water potential
Active Transport

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