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