Cell

• Basic unit of structure and function of the body.

• Highly organized molecular factory.

• Great diversity.

• Organ physiology derived from complex functions of the cell.

• 3 Principal Parts: plasma membrane, cytoplasm, nucleus

Cell

• Cell (plasma) membrane:– Lipid bilayer.– Is selectively permeable.– Separates cell’s internal structures from

ECF.– Participates in cellular communication.

All biological membranes have the same components

•Lipid

•Protein

Amphipathic: one part of molecule is hydrophilic and the other part is hydrophobic.

Fluid mosaic model

• Phospholipids:– Polar end with

phosphate group.– Nonpolar the rest

(hydrophobic)• Restricts passage of

H20 and H20 soluble ions

• Proteins:– Structural support.– Transport– Receptors

Membrane Fluidity

Lipid molecules do not “flip-flop”, but exchange with adjacent molecules in about 107 times per second.

Increase cholesterol contents decrease membrane fluidity

Function of plasma membrane: •Formation of cellular and subcellular compartments

•Maintenance of a stable intracellular milieu

•Regulation of intracellular metabolism

•Sensing and transduction of extracellular chemical signals

•Propagation of electrical signals that conduct message and or regulate the transport of substances across the membrane

•Endo- and exocytosis

Transmembrane movement

• Cell membrane separates ICF from ECF.

• Cell membrane is selectively permeable.

• Mechanisms to transport molecules and ions through the cell membrane:– Carrier mediated transport– Non-carrier mediated transport

Diffusion: •Molecules dispersed from regions of high concentrations to regions of lower concentrations.

Diffusion Through Cell Membrane

• Cell membrane permeable to:– Non-polar molecules (02)

– Lipid soluble molecules (steroids)

– Small polar covalent bonds (C02)

– H20 (small size, lack charge)

• Cell membrane impermeable to:– Large polar molecules (glucose)– Charged inorganic ions (Na+)

Rate of Diffusion

• Dependent upon:– The magnitude of concentration gradient.

• Driving force of diffusion.

– Permeability of the membrane.• Neuronal cell membrane 20 x more permeable to

K+ than Na+.

– Temperature.• Higher temperature, faster diffusion rate.

– Surface area of the membrane.• Microvilli increase surface area.

Diffusion

• Physical process that occurs:– Concentration difference across the membrane– Membrane is permeable to the diffusing

substance.

• Molecules/ions are in constant state of random motion due to their thermal energy.– Eliminates a concentration gradient and

distributes the molecules uniformly.

Osmosis Movement of water down its concentration gradient.

Osmosis

• Net diffusion of H20 across a selectively permeable membrane.

• 2 requirements for osmosis:– Must be difference in solute concentration on the 2

sides of the membrane.– Membrane must be impermeable to the solute.– Osmotically active solutes: solutes that cannot pass

freely through the membrane.

H20 moves by osmosis into the lower H20 concentration until equilibrium is reached (270 g/l glucose).

Molality and Osmolality

• Ratio of solute to H20 critical to osmosis.

• Use molality (1.0 m):– 1 mole of solute is dissolved in 1 kg H20.

• Osmolality (Osm):– Total molality of a solution.

• Plasma osmolality = 300 mOsm/l.

• NaCl ionized when dissolved in H20 forms 1 mole of Na+ and 1 mole of Cl-, thus has a concentration of 2 Osm.

• Glucose when dissolved in H20 forms 1 mole, thus has a concentration of 1 Osm.

Osmosis: •Movement of water down its concentration gradient.

Isomotic: two aqueous solutions that exist the same osmotic pressure

Hypoosmotic: one solution exerts less osmotic pressure than the other

Hyperosmotic: one solution exerts greater osmotic pressure than the other.

Regulation of Plasma Osmolality

• Maintained in narrow range.

• Reguatory Mechanisms:

• Osmoreceptors stimulate hypothalamus:– ADH released.

– Thirst increased.

Tonicity• The effect of a solution on the

osmotic movement of H20 in cells and tissues

• Isotonic solution: cells or tissues neither shrinks or swells when immersed in it.

• Hypotonic solution: water enter cells or tissues because of the high osmotic concentrations of the cytoplasm with respect to the solution, producing swelling.

• Hypertonic solution: water leaves cell because of the low osmotic concentrations of the cytoplasm with respect to the solution, causing shrinking.

Donnan Equilibrium: •If diffusible solutes are separated by a membrane that is freely permeable to water and electrolytes but totally impermeable to one ionic species, the diffusible solutes become unequally distributed between the two compartments.

Transmembrane movement

• Energy requirements for transport through the cell membrane:– Passive transport:

• Net movement down a concentration gradient.

– Active transport:• Net movement against a concentration gradient.

• Requires energy.

The membrane permeability of non-electrolyte is highly dependent on its oil/water partition coefficient

Extracellular substrate concentrations

Carrier-Mediated Transport

• Transport across cell membrane by protein carriers.

• Characteristics of protein carriers:– Specificity:

• Interact with specific molecule only.

– Competition:• Molecules with similar chemical structures compete

for carrier site.

– Saturation:• Carrier sites filled.

Transport maximum (Tm):Carriers have become saturated.

Competition:Molecules X and Y compete for same carrier.

Passive transport (facilitated diffusion)

Passive transport:– ATP not needed.

Powered by thermal energy.

- Involves transport of substance through cell membrane from higher to lower concentration.

Active Transport

• Movement of molecules and ions against their concentration gradients.– From lower to higher concentrations.

• Requires ATP.

• 2 Types of Active Transport:– Primary– Secondary

Primary Active Transport• ATP directly required

for the function of the carriers.

• Molecule or ion binds to carrier site.

• Binding stimulates phosphorylation (breakdown of ATP).

• Conformational change moves molecule to other side of membrane.

Na+ - K+ ATP-ase Pump

• Primary active transport.

• Carrier protein is also an ATP enzyme that converts ATP to ADP and Pi.

Secondary Active Transport

• Coupled transport.• Energy needed for

uphill movement obtained from downhill transport of Na+.

Facilitated Transport• Cotransport (symport):

– Molecule or ion moving in the same direction.

• Countertransport (antiport):– Molecule or ion is

moved in the opposite direction.

Transport macromoleculesEndocytosis: bulk uptake of material into a cell by membrane inpocketing to form an internal vesicle.

Pinocytosis: if the fluid is ingested

Phagocytosis: if solids are ingested

Exocytosis: fusion of the membrane of a vesicle to the plasma membrane and subsequent expulsion of the vesicle contents to the cell exterior

Junctions between cells

Gap junction: Specialized cell junctions that allow for electrical coupling are linked by tubular assemblies of proteins that are called connexons in vertebrates.

Tight junction: an area of membrane fusion between adjoining cells that prevents the passage of extracellular material between adjoining cells.

Water is transported by osmosis