physio 151 - membrane dynamics and transport

Cell Membrane Lipids

Consists of cholesterol, phospholipids, and sphingolipids.

Cell Membrane Proteins

Consists of integral, peripheral, and lipid-anchored proteins.

Cell Membrane Carbohydrates

Consists of glycoproteins and glycolipids.

Membrane Building Blocks

Cholesterol + phospholipids form the lipid bilayer.

Glycolipids and Glycoproteins

Glycolipids = phospholipids/sphingolipids + carbohydrates. Glycoproteins = carbohydrates + proteins.

Selectively Permeable

Means not everything can pass through the membrane.

Membrane Permeability to Molecules

Can pass: non-polar (hydrophobic) molecules. Cannot pass: macromolecules, charged ions, polar molecules.

Physical Requirements for Movement

Molecules move through the phospholipid bilayer, via a membrane protein, or in a vesicle.

Energy Requirements for Movement

Requires energy input from ATP (active transport) or no energy input (passive transport).

Simple Diffusion Requirements

Physical: passes directly through the phospholipid bilayer. Energy: none (passive).

Facilitated Diffusion Requirements

Physical: mediated transport requiring a membrane protein. Energy: none.

Active Transport Requirements

Physical: mediated transport requiring a membrane protein. Energy: requires energy (ATP).

Forms of Active Transport

Primary active transport and secondary active transport.

Forms of Passive Transport

Simple diffusion, osmosis, and facilitated diffusion.

Properties of Diffusion

Uses kinetic energy, no outside energy source, moves high to low concentration, and continues until equilibrium.

Factors Affecting Diffusion Rate

Higher concentration gradients, shorter distances, smaller molecules, and higher temperatures.

Molarity

The concentration of a chemical solute in a solution.

Concentration Gradient Directions

Down gradient = high to low concentration. Up gradient = low to high concentration.

Factors Affecting Diffusion Across a Membrane

Larger surface area, thinner membrane, larger concentration gradient, and higher membrane permeability.

Factors Affecting Membrane Permeability

Molecule lipid solubility, molecule size, and lipid composition of the membrane.

Osmosis

The movement of water, which requires non-penetrating solutes to happen.

Osmosis vs. Simple Diffusion

Osmosis moves water from low to high non-penetrating solute concentration. Simple diffusion moves solutes from high to low concentration.

Osmotically Active Solutes

Non-penetrating solutes.

Water Movement Rules

Water only moves with non-penetrating solutes. If water cannot move and solutes are permeable, diffusion occurs.

Tonicity vs. Osmolarity

Tonicity measures the number of non-penetrating solutes only. Osmolarity measures the total number of all solutes.

Hypertonic Solution

Has a higher number of non-penetrating solutes compared to the cell.

Hypotonic Solution

Has a lower number of non-penetrating solutes, but penetrating solutes raise the total solute count.

Isotonic Solution

Has the same amount of non-penetrating solutes as the cell, with penetrating solutes making the total higher.

IV Solutions and Tonicity

Dehydrated cells need hypotonic solutions; blood loss replacement needs isotonic solutions.

Penetrating vs. Non-Penetrating IV Solutes

NaCl (saline) is non-penetrating; dextrose (glucose) is penetrating.

Channel Proteins in Facilitated Diffusion

Rapid movement, water-filled passageways, creates open and gated channels for small molecules.

Carrier Proteins in Facilitated Diffusion

Must bind substances to transport, resulting in slower movement compared to channels.

Similarities of Channel and Carrier Proteins

Both use proteins and neither requires energy.

Cues for Gated Channels

Mechanically gated (sensory receptors), voltage-gated (neurons), and chemically-gated (neurotransmitter receptors).

Uniport Carriers

Carriers that move only one kind of molecule (e.g., glucose).

Cotransporter Carriers

Carriers that move more than one kind of molecule at a time (e.g., symport or antiport).

Symport vs. Antiport

Symport moves molecules in the same direction; antiport moves them in opposite directions.

Facilitated Diffusion vs. Active Transport

Facilitated diffusion is passive, high to low, reaches equilibrium. Active transport needs energy, low to high, creates concentration differences.

Primary Active Transport

Uses energy directly from high-energy ATP bonds to push molecules against their concentration gradient.

Secondary Active Transport

Uses potential energy stored in an existing concentration gradient to push other molecules against their gradient (ATP used indirectly).

Sodium-Potassium Pump Operation

Primary active transport that pumps 3 Na+ out and 2 K+ in for each ATP used, maintaining gradients and charge separation.

Properties of Carrier-Mediated Transport

Exhibits saturation, specificity, and competition for both active and passive transport.

Saturation in Carrier Transport

Transport rate depends on substrate concentration and the number of available carrier molecules.

Specificity in Carrier Transport

The ability of a carrier to move only one molecule or a group of closely related molecules.

Competition in Carrier Transport

A group of substances moved by a transporter compete for the same binding sites.

Vesicular Transport (Endo/Exocytosis)

Endocytosis, phagocytosis, and exocytosis move large molecules using membrane-bound vesicles and require ATP.

Exocytosis Machinery and Regulation

Rabs help dock vesicles, SNAREs facilitate membrane fusion, and regulated exocytosis is triggered by increased intracellular Ca2+.

Absorption vs. Secretion

Absorption moves items from the lumen to the ECF; secretion moves items from the ECF to the lumen.

Paracellular vs. Transcellular Transport

Paracellular transport goes through junctions between cells; transcellular transport goes through the epithelial cells themselves.