Lecture 5: Cell Membrane and Molecular Movement

Cell membrane

Semi-permeable barrier separating the inside of a cell from the outside.

Cell membrane structure

Phospholipid bilayer containing proteins, cholesterol, and carbohydrates; dynamic.

Phospholipid

Contains phosphate, glycerol, and two fatty acid tails.

Hydrophilic head

Polar phosphate-containing portion of a phospholipid that interacts with water.

Hydrophobic tails

Nonpolar fatty acid portions of a phospholipid that avoid water.

Peripheral proteins

Proteins located on the inner or outer surface of the membrane.

Integral proteins

Proteins embedded in the cell membrane.

Transmembrane proteins

Integral proteins that penetrate the membrane completely.

Membrane protein functions

Transport, recognition, receptors, adhesion, and enzymatic activity.

Cholesterol

Regulates membrane fluidity; prevents excessive rigidity in cold conditions and excessive movement in hot conditions.

Glycocalyx

Carbohydrate coat involved in adhesion, barrier formation, and recognition.

Fluid mosaic model

Describes the dynamic structure of the cell membrane.

Cell membrane functions

Allows cells to act as individual units, regulates molecular movement, and anchors enzymes.

Selective permeability

Membrane permeability based on molecular size, charge, shape, and lipid solubility.

Membrane transport

Controlled movement of nutrients, wastes, water, and other molecules into and out of cells.

Passive transport

Movement of molecules from high to low concentration without requiring energy.

Types of passive transport

Simple diffusion, facilitated diffusion, and osmosis.

Simple diffusion (dialysis)

Movement of solutes across a membrane from high to low concentration; examples include lipid-soluble compounds and dissolved gases.

Facilitated diffusion

Passive transport through carrier proteins or protein channels.

Carrier-mediated transport

Transporter proteins move specific solutes from high to low concentration; example: glucose carrier protein.

Protein channels

Transmembrane proteins that allow molecules to pass through based on size and charge; examples include ions and water-soluble compounds.

Osmosis

Movement of water across a membrane from high to low water concentration.

Osmotic pressure

Force of water generated by solute concentration; measures how strongly a solution draws in water.

Conditions for osmotic pressure

Solutes are not permeable, while water is freely permeable.

High osmotic pressure

A solution draws in more water.

Osmolality

Measure of osmotic pressure determined by the number of dissolved particles.

Osmolality calculation

Concentration multiplied by the number of particles produced by each dissolved solute.

1 M glucose osmolality

1 Osm because glucose exists as one particle in solution.

1 M NaCl osmolality

2 Osm because NaCl separates into two particles in solution.

Electrochemical driving force

Combined electrical and chemical forces driving passive transport.

Electrical driving force

Movement influenced by membrane potential; like charges repel and opposite charges attract.

Membrane potential

Difference in electrical potential across a membrane.

Chemical driving force

Movement influenced by a concentration gradient.

Factors affecting diffusion rates

Distance, gradient size, molecular size, and temperature.

Distance and diffusion

Shorter diffusion distance results in faster diffusion.

Gradient size and diffusion

A larger concentration difference results in faster diffusion.

Molecular size and diffusion

Smaller molecules diffuse faster.

Temperature and diffusion

Higher temperature increases diffusion rate.

Active transport

Energy-dependent movement of molecules against their concentration gradient using a transporter protein.

Primary active transport

Uses ATP directly to move molecules against their concentration gradient; example: Na+/K+ pump.

Na+/K+ pump

Uses one ATP to move three Na+ out of the cell and two K+ into the cell.

Secondary active transport

Uses a concentration gradient generated by primary active transport to move another molecule against its gradient.

Secondary active transport example

The Na+ gradient generated by the Na+/K+ pump drives glucose transport.

Cotransport

Movement of two types of molecules in the same direction.

Countertransport

Movement of two types of molecules in opposite directions.

Ion pumps

Transporter proteins that move ions such as Na+ and K+.

Exchange pumps

Transporter proteins that move two types of molecules at the same time.