Plasma Membrane
Bimolecular layer of lipids and proteins in a
constantly changing fluid mosaic
* Plays a dynamic role in cellular activity
* Separates intracellular fluid (ICF) from
extracellular fluid (ECF)
Phospholipids
75%
Phosphate heads: polar and hydrophilic
Fatty acid tails: nonpolar and hydrophobic
Glycolipids
5%
Lipids with polar sugar groups on outer membrane
surface
Cholesterol
20%
Increases membrane stability and fluidity
Integral proteins
Firmly inserted into the membrane (most are
transmembrane)
* Transport proteins (channels and carriers),
enzymes, or receptors
Peripheral proteins
Loosely attached to integral proteins
* Include filaments on intracellular surface and
glycoproteins on extracellular surface
* Enzymes, motor proteins, cell-to-cell links,
provide support on intracellular surface
Functions of Membrane Proteins
- Transport
- Receptors for signal transduction
- Attachment to cytoskeleton and extracellular
matrix - Enzymatic activity
- Intercellular joining
- Cell-cell recognition
Membrane Protein Functions
receptors, second-messenger systems,
enzymes, ion channels, carriers, cell-identity
markers, cell-adhesion molecules
Cilia
Hair like processes 7-10m long
* single, nonmotile primary cilium found on nearly every cell
* “antenna’ for monitoring nearby conditions
* sensory in inner ear, retina, nasal cavity, and kidney
Motile cilia
respiratory tract, uterine tubes, ventricles of the brain, efferent
ductules of testes
* sweep substances across surface in same direction
* power strokes followed by recovery strokes
Cystic fibrosis
hereditary disease in which cells make
chloride pumps, but fail to install them in the plasma
membrane
Microvilli
Finger like extensions of membrane (1-2 m)
* serves to increase cell’s surface area for absorption
* best developed in cells specialized in absorption (ex.
kidneys, stomach and intestines)
* gives 15 – 40 times more absorptive surface area
Gradient
difference in chemical concentration, charge,
temperature, or pressure between two points
Passive processes
No cellular energy (ATP) required
Active processes
Energy (ATP) required
What determines whether or not a
substance can passively permeate a
membrane?
- Lipid solubility of substance (lipophilic versus
lipophobic). - Channels of appropriate size
- Carrier proteins
5 Types of Passive Processes
Simple diffusion
* Carrier-mediated facilitated diffusion
* Channel-mediated facilitated diffusion
* Osmosis
* Filtration
Simple Diffusion
the net movement of particles
from area of high concentration to area of
low concentration
Factors affecting diffusion rate through a membrane
Temperature - temp., motion of particles
* Molecular weight - larger molecules move slower
* Steepness of concentrated gradient - difference, rate
* Membrane surface area - area, rate
* Membrane permeability - permeability, rate
