The Plasma membrane
The plasma membrane, also called the cell membrane separates the intracellular fluid within the
cell and the extracellular fluids outside the cell.
STRUCTURE OF PLASMA MEMBRANE – FLUID MOSAIC MODEL
Thin structure composed of a double layer of lipid molecules with protein molecules dispersed
Called the Fluid mosaic model because membrane proteins float about in the membrane lipid
Lipid bilayer is composed largely of phospholipids, some glycolipids and cholesterol and areas
called lipid rafts.
Phospholipid: has a phosphate group on two long chain lipids. The polar head is charged and
hydrophilic while the nonpolar tail is hydrophobic and uncharged.
Glycolipids: lipids that are attached to sugar groups. Only found on the outer surface
Cholesterol: formed between hydrophilic tails, stabilize the membrane, decrease mobility of the
phospholipids and the fluidity of the membrane. Too much cholesterol causes membranes to
lose its flexibility.
Lipid Rafts : assemblies of saturated phospholipids
MEMBRANE PROTEINS: allows to communicate with the environment
Integral Proteins: firmly inserted into the lipid bilayer. Most are transmembrane proteins that
span the entire membrane. Some transmembrane proteins cluster together to form channels or
pores. Others are involved in transport, some are receptors for hormones. All Integral proteins
have a hydrophilic and hydrophobic regions.
Peripheral Proteins: not embedded in the lipid bilayer. Attached loosely to integral proteins.
Include a network of filaments that help support the membrane from its cytoplasmic side. Some
peripheral proteins are enzymes. Involved in attachment functions, shape changes.
CYTOSKELETON: anchors to plasma membrane, can also interact with receptors
Ensemble of carbohydrates attached to lipids and proteins on extracellular face.
Sugar coating on plasma membrane provide cell recognition ex: sperm recognizes an ovum
Glycocalyx changes when cell becomes cancerous – can even change repeatedly to avoid
recognition by immune system
minor damage to the plasma membrane usually not a problem because it reseals naturally Cell Junctions
Three factors that act to bind cells together
Glycoproteins in the Glycocalyx act as adhesive
Wavy contours of the membranes of the adjacent cells act fit together
Special cell junctions form
− Fusion of series of integral protein molecules of adjacent plasma membranes to prevent passage
Ex: epithelial cells lining digestive tract keep digestive enzymes and micro-organisms entering
the bloodstream. (Importance: immune and toxin protection)
− Anchoring junctions: cytoplasmic face of each plasma membrane is covered by plaque
− Adjacent cells are held together by thin protein filaments that extend together from the plaque
− Thicker keratin filaments extend from the cytoplasmic side to anchor the plaque on the cell’s
opposite side (by this way desmosomes bind neighboring cells together as well as contribute to
a continuous internal network)
− This distributes tension throughout cellular sheet and reduces chances of tearing
− Abundant in tissues subjected to great mechanical stress ex: skin and heart muscle
− Communicating junction between cells
− Adjacent plasma membranes are very close and cells are connected by hollow cylinders called
connexons (composed of transmembrane proteins)
− Ions, simple sugars and other small molecules pass through these water filled channels from one
cell to the next
− Abundant in electically excitable tissues. Ex: heart and smooth muscle
FUNCTIONS OF PLASMA MEMBRANE
1. effective barrier between the intracellular & extracellular fluids
2. selectively permeable
3. allows the cell to respond to changes in the extracellular fluid
4. site of cell-to-cell interaction and recognition Membrane Transport
Cells are bathed in an extracellular fluid called Interstitial fluid (barrier between cytoplasm)
Interstitial fluid is a filtrate of blood and contains: salts, sugar, amino acids, vitamins, hormones,
metabolites, gases (2 and CO2), fatty acids, neurotransmitters, waste products.
Plasma membrane is selectively permeable
To maintain homeostasis and function normally, a cell must be extract needed nutrients , keep
valuable cell proteins inside & discard wastes.
Transport across Plasma membrane can be active or passive
− Passive transport by diffusion saves cellular energy
− Tendency of molecules or ions to move from an area with higher concentration to an
area with lower concentration along their concentration gradient.
− Greater the concentration, higher the collisions and faster the net movement/diffusion
− Driving force for diffusion is kinetic energy of the molecules
− Rate of diffusion depends on :
a) molecular size (smaller the faster)
b) temperature (warmer the faster)
c) gradient slope (higher, more potential)
− Diffusion ex: movement of ions across cell membrane, neurotransmitters between
− The plasma membrane is a hydrophobic barrier for diffusion therefore a molecules
a) Lipid soluble
b) Small enough to pass through membrane channels
c) Assisted by carrier molecule
Non-polar lipid soluble substances diffuse directly through the lipid bilayer.
Ex: 2 , C2 , fat soluble vitamins, urea, alcohol.
Molecule is moving down the concentration gradient. Facilitated Diffusion
Molecules unable to pass through the plasma membrane are transported by
a) By binding to a protein carrier in the membrane
b) Moves through water filled protein channels
Ex : Glucose, other sugars, some amino acids and ions.
Features of facilitated diffusion
b) Not ATP-requiring
c) Limited by carrier saturation
d) Movement down concentration gradient
e) Can be inhibited by certain substances
Carrier-mediated facilitated Channel-mediated facilitated
Lipid insoluble molecules (sugars and Channels are transmembrane proteins
amino acids) too large to pass through Substances like ions or water is
membrane pores/channels transported through aqueous channels
Carriers are transmembrane integral Channels are selective due to pore
proteins that are specific for each class size, charges of amino acid that line
of molecules channels
Alterations in the carrier allows Leakage channels are always open
molecules to bind and the carrier
Opening of gated channels are
protein move the binding site from one controlled by chemically or electrical
face of the membrane to the other signals
Can be limited by the number of Can be inhibited, show saturation and
protein carriers present. When all
carries are engaged it is said to be
saturated. 2. OSMOSIS
− The unassisted diffusion of water from an area of low to an area of high solute
concentration through a selectively permeable membrane
− Water moves through gaps between hydrophobic tails in the plasma membrane
− Water also moves freely through water specific channels constructed by
transmembrane proteins called aquaporins(AQPs) which allow single file diffusion of
− AQPs are abundant in red blood cells involved in water balance such as kidney tubule
− Osmosis occurs whenever the water concentrations or solute concentrations differ in
the two sides (as solute concentration increases, water concentration decreases)
− If distilled water is present on both sides no net movement occurs
− Osmolarity refers to the total concentration of all solute particles in a solution