Download Biological Membranes

Biological Membranes
Fluid Mosaic Model
- Membranes consist of a fluid lipid bilayer. Proteins are
embedded like tiles in a mosaic.
- Mosaic pattern not static; proteins constantly changing
positions.
Fluid Quality of Membranes
- most lipids & some proteins can drift laterally. Rare for
molecule to flip-flop across a membrane.
- Phospholipids move rapidly ( 2 µm) along membranes,
proteins are larger and move slowly.
- Membranes must be fluid to work properly.
Membrane Proteins
- proteins determine the membrane function.
- 2 major classes:
1. Integral Proteins – Firmly bound to the
membrane. Go through entire membrane.
(transmembrane proteins)
2. Peripheral Proteins – Bound loosely to exposed
regions of integral proteins. Can be easily removed
w/out disrupting the structure of the bilayer.
Biological membranes are selectively permeable – they allow
the passage of some things but not others.
Type of molecule
Gases
Hydrophobic
Large, Polar
Example
N2, O2, CO2
hydrocarbons
Glucose
Permeability
Freely permeable
Freely permeable
Not permeable
Passive Transport
- Diffusion of a substance across a biological membrane.
- Does not require energy.
1. Diffusion - net movement of a substance down a
concentration gradient.
2. Dialysis – diffusion of a substance across a semipermeable membrane.
3. Osmosis – diffusion of water across a selectively
permeable membrane.
4. Facilitated Diffusion – passive transport of ions or
molecules by a specific carrier protein in a
membrane. Transport proteins are similar to
enzymes:
a. They are specific for the solutes they transport.
b. Can be saturated w/ solute, so the maximum
transport rate occurs when all binding sites are
occupied with solute.
Example: Glucose transporter in Red Blood Cells keep the internal
concentration of glucose low by immediately adding a phosphate group to
entering glucose molecules (converts them to highly charged glucose
phosphates that can’t pass back through the membrane because glucose
phosphate is a different molecule) It doesn’t contribute to the glucose
gradient therefore a steep concentration gradient for glucose is continually
maintained & glucose rapidly diffuses into the cell, only immediately
changed to the phosphorylated form.
Active Transport
- Movement of substances across a biological membrane
against its concentration gradient or an electrochemical
gradient.
- Requires energy (usually ATP) & specific transport
proteins.
1. Sodium – Potassium Pump
 Exchange of sodium (Na+) for potassium (K+)
across the plasma membrane of animal cells.
 2 K’s in for every 3 Na’s out – electrochemical
gradient (- inside, + outside)
(http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/sppump.gif)
2. Cotransport
 Transport of a substance from a region of low
concentration to a region of high concentration by
coupling its transport to the transport of a substance
down its concentration gradient.
3. Exocytosis
 Trensport of materials out of the cell by fusion of
cytoplasmic vesicles with the plasma membrane.
 Used by secretory cells to export products like
hormones.
 Nerve cell also use exocytosis to release chemical
signals that stimulate other neurons of muscle cells.
4. Endocytosis
 Transport of materials into the cell by regions of the
plasma membrane that surround the material and
pinch off to form a cytoplasmic vesicle.
a. Phagocytosis – “cell eating”. Ingestion of large
solid particles.
b. Pinocytosis – “cell drinking”. Ingestion of
fluids and dissolved solids.
c. Receptor-mediated Endocytosis – Extracellular
molecules become bound to specific receptors
on the cell surface and then enter the cytoplasm
enclosed in vesicles.
Example: Cholesterol in blood is taken up by animal cells in this manner.
Cholesterol travels in the blood in particles called LDL’s which bind to
receptors on membranes then enter the cell by endocytosis. Familial
hypercholesterolemia ( have very high levels of cholesterol in blood) due
to LDL receptor being defective and the LDL particles can’t enter the
cells and it accumulates in the blood.
Intracellular Junctions
1. Desomsomes (Like Velcro®)
 Anchoring junction
 Hold cells together
 Allows cells to form strong sheets
Ex. Skin
2. Tight Junctions (Like Rain Coat Snaps)
 Membranes of neighboring cells are fused tightly
 Prevents the passage of materials through spaces
between cells
Ex. Intestinal wall cells
3. Gap Junctions (Like Rivets)
 Protein pores in membranes that allow
communication between the cytoplasm of adjacent
cells
 Pores can open or close, thus controlling the transfer
of small molecules and ions
Ex. Synchronized heart contractions of heart muscle
cells.