Chapter #7 – Membrane Structure And Function
Introduction
1. The plasma membrane is the edge of life, the boundary that separates the
living cell from its surroundings.
2. The plasma membrane exhibits selective permeability, that is, it allows some
substances to cross it more easily than others.
A. Cellular Membranes Are Fluid Mosaics of Lipids and Proteins
1. Lipids and proteins are stable ingredients of membranes, although carbohydrates
are also important.
a) The major structural components of cell membrane are phospholipids & proteins.
b) A phospholipids is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region.
c) The most abundant lipid in most membranes are phospholipids.
2. The currently accepted model for the arrangement of the molecules in the cell
membrane is the fluid mosaic model.
3. In the fluid mosaic model the membrane is a fluid structure with a “mosaic” of
various proteins embedded in or attached to a double layer of phospholipids.
B. Membrane Models: Scientific Inquiry
1. The first to propose that cell membranes are phospholipids bilayer were
Gorter and Grendel.
2. Building on the above idea, Hugh Davson & James Danielli were to answer
the question – where are the proteins located in the cell membrane. Davson
and Danielli proposed a sandwich model: the cell membrane is a phospholipid bilayer between two layers of proteins. ( #4.Add Singer & Nicolson…)
3. Cell biologists recognized two problems with this model.
a) The generalization that all cell membranes are identical. Membranes with
different functions differ in chemical composition & structure.
b) A more serious problem was the protein placement.
C. The Fluidity of Membranes
1. A membrane is held together primarily by hydrophobic interactions.
2. Most of the lipids and some of the proteins can drift about laterally.
3. Many other membrane proteins seem to be held virtually immobile by their
attachment to the cytoskeleton.
4. The steroid cholesterol, which is wedged between phospholipids molecules in
the plasma membrane of animal cells, has different effects on membrane fluidity
at different temperatures.
a) Cholesterol enables the membrane to stay fluid more easily when cell
temperature drops.
b) Membranes must be fluid to work properly.
D. Membrane Proteins & Their Functions
1. More than 50 kinds of proteins have been found in the plasma membrane of
red blood cells.
2. Proteins determine most of the membrane’s functions.
3. There are two major populations of membrane proteins: integral proteins and
peripheral proteins.
a) Some integral proteins are transmembrane proteins, which span the membrane.
b) Some integral proteins have a hydrophilic channel through their center that
allows passage of hydrophilic substances.
4. Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of
integral proteins.
E. The Role of Membrane Carbohydrates in Cell-Cell Recognition
1. Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell
from another, is crucial to the functioning of an organism.
2. Carbohydrates function as markers that distinguish one cell from another.
II. Membrane Structure Results in Selective Permeability
1. The cell membrane is selectively permeable, and substances do NOT cross the
barrier indiscriminately.
2. The cell takes up many varieties of small molecules & exclude others.
A. The Permeability of the Lipid Bilayer
1. Nonpolar molecules
a) Dissolve in the membrane & cross it with ease
b) If two are equally lipid soluble
1. The smaller one cross the membrane faster
2. The substances that pass the membrane easier are small & hydrophobic
3. CO2 move through plasma membrane very rapidly.
2. Polar molecules
a) Small polar, uncharged molecule will pass, but NOT very fast
b) Larger polar uncharged molecules find it even more difficult to penetrate
the hydrophobic core.
B. Transport Protein
1. Proteins play a key role in regulating transport.
2. To cross the membrane, polar molecules & ions generally require specific transport
proteins.
3. Some transport proteins, called channel proteins, function by having a hydrophilic
channel that certain molecules or atomic ions use as a tunnel through the membrane.
Example: Aquaporins – transport water molecules.
4. Other transport proteins, called carrier proteins, hold onto their passengers and
change shape in a way that shuttles them across the membrane.
III. Passive Transport is Diffusion of a Substance Across a Membrane With No Energy
Investment
1. Molecules have a type of energy called thermal motion (heat).
2. One result of thermal motion is diffusion, the spontaneous movement of a substance
down its concentration gradient.
a) Concentration gradient- the region along which the density of a chemical
substance decreases.
3. The diffusion of a substance across a biological membrane is called passive transport.
a) No work must be done for this process to take place. This is a spontaneous process.
A. Effects of Osmosis on Water Balance
1. The diffusion of water across a selectively permeable membrane is osmosis.
2. Osmosis is passive transport.
B. Water Balance of Cells Without Walls
1. When considering the behavior of a cell in a solution, both solute concentration &
membrane permeability must be considered.
2. Water diffuses out of a cell if the solution outside has a higher solute
concentration (hypertonic) than the cytosol & enters the cell if the solution has a
lower solute concentration (hypotonic).
3. If the concentrations are equal (isotonic), no net osmosis occurs.
4. Cells lacking walls (as in animals & some protists) are isotonic with their environments
or have adaptations for osmoregulation.Example- paramecium is equipped with a
contractile vacuole, an organelle that functions to force water out of the cell.
5. Plants, prokaryotes, fungi & some protists have relatively inelastic walls, so the cells
don’t burst when in a hypotonic environment.
C. Facilitated Diffusion: Passive Transport Aided by Proteins
1. In facilitated diffusion, a transport protein move many polar molecules & ions across
the membrane.
2. This process is passive transport.
3. Two types of transport proteins are channel proteins and carrier proteins.
IV.
Active Transport Uses Energy to Move Solutes Against Their Gradients
A. The Need for energy in Active Transport
1. To pump a solute across a membrane against its gradient requires work; the cell must
expend energy. This type of membrane traffic is called active transport.
2. Specific membrane proteins use energy, usually in the form of ATP, to do the work
of active transport. Example- sodium- potassium pump.
D. How Ion Pumps Maintain Membrane Potential
E. Cotransport: Coupled Transport by a Membrane Protein
1. A single ATP-powered pump that transports a specific solute can indirectly drive
the active transport of several other solutes in a mechanism called cotransport.
2. One solute’s “downhill” diffusion drives the other’s “uphill” transport – this is
cotransport.
V. Bulk Transport Across the Plasma Membrane Occurs by Exocytosis & Endocytosis
A. Exocytosis
1. In exocytosis, transport vesicles migrate to the plasma membrane, fuse with it, &
release their contents.
2. In endocytosis, molecules enter cells within vesicles that pinch inward from the plasma
membrane.
3. The three types of endocytosis are
a) phagocytosis
b) pinocytosis
c) receptor-mediated endocytosis