BIOL 600 Lecture Notes - Lecture 4: Lipid Bilayer, Membrane Lipids, Membrane Structure
26 May 2018
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AP Bio Chapter 7 Membrane Structure and Function
Lecture Outline
Overview: Life at the Edge
The plasma membrane separates the living cell from its nonliving surroundings.
This thin barrier, 8 nm thick, controls traffic into and out of the cell.
Like all biological membranes, the plasma membrane is selectively permeable, allowing
some substances to cross more easily than others.
Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins
The main macromolecules in membranes are lipids and proteins, but carbohydrates are
also important.
The most abundant lipids are phospholipids.
Phospholipids and most other membrane constituents are amphipathic molecules.
o Amphipathic molecules have both hydrophobic regions and hydrophilic regions.
The arrangement of phospholipids and proteins in biological membranes is described by
the fluid mosaic model.
Membrane models have evolved to fit new data.
Models of membranes were developed long before membranes were first seen with
electron microscopes in the 1950s.
o In 1915, membranes isolated from red blood cells were chemically analyzed and
found to be composed of lipids and proteins.
o In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a
phospholipid bilayer two molecules thick.
o The molecules in the bilayer are arranged such that the hydrophobic fatty acid
tails are sheltered from water while the hydrophilic phosphate groups interact
with water.
o Actual membranes adhere more strongly to water than do artificial membranes
composed only of phospholipids.
o One suggestion was that proteins on the surface of the membrane increased
adhesion.
o In 1935, H. Davson and J. Danielli proposed a sandwich model in which the
phospholipid bilayer lies between two layers of globular proteins.
o Early images from electron microscopes seemed to support the Davson-Danielli
model, and until the 1960s, it was widely accepted as the structure of the plasma
membrane and internal membranes.
o Further investigation revealed two problems.
First, not all membranes were alike. Membranes differ in thickness,
appearance when stained, and percentage of proteins.
Membranes with different functions differ in chemical composition
and structure.
Second, measurements showed that membrane proteins are not very
soluble in water.
Membrane proteins are amphipathic, with hydrophobic and hydrophilic
regions.
If membrane proteins were at the membrane surface, their hydrophobic
regions would be in contact with water.
In 1972, S. J. Singer and G. Nicolson presented a revised model that proposed that the
membrane proteins are dispersed and individually inserted into the phospholipid bilayer.
o In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids
are in maximum contact with water, and the hydrophobic regions are in a
nonaqueous environment within the membrane.
A specialized preparation technique, freeze-fracture, splits a membrane along the middle
of the phospholipid bilayer.
When a freeze-fracture preparation is viewed with an electron microscope, protein
particles are interspersed in a smooth matrix, supporting the fluid mosaic model.
Membranes are fluid.
Membrane molecules are held in place by relatively weak hydrophobic interactions.
Most of the lipids and some proteins drift laterally in the plane of the membrane, but
rarely flip-flop from one phospholipid layer to the other.
The lateral movements of phospholipids are rapid, about 2 microns per second. A
phospholipid can travel the length of a typical bacterial cell in 1 second.
Many larger membrane proteins drift within the phospholipid bilayer, although they
move more slowly than the phospholipids.
o Some proteins move in a very directed manner, perhaps guided or driven by
motor proteins attached to the cytoskeleton.
o Other proteins never move and are anchored to the cytoskeleton.
Membrane fluidity is influenced by temperature. As temperatures cool, membranes
switch from a fluid state to a solid state as the phospholipids pack more closely.
Membrane fluidity is also influenced by its components. Membranes rich in unsaturated
fatty acids are more fluid that those dominated by saturated fatty acids because the kinks
in the unsaturated fatty acid tails at the locations of the double bonds prevent tight
packing.
The steroid cholesterol is wedged between phospholipid molecules in the plasma
membrane of animal cells.
At warm temperatures (such as 37°C), cholesterol restrains the movement of
phospholipids and reduces fluidity.
At cool temperatures, it maintains fluidity by preventing tight packing.
Thus, cholesterol acts as a “temperature buffer” for the membrane, resisting changes in
membrane fluidity as temperature changes.
To work properly with active enzymes and appropriate permeability, membranes must be
about as fluid as salad oil.
Cells can alter the lipid composition of membranes to compensate for changes in fluidity
caused by changing temperatures.
o For example, cold-adapted organisms such as winter wheat increase the
percentage of unsaturated phospholipids in their membranes in the autumn.
o This prevents membranes from solidifying during winter.
Membranes are mosaics of structure and function.
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid
bilayer.
Proteins determine most of the membrane’s specific functions.
The plasma membrane and the membranes of the various organelles each have unique
collections of proteins.
There are two major populations of membrane proteins.
o Peripheral proteins are not embedded in the lipid bilayer at all.
Instead, they are loosely bound to the surface of the protein, often
connected to integral proteins.
o Integral proteins penetrate the hydrophobic core of the lipid bilayer, often
completely spanning the membrane (as transmembrane proteins).
The hydrophobic regions embedded in the membrane’s core consist of
stretches of nonpolar amino acids, often coiled into alpha helices.
Where integral proteins are in contact with the aqueous environment, they
have hydrophilic regions of amino acids.
o On the cytoplasmic side of the membrane, some membrane proteins connect to
the cytoskeleton.
o On the exterior side of the membrane, some membrane proteins attach to the
fibers of the extracellular matrix.
The proteins of the plasma membrane have six major functions:
1. Transport of specific solutes into or out of cells.
2. Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic
pathway.
3. Signal transduction, relaying hormonal messages to the cell.
4. Cell-cell recognition, allowing other proteins to attach two adjacent cells together.
5. Intercellular joining of adjacent cells with gap or tight junctions.
6. Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape
and stabilizing the location of certain membrane proteins.
Membrane carbohydrates are important for cell-cell recognition.
The plasma membrane plays the key role in cell-cell recognition.
o Cell-cell recognition, the ability of a cell to distinguish one type of neighboring
cell from another, is crucial to the functioning of an organism.
o This attribute is important in the sorting and organization of cells into tissues and
organs during development.
o It is also the basis for rejection of foreign cells by the immune system.
o Cells recognize other cells by binding to surface molecules, often carbohydrates,
on the plasma membrane.
Document Summary
Ap bio chapter 7 membrane structure and function. The plasma membrane separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Concept 7. 1 cellular membranes are fluid mosaics of lipids and proteins. The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important. Phospholipids and most other membrane constituents are amphipathic molecules: amphipathic molecules have both hydrophobic regions and hydrophilic regions. The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model. Membrane models have evolved to fit new data. Models of membranes were developed long before membranes were first seen with electron microscopes in the 1950s. In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins.
