Advanced Biology - Chapter 7 Structure & Function of Membranes Study Guide
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Singer & Nicolson developed the fluid mosaic model for membranes. This model proposed a phospholipid
bilayer with embedded proteins. Since the model’s development, there has been a revelation that there is a
higher percentage of protein present than originally anticipated.
Cholesterol aids in making membranes more fluid in cold temperatures and more stable (less likely to fall apart)
in warm temperatures. Cholesterol is present in certain areas, between phospholipids. The phospholipids also
play a role in maintaining membrane integrity. The fatty acid tails of these molecules may either be saturated or
unsaturated. Unsaturated tails have “kinks” in them due to the presence of double bonds. These kinks push the
phospholipids further away from each other, preventing them from packing tightly, enhancing fluidity.
Saturated fatty acid tails lack double bonds. As a result, the tails are straight, allowing the phospholipids to pack
more tightly. This reduces membrane fluidity. The evolutionary process of natural selection has allowed for the
presence of different phospholipid composition among different organisms. Thus, an organism living in a cold
environment is more likely to possess unsaturated fatty acid tails which preserves fluidity. Winter wheat, a cold
climate crop, is able to maintain membrane fluidity in this manner. The polyunsaturated tails in winter wheat
play a much larger role in its membrane fluidity that cholesterol molecules do. Organisms in warmer climates
are likely to exhibit saturated fatty acid tails preserving membrane integrity. Fish living in cold water exhibit this
trait.
The movement of phospholipids within the bilayer also accounts for fluidity and allows for membrane
reformation. The fact that they are not locked in place provides the opportunity for changing membrane
configuration. Phospholipids are extremely likely to move laterally within a layer, and do so at a staggering rate.
Yes, I said staggering! It makes one want to stagger. Phospholipids rarely move vertically from one layer of the
bilayer to the other. This is so because vertical movement would expose hydrophobic tails to water, which
chemically is not inclined to occur.
Proteins in a membrane may also vary by species. Membrane proteins that completely extend through the
bilayer are called transmembrane proteins. Integrins or integral membrane proteins. These proteins are
amphipathic, having at least one hydrophobic region as well as hydrophilic regions. The portion of the protein
that lies at the interior of the membrane is hydrophobic and hangs out with the hydrophobic fatty acid tails of
the phospholipids. The portions that extrude from the membrane on the cytoplasmic and extracellular sides of
the bilayer are water loving…..they love water. HYDROPHILIC!!!! In freeze-fracture technique, A membrane is
frozen and a knife is used to split the bilayer apart (think pulling a chunky peanut butter sandwich apart).
Examination of the separated bilayer reveals bumps on both separated surfaces. These bumps are INTEGRAL ,
transmembrane proteins that randomly remained attached to one of the two separated layers. Freeze fracture
proves that these integral proteins are transmembrane and penetrate through both layers of the bilayer.
Contrary to integral proteins are peripheral proteins. These proteins often attach to exposed areas of integral
proteins and are found on the membrane surface. They are not transmembrane proteins! Cell surface proteins
must be made by ribosomes and then transported outside of the cell. Do you think bound or free ribosomes
would produce these proteins? Which one? ……?
Carrier proteins are membrane proteins that assist in transporting substances across the membrane. Carrier
proteins have specific shapes for specific molecules, much like the “lock and key” or “induced fit” mechanisms of
enzyme-substrate interaction. It is this specificity that makes carrier proteins fun! Wooo-Hooo!!!!
Sometimes, polysaccharides or shorter sugar chains called oligosaccharides may attach to the extracellular
exposed surface of integral proteins in the membrane. These glycoproteins allow cells to recognize each other.
We call this cell-to-cell recognition. White blood cells have receptors on their surface that allow them to
recognize familiar cells that are harmless versus foreign cells that may be injurious to the organism.
Small, hydrophobic molecules pass most easily through membranes. These substances are small and non-polar.
They quickly move across membranes. Large, hydrophilic molecules have the most difficulty passing through a
membrane.
Water is polar. That means it would have difficulty passing through a membrane. Fortunately, special channel
proteins called aquaporins allow water to easily pass through the membrane.
Diffusion, osmosis, facilitated diffusion are all examples of PASSIVE TRANSPORT. They do not require energy
because their movement is from high to low concentration. Active transport is from low to high and requires
energy.
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Isotonic solutions have the same solute concentration as a cell. A cell placed in this solution will remain
unchanged due to dynamic equilibrium.
Hypertonic solutions have more dissolved solute than the cell does. As a result, solute moves into the cell and
WATER moves out. The cell shrivels and that’s that. You have a shriveled cell. I hope you’re happy.
Hypotonic solutions have less dissolved solute than the cell has. As a result, solute moves out of the cell and
WATER moves into the cell. The cell swells and possibly bursts. A cell placed in distilled water will do this.
Distilled water placed on and around a cell will also do this. 
Plant cells look different, compared to animal cells when exposed to hypertonic and hypotonic solutions.
Because of a rigid, unchanging cell wall that surrounds plant cells, attention must be directed toward the
membrane and cytoplasm that lies within the cell wall. A plant cell exposed to a hypertonic solution will show a
cytoplasmic area that has shrunk, while the cell wall is still intact. Plant cell shrinking is called plasmolysis. A
plant cell exposed to hypotonic solution will show swelling in the form of bulging cell walls but will never burst
like an animal cell can.
Protein pumps in membranes are able to generate millivolt voltages across membranes by pumping ions
(charged atoms) and changing ion concentrations on either side of the membrane. Pumps like these are called
electrogenic pumps. The voltage across the membrane is called the membrane potential. The sodiumpotassium pump is a great example of an electrogenic pump. It moves 3 Na+ ions to the outside of a nerve
membrane for every 2 K+ ions that are moved inside a nerve membrane. Pumps of this sort require ATP as the
mechanism involved is active transport. It is pumping ions against a gradient from low to high. The gradient is
electrochemical because there is a chemical factor - the concentration gradient, and an electrical factor – the
membrane potential and ion charges involved.
Ion concentrations in the body can be disrupted by viruses and bacteria that cause vomiting and diarrhea. The
loss of these vital ions has resulted in death, especially in decades past. Today, less deaths occur because
various hydrating drinks that contain needed electrolytes have been developed and can be administered to
these ill people.
Phagocytosis is cell “eating” and pinocytosis is cell “drinking”. Both fall under the endocytosis category and take
substances into the cell. Exocytosis is when substances are released to the outside of the cell.
In receptor-mediated endocytosis, the cell has receptors on the outside of the cell membrane. When a ligand
attaches to the receptor, endocytosis is stimulated. The engulfing process eventually forms a vesicle that has
receptors on the inner membrane surface.
Glycolipids are phospholipids with oligosaccharides attached to them. Used for cell-to-cell recognition but to a
lesser extent.
Proton pumps are active transport pumps that pump hydrogen ions across a membrane against a gradient. The
downhill backflow of these ions across the membrane is used to do work. This is how ATP is made in cellular
respiration.
Cotransport is when the backflow of H+ ions across a membrane is used to drive the transport of another
substance, such as glucose, across the membrane. See your book for a picture. A cotransporter protein
embedded in the membrane allows both H+ and sugar to pass across the membrane.