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Lecture 2

BIOL 1201 Lecture Notes - Lecture 2: Unsaturated Hydrocarbon, Membrane Fluidity, Semipermeable Membrane

Biological Sciences
Course Code
BIOL 1201
Bill Wischusen

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BIOL 1201 Section 1 Spring 2013
Membranes and membrane function (Chapter 7 of the textbook)
Why are membranes called phospholipid bilayers?
The most abundant lipids in most membranes are phospholipids.
Due to its fluid mosaic model.
Such a double layer of molecules could exist as a stable boundary between two
aqueous compartments because the molecular arrangement shelters the
hydrophobic tails of the phospholipids from water while exposing the hydrophilic
heads to water.
Phospholipids have a hydrophilic head and hydrophobic tail
What is meant by fluid mosaic?
The membrane is a fluid structure with a mosaic of various proteins embedded in
or attached to a double layer (bilayer) of phospholipids.
Mosaic of proteins drifting laterally in fluid bilayer of phospholipids.
How are membranes adjusted for different temperatures? How do the relative amounts of
saturated and unsaturated fats change?
Fish that live in extreme cold have membranes with a high proportion of unsaturated
hydrocarbon tails, enabling their membranes to remain fluid. Some bacteria and
archaea thrive at high temperatures where their membranes include unusual lipids
that may prevent excessive fluidity at such high temps.
Unsaturated hydrocarbon tails (kinked) prevent packing, enhancing membrane
Saturated hydrocarbon tails are packed together, increasing membrane viscosity.
Membrane fluidity adjusted by changing the ratio of unsaturated to saturated fatty
Cold environment unsaturated to make membrane more fluid; warm wants more
saturated to make membrane more solid.
What is meant by membrane fluidity? Why is this an important property?
Membranes must be fluid to work properly. When a membrane solidifies, its
permeability changes, and enzymatic proteins in the membrane may become
inactive if their activity requires them to be able to move within the membrane.
Membranes are not too solid, not too fluid. Just right to preserve membrane
Henrique and Hanson performed experiments with pigs and examined the effects on the
relative amounts of saturated and unsaturated fats. What did they find? One can see
similar patterns comparing species with different body temperature or cells grown at
different culture temperatures.
o Put the pigs in long underwear.
o Found that membrane fluidity can acclimate to temperature.
o Pigs raised wearing underwear in a hot room had subcutaneous fats with higher
melting point.
Transport of materials across the membrane
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What types of molecules can move easily across the membrane? (e.g. polar,
nonpolar, large, small)
Nonpolar hydrophobic and can therefore dissolve in the lipid bilayer of the
membrane and cross it easily.
Small, nonpolar molecules
gases, steroids, water
Transport Processes
What is simple diffusion?
Down concentration gradient. Depends on molecular movement. Does not require
supplied energy. Does not use a carrier molecule.
What is facilitated diffusion?
Difference is that it employs a carrier molecule. Moves down concentration
gradient. Depends on molecular movement. Does not require supplied energy.
What is active transport?
Energy and some sort of carrier molecule. Moves against the concentration
Work must be done; energy expended.
Active transport requires ATP. This is how it is different from facilitated diffusion.
For which processes are carrier molecules involved?
Facilitated diffusion
Active transport
Which processes require the input of additional energy?
o Active transport
What is a semi-permeable barrier?
Only some types of molecules can move through.
When equilibrium is reached in diffusion: Does movement of molecules stop? Does net
movement of molecules stop?
If a cell without a wall, such as an animal cell, is immersed in an environment that is
isotonic to the cell (iso=same), there will be no net movement of water across the
plasma membrane. Water diffuses across the membrane, but at the same rate in
both directions. In an isotonic environment, the volume of an animal cell is stable.
Movement of molecules does NOT stop. However, net movement does stop.
Membranes and membrane function: osmosis
The net movement of water across (through) a semi-permeable membrane.
o Diffusion of water
o From higher (few solutes) water potential to lower water potential.
What are aquaporins channel protein which facilitate water diffusion through the
Tonicity and aquatic organisms: In lecture we presented examples of aquatic animals
from freshwater and seawater: Bony fish, sharks and invertebrates. How do they
compare to their environments?
Tonicity the ability of a surrounding solution to cause a cell to gain or lose water.
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Isotonic with their environment like crabs; neither gain nor lose water.
Accumulate free amino acids to match the tonicity of seawater.
Sharks and rays marine elasmobranch fishes isotonic with seawater. They
accumulate urea (0.5 M) to help with their body solutions to match the solutes in
Freshwater bony fish (teleost fish) hypertonic to water: gain water.
Marine bony fish (teleost) hypotonic to seawater; lose water.
What is meant by hyper-, hypo- and isotonic? In which direction does water flow?
Isotonic same as the environment
o No net movement of water.
Hypertonic referring to a solution that, when surrounding a cell, will cause the cell
to lose water.
o Lower water potential; more solutes dissolved; tendency to gain water via
Hypotonic referring to a solution that, when surrounding a cell, will cause the cell
to take up water.
o Higher water potential; fewer solutes dissolved; hypotonic solutions will tend
to lose water via osmosis
Osmosis and the direction of net water movement
red blood cells would lyce or explode because water would be rushing into them.
Be able to analyze U-tube experiments involving a semipermeable barrier and
solutions with different concentrations of solutes in either arm of the U-tube.
What are aquaporins?
A channel protein in the plasma membrane of a plant, animal, or microorganism cell
that specifically facilitates osmosis, the diffusion of free water across the membrane.
Figures 7.2 to 7.6, 7.8, 7.11 to 7.15, 7.17, and 7.19
Enzymes and Energy (Chapter 8)
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