BIOLOGY 1A03 Study Guide - Midterm Guide: Lipid Bilayer, Lipid Raft, Fluid Mosaic Model
20 May 2018
School
Department
Course
Professor
1A03 Biology Kajiura: Friday 1-2pm LS 426
Module 1:Composition and structure of membranes
Learning objectives:
• identify that we are mosaics of cell types derived from different genetic origins
• recognize the 4 core macromolecules that make up all living cells
• describe the composition and function of cell membranes
• explain how fluidity and transport mechanisms can influence the movement of
substances across the cell membrane
Cells in our bodies
Cell division from fertilized embryo creates 10 trillion cells in adult human
Most cells in our bodies are prokaryotic (no nucleus eg bacteria)
Human Microbiome
Microbiome: population of microbes/microrganisms→ not visible to naked eye
10 000 species of microbes in a healthy individual
streptococcus salivarius: inhabitant in upper respitory tract and oral cavity. Member of a
collection of bacteria that contribute to the formation of dental plaque. Also fist microbe to
colonize germ-free newborn oral cavity, and gastrointestinal tract.
Straphylococcus haemolytius: o ou ski. If it stas thee, its haless ut it a e
pathogenic if it gets inside the body. Infection commonly leads to activation of the immune
system.
Bacteroides thetaiotamicron: predominant intestinal bacteria and break down plant materials
that we digest such as oat fibre.
Cells are made up of 4 classes of biological molecules
Cell: membrane bound structure containing macromolecules
1. nucleic acids (DNA and RNA)
2. Proteins
make structural elements of cell eg flagellum
3. Fats (phospholipids)
primary component of the cell membrane
4. Carbohydrates (polysaccharides)
important structural component of many cells (plant walls)
Membranes separate in internal environment from the external environment. Allows these
distinct environments to have a different chemical composition. The cell membrane is made up
of lipids. Each of these macromolecules has hydrophobic and hydrophilic properties that allow
staked lipid ilaes to fo. Have a hydrophobic (non-polar) interior and hydrophilic (polar)
surfaces. Cell membranes are very thin. Phospholipids are main components of cell
membranes. Each consists of polar group>phosphate group> glycerol>2 fatty acid tails
Different lipids can be found in cells:
Typically there are 16 or 18 carbons in a single chain
The bonds connecting the carbons may be single (saturated) or double bounds (unsaturated)
change the shape, size, and behaviour of the phospholipids and membranes
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Another important component is steroids eg cholesterol, characterized by 4 hydrocarbon ring
structure.
Kinked tail: double bonds
Straight tail: single bonds
Phospholipids form unique structures that are critically important for the cell. Water molecules
interact with head regions, but not the tails. The tails interact with the tails to form
hydrophobic cores. Phospholipids a aggegate ad fo lipid ielles. Thee ipotat for
absorption of fat soluble vitamins and complex lipids in the human body. They form
spontaneously without use of any energy.
Cell membranes are fluid.
The aet statioa eause eaes ae fluid. I othe ods, the a oe lateall
ithi the ell, foads/akads/left/ight ut, the at flip oe lae to the othe ithout
the use of a lot of energy.
Fluidity of membranes factors
1. The # of carbons in a hydrocarbon chain varies. Typically there are 16 or 18 carbons , the
longer chains pack together more tightly than the shorter chains, reducing the fluidity of
the membrane.
2. Double bonds within a hydrocarbon tail produce kinks and bends in the chain. This has
the effect of pushing neighboring phospholipids further apart and increasing
Fluidity. Unsaturated fatty acid tails can lead to kinks in hydrophobic tails that affect
overall permeability.
3. External environments such as temperatures promote fluidity. Interestingly, cold-
adapted organisms tend to have more unsaturated phospholipids in their membranes
4. Steroids such as cholesterol are found in the membranes of every cell in our body,
making up 50% of the molecules found in the bilipid membranes. Cholesterol molecules
constrain fluidity of the membranes by packing closely to neighboring phospholipids. At
low temperatures, phospholipid bilayers behave like many other fats (they solidify)
cholesterol helps to maintain fluidity by keeping the phospholipids apart from one
another
Within membranes, there are domains that demonstrate different degrees of fluidity. Some
domains are more fluid, and others are less. Regions of lower fluidity are called lipid raft and
can hold macromolecules together in the membrane. These rafts are found to gather proteins
involved in the same metabolic pathway or a collection of receptors on the surface of the cell.
What factors contribute to the variation of fluidity?
-Regions of the lipid raft is taller, due to the longer phospholipids hydrocarbon tails.
Phospholipid tails are straight rather than kinked because they are saturated. This allows the
phospholipids to pack together lowering the fluidity making it possible to hold the
macromolecules in the raft. A higher concentration of cholesterol decreases fluidity within the
microdomain of the lipid raft.
Movement across cell membrane
Fluidity changes the permeability of membranes.
- fluid membrane with - unsaturated fatty acids, + cholesterol → lower permeability
+ fluid membrane with + unsaturated fatty acids and - cholesterol → higher permeability
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Macromolecules can move laterally within the cell membrane, the fluidity of the cell membrane
also allows for transmembrane movements which includes movement of substances from
exterior to interior of the cell and vice versa. Changing the fluidity will alter how much and
quickly a substance will move across the membrane.
Cell membranes are selectively permeable.
The ability of cell membranes to control the traffic of substances into and out of the cell and its
organelles makes them selectively permeable. Small molecules and ions can cross the
membranes along a concentration gradient. (areas from high to low concentration) through the
process of diffusion. Small and non-polar molecules or hydrophobic molecules can pass through
the phospholipid bilayers relatively quickly, but charged, larger polar molecules have difficulty
in moving across the lipid bilayer.
Membranes transport can be made better!
Proteins embedded in the phospholipid bilayer can facilitate transport across a membrane. Cell
membranes are not exclusively made of up lipid macromolecules. They are actually a mosaic of
lipids, proteins and carbs. Cell membranes always contain proteins, and most also contain carbs
that can be attached to lipids or proteins. Bilayers can have protein channels that span the
entire width of the lipid bilayers. These channels allow efficient transport of substances across
the membrane.
Fluid mosaic model: membranes consist of proteins and carbohydrates embedded in a fluid
phospholipid bilayer. Membrane associated protein can be attached to the cell membrane on
the interior of exterior of the cell, or actually embedded in the bilayer. The proteins in the cell
membrane that enables transport of hydrophilic molecules across the cell membrane. A
scientific model is testable idea about the way that a system works.
Models make predictions about natural phenomena. The FMM predicts certain properties of
the cell membrane including ability of membrane components to move laterally.
Movement across the cell membrane:
Passive diffusion:
-from areas of high to low concentration (along gradient)
• A solute molecule comes in contact with and crosses the lipid portion of the plasma
membrane into the cell.
• No energy required
Passive transport:
-or facilitated diffusion: form areas of high to low concentration (along a gradient)
requires protein molecules that assist
• requires protein molecules that assists in the transmembrane movement of solutes
• no energy required
Active transport:
• -molecules move against the concentration gradient (energy from the hydrolysis of ATP)
can be classified as primary active or secondary active transport.
• primary active transport: direct expenditure of ATP energy
• secondary active transport: indirect expenditure of ATP energy
• both mechanisms transport substances against concentration gradient. Energy is
required
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