Chapter 7: Membrane Structure and Function
7.1: Cellular membranes are fluid mosaics of lipids and proteins
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Selective permeability – some substances move across the membrane more easily
Membrane made primarily of lipids and proteins, small amount of carbohydrate
Phospholipid – amphipathic molecule (has a hydrophobic and hydrophilic region
Fluid mosaic model – membrane is a fluid structure with a “mosaic” of a variety of proteins
embedded in it.
Membrane models: scientific inquiry
- 1972 – Model proposed that membrane proteins reside in the phospholipid bilayer with
hydrophilic regions protruding
- See figure 7.5
The fluidity of membranes
- Membrane is held together primarily by hydrophobic interactions (much weaker than covalent
bonds)
- Most lipids can shift laterally in the plane of the membrane
- Membranes with more phospholipids with unsaturated hydrocarbon tails remain fluid at lower
temperatures
- Cholesterol in the phospholipid molecules has different effects on the membrane at different
temperatures
o High temperatures – makes the membrane less fluid
- Membranes must be fluid to work properly
- If a membrane solidifies, its permeability changes as well
- Membranes that are too fluid cannot support protein function
Evolution of differences in membrane lipid composition
- Variations appear to be evolutionary adaptations that maintain the appropriate membrane
fluidity depending on the organism’s environmental conditions
- Animals in cold temperatures have bilayers that are rich in unsaturated hydrocarbon tails
Membrane proteins and their functions
- See figure 7.10
- Integral proteins
o Penetrate the hydrophobic interior
o Majority are transmembrane proteins
- Peripheral proteins
o Not embedded in the lipid bilayer at all, loosely bound to the surface of the membrane
- Functions
o Transport
o Enzymatic activity
o Signal transduction
o Cell-cell recognition
o Intercellular joining
o Attachment to the cytoskeleton and ECM
The role of membrane carbohydrates in cell-cell communication
- Important for cells to recognize it’s neighbouring cells from each other
o Fetal development – sorting tissues into organs
- Basis for the rejection of foreign cells by the immune system
- Glycolipids – carbohydrates covalently bonded to membrane lipids
- Glycoproteins – carbohydrate bonded to a protein
- Carbohydrates provide a binding site for cell recognition
Synthesis and sidedness of membranes
- Membranes have distinct inside and outside faces
- Two layers differ in specific lipid composition and protein directional orientation
7.2: Membrane structure results in selective permeability
- The ability to regulate the transport across cellular boundaries is essential to the cell’s existence
- Cells take in sugars, amino acids and other molecules, and waste products need to exit
The permeability of the bilayer
- Nonpolar molecules are hydrophobic (eg. Hydrocarbons, carbon dioxide, oxygen) and therefore
can dissolve in the lipid bilayer and cross easily without the aid of membrane proteins
- Hydrophobic interior impedes the direct passage of ions and polar molecules, and therefore
require membrane proteins to help in transporting
Transport proteins
- Aid in the transport of hydrophilic substances through the membrane (ions, polar molecules)
- Substances avoid contact with the lipid bilayer
- Channel proteins – have a hydrophilic channel that certain molecules or atomic ions use
- Aquaporins – channel proteins water uses to move across the membrane
- Carrier proteins – holds on to the molecules and change shape to shuttle them across the
membrane
7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
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Diffusion – The movement of any substance so they spread out evenly into the available space
o In the absence of other forces, a substance will diffuse from an area of high
concentration to an area of low concentration
o It diffuses against a concentration gradient
o No work is done, and energy is not required
- Passive transport – no energy required
Effects of osmosis on water balance
- See figure 7.14
- Diffusion of water – water diffuses across the membrane from low concentration to high
concentration
Water balance of cells without walls
- Tonicity – the ability of a surrounding solution to cause a cell to gain or lose water
- Isotonic – no net movement of water molecules
- Hypertonic – The cell has a higher solute concentration meaning the cell will lose water, shrivel
and probably die
- Hypotonic – The cell has a lower solute concentration than outside the cell, the cell will gain
water, and expand and lyse (burst)
- Osmoregulation – the control of solute concentrations and water balance
Water balance in cells with walls
- Walls are relatively inelastic and don’t expand and contract as much as animal cells
- Turgid – the cell is firm, occurs in a hypotonic solution (celery in water)
- Flaccid – The cell is limp, occurs in an isotonic solution
- Plasmolyzed – the cell wilts and can lead to death, happens in a hypertonic solution
Facilitated diffusion: Passive transport aided by proteins
- Transport of water and some hydrophilic solutes across the membrane
- Transport protein that spans the bilayer, most channels are specific and only transport some
substances
- Channel proteins – act as a corridor or tunnel to allow specific molecules to pass
- Ion channels – channel proteins that transport ions
- Gated channels – open or close in response to a stimulus
- Carrier proteins (glucose transporter) – undergo subtle change in shape that translocates the
solute-binding site across the membrane
- All move down a concentration gradient (from [low] to [high])
7.4: Active transport uses energy to move solutes against their gradients
The need for energy in active transport
- Active transport - Solutes are pumped against a [ ] gradient (from [high] to [low]), therefore
require energy
- All proteins involved are carrier proteins
- See figure 7.18
- Example Sodium-Potassium pump – Sodium is pumped out of the cell into the extracellular
matrix, and potassium is pumped into the cell
- ATP is the energy source for the pumps
- See figure 7.19
How ion pumps maintain membrane potential
- Cells have voltages across the plasma membrane (electrical potential energy)
o Cytoplasmic side is negative in relation to the extracellular side
o Called membrane potential
- Acts like a battery
- Because inside is negative, favours the movement of cations (positive ions) into the cell and
anions (negative ions) out of the cell
- Electrochemical gradient – combination of forces acting on the ion
- Ions diffuse against the electrochemical gradient
- Electrogenic pump – a transport protein that generates voltage across a membrane
- Proton pump – actively transports hydrogen ions out of the cell
Cotransport: Coupled transports by a membrane protein
- See figure 7.21
- “downhill” transport (passive) is coupled with the “uphill” transport (active)
7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Exocytosis
- Cell secretion by fusion of vesicles with the plasma membrane
- Used in exporting products, releasing transmissions through nerve cells
Endocytosis
- See figure 7.22
- Cell takes in biological molecules by forming new vesicles from the plasma membrane
- Used to take in cholesterol
- Ligand – term for any molecule that binds specifically to a receptor site on another molecule
- Provide a mechanism for rejuvenating the plasma membrane