5.1 Structure of Cell Membranes Flashcards
What are cells defined by?
membranes
What do membranes do?
physically separate cells from their external environment and define spaces within many cells that allow them to carry out their diverse functions
What is the main component of cell membranes?
lipids, they have properties that allow them to form a barrier in an aqueous environment
What are other components of membranes?
- proteins or embedded in or associated with the membrane
- carbohydrates are found attached to lipids (glycolipids), and proteins (glycoproteins)
What do embedded proteins in membranes do?
perform important functions such as transporting molecules
What are the major types of lipids found in cell membranes?
phospholipids
What are most phospholipids made up of?
glycerol backbone attached to a phosphate group and two fatty acids
Why are phospholipids amphipathic?
- head is hydrophilic/polar
- tail is hydrophobic/nonpolar
What does amphipathic mean?
having both hydrophilic and hydrophobic regions
How do amphipathic molecules behave in an aqueous environment?
spontaneously arrange themselves into various structures in which the polar head groups on the outside interact with water and the nonpolar tail groups come together on the inside away from water
results from tendency of polar molecules to exclude nonpolar molecules/groups
What is the shape of the structure determined by?
bulkiness of head group relative to hydrophobic tails
What are micelles?
spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed and wedge-shaped
What is a bilayer?
two-layered structure of the cell membrane with hydrophilic heads pointing outwards toward the aqueous environment and hydrophobic tails oriented inward away from the water
What structures do bilayers form? What does it explain
closed structure with an inner space because free edges would expose the hydrophobic chains to the aqueous environment
- explains why bilayers are effective cell membranes
- explains why membranes are self-healing
How are membranes self-healing?
small tears in a membrane are rapidly sealed by the spontaneous rearrangement of the lipids surrounding the damaged region because of the tendency of water to exclude nonpolar molecules
What are the consequences of the ability of phospholipids to form a bilayer when placed in water?
bilayer structure forms spontaneously, dependent only on the properties of the phospholipid and without the action of an enzyme, as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell
Why is the pH important?
it ensures that the head groups are in their ionized form and suitably hydrophilic
What are lipsomes?
an enclosed bilayer structure that surrounds a central space, spontaneously formed by phospholipids in environments with neutral pH like water
What can liposomes do as they form?
may capture macromolecules present in solution
What can liposomes do?
- form, break, and re-form in environments such as tidal flats
- can grow, incorporating more and more lipids from the environment
Why do lipids freely associate with one another?
because of the extensive van de Waals forces between their fatty acid tails
Describe van der Waals forces between fatty acid tails.
- weak
- easily broken and re-formed, so lipids molecules are able to move within the plane of the membrane, sometimes very rapidly
Why is the membrane said to be fluid?
because membrane lipids are able to move in the plane of the membrane
What does the degree of membrane fluidity depend on?
which types of lipids make up the membrane
Describe van der Waals forces in a single layer of the lipid bilayer.
strength between the lipid tails depends on the length of the fatty acid tails and teh presence of double bonds between neighbouring carbon atoms
What do longer fatty acid tails mean?
more surface is available to participate in van der Waals forces
the tighter packing that results tends to reduce lipid mobility
Describe saturated fatty acid tails.
- no double bonds
- straight
- tightly packed, reducing mobility
Describe unsaturated fatty acid tails.
- double bonds
- have kinks, reducing the tightness of packing and enhancing lipid mobility in the membrane
What is cholesterol?
an amphipathic lipid that is a major component of animal cell membranes
Describe the amphipathic nature of cholesterol.
- hydroxyl (OH) group is hydrophilic
- hydrophobic region consists of four interconnected carbon rings with an attached hydrocarbon chain
What does the amphipathic structure of cholesterol allow?
allows a cholesterol molecule to insert itself into the lipid bilayer so that its head group interacts with hydrophilic head group of phospholipids, while the ring structure participates in van der Waals interactions with the fatty acid chains
How does cholesterol increase or decrease membrane fluidity depending on the temperature?
at typical temperature in a cell:
- decreases fluidity because interaction of rigid ring structure with the phospholipid fatty acid tails reduces mobility of the phospholipids
low temperature:
- increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids
What does cholesterol do?
helps maintain a consistent state of membrane fluidity by preventing dramatic transitions from a fluid to a solid state
What are transporter proteins?
membrane proteins that move ions or other molecules across the cell membrane
What are receptor proteins?
membrane proteins that allow the cell to receive signals from the environment
What are enzymes?
proteins that catalyze chemical reactions
What are anchors?
membrane proteins that attach to other proteins and help to maintain cell structure and shape
What are the two groups of membrane proteins?
integral membrane proteins and peripheral membrane proteins
What are integral membrane proteins?
proteins that are permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself
What are peripheral membrane proteins?
proteins that are temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions
are easily separated from the membrane by simple experimental procedures that leave the structure of the membrane intact
What are transmembrane proteins?
(most) integral membrane proteins that span the entire lipid bilayer
What are the 3 regions of transmembrane proteins?
- two hydrophilic: one protruding from each face of the membrane
- one connecting hydrophobic that spans the membrane
What does the structure of transmembrane proteins allow?
allows for separate functions and capabilities of each end of the protein
ie. hydrophilic region on the external side of a receptor can interact with signalling molecules, whereas the hydrophilic region of that receptor protein on the internal side of the membrane often interacts with other proteins in the cytoplasm of the cell to pass along the message
What are peripheral membrane proteins associated with?
either the internal or external side of the membrane
What do peripheral membrane proteins interact with?
either with the polar heads of lipids or with integral membrane proteins by weak noncovalent interactions such as H-bonds
How are peripheral membrane proteins associated with the membrane?
transiently associated, and can play a role in transmitting info received from external signals, while others limit the ability of transmembrane proteins to move within the membrane and assist proteins in clustering lipid rafts
What is the fluid mosaic model?
model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins
