Biological Membranes Flashcards
what are the functions of plasma membranes?
- to control the movement of substances in and out of the cell
- selective permeability: keeps all of the components in the cell isolating organelles from the rest of the cytoplasm, allowing cellular processes processes to occur separately - a site for biochemical reactions
what is the structure of plasma membranes?
double membrane
phospholipid bilayer with embedded proteins
it has a hydrophilic phosphate head, a glycerol molecule and two hydrohpobic fatty acid tails.
how do phospholipids act in water?
they can form 2 structures: a micelle or a bilayer.
the hydrophilic phosphate heads face and interact with the water and the hydrophobic tails cluster together and point inwards, away from the water
micelle- forms a circle
bilayer- forms two layers
what are intrinsic proteins?
embedded within both layers of the membrane. they have amino acids with hydrophobic R-groups on their surface and they interact with the hydrophobic core of the membrane, keeping them in place
what are the two types of intrinsic proteins?
channel and carrier (both involved in transport across the membrane)
what are channel proteins?
they provide a hydrophilic channel that allows diffusion of polar molecules and ions down a concentration gradient through membranes
what are carrier proteins?
have a role in passive transport (diffusion) and active transport into cells. they bind to a molecule, undergo a change in shape and release the molecule on the other side (ions and large molecules).
what are glycoproteins?
intrinsic proteins that are embedded in the plasma membrane with attached carbohydrates (sugar)
what is the role of glycoproteins?
- adhesion (when cells join together to form tight junctions in certain tissues
- receptors for chemical signals (when the chemical binds to the receptor, a response is triggered in the cell which can cause a direct response or set off a cascade of events in the cell- this is called cell signalling)
what are glycolipids?
lipids with attached carbohydrate chains (sugar). these molecules are called antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
what are extrinsic proteins?
only present in one side of the bilayer. they have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins. they can be present in either layer and some move between layers
What is cholesterol in the context of cell membranes?
Cholesterol is a lipid molecule found within the phospholipid bilayer of animal cell membranes.
Where in the cell membrane is cholesterol located?
Cholesterol is positioned between phospholipid molecules in the bilayer, with its hydrophilic hydroxyl group near the phospholipid heads and its hydrophobic tail within the fatty acid tails, pulling them together. Stops the phospholipid molecules from grouping too closely and crystalising.
What are the main roles of cholesterol in the membrane?
Stability: Prevents the membrane from becoming too fluid at high temperatures.
Flexibility: Stops the membrane from becoming too rigid at low temperatures.
Barrier: Reduces permeability to small, water-soluble molecules and ions.
How does cholesterol affect membrane fluidity?
It interacts with phospholipid tails to make the membrane less fluid at high temperatures and prevents tight packing of phospholipids, maintaining fluidity at low temperatures.
Why is cholesterol important for membrane stability?
Its rigid ring structure adds strength to the membrane, especially in cells exposed to mechanical stress (e.g., red blood cells).
How does cholesterol contribute to selective permeability?
Cholesterol decreases the permeability of the membrane to small, polar molecules and ions, enhancing the barrier function.
what can diffuse across membranes?
-small, non-polar molecules (O2 and CO2) diffuse rapidly
-small, polar molecules (H2O and Urea) diffuse across much more slowly
- charged particles (ions) don’t diffuse and would require channel/carrier proteins
what are integral proteins?
transport molecules across the membrane or catalyse reactions
What are the two main factors affecting membrane structure?
Temperature and solvents.
What is the structure of the phospholipid bilayer?
It consists of phospholipids with hydrophilic phosphate heads, hydrophobic fatty acid tails, and embedded protein carriers or channels for facilitated diffusion and active transport.
What stabilises the structure of the phospholipid bilayer?
A:
Weak intermolecular forces, stabilise the bilayer by holding phospholipids together.
How does increasing temperature affect the phospholipid bilayer?
Higher temperatures increase the kinetic energy of phospholipids, causing them to move more and potentially break apart the intermolecular forces.
What happens to protein channels or carriers at high temperatures?
They may denature, as the bonds in their tertiary and quaternary structures are disrupted, which can affect membrane permeability.
How does increased temperature affect membrane permeability?
It generally increases permeability due to larger gaps between phospholipids, allowing more simple diffusion.
What is the effect of non-polar solvents on the membrane?
Non-polar solvents can interact with fatty acid tails, disrupting the weak intermolecular forces and increasing permeability and fluidity.
What is a common example of a non-polar solvent that affects membranes?
Ethanol (alcohol).
How does ethanol disrupt neural membranes?
It affects the permeability of the membrane, altering ion concentration differences, which can impair nervous transmission and lead to effects like intoxication.
How are alcohol wipes effective as disinfectants?
High concentrations of alcohol disrupt bacterial membranes, destroying the bilayer and killing bacteria.
What is the role of the hydrophobic core in the phospholipid bilayer?
It prevents polar molecules from passing freely, maintaining membrane selectivity.
