Lecture 6- Biological membranes Flashcards
What is the general design of a biological membrane called?
The fluid mosaic model
What are some functions of proteins embedded in the phospholipid bilayer?
Moving materials
Receiving chemical signals
What are carbohydrates associated with a membrane attached to?
Either lipids or protein molecules
What makes up the bulk of a membrane?
Lipids
What are the features of the hydrophilic region?
Phosphorus containing head, electrically charged (associates with polar water)
What are the features of the hydrophobic region?
Long, non-polar fatty acid tails
Associate with non-polar materials
What happens when a small hole forms on the phospholipid bilayer?
It seals spontaneously
What feature of phospholipid bilayers helps membranes fuse during vesicle formation or phagocytosis?
The capacity for lipids to associate with one another and maintain bilayer organisation
Given biological membranes have similar structure, what produces different types of bilayers?
They have different compositions
How can phospholipid composition change?
Fatty acid chain length
Degree of unsaturation
Polar (phosphate containing) groups present
Other than changing the phospholipids, how can the composition of a membrane be changed?
Up to 25% can be cholestrol
Cholestrol is important in membrane integrity
Cholestrol is common next to unsaturated fat
What 2 aqueous regions does the membrane separate?
The cytosol and the interstital fluid
What does the phospholipid membrane do to the membrane structure?
Stabilizes it
But keeps it flexible- it is fluid
What does fluidity of the membrane do?
Permits some molecules to move laterally (side to side)
What type of movement of a phospholipid molecule happens rarely?
It flips over to the other side, called flip-flop
Why is flip-flopping rare?
The polar part of each molecule would have to move through the hydrophobic interior.
What two factors affect membrane fluidity?
Lipid composition
Temperature
What type of membrane is more fluid?
Shorter chains
Unsaturated fatty acids
Less cholestrol
Why is membrane fluidity important for a membranes function?
Molecules move more slowly
What happens to membranes of organisms who cannot keep their bodies warm?
Molecules move more slowly, fluidity decreases- may adjust lipid composition
Where is changing lipid composition important?
Survival of plants and hibernating animals and bacteria over winter
How are membrane proteins distributed?
Asymmetrically
How many proteins molecules are there in a typical membrane?
1 per 25 phospholipid molecules
What regions do proteins embedded in phospholipid bilayers have?
Hydrophillic and hydrophobic regions
What are the hydrophilic regions of a membrane protein?
Stretches of amino acids with hydrophilic side chains
Domains interact with aqueous environment
What are the hydrophobic regions of a membrane protein?
Stretches of amino acids with hydrophobic side chains
Domains interact with interior of phospholipid bilayer
How are proteins embedded in the phospholipid bilayer seen?
Freeze-fracturing (electron microscopy)
How do proteins and lipids in the interior of membranes interact?
Non-covalentley
What are the two types of membrane protein?
Integral membrane protein
Peripheral membrane protein
What are integral proteins?
Proteins with a hydrophobic domain in the bilayer- long alpha helix regions that span the core
Hydrophilic heads
What are peripheral proteins?
- Do not have hydrophobic domains
- Not embedded in bilayer
- Polar/charged regions interact with regions on exposed parts of integral proteins or polar head of phospholipid molecules
How are some membrane proteins attached to the membrane?
Covalently to fatty acids or other lipid groups
What are proteins covalently bonded to fatty acids/lipid groups called?
A special type of integral protein
How are proteins distributed on inner and outer membrane surfaces?
Asymmetrically
What are integral proteins that protrude on both sides of the membrane called?
Transmembrane proteins
What do transmembrane proteins have?
Specific domains on each side of the membrane
Where are peripheral membranes localised?
One side of the membrane or the other
How does asymmetrical distribution affect the membrane?
Gives the two surfaces different properties
How do membrane proteins move around?
Relatively freely across the bilayer
What type of experiment demonstrates membrane protein movement?
Cell fusion
What happens in cell fusion experiments?
Two cells are fused
A single, continuous membrane forms
Proteins distribute uniformly around membrane
How are some proteins anchored/restricted?
Cytoskeleton- components below membrane attach to protein
Lipid rafts-semisolid lipids trap proteins within a region
What does it mean that membranes are dynamic?
They are constantly forming, transforming, fusing and breaking down
How are membranes constantly changing?
- Phospholipids are synthesised in SER
- Membrane proteins form on ribosomes
- Functioning membranes (ER buds away as vesicles join cis region, trans region buds join plasma membrane)
- Removal by phagocytosis
What serves as recognition sites?
Membrane carbohydrates on the outer surface
What may membrane carbohydrates be covalently bonded to?
lipids to form glycolipids
Proteins to form glycoproteins
What are glycolipids?
Carbohydrate covalently bonded to lipid
Carbohydrate extends outside plasma membrane to act as a recognition signal between cells
When does the carbohydrate of a glycolipid change?
When a cell becomes cancerous- allows white blood cells to target cancer for destruction
What is a glycoprotein?
Carbohydrate covalently bonded to a protein
What type of carbohydrate is bound to a glycoprotein?
Oligosaccharides (not usually exceeding 15 monosaccharides)
What do glycoproteins do?
Enable cells to be recognised by other cells and proteins
What two processes allow cells to arrange themselves in groups?
Cell recognition
Cell adhesion
How does cell recognition contribute to cells arranging themselves into groups?
One cell specifically binds to another cell of a certain type
How does cell adhesion contribute to cells arranging themselves into groups?
The connection between the two cells is strenghtened
The study of what organisms has revealed how cells associate?
Sponges- multi-cellular marine animal
How are the cells of a sponge separated?
Mechanically by passing through a fine mesh screen
In most cases, what type of binding of cells occurs?
Homotypic binding- the same molecules sticks out of both cells and exposed surfaces bind to each other
What is the other type of cell binding that can occur?
Heterotypic binding in which cells with different proteins bind
Give an example of heterotypic binding in mammals
Mammalian sperm and egg have different proteins which are complementary
Give an example of heterotypic binding in non-mammals.
Algae form similar appearing male and female reproductive cells with flagella
Heterotypic proteins on their flagella enable them to recognize each other
Where can cell junctions be seen easily?
Electron micrographs of epithelial tissues
What are the three types of cell junction?
Tight junctions, desmosomes and gap junctions
What do tight junctions do?
Seal tissues
What are tight junctions?
Specialized structures that link adjacent epithelial cells
What are tight junctions the result of?
Mutual binding of specific proteins in the plasma membranes of epithelial cells, forming a series of joints encircling each cell
What are the two functions of tight junctions?
- Prevent substances moving through spaces between cells
- Define functional regions of the membrane by restricting migration of membrane proteins/phospholipids
Why is it important that tight junctions prevent substances moving through spaces between cell?
Substances entering the body from the lumen of the intestine must pass through the epithelial cells that form the tight junction
Why is it important that tight junctions define functional regions of the membrane by restricting migration of membrane proteins/phospholipids?
Membrane proteins/phospholipids in the apical region (tip) facing lumen are different from those on the basolateral regions (facing body cavity)
How do the two functions of tight junctions effect movement?
They ensure the directional movement of materials into the body
What do desmosomes do?
Hold cells together by connecting adjacent membranes like spot welds or rivets
What does each desmosome have?
A dense structure called a plaque on the cytoplasmic side
What is attached to the plaque?
Cell adhesion molecules (CAMs) that stretch from the plaque through the plasma membrane of one cell, across intercellular space and through the plasma membrane of the adjacent cell
What else is the plaque attached to?
Fibers in the cytoplasm- the intermediate filaments of the cytoskeleton- made of keratin
What do the fibers attached to the plaque do?
Stretch across the cell to connect with another plaque on the other side to anchor on both sides
How do these fibers that attach plaques in desmosomes effect the properties of the tissue?
Provide great mechanical strength and stability to epithelial tissue
What are gap junctions?
Junctions which facilitate communication between cells
What are gap junctions made up of?
Specialised channel proteins called connexons
What property do plasma membranes of some cell types such as neurons, muscles cells and some eggs have?
They respond to electric charges carried on ions
They are electrically excitable
What are three other functions of some membranes?
- Help transform energy
- Organize chemical reactions
- Process information
How do some specialized membranes help to transform energy?
Inner mitochondrial membrane helps convert energy from fuel molecules into ATP
Thylakoid membranes help convert light energy into chemical bond energy
How do some specialized membranes help to organize chemical reactions?
- Cellular processes depend on a series of enzyme catalyzed reactions
- Membranes bring these molecules together in a sequential order
- More rapid and efficient chemical reactions
How do some membranes help to process information?
Binding of ligands to membrane proteins or carbohydrates initiates, modifies or stops a cell function
Give an example of a membrane helping to process information.
Insulin, a hormone, acts on specific target cells to elicit a response in that cell
How many subunits does the cholera toxin protein have?
2 subunits
One binds to cell surface glycolipid
What does the cholera toxin protein do once one subunit has bound to a glycolipid?
A change in the 3D shape of the toxin protein means the second subunit enters the cell
How does the second subunit of the cholera toxin protein effect the cell once inside?
It acts on an enzyme to modify a peripheral protein adenylate cyclase on the inner surface of the cells plasma membrane
What does modified adenylate cyclase do inside the cell?
It opens chloride channels in the membrane which results in Na+ and Cl- accumulating in the intestine and hence osmotic water loss
