Lect. 7 Membrane Structure - Function Flashcards
Membranes provide:
the structural basis for metabolic order
without membranes: no homeostasis, no metabolism
each contains specific proteins and precise enzymatic solutions and can carry out particular functions
Membranes form:
most of the eukaryotic cell’s organelles and therefore partition the cell into compartments
ex of a cellular membrane: plasma membrane
The plasma membrane forms:
a boundary between the cell and its surroundings.
The plasma membrane controls:
the traffic of molecules in and out of the cell and is selectively permeable.
The plasma membrane takes up:
substances needed by the cell and helps with the disposal of waste.
ex: Ions such as Na+ & K+ must travel across using a sodium/potassium pump (maintains resting potential of nerve cell)
All membranes of the cell, including the plasma membrane, are:
selectively permeable.
It allows some substances to cross more easily than others and block the passage of some substances completely.
The type of membrane transport used depends on:
the type of molecule looking to cross.
simple diffusion
membrane transport: carrier protein, channel protein
Describe the Fluid Mosaic Model
- cell membrane is composed of a phospholipid bilayer
- both phospholipids and proteins move laterally but rarely vertically (if at all)
- embedded and surface proteins interspersed throughout the bilayer
Membrane Fluidity
What happened when a human cell was fused with a mouse cell in an experiment?
The respective membrane proteins were eventually randomly distributed in the fused cell.
membrane proteins are not fixed
Membrane Fluidity
The fluid state of the membrane depends on:
the lipid components
Same principle that explains why butter is solid at room temperature whereas vegetable oil is liquid (saturated vs unsaturated)
Membrane Fluidity
In a living cell, at 37°, the bilayer remains as ____ as ____ at room temp.
In a living cell, at 37°, the bilayer remains as fluid as salad oil at room temp.
Membrane Fluidity
At lower temperatures, the motion of the fatty acids chains:
slows.
Membrane Fluidity
At a critical point, the membrane becomes a ____ and transport across the membrane ____.
At a critical point, the membrane becomes a solid gel and transport across the membrane ceases.
Membrane Fluidity
Many organisms have a way of maintaining optimal membrane fluidity: to compensate for temperature changes, they:
alter the FA content of their membrane lipids.
Membrane Fluidity - Altering Fatty Acid Saturation
ex: When the temperature goes down, the proportion of unsaturated FA ____ and the membrane stays fluid.
When the temperature goes down, the proportion of unsaturated FA goes up and the membrane stays fluid.
Membrane Fluidity - Altering Fatty Acid Saturation
A more fluid membrane has fatty acids with:
unsaturated hydrocarbon tails with kinks.
kink: =bond
Membrane Fluidity - Altering Fatty Acid Saturation
A more viscous membrane has fatty acids with:
saturated hydrocarbon tails.
Membrane Fluidity - FA Chain Length
The longer the FA chains, the ____ fluid the membrane is likely to be.
FA Chain Length can vary
The longer the FA chains, the less fluid the membrane is likely to be.
The number of potential vdW association b/w chains increases.
Membrane Fluidity - Cholesterol
Cholesterol acts as a:
Fluidity Buffer
Membrane Fluidity - Cholesterol
At low temps, cholesterol molecules act as:
spacers between the hydrocarbon chains of the FA which restricts vdW interactions that promote solidifying.
Membrane Fluidity - Cholesterol
At high temperatures, cholesterol:
prevents the membrane from becoming unstable.
The mosaic aspect of the membrane is due to:
the protein molecules that are associated with it.
2 major classes of membrane proteins:
- integral membrane proteins
- peripheral proteins
the difference b/w the 2 is defined by how they associate with the phospholipid bilayer
Describe integral proteins
- they are firmly bound to the membrane, penetrating the core of the lipid bilayer
- they are amphipathic
Amphipathic: the hydrophilic region extends out of the cell and/or into the cytoplasm; the hydrophobic regions interact with the FA tails.
Some integral proteins do not extend all the way through the membrane, while others are ____ integral proteins.
Some integral proteins do not extend all the way through the membrane, while others are transmembrane integral proteins.
Some transmembrane proteins span the membrane just once; others do so many times (up to ____ times.)
up to 24 times
The most common transmembrane structure:
α-helix
Describe peripheral proteins
- they are not embedded in the lipid bilayer
- they are located on the inner or outer surface of the membrane
How are the proteins of the membrane oriented?
They are asymmetrically oriented. When the 2 sides of the membrane are compared, more proteins are firmly attached to one side than the other.
The asymmetry of the bilayer is produced by:
the highly specific way each protein is inserted into the bilayer. Each side of the membrane has a specific characteristic.
Membrane proteins that will become part of the inner surface of the plasma membrane are made by:
free ribosomes in the cytosol.
Membrane proteins that will be associated with the outer surface of the cell are manufactured by:
bound ribosomes on the rough ER.
Outer surface membrane proteins are for the most part:
glycoproteins.
Describe how outer surface membrane glycoproteins are made
- sugars are added to certain parts of the proteins in the lumen of the ER
- transport vesicle brings glycoprotein to outer surface
- carb portion of the protein extends from the surface of the cell
Membrane Protein Functions
- attach the membrane to the cytoskeleton and to the external fibres of the ECM
- provide identification tags to cells
- form junctions b/w cells
- play a role in transport across membrane
- can act as enzymes w/in the membrane
- are involved in signal transduction
Particular proteins that aid in keeping the membrane in place are ____ linked to the cytoskeleton and to fibres of the adjacent ECM.
Particular proteins that aid in keeping the membrane in place are non-covalently linked to the cytoskeleton and to fibres of the adjacent ECM.
The ECM consists of a gel-like substance made of:
carbohydrates and fibrous proteins.
The most prominent carb in the ECM is:
collagen, which forms tough fibres.
Certain glycoproteins in the ECM, called fibronections, help:
- organize the matrix
- aid cells in attaching to the matrix by binding to proteins that extend from the plasma membrane (integrins)
Integrins are internal transmembrane proteins that:
- serve as membrane receptors for the ECM: they activate cell-signaling pathways that communicate info to the cell from ECM
- play a role in cell movement and in organizing the cytoskeleton, so that cells assume a definite shape
- anchor ECM to the microfilaments of the internal cytoskeleton
ex: Cancer cells can metastasize because:
they have lost the requirement to be anchored to the ECM
Providing identification tags to cells
The outer portion of the plasma membrane contains:
glycoproteins and glycolipids.
These molecules vary from species to species, from individual to individual w/in a species, and from cell to cell w/in an individual
Providing identification tags to cells
Membrane proteins:
- allows the immune system to recognize and reject foreign cells
- enables cells to sort themselves into tissues and organs during embryogenesis
Autoimmune diseases occur when:
antibodies produced by the immune system fail to distinguish what is “self” from what is foreign, attacking the body’s own tissues.
One of the most common autoimmune diseases is:
rheumatoid arthritis, which attacks the synovial membrane of joints
synovial fluid (secretes by synovial membrane) lubricates joints and nourishes articular cartilage
Cells in close contact with each other develop:
intercellular junctions
Animal cells are connected to each other by several types of junctions:
- anchoring junctions (desmosomes)
- tight junctions
- gaps or communication junctions
Plant cells are connected by:
plasmodesmata
Each desmosome consists of:
a pair of button-like discs associated with the cytosolic side of the plasma membrane of adjacent cells and the intercellular protein filaments that connect them.
The proteins that form tight junctions hold the cells together in actual physical contact which forms:
a sheet of tissue
Tight junctions b/w cells are so tight that substances cannot leak b/w them.
Ex: The sheet of tissue that lines the digestive tract and capillaries in the brain are formed by:
tight junctions between cells.
- The contents of the digestive tract are prevented from leaking into surrounding tissue.
- The brain-blood barrier prevents many substances in the blood from passing into the brain
Gap/Communicating junctions bridge space between cells like desmosomes, but the space they span is ____.
Gap/Communicating junctions bridge space between cells like desmosomes, but the space they span is narrower.
Gap/Communicating junctions contain channels connecting:
the cytoplasm of adjacent cells, thus allowing for communication.
Membrane proteins from adjacent cells line up to form a channel.
Gap junctions allow for rapid ____ and ____ communication between cells.
Gap junctions allow for rapid chemical and electrical communication between cells.
ex: cells in the pancreas are linked by gap junctions
If one group of cells receives a signal to secrete insulin, the signal is passed on to other pancreatic cells, ensuring a coordinated response
ex: Heart muscle cells are linked by ____ junctions that permit the flow of ions to synchronize contractions.
Heart muscle cells are linked by gap junctions that permit the flow of ions to synchronize contractions.
Plant cells do not need ____ since they have a cell wall for strength.
Plant cells do not need desmosomes since they have a cell wall for strength. But they do require connections through cell walls that are functionally equivalent to gap junctions.
Plants can dilate the channels. H2O, small molecules and ions can pass through these channels so that plant cells share H2O, nutrients, and chemical messages.
