B.2.1 SL Flashcards
How do lipid bilayers work as the basis of cell membranes?
The membrane is formed by amphipathic phospholipids which automatically form into bilayers in the water due to their hydrophobic tails which turn to each other to escape the water.
How do lipid bilayers work as barriers?
The membrane has low permeability to large molecules and hydrophilic molecules such as ions because of the hydrocarbon chains at the core of the membrane. This allows the membrane to serve as a barrier and create specific conditions inside of the cell.
Explain simple diffusion and give an example.
Simple diffusion is the net movement of particles from areas with higher concentration of molecules to lower concentration (moves with the gradient). Simple diffusion is not selective, molecules which can diffuse, will diffuse. An example of simple diffusion is gas exchange in the body where oxygen and carbon dioxide are diffused between the cell and the organism.
Explain integral and peripheral proteins in the membrane bilayer.
There are various proteins along and within the bilayer which have different forms and functions.
Membrane proteins are synthesized by bound ribosomes and brought to the cell membrane through exocytosis
- Integral proteins: imbedded and may span the lipid bilayer; they are able to establish hydrophobic interactions with the tails of phospholipids. Protein anchors to the membrane because of this. Can be a single helix, multiple or a rolled up sheet.
- Peripheral proteins: associated with the membrane surfaces and bind to one side of the bilayer or to an integral membrane protein. Embed into a monolayer through the face of the helix or attach to a lipid chain or carbohydrate. They ca also attach by interactions with other membrane proteins.
What is osmosis? describe the roll of aquaporins.
Osmosis is the free movement of water into and out of the cell. it is a type of passive diffusion which occurs through the membrane. Water particles move from an area of high water but low solute concentration to one of low water but high solute concentration to achieve equal levels of concentration. Aquaporin is a protein found in cells which increases the membrane permeability to water. Cells which function with rapid movements of water have high concentration of aquaporin:
- Kidney: reabsorbs water from urine
- Root hair cells: move water from soil to the roots
What are channel proteins and how do they work?
Channel proteins are integral proteins which transport only specific sizes, charges and shapes of particles. They are used for passive transport meaning they do not require the use of energy. They transport particles with the concentration gradient and are bidirectional (work both ways).
They work with gate channels which can open or close in response to a signal allowing specific ions only to diffuse through. They can be:
- Voltage: open and close In response to changes in the electrical potential of the cell membrane
- Ligand: open only to allow ions to pass through membrane only when a chemical messenger or ligand has bound to the channel
Some examples are:
- Sodium and potassium channels: channels in neuron cell membrane used during transmission of an action potential
- Calcium channels: in cell membranes used during synaptic transmission or to control muscle contractions
- Regulation of water concentration: occurs in the kidney collecting duct (many aquaporin channels)
What are pump proteins and how do they work?
Pump proteins are integral proteins which transport only specific sizes, charges and shapes of particles. they function through active transport meaning they need energy (ATP) to transport particles. They transport particles against the concentration gradient and are unidirectional (only work one way). Process of transport though pump proteins:
1. Particle enters the protein embedded into the membrane
2. ATP opens the protein leaving it in an unstable state
3. The particle is moved and released to the opposite side
4. The protein then closes putting it back in its stable state
Explain the selective permeability of the lipid bilayer.
The selective permeability of the bilayer only allows certain particles to cross it which lets it regulate the transport of materials entering and exiting the cell.
Variables that determine permeability:
- Particle size: the membrane is not permeable to large molecules because of the tightly packed phospholipids
- Hydrophobic and hydrophilic particles: the membrane is permeable to hydrophobic particles but only sometimes to small hydrophilic polar molecules. It is not permeable at all to hydrophilic particles with charge
particles that are unable to cross use channel or pump proteins depending on their shape.
What is the structure and function of glycoproteins and glycolipids in the membrane?
Glycoproteins and glycolipids are components of the membrane. they are formed by a short (3-10 sugar units) carbohydrate chain (oligosaccharide) which links to a protein or lipid. They are only found on the extracellular surface of the membrane and play a big part in cell adhesion and cell recognition. In cell to cell recognition glycoproteins on one surface are recognized by the receptors on another cell surface. This is very important in the immune system. They can form a layer on the outside of the plasma membrane called glycocalyx which creates a physical barrier to a cell and members of an adjacent can fuse by adhering
What are the components of a fluid mosaic model? (know how to draw it)
- Phospholipid bilayer: phospholipid chains facing each other
- Phospholipids: labelled with hydrophilic heads and hydrophobic tails
- Peripheral protein: globular structure at the surface of the membrane
- Integral protein: embedded globular structure spanning both sides of the membrane
- Glycoprotein: globular structure with protruding carbohydrate
- Glycolipid: phospholipid with a protruding carbohydrate
- Cholesterol: between adjacent phospholipids
