What is the difference between a micelle and a bilayer?
- phospholipid bilayers consists of double-tailed phospholipids that are cylindrical in shape while a micelle is composed of single-tailed phospholipids that are conical in shape
- the cylindrical phospholipids are able to “stack” while the conical ones can’t, their ends just join, and the heads face the water
- shape is dependent on both shape and chemistry
By what process are bilayers formed?
-bilayer is formed spontaneously when phospholipids are put in water (due to the hydrophilic heads moving towards the water and the hydrophobic tails moving away)
How do phospholipids move within the bilayer and why?
- they move freely laterally (rapid and frequently)
- this is because there are no covalent bonds holding the membrane together (held by hydrophobic and hydrophilic interactions)
- flip/flop movement is rare because the polar head is not stable in the hydrophobic core (H2O is excluded)
What consequence does the lateral movement of phospholipids have on the characteristics of the bilayer?
-it provides fluidity and flexibility to the bilayer
Explain the effect of high temp. on membrane fluidity
- phospholipids will have increased kinetic energy and will therefore move faster
- the heads will act like billiard balls and bounce off each other, resulting in a larger gap between the phospholipids
- as a result the membrane will become more fluid
Why is a higher temperature problematic for the membrane?
-increased fluidity and more gaps between the phospholipids means that there is a greater chance for some materials to get in or through the membrane, some of these materials can be harmful (greater permeability)
What are some adaptions to help the membrane keep its stability in high temperatures?
- longer hydrocarbon tail length (increased hydrophobicity), this results in the membrane being more stable
- decrease carbon-carbon double bonds (fewer kinks that further cause space between the phospholipids)
- increase cholesterol content (higher hydrophobicity and will act as a pylon-like obstacle that gets in the way of movement and forces the phospholipids to slow down)
Explain the effect of lower temperatures on membrane fluidity
- the phospholipids will have less kinetic energy and will bounce off each other less which reduces the gap between the phospholipids
- as a result, the membrane will become less fluid
Why is a lower temperature problematic for membranes?
- there are fewer gaps for material to get into or through the membrane, the membrane becomes less permeable
- organisms have to have a certain level of permeability in their membranes in order to avoid freezing to death and having some function in the membrane (eg: nutrients exchange)
What are some adaptions to help keep the membrane stable at low temperatures?
- Increase the number of carbon-carbon double bonds (kinks)
- Lower the tail length (lower hydrophobicity)
- Increase cholesterol (can act as a spacer between phospholipids and push them apart)
Draw integral and peripheral membrane proteins and the polar and non-polar amino acids interacting. Draw the whole membrane as well.
Check picture against notes
How can you identify an animal cell membrane?
- will have no cell wall
- no cholesterol in the membrane
What are the integral membrane proteins and their main function?
- are completely embedded in the bilayer of phospholipids and have both hydrophobic and hydrophilic regions
- their main function is to allow the big, polar molecules to pass across the membrane, which is restricted by the phospholipid bilayer
What are peripheral membrane proteins and their main function?
- do not cross the membrane, but can be attached to either side of the membrane or other proteins in the membrane
- involved in transportation to various locations in the cell, signaling, and maintaining cell shape and structure
What is the fluid mosaic model?
-states that the membrane is a fluid-structure (phospholipids) with a mosaic of different proteins embedded in or attached to a double bilayer of phospholipids
What are some different functions of proteins within the bilayer?
- transporter
- enzymes
- signal receptors
- recognition (blood type)
- cell-cell attachment
- attachment to extracellular matrix
Define permeable and state which molecules are permeable to the bilayer?
- a substance that can freely diffuse across the phospholipid bilayer
- have to be small and uncharged (to get through the hydrophobic core)
eg: CO2, O2, H2O
Define impermeable and state which molecules are impermeable to the bilayer?
-a substance that cannot cross the hydrophobic core because they are either too big or too polar (ions are the least permeable eg: H+)
Why are ions the least permeable substance to the bilayer?
-there is a stable entourage of H2O that accompanies the ion, it is more favorable to interact with the water than to be stripped of it to pass the phospholipid bilayer (hydrophobic core)
Why can water diffuse through the membrane by osmosis even though it is polar?
- undergoes a dipole moment-electrons that are usually found unequally across the bond will be found in the middle of the bond for a quick second and will become momentarily nonpolar, and will be able to pass through the membrane
- but water diffuses by osmosis very slowly
What is passive transport?
- happens spontaneously and is for permeable substances
- is the movement of molecules from areas of high concentration to a low concentration until an equilibrium is reached
eg: osmosis is the passive diffusion of water
What happens in a hypotonic environment?
- there is more solute inside the cell than outside the cell, so water rushed in as a result
- there is a risk that in an animal cell, the cell can burst (osmotic lysis)
- in plant cells, the cell wall prevents the ell, so there is a low chance the cell will burst
What happens in an isotonic environment?
-the concentration of solute inside the cell and outside the cell is the same, so there is no net movement of water
What happens in a hypertonic environment?
- more solute outside the cell than inside the cell, so the water rushed out
- in an animal cell, this can lead to the cell shrinking
- in a plant cell, this can result in plasmolysis (pulling away of membrane from the cell wall-connections to cell wall are being broken)
