Module 2 Section 5 - Biological Membranes Flashcards
Pyper
Give three functions of cell surface membranes (or four if you split one up).
- partially permeable barriers between and within cells & organelles maintaining different conditions (this also allows compartmentalisation where substances are held for chemical reactions) and can control the exchange of substances passing through them
- sites of chemical reactions (e.g. respiration)
- sites of cell communication (cell signalling, messenger molecules bind to receptors which can lead to changes in a cell)
What are cell membranes made of?
- Lipids (mainly phospholipids)
- Proteins
- Carbohydrates (which are usually attached to the two above)
fluid mosaic model
A model that describes the fluid and flexible nature of the cell membrane and the components it is made from.
Describe the fluid mosaic model (full description question).
- Phospholipids form a bilayer
-> this is fluid as they are constantly on the move - Proteins are scattered throughout the bilayer (like a mosaic/plum pudding)
- Glycoproteins/glycolipids are bonded to these
- Some cholesterol molecules are also in the bilayer
Why does the fluid mosaic model describe membranes as ‘fluid’?
- The phospholipids & proteins can move around via diffusion
- The phospholipids mostly move sideways, within their own layers
only really need to talk about the phospholipids in the top one for a 2 marker
Why does the fluid mosaic model describe membranes as a ‘mosaic’?
The different types of proteins randomly scattered throughout the bilayer (or between the phospholipids) move/float within it (although some may be fixed in position)
OR
The plasma membrane is made of lots of different molecules
Is the phospholipid tail hydrophobic or hydrophilic?
Hydrophobic
Role of phospholipids in the fluid mosaic model of cell membranes
Centre of bilayer is hydrophobic, so it acts as a barrier to stop water soluble substances (e.g. ions/polar substances) from diffusing through
Why can water diffuse through the cell membrane?
It’s polar, however it’s small enough to diffuse through
What can pass through the phospholipid bilayer (only through the phospholipids)?
Fat/lipid soluble substances (but not water soluble ones like ions/polar molecules).
Role of cholesterol in the fluid mosaic model of cell membranes
They give the membrane stability, as at high temperatures, they bind to the phospholids’ hydrophobic tails, causing them to pack more closely together. This makes the membrane less fluid and more rigid.
Some parts of cholesterol are hydrophobic, so it can create a further barrier to water soluble substances to stop them from moving through the membrane - cholesterol affects the permeability of the membrane.
What does cholesterol do when it’s cold?
Cholesterol prevents the phospholipids from packing too closely together, making the membrane more fluid and less rigid.
Role of proteins in the fluid mosaic model of cell membranes
- Control the movement of substances into and out of the cell
- Some form protein channels (pores) which allow small charged molecules through
- Some are carrier proteins which help transport bigger molecules and charged particles across the membrane (by active transport and facilitated diffusion).
- Some act as receptors for molecules (e.g. hormones) triggering a chemical reaction for cell signalling
glycolipid vs. glycoprotein
glyco = sweet (sugar)
glycolipid - carbohydrate bonded to a lipid
glycoprotein - carbohydrate bonded to a protein
Role of glycolipids & glycoproteins in the fluid mosaic model of cell membranes
- Form hydrogen bonds with surrounding water molecules to stabilise the membrane
- Act as receptors for messenger molecules in cell signalling
- Are antigens (which are involved with self-recognition)
Two factors that affect membrane permability
- Solvent
- Temperature
Explain why the phospholipid bilayer can melt.
Above 45°C, the increasing kinetic energy allows the phospholipids to move far away from each other, destroying the membrane’s structural integrity (ability to hold together) causing it to “melt”.
Explain how the type of solvent affects membrane permeability.
- Membrane is more soluble in the solvent
- Lipids dissolve in the cell membrane -> membrane loses its structure
- Increases membrane permeability
Explain how low temperatures affects membrane permeability.
- Phospholipids have little energy -> don’t move around lots -> low permeability
- Channel & carrier proteins denature -> increases permeability
- Ice can penetrate the membrane, leaving big pores when it melts -> increases permeability
Explain how temperatures between 0 and 45°C affects membrane permeability.
0-45°C:
* Phospholipids aren’t too closely packed together and so can move around
* Increasing temp increases kinetic energy -> permeability increases
Explain how high temperatures affects membrane permeability.
- Above 45°C, phospholipid bilayer melts -> membrane becomes more permeable
- Water expands, putting pressure on the membrane
- Channel and carrier proteins denature -> don’t control the transfer of substances -> permeability increases
Graph of alcohol concentration on membrane permeability
https://cdn.discordapp.com/attachments/1293960808828506224/1295071505683582996/image.png?ex=670d50d4&is=670bff54&hm=66ac284f71797a5cfc32b8104882a56e876736e025b575bf9698fe08d5685211&
pg 127 of CGP textbook
Graph of temperature on membrane permeability
https://cdn.discordapp.com/attachments/1293960808828506224/1295071805337239654/image.png?ex=670d511b&is=670bff9b&hm=18e48c8e45d3aae7be247ce1f2705dbf84b708f2d247170e16a29310c6b8c602&
pg 127 of CGP textbook
diffusion
The passive, net movement of molecules from an area of high concentration to an area of low concentration, down the concentration gradient, until equilibrium is reached. No external energy is required.
Simple diffusion
Where particles simply diffuse through the membrane.
Where can diffusion occur?
In solutions or gases
osmosis
The net movement of water molecules from a high water potential to a lower water potential, across a partially permeable membrane down the water potential gradient.
water potential (general definition)
The likelihood of water molecules to diffuse into or out of a solution.
water potential (pressure definition)
Pressure exerted by free water molecules on a membrane.
What is water potential measured in?
kPa
What is the water potential of:
a) distilled water?
b) a solution of glucose?
a) 0
b) negative
The more –ve the water potential, the ____ the concentration of the solutes.
stronger
hypotonic solution
A more dilute solution than the cells - has less concentrated solutes
hypertonic solution
A more concentrated solution than the cells - has more concentrated solutes
isotonic solution
The same concentration of solutes as the cell
What happens when a cell is placed in a hypotonic solution?
The solution has a higher water potential than the cell, so the net water movement is into the cell through the partially permeable plasma membrane down the water potential gradient via osmosis.
Animals cells swell and eventually burst, while plant cells’ vacuoles & cytoplasms swell and push against the cell wall. The cell becomes turgid.