Biological membranes Flashcards
What are the 3 main roles of membranes?
To act as partially permeable barriers
- between cell and environment
- between organelles and cytoplasm
- within organelles (mitochondria, chloroplasts)
To be sites of chemical reactions
To be sites of cell communication (cell signalling)
What does ‘partially permeable’ mean?
The ability to differentiate between types of molecules only allowing some through the membrane while blocking others.
How can substances pass through the membrane?
- dissolve in the lipid layer and diffuse through
- pass through protein channels or aquaphorins if water.
- small NON-POLAR molecules like oxygen, carbon dioxide nitrogen.
What is cell communication?
Occurs when chemicals bind to receptor molecules on the plasma membrane and cause a response within the cell. It helps to say in contact with the environment, stay in contact with adjacent cells and in multicellular cells the cells are spaced apart so they need to communicate to fulfil optimal bodily functions.
What is simple diffusion?
The movement of particles from a region of high concentration of that particle to a region of low concentration of that particle.
- KINETIC ENERGY
What is facilitated diffusion?
The movement of molecules from an area of high concentration of that molecule to an area of low concentration of that molecule across a partially permeable membrane via protein channels or carrier molecules.
- DOES NOT INVOLVE ATP
What molecules require protein channels to diffuse into the membrane?
Polar molecules or charged ions can’t interact with hydrophobic tails so they enter via water filled protein channels.
What happens when phospholipids vibrate?
Small pores appear allowing dissolved CO2 and O2 to get through as well as water.
Name 5 factors that affect the rate of simple diffusion.
- Temperature
- Concentration difference
- Surface area
4.Thickness of membrane - Size of molecule
Explain the effect of increasing temperature. (including protein molecules in the PLB)
- AS temperature increases the phospholipid molecules vibrate more due to increasing Ek.
- This increases membrane fluidity
- Makes it more permeable, creates temporary pores due to vibrations allowing large molecules to cross.
- Protein molecules vibrate more
- Proteins dentaure above 40c
- Once proteins are denatured the membrane is permanently destroyed with large holes.
Explain the effect of decreasing temperature.
- Saturated fatty acid tails become very compressed
- Membranes may contain unsaturated fatty acids, as they compress the kinks in their tails push adjacent phospholipids away.
- This maintains fluidity
- The presence of cholesterol lowers the temperature at which a membrane will lose its fluidity by disrupting the packing of tails.
What is endocytosis?
Phagocytosis (substance is solid), a segment of the membrane surrounds and encloses the particle and brings it into the cell enclosed in a vesicle, lyososmes fuse with the phagocytotic vesicle digesting the contents.
Pinocytosis (liquid)
What is exocytosis?
How large molecules are exported OUT of the cell.
- a vesicle containing them is moved towards the membrane and fuses to it.
- removes useful substances to the outside of the cell (E.g Insulin being moved from pancreatic cells to the blood stream).
Define osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
What is water potential?
The measure of the ability of water to move freely in a solution/ the relative tendency of water to move from one area to another.
Measured in kPa
Describe the terms: Isotonic, Hypotonic, Hypertonic.
Isotonic= Same, no net movement, equilibrium
HypOtonic= water potential is higher outside of cell, water moves in
HypERtonic= water potential is lower outside of cell, water moves out of cell
What happens to plant cells when placed in different solutions?
Isotonic= Nothing
Hypotonic= water moves in by osmosis down a WP gradient, plant cell wall prevents the cell from bursting, membrane pushes against wall. The cell is TURGID
Hypertonic= Water moves out of the cell by osmosis down a WP gradient, plant cell walls pull away from the membrane. PLASMOLYSED
What happens to animal cells when placed in different solutions?
Hypertonic= water osmoses out of cell across partially permeable membrane, shrivel and become CRENATED.
Hypotonic= water moves into cell down a WP gradient, cell bursts as plasma membrane breaks, it is CYTOLYSED/ HAEMOLYSED.
Isotonic= nothing happens,
How thick is the cell membrane?
7nm
What are intrinsic proteins?
- Transmembrane proteins
- Have amino acids with hydrophobic R groups on external surfaces.
- Interacts with hydrophobic core to keep them in place.
What are channel proteins?
- Hydrophilic channel
- Held in position by hydrophobic core and R group interactions
- Passage of movement for polar molecules
What are carrier proteins?
- Changes shape to transport molecules
- Can involve passive or active transport.
What are glycoproteins?
- Carbohydrate chains of varying lengths and shapes
- Involved in cell adhesion and as receptors for chemical signals (cell communication)
What are glycolipids?
- lipids with attached carbohydrate chains.
- Called cell markers or antigens and are recognised by the immune system as self or not self.
