Biological Membranes Flashcards
Roles of membranes
Sites of chemical reactions
Sites of cell communication (signalling)
Partially permeable barriers between
-> the cell and its environment
-> organelles and the cytoplasm and within the organelles
The fluid mosaic model is made up of what components
What role do phospholipids play
Form bilayer
Hydrophobic tail = inwards
Hydrophilic head = outwards
Act as a barrier to large, polar molecules
Only allow small, non-polar molecules to pass through by diffusion
Provides selective permeability
What role does cholesterol play
They fit in between the fatty acid tails within the bilayer
Stabilise the membrane structure
They regulate the fluidity of the membrane
What role do (general) proteins play
Forms pores (channels/carriers)
Can be intrinsic (transmembrane) or extrinsic
What role do glycolipids / glycoproteins play
Found sticking out of the surface of the bilayer
Act as antigens (recognise cells as self or foreign)
Cell signalling (allow communication across membranes & site of receptors for hormones & medicinal drugs)
Stabilise membrane & cell shape (by attaching to water molecules)
Cell adhesion (holds cells together in a tissue)
What role do membrane bound receptors play
Sites where hormones and drugs can bond to
Roles of membranes within cells
Role of cell surface membrane
- Control what substances enter & leave the cell
-> form partially permeable barriers between cell & its environments - cell signalling
The process of cell signalling and the role of the cell surface membrane
Factors that affect membrane structure / permeability
Temperature
Solvent concentration
pH
How does temperature affect membrane structure
How does solvent concentration affect membrane structure
Increases membrane permeability
1. Dissolve lipids in the membrane
2. Membrane then loses its structure
Different types of movement of molecules across the membrane
- Osmosis
- Passive processes
-> diffusion
-> facilitated diffusion - Active processes
-> endocytosis
-> exocytosis
-> active transport
What diffuses
Small non polar substances
What undergoes facilitated diffusion
This is diffusion using transport proteins (channel/carrier)
-> highly specific to molecules
For large polar molecules or ions
How do carrier proteins work
Can change shape
Used in active transport
How do channel proteins e.g. aquaporins -> allow charged shbstances & ions to diffuse through the cell membrane
- water filled pores
Used in facilitated diffusion
Have a fixed shape
Gated
Part of the channel protein on inside surface of membrane & can move to close & open the pore
How does endocytosis trasnsport LARGE materials INTO cells
Material is engulfed, forming small sacs (endocytic vacuole) and then absorbed into the cell
E.g. phagocytosis & pinocytosis
The bulk intake of solid (phago) & liquid materials (pino)
How does exocytosis work to transport materials out of cells
- content of vesicle packaged into secretory vesicles
-> secretory vesicles formed from golgi apparatus
-> secretory vessel travels to cell surface membrane
-> vesicle fuses with cell surface membrane
-> Contents of vesicle discharged to exterior
How does active transport work
-> movement of substances against the concentration gradient, from low to hifh
-> uses ATP & a carrier protein
(ATP needed for the carrier protein to change shape)
What is osmosis
The net movement of water across a partially permeable membrane from an area of high concentration to an area of low concentration down the concentration grsdient
2 ways water moves through the phospholipid bilayer
-> fitting between phospholipids (directly through the bilayer)
-> via protein channels (aquaporins)
Hypotonic vs isotonic vs hypertonic
Hypotonic -> solute conc outside of cells is lower than inside
Isotonic -> solute conc outside of cell = inside of cell
Hypertonic -> solute conc outside of cell > higher than inside
