2.1.5 Biological membranes Flashcards
What are the roles of membranes?
- Separates the cell’s components from its external environment.
- Regulates what goes into and out of cell.
- Release chemicals and contain receptors for cell signalling.
- Site of chemical reactions.
What are the components of plasma membrane?
- Phospholipids
- Cholesterol
- Membrane proteins
- Glycoproteins and glycolipids
What do phospholipids do?
- Hydrophilic phosphate head and hydrophobic fatty acid tail.
- Forms partially permeable membrane allowing small, lipid-soluble substances (e.g. oxygen and carbon dioxide) to enter and leave.
- Prevents polar (water soluble) substances from entering and leaving.
- Unsaturated fatty acids have kinks which INCREASE membrane fluidity when temperature drops.
What does cholesterol do?
-Stabilises cell membrane. (If temperature drops, cholesterol prevents phospholipids from packing too closely together increasing fluidity).
-Reduces lateral movement of phospholipids.
-Helps to prevent water/ions passing through the membrane.
What do membrane proteins do?
-Structural support
-Channel proteins allow ions to diffuse through (aquaporins for water).
-Carrier proteins allows large molecules (glucose and amino acids) to pass through via active transport.
What do glycoproteins and glycolipids do?
-Antigens (cells recognised as ‘self’ by immune system)
-Cell signalling (receptor for hormones)
-Help cells attach to each other (adhesion) to form tissues.
-Attach to water to stabilise membrane.
What is cell signalling and how does it work?
-Communication between cells.
-Cell recognition.
-Cells secrete specific signal molecule by exocytosis.
-Proteins, glycoproteins, glycolipids on surface of cells act as receptors.
-Shape of signal molecule is complementary to receptor site.
-Attachment causes changes within target cell.
What is diffusion?
- Net movement of particles down a concentration gradient, from a region of high concentration to low concentration.
- No energy is used so its an example of passive transport.
What is facilitated diffusion?
- Large polar molecules can only cross the phospholipid bilayer with the help of certain proteins (carrier and channel).
- The are highly specific.
What factors affect diffusion?
- Steepness of gradient
- Temperature
- Surface area
- Properties of molecules/ions
- Diffusion distance
What is active transport?
- Movement of substances against concentration gradient.
- Uses energy in the form of ATP.
- Uses a carrier protein.
What is endocytosis?
- The process of cells taking up material.
- The material can be solid (phagocytosis) or liquid (pinocytosis).
- The membrane engulfs the particle and pinches off into a membrane bound vesicle.
- Requires energy for ATP.
What is osmosis?
Net movement of water from a region of high water potential to a region of low water potential through a partially permeable membrane.
What is water potential?
- The tendency of water molecules in a system to move.
- The more solute in a solution, the lower the water potential because water molecules bind to solute molecules reducing the number of water molecules that are free to diffuse.
Language of water potential.
Hypertonic - solution has higher solute concentration than cell (lower water potential).
Isotonic - solution and cell have same solute concentration (no net movement of water).
Hypotonic - solution has lower solute concentration than cell (higher water potential).
