2 - MZH - Biological membranes Flashcards
3 things that living cells must do? Examples?
- Take in molecules e.g. Oxygen and nutrients
- Extrete metabolic waste e.g. CO2
- Produce materials for secretion e.g. Enzymes and hormones
Name 3 roles of membranes
- Partially permeable barriers between the cell and its environment and between organelles and the cytoplasm and within organelles.
- Sites of chemical reactions
- Cell signalling
What is compartmentalisation? Purpose of it and give an example?
Formation of seperate membrane-bound areas in a cell is called compartmentalisation.
This allows many activities to take place in a cell without interfering with each other.
(Consider it a bit like division of labour!)
E.G Mitochondria
Name the basic structure model that biological membranes are based on?
Why is it called that?
Fluid-mosaic model
- Fluid* = The individual phospholipids aren’t fixed into place and can move i.e. they’re no static
- Mosaic* = Looks like the tiles of a mosaic from bird’s eye view
What is true about the molecules that make up a membrane?
They are not static. In reality the molecules in a membrane move about.
- They move laterally about the membrane
- They leave the membrane to join vesicles in the cell’s cytoplasm
- They join the membrane from vesicles in the cell’s cytoplasm
List the components of a cell membrane? (5)
Phospholipids
Cholesterol
Proteins (extrinsic and intrinsic)
Glycolipids
Glycoproteins
Phospholipids? (component)
Thickness?
What can and can’t pass through?
Examples of substances that can pass through (2)
An exception?
-Contains hydrophilic head (glycerol and phosphate head) and hydrophobic tail (fatty acid tail).
-Approx 7nm thick
- Prevents the entry or exit of any ions or polar (water soluble) molecules.
- Lipid soluble substances can diffuse freely across membranes.
- CO<strong>2</strong> and O2 can pass through freely.
- H2O is polar but can pass through as well (it’s small in size).
Cholesterol? (component)
Purpose and reason for it being there?
Hydrophilic or hydrophobic? Purpose?
- Occurs only in animal cells.
- Strengthens the membrane (provides structure) as there’s no cell wall.
- Very hydrophobic, prevents leakage of water and ions out of the cell.
Extrinsic proteins? (component)
Where are they found?
Functions? (3)
-Found on only one side of the phospholipid bilayer and never extended completely across it.
Functions:
- Receptor sites combining with specific messenger molecules (e.g. hormones). The messenger molecules trigger particular activities in the cell.
- Enzymes e.g. as enzymes on the surface of cells slining in the small intestine.
- Mechanical support to membrane
Intrinsic proteins? (component)
What is it also called?
Examples (2)? Do they deal with passive or active transport?
Also called Transmembrane protein.
- Channel proteins and carrier proteins are intrinsic proteins involved in transport across membranes.
- Channel proteins = Allow the passive movement of polar molecules and ions down a concentration gradient through membranes. (Facilitated diffusion).
- Carrier proteins - Important role in both passive transport (down a conc gradient) and active transport (against a conc gradient). This involves the shape of the protein changing.
Glycolipids? (component)
What is it?
Functions (3)
Carbohydrate chain attatched to a phospholipid molecule. Carbohydrate chain extrends into the watery environment surrounding the cell.
Functions:
- Act as recognition sites e.g. ABO blood system
- Maintain stability
- Help cells attach to each other to form tissues
Glycoproteins? (component)
What is it?
Functions? (3) Give exmaples to 2 of the functions
Carbohydrate chain attached to many of the extrinsic proteins on outer surface of membrane.
Functions:
- Recognition sites for hormones and neurotransmitters
- Act as antigens (cell recognition) e.g. lymphocytes can recognise an organism’s own cells.
- Help cells attach to form tissues.
Temperature affecting membrane structure & permeability
When it rises and when it falls?
If temp rises:
- Phospholipids have more KE and will move more.
- Make membrane more fluid and it’ll begin to lose it’s strucutre if the temperature reaches too high.
- Increases permeability and may allow harmful substances in.
- Carrier & channel proteins will be denatured.
If temp falls:
- Bilayer becomes rigid. Membrane is less fluid impairing the cells ability to move and grow.
- Decreases permeability, limiting the movement of molecules in/out of cells.
Solvents affecting the permeability of a cell membrane
- Organic solvents will dissolve membranes, disrupting cells.
- Pure/very strong alchoholic solutions are toxic as they destroy cells in the body.
- Less conc solutions of alcohols will not dossolve membranes but will still cause damage.
- Transmission of nerve impulses are disrupted if the cell membranes of neurones are damaged.
- Happens in the brain as result of consumption of alcoholic drinks.
Cell signalling and membrane receptors
How do cells communicate?
Cells communicate by:
- Direct contact
- Long distances e.g. via hormones
- Membrane receptors play a key role* as they have a specific shape that is complementary to another molecule.
Forms a receptor/messenger complex and triggers a reaction inside the cell.
Give a generalised exmaple of cell signalling and membrane receptors
