W5 Plasma membranes Flashcards
What does a cell membrane do?
It is a boundary that separates the cell from its surroundings.
A barrier to prevent the contents of the cell from escaping and mixing with the surrounding environment
How are lipids arranged in a cell membrane?
A bilayer (of phospholipids)
(two-layer sheets)
Phospholipids are amphipathic. What does this mean?
They contain hydrophobic and hydrophilic regions
Hydrophobic Fatty acid tail
Hydrophilic Phosphate head
What is the fluid mosaic model?
It is a dynamic and complex structure of the plasma membrane
Fluid= viscosity of lipid bilayer
Mosaic=Besides phospholipids, Glycolipids, sterols (cholesterol
in mammalian cells) and various proteins and glycoproteins are contained
Phospholipids can lead to..
Rapid lateral diffusion
Spin in place
Flexion (contraction movement)
What does cholesterol do?
Modulates the membrane fluidity and improves its stability. Stiffens the membrane, reducing molecules passage
Maintains rigidity
What are glycolipids
Lipids containing sugars representing their hydrophilic head
Are cholesterol and Glycolipids amphipathic?
YES
Where are membrane proteins synthesised?
Rough ER
What are the functions of membrane proteins?
Transport of molecules
Enzymatic activity
Cell communication
Structural support
What are glycoproteins?
Short chains of sugars linked to amino acids/protein
What is passive transport?
Movement of molecules or ions across membranes without energy expenditure
Movement DOWN the concentration (or electrical) gradient
Movement from areas with high concentrations to a low concentration of the molecule, with the aim to reach an equilibrium
What are the 3 types of passive transport?
Simple diffusion, Osmosis, Facilitated diffusion
What is diffusion?
FREE movement of molecules across the lipid bilayer
* No energy (ATP) cost (passive)
* Down their gradient concentrations (until an
equilibrium is reached)
Which molecules can diffuse through the membrane?
Gases e.g. O2 and CO2
Small, uncharged polar molecules e.g. Urea and Ethanol
What is osmosis?
The spontaneous movement
(passive transport) of water/solvent across a
semipermeable membrane from a solution with higher water potential (lower solute concentration) to a solution with lower water potential (higher solute concentration)
- Water potential of a solution is inversely proportional to the solutes concentration
What is osmotic pressure?
Hydrostatic pressure required to stop the net Flow of water across a membrane separating solution of different water potentials
What is an:
isotonic solution
hypertonic solution
hypotonic solution
Isotonic Solution – a solution with the same concentration of solutes both inside and outside the cell.
Hypertonic Solution – a solution that has a higher solute concentration (water moves out)
outside the cell than inside.
Hypotonic Solution – a solution that has a higher solute concentration inside the cell than outside (water moves in)
What is facilitated diffusion
The passive movement of molecules down a
concentration gradient (or electric potential) via a
transport protein (transmembrane proteins):
Protein channel (ions or water)
Protein carrier/transporters (of specific small
molecules or ions)
* Highly specific (glucose vs. fructose)
* Reversible
* No energy (ATP) cost
Symporters and anti porters are associated with..?
Active transport
What are protein channels?
- Channels (pore membrane forming) form a hydrophilic
“tube” across the membrane through which specific
molecules/ions diffuse (down a concentration gradient) - Do not interact with the molecules, although specific
- Move at a very rapid rate
- Passive transport (no energy cost)
What are symporters and antiporters?
Symporters moving 2 different molecules across the cell membrane (same direction)
Antiporter moving 2 distinct molecules across the membrane (opposite directions)
E.g. Na+-K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in.
What is active transport?
- Moves substances against their concentration or electrochemical gradients
- Requires energy, usually in the form of ATP
- Performed by specific proteins embedded in the membranes
- Allows for stockpiling
What are the 2 types of bulk transport?
Exocytosis and Endocytosis.
What happens during exocytosis?
- Secretion or excretion (from inside
to outside) e.g. insulin (protein hormone) by
pancreatic cells - Transport vesicles (from Golgi) migrate to
the plasma membrane, fuse with it, and
release their contents
What happened during endocytosis?
Uptake (from outside to inside) of macromolecules and large particles
*Invagination of the cell surface to form an intracellular membrane-bounded vesicle
containing extracellular fluid
What are the 3 types of endocytosis?
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
What is an inactive enzyme called in the absence of its essential cofactor?
Apoenzyme
