Plasma membrane Flashcards
What are the main functions of the plasma membrane?
Boundary that separates the cell from its surroundings. Found in all cells.
* Internal membranes of organelles allow
compartmentalisation (for a specific
composition → specific reactions/functions)
* It exhibits selective permeability, allowing only specific molecules to pass through
* It plays a key role in interacting with other cells (glycoproteins) & cell signalling
* It provides structural integrity to the cell and protects its contents
What makes up the mosaic part of the fluid mosaic model?
▪ Phospholipids,
▪ Glycolipids,
▪ sterols
(cholesterol in
mammalian cells)
▪ various proteins
▪ glycoproteins
What makes up the fluid part of the fluid mosaic model?
- Dynamic, fluid and flexible
▪ Phospholipids and proteins move laterally within the layer
What are phospholipids? and what are they composed of?
- Phospholipids (75% lipids of the membrane) and are arranged in two-layer sheets
- Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions
What are the hydrophilic heads?
▪ a phosphate group and a glycerol
▪ Polar, allowing interacting with water
What are the hydrophobic tails?
▪ Nonpolar, repelling water and
interacting with other lipid tails
▪ Self-Assembly into the bilayer to
shield water. No energy needed
What can phospholipid fluidity in the membrane lead to?
➢Rapid lateral diffusion within the plane (107 times/sec)
➢ Spin in place (rotations – up to 500/sec.)
➢ Flexion (contraction movement)
➢ Very rarely, Flip-flop (side-to-side movement)
What other lipids are there in eukaryotic cell membranes?
- Cholesterol (20%)
- Glycolipids (5%)
What are the functions of cholesterol?
➢ Stabilise the membrane → affecting its
fluidity and stiffness
➢ Improve stability
➢ Reduce permeability ꜜmolecule passage
What are the functions of glycolipids?
▪ Sugar groups (hydrophilic) face outwards → asymmetry
What are the two types of membrane proteins?
Integral proteins
Peripheral proteins
What are integral proteins?
➢ Trans-membrane proteins spanning the membrane
➢ Integral proteins facing only one side
How do integral proteins have ampithatic natures?
▪ hydrophobic amino acids lie in the bilayer
▪ hydrophilic regions facing the aqueous environment
What are the functions of membrane proteins?
Major functions of membrane proteins
➢Transport of molecules
➢Enzymatic activity
➢As receptors in cell signalling
➢Structural support (linked to cytoskeleton)
What are the functions of glycoproteins?
➢Protection from mechanical damage
➢cell-to-cell communication
Describe the conditions of diffusion
Movement of small, nonpolar
molecules* that can diffuse
directly through the lipid bilayer
▪ No energy cost
▪ Down their conc. gradient
What are small and non polar molecules that diffusion can move?
O2, CO2, steroid hormones
+ small uncharged polar molecules like urea and ethanol
Describe osmosis
- Movement of water across a cell
membrane from an area with higher
water potential (lower solute conc.)
to an area with lower water
potential (higher solute conc.)
How does water and solute potential differ in osmosis?
- Water potential is inversely proportional to the solute’s concentration
▪ as solute concentration increases, water potential decreases. - When the solutes are unable to across, osmosis balances solute concentrations, to achieve equilibrium
What is an isotonic solution and how does this effect net movement?
Isotonic Solution - Solute concentration is equal inside and outside the cell
* No net water movement
What is an hypertonic solution and how does this effect net movement?
Hypertonic Solution - Solute concentration is higher outside than inside the cell
* Water moves out of the cell → cell shrinkage
What is an hypotonic solution and how does this effect net movement?
Hypotonic Solution Solute concentration is lower outside than inside the cell
* Water moves into the cell → cell swelling
What is facilitated diffusion?
The passive movement of molecules down a
concentration gradient (or electric potential) via a
transport protein (transmembrane proteins):
➢ Protein channel
➢ Protein carrier/transporters
What molecules apply to facilitated diffusion?
➢ Specific ions (e.g. Cl-)
➢ Hydrophilic small molecules (e.g. glucose)
➢ Water facilitated diffusion (via Aquaporins)
What are the properties of a protein channel?
Form a hydrophilic “tube/corridor” across the membrane
through which specific molecules/ions diffuse
▪ Specific
▪ No molecule-channel interaction
▪ Rapid rate
▪ Can be Gated → open or close in response to signals
(e.g. voltage changes in neurons)
What is a uni porter?
Uniporter → one molecule/ion interaction at a time
1) The solute interacts with the protein carrier
2) Triggering a change in shape that translocate the
solute across the membrane
What are properties of protein carriers in f diffusion?
*Uniporter
*specificity
*slower rate than protein channels
Describe what makes up 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
cell/organelle membrane or vesicles - To maintain concentration gradients
What are the two types of protein carriers in active transport?
➢ symporter
➢ antiporter
What is a symporter?
moving 2 distinct molecules in
the same direction across the cell membrane
What is an antiporter?
moving 2 molecules in opposite directions across the membrane
✓ E.g. Na+-K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in
What are two main types of vesicle transport? When are they required?
- required in bulk transport
- Exocytosis
- Endocytosis
What is exocytosis?
from inside to outside cell
Transport vesicles (from Golgi) fuse with the
plasma membrane, and release their contents
➢ insulin from pancreatic cells
What is endocytosis?
Uptake of macromolecules and
large particles (from outside to inside)
* Substances are brought into the cell by
engulfing them in a membrane-bound vesicle
What are the three types of ENDOcytosis?
➢ Phagocytosis (“cellular eating”)
➢ Pinocytosis (“cellular drinking”)
➢ Receptor-mediated endocytosis
