Principles of Cell Function (1) Flashcards
Cellular membranes
Cellular membranes are fluid mosaics of proteins and a phospholipid bilayer.
Phospholipids
Phospholipids are abundant amphipathic molecules and components of cellular membranes. They consist of hydrophilic heads (phosphate group) and hydrophobic tails (two fatty acids).
Movement of phospholipids
Lateral movement - often
Flip-flop - once per month
Describe how phospholipids affect membrane fluidity
Unsaturated carbon tails of the phospholipids have kinks which make it difficult for the phospholipids to layer, making the membrane more fluid.
Saturated fatty acid tails - viscous
Cholesterol
Prevents movement of the phospholipids as a result of mechanical force or temperature change.
Six major functions of membrane proteins
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition (glycol-protein)
- Intercellular joining
- Attachment to the cytoskeleton and extracellular matrix (ECM)
What are lipid bilayers impermeable to?
Ions
Small hydrophilic molecules (glucose
Macromolecules like proteins and RNA
What are lipid bilayers permeable to?
Some permeability to water molecules and other SMALL, UNCHARGED (lipophilic), molecules like oxygen and carbon dioxide.
What is passive transport?
Diffusion of a substance across a membrane with no energy investment.
Diffusion: with time - due to random motion molecules become equally distributed. i.e. to eliminate concentration gradients - provided molecules can cross the membrane.
Describe osmosis
Diffusion of water through a selectively permeable membrane into another aqueous compartment containing solute at a higher concentration. (water wants to be at equilibrium)
Plasma membrane is semi-permeable!
What is tonicity?
The ability of a solution to cause a cell to gain or lose water
Describe isotonic, hypertonic and hypotonic solutions
Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.
Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water.
Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water.
What are the effects of different solutions on erythrocytes and plant cells?
Hypo: RBC lysed, plant cell turgid
Hyper: RBC shrivelled, plant cell plasmolysed
Iso: RBC normal (300osmol/L), plant cell flaccid
Channel proteins
Channel proteins provide facilitated diffusion. They have a hydrophilic channel that certain molecules or ions can use as a tunnel
- Aquaporins
- Ion channels (open or close in response to stimulus)
Transport proteins
Allow passage of hydrophilic substances across a membrane.
A transport protein is specific for the substance it moves
Facilitated diffusion or active transport
Carrier proteins
Bind to molecules and and change shape to shuttle them across the membrane
Passive transport
Down a concentration gradient
Active transport
Against a concentration gradient
Requires work - from energy of ATP
e.g. Na+/K+-ATPase = sodium-potassium pump
Electrogenic pump
transport protein that generates a voltage across a membrane
Sodium-potassium pump
major electrogenic pump of animal cells
EP - transport protein that generates a voltage across a membrane
Proton pump
main electrogenic pump of plants, fungi, and bacteria
EP - transport protein that generates a voltage across a membrane
Cotransport
occurs when active transport of a solute drives transport of another solute
e.g. plants commonly use the gradient of protons generated by proton pumps to drive the active transport of other nutrients into the cell
Bulk transport
Endocytosis and exocytosis
Types of endocytosis:
Phagocytosis -engulfing particles
Pinocytosis - cells drinking
Receptor-mediated endocytosis
Example of exocytosis:
Neurotransmitter release from neurons
Cell to cell communication stages
- Reception
- Transduction
- Response
Ion channel receptors
Plasma membrane receptor
Na+ channels open by ligand - fast neurotransmission
G protein-coupled receptors
Plasma membrane receptor
7 transmembrane-spanning regions - all aspects of physiology and pharmacology
First messenger —> receptor —> g-protein —> Second messengers—> Kinase —> cellular response
Adrenaline (first messenger) binds to a GP linked receptor. Interacts with heterotrimeric G proteins which binds to a target enzyme to make a second messenger (like cAMP). This causes protein kinase A to induce a cellular response.
Tyrosine kinase linked receptors
Plasma membrane receptor
e.g. insulin receptors - metabolism, cell growth, cell reproduction
Insulin binds to tyrosine kinase linked receptor. Kinases transfer phosphates and activate or inactivate target proteins.
Steroid receptors
Intracellular receptor
Protein phosphorylation (in TKLRs)
Causes conformational change
Protein-protein interactions
Change in cellular location
These changes lead to activation or inactivation of the target proteins
