Module 1 Principles of Cell Function

WHat is the fluid mosaic model of the cell membrane

a model which describes a cell membrane as a fluid lipid bilayer of ampipathic phosphilipd molecules wirh a mosaic of embedded proetins

what makes phospholipids amphipathic

Hydrophilic (polar) head and Hydrophobic (nonpolar) tail

how do phospholipids move in the membrane

they undergo rapid lateral movement (~10^7 times/sec) but rarely flip flop (up and down) ~ once per month

how does cholesterol affect membrane fluidity

it stabilises fluidity - reduces movement at high temps and prevents too much packing at low temps - sort of like a buffer

WHat are the main types of membrane proteins

integral membrane proteins (embedded in bilayer) and peripheral proetins (loosely bound to the surface)

List 6 major functions of membrane proteins

Transport (of substances in and out), enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, Attachment to the cytoskeleton and extracellular matrix (ECM)

How do we know membranes are fluid

cell fusion experiments show mixing of proteins between fused cells over time

which molecules can cross lipid bilayers easily

small uncharged molecules and sometimes water

which molecules are unable to cross the bilayer unaided

Ions (Na+ K+ Ca2+ Cl-) polar molecules like glucose and macromoleciules like proteins/RNA

Define passive transportr

movement of substances across a membrane where they travel down the concentration (High conc —> low conc) without energy input

What is osmosis

diffusion of water molecules across a selectively permeable membrane from low to high solute concentration

What are osmotica

osmotically active solutes - ions (Na+ K+), sugars, proteins, nutrients

Define Tonicity

it is a cells ability to lose or gain water in response to a solution

describe what happens to a red blood cell in hypertonic, hypotonic and isotonic solution.

Hypotonic (Hippo) —> the cells swells and may burst, Hypertonic —> cells crenate/shrink, Isotonic —> nothing happens

how do you calculare osmolarity -

multiply the number of particles in the solution by the concentration

What is facilitated diffusion

passive diffusion with the aid of transport proteins (channels or carriers) down a concentration gradienr - usually only applies for hydrophillic substanced since hydrophobic substances tend to be able t pass through anyway

How do channel and carrier protein work.

Channel Proteins: Open and close in response to stimuli. Carriers/transporters: bind to molecules and change shape to shuttle them across the membrane

Examples of channel and carrier proetins

aquaporins, ion channels; Carriers: glucose transporters, amino acid transporters.

DO channel proetins mediate active transport

No - only carrier/transport proteins can mediate active transport

What is active transport

movement of solutes against a concentration curve - low to high conc - and requires the use of energy such as ATP

example of an electrogenic pump in animal

Na+/K+ - ATPase (pumps 3 Na+ out, 2 K+ in) generating a membrane voltage

What is the main electrogenic pump in plants, fungi, and bacteria?

Proton pump.

What is cotransport?

When active transport of one solute drives transport of another (e.g., Na⁺-glucose cotransporter in animals).

What is exocytosis with an example in the human body

mass release of materials in the body via vesicles fusing with the plasma membrane; e.g. neurotransmitter release

What are the three types of endocytosis and how do they work

Phagocytosis - engulfing larger particles (eating) wrapping around the particles, Pinocytosis - cells drinking (engulfing liquid), receptor mediated endocytosis - specific ligand binds to receptor

WHat are the rwo major types of Cell signalling

Local signalling (neighbouring cells) and long distance signalling (travelling thorugh bloodstream tro rest of body)

Types of Local Signalling

Paracrine Signalling - signalling cell acts on target cells by secreting molecules of a local regulator. can also secrete molecules which itself has receptors for

Synaptic signallin - nerve cell releases neurotransmitter molecules into synapse, stimulating target cell

Types of long-distance signalling

endocrine (hormonal) signalling - hormones released into body fluids, mainly blood

what are the stages of cell signalling

Reception - cell receives signalling molecules, Transduction - cell changes the signal into a nother form within the cell, Response - activation of cellular resposne

How do signalling moelcules act on the cell

they interact with the cell receptors whcih exist on the surface of the cell or inside the cell (for hydrophobic signalling molecules)

Name the 4 receptor families

Plasma Membrane Receptors: Ion Channel receptors, G protein-coupled receptors, receptor tyrosine kinases.

Intracellular Receptors: Steroid receptors

Ion Channel Receptors

Ligand-gated ion channel - allows particles who cant diffuse normally to travel down the conc. gradient rapidly

G protein-coupled receptors

Involved in the nervous system - fast. Targetted by 50% of current drugs. Activated by Light, ions, hormones, peptides, neurotransmitters. check lecture for proper rundown

Receptor Tyrosine Kinases

E.g. Insulin receptors . Ligand binding causes dimerisation (the two receptors connect to form 1). The Tyrosine molecules in the receptor’s intracellular region are phosphorylated, which fully activated the receptor. The phosphorylated groups act as docking sites for cellular signalling proteins. Therefore, there are multiple possible signalling pathways.

Steroid receptors

Slow - acting throughout the body. Intracellular and activated by hormones. Hormone enters the cell and binds to the receptor forming a complex. this conmplex travels into the nucleus and acts as a transcription factore, changibng gene expression.