Cell biology 1.3 Membrane structure

Describe the amphipathic properties of phospholipid molecules.

Amphipathic molecules have both hydrophilic (water-loving) and hydrophobic (ew water-hating) regions Phospholipids consist of a polar organic molecule (e.g. choline, serine), a phosphate group, a glycerole molecule, and two fatty acid tails (may be saturated or unsaturated)

Outline that phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Bc of the amphipathic properties, phospholipids spontaneously arrange in a bilayer. The hydrophobic tails face inwards and are shielded from the watery environment on the outside. Properties of the bilayer: - Weak hydrophobic interactions between the tails holds the bilayer together. - Hydrophilic / hydrophobic restrict the passage of many substances, as most substances are either hydrophilic or -phobic. - Individual phospholipids can move within the bilayer, allowing membrane fluidity and flexibility. - Fluidity allows for endocytosis and exocytosis, which is the spontaneous breaking and reforming of membranes

Draw the fluid mosaic model.

Checklist: - individual phospholipid molecules are shown by using the symbol of a circle with two parallel lines attached - a range of membrane proteins is shown, incl peripheral and integral - following labels are included: phospholipid bilayer, phospholipid molecule, glycoprotein, glycolipid, integral and peripheral proteins, and cholesterol

State that cholesterol is a component of animal cell membranes.

Cholesterole is a steroid made up of a non-polar part, including four ring structures, a hydrocarbon tail, and a polar hydroxyl group. Cholesterol fits between phospholipids in the cell membrane with its hydroxyl (OH) group by the hydrophilic heads and the hydrophobic rings by the hydrophobic tails

Analyse the evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

Davson and Danielli proposed the "protein-lipid sandwich" structure of the cell membrane. this was due to their observation of two dark parallel lines with a light region in between. Proteins appear dark and lipids appear light -> suggested that the phospholipid bilayer is embedded between two layers of proteins

Analyse the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Freeze-etched electron micrograph images of the membrane: - cells are rapidly frozen and fractured. Fracture occurs along lines of weakness, e.g. the center of membranes. Globular structures throughout the membrane were interpreted as transmembrane proteins (not accounded for in Dav-Dan) Cell fusion experiments: - shows that protein molecules can move from place to place on the cell membrane Improvements in techniques: - allowed them to extract membrane proteins and determine their chemical and physical properties -> they vary in shape and size and properties

Differentiate between integral and peripheral proteins.

Integral proteins are permanently attached to the membrane, and typically span across the bilayer Perhipheral proteins are temporarily attached by non-covalent interactions, and associate with one surface of the membrane

Discuss the role played by cholesterol in the fluidity of animal cell membranes.

It acts as a regulator of membrane fludity. - At high temperatures, it stabilises the membrane and raises the melting point. - At low temperatures, it prevents phospholipids from packing too close together (which could lead to stiffening.

Functions of membrane proteins mnemonic: Jet rat

Junctions - Serve to connect and join two cells together Enzymes - Fixing to membranes localises metabolic pathways Transport - Responsible for facilitated diffusion and active transport Recognition - May function as markers for cellular identification Anchorage - Attachment points for cytoskeleton and extracellular matrix Transduction - Function as receptors for peptide hormones

Outline that membrane proteins are diverse in terms of structure, position in the membrane and function.

Non-polar amino acids associate directly with the lipid bilayer. Polar amino acids are located internally and face watery environments. Transmembrane proteins typically form single helices (enzymes, transporters, receptors) or beta barrels (common for channel proteins).

Explain how cholesterol affects the permeability of mammalian membranes to some solutes.

The membrane fluidity effects how permeable the structure is to solutes - too fluid -> too much permeability - too stiff -> not enough permeability

List the functions of membrane proteins.

Transport: Protein channels (facilitated) and protein pumps (active) Receptors: Peptide-based hormones (insulin, glucagon, etc.) Anchorage: Cytoskeleton attachments and extracellular matrix Cell recognition: MHC proteins and antigens Intercellular joinings: Tight junctions and plasmodesmata Enzymatic activity: Metabolic pathways (e.g. electron transport chain)