B2.1.13 Membrane fluidity and the fusion and formation of vesicles

membrane fluidity

phospholipids can move around and switch positions within the bilayer

• important to molecule diffusion, interaction between proteins, membrane fusion, etc.

factors affecting membrane fluidity

temperature, fatty acid tail length, fatty acid saturation, presence of cholesterol

membrane fluidity in lower temperatures

high viscosity (not fluid/flexible), densely packed, more rigid, not permeable enough

membrane fluidity in higher temperatures

lower viscosity, less densely packed, won’t hold shape, too permeable

effect of longer phospholipid tails on membrane fluidity

more interactions between tails possible, less fluid

effect of saturated fatty acid tails on membrane fluidity

structure of fatty acid tails allows more phospholipids to press together closely, stronger intermolecular forces can decrease fluidity and cause higher melting point

effect of unsaturated fatty acid tails on membrane fluidity

bends in tail mean phospholipids cannot pack together neatly, weaker intermolecular attraction, lower melting point

• used in adaptation to colder temperatures to allow membrane fluidity

Arabidopsis (watercress) and saturation of fatty acids

at high temperatures, organisms increase saturation of fatty acids, saturation of membrane lipids can vary within the body of a single organism

cholesterol structure

amphipathic (both hydrophilic and hydrophobic parts)

• polar hydroxyl HO group linked at one end

• 4 linked hydrocarbon rings

• non-polar hydrocarbon tail linked to other end

cholesterol orientation within the membrane

• polar hydroxyl groups form hydrogen bonds with phosphate head, exposed to water

• hydrocarbon rings and tail within membrane’s hydrophobic core

cholesterol at high temperatures

restrains movement of phospholipid fatty acids, stabilizing membrane by making membrane less fluid and reducing permeability

cholesterol at low temperatures

prevents stiffening of membrane, preventing tight packing of fatty acid chains, maintaining membrane fluidity

formation of vesicles

pinched off membranes, formed from cell membrane (off plasma membrane, ER membrane, Golgi membrane), allowed by membrane fluidity

endocytosis

cell activity transports water and solutes into the cell by engulfing them into vesicles

• plasma membrane folds inwards forming a cavity that fills with extracellular fluid, dissolved molecules, food particles, foreign matter, pathogens, or other substances

• the plasma membrane folds back on itself until the ends of the in-folded membrane meet, trapping fluid inside the vesicle

• the vesicle pinched off from the membrane as the ends of the in-folded membrane fuse together

• vesicle breaks away from cell membrane and moves into cytoplasm

• cell membrane has gotten smaller

macrophages as an example of endocytosis

type of white blood cell which can engulf pathogens when fighting infection

amoeba as an example of endocytosis

this organism and other single celled organisms engulf other organisms as a food source

the fetus and endocytosis

supplied with proteins such as antibodies taken from the mother’s blood at the placenta

exocytosis

cell activity transports molecules out of the cell into extracellular space

• vesicles containing molecules are transported from within cell to cell membrane

• vesicle membrane attaches to the cell membrane

• fusion of vesicle membrane with the cell membrane releases vesicle contents outside the cell

• cell membrane has grown larger

secreted by exocytosis to extracellular environment

signaling hormones, neurotransmitters

proteins which become bound to vesicle membrane to become part of cell membrane

channels, pumps, recognition proteins, adhesion proteins, receptor proteins

neurotransmitter as an example of exocytosis

released from a presynaptic membrane

hormones as an example of exocytosis

secreted from endocrine glands (ex. insulin and glucagon from the pancreas)

contractile vacuole as an example of exocytosis

removal of excess water in some unicellular organisms

egg cell during fertilization as an example of exocytosis

release of cortical granules to prevent polyspermy

types of gaated ion channels

ligand-gated, mechanically-gated, voltage-gated

ligand-fated ion channels

ligand binds, ion channel opens to ions, ligand displaced, ion channel closed

mechanically-gated ion channels

ion channels open due to accumulation of charge

voltage-gated ion channels

sodium and potassium channels