Membranes What are they made of? How does design affect function? What variations are there? How do things get in and out? Refer to chapter 5 in text.

Compare/contrast These two membrane models: Account for components, relative amounts, abilities of structural materials. How would you distinguish? Davson-Danielli model dates from What did they know? What did they not know?

Plasma Membranes diagram and label, including: phospholipid bilayer (next slides) cholesterol (membrane fluidity) glycoproteins (identity/recognition) integral proteins (spanning the membrane: transport) peripheral proteins (on surface only) aquaporins are water channels: numerous, not gated, allow free osmosis.

Review of phospholipids ←A typical lipid, composed of a glycerol backbone and 3 (saturated) fatty acids, joined by ester linkages, due to dehydration. Lipids are hydrophobic. For membranes, one fatty acid is replaced by a phosphate group, which is hydrophilic.→ This structure defines how phospholipids interact.

The phospholipids are NOT bonded to one another, shifting freely laterally…. ← note role of saturation on membrane fluidity

and … proteins move freely within the membrane: Fluid mosaic model Not all proteins float about. Many are anchored by cytoskeleton or ECM…

A variety of functions performed by membrane proteins: passive and active e.g. hormone reception Golgi, ERs, mitochondria… desmosomes (adhesion), gap, and tight junctions MHP, immunology

Membranes are good at protecting the insides… …yet stuff has to get in and out, too. But not all stuff. And some stuff only some times.

Diffusion is the spontaneous movement of solutes from an area of higher solute concentration to an area of lower solute concentration (down its concentration gradient), until balance is reached. Molecules keep moving, but there is no NET movement. (No barriers need be involved.) - Molecular energy provides Brownian motion (random).

Passive Transport - No cellular energy is expended. - Solutes are going down their concentration gradient, - across a semi-permeable membrane. 1. Simple Diffusion Some things can cross a plasma membrane by diffusing through, but not many, because of the bilayer structure. * O 2 (non-polar) is an example. * Filtration under pressure is a special case of this (like in kidney glomerulus and liver).

Osmosis is the specific case of diffusion of water molecules across a semi-permeable barrier BUT water, because it is polar- can’t osmose across a biological membrane, so there are aquaporins, channels that enable it. - note that water’s higher concentration is in the area of lower solute concentration. Campbell CD for animation 7.3… Know hyper-, hypo-, and isotonic How would this relate to tissues to be used in transplants?

2. Facilitated Diffusion Some things are too large, or too polar, to easily cross, so transport proteins or channel proteins (aquaporins) help... Picture a revolving door, with a bouncer to ensure specificity. * Glucose (too polar) is an example. - note, still no energy used, - note still going down concentration gradient. Passive Transport (cont.) Passive transport protein(Gated) channel

Active Transport - Cellular energy is expended. - Solutes are going against their concentration gradient… - Protein pumps - link ATP breakdown (energy release) to shoving solute across membrane, (pay attention to Na/K pump in axons in anatomy unit), or cotransport. - link transport to dissolution of something else’s gradient: (Second transportee may be active, or osmosis)

“Bulk transport” Endocytosis - phagocytosis (food particles) - pinocytosis (liquid droplets), - (also receptor-mediated, more specific, endocytosis), and Exocytosis are also forms of active transport, because energy is expended to merge membranes. Active Transport (cont.)

Consider the AP lab on this topic: In what ways does it demonstrate cell membrane function? In what ways does it NOT? Make a table to compare active and passive transport Explain how structure serves function with respect to transmembrane proteins. Fluid-mosaic nature of plasma membranes: What is it? Why does it matter?

plasma membranefiltration phospholipid bilayerfacilitated diffusion cholesteroltransport protein glycoproteinchannel protein integral proteinactive transport peripheral proteinendocytosis phospholipidphagocytosis hydrophilicpinocytosis hydrophobicexocytosis fluid mosaic modelprotein pump diffusionhypotonic osmosishypertonic passive transportisotonic simple diffusionsaturation concentration gradient cotransport aquaporins