General Biology/Cells/Membranes

Biological membranes

Biological membranes surround cells and serve to keep the insides separated from the outsides. They are formed of phospholipid bilayers, which by definition are a double layer of fatty acid molecules (mostly phospholipids, lipids containing lots of phosphorus).

Proteins serve very important functions in cellular membranes. They are active transports in and out of the cell, acting as gatekeepers. They relay signals in and out of the cell. Proteins are the site of many enzymatic reactions in the cell, and play a role in regulation of cellular processes.

Phospholipid

Phospholipid bilayer

basis of biological membranes and cellular organisms
contains a charged, hydrophilic (attracted to water) head and two hydrophobic (repelled by water) hydrocarbon tails
In presence of water, phospholipids form bilayer
- maximize hydrogen bonds between water
- creates barrier to passage of materials
- fluid mosaic model shows horizontal (common) and "flip-flop" (rare) movement of phospholipids

Fluid mosaic model

Current model of membrane
Phospholipid bilayer
- Phospholipids
Move freely in lipid layer, but rarely switch layers
Different phospholipids in each layer in different organelles
- Glycolipids
- Sterols (cholesterol in animals)
Transmembrane proteins "float" in fluid lipid bilayer
- also called intrinsic, integral proteins
Exterior (extrinsic, peripheral) proteins

Membrane proteins

Transport channels
Enzymes
Cell surface receptors
Cell surface identity markers
Cell adhesion proteins
Attachments to cytoskeleton

Integral membrane proteins

Anchoring to membrane
- Protein has attached phosphatidylinositol (GPI) linkage, anchors protein in outer layer (no picture)
- Protein has one or more hydrophobic transmembrane domains
-helix
-sheet

Channel protein Transport across membranes * Diffusion

- From higher concentration to lower concentration
- Membranes are selectively permeable
Ions diffuse through membrane channels
- Selective
- Movement determined by diffusion and voltage differences
Facilitated diffusion
- Carrier protein, physically binds transported molecule
Osmosis
Diffusion of water down concentration gradient
In cell: various solutes (amino acids, ions, sugars, etc.)
- interact with water, e.g., hydration shells
Water moves through aquaporin channels into cell
Depends upon the concentration of all solutes in solution
- Hyperosmotic solution: higher concentration of solutes
- Hypoosmotic solution: lower concentration of solutes
- Isoosmotic solution: solute concentrations equal
Water moves from hypoosmotic solution to hyperosmotic solution

Osmotic pressure Bulk transport

Endocytosis: energy requiring
- Phagocytosis
Solid material, typically food
- Pinocytosis
Primarily liquid

** Receptor-mediated endocytosis

Pits on cell surface coated with clathrin and receptors
Bind specific proteins
Exocytosis
- Discharge of materials from vesicle at cell surface

Receptor-mediated endocytosis

Active transport

Energy required (usuallyننةىة
Highly selective
Works against concentration gradient
Many examples, e.g., Na⁺/K⁺ pump

Cotransport (coupled transport)

Does not use ATP directly
Molecule is transported in connection with another molecule that is moving down a concentration gradient
- Example: Na⁺ gradient is established by a Na⁺ pump, with higher concentration on outside of cell. Cotransport channel carries Na+ and another molecule (e.g. glucose) into cell
May involve proton (H⁺) pumps (chemiosmosis - ATP production)

This text is based on notes very generously donated by Dr. Paul Doerder, Ph.D., of the Cleveland State University.