2. cell membrane physiology I

phospholipid bilayer, membrane proteins, and membrane carbohydrates

- continuous layer around the cell

- barrier to water soluble substances (not to lipid soluble molecules and small molecules like oxygen and CO2)

transport proteins, receptor proteins, enzymes, joining proteins, and identifying proteins

major histocompatibility complex (MHC) proteins

- on the surface of all cells except red blood cells

- identify the cell as part of the body (not foreign)

- anchor cell membrane to cytoskeleton or an adjacent cell

two types:

- junctional proteins between cells (desmosomes, tight junctions, gap junctions)

- extracellular fibers (usually glycoproteins)

—  control chemical reactions on outer or inner surface

Examples:

- acetylcholinesterase

- Na⁺/K⁺- ATPase - all cells have these

—  can bind specific extracellular molecules (= ligands) like hormones and neurotransmitters (nt)

example: glucose uptake (by skeletal muscle cells or adipose tissue):

- insulin binds to receptor on skeletal muscle or adipose tissue

- triggers movement of more glucose transporters to cell membrane

- ⇑ glucose movement from blood into cells

channels and carrier proteins

- form pore in membrane through which selected ions/molecules can move freely

- selectively permit channel-mediated facilitated diffusion of water & specific ions

- Can be: gated (can open or close – when signaled) or non-gated (leakage channels, always open)

- bind to solute (ion/molecule) and help carry it across membrane

- allow protein carrier-mediated facilitated diffusion OR active transport

- e.g. glucose transporters

the ability of cells to maintain the composition of the intracellular (ICF) and extracellular (ECF) fluids and to move solutes and/or solvents across the cell membrane

substance dissolved in a solution

substance solute is dissolved in (e.g. water, some lipids, alcohol)

passive and active transport

—  no energy required (no ATP)

—  movement from a high to low concentration (i.e. down its concentration gradient)

—  the greater the difference in concentration = the more molecules want to move

Simple diffusion (solute movement), channel-mediated facilitated diffusion (solute movement), carrier-mediated facilitated diffusion (solute movement), osmosis (solvent movement), and Bulk flow

- passive transport, solute movement

- solute diffuses directly through cell membrane bilayer ∴ small and lipid soluble molecules (O₂, CO₂, etc.)

- passive transport, solute movement

- ions diffuse through membrane via protein channels – the solute does not bind to the protein, but moves freely through the opening created by the channel

- passive transport, solute movement

- large, charged or water soluble molecules diffuse across membrane using a specific carrier protein – must bind to protein to be transported

- e.g. glucose movement into liver or skeletal muscle

- solvent movement passive transport

- movement of H₂O across a semipermeable membrane (permeable to H₂O) due to [H₂O] difference (H₂O moves down its concentration gradient) via pores (channels) or across the membrane bilayer

- the pressure required to prevent the movement of water across a semipermeable membrane due to osmosis

- the concentration of solutes in a solution.

- Pure water has an osmotic pressure of 0, and the greater the solute concentration, the higher the osmotic pressure will be

- high osmotic pressure = high [solute]/low [H₂O]

- low osmotic pressure = low [solute]/high [H₂O]

- response of a cell immersed in a solution

- depends on solute concentration (and permeability of the cell membrane to the solute)

hypotonic solution, hypertonic solution, and isotonic solution

—  ECF has lower osmotic pressure (higher concentration of water) than ICF (cytosol)

—  cell swells (takes in water)

- the burst/rupture of a cell, caused by swelling in a hypotonic solution

- rupture of a red blood cell (RBC) = hemolysis

—  ECF has higher osmotic pressure (lower concentration of water) than ICF (cytosol)

—  cell shrinks (loses H₂O)

—  ECF and ICF have equal osmotic pressures

—  cell neither swells nor shrinks

—  injecting 10% sucrose solution (hypertonic) will move water to blood stream

—  e.g. use to reduce brain edema (swelling)

- Concentration of solutes in body fluids must be maintained within narrow limits or cells will die

- extracellular fluids (ECFs): blood plasma, interstitial fluid (ISF)

- intracellular fluid (ICF)

- the concentration of the blood increases, and therefore the osmotic pressure of the blood increases

- fluid moves from the tissues into blood (dehydration of tissues)

- the response will include thirst and a decrease in renal H₂O loss which leads to decreased urine production

- passive membrane transport

- movement of fluids (and solutes) due to a pressure gradient (moves from high pressure to low pressure)

- the pressure of a fluid pressing against a surface

- examples of surfaces: cell membrane, blood vessel wall (blood pressure)

- fluid flows out of the capillary (= filtration)

- If ISF has higher pressure than blood, fluid flows from ISF into the capillary (= absorption)

- Require energy (ATP) to move ions/molecules against their concentration gradient

- types: active transport and vesicular transport

- a cell membrane active process

- substances move against concentration gradient (from low to high)

- always protein carrier-mediated

Primary (1°) Active Transport and Secondary (2°) Active Transport

- a type of active transport

- molecular pumps - ATP breakdown is directly part of transport process

- e.g. Na⁺/K⁺-ATPase Pump ---> 3 Na⁺ out of cell and 2  K⁺ in per ATP

- a type of active transport

- cotransport (use of ATP is indirect)

- a Na⁺ gradient is established by Na⁺/K⁺-ATPase (ATP use step)

- glucose & Na⁺ both must bind to the carrier protein and are cotransported into the cell ⇒ Na⁺ moving down its concentration gradient drives in glucose against its concentration gradient (transport step)

- a type of active process

- substance is surrounded by a membrane within the cell (vesicle)

- types: endocytosis (movement into a cell) and exocytosis (movement out of a cell)

- Phagocytosis: large items into cell (like bacteria), like the cell eating

- Pinocytosis: fluids (+ dissolved substances), like cell drinking

- type of vesicular transport

- the process of moving large molecules out of the cell, which are vesicles containing hormones, enzymes, neurotransmitters, etc

- the vesicles fuse with cell membrane releasing contents into ECF (triggered by the rise in cytosolic calcium ions Ca++)