3.3 Membrane Transport

Selectively permeable

The plasma membrane is selectively permeable - it does this through passive or active mechanisms

Passive Mechanism

Require no ATP from the cell

Filtration

Diffusion

Osmosis

Active Mechanisms

Consume ATP

Active transport or Vesicular transport

Carrier-mediated mechanisms use membrane protein to transport substances from one side to the other

Filtration

Physical pressure forces fluid through a selectively permeable membrane such as a coffee filter

Body example is blood capillaries

This is how water, salt, nutrients, and solutes are transferred from bloodstream to tissue and how kidneys filter out blood cell and proteins

Simple Diffusion

Net movement of particles from place of high concentration to low concentration from constant, spontaneous movement

Substances move down their concentration gradient

Some factor that contribute to how fast something goes through diffusion

• temperature - when warmer, moves faster

• molecular weight - light particles are faster

• Steepness of concentration gradient - the steeper the gradient, the more quickly they diffuse

• Membrane surface area - apical surface of cells special in absorption

• Membrane permeability - how permeable something is

Osmosis

Net flow of water from one side of a selectively permeable membrane to another

Imbalance in osmosis can cause diarrhea, constipation, hypertension and edema

Osmolarity

Osmotic concentration of body fluids effect cellular function that is important to understand the units which is is measured - milliosmoles per liter

Tonicity

Ability of a solution to affect the fluid volume and pressure in a cell

Solute cannot pass through a plasmamembrane, remaining more concentrated on one side thant the other, causing osmosis

Hypotonic / hypertonic / isotonic

Lower concentration of non-permeating solutes than the intracellular fluid (ICF). Absorb water, swell, and may burst - hypo

Higher concentration of nonpermeating solutes than the ICF, cells may lose water and shrivel, they can then die of torn membranes - hyper

Total concentration of nonpermeating solutes is same as ICF - no change in cell shape or volume - isotonic

Carrier-mediated Transport

Solutes bind to carrier into plasma membrane, changes shape and releases solute onto the other side

Carriers act like enzymes in that the solute is a ligand that binds to a specific receptor site on the carrier

Carries have specificity

Carriers can have saturation, as solute concentration rises the rate of transport increases only up to a point, when all are full the solute cannot be added

Types of transport

Uniport - carries only one type of soulte

Cotransport - move two or more solutes through the membrane in the same direction (carrier protein who does this is symport)

Countertransport - two or more solutes in opposite directions (carrier protein is antiport)

Mechanisms of Carrier Mediated Transport

Facilitated diffusion: carrier mediated transport of a solute through a membrane down its concentration gradient, doe not use ATP

Primary active transport: carrier moves through a substance through a cell membrane up its concentration gradient using ATP

Secondary active transport: require energy input, and uses ATP as a secondary source of energy

Sodium potassium Pumps Na+-K+ Pump

Primary active transport

Extremely important - the body can exchange 30 million sodium ions and 20 million potassium ions per second!!!

Its functions include

1) Secondary active transport: maintains steep Na+ gradient across membranes

2) Regulates cell volume: some anions “fixed anion” are confined to cells and cannot penetrate plasma membrane, they attract and maintain cations. If this went unchecked, osmotic swelling would occur. Cell swelling causes Na-K pumps to activate and each cycle is part of a negative feedback loop removing 1 extra anion then it brings in thus reducing/preventing cell swelling

3) Maintenance of membrane potential: the inside of the membrane is more negatively charges and the outside is more positively charged. The unequal distribution of ions on either side of the membrane is maintained by the Na-K pump and is important for excitability for nerve and muscle cells

4) heat production: thyroid hormones stimulate cells to produce more Na-K pumps, as they consume ATP they release heat, which helps when it is cold outside and we lose body heat

Vesicular transport

Moves large particles, fluid, or many molecules at once through membrane contained in bubble like vesicles

Endocytosis (phagocytosis, pinocytosis, and receptor mediated endocytosis)

Vesicular transport bringing matter into the cell using motor proteins that use ATP

Phagocytosis - “cell eating” process of engulfing partiles like bacteria, dust, and cellular debris using phagosomes. Macophages phagocytize 25% of their own volume per hour. Occurs only in some cells

Pinocytosis - cell drinking process of taking in droplet of EFC containing molecules the cell can use. Occurs in all cells.

Receptor mediated endocytosis - Selective form of phago or pino cytosis. Cells can take in specific molecules from the ECF with minimum of unneeded matter.

Exocytosis

Releasing matter from the cell (EXIT) using motor proteins that use ATP