Cell transport

crosses bilayer on its own

• gases

• small uncharged polar molecules

does not cross bilayer on its own

• large uncharged polar molecules (needs transport protein)

• Ions (need channel or pump)

• charged polar molecules (they cant cross, must stay in)

simple diffussion

passive transport

molecule dissolves in the lipid and passes through down the gradient

no energy, no protein needed

facilitated diffusion

passive transport

there is a protein that decided what to allow, no energy

channel

open pore

• aquaporins, Na+/K+ channels

carrier

binds and changes shape

• GLUT transporters for glucose

pump

spends energy to go up hill in the concentration gradient

active transport

osmosis

the movement of water across a semipermeable membrane, from where solute is dilute to where it is concentrated. Nothing pushes the water. It simply diffuses down its own gradient.

osmotic pressure

The more particles in solution, the higher the osmotic pressure — and it is the NUMBER of particles that counts, not what they are.

osmolarity

A measurement. The total number of dissolved particles per liter, whatever they are.

Plasma sits at roughly 290–310 mOsm/L in the domestic species.

tonicity

A prediction. What a solution will DO to a cell placed in it.

It counts only the particles that cannot cross the membrane — because only those hold water where they are.

isotonic

cells keep its normal shape

0.9% NaCl

Water always moves towards the higher solute concentration. Decide which side that is and the direction, and the consequence, follow automatically.

hypotonic

cell swells then bursts- hemolysis

pure water

Water always moves towards the higher solute concentration. Decide which side that is and the direction, and the consequence, follow automatically.

hypertonic

cells shrinks and crenates

7.2% NaCl

Water always moves towards the higher solute concentration. Decide which side that is and the direction, and the consequence, follow automatically.

primary active transporters

• ATP driven

• NA+/K+ ATPase

Secondary active transporters

•  SGLT1 & SGLT2 (Sodium-Glucose Cotransporters)

Na⁺ and glucose bind the same carrier. Na⁺ falls down its gradient and drags glucose up against its own.

Na/K pump

3 Na out, 2 K in, for every ATP (remember Nokia 321 mnemonic)

• uphill (costs energy)

primary vs secondary active transport

primary spends ATP directly (Na+/K+ pump)

secondary spends a gradient that ATP built earlier

exocytosis

out of the cell

both consume energy

• constitutive

• regulated

constitutive exocytosis

• default

• A steady stream of vesicles leaves the trans Golgi network in every cell, all the time. Delivers new membrane and secretes whatever is in the vesicle.

regulated exocytosis

Cargo is stored in secretory vesicles and released only on a signal/demand.

Hormones, neurotransmitters, digestive enzymes, mast cell granules.

endocytosis

into the cell

both consume energy

• phagocytosis

• pinocytosis

• receptor-mediated

phagocytosis

• endocytosis

• cell eating, large particles.

• triggered not constant

pinocytosis

Cell drinking. Small vesicles, ~100 nm, taking in fluid and whatever is dissolved in it. Continuous, in every cell

receptor mediated endocytosis

Selective. Cargo binds a specific receptor, concentrates in a clathrin-coated pit, and comes in enriched.

transcytosis

combination of endocytosis (cell takes extracellular cargo), intracellular transport (vescicle moves and carries cargo inside cell) and exocytosis (vesicle reaches the opposite of the cell

0.9% NaCl

isotonic (tonicity in patient)- volumen replacement, stays in extracellular space

LRS

slightly hypotonic

• balanced maintenance and replacement fluid

5% dextrose in water

effectively hypotonic - not volume expander

7.5% NaCl

strongly hypertonic - small volume resucitation - pulls water into vasculature fast