Transport across biological membranes

passive diffusion

small uncharged molecules diffuse across bilayer membrane down concentration gradient

diffusion occurs until molecules are randomly distributed on both sides of the membrane

facilitated diffusion

Specific integral membrane proteins transport solutes down a concentration gradient

allows polar and charged molecules to rapidly pass through the membrane without contacting the hydrophobic bilayer

active transport

pumping of ions or other charged or polar molecules AGAINST the gradient

this is endergonic and must be coupled to an exergonic reaction

uses transport protiens either pumps or cotransporters

osmosis

form of passive diffusion involving water

water moves to higher concentration of solute to make it even amount of concentration

aquaporin

protein channels in cell membranes that facilitate the transport of water across cell membranes, allowing for efficient water movement in and out of cells

hypotonic

the solution outside the cell has less solute (like salt) and more water compared to the inside of the cell

isotonic

Outside the cell: Same solute concentration as inside the cell

hypertonic

Outside the cell: Higher solute concentration, less water

ion channel

integral membrane proteins from a pore Ion able to go down concentration gradient

channels open or close in response to signals

carrier proteins

integral membrane protein bind specific solute on one side of membrane, structure of protein changes as it carries solute across the membrane

glucose uniport

an example of a carrier protein that carriers glucose into the cell

pump

a pump proteins in the membrane uses energy from ATP to move molecules

this energy allows the protein to push molecules across the membrane

co-transporter

move two substances at once across a membrane

They use the energy stored in a gradient of one molecule (like H⁺ ions) to help move another molecule (like sucrose) against its own gradient — without directly using ATP

sodium-potassium ATPase

it pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell using 1 ATP

This keeps sodium high outside and potassium high inside the cell

Super important for nerve signals, muscle contractions, and maintaining cell volume

exocytosis

fusion of membrane-bound vesicles with the plasma membrane

vesicle merges with the membrane wall to release proteins into cell

endocytosis

uptake of extracellular material in the membrane-bound vesicles

membrane wall uptakes proteins and detaches from wall

2 types

phagocytosis

1 type of endocytosis

cell engulfs a particle (celling eating)

pinocytosis

1 type of pinocytosis

cell takes up external fluid with dissolved solutes (cell drinking)

receptor-meditated endocytosis

It’s a specific and efficient way cells bring in large or important molecules (like hormones, cholesterol, or vitamins) from the outside

describe how the size, polarity, and charge of solutes influences their ability to diffuse across pure lipid bilayers

if small and uncharged, able to just pass through, if polar or charged molecules able to pass through with proteins with facilitated diffusion, if ions, charged or polar molecules can pass through active transport which uses energy to move them accross

compare and contrast passive diffusion, facilitated diffusion and active transport with reference to the roles of energy and transport proteins in these processes

both passive and facilitated diffusion don’t require energy, but active transport requires energy

faciltiated and active transport can do both polar molecules but passive can only do nonpolar

describe how the concentration of solutes on either side of membrane influences the movement of water via osmosis

water will move to the side with higher concentration to even the concentration.

compare and contrast the roles of carrier proteins and channels in facilitated diffusion

both carrier polar charged molecules across, ion channels open from signaling, letting the molecules go through while the carrier proteins carry them by binding to the specific molecule across

compare and contrast the roles of co-transporters and pumps in active transport

both use energy to move molecules against the gradient across pumps use ATP as energy to pump cotransporter uses indirect ATP as one goes across another goes

explain how the sodium-potassium ATPase maintains the electrical potential across the plasma membranes

pumps out 3 Na+ molecules and pumps 2K+ into the cell he inside of the cell becomes more negative compared to the outside.

This difference in charge across the membrane is called the membrane potential

briefly describe the functions of exocytosis and endocytosis

exocytosis takes material from the cell and endocytosis merges with the cell bringing material into the cell

compare and contrast pinocytosis, phagocytosis, and receptor-mediated endocytosis

pino- cell eating takes up large particles

phag - cell drinks just takes extra fluid

receptor - Cell selectively engulfs specific molecules by using receptors on its surface