APHY 201 Module 01 Membrane Transport

molecules using membrane transport

Ability to pass through membrane determined by lipid solubility, solute size and shape, and electrical charge. Uncharged particles preferred to cations (+) preferred to anions (-). Large, non-lipid soluble, and negatively charged (anions) least likely to diffuse into cells.

How is selective permeability of a cell determined?

By number and types of carriers and channels available. If the cell wants more/less of a substance, then the cell produces more/less of the specific proteins (carriers or channels) to move it into or out of the cell.

carrier mediated transport characteristics

Each solute requires a specific carrier. Does not require energy. Faster than non-carrier mediated diffusion. Reaches a maximum rate when all the sites are full (saturated).

the concentration gradient of what ion contributes the most to membrane potential?

K+

net diffusion

Measurable difference of particles from an area of greater concentration to an area of lesser concentration. Occurs until the concentration of the diffusing substance is uniform (diffusion equilibrium), then net diffusion ceases and simple diffusion continues. Collisions cause molecules to move down or with their concentration gradient.

vesicle

Membranous sac of large particles, macromolecules, and fluids formed by a portion of membrane pinching off from organelles or cell membrane

osmosis

Movement of solvent (e.g., water) across selectively permeable membrane; water can move through an aquaporin or plasma membrane

carrier saturation

Occurs when all carriers are engaged (transport maximum). Rate of diffusion based on number of binding sites available for transport. More carriers = faster transport

fluid mosaic model of plasma membrane

Polar, hydrophilic heads and nonpolar, hydrophobic tails of phospholipids arrange with embedded proteins and cholesterol into a phospholipid bilayer to form the plasma membrane of the cell. Phospholipids and membrane proteins can move laterally within the membrane unless held by the cytoskeleton.

simple diffusion

Substances diffuse using continual random movement of particles among one another in a liquid or gas. Uniform spreading out of molecules due to their random intermingling and molecular motion. E.g., oxygen, carbon dioxide, lipids, vitamins.

electrochemical gradient

Tendency of an electrically charged solute, such as K+, to move across the membrane arises from three factors: the difference in the concentration of the solute the charge of the solute molecule the difference in voltage between the two sides of the membrane

Water moves by osmosis if there is a difference in water concentration (osmotic pressure) until?

Water moves by osmosis until hydrostatic pressure (back pressure of water on membrane) and osmotic pressure (tendency of water to move into cell by osmosis) equalize.

plasma membrane

a phospholipid bilayer that creates a barrier between the inside of a cell (intracellular fluid) and the exterior (extracellular fluid); acts in transporting substances into or out of the cell; externally facing proteins acts as receptors, transport proteins, and in cell-to-cell recognition

interstitial fluid

a solution that bathes tissue cells; it is the main component of extracellular fluid; it is found in the interstitial spaces (spaces between cells); contains thousands of substances, e.g., amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products

tonicity

ability of solution to alter cell's water volume

vesicular transport

active transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles; requires cellular energy (e.g., ATP)

hydrostatic pressure

back pressure of water on membrane due to the force of gravity

exocytosis

cellular process where cells eject waste products or chemical transmitters (such as hormones) from the interior of the cell

facilitated diffusion

certain hydrophilic molecules (e.g., glucose, amino acids, and ions) transported passively using specific membrane proteins - carriers or channels.

secondary active transport

cotransport driven by the concentration gradient created by primary active transport; indirectly requires ATP; only functions when Na-K pump is also functioning

membrane potential

difference in electric potential between the interior and the exterior of a biological cell. With respect to the exterior of the cell, typical values of membrane potential range from -40 mV to -80 mV.

voltage

difference in electric potential energy between two points (for cells, opposite charged ions separated by the cell membrane)

types of passive transport

diffusion (includes simple diffusion, carrier- and channel-mediated facilitated diffusion, osmosis) and filtration, dialysis

How does the RMP come about, and how is it maintained?

diffusion of ionic imbalances that polarize the membrane, and active transport processes maintain the membrane potential

active membrane transport

energy (ATP) required; occurs only in living cell membranes; substance moves up or against its concentration or pressure gradient; utilizes pumps and vesicular transport

primary active transport

energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane; Na-K pump

symport

integral membrane protein that is involved in the transport of many differing types of molecules across the cell membrane; a type of cotransporter Ex: symporter works in the plasma membrane and molecules like glucose are transported across the cell membrane at the same time as sodium (Na)

Na-K pump

maintains the resting membrane potential of cells; Moves 3 Na out and 2 K into the cell while utilizing 1 ATP with each pump.

secretory vesicles

membrane bound bubbles that carry chemical substances like lipids and proteins to the cell membrane where they are released or secreted from the cell

channel

membrane protein with a pore for substrates to pass through; mostly ions selected on basis of size and charge. Leakage channels (always open); Gated channels (controlled by chemical or electrical signals).

Additional factors that affect rates of diffusion

molecular configuration (shape), ability of diffusing substance to dissolve in lipids, and molecular charge (if present) on the diffusing solute. Large, non-lipid soluble, and negatively charged least likely to diffuse.

passive membrane transport

no cellular energy (ATP) required; substance moves down its concentration or pressure gradient

When does osmosis occur?

occurs when water concentration is different on the two sides of a membrane

carrier

protein carrier specific for one chemical; binding of substrate causes transport protein to change shape

hypertonic

solution with higher non-penetrating solute concentration than cytosol; cell volume shrinks due to water leaving the cell by osmosis

hypotonic

solution with lower non-penetrating solute concentration than cytosol; cell volume increases causing cell to swell and burst as water continues to flow in by osmosis

isotonic

solution with same non-penetrating solute concentration as cytosol; cell volume stays the same; equivalent to 0.9% NaCl solution

selectively permeable

some molecules pass through easily; some do not (impermable); selectivity due to polarity, molecular size, and gates/channels/carriers

polarized

state of a plasma membrane of an unstimulated neuron or muscle cell in which the inside of the cell is relatively negative in comparison to the outside

osmotic pressure

tendency of water to move into cell by osmosis

endocytosis

transport into cell; includes phagocytosis, pinocytosis, receptor-mediated endocytosis

functions of plasma membrane proteins (6)

transport, receptors for signal transduction, attachment to the cytoskeleton and extracellular matrix, enzymatic activity, intercellular joining, cell-to-cell recognition

types active transport processes

two types of active processes (active transport and vesicular transport); both require ATP to move solutes across a living plasma membrane because solute too large for channels or not lipid soluble or not able to move down concentration gradient

receptor-mediated endocytosis

type of endocytosis; allows specific endocytosis and transcytosis; cells use to concentrate materials in limited supply; uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins

pinocytosis

type of endocytosis; cell drinking; process by which a cell takes in liquid (ECF) from the surrounding environment; requires ATP

phagocytosis

type of endocytosis; cell eating; process in which extensions of cytoplasm surround and engulf large particles and take them into the cell; requires ATP

resting membrane potential

voltage that exists across the plasma membrane during the resting state of an excitable cell

aquaporin

water specific channel; allows water to move rapidly down its osmotic gradient

concentration gradient

Difference in the concentration of solutes in solution across the membrane. ECF Na+ = 136-145 mmol/L, ICF Na+ = 12 mmol/L