transport across cell membrane

cell biology exam 3

simple diffusion

the movement of molecules from an area where they are more concentrated to an area where they are less concentrated without help from any additional molecules

facilitated transport

movement of molecules across a membrane with assistance from a transport protein

moves slowly across membranes

large molecules, polar molecules, charged molecules, hydrophilic molecules

moves more rapidly across membranes

small molecules, nonpolar molecules, neutral molecules, hydrophobic molecules

membrane potential

small electrical imbalances at the membrane create a voltage difference

• potential energy

• critical for the function of electrically excitable cells such as neurons

• helps move some ions across the membrane

electrochemical gradient

driving force that determines which way an ion will move across a membrane

transport protein function

• facilitate the movement of select substances across cell membranes

• each protein allows only 1 or a few molecules to pass

passive transport

doesn’t require energy

• direction depends on concentration

• molecules flow from areas of higher concentration to areas of lower concentration

active transport

requires energy

• moves a solute “uphill” against its concentration gradient

• carried out by special types of transporters called pumps

aquaporins

specialized channels that facilitate the flow of water

• found in cells that move large volumes of fluid

osmolarity

the total solute concentration inside a cell

• usually higher than the environment (more solute inside cell than outside)

osmosis

the movement of water from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration)

• water tends to move inside the cell

• balance is important for cells, and different cells cope in different ways

pumps

actively transport a solute against its electrochemical gradient

• 3 types of pumps

gradient driven pump

• exploit solute gradients to mediate active transport

• use potential energy from the electrochemical gradient

• electrochemical Na+ gradient drives the transport of glucose across the plasma membrane of animal cells

Na+ - K+ ATPase

• uses ATP to pump sodium out of the cell and potassium into the cell

Na+/ K pump

• in animal cells uses energy supplied by ATP to expel Na+ and bring in K+

• generates a steep concentration gradient of Na+ across the plasma membrane

Calcium pumps

keep the cytosolic Ca²⁺ concentration low

• calcium can bind to proteins and alter their activity, so it’s important to keep intracellular calcium

• calcium influx is also used as a cellular signal, so it’s important to keep intracellular calcium low

• ATPases that work similarly to the Na/K pump

• ER membrane also has calcium pumps that pump calcium into the ER

potassium leak channels

allow potassium to slowly leak out of the cell, creating additional loss of positive charge inside the cell

ion channels

• created a hydrophilic pore in the membrane to allow water soluble molecules through

• channels are very specific for certain molecules

• movement through channels is passive and the molecule moves down it’s concentration gradient

gated ion channels

• can open and close

• frequently do this random

• can also be stimulated to open or close by different signals

stimuli that inflience opening and closing of gated channels

3 types gates

• mechanically gated

• voltage gated

• ligand gated

action potnential

allow long distance communication along nerve cells

• mediated by voltage gated channels

voltage gated calcium channels

• covert electrical signals to chemical signals