Functions and importance of cell membranes Flashcards
Diffusion
The process of random movement of molecules from an area of high concentration to an area of low concentration (a concentration gradient)
Diffusion Across A Membrane
Membranes act as a natural barrier to diffusion of some molecules
Cell membranes are selectively permeable
Some solutes will move through the membrane, whereas others will be prevented from crossing it
Molecules that can move across the membrane will do so until their concentrations are equal on both sides of the membrane
3 ways that molecules move across membrane
- simple diffusion
- facilitated diffusion
- active transport
Simple diffusion
- small + nonpolar substances move across a membrane UNASSISTED
- small tunnels in membrane open
Some simple diffusion occurs through integral membrane proteins that allow ions and water to diffuse into and out of the cell
Facilitated diffusion
Solute molecules cannot diffuse through the membrane on their own
Combine with carrier molecules in the membrane that change shape (i.e. alter their tertiary or quaternary structure) to allow the solute to pass into or out of the cell
These carrier molecules may be integral membrane proteins
Some allow bi-directional diffusion, others only allow diffusion in one direction (uniports)
Usually used for larger molecules and hydrophilic molecules that cannot cross the phospholipid bilayer of the membrane by themselves
Passive vs. Active transport
passive
* no energy expended
* high concentraton –> low concentration
active
* energy required
* low concentration –> high
this is why energy is needed, as it is hard to move things AGAINST the concentration gradient, shhhhhhh
Active transport: primary
Requires energy from ATP in order to force the solutes to move against the concentration gradient
Ex. The Na+/K+ Pump
describe the process of primary active transport
- protien in membrane binds w intracellular sodium (inside the cell)
- atp phosphorylates bind w the soduim already bonded
- sodium is like screw you and goes OUT the cell the phosphor causes the protien shape to change, this is how the sodium is released
- extracellular (outside) potassium binds to the exposed sites
- ATP just leaves bc they need that roster yk hehe
- this causes the protein to change, releasing potassium INTO the cell
- repeat!
Secondary active transport
Requires the movement of other solutes with the concentration gradient to provide the energy needed to “drive” the transport of other molecules against the concentration gradient
Coupled transport systems – both solutes are required to move across the membrane
Symport: both solutes move in the same direction
Antiport: solutes move in opposite directions
Ex. Na+ diffusion from the extracellular space into the cytoplasm through an integral membrane protein is required to move glucose against its concentration gradient into the cytoplasm through the same protein
What is osmosis?
Passive transport
Movement of water through membranes
Occurs when water can move through a membrane, but the solute cannot. Thus water moves from a low concentration of solute to a high concentration of solute
This can also be stated as: water moves from an area of high concentration (of water) to an area of low concentration (of water)
main idea: equalizes the concentration by adding more water instead of moving particles like diffusion
Isotonic
If there is an equal concentration solutes on both sides of the membrane, the solution is isotonic
Water does not move in or out of the cell
stays the same
hypertonic solution
If there is an unequal concentration of solutes on both sides of the membrane, the solution with the higher concentration of solutes is the hypertonic solution
Water will move into the hypertonic solution.
If red blood cells are hypertonic to the environment around them, water will move into the cells and they will burst
If the environment around red blood cells is hypertonic to the cells, water will move out of the cells and they will shrink and shrivel
hypotinic
If there is an unequal concentration solutes on both sides of the membrane, the solution with the lower concentration of solutes is the hypotonic solution
Water will move out of the hypotonic solution.
If red blood cells are hypotonic to the environment around them, water will move out of the cells and they will shrink and shrivel
If the environment around red blood cells is hypotonic to the cells, water will move into the cells and they will burst
Large Molecules and Food Particles
Large molecules and food particles are ingested by endocytosis
Cell surface invaginates (folds to make a small pocket) that is lined by the cell membrane
Invagination continues until the two ends of the cell membrane fuse and a vesicle is formed
If the contents are food, the vesicle is fused with a lysosome and its contents are digested
Types of Endocytosis
Pinocytosis: cell drinking
Tiny, liquid-containing vesicles are formed
Phagocytosis: cell eating
Particles or entire cells are engulfed and trapped in large vesicles
Immune system (i.e. phagocytes, macrophages) uses phagocytosis to defend the body against foreign cells
Receptor Mediated
receptors on the cell surface interact with particles outside the cell, causing endocytosis
exocytosis
Opposite process to endocytosis
Cellular vesicle fuses to the membrane and expels its contents outside the cell
Method for getting rid of waste or delivering needed macromolecules to the extracellular fluid
