Diffusion and osmosis Flashcards
How permeable is the Plasma membrane?
It is selectively permeable, meaning only some molecules can pass freely across the membrane
Movement of a molecule across the plasma membrane depends on its size and if it can pass through the lipid core of the membrane
Gases (CO2 and O2), hydrophobic molecules (benzene) and small polar molecules (H2O and Ethanol) can pass through. Large polar molecules (Glucose and Amino acids) and charged molecules/ions (H+, Cl-, Na+ and Ca2+) cannot pass through
What is a concentration gradient?
Difference in concentration across two areas. The greater the difference, the greater the gradient
What are the two kinds of transport across the Plasma membrane, and what do they encompass?
Passive transport
- Does not require cellular energy
- Occurs down the gradient (high to low conc.)
- Simple diffusion, facilitated diffusion (channels and carriers) and osmosis
Active transport
- Requires cellular energy (ATP)
- Occurs against the gradient (low to high conc.)
- Primary active transport, secondary active transport and vesicular transport (endocytosis and exocytosis)
What is Diffusion?
The movement of molecules from an area of high concentration to an area or low concentration (down the gradient)
Molecules are in a constant random motion and collins cause the molecules to spread out
The molecules will eventually reach an equilibrium
What factors affect the rate of Diffusion?
- Size of the concentration gradient (bigger gradient = faster diffusion)
- Membrane surface area (bigger surface area = faster diffusion)
- Size of the molecule (small molecules diffuse more quickly than larger molecules)
- Diffusion distance (decreasing diffusion distance = increasing diffusion rate)
- Lipid solubility of the molecule (hydrophobic molecules move across the membrane more easily)
What is Simple diffusion?
Molecules that are small and/or hydrophobic can diffuse through the plasma membrane unaided, such as CO2 or O2
What is Facilitated diffusion?
Molecules that are large or hydrophilic can only diffuse across the plasma membrane through carriers or channel (‘facilitated’)
Carriers interact with the molecule and move them to the other side, and channels act like tunnels through the membrane
What is Osmosis?
The diffusion of water across a selectively permeable membrane
The movement of water from an area of high water concentration to an area of low concentration
The movement of water from an area of low solute concentration to an area of high solute concentration
What are the different kinds of Solutions and solutes
Solution is a combination of a solute and solvent
Solvent is a liquid that substances dissolve into (usually water in biology)
Solute is a substance that dissolves in liquids
Penetrating solutes can cross the plasma membrane, whereas non-penetrating solutes can’t
What happens if the membrane is permeable to solutes and water?
Both can move until they reach an equilibrium. The amount of water and solutes is balanced across the membrane
What happens if the membrane is impermeable to solutes, but permeable to water?
The water will diffuse across the membrane (osmosis), but the solutes won’t. The amount of water becomes unbalanced across the membrane
What is Osmolarity?
The number of solute particles per litre of solution
Biological solutions are measured in milliosmoles/litre (mOsm/L)
Most body fluids have a particular concentration, usually 300 mOsm/L
What are the two fluid compartments in the body?
ICF and ECF (plasma and interstitial fluid)
ECF accounts for ⅓, and ICF ⅔
What is Tonicity?
The ability of a solution to change the shape (tone) of a cell by changing the cell’s internal water volume (via osmosis)
This depends on the concentration of non-penetrating solutes on either side of the cell membrane
What is Isotonic?
When the ECF concentration is the same as ICF
Water will not flow in or out of the cell, so the shape and volume of the cell will stay the same
