Transport across membranes Flashcards
What is the definition of diffusion?
The net movement of particles from a region of higher concentration to a region of lower concentration
Is diffusion active or passive?
Passive, meaning it will occur without need for energy
Why does diffusion occur?
Because particles have kinetic energy (are moving), and an unequal distribution of particles will eventually become an equal distribution (reach equilibrium)
Once diffusion equilibrium is reached, what happens to the particles?
The particles continue to move, however the movement are equal in both directions resulting in no net change
What is the trend between diffusion distance and rate of diffusion and why?
As diffusion distance increases, rate of diffusion will decrease, as the number of collisions between particles will increase
How does temperature impact rate of diffusion and why?
As temperature increases, rate of diffusion also increases as particles have more kinetic energy so move more quickly
How does concentration difference impact rate of diffusion and why?
The greater the difference in concentration between two regions the faster the rate of diffusion, as the overall movement from a higher to lower concentration will be larger
What is simple diffusion?
Diffusion in the absence of barriers or membranes
How does polarity affect ability to diffuse through membranes?
Non-polar molecules (such as O2) can diffuse freely through membranes, whereas charged/polar species struggle to diffuse freely
Why do charged and polar species struggle to diffuse through membranes?
As the hydrophobic interior of the membrane repels charged/polar species
Why are membranes described as partially permeable?
As some species diffuse freely, whereas others (like ions and polar molecules) cannot easily diffuse
How does surface area of a membrane impact rate of diffusion?
The larger the surface area, the higher the rate of diffusion
How does thickness of membrane impact rate of diffusion?
The thicker the membrane (the greater the diffusion distance), the slower the rate of diffusion
What feature of membranes allow ions/polar molecules to pass through?
Membranes contain transport proteins, which polar/charge molecules can use to pass through the membrane
What is diffusion across a membrane using transport proteins known as?
Facilitated diffusion
Why are membranes with transport proteins known as selectively permeable?
As most transport proteins are specific to one molecule or ion
What polar molecule can freely pass through membranes and why?
Water, despite being polar, can pass through the membrane without facilitated diffusion, as it is so small
What are the two types of transport proteins?
Carrier and channel proteins
How do channel proteins allow ions and polar molecules to pass through?
Channel proteins provide a hydrophilic channel that ions and polar molecules are able to transport through down a concentration gradient
How do carrier proteins allow ions and polar molecules to pass through?
Carrier proteins change shape when a particular molecule binds to them, allowing the species to pass through
Does facilitated diffusion require energy?
Both diffuse and facilitated diffusion are passive processes, meaning they do not require energy
What is the definition of active transport?
The movement of particles from a region of lower concentration to a region of higher concentration
What does active transport into a cell require?
Energy and carrier proteins
What is the first step of active transport into a cell via carrier proteins?
The molecule/ion to be transported binds to receptors of the carrier protein on the outside of the cell
What is the second step of active transport into a cell via carrier proteins, after the molecule has bound to the carrier protein?
On the inside of the cell, ATP binds to the carrier protein and is hydrolysed into ADP and a phosphate ion
What is the third step of active transport into a cell via carrier proteins, after ATP has been hydrolysed?
The phosphate ion from the ATP binds to the carrier protein, causing the protein to change shape and open to allow the molecules to enter
What is the fourth step of active transport into a cell via carrier proteins, after the molecules have entered the carrier protein?
The phosphate ion is released and recombines with ADP to form ATP, and consequently the protein returns to its original shape, allowing the molecules to enter
What is the definition of osmosis?
The diffusion of water across a partially permeable membrane, from a region of higher water potential to lower water potential
What is water potential?
The pressure exerted by water molecules as they collide with a membrane or container
What is the highest water potential possible?
0kPa is the water potential of pure water, and so is the highest possible water potential. All other water potential values are negative
What is crenation?
Where animal cells are placed in solutions of lower water potentials, they will lose water to the solution via osmosis, and will appear to shrink/shrivel
What is cytolysis?
Where animal cells are placed in solutions of higher water potentials, they will gain water from the solution via osmosis. This will increase the hydrostatic pressure of the cell, causing the cell to stretch and eventually burst
What is turgor pressure?
The pressure of plant cell surface membranes against the cell wall
What happens to plants cells when they are placed in solutions of higher water potential?
Water will enter the cell until the turgor pressure is high enough so no more water can enter
Why do plant cells tend to not burst?
As they have strong and rigid cell walls
What is plasmolysis?
Where plant cells are placed in solutions of lower water potential, causing water to leave via osmosis and the cell surface membrane to peel away from the cell wall
