diffusion Flashcards
why do cells need to receive raw material
for the biochemical reactions and processes that go on in cells to sustain life
what do cells make when they respire
ATP
what does ATP provide
cellular energy to drive biochemical reactions
what do cells need to respire
they need oxygen and glucose and need to remove the toxic metabolic ewaste products such as carbon dioxide.
what else do cells need to do apart from respiring
to export some of the molecules they make, such as enzymes, hormones or other signalling molecules
what does active and passive transport refer to
the exchange of substances between cells and their environment, or between membrane bound compartments within cells and the cell cytosol
what does all movement require
energy usually in the form of ATP
what does passive movement utilise
passive transport utilises energy from the natural motion of particles rather than the energy from another source. they do not use ATP
what is diffusion
the net movement of particles (atoms, molecules or ions) from a region of high concentration to a region of low concentration
what kind of process is diffusion
a passive process
at what point does diffusion continue to occur
until there is a concentration equilibrium between the two areas
why does diffusion happen
the particles in a gas or liquid have kinetic energy. this movement is random and an unequal distribution of particles eventually becomes an equal distribution.
what does equilibrium actually mean
it doesnt mean the particles stop moving, the movement is just equal in both directions
why is diffusion fast over short distances
particles move at high speeds which are constantly colliding which slows down their overall movement. so diffusion is faster when the pathway is shorter. as diffusion distance increases, the rate of diffusion slows down because more collisions take place
what is a concentration gradient
a difference in concentration. it goes from high to a low concentration
where does diffusion take place along the concentration gradient
DOWN the concentration gradient
what acts as a barrier in the membrane
the non polar, hydrophobic tails of phospholipid molecules.
if a molecule is small and less polar what does this mean
it will diffuse across the membrane easier and faster
when does simple diffusion take place
for small non polar molecules.
what types of molecules can easily pass through the lipid bilayer
small non polar molecules in a high concentration on one side of the membrane can easily pass through the bilayer. lipid soluble molecules and very small molecules can diffuse directly through the bilayer
give examples of molecules can pass through cell membranes in simple diffusion
oxygen and carbon dioxide
how do fat soluble molecules such as steroid hormones diffuse through cell membrane
even if they are larger, they diffuse through cell membranes as they dissolve in the lipid bilayer. they still move down their concentration gradient.
is water soluble or insoluble in lipids
insoluble because they are polar
what is the name of the specific water channel protein
aquaporins
what is the role of aquaporins
in membranes where a very high rate of water movements are required aquaporins allow water molecules to cross the membrane
which molecules diffuse freely down the concentration gradient
non polar molecules
what does the hydrophobic interior of the membrane do
repels substance with a positive or negative charge so they can’t easily pass through.
how fast do polar molecules diffuse through
polar molecules with a slightly negative or slightly positive charge diffuse through at a slow rate
which size molecules diffuse through faster
smaller molecules diffuse faster than larger molecules
how is the concentration gradient maintained
molecules entering cells pass into organelles and then used for metabolic reactions. this maintains the concentration gradient and keeps more of the molecules entering the cell.
give examples of how the concentration gradient is maintained
oxygen diffusing into the cytoplasm of respiring cells then diffuse into mitochondria and used for aerobic respiration
carbon dioxide diffusing into the palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and used for photosynthesis
how is the rate of simple diffusion affecting
simple diffusion relies on the molecules own kinetic energy. so factors altering the kinetic energy will affect the rate of diffusion
how does temperature affect the rate of diffusion
if temperature increases, molecules have more kinetic energy so rate of diffusion increases. as they lose heat, diffusion slows down
how does the distance of diffusion affect the rate of diffusion
the thicker the membrane across which molecules diffuse, the slower the rate of diffusion
how does surface area affect the rate of diffusion
more diffusion takes place across a larger surface area. cells specialised for absorption have extensions have microvilli which increase surface area
how does the size of the diffusing molecules affect the rate of diffusion
smaller ions or molecules diffuse more rapidly than large molecules
how does the concentration gradient affect the rate of diffusion
the steeper the gradient, the faster the diffusion down the gradient.
what is the phospholipid bilayer a barrier to
polar molecules and ions
what do membranes contain that allow polar substances to pass through
channel proteins
what is facilitated diffusion
diffusion across a membrane which uses channel proteins
how do small ions diffuse through facillitated diffusion
through channel proteins embedded within the cell membrane. some of these open or close in response to a specific messenger or a change in voltage across the membrane
what is special about membranes with protein channels
they are selectively permeable as most protein channels are specific to one molecule or ion.
what else can facilitated diffusion involve
carrier proteins
what is special about carrier proteins
they can change shape when a specific molecule binds
what is similar about simple and facilitated diffusion
they both occur down the concentration gradient
what does the rate of facilitated diffusion depend on
temperature, concentration gradient, membrane surface area and thickness but also affected by the number of channel proteins present
how does the number of channel proteins affect facilitated diffusion
the more protein channels, the higher the rate of diffusion
what do cholesterol molecules within the membrane do
they reduce the permeability of the membranes to small water-soluble molecules.
how do glucose molecules pass through the membrane
glucose molecules are too large to diffuse through the water filled protein channel in a membrane but they can bind to a transmembrane carrier protein which then opens to allow the glucose to pass out on the other side of the membrane
how are cells able to control the types of molecules that pass in or out
different cell types have membranes with differing proportions of transmembrane protein channels and carrier proteins.
describe the mechanism of diffusion for lipid soluble molecules
- lipid soluble molecules diffuse through the plasma membrane from a region of high concentration outside the cell to a region of low concentration in the cytoplasm until they reach equilibrium
describe the mechanism of diffusion for lipid insoluble solutes
- lipid insoluble solutes (such as sugars or amino acids) can’t diffuse freely through the lipid bilayer due to their size or polarity. the carrier molecule binds with a molecule, such as glucose on the outside of the cell membrane. the carrier molecules changes shape and releases the molecule on the inside of the cell membrane
describe the mechanism for diffusion for small lipid insoluble molecules
they travel through channel proteins. these open or close in response to a specific messenger. these are gated channel proteins
describe the mechanism of diffusion for the transport of water
through aquaporins which are channel proteins. they travel by diffusion or osmosis.
how does diffusion occur in neurones
neurone plasma membranes have many channels specific to either sodium or potassium ions. the diffusion of these ions into and out of the neurone axon is crucial for the conduction of nerve impulses. at synapses, there are also calcium ion channels and chloride ion channels.
how does diffusion occur in epithelial cells
the plasma membranes of epithelial cells that line your airways have chloride ion channels and these play a crucial role in regulating the composition of mucus to trap pathogens and particles.
how is the rate of diffusion calculated
using Ficks law
surface area x difference in concentration/length of diffusion path (membrane thickness)
