lecture 15 Flashcards
Membrane Transport
what is the need for substance transport across membranes?
needed for nutrients, waste, cell signaling
what can freely diffuse across the membranes?
only very small polar molecules like water and nitric oxide and lipophilic molecules can passively diffuse through the lipid bilayer; other molecules require a transport system to cross the bilayer
how do substances generally flow?
from high concentration to low concentration
how do non polar substances pass through the bilayers?
they directly and passively
how do polar or charged substances move across the membrane?
through facilitated diffusion which requires a transport protein
what happens when you need to transport substances against the electrochemical gradient?
energy is required and an active transport is needed, we see this through ATP hydrolysis and light energy
how are facilitated and simple transport common?
both are passive, meaning that no energy is required to move the particle across the membrane. Concentration gradient drives the transfer.
how can we define simple diffusion?
Simple diffusion is defined as molecules passing through the membrane without energy input and without a specific transporter protein. In the figure above, two examples of simple diffusion are shown. The truest form of simple diffusion is the substance passing through the actual membrane.
how do channel proteins work?
This channel is just a pore through which many things can pass. It is a hole in the membrane to allow substances through. The channel doesn’t really do anything but provide a way through the membrane. It can be opened or closed, but it does not participate otherwise. It cannot be saturated
how does facilitated diffusion work?
the channel is a participant in the transport process. It will bind specific molecules or ions, thus providing selectivity. The molecule will pass through the channel and have multiple interactions with the channel on the way through. However, no energy is required to move the substance, so the transport is still passive. Some may argue that the pore protein is also facilitated diffusion, saying that the substance transported could not pass in the absence of the pore. This is true, but because the pore does not play any role in the transport process apart from making a hole in the membrane, it is classified as simple diffusion.
what is sodium concentration like inside and outside the cell?
Na+ concentration is high outside cells but low inside cells.
what is potassium concentration like inside and outside the cell?
K+ concentration is high inside cells but low outside cells.
what is calcium concentration like inside and outside the cell?
Ca2+ concentration is high outside cells, and very low inside cells.
what is chlorine concentration like inside and outside the cell?
Cl- is high outside cells but low inside cells.
why is keeping note of these ion concentrations important?
These ions affect the osmolarity of these fluids (which is balanced in normal homeostais), which in turn affects water flow into and out of cells.
what is endocytosis?
when the membrane recognizes something on the outside, like proteins or larger molecules, and so they are enclosed by a vesicle and then brought in, opposite in exocytosis
what is the chemical potential?
high concentration on one side of the membrane and a low concentration on the inside and if its simple diffusion then the high concentration will push and molecules will go across the membrane
considering the equation, ΔG = RT ln { [A]in / [A]out} and [A]in / [A]out < 1, what does it generally mean if outside number is a big number and inside is a small number? what does this mean for the log function? name example
you get a number less than one, and so this means the ΔG is negative = spontaneous; Recall that for log functions, for ln(x), if 0 < x < 1, the solution to ln(x) will be negative. If x > 1, the solution to ln(x) will be positive; we see this with CO2 diffusing across the red blood cells because the CO2 is being consumed and carbonic acid is being produced which continues to drive the gradient
how does an electrochemical potential gradient develop?
for example, if we take unequal concentrations of potassium then potassium ions will move from areas of higher to lower concentration and this is seen as a chemical potential and an electrical potential develops as positive charge builds up and eventually the electrical potential will balance the chemical potential
