Membrane transport.
So, how do substances move into and out of a cell?
Well, cells have a plasma membrane that acts like a fence and this
plasma membrane is what regulates what moves into and out of a cell.
The membrane is comprised mainly of phospholipid molecules and
has protein molecules scattered throughout its structure.
The phospholipid molecules have a polar hydrophilic water
loving head and two non polar hydrophobic water fearing tails.
The membrane is formed as the heads face out and the
tails face in forming a bilayer as you can see here.
The inner part of the cell called the cytosol is here, and the outside
of the cell is surrounded by a fluid called interstitial fluid.
Substances move into and out of a cell through several
different processes called membrane transport.
There are two main processes, passive transport
processes and active transport processes.
The main difference between the two is that passive
processes do not require energy expenditure
and active processes do require the cells to expend energy.
Let’s start by looking at the passive processes, which
include simple diffusion, facilitated diffusion and osmosis.
Diffusion is the movement of a substance from where it has
a high concentration to where it has a low concentration,
or the tendency of a substance to spread out evenly over a given space.
For instance, when a sugar cube is dissolved in water.
Over time the cube will dissolve and eventually spread
out evenly in the water, until it reaches equilibrium.
Diffusion occurs down a concentration gradient, which is a
difference in concentration of a substance between two areas.
So, the sugar molecules will move from an area of
high concentration to an area of low concentration.
Cellular diffusion is when diffusion of a solute, which is
a dissolved substance, occurs across the plasma membrane
from an area of high concentration to an area of low concentration.
This is dependent on the concentration of a substance in the
interstitial fluid outside the cell, and the cytosol inside the cell.
This can occur through simple diffusion or facilitated diffusion.
Simple diffusion occurs with solutes that are small and non polar.
By being non polar they can move in between phosphoipid molecules that form
the plasma membrane because the interior region of the membrane is non polar.
Some of the materials that move by simple diffusion include
the gases oxygen and carbon dioxide and small fatty acids.
So, if there is a higher concentration of oxygen O2 molecules outside of a cell,
they can move down the concentration gradient, across the membrane without
assistance, and into the cell as long as the concentration gradient exists.
And if there is a higher concentration of carbon dioxide CO2 molecules inside a cell,
they can move across the membrane without assistance,
out of the cell into the interstitial fluid.
So, again, simple diffusion is when non polar
molecules pass directly through a membrane.
The second type of diffusion is facilitated diffusion.
This applies to solutes that are small and either charged or polar.
Because these solutes are polar, the non polar phospholipid bilayer blocks them
from passing through the membrane and into or out of the cell by simple diffusion.
However, they can pass into and out of the cell with the assistance of
plasma membrane proteins through a process called facilitated diffusion.
There are two types of facilitated diffusion, channel
mediated diffusion and carrier mediated diffusion.
The difference between the two is the type of transport
protein used to move the substance across the membrane.
Channel mediated diffusion is when a ion, which is a charged particle where
its total number of electrons does not equal its total number of protons
giving it a positive or negative charge, moves across
the membrane through a water filled protein channel.
Each protein channel is typically specific for one
type of ion, and there are two types of channels,
a leak channel, which is continuously open, and a
gated channel, which only opens due to a stimulus,
and only stays open for a fraction of a second.
So, for a sodium positive ion, it can pass through
a sodium positive leak channel continuously
and a gated sodium positive channel will only open due to a
stimulus to allow the ion to pass through into the cell.
Carrier mediated diffusion involves the movement of polar molecules such as
simple sugars or simple carbohydrates and amino acids across the membrane.
This is accomplished by a carrier protein,
which actually changes shape in the process.
For instance glucose binds to a carrier protein, which changes shape
and moves the glucose molecule to the other side of the membrane.
Now for osmosis.
Osmosis is the passive movement of water through a selectively permeable membrane.
This occurs when there is a difference in concentration
of water on either side of the membrane.
This can happen in one of two ways, water can slip between
the phospholipid molecules that make up the plasma membrane,
or through integral protein water channels that are called aquaporins.
The plasma membrane is not permeable to most solutes,
such as charged, polar and large substances,
so for example, one side of the membrane, the cytosol or
interstatiul fluid, can have more solutes than the other side.
Let's say the interstitial fluid has 3 percent solutes and 97 percent
water, and the cytosol side has 1 percent solutes and 99 percent water.
In this example water will move down its concentration gradient from the 99
percent cytosol side to the 97 percent interstitial side to achieve equilibrium.
Now let’s look at active processes.
Active processes require the use of cellular energy for membrane transport.
There are two types of active processes, active transport and vesicular transport.
Active transport is the movement of a solute against its concentration gradient,
or going from an area of low concentration to a place of higher concentration.
Vesicular transport is the transport of large substances across the plasma
membrane by a vesicle, which is a membrane bound sac filled with materials.
Active transport has two types, primary active
transport and secondary active transport.
In primary active transport cellular protein pumps called ion pumps
move ions across the membrane, against their concentration gradient.
Let’s take a look at an example.
Here we have a membrane along with a sodium
positive, and potassium positive protein pump.
Three sodium ions and ATP bind to the pump.
ATP being like fuel or the form of cellular energy.
In order to release that energy ATP breaks down into ADP and P.
P binds to the pump and the release of energy causes the pump to
change shape are release the sodium positive ions into the interstitial fluid.
Two positive potassium ions bind to the pump and the P
produced by the ATP is released into the cytosol of the cell.
The transport protein, or pump reverts back to its orginal shpe, releasing the
positive potassium ions into the cytosol, and the pump is ready to do its job again.
In secondary active transport a substance is moved against its concentration gradient
by using energy provided by the movement of a
second substance down its concentration gradient.
So, a substance that is moving from a place of high
concentration to low concentration will provide
the energy to move a second substance from a place
of low concentration to high concentration.
There are two types of secondary active transports,
symport, where two substances are moved in the same direction and
antiport, where two substances are moved in opposite directions.
In our example here we have positive sodium ions moving from high concentration
outside of the cell to lower concentration inside of the cell.
A symporter will use the energy created by this to move a glucose
molecule from a low concentration area to a high concentration area.
And here we have an antiporter,
again using the energy from moving positive sodium
ions from high concentration outside of the cell
to lower concentration inside of the cell an H+
is moved up its gradient and outside of the cell.
Vesicular transport involves the transport of larger substances, such as
proteins or large carbohydrate polysaccharides,
across the plasma membrane.
There are two types of vesicular transport, exocytosis and endocytosis.
In exocytosis, materials are secreted from the
cell to the interstitiual fluid outside of the cell.
Typically, membranous vesicles formed by the golgi
apparatus transport materials to the cells membrane.
At the membrane the phospholipid molecules that make up the vesicle
fuse the phospholipid molecules that make up the plasma membrane,
and the contents of the vesicle are released outside of the cell.
The vesicle membrane now becomes part of the plasma membrane.
In endocytosis the plasma membrane kind of traps a substance by folding inward.
The lipid bylayer then fuses to form a vesicle surrounding the substance.
There are three main forms of endocytosis, phagocytosis,
pinocytosis, and receptor mediated endocytosis.
In phagocytosis a large particle is engulfed by the newly
formed vesicle and this vesicle fuses with a lysosome,
which is a membranous vesicle that contains digestive enzymes
that break down the particle into its component molecules.
Phagocytosis is also called cell eating.
Pinocytosis, which is also referred to as cell
drinking, is when the plasma membrane folds inward
and engulfs droplets of interstitial fluid that contain
dissolved solutes that can be used by the cell.
Receptor mediated endocytosis involves using
receptors on the outside of the plasma membrane.
These receptors bind with molecules in the in the interstitial fluid and
the membrane folds enclosing the receptors and the bound molecules,
to form a vesicle for transport within the cell.
One last thing to note, both vesicular transport
processes, endocytosis and exocytosis require energy.
And that be the basics on transport across a membrane.