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.