The Cell Membrane Essay, Research Paper
Homeostasis is essential to the cell s survival. The cell membrane is
responsible for homeostasis. The membrane has a selective permeability
which means what moves in and out of the cell is regulated. Amino
acids, sugars, oxygen, sodium, and potassium are examples of substances
that enter the cell. Waste products and carbon dioxide are removed from
the cell. All of these substances cross the membrane in a variety of
ways. From diffusion and osmosis, to active transport the traffic
through the cell membrane is regulated.
Diffusion is the movement of molecules form one area of higher
concentration to an area of lower concentration. Concentration gradient
causes the molecules to move from higher concentration to a lower
concentration. The side of the membrane that has the higher
concentration is said to have the concentration gradient. It drives
diffusion because substances always move down their concentration
gradient. The pressure gradient also plays a role in diffusion. Where
this is a pressure gradient there is motion of molecules. The pressure
gradient is a difference in pressure between two different points. If
the concentration of one side of the membrane is greater than the
molecules will travel from the higher to lower concentration.
Eventually there will be a dynamic equilibrium and there will be no net
movement of molecules from one side to the other.
Osmosis is the diffusion of water. Like diffusion, the water moves
from a region of higher water potential to a region of lower water
potential. Solutions have three different stages that the solutes can
be classified in: isotonic, hypertonic, and hypotonic. Isotonic is when
the solutions have equal amounts of solutes. Like equilibrium, there is
no net change in the amount of water in either solution. When the
solutions have different concentration of solutes then the one with less
solute is hypotonic and the one with more solute is hypertonic.
Hypotonic takes in the solute from the hypertonic side that gives away
the solute. There will be a net movement in these types of solution.
The molecules will move from the hypotonic solution into the hypertonic
solution.
The third way a substance can cross the cell membrane is through
facilitated diffusion. This occurs when special carrier proteins carry
solutes dissolved in the water across the membrane by using active
transport. When the concentration gradient can not allow travel from
one side of the membrane to the other fast enough for the cell s
nutritional needs, then facilitated diffusion is used. The transport
protein is specialized for the solute it is carrying, just as enzymes
are specialized for their substrate. The transport protein can be
changes or blocked just like enzymes. There are several theories on how
proteins facilitate diffusion. In one case t
like a revolving door. The protein opens on one end and accepts the
solute and then closes. It opens on the other end and releases the
solute. Another way proteins facilitate diffusion is proteins that
extend over the membrane can provide a shuttle that the selective
solutes may cross. There are proteins where their job is to open and
close a gated channel. Polar molecules and ions blocked by the
phospholipid bilayer diffuse through it by facilitated diffusions.
Intercellular joining is when cell s membranes hook together. Cell to
cell recognition and when different parts of the cytoskeleton bond to
proteins, are both ways that substances may cross the membrane through
facilitated diffusion.
Active transport moves the solutes against their concentration
gradients. Proteins use ATP energy to pump certain substances through
the membrane. One way ATP powers active transport is by a
sodium-potassium pump. The pump exchanges sodium for potassium across
the plasma membrane of animal cells. ATP powers it by transferring a
phosphate group to the protein. Large molecules are transported across
the plasma membrane by vesicles, this is called exocytosis. Vesicles,
which form in the Golgi apparatus, are moved by the cytoskeleton to the
plasma membrane. When the plasma membrane and the vesicle membrane
touch the lipid molecules rearrange themselves so that the two membranes
are fused together. The substance inside the vesicle now overflows
outside of the cell. The opposite method, substances moving into the
cell, is called endocytosis. There are three steps involved in
endocytosis. The first step, phagocytosis, takes in macromolecules and
packages hem membrane-enclosed sac. After the sac fuses with enzymes it
is digested. In the next step, called pinocytosis, the cell engulfs the
sac and its dissolves solute. The final step is receptor-mediated
endocytosis, where only specific substances attach to the receptor
sites. Endocytosis brings substances outside the cell into the cell.
Active transport has one final way of transporting substances across the
cell membrane, and that is called phosphorylation. ATP is made up of
three phosphates that can power active transport by transferring one of
its phosphate groups to a protein. When ATP s phosphate group is
hydrolyzed and the phosphate group is transferred we call it
phosphorylation.
In review, substances cross the cell membrane in two different ways:
passive and active transport. Passive transport includes osmosis,
diffusion, and facilitated diffusion. None of these techniques use
energy. Active transport includes endo and exocytosis, sodium-potassium
pumps. and phosphorylation, all of which expend energy to transport
substances across the cell membrane. The cell has the ability to
regulate what crosses its cellular boundaries.
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