How Various Mechanisms By Which Substances Cross

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 the transport protein acts

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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