Substances Across The Cell Membrane example essay topic

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 bi layer diffuse through it by facilitated diffusion's. Intercellular joining is when cell's membranes hook together. Cell to cell recognition and when different parts of the 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. Vesicles, which form in the Golgi apparatus, are moved by the 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. There are three steps involved. The first step, phagocytosis, takes in macromolecules and packages hem membrane-enclosed sac. After the sac fuses with enzymes its digested. In the next step, called, the cell engulfs the sac and its dissolves solute. The final step is receptor-, 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. 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. 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, sodium-potassium pumps. and, all of which expend energy to transport substances across the cell membrane. The cell has the ability to regulate what crosses its cellular boundaries.