Cell Transport In The Human Body

There are transport processes of the cells in the human body.

The physical basis of the body is a chemical substance called protoplasm. It is still a mysterious substance in the world as it alone has life. There are various elements composing protoplasm but only eighteen seem necessary in composing protoplasm. Protoplasm is complied largely of elements relatively scarce outside living matter. These elements are carbon, hydrogen, oxygen and nitrogen. The protoplasm forms units called cells. The human body is so complex that it consists of trillions of these cells. Human cells are microscopic in size and can only be seen when magnified by a microscope. Cells differ more in shape than in size as some are flat, some brick shaped, some threadlike and others have irregular shapes.

Different tasks are performed by cells in the human body grouping together. They will vary in size and form but are all built to the same plan. Each is a mass of protoplasm differentiated into a main mass of cytoplasm and a smaller spherical nucleus near the cell's center. Cytoplasm or "living matter," is a substance that exists only in cells. It is the specialized living material of cells. It lies between the plasma membrane and the nucleus. Each cell in the human body is surrounded by a thin membrane, the plasma membrane. This membrane separates the cell contents from the dilute salt water solution called interstitial fluid or tissue fluid, that bathes every cell in the body. The cell has other important parts, some serving as centers for enzymatic action in energy conversion. In the nucleus is the chromatin network, which in cell division divides into a specific number of chromosomes bearing the genes of heredity.

The cells in the body are in two main divisions, the somatic cells and the germ cells. The somatic cells make up the body cells and carry on the basic functions of living. They live only for the generation. The germ cells perform only one function, reproduction. They are immortal, stable in their constitution; but they can be influenced by environmental factors, as evolution indicates.



The contents of the cell are separated from the tissue fluid by the plasma membrane. This membrane will allow some substances to enter the cell and then allow others to leave. There is much movement in both directions through cell membranes. There are wastes, gases, foods, water molecules and other substances moving in and out endlessly. There are processes that allow this movement. They are classified in two general headings, passive and active transport.

Active transport requires the expenditure of energy by the cells. It is an uphill movement of a substance through a living cell membrane. Adenosine triphosphate or ATP is a chemical substance that gives the energy required for the active transport process. ATP is produced in the mitochondria using energy from nutrients and is capable of releasing that energy to do work in the cell. There is a breakdown of APT and a use of energy that makes this process possible. Cellular energy is required to move substances from a low concentration to a high concentration. Because the formation and breakdown of ATP requires complex cellular activity, active transport mechanisms can take place only through living membranes.

An ion pump that is a specialized cellular component makes possible a number of active transport mechanisms. It is a protein structure in the cell membrane called a carrier. Energy is used by the ion pump from ATP to actively move ions across cell membranes against their concentration gradients. This active transport moves a substance in an uphill direction just as a water pump, for example, moves water uphill. There are different ion pumps that are required to move different types of ions. The calcium pumps only move calcium ions. The sodium pumps only move sodium and potassium pumps only move potassium. Some of these ion pumps are coupled to one another so that more than one substance can be moved at one time. Some ion pumps are coupled with other specific carriers that transport glucose, amino acids, and other substances.

Phagocytosis is a way that a cell can use its active transport mechanism to move an object or substance through the plasma membrane and into the cytoplasm. Phagocytosis is from a Greek word meaning, "to eat." This process permits a cell to engulf and literally "eat" foreign material. Phagocytosis will even contain white blood cells that destroy bacteria. By this method the cell membrane forms a pocket around the material to be moved into the cell and, by expenditure of energy from ATP, the object is moved to the interior of the cell. After it is inside the cytoplasm, the bacterium fuses with a lysosome and is destroyed. Pinocytosis is an active transport mechanism used to incorporation fluids or dissolved substances into cells. Pinocytosis is from a Greek word meaning "drink."

The movement of substances in passive systems is considered a "down concentration gradient." Substances in passive systems move from a region of high concentration to a region of low concentration until they reach equal proportions on both sides of the membrane. An example of a passive transport process is diffusion. This is the process by which substances scatter themselves evenly throughout an available space. Energy is not required for this movement through space. Two specialized examples of diffusion are osmosis and dialysis. Diffusion occurs in both of these across a selectively permeable membrane. The plasma membrane of a cell is said to b selectively permeable because it permits the passage of certain substances but not others. For the cell to let some substances such as nutrients enter the cell and to exclude others this permeable process has to occur. Osmosis is the diffusion of water across a selectively permeable membrane. In the case of dialysis substances called solutes, that are dissolved particles in water, move across a selectively permeable membrane by diffusion.

Filtration is a movement of water and solutes through a membrane because of a greater pushing force on one side than the other of the membrane. Hydrostatic pressure is the name of the force that is the force or weight of a fluid pushing against a surface. Filtration always occurs down hydrostatic pressure gradient. When two fluids have unequal hydrostatic pressure they are separated by a membrane. Water and diffusible solutes or particles filter out of the solution that has the lower hydrostatic pressure. This filtration process is the same process that is responsible for urine formation in the kidney.

Very severe diseases can occur if any of the above processes fails as they are so important to cell survival. Cystic fibrosis is an inherited condition in which chloride ion pumps in the plasma membrane are missing. This causes abnormally thick mucus in the lungs and impairs normal breathing and leads to lung infections. Duchenne muscular dystrophy is another inherited lethal muscle disease resulting from "leaky" membranes in muscle cells. Calcium enters affected muscle cells through the leaky membranes and chemical reactions occur that destroy the muscle, causing life-threatening paralysis.

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