All cellular functions depend on energy generation and transportation of substances within and among cells. In order to function properly a cell must maintain a stable internal environment and therefore the transport of materials has to be achieved without an excessive build-up of chemicals. The term cell respiration refers to the controlled exchange of nutrients (such as oxygen and glucose) and waste (such as carbon dioxide) by the cell to activate the energy needed for the cell to function.
In order for cells to carry out their work they need to produce enough energy or fuel. Fuel is provided by glucose from carbohydrate metabolism and in order for the glucose to be released or 'oxidised', oxygen is absorbed from the respiratory system into the bloodstream.
Cells are bathed in a fluid known as tissue fluid or interstitial fluid, which allows the interchange of substances between the cells and the blood, known as internal respiration.
The body's internal transport system, the blood, carries oxygen from the respiratory system and nutrients such as glucose from the digestive system to the cells and these are absorbed through the cell membrane in several different ways: diffusion, osmosis, active transport and filtration. When certain molecules are needed, such as glucose, the cell will take these in and discard other materials in order to preserve the equilibrium.
As chemicals become concentrated outside the cell, a flow of small molecules takes place through the cell membrane until a balance exists. This process in which small molecules move from areas of high concentration to those of lower concentration is called diffusion. Diffusion is the basis by which the cells lining the small intestines take in digestive products to be utilised by the body.
Fig 1.4 Diffusion: the process in which small molecules move from an area of high concentration to lower concentration
The cell membrane
The cell membrane
This process refers to the movement of water through the cell membrane from areas of low chemical concentration to areas of high chemical concentration. This process allows for the dilution of chemicals, which are unable to cross the cell membrane by diffusion, in order to maintain equilibrium within the cell.
Fig 1.5 Osmosis: the movement of water through the cell membrane from low to high chemical concentration
Area of low chemical concentration
Area of high chemical concentration
This is an energy-dependent process in which certain substances (including ions, some drugs and amino acids) are able to cross cell membranes against a concentration gradient. This is the process, using chemical energy, by which the cell takes in larger molecules that would be otherwise unable to enter in sufficient quantities. Carrier molecules within the cell membrane bind themselves to the incoming molecules, rotate around them and release them into the cell. This is the means by which the cell absorbs glucose.
This is the movement of water and dissolved substances across the cell membrane due to differences in pressure. The force of the weight of the fluid pushes against the cell membrane, thereby moving it into the cell. One site of filtration in the body is in the kidneys. Blood pressure forces water and small molecules through plasma membranes of cells and the filtered liquid then enters the kidneys for filtration.
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