Despite their many differences, human cells have several similar structural features: a cell membrane, a nucleus, and cytoplasm and cell organelles. Red blood cells are an exception because they have no nuclei when they mature. The cell membrane forms the outer boundary of the cell and surrounds the cytoplasm, organelles, and nucleus.
Also called the plasma membrane, the cell membrane is made of phospholipids, cholesterol, and proteins. The arrangement of these organic molecules. The phospholipids are diglycerides and form a bilayer, or double layer, which makes up most of the membrane. Phospholipids permit lipid-soluble materials to easily enter or leave the cell by diffusion through the cell membrane. The presence of cholesterol decreases the fluidity of the membrane, thus making it more stable.
With the exception of mature red blood cells, all human cells have a nucleus. The nucleus is within the cytoplasm and is bounded by a double-layered nuclear membrane with many pores. It contains one or more nucleoli and the chromosomes of the cell.
Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration (that is, with or along a concentration gradient). Diffusion occurs because molecules have free energy; that is, they are always in motion. The molecules in a solid move very slowly; those in a liquid move faster; and those in a gas move faster still, such as when ice absorbs heat energy, melts, and then evaporates
OSMOSIS may be simply defined as the diffusion of water through a selectively permeable membrane. That is, water will move from an area with more water present to an area with less water. Another way to say this is that water will naturally tend to move to an area where there is more dissolved material,
The word facilitate means to help or assist. In facilitated diffusion, molecules move through a membrane from an area of greater concentration to an area of lesser concentration, but they need some help to do this.
Active transport requires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of molecules causes them to move to where there are fewer of them. Active transport is therefore said to be a movement against a concentration gradient.
The process of filtration also requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure. Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure.
As mentioned at the beginning of this chapter, human cells work closely together and function interdependently. Each type of human cell makes a contribution to the body as a whole. Usually, however, cells do not function as individuals, but rather in groups. Groups of cells with similar structures and functions form a tissue, which is the next level of organization.