Cell Membrane - Structure of the Cell

Structure of the Cell

The human body contains two general classes of cells: sex cells and somatic cells. Sex cells (also called germcells or reproductive cells) are either the sperm of malesor the oocytes of females. Somatic cells (derived from the term soma, meaning “body”) include all other cells in the human body. This chapter focuses on somatic cells.

The three major parts to a cell are the cell mem-brane, the nucleus, and the cytoplasm. The cell membrane encloses the cell, its nucleus, various organelles, and its cytoplasm. The nucleus contains the cell’s genetic material and controls its activities. The cytoplasm fills out the cell and its shape. Organ-elles are microscopic, specialized cell structures that perform specific functions required by the cell (FIGURE 3-1 ). The organelles fulfil functions and pro-cesses that are vital to the life of cells, tissues, and organisms.

Cell Membrane

The cell membrane (also called the plasma membrane) controls movement of substances both intoand out of the cell. The cell membrane allows selective communication between the intracellular and extracellular compartments while aiding in cellular movement. It gives form to the cell and is also where much of the cell’s biological activities are conducted. Molecules in the cell membrane form pathways that allow the signals from outside the cell to be detected and transmitted inside. When cells form tissues, the cell membrane assists by adhering the cell to other cells.

Each cell’s membrane is extremely thin and delicate, able to stretch to differing degrees. There are usually tiny folds on the surface, which help to increase its surface area. Only certain substances can enter or leave each cell (a condition known as selectivepermeability). Cell membranes can be differentially­permeable or semipermeable. A semipermeable membrane allows certain elements to pass through but not others.

Lipids and proteins are the primary substances that make up cell membranes, usually in a double layer of phospholipid molecules (FIGURE 3-2). The phosphate portion forms the outer surface, with the fatty acid portion forming the inner surface. Sub-stances such as oxygen and carbon dioxide, which are soluble in lipids, can easily pass through this double layer (also called a bilayer). Other substances such as amino acids, proteins, nucleic acids, certain ions, and sugars cannot pass through this layer. Cholesterol in the inner cell membrane helps to keep the membrane stable. The phospholipids organize themselvesin a bilayer to hide their hydrophobic tail regions and expose the hydrophilic regions to water. This process does not require energy and forms a layer that is the wall between the inside and outside the cell.

The proteins in a cell membrane are classified according to where they are positioned. They also may have different shapes such as fibrous, globular, or rodlike. Proteins can move in the cell membrane because they are enclosed in an oily background. Cell membrane proteins may form receptors for hormones or growth factors, transport substances across the cell membrane, and form selective channels that deter-mine which types of ions can enter or leave the cell. On the cell membrane’s outer surface, proteins may extend outward, marking the cell as a component of a particular tissue or organ. Many proteins are attached to carbohydrates to form glycoproteins.

1. Name the three major parts of a cell and the function of the cell membrane.

2. Explain how a semipermeable cell membrane functions.