Biology of the Cell

The smallest living entity of an organism is the cell. In contrast to single-celled organisms that are independent entities, the cells of higher organisms form functional units. In accordance with their function, the cells are differentiated by size, shape, and the degree of definition of certain characteristics.
For all cells of the body there are a certain basic structure and numerous basic properties. The basic properties include the ability to divide and to sense and respond to stimuli.
Basic Cell Structure
By and large, the cell consists of the cytoplasm containing the cell organelles, the nucleus, and the cell membrane surrounding the whole structure.
Cell Membrane
The cell membrane, also known as the elementary membrane, consists of a double lipid layer, in which the fat-soluble components face each other while the water-soluble parts form the inner and outer boundaries (a three-layered structure). The lipid molecules are infiltrated with proteins. The outer side of the membrane is covered by a glycocalyx. An elementary membrane also surrounds the cell organelles and the nucleus.
Cytoplasm and Cell Organelles
The cytoplasm consists of the intracellular fluid (cytosol), the cell organelles, and various cell inclusions (phakeroplasm). The cell organelles are responsible for the cell’s metabolism.
Endoplasmic Reticulum (ER)
Present in all cells except erythrocytes; serves intracellular material transport; protein synthesis (granular ER); lipid and hormone synthesis (smooth ER).
Ribosomes
No elementary membrane; multienzyme complexes made up of proteins and rRNA molecules that link amino acid chains for protein synthesis.
Free ribosomes: intracellular proteins (enzymes, etc.). Ribosomes in the ER (granular ER): exported proteins (glandular secretions, etc.).
Golgi Apparatus
Present in all cells except erythrocytes; uptake and discharge products of synthesis in the form of membrane-bounded transport vesicles that are flushed from the cell (secretory vesicles) and serve the renewal of the cell membrane or take part in intracellular digestion as primary lysosomes.
Lysosomes
“Digestive organs” of the cell; with the aid of enzymes they degrade cell-alien structures and the cell’s own decaying organelles.
Centrioles
Build the spindle fibers during cell division.
Mitochondria
“Power stations” of the cell; here nutrients (proteins, fats, carbohydrates) are metabolized essentially to CO2 and H2O, generating the energy necessary for metabolism (e. g., muscle contraction, synthesis of structural substances), which is then stored in the form of ATP.
Cell Nucleus
Present in all cells except erythrocytes; the nucleus contains the nucleolus (production of rRNA ⇒ protein biosynthesis) and the chromosomes, carriers of the hereditary factors (genes). Human nuclei contain 23 chromosome pairs (23 paternal, 23 maternal ⇒ diploid chromosome set); the 23rd pair determines sex.
The appearance of the nucleus and of the chromosomes changes with the individual phases of cell division.
During the interphase (working phase of the cell) between two cell divisions (mitoses) the genetic material is duplicated and chromosomes form, each with two chromatids joined by a constriction (centromere). Each chromatid consists of one molecule of DNA (deoxyribonucleic acid). The basic units contained in DNA, the nucleotides, are each composed of one base (adenine, cytosine, guanine, or thymine), a sugar (deoxyribose), and an acid phosphate radical. DNA contains the complete hereditary matter in the form of genes.
Each unit of information comprises three bases (triplet, codon) in varying combinations. Each triplet represents the information for one amino acid. One gene consists of about 300−3000 base triplets and provides the information for one protein. This genetic code is the same for all living things and contains the information for the biosynthesis of proteins, the most important structural and energizing  substances in all organisms.
Protein Biosynthesis
Single-stranded RNA synthesized in the nucleus copies the genetic code (transcription) and brings the message to the ribosomes, the site of protein biosynthesis. Each copied triplet represents one amino acid in the final protein. tRNA molecules, also synthesized in the nucleus, bind amino acids in accordance with the genetic code (according to the sequence of triplets) and transport them to ribosomes, where they are linked into proteins with the aid of enzymes. Each tRNA is specific for one amino acid.
Cell Division (Mitosis)
Chromosomes containing two chromatids are created by the duplication of genetic material during interphase. This process is necessary for the transmission of genetic information to the daughter cells. Mitotic division of cells makes possible growth and the renewal of cells.
Reduction or Maturation Division (Meiosis)
Two successive cell divisions lead to the creation of male or female sex cells with half the chromosome complement (haploid cells).
First maturation division: The (homologous) paternal and maternal chromosomes lying next to each other separate, which leads to the exchange of homologous fragments by “crossing-over.” This results in two daughter cells with haploid chromosome sets.
Second maturation division: Corresponds to a normal mitosis. The chromatids of the chromosomes separate again. The two daughter cells create four mature sex cells with a haploid set of chromosomes.
Fertilization creates a new diploid set of chromosomes. The actual reason for meiosis is the restructuring and recombination of the chromosomes, that is, the shuffling of genetic material.
Intracellular and Extracellular Fluid
The body of an adult person consists of 60% water, two-thirds of which is intracellular, one-third extracellular. Of the 14 liters of extracellular fluid, three-quarters occupy the interstitial spaces and onequarter the vascular system.
Membrane Potential
In the extracellular fluid, sodium is the predominant cation and chloride is the predominant anion; in the intracellular fluid, the predominant cation is potassium and proteins are the predominant anions.
By the differential distribution of ions in the intracellular and extracellular spaces, a potential difference is created across cell membranes (membrane or resting potential). This is caused by the active accumulation of potassium inside the cell (ATP-dependent Na+−K+ pump).
Solid and Fluid Transport
Transport processes between the cells and their environment play an important role in the maintenance of the “internal milieu” (homeostasis). A distinction is made between passive and active (energy-dependent) transport processes. Passive processes include free diffusion (e. g., of O2, CO2, H2O), facilitated diffusion (e. g., of glucose and amino acids in the cells of the intestinal mucosa), osmosis, and filtration (e. g., of glucose and amino acids in the capillaries of the tissue). Active processes include active transport (e. g., of ions) as well as endocytosis and exocytosis (e. g., of proteins).