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).

Endocytosis and Exocytosis

Large molecules, such as proteins, enter (endocytosis) or exit (exocytosis) through the cell membrane by so-called vesicular transport. During this process, substances are attached in part to the outside of the cell by membrane-bound receptors, enclosed by a part of the plasma membrane, and moved into the interior of the cell as a membranewrapped vesicle (receptor-mediated endocytosis). Depending on the size of the absorbed particle, this process may also be called pinocytosis or phagocytosis.
In exocytosis, products synthesized in the cell are enclosed in membranous vesicles and, by coalescence of these vesicles with the inside of the plasma membrane, reach the extracellular space. In this way, the transmitter substances in the endings of nerve cell processes are liberated at the synapses. The secretory products of most glandular cells leave the cell interior in similar fashion. Endocytosis and exocytosis are dependent on the action of ATP.

Exocytosis and endocytosis

Active Transport

Active transport is the transport of substances through the cell membrane by means of an energy-consuming transport system (transport ATPase). Here, again, ATP serves as universal fuel. Such a transport process can move a substance through the membrane against a concentration gradient. Thus cells have the ability to maintain in their interior stable ion concentrations, for example, that are clearly different from their concentrations in the extracellular fluid. These active transport processes are served by specialized proteins in the cell membrane that can move several ions simultaneously. In this process, the coupled transport of substances can occur in the same direction (cotransport) or in opposite directions (countertransport). For instance, in the kidney the transport of amino acids is coupled with an active Na+ transport. Additionally, active ion transport through cell membranes is necessary for the formation of  membrane or resting potentials.

Filtration

Filtration occurs when water and any dissolved particles are pushed through cell membranes or pore systems by a hydrostatic pressure difference. Pores occur, for instance, when there are small spaces between endothelial cells (intercellular clefts) or holes (fenestrations) in the cell membranes. Such a process is found in the capillaries of the tissues. The term ultrafiltration is used when, in the course of filtration processes such as that in the capillaries of the renal corpuscles, larger blood components are retained or dissolved molecules are separated out because of their size or charge.

Osmosis and Osmotic Pressure

When two solutions containing different concentrations of the same solute are separated by a partly permeable membrane, a so-called semipermeable membrane, osmosis can take place. In osmosis the semipermeable membrane allows the solvent, but not the solute, to pass.Water diffuses through the membrane toward the solution of higher concentration, until equilibrium is attained. During this process the volume of the side that initially contained the higher concentration increases. The pressure that must be applied to this side to reverse the process of osmosis is called the osmotic pressure. It is expressed inmmHg or in the SI units of pascals (Pa) or kilopascals (kPa). When such a measurement is applied, it is found that the osmotic pressure depends only on the number of dissolved particles in a defined volume, and not on their size or charge.
The cell membranes are more or less semipermeable membranes, since the lipid layer is less permeable to charged molecules such as ions and proteins. The osmotic pressure of the extracellular fluid depends on its content of protein and salts and corresponds approximately to that of a 0.9% solution of NaCl. Such a physiological salt solution is isotonic (that is, it is in osmotic equilibrium with the cell). Consequently, cells bathed in hypertonic (more concentrated) solutions lose water and shrink, while in hypotonic (less concentrated) solutions they take up water and swell. The organism therefore endeavors by special regulatory mecha nisms to keep the osmotic pressure of the extracellular fluid as constant as possible. Because of the good permeability of cell membranes to water, these mechanisms lead to a more or less constant osmotic pressure in the cell interior.

Thetermcolloidosmoticpressureisusedwhen,for instance, proteins for which the capillary wall is impermeable are dissolved in the blood plasma and not in the interstitial fluid. They create an osmotic pressure difference of about 25mmHg (3.3 kPa) between the interstitial fluid and the capillary space. Thiswould lead to amovement of fluid into the vessels if the hydrostatic blood pressure active inside the blood vessels were not opposed to it. Since the blood pressure at the beginning of the capillaries (37 mmHg)is greaterthanthe colloidosmotic pressure, fluid is actuallyfiltered into the interstitial space.

Development of osmotic pressure in a semipermeable membrane.

Solid and Fluid Transport

The specific transport processes that take place in the microscopic realm, e. g., between the cells on the one hand and the blood capillaries and their surrounding cells on the other, can be divided into essentially passive (diffusion, osmosis, and filtration) and active (energy-dependent) transport processes (active transport, endocytosis, exocytosis).

Membrane or Resting Potential of a Cell

Because the ions are distributed unevenly between the intracellular and extracellular spaces, a potential difference, known as the membrane potential, is created at the cell membrane. This creates a negative charge in the interior of the cell relative to the extracellular space, the so-called resting potential. This potential difference can be measured with sensitive instruments and is about 60−80mV.
The reason for the negative potential inside the cell with respect to its surroundings lies in the differential distribution of ions between the intracellular and extracellular spaces. Thus, the intracellular potassium concentration is about 35 times greater than the extracellular concentration, while proteins are the preponderant anions inside the cell. Sodium ions dominate in the extracellular space, balanced on the negative side by chloride anions. The accumulation of potassium ions inside the cell is a specific activity of almost every cell and represents one of its most important active transport processes. This “ion pump” transports potassium ions into the cells and, to balance this, transports sodium ions out. It is therefore also called the sodium−potassium (Na+−K+) pump. It includes an ATP-splitting enzyme (sodium−potassium ATPase, Na+,K+-ATPase). This reaction liberates the energy required for ion transport. The cell membrane is impermeable to ions, so there are membrane pores (channels) forNa+, K+, and Cl−, but not for protein anions. During resting potential, the K+ channels are often open, but the Na+ and Cl− channels are mostly closed. Because of the concentration difference, the K+ ions have a tendency to diffuse outward. However, the diffusion of positively charged potassium ions out of the cell is limited by the negatively charged protein anions, which cannot cross the membrane because of their size. The diffusion of even a few potassium ions out of the cell leaves anions with the opposite (negative) charge (protein anions) on the inside of the cell membrane, so that the interior of the cell is negatively chargedwith respect to its surroundings. The resting potential is therefore also known as the diffusion potential. The diffusion of ions outwardthrough the membrane pores is independent of the Na+−K+ pump.
The energy-consuming ion pumps can be impeded or blocked by lack of oxygen (failure of ATP production) or by metabolic poisons (e. g., cyanide), leading to severe disturbances in the specific performance of a cell. The initiation and propagation of nerve or muscle cell excitation depends on brief membrane potential changes (action potentials) (see Chapter 3: Nerve Tissue).