ROOT PRESSURE
If the stem of a young seedling is cut off just above the soil, the stump will often exude sap from the cut xylem for many hours. If a manometer is sealed over the stump, positive pressures can be measured. These pressures can be as high as 0.05 to 0.5 MPa. Roots generate positive hydrostatic […]
Diffusion
The net movement of molecules from region of high concentration to regions of low concentration that is, down the concentration gradient. Water molecules in a solution are not static they are in continuous motion, colliding with one another and exchanging kinetic energy. The molecules intermingle as a result of their random thermal motion, this random […]
WATER IN THE SOIL
The water content and the rate of water movement in soils depend to a large extent on soil type and soil structure. At one extreme is sand, in which the soil particles may be 1 mm or more in diameter. Sandy soils have a relatively low surface area per gram of soil and have large […]
WATER IN PLANT LIFE
Water makes up most of the mass of plant cells. Each cell contains a large water-filled vacuole. In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole. Water typically constitutes 80 to 95% of the mass of growing plant tissues. Common vegetables such as carrots and […]
Epistasis
It is the type of gene interaction in which one gene locus masks or modifies the phenotype of other gene locus In trying to find evidence of gene interaction, one approach is to look for cases of a type of gene interaction called epistasis. This word means “stand upon,” referring to the ability of a […]
Recessive Lethal Alleles
Many mutant alleles are capable of causing the death of an organism; such alleles are called lethal alleles. The human disease alleles provide examples. A gene whose mutations may be lethal is clearly an essential gene. The ability to determine whether a gene is essential is an important aid to research on experimental organisms, especially […]
Protein Synthesis Inhibitors
Many antibiotics inhibit protein synthesis by binding with the bacterial ribosome and other components of protein synthesis. Because these drugs discriminate between bacterial and eukaryotic ribosomes, their therapeutic index is fairly high but not as high as that of cell wall synthesis inhibitors. Several different steps in protein synthesis can be affected by drugs in […]
Cell wall Inhibitor
Penicillins Most penicillins (e.g., penicillin G or benzylpenicillin) are derivatives of 6-aminopenicillanic acid and differ from one another with respect to the side chain attached to the amino group. The most crucial feature of the molecule is the B-lactam ring, which is essential for bioactivity. Many penicillin resistant bacteria produce penicillinases (also called beta-lactamases), enzymes […]
Characteristics of Antimicrobial Drugs
As Ehrlich so clearly saw, a successful chemotherapeutic agent has selective toxicity: it kills or inhibits the microbial pathogen while damaging the host as little as possible. The degree of selective toxicity may be expressed in terms of (1) the therapeutic dose-the drug level required for clinical treatment of a particular infection, and (2) the […]
Penicillin
Penicillin, the first true antibiotic because it is a natural microbial product, was first discovered in 1896 by a twenty-one-yearold French medical student named Ernest Duchesne. His work was forgotten until Alexander Fleming accidentally rediscovered penicillin in 1928. After returning from a weekend vacation, Fleming noticed that a Petri plate of Staphylococci also had mold […]