high school biology textbooks american association

Prokaryotic cells lack nuclear membrane and the genetic material is found in a compact region called nucleoid. The chromosome is made up of DNA and has associated proteins. It is about 1.2 mm in length in the bacterium Escherichia coli. The chromosome is highly coiled and super ..

These soils contain low amount of nutrients. Due to weathering and high rates of leaching, these soils have poor nutrients. Many plants growing is such nutrient poor soils, possess mycorrhizae, which is an association of roots with fungi. Mycorrhizae help in absorption of nutri..

>Sickle cell anaemia is very common in Africa and in the American blacks. Due to the high incidence of diseases, the average life span in this population is only about 40 to 45 years. The disease is controlled by a single pair of alleles. Though heterozygotes are apparently unaf..

From the seminiferous tubules, the sperms are passed into a network of 10-12 ducts called the efferent ducts or the vasa efferentia . They are then passed into a highly coiled tubular part called the epididymis . Epididymis is an organ that extends from the top of the testis along its ..

. It receives motor or afferent nerves from the brain and spinal cord. The contractions may be slow or rapid but always arrhythmic and not sustained. The muscle easily experiences fatigue since the energy expenditure is very high. It is described as voluntary since its functioning is unde..

The rainfall patterns in the world are governed by differential heating of air layers over different latitudes. Differential heating of air is the main reason for different convectional air current movement from one part of globe to other. The rainfall is associated with these convection..

Fluid Mosaic Model - In the year 1972 Singer and Nicolson proposed a model for explaining the membrane structure, taking into account all the known facts. According to this model, cell membrane consists of a highly viscous fluid matrix of a bilayer of phospholipids having globular protein..

. When the number is less, the chromosomes are larger in size. A metaphase chromosome shows two identical components called chromatids, joined by a centromere (primary constriction). Each chromatid has two arms. A secondary constriction may occur sometimes. Such a chromosome is called sat-chromosom..

In human beings, sexual reproduction takes place. Sexes are separate and hence they are known to exhibit sexual dimporphism. Males have sex organs called the tetes while the females have sex organs called the ovaries. Even in the external appearance, males and females show differences. Some of the ..

. Polyploidy may be caused by fertilization of an egg by more than one sperm failure of meiosis during gamete formation. Polyploidy can artificially be introduced by a chemical called colchicine or by crossing diploids with tetraploids. Although triploids exhibit sterility, polyploidy is useful in ..