The tricarboxylic acid cycle (TCA cycle) is a series of enzyme-catalyzed chemical reactions that form a key part of aerobic respiration in cells. This cycle is also called the Krebs cycle and the citric acid cycle. This shows the TCA cycle in the context of what is happening in the inner mitochondrial membrane. The TCA cycle is amphibolic; i.e., it serves as a catabolic and an anabolic pathway. Reactions that utilize intermediates of the cycle as precursors for the biosynthesis of other molecules are as follows. This reaction takes place in the cytoplasm and is a source of acetyl-CoA for fatty acid biosynthesis.

The TCA cycle is a central pathway into which many metabolites feed. It consists of a number of reactions which generate NADH and FADH₂ which can in turn be used by the oxidative phosphorylation pathway to generate ATP. The TCA cycle occurs in the matrix of the mitochondria. The citric acid cycle is a series of reactions that produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2. The regulation of the rate of oxidative phosphorylation by the ADP level is called respiratory control or acceptor control. The level of ADP likewise affects the rate of the citric acid cycle because of its need for NAD+ and FAD. The physiological significance of this regulatory mechanism is evident. The major significance of the citric acid cycle is to act as the final common pathway for the oxidation of carbohydrates, lipids and proteins, since glucose, fatty acids and many amino acids are all metabolised to acetyl CoA.

Linnaeus' book was called Systema Naturae.

Secondary growth in roots

• In the dicot root, the vascular cambium is completely secondary in origin.
• Vascular cambium originates from the tissue located just below the phloem bundles, a portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring, which later becomes circular.

Carl Linnaeus was a famous botanist, zoologist and a physician who brought about binomial nomenclature. Binomial nomenclature is a naming system in classification of species.
He was famous for his different books published which laid foundation for nomenclature. He was known for two works namely Species Plantarum (1753) for plants and System Naturae (1758). These two books are still used in naming plants and animals.  

Basically bilateral and protostomial eucoelomate eumetazoans whose soft body (L., mollis or molluscum = soft) is unsegmented and enclosed within a skin–fold (mantle) which usually secretes a calcareous shell.

- About 80,000 existing and 35,000 extinct species known. Aquatic or terrestrial.

- Snails and mussels found along banks of freshwater bodies and the oysters, shanks (shoes conch–shell is commonly blown in religious ceremonies) and cowries of the sea are common molluscs.

Malacology or Conchology is the branch of study of molluscs and their shells.

Molluscs are of some economic importance; man uses some as food; shells of some are used for making buttons, beads, etc

Sexual reproduction is a natural way of reproduction in humans, animals and the majority of plants also choose to reproduce sexually. This type of reproduction is more complex and lengthy as compared to asexual reproduction. Moreover, reproducing sexually gives the benefit of variation and offsprings are unique. Sexual reproduction consists of a set of events and can be divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization.

Phylum Arthropoda is the largest phylum of Animalia which includes insects.
They have organ-system level of organization, they are bilaterally symmetrical, triploblastic, segmented and coelomate animals.
The body is covered by chitinous exoskeleton.
The body consists of head, thorax and abdomen and have jointed appendages.
Respiratory organs are gills, book gills, book lungs or tracheal system.
Circulatory system is of open type, sensory organs like antennae, eyes, statocysts are present, and excretion takes place through Malpighian tubules.

Root apical meristem occupies the tip of a root while the shoot apical meristem occupies the distant most region of the stem axis.

Root pressure is caused by active transport of mineral nutrient ions into the root xylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential. Water then diffuses from the soil into the root xylem due to osmosis. Root pressure is caused by this accumulation of water in the xylem pushing on the rigid cells. Root pressure provides a force, which pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallest trees.

Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants.Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to pull back into the pores of the cell wall. Inside the pores, the water forms a concave meniscus. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull only works because the vessels transporting the water are very small in diameter, otherwise cavitation would break the water column.

A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes. Often, the genes carried in plasmids provide bacteria with genetic advantages, such as antibiotic resistance. A plasmid is a small, often circular DNA molecule found in bacteria and other cells. Plasmids are separate from the bacterial chromosome and replicate independently of it. They generally carry only a small number of genes, notably some associated with antibiotic resistance.

These tissues have the capability to develop by swift division.  They assist in the major growth of the vegetation. Growth in length and growth in diameter of the plant are carried about by these cells. The Meristematic cells are cubical, living cells with a big nucleus.  These cells are meticulously crammed with no intercellular spaces. Depending on the section where the meristematic tissues are existing, they are categorized as meristems intercalary, lateral and apical.

1. Apical meristem is existent at the growing tips or apical of stems and roots. Apical meristem upsurges the length of the plant.
2. Lateral meristem is existent in the radial portion of the stem or root. Lateral meristem upsurges the thickness of the plant.
3. Intercalary meristem is found at the internodes or at the base of the leaves. Intercalary meristem upsurges the size of the internode.

Histamine is not only released when the body encounters a toxic substance, it is also released when mast cells detect injury. It causes nearby blood vessels to dilate allowing more blood to reach the site of the injury or infection. The Fc region of immunoglobulin E (IgE) becomes bound to mast cells and basophils and when IgE's paratopes bind to an antigen, it causes the cells to release histamine and other inflammatory mediators. These similarities have led many to speculate that mast cells are basophils that have "homed in" on tissues. Histamine is involved in the inflammatory response and has a central role as a mediator of itching. ... Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.

Metaphase is the best stage to count the total number of chromosomes in any species and to establish a detailed study of the morphology of the chromosomes. As mitotic cells are easy to obtain, morphological studies are generally based on mitotic metaphase chromosomes.

A cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells.

All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular).  Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy.

It is necessary to classify organisms because:

• Classification allows us to understand diversity better.
• It helps in the identification of living organisms as well as in understanding the diversity of living organisms.
• Classification helps us to learn about different kinds of plants and animals, their features, similarities and differences.
• It enables us to understand how complex organisms evolve from simpler organisms.
• To understand and study the features, similarities and differences between different living organisms, they are grouped under different categories.
• Classification is a tool which helps us to deal with a great diversity of living forms.
• It is essential to understand the inter-relationships among the different groups of organisms.
• Classification forms a base for the development of other biological sciences.