The hypothesis that eukaryotic cells evolved from a symbiotic association of prokaryotes—endosymbiosis—is particularly well supported by studies of mitochondria and chloroplasts, which are thought to have evolved from bacteria living in large cells. The endosymbiotic theory explains how eukaryotic cells evolved. The large and small cells formed a symbiotic relationship in which both cells benefited. Some of the small cells were able to break down the large cell's wastes for energy. ... They became the chloroplasts of eukaryotic cells.

Viroids are free RNA molecules of low molecular weight without any protein coat while viruses can have either RNA or DNA molecules encapsulated in a protein coat. Viroids are smaller in size than the viruses. Viroids infect only plants whereas virus infects all types of organisms. In viroids, protein coat is absent whereas in viruses a protein covering or a coat called as capsid is present around the genetic material.

The Watson-Crick Model of DNA (1953) Deoxyribonucleic Acid (DNA) is a double-stranded, helical molecule. It consists of two sugar-phosphate backbones on the outside, held together by hydrogen bonds between pairs of nitrogenous bases on the inside.

Watson and Crick DNA Model
DNA stands for Deoxyribonucleic acid which is a molecule that contains the instructions an organism needs to develop, live and reproduce.
It is a type of nucleic acid and is one of the four major types of macromolecules that are known to be essential for all forms of life.

Mycorrhiza is a mutually beneficial or symbiotic association of fungi Avith roots of higher
plants. Orchids grow as epiphyte on plants, have their roots associated with fungi right from the seedling stage. Many orchids cannot survive without mycorrhizae associations. Many forest trees such as pines appear stunted if mycorrhizal associations are absent. In presence of these associations, these plants absorb 2-3 times more nitrogen, potassium and phosphorus compared to without them.

The digested food is assimilated into the body of the living organisms which is used mainly for two purposes:

1. It is used as a fuel to get energy for various life processes.
2. It is used as a material for the growth and repair of the body.

The process of releasing energy from food is called respiration. It involves taking in oxygen into the cells, using it for releasing energy by burning food, and then eliminating the waste products from the body.

The mechanism by which an organism obtains oxygen from the air and releases carbon dioxide is called breathing.

Difference between breathing and respiration:-

The respiration takes place inside the cells of the body so it is known as cellular respiration. The energy released during cellular respiration is immediately used to synthesize a molecule called ATP which is used to fuel all other activities in the cell. In these processes, ATP is broken down giving rise to a fixed amount of energy which can drive the endothermic reactions taking place in the cell.

Diffusion is insufficient to meet the oxygen requirements of large multicellular organisms like humans because the volume of human body is so big that the oxygen cannot diffuse into all the cells of the human body quickly and oxygen will have to travel large distances to reach each and every cell of the body.

Large organisms contain a respiratory pigment called haemoglobin which carries the oxygen from the lungs to all the body cells very efficiently.
A terrestrial animal has an advantage over an aquatic animal in regard to obtaining oxygen for respiration that it is surrounded by an oxygen rich atmosphere from where it can take any amount of oxygen.

The aquatic animals use the oxygen dissolved in water to carry out respiration. The terrestrial animals obtain oxygen from air. The rate of breathing in aquatic animals is much faster than terrestrial animals because the amount of oxygen dissolved in water is low as compared to the amount of oxygen dissolved in air.

There are two ways in which glucose is oxidized to provide energy in various organisms:
(i) Anaerobic respiration – The respiration which takes place without oxygen is called anaerobic respiration.
Example: Yeast and some bacteria break down glucose into ethanol and carbon dioxide.

Anaerobic respiration takes place in human muscles during vigorous physical exercise because oxygen gets used up faster in the muscle cells than can be supplied by the blood.

(ii) Aerobic respiration – The respiration which uses oxygen is called aerobic respiration.
Example: Plants and animals break down glucose completely into carbon dioxide and water to release energy.

The end-products of anaerobic respiration in muscles of human beings is lactic acid and in yeast cells, ethanol and carbon dioxide are released. Carbon dioxide and water are released during aerobic respiration. Large amount of energy is released in aerobic respiration as compared to anaerobic respiration.

Difference between aerobic respiration and anaerobic respiration:

When you swallow food, it passes from your mouth down into your throat (pharynx). From there, the food moves down through a long tube (the esophagus) and into your stomach. This journey is made possible by a series of actions from the muscles in these areas. Dysphagia happens when there is a disruption in the swallowing process as food and liquids pass through your mouth, throat, and esophagus. 

The pharynx is also part of the system that brings air into your lungs. When you breathe, air enters your mouth and moves into the pharynx. The air then goes down into your main airway (trachea) and into your lungs. A flap of tissue called the epiglottis sits over the top of the trachea. This flap blocks food and drink from going down into the trachea when you swallow. But in some cases, food or drink can enter the trachea causing aspiration. It may go down as you swallow. Or it may come back up from the stomach. A person with dysphagia is much more likely to aspirate.

Dysphagia is more common in older adults. Stroke is a very common cause of both dysphagia and aspiration. About half of people who have had a stroke also have dysphagia. About half of people with dysphagia have aspiration. About one-third of these people will need treatment for pneumonia at some point.

 The functions of centrioles are:

• The main function of centriole is to help with cell division in animal cells.
• The centrioles also help in the formation of the spindle fibers that separate the chromosomes during cell division (mitosis).
• The second function of centrioles that we will focus on is celiogenesis. Celiogenesis is the formation of cilia and flagella on the surface of cells. Cilia and flagella help in the movement of cell.

The vegetable ghee may contain trans fat. Trans fats are said to cause serious health conditions.” In fact, since vegetable ghee is so high in trans fats, it is considered extremely harmful for people, especially those who live sedentary lifestyles. The higher concentration of the nutrients in the ghee is due to its higher concentration of fat. A 2013 study determined that ghee isn't as harmful to heart health as it may appear. Ghee has been blamed for heart disease in Asian Indians populations because of the high amounts of artery clogging saturated fat.

Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plant hormone. It is thought that the selective breeding (albeit unconscious) of crop strains that were deficient in GA synthesis was one of the key drivers of the "green revolution" in the 1960s, a revolution that is credited to have saved over a billion lives worldwide.

Characteristics of Phylum Chordata

1. (Noton; back and chorda; cord).
2. Kingdom: Animalia
3. Presence of a notochord
4. They are backboned animals (vertebrates),
5. Most of the living chordates are familiar vertebrate animals.
6. Presence of dorsal hollow nerve cord
7. Blood vascular system: Present, closed type
8. Ventral heart, hepatic portal system and RBC are present.
9. Germ layer: Triploblastic.
10. Symmetry: bilateral symmetry body.
11. Coelom: Present. Well developed
12. Presence of gill (pharyngeal) slits
13. Presence of post **** tail

Phylum Chordata is divided into four sub-phylum:

1. Hemichordata,
2. Urochordata,
3. Cephalochordata
4. vertebrata or Craniata

Factors affecting enzyme activity

Enzyme activity can be affected by a variety of factors, such as temperature, pH, and concentration.

Enzymes work best within specific temperature and pH ranges, and sub-optimal conditions can cause an enzyme to lose its ability to bind to a substrate.

• Temperature: Raising temperature generally speeds up a reaction, and lowering temperature slows down a reaction. However, extreme high temperatures can cause an enzyme to lose its shape (denature) and stop working.

• pH: Each enzyme has an optimum pH range. Changing the pH outside of this range will slow enzyme activity. Extreme pH values can cause enzymes to denature.

• Enzyme concentration: Increasing enzyme concentration will speed up the reaction, as long as there is substrate available to bind to. Once all of the substrate is bound, the reaction will no longer speed up, since there will be nothing for additional enzymes to bind to.

• Substrate concentration: Increasing substrate concentration also increases the rate of reaction to a certain point. Once all of the enzymes have bound, any substrate increase will have no effect on the rate of reaction, as the available enzymes will be saturated and working at their maximum rate.