Nutrition can be defined as the process of acquiring source of energy from the outside to the inside of the body. Nutrients are the different sources of energy acquired by living organisms by the process of nutrition.

Photosynthesis is the process by which plant cells prepare food (carbohydrates) from inorganic raw materials such as carbon dioxide and water in presence of sunlight and chlorophyll.
\({ 6CO }_{ 2 }+{ 6H }_{ 2 }O\xrightarrow [ Chlorophyll ]{ Sunlight } { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+{ 6O }_{ 2 }\)
The process of photosynthesis takes place in two phases:
Light reaction - It requires light which is absorbed by chlorophyll molecules to split water molecules into hydrogen and oxygen. ATP and NADPH₂ are produced during this phase and it occurs in the thylakoid membrane of the chloroplasts.
Dark reaction - It does not require light energy and it occurs in the stroma of chloroplasts, In this phase, the ATP and NADPH₂ formed during light reaction are utilised and CO₂ is reduced to form carbohydrates.
The four factors affecting the rate of photosynthesis are:
(i) Sunlight - Sunlight provides the energy required for the preparation of food by plants. In the absence of sunlight, photosynthesis cannot take place.
(ii) Carbon dioxide - Carbon dioxide acts as the substrate for photosynthesis. During this process, carbon dioxide and water are converted into carbohydrates.
(iii) Water - During photosynthesis, water is photolysed into oxygen and hydrogen and it provides the energy for photosynthesis.
(iv) Chlorophyll - Chlorophyll is the green pigment present in the leaves which absorbs sunlight

The mechanism of urine formation includes three steps namely,
(i) Glomerular ultra-filtration
(ii) Tubular reabsorption
(iii) Tubular secretion
(i)Glomerular Ultrafiltration:
Ultrafiltration of blood takes place inthe Malphighian body which acts as a biological filter. A Malphighian body comprises of Bowman's capsule and glomeruli.
The blood from the kidney flows through the capillaries of glomerulus where the blood pressure, is comparatively high and so brings about effective filtration. The hydrostatic pressure of the blood is always greater than the opposing pressures existing in the plasma protein and renal capillaries. Thus, the available net filtering force is chiefly responsible for glomerular filtration. The fluid in the capsule which is obtained by this process is termed glomerular filtrate. In man it is about 125ml/min. In 24 hours, the total volume of glomerular filtrate is 170 to 180 litres.
(ii) Tubular Reabsorption: Reabsorption takes place in the 'uriniferous tubules. Reabsorption of useful substances is a differential or selective process. The glomerular filtrate contains many useful substances such as glucose, amino acids, mineral salts and vitamins dissolved in large amount of water. Substances such as glucose, sodium and calcium, are actively reabsorbed in large quantities. Substances like urea and uric acid etc are reabsorbed in small quantities by a simple diffusion process or passive reabsorption. Substances like creatinine are not reabsorbed. They are completely eliminated.
Proximal convoluted tubule is responsible for the reabsorption of water, glucose, sodium phosphate and bicarbonates. On entering the distal convoluted tubule, the urine becomes nearly isotonic to the surrounding tissue fluid due to the active transport of sodium and passive transport of water. As urine (isotonic) passes into the collecting tubule it becomes once again hypertonic by the osmotic reabsorption of water under the influence of the hormone ADH. Thus the urine formed contains 96% of water, 2% urea and 2% of the metabolic products.
(iii) Tubular Secretion: This is the final step in the formation of urine during which the walls of tubule actively remove additional waste substances which are harmful to the body from the blood that have escaped filtration.

Food is prepared in the green leaves of the plant by combining carbon dioxide and water in presence of sunlight and chlorophyll. The carbon dioxide required for photosynthesis is taken by the plants from air. The carbon dioxide enters the leaves through tiny pores called stomata. The water required is taken from soil. The sunlight provides the energy required to carry out the preparation of food. The green pigment called chlorophyll present in green leaves help to absorb energy from sunlight. The oxygen produced as a by-product goes into the air.
The food prepared by the green leaves is in the form of glucose. This glucose is sent to different parts of the plant. The excess glucose is changed into starch and is stored in the leaves of the plant. The food prepared by photosynthesis provides all the energy to a plant which it needs to grow. Thus, it is clear that the green plants convert sunlight energy into chemical energy. by making food. When we eat plant foods such as fruits, vegetables, food grains etc., the chemical energy stored in them is released in our body during respiration.
During the process of photosynthesis, the following three steps take place:
(i) Absorption of sunlight energy by chlorophyll.
(ii) Conversion of sunlight energy into chemical energy and splitting of water into hydrogen and oxygen using sunlight energy.
(iii) Reduction of carbon dioxide to form carbohydrates using the chemical energy.

(i)Anchoring the plant
(ii)Source of water and minerals
(iii)Availability of oxygen for respiration of root cells
(iv)Symbiotic association with microbes.

The alimentary canal is nearly 9 m long. It includes -
1. Mouth and Buccal Cavity: It is the upper most opening of human digestive system which gives passage if for ingestion of food. Buccal cavity takes in the food cuts the food into small pieces and chews them to make it soft or easy swallowing. Buccal cavity also has three pairs of salivary glands. These glands secrete saliva
which contains water, salts, mucin and an enzyme ptyalin. Ptyalin is salivary amylase which splits starch and glycogen into maltose.
2. Pharynx: It acts as a passage for the food from buccal cavity to oesophagus.
3. Oesophagus: It is a long and tubular structure which pushes the food rhythmically forward.
4. Stomach: It is a large organ which expands when food enters it. It serves four functions:
(i) Storage of food - Carbohydrates gets digested till the salivary enzyme ptyalin is destroyed by HCI, secreted in the stomach.
(ii) Churning of food - The stomach wall produces muscular contraction, for the food to get churned and to mix it thoroughly with gastric juice.
(iii) Partial digestion - There are three types of gastric juice in stomach namely HCI, protein digesting enzyme (pepsin) and mucus.
(iv) Exit of food - The stomach releases food into the small intestine in a controlled and regulated manner.
Functions of the Gastric Juices
(i) HCI - Makes the tnedium acidic, kills the germs present in the food.
(ii) Pepsin - Acts in acidic medium, breaks proteins into peptone.
(iii) Mucus - Protects the inner lining of the stomach from the action of the acid under normal conditions.
(iv) Gastric juice - Also contains some gastric lipase which partially breaks down lipids.
(v) Regulation of flow of food - Stomach regulates the flow of partially digested food into the small intestine.
5. Small intestine: It is the longest part of digestive system. It is a narrow tube divided into three parts 
(i) duodenum
(ii) jejunum and 
(iii) ileum. It is the site of the complete digestion of proteins, carbohydrate and fats. Duodenum receives the secretion of bile from bile duct and pancreatic juice from pancreatic duct.
Bile juice
(i) It is secreted by liver and stored temporarily in the gall bladder.
(ii) It has bile pigments and bile salts.
(iii) It has no enzymes and thus has no chemical action on food, but breaks down the fat molecules into small globules.
(iv) This process is called emulsification of fats. Pancreatic juice
(i) It includes pancreatic amylase which breaks down starch, pancreatic lipase which breakdown lipids and trypsin for digesting proteins.
(ii) The secretions of pancreatic duct make the medium alkaline, which is favourable for the action of pancreatic enzymes.
Pancreas secretes hormones insulin and glucagons. The walls of small intestine contains glands which secrete intestinal juice called succus entericus which helps in digesting protein A into amino acids, carbohydrates into glucose, fats into fatty acids and glycerol. The digested food is taken up by the walls of the intestine which has numerous fingers like projection called villi. Villi are richly supplied with blood vessels which the absorbed food to obtain energy. The presence of villi and
blood capillaries increases the capacity of absorption by the wall of intestine
6. Large intestine: It includes three parts namely caecum, colon and rectum. It is shorter but wider than small intestine. It secretes mucus for lubrication. The undigested food is collected as faeces egested out through anus which is regulated by anal sphincter.