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Factors Affecting Photosynthesis


External factors

 Sunlight provides the necessary energy required for the process. The rate of photosynthesis depends on the quantity and quality of light. The rate increases with the increase in light intensity. In very weak light, no starch grains are formed except in shade by loving plants. Very intense light has an adverse effect on the protoplasm of the leaf. In blue and red lights, the rate of photosynthesis is maximum, while in green light, the rate is minimum.

Carbon dioxide:
 The rate of photosynthesis increases with an increase in carbon dioxide concentration up to a certain level. Beyond that, carbon dioxide concentration has no effect on the rate of photosynthesis. On the contrary, it may prove inhibitory.

 Low temperature has an inhibitory effect on the rate of photosynthesis because the activity of the enzymes is lowered. Increase in temperature initially accelerates the rate, but high temperature inhibits photosynthesis.

 About 1% of water absorbed by the roots is utilised in photosynthesis. Hence, it is rarely a limiting factor. Decrease in water causes closure of stomata, thereby reducing photosynthesis.

Oxygen: Accumulation of oxygen retards the rate of photosynthesis.

Internal factors

 It is essential for photosynthesis. In the absence of chlorophyll, leaf cannot absorb the required solar energy for the plants. An etiolated leaf cannot carry out photosynthesis.

Accumulation of photosynthetic products
: Accumulation of sugar or starch in the assimilating cells slows down the process of photosynthesis or even brings it to a standstill.

Some interesting facts about photosynthesis:

  • Nine-tenth of photosynthesis in the world is undertaken by algae of oceans
  • The green alga, Chlorella, is the most preferred in research on photosynthesis
  • The enzyme rubisco, which is involved in dark reaction, is the most abundantly found enzyme on this earth

Nutrition in Animals

Nutrition in higher animals involves ingestion, digestion, absorption, assimilation and egestion.


Ingestion: It is the intake of food including solid food and water.  

Digestion: It is the process of breaking down of molecules into smaller units so that they could pass through the cell membranes of the wall of the gut.  

Absorption: It is the intake of soluble digested nutrients into blood.  

Egestion: It is the elimination of undigested food from the body.
  • Amoeba engulfs food by the process of phagocytosis
  • Grasshopper is herbivorous and it possesses a well-developed digestive system
Human Digestive System Man consumes both plant and animal products. Hence, he is an omnivore. Digestion in man is of two types, namely mechanical and chemical digestion.  

Mechanical digestion is breaking up of larger particles of food into a fine paste with the help of teeth and tongue.  

Chemical digestion is the hydrolysis of food into simplest particles by the action of enzymes. The human digestive system consists of alimentary canal and associated digestive glands. Alimentary canal runs from mouth to anus including regions such as pharynx, oesophagus, stomach, small intestine and large intestine. Also, some accessory digestive organs, such as salivary glands, pancreas, liver and gall bladder are connected to the main system by a series of ducts.
  • The mouth is an opening and is protected by upper and lower lips. It leads to the oral cavity with two lateral walls (cheeks), an anterior roof (hard palate), a posterior roof (soft palate) and a pair of jaws (the upper maxilla and lower mandible).
  • Each jaw holds four types of teeth on both sides, namely 2 incisors, which are chisel-shaped and used for cutting, chopping or gnawing; 1 canine, which is more pointed and used for ripping or shredding; 2 premolars and 3 molars, which are broad and used for shearing, crushing and grinding.
    • Man possesses heterodont, diphyodont and thecodont dentition.
    • The dental formula of permanent set of teeth in man is   (i) 2/2, (c) 1/1, (pm) 2/2, (m) 3/3 = 32 Each tooth has three layers, namely crown, neck and root. Crown remains capped with hard enamel, formed principally of calcium phosphate. It forms the chewing surface of the tooth. Both crown and root are covered by a layer of bony dentine inside which is a central pulp having nerve and blood supplies.
    • The oral cavity also receives saliva through the ducts of the salivary glands. Salivary glands secrete salivary amylase or ptyalin, an enzyme that begins the breakdown of starch into glucose. Bicarbonate ions in saliva neutralise the acids in food.
    • This mixture of food and saliva is then pushed into the pharynx and oesophagus. The oesophagus is a muscular tube whose muscular contractions propel food to the stomach.
    • Epithelial cells lining the inner surface of the stomach secrete gastric juices. Gastric juice contains hydrochloric acid, pepsinogen, a small amount of gastric lipase, mucus and rennin.
    • HCl secreted by the parietal cells lowers the pH of the stomach so that pepsin is activated.
    • Pepsin secreted by the chief cells is an enzyme that controls the hydrolysis of proteins into peptides.
    • Gastric lipase partially breaks down lipids.
    • Pepsin activates prorenin into rennin. Rennin is the curdling enzyme which brings about the curdling of milk.
    • Stomach also mechanically churns the food. Chyme, the mix of acid and food in the stomach, leaves the stomach and enters the small intestine.
    • The small intestine is a coiled and narrow tube, which can be markedly divided into three regions, namely the proximal duodenum, middle jejunum and distal ileum. The wall of the small intestine is provided with several long finger-like projections called villi, which increase the surface area of the lining of intestine. This enhances the ability of absorption by the wall of the intestine.
    • Duodenum starts after the pyloric end of the stomach. It is a U-shaped structure and receives the secretion of common duct, which brings the secretions of liver and pancreas.
    • Liver is the largest digestive gland in humans. It is covered by Glisson’s capsule. It does not produce any enzymes but secretes bile which helps in emulsification of fats. Liver also plays an important role in glycogenesis and glycogenolysis.
    • Pancreas is situated between the stomach and loop of duodenum. It is both exocrine and endocrine gland. The enzymes produced by pancreas include trypsin, pancreatic amylase and steapsin. The hormones produced by the pancreas are insulin by β cells and glucagon by α cells.
    • Both bile and pancreatic juice enter duodenum through a common duct. In the duodenum, trypsin and pancreatic amylase digests protein and starch, respectively, while the bile emulsifies fat.
    • Partially digested proteins, carbohydrates and emulsified fats enter the jejunum and ileum, where intestinal juice called succus entericus completes the digestion of proteins into amino acids, carbohydrates into glucose and fats into fatty acids and glycerol.
    • The digested food then undergoes absorption by the wall of the intestine that contains blood capillaries. The simple molecules of food, which have been absorbed in the blood stream, reach the cells and are ready for utilisation. Utilisation of food is also part of nutrition, and is termed assimilation.
    • The small intestine opens into the large intestine. The large intestine is made up by the colon, caecum, appendix and rectum. Material in the large intestine is mostly indigestible residue and liquid. Rectal wall absorbs water from it and the faecal matter is egested out through anus.
Diseases and Disorders of Digestive System
  • Excess secretion of gastric juice, particularly in an empty stomach, erodes the inner lining of the stomach. This erosion causes lesions or round depressions in the stomach walls called peptic ulcers.
  • Acidity is caused by anxiety, high mental tension and consuming hot, spicy and oily food.
  • Obesity is caused by over eating and lack of exercise.
  • Indigestion is caused by untimely, unhygienic and excessive food consumption.
  • Constipation is caused by consuming food that does not contain roughage.
  • Jaundice is caused by consuming alcohol, drug addiction, viral infection and physiological disorders.

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