Work Text:
Nutrition is the process by which organisms obtain food and energy for growth, repair and maintenance of the body.
Digestion
Physical digestion: Mechanical break up of food into smaller particles, increasing surface area to volume ratio. (E.g. chewing, peristalsis, etc.)
Chemical digestion: Chemical break down of large molecules in food into smaller soluble molecules which can be absorbed. Involves hydrolytic reactions catalysed by digestive enzymes.
Mouth- Physical digestion in the form of chewing. This increases the surface area to volume ratio of food in contact with saliva containing amylase.
Chemical digestion is also present in the form of salivary amylase being secreted by salivary glands. The tongue mixes food with amylase and rolls it to the back of the mouth.
Stomach- Physical digestion (churning of the stomach). Gastric glands secrete gastric juices that contain HCl (hydrochloric acid), which stops the action of salivary amylase, kills viruses and bacteria and converts inactive pepsinogen into pepsin. Said conversion is for the digestion of protein only (chemical digestion).
Small intestine- Chemical digestion. Secretion of digestive enzymes, bile and pancreatic juice secreted by the lining of the duodenum. Consists of the duodenum, jejunum and ileum.
Large intestine- Consists of the colon and the rectum.
Other organs:
Oesophagus- A narrow, muscular tube that extends to the stomach. It is made of both longitudinal and circular muscles that are antagonistic, meaning that they oppose each other.
When the longitudinal muscles contract, the circular muscles relax and vice versa. These muscles allow peristalsis, a process where rhythmic, wave like muscle contractions are used to help push food down the oesophagus, to occur.
Pancreas- The pancreas secretes pancreatic juice through the pancreatic duct into the duodenum. Said pancreatic juice consists of amylase, protease (also known as trypsin) and lipase.
Pharynx- The pharynx connects the buccal cavity to the oesophagus and pharynx.
Food and inhaled air pass through here. The epiglottis function to prevent swallowed food from entering the trachea by closing the opening, preventing choking.
Gallbladder- Stores bile produced by the liver for peristalsis (breaking down of fats).
Absorption
The process where digested food substances are absorbed into the body cells.
Small intestine- Absorbs 90% of water and mineral salts. The small intestine is long and folded and is lined with finger-like projections called villi.
These villi have epithelial cells, which contain many microvilli. These increase the surface to volume ratio to increase the rate of diffusion of food molecules into the villi.
The epithelium of the villi is one-cell thick, decreasing the diffusion distance for the food molecules to be absorbed into the microvilli. The epithelial cells have many mitochondria in order to release energy for active transport.
Each villus is supplied with blood capillaries and lacteal to transport the absorbed amino acids, glucose and fats to maintain a steep concentration gradient.
Large intestine- No digestion occurs in the large intestine. Absorption of water via osmosis and absorption of vitamins and mineral salts via active transport occurs.
Undigested food is stored in the rectum temporarily and released at the anus as faeces vis egestion.
Assimilation
Assimilation is the process whereby some of the absorbed food substances are converted into new protoplasm or used to provide energy.
Hepatic portal vein- Glucose and amino acids are absorbed into the blood capillaries. The blood capillaries unite to form larger blood vessels which unite to form a larger vein (hepatic portal vein). Said vein transports nutrients from the small intestine to the liver. Nutrients leave the liver by the hepatic vein.
Transportation of fats (hepatic artery)- Fats are carried by lacteal into the lymphatic capillaries. The lymphatic capillaries join to form larger lymphatic vessels. Fats are transported into the liver via the hepatic artery.
Utilisation of:
Glucose- Broken down during respiration to release energy.
Excess glucose is converted into glycogen in the liver. A hormone called insulin produced by the islet of Langerhans in the pancreas, stimulates the liver.
Amino acids- Converted into new cytoplasm used for the growth and repair of worn-out cells. Also used to form enzymes and hormones.
Excess amino acid is deaminated into urea in the liver.
Fats- When glucose is short in supply, fats are hydrolysed to release energy. If not used for said purpose, fats are used to build protoplasm (e.g. cell membrane).
Excess fats are stored as adipose tissues, which act as shock absorbers in order to protect the organs.
Functions of liver
Production of bile- Bile is temporarily stored in the gallbladder and emulsifies fats through chemical digestion for faster digestion by lipase (increase surface area to volume ratio).
Deamination of amino acids- The process by which amino groups are removed and converted into urea.
Amino groups are removed and converted into urea which is removed by the body as urine.
Carbon groups are converted into glucose in the liver.
Urea is transported to the kidneys to be excreted as urine.
Amino groups: Amino acid —(deamination)—amino group—(converted into)— ammonia —(converted into)— urea
or
Carbon groups: Amino acid —(converted into)—carbon residue—(converted into)— glucose —(excess converted into)— glycogen
Regulation of blood glucose concentration-
Insulin and glucagon are hormones released by the Islets of Langerhans. They help to regulate the level of blood glucose in the blood.
When the blood glucose concentration rises, insulin sends signals to the liver. The liver converts glucose into glycogen for storage and the blood concentration decreases.
When the blood glucose concentration falls, glucagon sends signals to the liver. The liver converts glycogen into glucose to be released into the bloodstream and the blood glucose concentration increases.
Breakdown of hormones- Hormones are broken down in the liver.
Breakdown of alcohol- The liver detoxifies harmful substances into harmless ones. Alcohol is broken down in the liver. The liver cells contain an enzyme which breaks down alcohol into compounds that can be used in respiration.
Effects of excessive alcohol consumption
Digestive system- Alcohol stimulates acid secretion in the stomach. Excess stomach acid increases the risk of gastric ulcers. Prolonged alcohol abuse leads to liver cirrhosis. Cirrhosis may haemorrhage, leading to liver failure and death.
Nervous system- Alcohol acts as a depressant, reduces self-control, increases reaction time, has long term effects on the brain and affects social relationships (social implications).