8 Human nutrition (8.1, 8.2 & 8.3)

Lack of Vitamin C causes fibres in connective tissue of skin and blood vessels to not form properly, leading to bleeding under the skin, particularly at the joints. Swollen, bleeding gums, poor healing of wounds and feeling constantly tired, weak and irritable are symptoms of scurvy. In severe cases, teeth may fall out
Lack of vitamin D causes rickets, where bones become soft and deformed (this is because vitamin D is needed for absorption of calcium into the body, which is a key component of bones and teeth)
Lack of calcium causes muscle aches, cramps and spasms, with numbness of hands, feet and around the mouth. Teeth and bones become soft and more easily breakable. Deficiency can lead to rickets linked to vitamin D deficiency and osteoporosis
Lack of Iron causes anaemia - where there are not enough red blood cells, so tissues do not get enough oxygen delivered to them (this is because iron is a key component of haemoglobin)

3. Understand the concept of a balanced diet

A balanced diet is a diet that contains all the essential nutrients in the correct proportions to maintain good health. The nutrients needed are carbohydrate, fat, protein, vitamins, minerals, fibre(roughage)and water.

8.2 Human digestive system

1. Identify the main regions of the digestive system: mouth, salivary glands, oesophagus, stomach, small intestine(duodenum and ileum), pancreas, liver, gall bladder and large intestine(colon, rectum and anus)

2. Explain why most foods must be digested before they can be absorbed

Food particles consist of large molecules which cannot be absorbed by the body. Hence, they must be broken down into smaller soluble molecules so that they can be easily digested in the epithelium lining of the small intestine through the walls of the blood vessels into the blood

3. Describe physical digestion as the breakdown of food into smaller pieces without chemical change to the food molecules

4. Describe chemical digestion as the breakdown of large molecules into small molecules

5. State that physical digestion increases the surface area of food for the action of enzymes in chemical digestion

Examples: chewing by teeth, bile emulsifying fats, action of muscles in the stomach

6. Identify the types of human teeth (incisors, canines, premolars and molars)

7. Describe the structure of human teeth, limited to: enamel, dentine, pulp, nerves and cement, and understand that teeth are embedded in the gum

A tooth is embedded in the gum. There are two major parts of a tooth - the crown [which is visible above the gum line]and the root, which is embedded in the gum. The crown is covered by a layer of enamel, which is a very hard substance and prevents the tooth from any external damage. Inside the layer of enamel is a softer layer called the dentine. Inside the dentine is the pulp cavity, which contains nerves and blood vessels. These nerves and blood vessels enter the teeth through a small hole at the base of the root. The cement in the root enables the tooth to grip its bony socket in the jaw.

8. Describe the functions of the types of human teeth in the physical digestion of food

Incisors - biting and cutting food
Canines - tearing, holding and biting
Premolars and molars: Chewing and grinding food

9. Describe the functions of the main regions of the digestive system, limited to:

(a) mouth – ingestion, physical digestion, chemical digestion of starch by amylase

Ingestion is the act of taking food into the mouth. The teeth help chew the food to break it down into smaller pieces and help increase the surface areas so that enzymes in the saliva [salivary amylase] can act on it easily and start digesting starch into maltose. Saliva also helps lubricate the food and helps the small particles to stick together so that they can be easily swallowed down the oesophagus.

(b) salivary glands – secretion of saliva containing amylase

(c) stomach – physical digestion, chemical digestion of protein by protease, presence of hydrochloric acid in gastric secretions

The main function of the stomach is to store food and turn it into a liquid. When food enters the stomach, the muscles in the stomach alternately contract and relax, churning and mixing the food with the gastric juices to give a creamy liquid, increasing the surface area for digestion to occur efficiently
The two most important gastric juices are pepsin and renin, which act on proteins and convert proteins into smaller molecules called polypeptides. Renin particularly curdles the milk protein - caseinogen - into insoluble casein so that it remains in the stomach for an enough period to digest the proteins in milk by pepsin
Presence of HCl stops the action of salivary amylase - converts the inactive forms of pepsin and renin into active forms - kills bacteria and germs in the food - providing a slightly acidic environment for enzymes to work at optimum pH.
Pyloric sphincter is a valve that relaxes and contracts to allow digested food to pass into the small intestines at a time

(d) small intestine(duodenum and ileum) – chemical digestion of starch by amylase, maltose by maltase, protein by protease, lipids by lipase

When food enters the small intestine, pancreatic juice and bile are released and neutralise the acidic mixture, turning it into a slightly alkaline that enzymes can work at the optimum pH.
Pancreatic juice contains pancreatic amylase, which digests remaining starch to maltoseand maltase digests the maltose to simple sugars like glucose.
Pancreatic protease[trypsin] digests the polypeptides into simple amino acids
Bile emulsifies the lipids - breaking them into small droplets to increase the surface area so that they can be easily digested by pancreatic lipase to fatty acids and glycerol

(e) liver – production of bile and storage of glycogen

Bile is a green watery fluid made in the liver and is sent to the duodenum through the bile duct. It helps in the digestion of lipids and provides an alkaline environment in the small intestine for enzymes to work at an optimum pH. The green colour of the bile is caused by the bile pigments formed by the breakdown of haemoglobin in the liver.

(f) Gall bladder – storage of bile

(g) pancreas – alkaline secretion containing amylase, protease[trypsin] and lipase

(h) ileum and colon – absorption

(i) rectum and anus – egestion

The semi-solid waste is called the faeces. It is passed into the rectum by peristalsis and is passed out at intervals through theanus. The undigested material may spend from 12 to 24 hours in the intestine. The process of passing out the faeces is called egestion.

10. Describe the functions of amylase, maltase, proteaseand lipase, listing the substrates and end-products, limited to:

(a) Amylase breaks down starch to maltose [in the mouth]

(b) Maltase breaks down maltose to glucose [in the small intestine]

(c) Protease (pepsin and trypsin) breaks down protein to amino acids [in the stomach and small intestine]

(d) Lipase breaks down lipids to fatty acids and glycerol [in the small intestine]

11. Describe the function of hydrochloric acid in the stomach as killing ingested bacteria

12. Understand that the different proteases present in the stomach and the duodenum work best at different pH levels

13. Outline the role of bile in emulsifying fats to increase the surface area for the chemical digestion of fat to fatty acids and glycerol by lipase

Bile emulsifies the lipids - breaking them into small droplets to increase the surface area so that they can be easily digested by pancreatic lipase to fatty acids and glycerol

14. Describe peristaltsis as waves of contractions of longitudinal and circular muscles, which move food through the digestive system

8.3 Absorption and assimilation

Absorption by diffusion, osmosis and active transport is the movement of nutrients from the intestines into cells lining the digestive system and then into the blood. The small intestine is the region where nutrients are absorbed. Nearly all the absorption of digested food and most of the water takes place in the ileum [last part of the small intestine]. Some water is also absorbed from the colon [part of the large intestine]. The ileum is suited for absorption because it is quite long and provides a large surface area to absorb the digested food.
Inside, it has folds with tiny finger-like structures called villi, which increase its surface even more.
The lining epithelium is very thin, and the fluids can pass rapidly through it. The outer membrane of each epithelial cell has microvilli, which increase the exposed surface of the cell.
There is a dense network of blood capillaries in each villus.
When food is digested into small molecules, they pass into the cells of the villi and then into the blood vessels. From there, they are carried away in the bloodstream. The blood from the intestine goes to the liver through a large vein called the hepatic portal vein. The liver can store or change these digestion products before releasing them back into the bloodstream.
Some of the fatty acids and glycerol combine again to form fats inside the cells of the intestine. These fats then go into small vessels called lacteals, which are part of the lymphatic system. The fluid from the lacteals flows through the lymphatic system, which eventually empties into the bloodstream.

3. Understand that assimilation is the uptakeand use by cells of nutrients from the blood

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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