nutrition in humans

1. describe the functions of various parts of the digestive system in relation to ingestion, digestion, absorption, assimilation and egestion of food

Mouth

Ingestion, 

• food enters the body through the mouth which leads to the buccal cavity 

• Buccal cavity: inside the mouth, the beginning of  the alimentary canal

Physical digestion,

• teeth breaks down large pieces of food into smaller pieces, increases surface area-to-volume ratio of  food so that salivary amylase can act on it more efficiently (chewing)

Salivary glands

Chemical digestion,

• secretes saliva in the mouth

• Saliva contains salivary amylase (optimum pH 7), which digests starch to maltose

Oesophagus

• No digestion takes place

• Bolus (plural: Boli) is swallowed and enters the Oesophagus via the pharynx

• Boli is formed when the tongue rolls the food into small, slippery, round masses

Stomach

Chemical digestion,

• presence of food stimulates the gastric glands to secrete gastric juice into the stomach cavity

• Gastric juice contains HCL, mucus (protects the stomach walls against being digested by the enzymes it produces, moistens the food to allow easy movements within the stomach) and pepsin

HCL,

• stops the action of salivary amylase by denaturing it

• Provides low pH environment, about pH 2, which is the optimum pH for protease to digest proteins 

• Kills certain potentially harmful micro- organisms in the food

• Protease digests proteins into polypeptides 

Physical digestion,

• peristalsis in the stomach wall churns and breaks up the food, mixes the food well with gastric juice

Duodenum

Chemical digestion,

• Chyme enters the duodenum through the pyloric sphincter (relays and opens) and stimulates the pancreas, gall bladder and epithelial cells in the small intestine.

• Chyme is formed when food remains in the stomach for 3-4 h to form partially digested food which is liquefied

• Pancreas secretes pancreatic juice, which passes through the pancreatic duct into the duodenum

• Pancreatic juice contains pancreatic amylase, protease and pancreatic lipase

• epithelial cells produces maltase, protease, lipase

• pancreatic juice, intestine juice and bile are all alkaline

• Neutralise the acidic chyme

• Provide a suitable alkaline medium (pH 8) for the action of pancreatic and intestinal enzymes

Physical digestion 

• gall bladder releases bile, which passes through the bile duct into the duodenum

• Bile does not contains enzymes, cannot digest food

• Bile speeds up the digestion of fats

Pancreas

Chemical digestion 

• Pancreas secretes pancreatic juice, which passes through the pancreatic duct into the duodenum

• Pancreatic juice contains pancreatic amylase, protease and pancreatic lipase

Assimilation

• releases insulin produced by the islets of langerhans to stimulate liver cells and convert excess glucose into glycogen 

• Releases glucagon produced by the islets of langerhans to stimulate liver cells and convert glycogen into glucose for energy

Gall bladder

Physical digestion 

• gall bladder releases bile, which passes through the bile duct into the duodenum

• Bile does not contains enzymes, cannot digest food

• Bile speeds up the digestion of fats by emulsifying fats

• Lower the surface tension of the fats p, reduce the attractive forces between the fat molecules

• Causes fats to break into tiny fat droplets suspended in water forming an emulsion

• Emulsification increases the surface area-to-volume ratio of the fats, speeding up their digestion with lipase

Liver

Assimilation (process whereby some of the absorbed nutrients are converted into new cytoplasm or used to provide energy)

• in the liver, most of the absorbed sugars are converted into glycogen and stored

• Some glucose leaves the liver and is distributed around the body to be assimilated and broken down during tissue respiration to release energy for the vital activities of the cells

• Excess amino acids are deaminated in the liver 

• Amino acids that enter the cells are converted to new cytoplasm that is used for growth and repair of worn-out parts of the body, used to form hormones and enzymes

Ileum

Absorption (process whereby digested food substances are absorbed into the body cells)

• Glucose and amino acids are absorbed by diffusion or active transport depending on the conc. of glucose and amino acids in the blood capillaries and the lumen of the small intestine 

• Glycerol and fatty acids diffuse into the epithelium and combine to form minute fat globules that enters the lacteals

Colon

• Absorbs remaining water in the undigested food

Rectum

Egestion (removal of undigested material in the body)

• Undetected and unabsorbed matter is stored temporarily in the rectum before discharged as faeces through the anus 

Anus

Egestion

• Faeces leave the body through the anus

2. Describe peristalsis in terms of rhythmic wave-like contractions of the muscles to mix and propel the contents of the alimentary canal

Definition 

• Rhythmic, wave-like muscular contractions in the wall of the alimentary canal 

• circular muscles contract (inner layer), they constrict the lumen 

• Longitudinal muscles contract (outer layer), they shorten and widen the lumen

• Circular and longitudinal muscles are antagonistic muscles 

• Circular contract, longitudinal relax, wall of the gut constricts, gut becomes narrower and longer, food is squeezed or pushed forward

• Longitudinal contracts, circular relax, gut dilates and becomes wider and shorter, widens the lumen for food to enter

3. Describe the function of enzymes in digestion, listing the substrates and end products

Amylase

• Amylase digests starch into maltose

Maltase 

• Maltase digests maltose into glucose

Protease

• Pepsin & trypsin 

• Pepsin digests proteins into polypeptides in the stomach

• Protease digests undigested proteins into polypeptides in the small intestine

• Protease further digests polypeptides into amino acids in the small intestine 

Lipase

• Lipase digests fats into fatty acids and glycerol 

4. Explain how the structure of a villus, including the capillaries and lacteal, is suited for its function of absorption

• The inner surface of the ileum is folded extensively and has numerous minute finger-like projections called villi, increases the surface area for absorption 

• The epithelium is only one cell thick, provide a short diffusion distance for nutrients to pass through

• The cells of the epithelium have many microvilli to further increase the surface area of the small intestine

• Each villi has many blood capillaries, allows blood to transport the absorbed glucose and amino acids in order to maintain a diffusion gradient

• Each villi contains a lacteal that transports absorbed fats away and maintain a steep diffusion gradient

• Epithelial cells contain many mitochondria, provide energy for active transport of nutrients into the villi 

5. State the function of the hepatic portal vein 

• the transport of blood rich in absorbed nutrients from the small ingestion to the liver

6. State the role of the liver in:

Conversion of glucose to glycogen and vice versa

When blood glucose level is above normal,

• Insulin is secreted and the liver cells are stimulated to convert  glucose into glycogen for storage, blood glucose level decreases to normal

• Glucagon is secreted and the liver cells are stimulated to convert glycogen into glucose to be released into the bloodstream, blood glucose level increases to normal

Fat digestion 

• Helps the digestion of fats by secreting bile

Deamination of amino acids and formation of urea

• Excess amino acids are transported to the liver

• Amino groups are removed and converted into urea

• Deamination: process whereby amino groups are removed from amino acids and converted to urea

• Deaminated amino acids are converted into glucose in the liver, excess glucose is converted into glycogen 

Breakdown of alcohol (detoxification)

• Liver cells convert harmful substances into harmless ones

• Alcohol is broken down in the liver

• Alcohol can cause damage to the digestive system and slow down brain functions

• Liver cells contain an enzyme which breaks down alcohol compounds that can be used in respiration, providing energy for cell activities

Breakdown of hormones (detoxification)

• After they have served their purpose, they are broken down in the liver

7. Outline the effects of alcohol consumption on the brain, digestive system, long term effects of excessive consumption and the social implications.

Brain

Reduces self-control 

• person becomes carefree as alcohol takes away his self-control

• May do thing that he regrets after the effects of the alcohol have worn off

Effect on reaction time

• more alcohol consumption, other observable effects of intoxication occurs, eg. slurred speech

• Blurred vision and poor muscular coordination makes him clumsy and unable to walk steadily

• Judgement deteriorates and tends to underestimate speed

• May dive faster with less caution 

• His reaction becomes slower

• Higher tendency to be involved in a traffic accident

Depressants

• slows down some brain functions

Digestive system

• Alcohol stimulates acid secretion in the stomach

• Excess stomach acid increases the risk of gastric ulcers

Long term effects of excessive consumption 

Brain

‘Wet brain’

• type of dementia caused by brain damage

• Alcohol interferes with the absorption of vitamin B1 in the small intestines

• Vitamin B1 is part of an enzyme needed to break down sugar to release energy for brain activities, eg. the conduction of nerve impulses

Shrinkage of brain volume

• Brain becomes smaller than normal, especially the region associated with memory and reasoning

During pregnancy, affect the development of fetus

• lead to lifelong physical, mental and behavioural problems

Social implications

• Addicted to alcohol

• Unable to stop drinking until he’s drunk

• Body becomes dependent in alcohol, becomes an alcoholic

• may neglect their work and families

• May exhibit violent behaviours, especially towards family members

• May tend to commit crimes