chapter 6: nutrition in humans

Question 1

what is nutrition?

Answer 1

• process by which organisms obtain food and energy for growth, repair and maintenance of the body

Question 2

what processes do nutrition consist of in humans?

Answer 2

• feeding/ingestion: food taken into the body
• digestion: large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells
• absorption: digested food substances are absorbed into the body cells
• assimilation: some of the absorbed food substances are converted into new protoplasm or used to provide energy

Question 3

how long is the alimentary canal and where is most of in located at?

Answer 3

• nine meter long tube that extends from the mouth to the anus
• most of it is coiled in the abdominal cavity
  > it makes us a large part of the organ system for nutrition

Question 4

what are the parts in the mouth and buccal cavity and what are its functions?

Answer 4

• food enters through the mouth into the buccal cavity
  in the mouth:
  teeth: chewing action of the teeth breaks down large pieces of food into smaller pieces
  > increases SA to V ratio of food so enzymes can act on it more efficiently

salivary glands: secrete saliva into the mouth
> saliva flows into the buccal cavity via salivary ducts (tubes)
> moistens the food and makes it easier to swallow

tongue: the tongue helps to mix the food with saliva
>and roll food into boli
*taste buds on the tongue help you identify and select suitable foods

Question 5

what is the pharynx and what is its function?

Answer 5

• part of the gut that connects the buccal cavity to the oesophagus and the larynx (voice box)
• also leads to the trachea (windpipe)
• larynx has a slit-like opening called the glottis
• trachea leads to the lungs

Question 6

what is a diaphragm?

Answer 6

-a sheet of muscle that separates the thorax from the abdomen which plays a part in breathing

Question 7

what is the oesophagus? and what muscles does it contain?

Answer 7

• a narrow, muscular tube
  > passes through the thorax (chest) and the diaphragm to join the stomach
• diaphragm: a sheet of muscle that separates the thorax from the abdomen which plays a part in breathing
• wall of oesophagus contains two layers of muscle which are present along the whole gut from the oesophagus to the rectum:
  (longitudinal muscles): on the outer side of the gut wall
  (circular muscles): on the inner side of the gut wall
  > both muscles produce long, slow contractions which moves food along the gut via peristalsis

Question 8

define peristalsis.

Answer 8

• it is the rhythmic, wave-like muscular contractions in the wall of the alimentary canal

Question 9

describe the process of peristalsis.

Answer 9

• enables the food to be mixed with digestive juices & pushes or propels the food along the gut
• circular muscles constrict the lumen
  >whereas longitudinal muscles shorten and widen the lumen
• both muscles are antagonistic muscles: means that one set of muscles contracts, the other set relaxes
• when the circular muscles contract, the longitudinal muscles relax
  > gut becomes longer and narrower > food is pushed forwards
• when the longitudinal muscles contract, the circular muscles relax
  > gut dilates and becomes shorter and wider > widens lumen for food to enter

Question 10

what is a stomach and what glands does in contain?

Answer 10

• stomach is a distensible (can be stretched or expanded) muscular bag
  > with thick and well developed muscular walls
• when it is fully distended, it sends signals to the brain that is it sated
• stomach walls have numerous pits
  > pits lead to gastric glands that secrete gastric juice into the stomach cavity
  -gastric juice contains pepsinogen, prorennin and hydrochloric acid
• HCl- kills most bacteria present in food, provides an optimal pH for pepsin activity and denatures salivary amylase
• stomach is able to store food for hours

Question 11

what is the pyloric sphincter and where is it located?

Answer 11

• a ring of muscle located at the place where the stomach joins the small intestine
• when this ring contracts > entrance to the small intestine closes
• when ring relaxes > the entrance opens, allowing food to pass from the stomach into the small intestine

Question 12

what is small intestine and what does it consist of?

Answer 12

• u-shaped duodenum (1) , the jejunum (2), much coiled ileum (3)
  > duodenum: receives chyme from the stomach and where most of the chemical processes take place
  > ileum: mostly where absorption of digested food products and water takes place
• small intestine about 6m long
• lining of the walls of small intestines contain glands which secrete digestive enzymes

Question 13

what is the large intestine and what does it consist of?

Answer 13

• shorter but much broader than small intestine
• 1.5 meters long and consists of:
  > colon
  > rectum ( a short muscular tube)
• faeces are stored temporarily in the rectum
  > when rectum contracts, faeces expelled through anus
• main function of the colon:
  > absorb water and mineral salts from undigested food material
• no digestion occurs

Question 14

what is at the junction of the colon and small intestine?

Answer 14

• caecum and appendix
• caecum: sac-like structure
• attached to it, is the tubular appendix
• in humans, caecum and appendix have no specific function
• appendix can become infected and inflamed, causing appendicitis

Question 15

what are the accessory organs?

Answer 15

• organs that are not part of the alimentary canal
  >liver, gall bladder and pancreas

Question 16

what is the liver and what does it consist of?

Answer 16

• dark red and made of five lobes, three on the right and two on the left
• attached to the lower surface of the liver:
  > hepatic portal vein, hepatic vein, hepatic artery
• liver cells PRODUCE and SECRETE bile
• bile: an alkaline greenish-yellow liquid
  > containing bile salts and bile pigments
• bile pigments gives bile its colour
  > they are waste products and are removed with the faeces or undigested matter

Question 17

what is the gall bladder?

Answer 17

• bile in temporarily stored in the gall bladder
  > bile contains bile salts that emulsify fat droplets
• gall bladder: greenish-yellow bag attached to the liver
• when gall bladder contracts, bile flows into the duodenum via the bile duct

Question 18

what is a pancreas?

Answer 18

• gland connected to the duodenum by pancreatic duct
• bile duct joins pancreatic duct opens into the duodenum
• pancreas produces pancreatic juice
  > contains digestive enzymes
  > also secretes the hormones: insulin and glucagon
• these hormones play an important role in controlling the concentration of glucose in the blood and carbohydrate metabolism

Question 19

what is digestion and what are the two processes involved?

Answer 19

• digestion: the process whereby large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells

> physical digestion and chemical digestion

Question 20

what is physical digestion? and where does it occur?

Answer 20

• involves the mechanical break-up of food into smaller particles
  > in mouth when you chew food by the action of teeth and tongue
  > in the stomach, the continual contractions and relaxations of the muscles in the stomach wall > cause a churning action&raquo_space;>
  »> breaks up food particles and mixes them with the digestive enzymes
• physical digestion increases the surface area to volume ratio of ingested food
  > so digestive enzymes can act on food more efficiently

Question 21

what is chemical digestion?

Answer 21

• is the breaking down of large molecules in food such as: proteins, carbohydrates and fats
  > into small soluble molecules which can be absorbed
• this involves hydrolytic reactions catalysed by digestive enzymes

Question 22

describe the processes that takes place in the mouth

Answer 22

1. food in mouth stimulates salivary glands to secrete saliva
2. saliva is mixed with food (mucus in saliva helps to soften the food)
   - saliva contains an enzyme called salivary amylase which digests starch to maltose
   - the pH of saliva is neutral ( arnd 7) salivary amylase is active at this pH
3. chewing breaks the food up into smaller pieces
   > increases the surface area to volume ratio for salivary amylase to work on
4. tongue rolls the food into small, slippery, round masses ( boli or bolus)
5. the boli are swallowed and passed down into the oesophagus via the pharynx
6. peristalsis in the walls of the oesophagus pushes each bolus of food down the stomach

Question 23

describe the digestive process that takes place in the stomach.

Answer 23

1. presence of food in the stomach > stimulates gastric glands to secrete gastric juice into the stomach cavity
2. peristalsis in the stomach wall churns and breaks up food
   > also mixes the food with the gastric juice
   - gastric juice: a dilute solution of hydrochloric acid (arnd pH 2), mucus and pepsin
3. the dilute hydrochloric acid:
   - stops the action of salivary amylase by denaturing it
   - changes the inactive form of pepsinogen in gastric juice to the active form, pepsin
   - provides a slightly acidic medium suitable for the action of pepsin
   - kills certain potentially harmful microorganisms in food
4. pepsin digests proteins into polypeptides
5. food normally remains in the stomach for about three to four hours
   > partly digested food becomes liquefied, forming chyme
6. chyme passes in small amounts into the duodenum when the pyloric sphincter relaxes and opens

Question 24

what are the digestive processes that take place in the small intestine?

Answer 24

1. chyme enters the small intestine > stimulates:
   - the pancreas to secrete pancreatic juice, containing the enzymes: pancreatic amylase, trypsin, and pancreatic lipase > pancreatic juices passes through the pancreatic duct into the duodenum

• gall bladder to release bile > does not contain enzymes so does not digest food
  > bile salts speed up the digestion of fats
  > bile passes through the bile duct into the duodenum
• the epithelial cells in the small intestine to produce maltase, peptidases and lipase

2. food now comes into contact with pancreatic juice, bile and intestinal juice { all alkaline}
   - neutralises the acidic chyme
   - provide a suitable alkaline medium for the action of the pancreatic and intestinal enzymes

Question 25

describe the process of carbohydrate digestion.
(what carbohydrates food has)
( where it begins)
( the end rpoducts)

Answer 25

• food we eat may contain starch and other carbohydrates such as lactose, sucrose and cellulose

-starch digestion begins in the mouth
> as food does not remain long in the mouth only a little starch is digested by salivary amylase into maltose
- no digestion of carbohydrates occurs in the stomach
- when they enter the small intestine:
starch —————————>maltose———–>glucose
pancreatic amylase maltase

lactose ——————> glucose + galactose
lactase

sucrose —————-> glucose + fructose
sucrase

• end products are simple sugars such as: glucose, fructose and galactose ( which can be absorbed into the bloodstream)
• cellulose is NOT digested at all in humans

Question 26

describe protein digestion

Answer 26

proteins are digested by proteases:
- some protein digestion begins in the stomach
> pepsin digests proteins to polypeptides
- the undigested proteins which enter small intestine
> digested by trypsin into polypeptides
- polypeptides produced are further digested into amino acids by peptidases
> end products are amino acids which can be absorbed

proteins ———> polypeptides
trypsin

polypeptides ————-> amino acids
peptidases

Question 27

describe fat digestion.

Answer 27

• in the small intestine, bile salts emulsify fats
  > they lower the surface tension of the fats by reducing the attractive forces between the fat molecules
• this causes the fats to break into tiny fat droplets suspended in water > forming an emulsion
• THIS IS A PHYSICAL BREAK UP
• emulsification increases the surface area to volume ratio of the fats
  > speeding up their digestion by lipase
• emulsified fats are digested by lipases
  > end products: fatty acids and glycerol

Question 28

what is absorption?

Answer 28

• it is the process whereby digested food substances are absorbed into the body cells
• small intestine: products of digestion like simple sugars, amino acids, fatty acids and glycerol are absorbed throughout the small intestine esp the ileum
• water and mineral salts are absorbed by the small intestine and colon
• small intestine absorbs most of the water that enters the alimentary canal
• from the small intestine, about 1-1.5 L of water pass into the large intestine daily
  > colon absorbs 90% of that

Question 29

how is the small intestine adapted for absorption?

Answer 29

1. the surface area of the small intestine is increased:
   - inner surface of the small intestine is folded
   - the folds bear numerous minute finger-like projections called villi
   - under the microscope, we see that the epithelial cells of the villi have numerous microvilli to increase surface area
2. the villi have thin walls or membranes ( epithelium is only one cell thick)
   > makes it easy for the nutrients to diffuse through it to the bloodstream
3. the small intestine is long to provide sufficient time for absorption
4. in each villus, is a lacteal or lymphatic capillary surrounded by blood capillaries
   > the lymphatic capillaries transports fats while the blood capillaries transport sugars and amino acids away from the small intestine
   - this continual transport of digested food substances maintains the concentration gradient for digested food substances

Question 30

what does the rate of absorption of digested food substances depend on?

Answer 30

• the surface area
• the membrane that separates the food substances from the blood capillaries
• the concentration gradients of individual food substances

Question 31

how does absorption take place in the intestines?

Answer 31

• glucose and amino acids are absorbed by diffusion into the blood capillaries of the villi
• glucose and amino acids are also absorbed by active transport when,
  > there is a lower concentration of these digested food substances in the lumen of the small intestine than the blood capillaries
• glycerol and fatty acids diffuse into the epithelium
  > they combine to form minute fat globules which enter the lymphatic capillary

Question 32

what happens to undigested and unabsorbed matter?

Answer 32

• they are stored temporarily
  > mainly in the rectum before it is discharged as faeces through the anus
• this removal of undigested matter from the body is called: egestion or defecation

Question 33

what is assimilation?

Answer 33

• the process whereby some of the absorbed food substances are converted into new protoplasm or used to provide energy

Question 34

how are glucose and amino acids transported and utilised?

Answer 34

glucose is used by all the cells as a source of energy

• after absorption, the blood in the villi is now rich in nutrients
  > the blood capillaries unite to form a larger blood vessels, which in turn unite to for a large vein
  > HEPATIC PORTAL VEIN
  > it transports nutrients to the liver
• in liver, most absorbed sugars are converted into glycogen and stored
• some glucose is transported by the blood leaving the liver and is distributed around the body
  > it is assimilated then oxidised during tissue respiration
  > to release energy for the vital activities of the cells
• excess glucose is returned to the liver and stored as glycogen
  > a hormone (insulin), produced by the islets of Langerhans in pancreas
  > stimulates the liver to convert excess glucose to glycogen
• when body needs E, body converts glycogen back into glucose
  > glucose is transported by blood to the body cells
• amino acids which enter the cells are converted into new protoplasm that is used for growth and repair of worn-out parts of body
• amino acids also used to for enzymes and hormones
  > excess amino acids are deaminated

Question 35

how are fats transported and utilised?

Answer 35

• fats are absorbed into the lymphatic capillaries
• the lymphatic capillaries join to form larger lymphatic vessels > which discharge fats into the bloodstream
• blood carries fats to all parts of the body especially liver
  > here, fats converted into forms that can either be broken down or stored
• under normal conditions when there is a sufficient supply of glucose, fats are not broken down
  > instead fats are used to build protoplasm
  > eg. cell membranes
• when glucose is in short supply
  > eg. during fasting, fats are broken down to provide the energy needed for vital activities of the body
• excess fats are stored in the special tissues called adipose tissues
  > which occur beneath the skin and around the heart and kidneys
  > protects these organs by acting as shock absorbers

Question 36

what are the functions of liver?

Answer 36

• regulation of blood glucose concentration
  -production of bile
• iron storage
• protein synthesis
• deamination of amino acids
• detoxification

Question 37

how does the liver regulate blood glucose concentration?

Answer 37

• liver plays a key role in carbohydrate metabolism by keeping the amount of glucose in the blood constant, especially after a heavy meal or during fasting

after a heavy meal:
too much glucose in the blood—————>
-insulin secreted stimulates the liver
(liver converts glucose into glycogen for storage)
> blood glucose concentration decreases

during fasting:
too little glucose in blood —————->
glucagon secreted
(liver converts glycogen into glucose to be released into the bloodstream)
>blood glucose concentration increases

Question 38

describe the production of bile from the liver.

Answer 38

• liver plays an important role in fat digestion
• helps the digestion of fats by secreting bile
• bile stored temporarily in the gall bladder before use

Question 39

how does the liver store iron?

Answer 39

• red blood cells of the body become worn out after sometime,
  >in mammals, these cells do not have nuclei and are destroyed in the spleen, a gland found near the liver
• their haemoglobin is brought to the liver
• the liver breaks down the haemoglobin and stores the iron released in the process
  >bile pigments are also formed from the breakdown of the haemoglobin

Question 40

how does the liver synthesise proteins?

Answer 40

• liver synthesises proteins found in blood plasma from amino acids in the diet
  > plasma proteins include: prothrombin and fibrinogen
  >which are essential for clotting blood

Question 41

what is deamination?

Answer 41

the process by which amino groups are removed from amino acids and converted into urea

Question 42

how does the liver carry out deamination of amino acids?

Answer 42

• excess amino acids are transported to the liver
• their amino groups are removed and converted into urea
  > DEAMINATION
• urea is then excreted by the kidneys in the urine
• the remains (carbon residues ) of the deaminated amino acids are converted into glucose in the liver
• any excess glucose formed in this way is converted into glycogen

Question 43

how does the liver carry out detoxification?

Answer 43

• harmful substances may be absorbed into the blood from the gut
• eg. the food preservative benzoic acid is made harmless by the liver cells
• the process of converting harmful substances into harmless ones is known as detoxification
• alcohol also broken down in the liver
• liver cells contain an enzyme called alcohol dehydrogenase
• this enzyme breaks down alcohol to a compound called acetaldehyde
• acetaldehyde can be broken down further to compounds that can be used in respiration to provide energy for cell activities

Question 44

what are the effects of excessive consumption of alcohol?

Answer 44

• harmful effects on the digestive system
• harmful effects on the nervous system
• social implications

Question 45

why does alcohol have harmful effects on the digestive system?

Answer 45

• alcohol stimulates acid secretion in the stomach
• excess stomach acid increases the risk of gastric ulcers
  -prolonged alcohol abuse may lead to cirrhosis of the liver
  >a disease in which the liver cells are destroyed and replaced with fibrous tissue
  > making the liver less able to function
• patients with alcoholic cirrhosis may hemorrhage or have bleeding in the liver
  > this can lead to liver failure and subsequently death.

Question 46

why does alcohol have harmful effects on the nervous system?

Answer 46

1. depressant
   - alcohol is a depressant
   - it slows down some brain functions ( effects vary from one person to another)
2. reduced self-control
   - under the influence of alcohol,
   > person becomes carefree as alcohol takes away his inhibitions
   - some may take personal and social liberties which they may regret after the effects of alcohol have worn off
3. effect on reaction times
   - observable effects of intoxication, like slurred speech occur
   - blurred vision and poor muscular co-ordination make him clumsy and unable to walk steadily
   - his judgement deteriorates and he tends to underestimate speed
   - many traffic accidents are caused by drunk drivers

Question 47

how does alcohol affect social implications?

Answer 47

• when a person drinks frequently, he can become addicted
• he is unable to stop drinking until he is drunk and his body becomes dependent on alcohol > becomes an alcoholic
• alcoholics are liabilities to society

> they may neglect their work and families, and exhibit violent behavior especially towards family members

> many crimes have been committed by people under the influence of alcohol

Question 48

what enzymes are in intestinal juice?

Answer 48

peptidase
lipase
maltase, sucrase, lactase

Question 49

what enzymes are in pancreatic juice?

Answer 49

trypsin
amylase
lipase