7 - Human Nutrition

• Dietary sources:

  • diet = the food that an organism consumes daily
  • human beings require these 7 types of nutrients:
  • carbohydrates
  • proteins
  • fats
  • vitamins
  • minerals
  • water
  • fibre
  • a diet which contains all these components is called a balanced diet
  • balanced diet = diet that contains all the essential nutrients in the correct proportions to maintain good health
• Carbohydrate

  • Source of energy
  • good food sources:
  • Rice, potato, sugary foods (e.g. cake, jam, honey)
• Fat/oil (fats are solid at room temperature, oils are liquid) -

  • source of energy (twice as much as carbohydrate)
  • used as insulation against heat loss, for some hormones, in cell membranes, for insulation of nerve fibres
  • good food sources
  • Butter, milk, cheese
• Protein -

  • Growth, tissue repair, enzymes, some hormones, cell membranes, hair, nails
  • Can be broken down to provide energy
  • good food sources:
  • Meat, fish, eggs
• Vitamin C -

  • Needed to maintain healthy skin and gums
  • a deficiency can lead to scurvy
  • good food sources:
  • Citrus fruits, tomato, mango
• Vitamin D -

  • Needed to maintain hard bones
  • helps in absorption of calcium from small intestine
  • deficiency can lead to rickets
  • good food sources:
  • Milk, cheese, sunlight
• Iron -

  • Needed for formation of haemoglobin in red blood cells
  • a deficiency can lead to anaemia
  • good food sources:
  • red meat, eggs, chocolate
• Calcium -

  • Needed to form healthy bones and teeth
  • helps for normal blood clotting
  • a deficiency can lead to rickets
  • good food sources:
  • Milk, cheese, fish
• Dietary fibre (roughage) -

  • This is cellulose, which adds bulk to undigested food passing through the intestines, maintaining peristalsis
  • a deficiency can lead to constipation
  • good food sources:
  • Vegetables, fruit, wholemeal bread
• Water -

  • Formation of blood, cytoplasm, as a solvent for transport of nutrients and removal of wastes (as urine)
  • enzymes work only in solution
  • good food sources:
  • Drinks, fruit, vegetables
• The causes of scurvy -

  • A shortage of vitamin C can lead to a deficiency disease called scurvy.
  • Fibres in connective tissue of skin and blood vessels do not form properly, leading to bleeding under the skin.
  • Other symptoms are: feeling constantly tired, weak and irritable, with joint pains and swollen, bleeding gums
  • In severe cases, the teeth can fall out.
• The causes of rickets -

  • A shortage of vitamin D or calcium can lead to a deficiency disease called rickets.
  • The symptoms are soft bones that become deformed.
  • Sufferers may become bow legged.
• Mouth -

  • Food is ingested here. It is mechanically digested by cutting, chewing and grinding of teeth.
  • Saliva is added.
• Salivary glands -

  • Produce saliva containing the enzyme amylase to begin the chemical digestion of starch.
  • The water in saliva helps lubricate food and makes small pieces stick together.
• Oesophagus -

  • Boluses (balls) of food pass through, from mouth to stomach.
• Stomach -

  • Muscular walls squeeze the food to make it semi-liquid.
  • Gastric juice contains protease to chemically digest protein and hydrochloric acid to maintain an optimum pH (1–2.5).
  • The acid also kills bacteria.
• Duodenum -

  • The first part of the small intestine. It receives pancreatic juice containing protease, lipase and amylase.
  • The juice also contains sodium hydrogencarbonate, which neutralises stomach acid, giving a pH of 7–8.
  • Carbohydrates, fats and proteins are digested here.
• Ileum -

  • The second part of the small intestine.
  • Enzymes in the epithelial lining chemically digest maltose and peptides.
  • Its surface area is increased by the presence of villi, which allow the efficient absorption of digested food molecules.
  • Most water is reabsorbed here.
• Pancreas -

  • Secretes pancreatic juice into the duodenum for chemical digestion of proteins, fats and starch.
• Liver -

  • Makes bile, which is stored in the gall bladder (bile is not an enzyme).
  • Bile contains salts that emulsify fats, forming droplets with a large surface area to make digestion by lipase more efficient.
  • It does not change the fat molecules chemically – it is just the droplet size that changes from large to small due to the action of bile.
  • Digested foods are assimilated here – for example, glucose is stored as glycogen.
  • Surplus amino acids are deaminated
• Gall bladder -

  • Stores bile, made in the liver, to be secreted into the duodenum via the bile duct.
• Colon -

  • The second part of the large intestine.
  • It reabsorbs water from undigested food, resulting in the formation of faeces.
  • It also absorbs bile salts to pass back to the liver.
• Rectum -

  • Stores faeces until they are egested.
• Anus -

  • Has muscles to control when faeces are egested from the body.
• Physical digestion -

  • the breakdown of food into smaller pieces without chemical change to the food
  • increases the surface area of food for the action of enzymes in chemical digestion
• Incisor -

  • position in mouth = front
  • description = chisel-shaped (sharp edge)
  • function = biting off pieces of food

  
• Teeth -

  A) Premolars

  B) Canine

  C) Incisors

  D) Molars

  
• Main organs of the digestive system

  A) mouth

  B) oesophagus

  C) stomach

  D) pancreas

  E) colon

  F) rectum

  G) gall bladder

  H) bile duct

  I) liver

  J) duodenum

  K) ileum

  L) anus

  
• Canine -

  • position in mouth = either side of incisors
  • description = slightly more pointed than incisors
  • function = similar function to incisors

  
• Premolar

  • position in mouth = behind canines
  • description = have two points (cusps), one/two roots
  • function = tearing and grinding food

  
• Molar -

  • position in mouth = back of the mouth
  • description = have four/five cusps, two/three roots
  • function = chewing and grinding food

  
• Structure of a molar tooth -

  A) enamel

  B) gum

  C) pulp

  D) dentine

  E) cementum

  F) periodontal membrane

  G) crown

  H) root

  
• Description of tooth part -

  • Enamel
  • hardest tissue in the body
  • protects the inner part of the tooth
  • made from calcium salts
  • Dentine 
  • harder than bone
  • made from calcium ions deposited on a mesh of collagen fibre
  • Pulp 
  • contains blood vessels and nerve endings to detect pain (a tooth is a living structure)
  • Cementum 
  • similar composition to dentine
  • anchors the tooth into the jaw
  • Periodontal Membrane
  • anchors the cementum of the tooth to the jawbone
  • contains many nerve endings to detect pressures when chewing
• Bile: 

  • made in the liver, stored in the gallbladder and transferred to the duodenum by the bile duct
  • has no enzymes but contains bile salts 
  • bile salts emulsify the fats, breaking them up into small droplets with large surface area
  • digests lipase more efficiently
  • is slightly alkaline as it contains sodium hydrogencarbonate, and has the function of neutralising the acidic mixture of food and gastric juice as it enters the duodenum
  • this is important because the enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate
• Enzymes -

  • carbohydrase = acts in pancreas, salivary glands and small intestine
  • amylase = acts in the duodenum
  • maltase = acts in the small intestine
  •  protease = acts in stomach, pancreas, and small intestine
  • pepsin = acts in stomach
  • trypsin = acts in small intestine
  • lipase = small intestine
• Chemical digestion:

  • the process by which large insoluble molecules are broken down into small soluble molecules
  • enzymes speed up the process

  Main places of chemical digestion -

  • mouth
  • stomach
  • small intestine
• Amylase -

  • site of action = mouth, duodenum
  • special conditions = slightly alkaline
  • substrate digested = starch
  • end product(s) = Simple reducing sugars (e.g. maltose, glucose)
• Protease -

  • site of action = stomach, duodenum
  • special conditions = acid in stomach, alkaline in duodenum
  • substrate digesteed = protein
  • end product(s) = amino acids
• Lipase -

  • site of action = duodenum
  • special conditions = alkaline
  • substrate digesteed = fats and oils (lipids)
  • end product(s) = fatty acids and glycerol
• Digestion of starch:

  1. starch is digested in two places in the digestive system:

  • by salivary amylase in the mouth and by pancreatic amylase in the duodenum

  1. amylase works best in a neutral or slightly alkaline pH and converts large, insoluble starch molecules into smaller, soluble maltose molecules
  2. maltose is a disaccharide sugar and is still too big to be absorbed through the wall of the intestine
  3. maltose is broken down to glucose by the enzyme maltase 

  • this is present on the membranes of the epithelial cells of the villi in the small intestine