Ch 6 Nutrition in Humans 🔋

• Nutrition is a process in which organisms obtain food for energy, growth, and maintaining health.
• Organisms that make their own food are called autotrophs. This mode of nutrition is called autotrophic nutrition.
• Organisms that depend on other organisms for food are called heterotrophs. This mode of nutrition is called heterotrophic nutrition.
• The three main types of heterotrophic nutrition are holozoic nutrition, saprophytic nutrition, and parasitic nutrition.
• Animals like humans carry out holozoic nutrition. They take in complex food and digest it inside the body.
• Many fungi like mould and bacteria carry out saprophytic nutrition. They are known as saprophytes. They break down the food outside the body and then absorb the soluble nutrients into the body.
• Parasites can be animals, plants, fungi or bacteria. They live on or inside the body of other organisms and obtain food from them. An example is tapeworms, that live in the small intestine of animals and feed on the digested food inside.
• Humans take in food containing large, complex molecules. These molecules have to be broken down before our cells can use them because:

  • The membrane of our cells is differentially permeable. Large, complex food molecules cannot pass through. They have to be broken down into small, soluble molecules, so that they can enter the cells.
  • The food molecules are structurally different from those making up our body. Our body has to break them down and use the small molecules to build our own molecules.
• The nutrition in humans consists of five main processes: ingestion, digestion, absorption, assimilation, and egestion.
• During ingestion, food is taken in through the mouth.

  During digestion, food is broken down into small, soluble food molecules in our digestive system.

  During absorption, the small, soluble food molecules enter our circulatory system.

  During assimilation, the absorbed food molecules are taken up by cells for metabolism.

  During egestion, the undigested and unabsorbed materials are removed from the body as faeces.
• The human digestive system consists of the alimentary canal and its associated digestive glands.

  • The alimentary canal is a long muscular tube that begins at the mouth and ends at the anus. After food is taken into our mouth, it is pushed along the alimentary canal in one direction. On the way, food is digested and useful materials are absorbed. Undigested and unabsorbed materials are egested through the anus.
  • The digestive glands produce digestive juices, which are released into the alimentary canal to aid digestion.
• Parts of the alimentary canal include the mouth cavity, pharynx, oesophagus, stomach, small intestine(containing duodenum and ileum), large intestine(containing caecum, appendix, colon and rectum), and anus.
• Parts of the digestive glands include the salivary glands, liver, and pancreas.
• The process of chewing food into small pieces is called mastication.
• We have four types of teeth: incisors, canines, premolars, and molars.
• The arrangement and number of our teeth are the same on the upper and lower jaws.
• The incisor is for biting and cutting food. It is chisel-shaped with flat, sharp edges. It has one root.

  The canine is for tearing flesh. It is pointed and curved. It has one root.

  The premolar is for crushing and grinding food. It has a broad top with cusps. It has one or two roots.

  The molar is for crushing and grinding food. It has a broad top with cusps. It has two or three roots.
• Humans have two sets of teeth. The first set is the milk teeth which appears during the first two years of life. The second set is the permanent teeth, which replaces the milk teeth between the ages of 6 and 12 after the milk teeth fall out.

  Permanent teeth cannot be replaced if they are damaged or lost.
• Dentition refers to the type, number and arrangment of teeth in the jaws. It can be represented by a dental formula, which shows the numbers of different types of teeth on each side of the upper and lower jaws.
• The total number of human milk teeth is 20. The dental formula is 2102/2102.

  The total number of human permanent teeth is 32. The dental formula is 2123/2123.

  The formula is incisor + canine + premolar + molar.
• A tooth is divided into three regions: the crown, the neck, and the root
• Each tooth consists of three layers:

  • Enamel:
  • Outermost layer of the crown
  • Non-living, mainly made of calcium salts
  • Hardest tissue in the body, protecting the tooth from wear due to chewing
  • Replaced by cementum around the roots, which attaches the tooth to the jawbone through the periodontal membrane
  • Dentine:
  • Bone-like substance with a large amount of calcium salts, not as hard as enamel
  • Living tissue containing strands of living cytoplasm
  • Pulp cavity:
  • Contains living cells, blood vessels, and nerve fibers
  • Blood vessels supply oxygen and nutrients to the tooth and remove wastes
  • Nerve fibers detect temperature and pressure
• After ingestion, the food is moved along our alimentary canal and digested. Digestion occurs physically and chemically.
• Physical digestion is the breaking down of food into smaller pieces by physical actions. It does not change the chemical structure of the food, but it increases the surface area of the food for digestive juices to act on.

  The resultant food pieces in physical digestion are not yet small enough for absorption. They must be further broken down into smaller molecules by chemical digestion.
• Chemical digestion involves chemical reactions in which large, complex food molecules are broken down into small, soluble molecules. The reactions are catalysed by digestive enzymes. Carbohydrases, lipases and proteases are three main types of digestive enzymes in our digestive system. They break down carbohydrates, lipids, and proteins respectively into small soluble molecules that are ready for absorption.
• Small food substances like water, vitamins and minerals do not need chemical digestion and can be absorbed directly.
• Carbohydrases break down starch and disaccharides into monosaccharides.

  Lipases break down triglycerides into glycerol and three fatty acids.

  Proteases break down proteins into amino acids.
• Saliva, produced by the salivary glands, contains salivary amylase, mucus, and water.

  1. Salivary amylase is a carbohydrase that catalyzes the breakdown of starch into maltose.
  2. Mucus binds food particles together and moistens and lubricates the food to make it easier to swallow.
  3. Water dissolves soluble substances in the food so we can taste it.
• Mucus is found in the mouth cavity, small intestines and stomach.
• Gastric juice


  Acidic digestive juice produced by the gastric glands
• Gastric juice is the only acidic digestive juice in the human digestive system
• Components of gastric juice


  • Pepsin
  • Hydrochloric acid
  • Mucus
• Pepsin


  A protease that catalyzes the breakdown of proteins into peptides, works best in an acidic medium
• Hydrochloric acid


  Provides an acidic medium for pepsin to work properly, kills bacteria in food
• Mucus


  Protects the stomach from being digested by the protease
• Bile is an alkaline digestive juice produced by the liver and stored in the gall bladder. It contains bile salts, bile pigments, and sodium hydrogencarbonate.

  1. Bile salts emulsify lipids into small oil droplets. (physical digestion) it increases the surface area for lipidases to act on.
  2. Sodium hydrogencarbonate neutralizes the acidic chyme. It provides alkaline medium for the digestive enzymes to work properly.
  3. Bile pigments are excretory products that are passed out in feces.
• Pancreatic juice is a digestive juice produced by the pancreas. It contains amylase, proteases, lipases and sodium hydrogencarbonate.

  1. Amylase is a carbohydrase that catalyzes the breakdown of starch into maltose.
  2. Proteases catalyze the breakdown of protein into peptides and peptides into amino acids.
  3. Lipases catalyze the breakdown of lipids into 3 fatty acid molecules and glycerol.
  4. Sodium hydrogencarbonate neutralizes the chyme and provides an alkaline medium for the digestive enzymes to work properly.
• Intestinal juice is produced by the intestinal glands. It consists of mucus and sodium hydrogencarbonate. The mucus protects the intestine from being digested by the protease and the sodium hydrogencarbonate neutralizes the acidic chyme and provides alkaline medium for digestive enzymes to work properly.
• Digestive enzymes are located on the membrane of the epithelial cells of villi but NOT secreted from intestinal glands.

  Ex: maltase, lactase, sucrase, peptidase
• Most of absorption takes place in the small intestine, mostly in the ileum. when food molecules enter the circulatory system.