Human Nutrition

• Carbs- needed for energy.

  Fats- needed for energy, making cell membranes, acting as insulator (stored in adipose tissue under skin) to reduce heat loss and providing mechanical protection to body organs by forming a layer around them.

  Proteins- needed to build new cells, for growth and making proteins.

  Vitamins- organic substances needed in small amounts.

  Minerals- inorganic substances needed in small amounts.

  Fibre- needed to keep alimentary canal in working order to prevent constipation.

  Water- needed to get rid of waste products and is an important solvent.
• Vitamins:-

  C- makes stretchy protein, collagen, and keeps tissues in good repair: citrus fruits, raw vegetables. Deficiency- scurvy, causes pain in muscles and joints and bleeding from gums and other areas.

  D- helps in calcium absorption and makes bones and teeth: butter, egg yolk, mostly sunlight. Deficiency- rickets, bones become soft and deformed.
• Minerals:-

  Calcium- for bones and teeth and blood clotting: milk, bread. Deficiency- brittle bones and teeth and poor blood clotting.

  Iron- for making haemoglobin: egg yolk, dark green vegetables. Deficiency- anaemeia, not enough red blood cells so tissues don't get enough oxygen.
• Fibre:-

  Muscles in alimentary canal perform peristalsis when food is present. Soft foods don't stimulate muscles but harder, less digestible foods do. All plant food has fibre because plant cells have cellulose cell walls which humans can't digest. Brown rice and bran are good sources of fibre.

  Peristalsis- rhythmic muscular contractions that move food through the alimentary canal.
• Water:-

  Cytoplasm is a solution of many substances in water. the spaces between our cells are also filled with watery liquid. Metabolic reactions can only take place if the reacting chemicals are in solution. If a cell dries out, reactions stop and cell dies. Plasma is mostly water, with dissolved substances. Water dissolves enzymes and nutrients. Kidneys remove urea from blood. Urea is dissolved in water to form urine.
• Digestive process:-

  Ingestion- taking food and drink into the body.

  Digestion- breakdown of food.

  Absorption- movement of nutrients from the alimentary canal into the blood.

  Assimilation- uptake and use of nutrients by cells.

  Egestion- removal of undigested food as faeces.

  Physical digestion- breakdown of food into smaller pieces, without chemical changes happening to the molecules.

  Chemical changes- breakdown of large molecules into smaller ones so they can be absorbed.
• The mouth:-

  Food is ground by the teeth to increase its surface area.

  Tounge mixes food with saliva and forms it into a ball that can be swallowed.

  Saliva is made by salivary glands. It's a mixture of water, mucus, and amylase. Water dissolves substances in food, allowing us to taste them. Mucus helps the food to bind together to form a ball and lubricates it so it slides down the oesophagus. Amylase begins to digest starch.
• The oesophagus:-

  Front tube is trachea and behind it is oesophagus.

  Hole in centre of oesophagus down which food goes is called lumen.

  All blood vessels also have this.

  Entrance to the stomach from oesophagus is closed by a sphincter muscle which relaxes to let food enter the stomach and contracts to close the entrance again.
• The stomach:-

  Its strong, muscular walls contract and relax to mix food with the enzymes and mucus.

  Walls contain goblet cells along with cells that produce enzymes and others that make hydrochloric acid. Enzymes produced are proteases.

  The HCL has a pH of 2 and kills harmful microorganisms in food.

  Can store food so after one-two hours, sphincter muscle at the bottom of stomach opens and lets the partly digested food into the duodenum.
• The small intestine:-

  part between the stomach and colon.

  about 5m long and if quite narrow.

  first part, nearest to stomach, is duodenum and last part, nearest to colon, is ileum.

  pancreas lie just under the stomach and a tube called pancreatic duct leads from here to the duodenum.

  pancreatic juice is made by pancreas and flows along the pancreatic duct to the duodenum.

  pancreatic juice contains different enzymes so chemical digestion continues in duodenum.

  digested nutrients and water are absorbed into the blood in the ileum.
• The large intestine:-

  made up of colon and rectum.

  colon absorbs most of the water that remains in the food.

  rectum stores undigested food as faeces, which is egested from the body through the anus.
• The liver:-

  secretes bile.

  bile is stored in the gall bladder.

  bile flows along the bile duct and mixes with food when it enters the duodenum.
• Teeth:-

  bite pieces of food and grind, crush, or chop it into smaller pieces, giving the food a larger surface area and making it easier for the enzymes to work on the food.

  helps any soluble molecules or ions in food to dissolve in the saliva.
• Structure:-

  tooth is embedded in the gum.

  part of tooth above the gum is covered in enamel which is the hardest substance made by animals so it's difficult to break or chip it but it can be dissolved by acids.

  sweet foods left on teeth are fed on by bacteria that produce acids that dissolve the enamel and cause decay.

  under enamel is dentine, which is like bone, it's hard but not as hard as enamel and has channels which contains living cytoplasm.
• In the middle of the tooth, there are nerves and blood vessels (pulp). The blood vessels supply the cytoplasm in the dentine with nutrients and oxygen.

  Part of tooth that's embedded in the gum is covered with cement. This has fibres growing out of it which attach the tooth to the bone in the jaw but allow it to move slightly when biting or chewing.
• Incisors- sharp-edged, chisel-shaped teeth at front of the mouth, used for biting off pieces of food.

  Canines- pointed teeth at either side of the incisors, used similarly to the incisors.

  Premolars and molars- large teeth towards the back of the mouth, used for chewing food. In humans, the ones right at the back are called wisdom teeth which don't grow until much later in the person's development.
• Amylase- secreted by the salivary glands and pancreas, in the mouth and duodenum.

  Protease- secreted by the walls of the stomach and the pancreas, in the stomach and duodenum.

  Lipase- secreted by the pancreas, in the duodenum.
• Optimum temperature for human digestive enzymes- 38 degrees celsius

  Optimum pH for protease enzymes in stomach- 2

  Optimum pH for enzymes in duodenum- just above 7, because bile and pancreatic juice are present, they contain alkaline substances which neutralize the acidic mixture that comes into the duodenum from the stomach.
• Amylase breaks down into maltose, made of two glucose molecules linked together. Maltose molecules are too big to be absorbed so an enzyme, maltase, breaks down the maltose into glucose molecules. Maltase is secreted by the cells lining the small intestine (epithelium). Maltase doesn't go into the lumen of the small intestine but remains attached to the epithelial cells, sitting on their cell membranes. So, maltose is digested on the surface of the epithelial cells.

  starch (amylase on top of arrow)-->maltose (maltase on top of arrow)-->glucose
• Protease is secreted into the stomach and duodenum. Protease produced in the stomach is called pepsin and is secreted from the stomach walls in gastric juice which also contains HCL. Protease produced in the pancreas and working in the duodenum is called trypsin. Pepsin and trypsin both break protein molecules apart to produce amino acids which can be absorbed.
• Bile emulsifies fats. Fats and oils are insoluble in water so they stay as little droplets that don't fully mix into the watery fluids in the digestive system so bile breaks up large drops of fat or oil into tiny droplets which can disperse among the watery liquids.
• Emulsification is physical, not chemical. It increases the surface area of fat droplets so the emulsified fats are much easier for the lipase to access. Lipase can then chemically digest the fats.
• The broken-down molecules of carbs, proteins, and fats are soluble and small enough to be absorbed into the blood. Some molecules and ions in food are already small enough to be absorbed such as mineral ions, vitamins, and water (some water is absorbed into the colon)
• Villi:-

  each villus (singular) is about 1mm long.

  cell membrane on the surface of each villus is folded to form many tiny microvilli where maltase breaks down maltose into glucose and through these membranes glucose, amino acids, water, vitamins, minerals, fatty acids and glycerol are absorbed.

  Most of these substances pass into the blood capillaries inside the villus. The blood capillaries from all the villi link up to join a vein, called the hepatic portal vein, which takes all these substances to the liver.
• Glucose may be changed into glycogen, by the liver cells, for storage. Amino acids are used by the liver cells to make many kinds of proteins, some of which are returned to the blood to be transported around the body, and excess amino acids are broken down by the liver to form urea. Fatty acids and glycerol pass into the lacteals. The contents of the lacteals eventually empty into the blood.
• The villi hugely increase the surface area os the epithelium of the small intestine. As each villus is covered with thousands of microvilli, the total surface area is increased even more which greatly speeds up the absorption rate.