Chapter 6: Nutrition in Humans

6.1 Holozoic Nutrition Nutrition is the intake of food and processes of converting food substances into living matter Plants manufacture their own food – autotrophic Animals cannot manufacture their own food, dependent on eating plants or other animals – heterotrophic Mode of feeding organic matter is known as holozoic nutrition

6.1 Holozoic Nutrition Heterotrophic Nutrition Ingestion/Feeding – Taking food into body Digestion Mechanical – chopping and grinding food with teeth and muscular churning of food, in stomach Chemical – breaking large insoluble molecules into small, soluble ones (using enzymes) Absorption – taking digested food into bloodstream Assimilation – using absorbed food in metabolic processes Any food which cannot be digested/absorbed is passed out of the gut during egestion

6.2 Mammalian Digestive System Consist of the gut (alimentary canal) and glands associated with it (9m long) Includes mouth, pharynx, oesophagus, stomach, small intestine and large intestine Other structures include salivary glands, pancreas, liver and gall bladder

6.2 Mammalian Digestive System The Mouth and buccal cavity Opening where food is ingested into buccal cavity (mouth cavity) Buccal cavity processes food: Teeth mechanically digest the food (increase surface area for enzymes to act on) Salivary glands secrete saliva containing enzyme salivary amylase to digest starch into maltose Tongue rolls the food into balls or boli (bolus) and pushes them to the back of the buccal cavity for swallowing

6.2 Mammalian Digestive System Pharynx Pharynx is the part of the gut leading from mouth to oesophagus and windpipe Larynx (voice box) which lies below pharynx has a slit-like opening called glottis Pharynx is a passage for both food and air

6.2 Mammalian Digestive System Pharynx Food entering windpipe is prevented by a piece of flap-like cartilage called epiglottis Epiglottis is above larynx just behind root of tongue During swallowing, larynx moves up to be covered by epiglottis and thus preventing food from entering windpipe If small particles get into the larynx or windpipe, violent coughing results to force them out and prevent choking

6.2 Mammalian Digestive System Oesophagus (Gullet) Muscular tube connecting mouth cavity and stomach Wall of the oesophagus is made up of 2 layers of muscles These muscles are present along the gut from the oesophagus to rectum

6.2 Mammalian Digestive System Oesophagus (Gullet) > Peristalsis The two layers of muscles cause rhytmic, wave-like contractions of the gut walls. This movement is known as peristalsis This process enables: Enables food to be mixed with the digestive juices Moves the food along the gut The circular and longitudinal muscles are antagonistic muscles (one set of muscles contracts, other set relaxes)

6.2 Mammalian Digestive System Oesophagus (Gullet) > Peristalsis When circular muscles contract, longitudinal muscles relax  wall of gut constricts becoming narrower and longer  Food in gut is squeezed/pushed forward When longitudinal muscles contracts, circular muscles relaxes  Wall of gut dilates becoming wider and shorter  Widens lumen for the food to enter

6.2 Mammalian Digestive System Oesophagus (Gullet) > Peristalsis

6.2 Mammalian Digestive System Stomach Distensible muscular bag with thick and well developed muscular walls Peristalsis of stomach wall churns (mechanical digestion) food for up to 4 hours to mix with gastric juices Mucous coat of stomach wall has numerous pits that secrete gastric juices. Gastric juice consists of concentrated hydrochloric acid (pH2) and enzymes (rennin and pepsin)

6.2 Mammalian Digestive System Stomach > Hydrochloric Acid Hydrochloric acids: Stops action of salivary amylase Changes inactive forms of enzymes in gastric juice to the active forms Provides an acidic medium suitable for the action of the gastric enzymes Kills germs and certain potential parasites in food

6.2 Mammalian Digestive System Stomach Partly digested food becomes liquefied, forming chyme Chyme passes in small amounts through a ring of muscle called the pylorus / pyloric sphincter, which relaxes to allow the food to enter the duodenum

6.2 Mammalian Digestive System Small Intestine Final place of digestion Place where absorption of nutrients take place Consists of: Duodenum, Jejunum, ileum

6.2 Mammalian Digestive System Small Intestine > Duodenum U-shaped First part of small intestine (most digestion occurs here) About 30m long Receives bile (produce by liver) via bile duct from gall bladder (stores bile) Receives pancreatic juice from pancreas, through pancreatic duct Releases digestive juice from its walls

6.2 Mammalian Digestive System Small Intestine > Duodenum Chyme stimulates intestinal glands to secrete intestinal juice Food comes in contact with pancreatic juice, bile and intestinal juice All 3 fluids are alkaline which neutralize the acidic chyme and provide suitable alkaline medium for the action of pancreatic and intestinal enzymes

6.2 Mammalian Digestive System Small Intestine > Duodenum Pancreatic juice contains pancreatic amylase, pancreatic lipase and protease (trypsinogen) Intestinal juice contains enterokinase, erepsin (peptidase), maltase, sucrase, lactase, intestinal lipase

6.2 Mammalian Digestive System Large Intestine 1.5 m long Consists of : Colon, Rectum (stores faeces temporarily) Absorbs water and mineral salts No digestion takes place here

6.2 Mammalian Digestive System Organs and glands related gut > Liver Largest gland Dark red with 5 lobes Attached with 3 blood vessels Hepatic portal vein: Transport digested products from intestines to liver hepatic vein: carries deoxygenated blood away hepatic artery: carries oxygenated blood to liver Secretes bile (alkaline greenish-yellow fluid)

6.2 Mammalian Digestive System Organs and glands related gut > Gall Bladder Stores bile temporarily Greenish yellow bag attached to liver Contracts to release bile via bile duct into duodenum

6.2 Mammalian Digestive System Organs and glands related gut > Pancreas Connected to duodenum via pancreatic duct Produces pancreatic juice (Digestive enzymes) Pancreatic amylase Pancreatic lipase Trypsinogen Secretes hormones insulin and glucagon

6.3 Digestion A process where large insoluble molecules are broken down to smaller, soluble and diffusible molecules. It can be broken down as: Physical/Mechanical Chemical

6.3 Digestion Physical Digestion Mechanical breakdown of food in: Mouth by the action of teeth (chewing) Stomach by churning of food Increase surface area to volume ratio for enzyme to acts on.

6.3 Digestion Chemical Digestion Breakdown of food involving hydrolytic reactions catalysed by enzymes Takes place in Mouth, Stomach, Small Intestine (SMS)

6.3 Digestion: The Digestive Process Mouth Both Physical and chemical process Physical – chewing by teeth to increase surface area for enzyme to act on Food In mouth stimulates salivary glands to secrete saliva Salivary glands secrete saliva containing enzyme amylase to digest starch into maltose and protein mucin Mucin is sticky thus binding food together and lubricates it pH 7

6.3 Digestion: The Digestive Process Oseophagus No digestion occurs here Peristalsis and gravity help to push food down to stomach.

6.3 Digestion: The Digestive Process Stomach Physical churning of food Chemical digestion (Digestion of proteins starts) Digested by proteases - rennin and pepsin Carbohydrates digestion stops Stomach contains gastric juice and it contains: Pepsin = digest proteins to polypeptides Rennin which clots or curdles milk proteins by converting soluble protein caseinogens into casein dilute hydrochloric acid

6.3 Digestion: The Digestive Process Stomach > Hydrochloric Acid Stops action of salivary amylase by denaturing it Changes inactive forms of enzyme pepsinogen and prorennin in gastric juice to active forms pepsin and rennin respectively. Partly digested food in stomach is called chyme Chyme passes into the duodenum when they pyloric sphincter relaxes and opens

6.3 Digestion: The Digestive Process Small Intestine Chyme stimulates the release of 3 fluids into the duodenum: Pancreas secretes pancreatic juice into pancreatic duct Intestinal glands secrete intestinal juice Gall bladder releases bile into bile duct Digestion of Carbohydrate and protein continues Fat digestion starts

6.3 Digestion: The Digestive Process Pancreatic Juice Pancreatic amylase which digests starch to maltose Trypsin which digests proteins to polypeptides. Trypsin is produced as inactive trypsinogen are converted to the active trypsin by intestinal enetrokinase Lipase which digests fats to fatty acids and glycerol Intestinal Juice Maltase which digests maltose to glucose Enterokinase – Converting inactive trypsinogen to active trypsin Erepsin – digest polypeptides to amino acids Lipase – digest fats to fatty acids and glycerol Bile Emulsify (breaking down into smaller molecules) fats

6.3 Digestion : Carbohydrates Starch Salivary Amylase (In Mouth) Maltose Pancreatic Amylase No digestion of starch in stomach (amylase are denatured) Maltose (in small intestine) Glucose

6.3 Digestion : Proteins Starts in stomach : acidic – pH2 Enzymes pepsin and rennin – pH 7 Continues in small intestine : alkaline- pH9 enterokinase Trypsinogen Trypsin In Small intestine Trypsin Proteins Polypeptides Erepsin Polypeptides Amino acids

6.3 Digestion : Fats Starts in small intestine Bile emulsifies fats, i.e.: lowers the surface tension of fats and breaks them up into smaller fat globules Surface area increased for action of lipases Fats + Bile  Emulsion Fatty Acids + Glycerol Gall bladder secretes

6.4 Absorption Occurs in ileum Active transport takes place (requires energy) Rate of absorption depends on: Length of Small Intestine Inner wall of intestine has numerous folds Presence of villi (singular: villus) Presence of microvilli on villi

6.4 Absorption Villi have thin wall (one cell thick) Absorbed nutrients transported away quickly resulting in concentrating gradient All these speeds up diffusion for efficient absorption.

6.4 Absorption Sugar, amino acids and mineral salts pass into blood capillaries via process of active transport Glycerol and fatty acids enters into epithelium before combining into minute fat globules Fat globules enters the lacteals/ lympathic capillary.

6.4 Absorption Undigested and unabsorbed materials Stored temporarily in rectum Discharged as faeces through anus Process of removal is known as egestion / defacation

6.5 Functions of Liver Regulate blood glucose level Production of bile Iron storage Protein synthesis Deamination of amino acids Detoxification

6.5 Function of Liver Regulate blood glucose level When blood glucose is too high (especially after meal), liver alerts pancreas Islets of langerhans in pancreas will produce insulin Insulin convert glucose into glycogen.

6.5 Function of Liver Regulate blood glucose level Glycogen After Lunch Insulin Glucose Blood Glucose High Stored in Liver and Muscles Liver Islets of Langerhans Pancreas High Blood Glucose Concentration

6.5 Function of Liver Regulate blood glucose level Glucagon Glucose Glycogen Liver (Low blood glucose) Pancreas Islets of Langerhans Low Blood Glucose Concentration

6.5 Function of Liver Production of Bile Helps in digestion fats (emulsifies fats) Stored temporarily in gall bladder Protein Synthesis Forms amino acids Albumins, globulins, fibrinogen found in blood plasama

6.5 Functions of Liver Iron Storage Spleen (near liver)  Destroys red blood cells Haemoglobin in red blood cells destroyed by liver > Storing of Iron Bile pigments also formed during haemoglobin breakdown.

6.5 Function of Liver Deamination of Amino Acids Carbon Residue Amino Group converted converted Ammonia Glucose converted converted Urea Glycogen

6.5 Functions of Liver Detoxification Converting harmful substances in harmless ones. Example: Alcohol dehydrogenase (a type of enzyme) breaks down acetaldehyde in alcohol Alcohol stimulates secretion of stomach acid which may lead to stomach ulcers Prolonged alcohol abuse may lead to cirrhosis (Damage of liver cells)