Biology - Nutrition and Digestion - Quick Guides | Biology

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Study GuideBiology–Nutrition and Digestion1. Introduction to Nutrition and DigestionNutritionincludes all the activities by which living organisms obtain materials from their environmentand move them into their cells. Once inside the cells, these materials aremetabolized, meaning theyare chemically processed to build important substances such as:•structural components of the body•enzymes•energy-rich compounds•other materials needed for lifeAll of the elements and compounds taken into an organism for these purposes are callednutrients.1.1 Nutrition in AnimalsAnimals, including humans, areheterotrophs. This means theycannot make their own foodandmust obtain nutrients by eating other organisms. The nutrients animals consume are already formedorganic molecules.However, these molecules usually cannot beused right away. They are often too large or complex tobe absorbed directly by the body.1.2 DigestionTo make nutrients usable, animals must break food down intosimpler formsthat can be absorbedinto the bloodstream and transported to cells. This process is calleddigestion.Digestion allows the body to:•absorb nutrients efficiently•deliver them to cells•use them for energy, growth, repair, and maintenance

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Study Guide2. The Human Digestive System: A Clear and Friendly GuideThe human digestive system helps the body turn food into energy and building materials. Large,complex food molecules are broken down into smaller pieces that the body can absorb and use. Thisprocess takes teamwork. Many organs and enzymes work together to digest food efficiently.The main parts involved in digestion are themouth, esophagus, stomach, small intestine, largeintestine, and liver.2.1 Mouth: Where Digestion BeginsThemouth, also called the oral cavity, is the starting point of digestion. Its job is to receive food andbegin breaking it down.2.1.1 Teeth and Mechanical DigestionDigestion in the mouth begins withchewing, which breaks food into smaller pieces. Humans havefour types of teeth, each with a special role:•Incisors: Flat, chisel-shaped teeth used for cutting and biting food•Canines: Pointed teeth used for tearing food•Premolars and molars: Broad, ridged teeth used for grinding and crushing food2.1.2 Saliva and Chemical DigestionAs you chew, food mixes withsaliva, a sticky fluid that softens food and helps hold it together. Salivais produced by three pairs of salivary glands:•Parotid•Submaxillary•SublingualSaliva contains an enzyme calledamylase, which begins chemical digestion by breaking starch intosmaller sugar molecules calledmaltose.

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Study Guide2.1.3 Forming the BolusThetonguemoves food around and shapes it into a soft mass called abolus. Once ready, the bolusis pushed into thepharynx (throat)and then into the esophagus.2.2. Esophagus: Moving Food to the StomachTheesophagusis a muscular tube located behind the windpipe. It connects the mouth to thestomach.Food travels through the esophagus byperistalsis, which is a series of wave-like musclecontractions. These contractions push the bolus downward, and gravity also helps move the foodtoward the stomach.2.3 Stomach: Mixing and Protein DigestionThe esophagus connects to thestomachjust below the diaphragm. A ring of muscle called thecardiac sphinctercontrols the opening. It relaxes to let food in and then closes to keep food frommoving backward.2.3.1 Structure and FunctionThe stomach is a stretchy, pouch-like organ in the upper abdomen. Its muscles churn food and mix itwithgastric juices, turning it into a soupy mixture calledchyme.The stomach has several important jobs:•Stores food temporarily•Mixes food with digestive juices•Begins the digestion of proteins2.3.2 Enzymes and AcidsSpecial cells in the stomach lining release digestive substances:•Chief cellssecretepepsinogen, an inactive enzyme•Parietal cellsreleasehydrochloric acid (HCl)

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Study GuideThe acid changes pepsinogen intopepsin, an enzyme that breaks large protein molecules intosmaller pieces calledpeptides.2.3.3 Protection of the StomachAnother type of cell secretesmucus, which coats the stomach lining and protects it from acid. If thismucus layer fails and too much acid damages the lining, anulcercan form.2.4 Small Intestine: Digestion and AbsorptionThe chyme leaves the stomach through a sphincter and enters thesmall intestine, where mostdigestion and absorption occur.2.4.1 StructureThe adult small intestine is about23 feet longand has three sections:•Duodenum: First 10–12 inches•Jejunum: Next 10 feet•Ileum: Final 12 feetThe inner lining has tiny fingerlike projections calledvilli, and each villus has even smaller structurescalledmicrovilli. These greatly increase surface area, allowing more nutrients to be absorbed.2.4.2 Chemical Digestion in the DuodenumMost chemical digestion happens in theduodenum. Enzymes from the small intestine and pancreaswork together here. In addition,bileenters from the gallbladder to help digest fats.2.4.3Carbohydrate DigestionDifferent enzymes break down different sugars:•Amylase: Starch•Maltase: Maltose

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Study Guide•Sucrase: Sucrose•Lactase: Lactose2.4.4Fat Digestion•Bilebreaks large fat globules into smaller droplets•Lipasethen digests fats into smaller moleculesBile is made in theliverand stored in thegallbladder, a small sac under the liver.2.4.5Protein DigestionProteins are broken down by severalenzymes:•Trypsinandchymotrypsin(from the pancreas) break peptides into smaller peptides•Peptidasesturn peptides intoamino acids2.4.6Nucleic Acid DigestionNucleasesbreak nucleic acids intonucleotides.2.5 Absorption in the Jejunum andIleumMost absorption occurs in thejejunum. Digested nutrients enter the cells of the villi and then moveinto:•Capillaries(for sugars, amino acids, and other nutrients)•Lacteals(for fats, which enter the lymphatic system)Some nutrients move bydiffusion, while glucose and amino acids usefacilitated diffusion.Theileumcompletes absorption. Any material that is not digested or absorbed moves on to the largeintestine.

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Study Guide2.6 Large Intestine: Water Recovery and Waste RemovalThesmall intestineconnects to thelarge intestinein the lower-right abdomen at a pouch called thececum. Attached to the cecum is theappendix, which is believed to be a leftover structure fromhuman ancestors.2.6.1 Structure and FunctionThe large intestine, also called thecolon, has three main sections:•Ascending colon•Transverse colon•Descending colonEach section is about one foot long.The main functions of the large intestine are:•Absorbingwater•Storing andprocessing waste•Eliminating solid waste from the body2.6.2 Feces FormationAfter water is absorbed, the remaining material becomesfeces, which include:•Undigested food (such as cellulose)•Billions of mostly harmless bacteria•Bile pigments and other waste materialsFeces are stored in therectumand leave the body through theanus, completing digestion.2.7 Liver: Processing and RegulationTheliverplays a major role after digestion by processing nutrients absorbed into the bloodstream.

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