Honors Biology: Digestive System

Stages of Food Processing (and defenitions)

1. Ingestion - eating

2. Digestion - breaking down food into smaller components (monomers that can be absorbed) 2 types

3. Absorption - removes monomers and nutrients from the alimentary canal and places them in the bloodstream

4. Elimination - removal of indigestible substances (waste).

Types of Digestion

• Mechanical Digestion → physical breakdown of food

  • Increases the surface of food to be chemically digested (increases access to food material by enzymes)

  • ex. oral cavity (chewing)

  • ex. stomach churning 

• Chemical Digestion → enzymatic breakdown of food into monomers

  • Macromolecules in food to monomers that the body can use

  • use hydrolysis

  • Polypeptide → Amino Acid (Stomach)

  • Polysaccharide → Monosaccharide (Starts in the Oral Cavity)

  • Polynucleotide → Nucleotide (Stomach)

  • Triglyceride → Fatty Acids + Glycerol (small intestine)

Alimentary Canal 

• digestive tube with an opening at each end

• divided into specialized compartments which each of which has a structure designed to allow for a specific function

• Also Accesory Glands

Sphincters

rings of contractile muscle that control flow of material between components of alimentary canal

Oral Cavity (Mouth)

3 Stages of Food Processing:

1. ingestion

2. mechanical digestion

3. chemical digestion (saliva provides enzymes, but partial)

creates a bolus

Saliva (Components and Role in Digestion)

• Produced by salivary glands when eating is anticipated (mouth watering)

• Components

  • Salivary amylase - digests starch (amalose)

  • mucus (H₂O, salts, and slippery glycoproteins) - forthe lubrication of food

  • buffers - neutralize food acid to protect soft tissue

  • anti-bacterial agent (lysozyme) → kills bacteria

Esophagus 

• A tube of smooth muscle that connects the oral cavity and the stomach

• know the swallowing reflex which occurs here/ in pharynx

• Food moved through it using peristalsis (contraction of muscles to move food)

Sphincter into the Esophagus 

esophogeal sphincter

Sphincters into and out of the Stomach

cardiac sphincter and pyloric sphincter

Stomach (Role in Digestion)

• mechanical digestion - churning of food (muscle contractions → smooth muscle so involuntary)

• chemical digestion - enzymatic digestion of proteins (begin), through the additon of gastric juice.

• Creates acid chyme as an end product, which is then slowly squirted into the duodenum

Gastic Juice

• digestive fluid made in the stomach for chemical digestion

• produced by gastric glands on the stomach walls

• consists of mucus, pepsinogen, and hydrochloric acid

Gastic Glands (cell types and functions)

1. Mucous Cells → make mucus, which helps to protect the cells of the stomach wall

2. Parietal Cells → make H⁺ and Cl^- → combine in the lumen to form. Why don’t Parietal Cells make HCl? If HCl were made in gastric glands, then the cells would be dissolved.

3. Chief Cells → make pepsinogen inactive protein enzyme that is a precursor to pepsin (active protein enzyme). Chief cells are of CHIEF importance, i.e. breaking down proteins.,

Pepsinogen and Pepsin (Funtion and How)

• Pepsinogen is an inactive precursor activated by hydrochloric acid (HCl) into pepsin.

• Pepsin is a protein-digesting enzyme that functions optimally at pH=2.

• It exhibits positive feedback, where activated pepsin aids in converting more pepsinogen.

  • Chief cells produce pepsinogen, not active pepsin, to prevent self-digestion of stomach cells.

Function of HCl in the Lumen

• converts inactive pepsinogen into active pepsin

• denaturization of proteins and DNA

• kills bacterica and other pathogens

Production of Gastric Juice

• See/smell/taste food -> gastric juice production starts

• Food in stomach -> stretches stomach wall -> gastrin (hormone) released

• Gastrin circulates in the blood back to the stomach -> signals the stomach to produce more gastric juice (acidic)

Negative Feedback Loop of Gastrin

• Gastrin tells the stomach to make gastric juices (acidic)

• Lumen gets too acidic (there is a lot of gastric juice)

• This is the indicator that tells the stomach to stop making gastrin.

  • Low gastrin means no gastric juice is made.

Structure of Small Intestine

• Structured to maximize the surface area for each function with folds, villi, and microvilli

• Divided into 3 sections: duodenum, jejunum, and ileum

• Acid Chyme enters the small intestine from the stomach through the pyloric sphincter.

Stages of Food Processing in the Small Intestine

• chemical digestion → occurs in the duodenum (first 25 cm)

  • Note: Prior to entering the small intestine, only a small amount of chemical digestion was started and not finished

• absoption → occurs in the jejunum and the ileum

Chemical Digestion in the Small Intestine (parts)

Several materials combine to complete chemical digestion

• Acid Chyme

  • slowly squirted through the pyloric sphincter into the duodenum

  • Why? Allows for time for neutralization of acid, to protect enzymes and protein cells in the duodenum, and to allow for maximum digestion

• Pancreatic juice (from the pancreas) made of:

  • alkaline solution (high pH→ basic) helps to neutralize pH 2 to protect the intestinal walls and not to denature the enzymes in pancreatic juice

  • digestive enzymes: know one for each macromolecule

    • Polysaccharides → pancreatic amylase

    • Polypeptides → peptidase and pepsin

    • DNA and RNA → nulease

    • Fats → lipase

• Note: Digestive enzymes are also released from the walls of the small intestine

• Bile Salts (from liver + gallbladder, transported through bile ducts)

  • used for the emulsification of fats so they can be digested.

  • Has a polar and nonpolar side (amphipathic), so they can react with fats and water

Nutrient Absorption In The Small Intestine

• The small intestine has folds which increase surface area.

• On top of these folds are finger-like tissues called villi, where blood vessels are located.

• Villi are made up of epithelial cells which have tiny microvilli on top of them.

• Channel proteins let monomers into the epithelial cells and transport proteins take those monomers and put them into the capillaries which then take them to the bloodstream

• Fats are transported a different way: 

  • Their monomers are brought into the epithelial cells where they are reassembled into fat. 

  • They then are transported in the lymphatic system and the lacteal.

• Absorbed nutrients enter the hepatic portal vein for delivery to the liver.

Liver

Has 3 functions

• Building molecules → Uses absorbed monomers from the small intestine, transported through the hepatic portal vein, to make useful, biological molecules.

• Drug detoxification → detoxifies blood before it is pumped to the rest of the body. Toxins and drugs are made less harmful through a chemical reaction

• producing and transporting bile

Large Intestine Structure (Parts and Purpose)

• Parts: ileocecal sphincter, cecum, ascending colon, transverse colon, descending colon, sigmoid, rectum, anus.

• Unabsorbed food material is emptied into the cecum and is moved through the tube by peristalsis

• Purpose: to absorb water and nutrients from the feces.

  

Stages of Food Processing That Occur in the Large Intestine

• Absorption

  • water → reabsorbed into the blood vessels that are around the LI. As water is absorbed, solid waste is produced

  • vitamins produced by gut bacteria. A large population of bacteria that live in the colon

Accessory Glands

Note: food material does not enter the accessory glands (not part of the alimentary canal) but are needed in the digestive process

• Salivary Glands (in the mouth; make saliva)

• Liver (produces and releases bile salts, which digest triglycerides) 

• Gallbladder (stores and releases bile salts, which digest triglycerides)

• Pancreas (makes and releases digestive enzymes/pancreatic juice)