Human Digestion and Metabolism: Organs, Enzymes, and Processes

Intracellular digestion

Digestion inside the cell (plants, single-celled organisms)

Extracellular digestion

Digestion outside cells (most animals)

Human digestion

Breaks down macromolecules so they can be absorbed and used as matter and energy

Physical digestion

Physically breaks food into smaller pieces; increases surface area so it breaks down faster

Chemical digestion

Enzymes break food into smaller molecules to be absorbed

Digestive system

Digestive tract (large tube with organs) plus organs that deliver fluids that aid digestion

Accessory organs

Salivary glands, liver, gallbladder, pancreas (not part of the digestive tract)

Teeth (mouth)

Mechanical digestion

Saliva

Chemical; softens food and secretes salivary amylase, which begins breaking starch into a disaccharide

Salivary amylase

Breaks starch into maltose (a disaccharide) in the mouth

Epiglottis

Flap that blocks the trachea to prevent food and water from entering

Esophagus

2 layers of muscle; pharynx to stomach; peristalsis moves food

Peristalsis

Muscle contractions that move food through the esophagus

Stomach (muscle)

3 layers of smooth muscle; contracts to churn food (mechanical digestion)

Stomach chemical digestion

Gastric glands secrete mucus, hydrochloric acid, and pepsin

Mucus (stomach)

Lubricates and protects the stomach lining from acids and enzymes

Hydrochloric acid (stomach)

Activates enzymes, dissolves minerals, kills bacteria

Pepsin

Breaks complex proteins into smaller polypeptides (and amino acids)

Pepsinogen

Inactive form of pepsin secreted by stomach gland cells; activated by hydrochloric acid

Gastrin

Hormone released when food enters the stomach; stimulates secretion of hydrochloric acid

Sphincter

Thick ring of muscle that opens and closes to allow food in and out of the stomach

Cardiac sphincter

Between the esophagus and stomach

Pyloric sphincter

Between the stomach and small intestine

Liver functions

Stores glycogen, detoxifies substances from blood, secretes bile

Is the liver part of the digestive tract?

No, it is an accessory organ

Bile

Made by the liver, stored in the gallbladder; physically breaks large fat droplets into small droplets (mechanical digestion)

Does bile contain digestive enzymes?

No

Gallbladder

Stores bile until it is needed for digestion

Cirrhosis

Liver disease shown in the notes (scarred, damaged liver)

Pancreas

Not part of the digestive tract; secretes pancreatic fluid (trypsin, amylase, lipase, nuclease) into the small intestine and neutralizes stomach fluids to raise the pH

Pancreatic juice

Enters the small intestine through the pancreatic duct; shifts pH from acidic to basic

Small intestine (function)

Chemical digestion and absorption; receives secretions from gallbladder and pancreas

3 sections of the small intestine

Duodenum, jejunum, ileum

Villi

Fingerlike projections in the small intestine lining; increase surface area; contain blood vessels and lymph; absorb water and nutrients

Trypsin

Chemical digestion of proteins into amino acids (small intestine)

Amylase

Chemical digestion of starch/complex carbohydrates into simple sugar

Lipase

Chemical digestion of fat into fatty acids and glycerol

Nuclease

Chemical digestion of nucleic acids into nucleotides

Which enzyme digests proteins in the stomach?

Pepsin

Which enzyme digests proteins in the small intestine?

Trypsin

Large intestine (colon) functions

Reabsorbs water from food material; absorbs vitamins produced by bacteria; eliminates feces

Parts of the large intestine

Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum

E. coli in intestines

Bacteria that produce vitamins as they digest cellulose

Where does carbohydrate digestion begin?

Mouth (salivary amylase)

Where does protein digestion begin?

Stomach

Where does fat digestion mainly occur?

Small intestine (bile + lipase)

Digestion of fats: role of bile vs lipase

Bile = physical breakdown of fat droplets (more surface area); lipase = chemical, splits fats into fatty acids and glycerol

Metabolism

All the chemical activities/changes that take place continuously in a cell or organism

Two types of metabolism

Synthesis (building up) and decomposition (breaking down)

Anabolic reaction

Synthesis; endothermic; builds large molecules from small ones; uses free energy and stores it in new molecules

Catabolic reaction

Decomposition; exothermic; breaks large molecules into small ones; releases free energy (and heat)

Exothermic reaction

Releases energy; products have less potential energy than reactants

Endothermic reaction

Absorbs energy; products have more potential energy than reactants

Synthesis = ?

Endothermic / anabolic

Decomposition = ?

Exothermic / catabolic

Reaction speed

Increased by increasing temperature or concentration

Enzymes

Specialized proteins that lower the activation energy needed for a reaction to proceed

Catalyst

Chemical (like an enzyme) that lowers activation energy and speeds up reactions by bringing reactants together and aligning them

Are enzymes used up in a reaction?

No, they are recycled

Activation energy

Energy required for a reaction to begin

Active site

Small area of the enzyme's tertiary structure where the specific reaction is catalyzed

Substrate

The starting molecule that fits into the enzyme's active site

Enzyme-substrate complex

Enzyme and substrate bound together; lowers activation energy so substrate becomes product

Enzyme specificity

Each type of enzyme catalyzes only one or a few specific reactions

Noncompetitive inhibition

Molecule attaches to the enzyme (not at the active site), causing a shape change so the substrate can't bind

Competitive inhibition

A molecule other than the substrate blocks the active site so the substrate can't insert

Denaturation (enzyme problem 3)

Active site changes due to temperature or pH changes so the substrate can't bind

ATP

Adenosine triphosphate; chemical energy used to store and release energy in the cell

ATP structure

Adenine (base), ribose (sugar), and 3 phosphates

Energy release from ATP

Breaking the bond between the 2nd and 3rd phosphate releases energy; ATP becomes ADP

Energy storage in ATP

Forming a bond between the 2nd and 3rd phosphate stores energy (ADP to ATP)

ADP

Adenosine diphosphate; ATP after it gives up one phosphate group

ATP/ADP cycle

ATP is continually synthesized and broken down; ADP + phosphate + free energy from breakdown of organic compounds forms ATP

Phosphoanhydride bonds

Bonds between the phosphate groups in ATP

Oxidation

Removing electrons (usually on hydrogen); releases free energy

Reduction

Gaining electrons; gains energy

Decomposition example

Breakdown of glycogen in muscle cells; glucose breaks down to CO2 and water, releasing energy as ATP

Biosynthesis

Requires input of free energy; simple molecules (substrate) form complex molecules/macromolecules (products)

Biosynthesis example

Using ATP energy to add a glucose molecule to glycogen