Digestive system

Function of the digestive system

Primary function: breaks down macromolecules into substances the body can use

Other functions: house helpful bacteria, excrete solid waste

Digestion Digestion – the breakdown of macromolecules into components more easily be absorbed/utilised by the body

Biologists consider the digestive system an external surface

Why do you think that is?

Digestion is mechanical AND chemical

Mechanical digestion◦ Physical breakdown of large food portions

Chemical digestion◦ Molecular degradation of foodstuffs by chemical substances (acid, enzymes, etc.)

Enzymes & digestion Enzymes – proteins that catalyse metabolic chemical reactions

An enzyme’s structure determines its function

Denaturation = the loss of protein structure◦ Caused by low/high pH and high temperature

Enzymes & digestion cont’d

Digestive enzymes rapidly break down macromolecules into smaller subunits (monomers) for absorption into the body

Enzymes are named for what they act on and end in “______ase”◦ eg. Lactase breaks down lactose

◦ Sucrase breaks down _________

◦ ______ breaks down lipids (fats)

Enzymes are present throughout theGI tract

Enzymes are everywhere…

The mouth Digestion begins in the mouth

◦ Mechanical digestion = teeth◦ Chemical digestion = enzymes begin breaking down sugars

Tongue & salivary glands◦ Produce saliva to dampen solid food◦ Saliva contains digestive enzymes◦ Tongue presses softened food against hard

palate forming a bolus (ball) for swallowing

Pharynx◦ Swallowing causes epiglottis to direct food

down the esophagus

Esophagus◦ Muscular tube that carries bolus to the stomach via peristalsis

Peristalsis◦ Radial contraction of muscles that propagates a wave down a

tube

The esophagus & peristalsis

Small intestine

The stomach Stomach

◦ Site of chemical digestion by HCl (pH 1.5 – 3.5)◦ Mucus-lined walls protected from damage◦ Acid may splash up into unprotected

esophagus, causing heartburn

Why are the walls wrinkled?

◦ Food may enter small intestine via pyloric sphincter only when liquefied (chyme)

Accessory organs The initial segment of the small intestine, the duodenum, is where accessory organs contribute to digestion

Accessory organs:◦ Liver – produces bile to breakdown

lipids (fats)◦ Gall bladder – stores excess bile◦ Pancreas – produces additional

digestive enzymes (insulin, lipase, etc.)

◦ The bile duct adds bile/digestive enzymes to the duodenum

The small intestine Small intestine (duodenum, jejunum & ileum)

◦ Responsible for absorbing majority of nutrients (into blood stream)◦ ~6m long and houses some bacteria that aid in digestion◦ Maximizes surface area to boost absorption

Nutrient absorption To maximize surface area, the small intestine is lined with two structures:1. Villi

◦ finger-like projections on inner surface of SI

2. Microvilli◦ hair-like projections on surface of villi

These form the absorptive surface where nutrients enter the bloodstream (‘blood-gut’ barrier)

Large intestine & fecal formation

Large intestine (AKA the colon)◦ Absorbs water from slurry entering from SI◦ Removal of water solidifies leftovers into feces for excretion◦ Houses primary gut microflora

◦ Bacteria digest unused macromolecules to yield useful nutrients (eg. E. coli produce vitamin K)◦ ~1x1014 bacterial cells (we know about ~5% of them)◦ Bacteria may produce organic gases while metabolising

leftover nutrients… (what does this cause?)

Rectum, anus & defecation

Rectum◦ Tail end of colon◦ Site of fecal collection (when full, time to poop!)

Anus◦ Sphincter that controls departure of fecal matter

(defecation)

Fun fact: 1/3 the dry mass of every poop is bacterial cells (this is why we ALWAYS wash our hands post-poop)

Try this… Pg. 266, #1 – 8 Pg. 274, #1 – 5, 8, 10