IBDP Biology Units 6 and 11

Alimentary Canal

Organs through which food passes (oesophagus, stomach, small & large intestine)

Accessory Organs

Aid in digestion but do not actually transfer food (salivary glands, pancreas, liver, gall bladder)

Oesophagus

Transports food to stomach

Stomach

Stores and churns food

Small Intestine

Absorbs nutrients

Large Intestine

Absorbs water and ions

Salivary Glands

Moistens food into a bolus

Pancreas

Releases digestive enzymes

Liver

Detoxifies certain molecules

Gall Bladder

Stores/concentrates bile

Rectum

Stores and expels feces

Mechanical Digestion

Chewing (mouth), churning (stomach).

Movement of Food

Peristalsis (longitudinal muscle rhythmically contracts and relaxes) and Segmentation (circular smooth muscle in the small intestine contracts and relaxes)

Chemical Digestion

Food is broken down by chemical enzymes in the stomach acids, bile (gall bladder), enzymes (pancreas & liver)

Four Main Tissue Layers of the Small Intestine

Serosa (protective outer layer), Muscle Layer (outer layer of longitudinal muscle and inner layer of circular muscle), Submucosa (composed of connective tissue separating the muscle layer from the innermost mucosa), Mucosa (highly folded inner layer)

Features of Villi (hint: Mr. Slim)

Microvilli – Ruffling of epithelial membrane further increases surface area

Rich blood supply – Dense capillary network rapidly transports absorbed products

Single layer epithelium – Minimises diffusion distance between lumen and blood

Lacteals – Absorbs lipids from the intestine into the lymphatic system

Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices

Membrane proteins – Facilitates transport of digested materials into epithelial cells

Absorption

Digested food monomers must pass from the lumen into the epithelial lining of the small intestine, tight junctions between epithelial cells occlude any gaps between cells, different monomers undertake different methods for crossing the apical and basolateral membranes

Secondary Active Transport

• A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)

• Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na⁺)

Facilitated Diffusion

• Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane

• Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)

• Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion

Osmosis

• Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)

• The absorption of water and dissolved ions occurs in both the small and large intestine

Simple Diffusion

• Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane

• Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood

Endocytosis

• Endocytosis involves the invagination of the plasma membrane to create an internal vesicle containing extracellular material

• Vesicle formation requires the breaking and reforming of the phospholipid bilayer and hence is an energy-dependent process

• In the intestines, vesicles commonly form around fluid containing dissolved materials (pinocytosis – cell ‘drinking’)

• Pinocytosis allows materials to be ingested en masse and hence takes less time than shuttling via membrane proteins

Two Types of Starch

Amylose: linear chains, soluble in water, digested into maltose subunits.

Amylopectin: branched chains, insoluble in water, digested into branched chains called dextrins

Role of Pancreas in Starch Digestion

Produces the enzyme amylase, and the hormones insulin and glucagon