Digestion and Absorption

• What is digestion?

  The process in which relatively large, insoluble biological molecules in food are hydrolysed into smaller, soluble molecules that can be absorbed across the cell membranes into the bloodstream and delivered to cells in the body
• The products of digestion are used to either provide cells with energy or to build other molecules for cell growth, repair and function.
• What is the use of the products of digestion?


  To provide cells with energy or to build other molecules for cell growth, repair and function
• Enzymes are specific to one substrate
• Enzymes are responsible for breaking down polymers into sections, and other enzymes may then need to break them down further into their monomers
• Epithelial cells have membrane bound enzymes and also carrier proteins for specific molecules to aid digestion.
• Where does digestion begin?


  In the mouth
• What is the function of teeth in digestion?


  Breaks down food into smaller pieces, this increases its surface area to volume ratio
• What is the tube connecting the mouth and the stomach called?

  The oesophagus
• The contractions of the smooth muscle in the wall of the oesophagus help move food down towards the stomach.
• Protein digestion begins in the stomach
• Carbohydrate digestion begins in the mouth
• How does the stomach aid the digestion of molecules?

  The glandular tissue produces enzymes and stomach acid, and the muscular tissue churns the food, mixing it with the enzymes and acid.
• How does stomach acid aid protein digestion?

  The acid helps to unravel proteins tertiary structure to enable enzyme activity, as well as lowering the pH, which is optimal for stomach enzymes to work. The lowered pH is also detrimental to any microorganisms present in the food
• What are the three sections of the small intestine?

  Duodenum, jejunum, ileum
• The wall of the small intestine contains smooth muscle tissue, which rhythmically contracts (peristalsis) to move food along.
• Carbohydrate, protein and lipid digestion occurs mainly in the duodenum, catalysed by enzymes produced in the pancreas and the small intestine itself.
• Soluble food molecules are absorbed by diffusion and active transport into the bloodstream across the wall of the small intestine.
• The ileum is long and lined with finger-like villi to increase the surface area over which absorption can take place, including water absorption
• What is the function of the large intestine?

  Absorbs any water remaining in food that was not able to be digested along with vitamins and minerals
• Where is undigested food material stored?

  The rectum
• The internal walls of the small intestine are folded into projections called villi, about 1mm in length, which increase the surface area
• What are the benefits of villi in the small intestine?

  Increase the surface area, short diffusion pathway, lots of capillaries to help maintain concentration gradients and containing muscles
• Villi have thin walls, around one cell thick, in order to help maintain a short diffusion pathway and thus increase the rate at which substances are transported
• Villi have lots of capillaries to help maintain the concentration gradients and lost by constantly transporting absorbed nutrients away
• Villi contains muscles and can move, this helps them to mic the contents of the ileum so that the villi always have new material to absorb nutrients from. This also helps to maintain the concentration gradient.
• Epithelial cells line the ileum and colon and are specialised for absorption
• What are some of the adaptions of epithelial cells?

  Contain microvilli, which increase the surface area.

  Have many mitochondria to provide energy for active transport via ATP.

  Large amounts of carrier proteins for active transport

  Large amounts of channel proteins for facilitated diffusion
• Enzymes are substrate specific, meaning that different enzymes digest and breakdown the different biological molecules contained in food
• Digestive enzymes are extracellular enzymes, working outside of cells
• What are the three main types of digestive enzymes?

  Carbohydrases, proteases and lipases
• Where does the digestion of carbohydrates take place?

  In the mouth and small intestine
• Amylase enzyme in saliva is produced by the salivary glands and aids digestion by breaking the glycosidic bonds in starch to form maltose
• Pancreatic amylase is produced by the pancreas and released into the small intestine where it hydrolyses the glycosidic bonds in starch to form maltose
• The small intestine produces membrane bound disaccharides and amylase in the cell membrane of epithelial cells
• Membrane bound disaccharides produced by the small intestine hydrolyse the glycosidic bonds in disaccharides to turn them into monosaccharides
• What are the three membrane bound enzymes?

  Maltase, sucrase and lactase
• Maltase catalyses the breakdown of maltose into alpha glucose
• Sucrase catalyses the breakdown of sucrose into alpha glucose and fructose
• Lactase catalyses the breakdown of lactose into glucose and galactose