digestion
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Digestion. IB Core Topic V. Processing Food. Ingestion: taking food into the digestive system Digestion: breaking down large molecules into smaller molecules that can be absorbed - PowerPoint PPT PresentationTRANSCRIPT
DigestionIB Core Topic V
Processing Food Ingestion: taking food into the digestive system Digestion: breaking down large molecules into
smaller molecules that can be absorbed Absorption: Transferring nutrients from the
digestive system to the circulatory system for transporting around the body
Assimilation: The body’s processing of digested, absorbed substances (e.g. transamination and deamination of animo acids)
Egestion: Passing indigestible matter out of the body
The Alimentary Canal
Basic 4-layered structure:1. Inner lining – contains goblet cells that secrete
mucus for lubrication2. Nerves, blood and lymph vessels3. Involuntary muscle layer that mixes food and
causes peristalsis (pushing food along)4. Peritoneum – outer, lubricated layer
Alimentary Canal Sections
Oesophagus – “food pipe” Epiglottis stops food going down the trachea Uses peristalsis to carry food to the stomach
Stomach – muscular bag for storing and digesting food Cardiac sphincter relaxes to let food in Holds ~ 1litre of food for ~ 4hrs
The Stomach Stomach lining is heavily folded, containing
gastric glands Glands secrete gastric juice
Mucus – lines stomach to prevent self-digestion Pepsinogen – converted to pepsin (protease) in low
pH conditions and by pepsin (autocatalytic) Hydrochloric acid – kills micro-organisms, gives
optimum pH for pepsin, loosens fibrous/cellular food and deactivates salivary amylase
Young mammals have rennin, and enzyme that converts caesinogen (soluble milk protein) to caesin (insoluble) so it can be digested by pepsin
Food + gastric juice = CHYME
Duodenum 1st section of the small intestine Passage of food from stomach controlled
by pyloric sphincter Surface is highly folded, containing villi
(finger-like projections) Cells on surface of villi have further
projections – microvilli. These form the brush border (massive S.A.)
The Pancreas
Contains the following: water and bicarbonate ions to neutralise
stomach acids Pancreatic amylase, to continue the
breakdown of starch to maltose Lipase, to break down lipids to fatty acids and
glycerol Trypsinogen, precursor of the protease trypsin
Gall Bladder Produces bile, which contains:1. Bile pigments2. Bile salts
alkali, to neutralise stomach acid Emulsify fats, to increase SA for lipase
action Assists in absorption of fat-soluble vitamins
(A, D and K)The acidic chyme is converted to chyle by the
addition of bile and pancreatic juice
Jejunum and Ileum Length of the small intestines are covered in villi The structure of the villi reflect their function
(absorption of nutrients). Large surface area Very long (~8m) Extremely thin lining (1 cell thick) Rich blood supply – lots of capillaries
Capillaries absorb small substances (e.g. glucose, water, amino acids, vitamins, minerals) by a combination of diffusion (with the concentration gradient) and active transport (against the concentration gradient
Large intestines Consists of caecum, appendix, colon, rectum
and anus Caecum and appendix are thought to be vestigial in humans: herbivores host cellulase-secreting bacteria here
Large intestines host bacteria that produce vitamins (K, B2 group), but some can cause infection (appendicitis)
Colon reabsorbs water – undigested liquid from small intestines is converted to semi-solid faeces (colour due to bile pigments)
Faeces are stored in the rectum and pass out of the anus under the control of the anal sphincter