Biology - Digestive System

• Digestion


  The chemical and mechanical breakdown of proteins, fats and carbohydrates into smaller molecules, which can be absorbed into cells
• Organs that assist in the human digestive system by carrying out basic functions of:

  • Ingestion of food
  • Mechanical and chemical digestion of food
  • Movement of food through the alimentary canal
  • Absorption of digested food to then be transported to cells
  • Egestion of waste materials
• Alimentary Canal


  The passage from the mouth to anus along which food passes within the body
• Layers of the alimentary canal

  • Inner Mucosa
  • Sub-mucosa
  • Muscular Externa
  • Serosa
• Mucosa


  Moist membrane which lines the cavity, forms villi, functions include secretion of mucous and digestive enzymes, absorption of end products into blood, protection against disease
• Mechanical digestion in the mouth


  Chewing action of teeth and jaw breaks food into smaller particles
• Chemical digestion in the mouth

  Food is mixed with saliva containing digestive enzymes
• Types of teeth and their functions

  • Incisors - biting
  • Canines - tearing
  • Premolars - crushing and grinding food
  • Molars - crushing and grinding food
• Saliva


  Contains salivary amylase (breaks down starch), mucous (lubricates mouth and food), and antibodies (kill bacteria in food)
• Swallowing


  1. Food forms a bolus in the mouth and is pushed to the pharynx

  2. Food moves from the pharynx through the oesophagus to the stomach

  3. The epiglottis covers the trachea
• Enzymes


  Specific enzymes act on specific substrates, catalyse reactions best at normal body temperature (~37°C)
• Enzymes and their substrates

  • Lipases - act on lipids and fats
  • Proteases - act on proteins (e.g. pepsin)
  • Carbohydrases - act on carbohydrates (e.g. amylase)
  • Nucleases - act on nucleic acids
• Stomach


  Mucosa contains gastric glands which secrete gastric juice (HCl, mucus and digestive enzymes)
• Mechanical digestion in the stomach

  1. Waves of muscular contraction move along the stomach wall

  2. Unique oblique muscle layer enables the stomach to contract in a variety of ways to churn and mix food and juices

  3. Food is converted into a thick liquid called chyme
• Chemical digestion in the stomach

  Pepsin (gastric protease) breaks down proteins into polypeptides and then into amino acids
• Pyloric sphincter


  Thickening of the circular muscle at the lower end of the stomach which prevents the stomach contents moving through unless moved by peristalsis
• Small intestine


  Longest part of the alimentary canal (~6m), divided into 3 regions: duodenum, jejunum, ileum
• Specialisations of the small intestine

  • Permanent folds that extend into the interior
  • Villi (finger-like projections extending from the folds)
  • Microvilli (microscopic projections on the surface of the villi)
  • Glands in the mucosa produce intestinal juice
• Digestion in the small intestine

  1. Segmentation occurs (mechanical digestion)

  2. Chemical digestion is completed by pancreatic juice, bile, and intestinal juice
• Pancreatic juice


  Alkaline (bicarbonate) (pH 8) to help neutralize acid from stomach, contains enzymes
• Bile


  Contains bile salts for digestion of fats (but no enzymes)
• Intestinal juice


  Secreted by glands along the length of the small intestines, contains many enzymes mainly for the digestion of proteins, carbohydrates and lipids
• Absorption in the small intestine

  Digested material moves through the cells lining the outside of the villi and into the blood and lymph vessels
• Absorption and assimilation


  Molecules that enter through the walls of the small intestine have been absorbed, assimilation occurs when these molecules are used for building new, larger molecules
• Large intestine


  Approximately 1.5 m long, has a larger diameter than the small intestine, no villi, no enzymes secreted, main function is to absorb water and minerals
• Defecation


  Removal of semi-solid material from the rectum via peristalsis, faeces expelled contains water, undigested food material, bacteria, bile pigments, and cellular remains
• Types of digestive systems

  • Hindgut fermenter
  • Foregut fermenter
  • Monogastric organisms
• Autotrophs


  Organisms that produce their own energy by converting solar energy (from the sun) into glucose in a process called photosynthesis. This glucose is then converted to energy in the form of ATP in a process called respiration.
• Plant cells


  • Specialised
  • Organised into specialised tissues, organs and organ systems which work together to allow the organism to grow and function normally
• Main systems in plants

  • Root system
  • Shoot system
  • Reproductive system
• Plants have specialised cells that work together to form specialised tissues.
• Root hair cells


  • Contain hair-like extensions to the membrane surrounding the root cells
  • Projection increases the surface-area-to-volume-ratio (SA:V) maximising the rate at which water diffuses from the soil into the plant by osmosis
  • Long thin shape maximises the rate at which oxygen moves into the cell and carbon dioxide is removed
• Root hair cell function

  1. Absorb water by osmosis

  2. Undergo cellular respiration to create the energy they need to grow and repair
• Palisade mesophyll cells


  • Located on the upper side of the leaf just below the almost transparent cells of the upper epidermis
  • Coated in a waxy layer that prevents water loss
  • Where most of the photosynthesis within the leaf occurs
  • Tightly packed together and close to the surface of the leaf to maximise light absorption
  • Contain a lot of chloroplast which allow them to perform photosynthesis
• Waxy layer on upper epidermis cells

  Prevents water loss
• Palisade mesophyll cells


  Where most photosynthesis occurs
• Chemical equation for photosynthesis
• Guard cells (stomata)


  • Waxy cuticle layer prevents gas or water from entering or leaving the leaf
  • Only way gases and water vapour is able to be transported into/out of the cell is through pores in the epidermis called stomata
  • Pores formed by two chloroplast containing guard cells which open and close allowing gases and oxygen to leave and enter the leaf
  • Respond to light, opening at day break and closing at night, and hydration levels
  • If the leaf is losing too much water the stomata will close, if it is humid and they are well hydrated the stomata remain opened
• Explaining how guard cells maintain balance

  1. Observation: Guard cells regulate water loss by closing and opening stomata

  2. Linking statement: When guard cells need to allow gas exchange for photosynthesis, they take up water, swell and become turgid opening the stomata

  3. Inference: When plants need to reduce water loss, the guard cells release water and become flaccid causing the stomata to close
• Percentage of stomata open over time

  Varies between different plant types (e.g. geranium vs pineapple)