Tissues

• Layers of the ileum


  • Serosa
  • Muscularis externa
  • Submucosa
  • Muscularis mucosa
  • Mucosa
• Muscularis externa


  Consists of an outer layer of longitudinal muscle and an inner layer of circular muscle. Contractions of the longitudinal muscle cause pendular movement and contractions of the circular muscle cause local constrictions which help churn and mix the food.
• Submucosa


  Largely composed of connective tissue and contains many blood vessels and lymphatic vessels which are crucial in transporting absorbed food products
• Muscularis mucosa


  Thin layer of muscle that lies between the submucosa and the mucosa. It is important in moving the villi that are present in the mucosa, thus increasing contact with digested food in the gut lumen.
• Mucosa


  The layer in contact with the food in the gut lumen. It is highly specialised with a substantially increased surface area due to the presence of villi and microvilli.
• Columnar epithelial cells of the villus


  • Have their surface area extended by the presence of numerous microvilli that give a brush border appearance
  • Many digestive enzymes occur within the cell surface membrane, where the presence of microvilli further increases surface area, increasing the number of these enzymes that can make contact with the food
  • Particularly rich in mitochondria for the active transport of some absorbed food products
• Absorption of food products


  1. Active transport of glucose and amino acids into epithelial cells

  2. Diffusion of lipid soluble products such as glycerol and fatty acids into the cells lining the villi

  3. Pinocytosis involved in absorption of some substances like antibodies
• Lacteals


  Part of the lymphatic system that transport lipids (fatty acids and glycerol) out of the villi
• Crypts of Lieberkuhn


  Intestinal glands that produce new cells that constantly move up the surface of the villi. They have an anti-microbial function in protecting the neighbouring dividing cells.
• Leaf structure


  • Upper epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Vascular tissue
  • Lower epidermis
• Upper epidermis


  Protective layer covered in a waxy cuticle to reduce water loss
• Palisade mesophyll


  Primary photosynthesising region, with cells arranged perpendicular to the upper epidermis to maximise light absorption
• Spongy mesophyll


  Loosely and irregularly arranged cells with intercellular air spaces that facilitate gas exchange. They have fewer chloroplasts than palisade cells.
• Vascular tissue


  Xylem vessels transport water and minerals up the plant, phloem sieve tubes transport sugars produced in photosynthesis away from the leaf
• Lower epidermis


  Also covered in a waxy cuticle, but thinner than the upper epidermis. Contains stomata controlled by guard cells for gas exchange.
• The leaf is primarily an organ for photosynthesis and is well adapted for gas exchange.
• The large surface area of most leaves is a further adaptation for photosynthesis, both through providing more palisade cells in close proximity to the upper leaf surface and also to facilitate gas exchange.
• In plants such as cacti, adapted to arid habitats, needle-shaped leaves have evolved to decrease the surface area across which water loss can occur.
• Serosa - the outermost layer of the peritoneum, composed of loose connective tissue