Tissues

• Tissues


  Groups of cells of the same type that perform a common function
• Organs


  Structures made up of different tissues that work together to perform a specific function
• Most organisms are multicellular and their bodies contain a variety of different cells
• Cell differentiation


  Cells developing different structures and shapes to carry out different functions
• Tissue


  Cells that differentiate in the same way, e.g. cardiac muscle
• Organ


  Different tissues combining to form an organ, e.g. the heart
• System


  Organs working together, e.g. the circulatory system
• Organism


  All the systems in an individual
• In this unit we are going to study the structure and function of two organs: the ileum (mammalian organ) and a mesophytic leaf (plant organ)

  
• Ileum


  A muscular and glandular tube suspended by connective tissue (mesentery)

  
• Wall of the ileum


  • Outer layer of fibrous connective tissue (serosa)
  • External muscle layers (muscularis externa)
  • Connective tissue (submucosa)
  • Innermost layers (mucosa)
• Mucosa


  The innermost layer of the gut, with a folded lining and numerous finger-like villi

  
• Mucosa


  • Crypts of Lieberkuhn (glands) that secrete intestinal juice
  • Continually dividing stem cells at the base of the crypts that produce new columnar cells for the villi
  • Paneth cells that produce anti-microbial substances
  • Disaccharidease and peptidase enzymes on the epithelial cell surface membranes
  • Columnar epithelium with goblet cells that produce mucus
  • Epithelial surface with microvilli/brush border to increase surface area
  • Network of blood capillaries in each villus that absorb products of digestion/sugars
  • Lacteal in each villus that absorbs the majority of fats
• Absorption in the small intestine

  1. Diffusion of fatty acids, glycerol and most vitamins

  2. Facilitated diffusion of molecules like fructose

  3. Active transport of glucose, amino acids, dipeptides and some salts

  
• Submucosa


  • Layer of involuntary muscle (muscularis mucosa) that allows movement of the villi
  • Rich supply of blood and lymph vessels that carry absorbed nutrients away
  • Connective tissue that provides support
• Muscularis externa


  • Outer longitudinal layer with muscle fibres arranged along the gut
  • Inner circular layer with muscle fibres arranged around the gut
  • Circular and longitudinal muscles work antagonistically to cause peristalsis
• Serosa


  • Outermost layer containing connective tissue that protects the gut
  • Mesentery connects to the peritoneum and supports the coiled intestine

  
• You could be asked to draw a block diagram of the ileum or leaf using a photograph in the exam
• Mesophytic leaf


  The plant organ specialised for photosynthesis, attached to the stem by a stalk
• Structure of a leaf

  • Upper epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Xylem
  • Phloem
  • Lower epidermis
  • Stomata

  
• Adaptations of leaf tissues

  • Large surface area for efficient light absorption
  • Epidermis with waxy cuticle to allow light through
  • Palisade layer with tightly packed cylindrical cells and more chloroplasts
  • Spongy mesophyll with large air spaces for gas diffusion
  • Vascular bundles with xylem vessels and phloem sieve-tubes
  • Lower epidermis with stomata to regulate gas exchange and water loss
  • Thin leaf structure to minimise diffusion distances
• The leaf is an organ with adaptations for maximising photosynthesis while minimising transpirational water loss