tissues and levels of organisation

• How do small, unicellular organisms get nutrients?

  by diffusion
• cell size is constrained by SA:Vol ratio = as the cell gets larger, the ratio decreases (not good)
• larger organisms don't have larger cells, they have more cells (multicellular)
• connecting cells - joining cells together makes a tissue
• 3 types of cell-to-cell junctions:

  • tight junctions
  • gap junctions
  • anchoring junctions
• cells > tissues > organs > organ systems
• tight junction:

  • adjacent plasma membranes
  • strands of transmembrane proteins
  • intercellullar space
• gap junction:

  • adjacent plasma membranes
  • gaps between cells
  • connexons - composed of connexins
• 3 types of anchoring junction

  • desmosomes
  • adherens
  • hemidesmosomes
• tight junctions - create a continuous semi-permeable membrane connection, controlling the movement of solutes and liquids
• adherens:

  • found below tight junctions
  • glycoprotein = cadherin
  • cadherin forms a zipper-like structure in the gap between two cells
  • actin filaments join together around the cells
• desmosomes:

  • commonly found in skin cells and cardiac cells
  • responsible for joining two cells together
  • transmembrane glycoprotein = cadherin
  • intermediate filament = keratin
• hemidesmosomes:

  • attach cells to extracellular filaments in the basement membrane
  • attachment helps stabalise position of epithelial cells and anchors it to underlying tissue
  • integrins attach to basal lamina in basement membrane
• gap junctions:

  • two cells are held together by two embedded, interlocking transmembrane proteins (connexons)
  • common among epithelial cells where the movement of ions helps coordinate the functions e.g beating of cilia
• 4 main types of animal tissues:

  • epithelium
  • connective
  • muscle
  • nervous
• epithelium tissues - line cavities, open spaces and surfaces
• connective tissue - connect tissues together, provide support
• muscle tissue - generate movement
• nervous tissue - generate and send electrical signals
• epithelia categorised by:

  • number of layers
  • shape of cells
• (epithelia) number of layers:

  • simple = 1 layer
  • stratified = multiple layers
• (epithelia) shape of cells:

  • squamous
  • cuboidial
  • columnar
  • transitional
• almost all substances coming in/out of the body must pass through an epithelium
• characteristics of epithelia:

  • top and bottom surface
  • supported by connective tissue
  • no blood supply
  • regenerative
• functions of epithelia

  • absorption
  • protection
  • secretion
  • sensory perception
  • filtration
  • excretion
• connective tissue:

  • connects different tissues/ gives body structure
  • consists of cells embedded in a non-cellular matrix with fibres
• squamous cells

  • cells appear thin and flat like scales
• cubodial cells

  • appear like little boxes
• columnar cells

  • tall and relatively slender rectangles
• transitional:

  • cells can change between squamous and cubodial in shape
  • found in bladder - can undergo repeated stretching without damage
• Types of fibres

  • collagen
  • elastin
  • reticular fibres
• collagen:

  • long, straight and unbranched
  • consists of bundle of fibrous proteins
  • tendons and ligaments are made of collagen
  • provides tensile strength
• reticular fibres

  • made of same sub-units as collagen but arranged differently
  • thinner than collagen, branched
  • form an interwoven network called stroma - stabalises position of functional cells e.g in organs like liver, or blood vessles and nerves
• elastic fibres:

  • important structural protein
  • branched and wavy
  • arranged as fibres or discontinuous sheets in the ECM of skin
  • provides stretch and elastic recoil
• ECM = extracellular matrix
• types of cells

  • fibroblasts
  • chondrocytes
  • osteocytes
  • adipocytes
• fibroblasts - secrete ECM and fibres
• type of cells:

  • fibroblasts
  • chondrocytes
  • osteocytes
  • adipocytes
• fibroblasts - secrete ECM and fibres
• Chondrocytes - secrete ECM in cartilage