Animal and plant cells

• Cell


  The basic functional unit of life
• Cell Theory


  • All organisms are composed of one (unicellular) or more cells (multicellular)
  • Cells are the basic functional units of life and the basic units of organisation of multicellular organisms
  • Cells arise by the division of pre-existing cells
• Histology


  The microscopic study of the structure of animal and plant tissues
• Histology procedure


  Examining cells and tissues under a light microscope or electron microscope, which have been sectioned, stained and mounted e.g. on a microscope slide
• Tissue


  An organised group of cells, working together as a functional unit
• Organ


  A structure that serves a specialized function and consists of several tissues
• System


  Groups of tissues and organs that work together to perform one or more functions
• Types of animal tissues

  • Epithelial tissue
  • Connective tissue
  • Muscle tissue
  • Nervous tissue
• Epithelium


  A tissue composed of cells that line the cavities and surfaces of structures throughout the body

  
• Epithelial tissue


  • Main function is to act as a barrier and form a protective layer
  • Cells typically form layers of closely packed cells
  • The bottom layer of cells rests on a basement membrane
• Cell polarity


  Epithelial cells show apical-basal polarity, meaning the cell is different at its two ends
• Types of epithelia


  • Simple
  • Pseudostratified
  • Stratified
• Simple squamous epithelium


  Made up of flattened cells with a characteristic bulging nucleus, low amount of cytoplasm, and irregular (tessellated) edges when observed in surface view

  
• Simple squamous epithelium


  • Provides a thin layer across which diffusion can occur readily
  • Provides relatively friction-free surfaces for fluids to flow over them
• Simple cuboidal epithelium

  Made up of roughly cube-like cells with the nucleus typically occupying the centre of the cell
• Simple cuboidal epithelium


  • Rich in specific transport channels
  • Functions in secretion and absorption
• Simple columnar epithelium


  Made up of elongated cells with the nucleus at the basal end and a lot of cytoplasm

  
• Simple columnar epithelium


  • Presence of microvilli (forming a brush border) indicates increased surface area for secretion and absorption
  • Presence of cilia indicates movement of substances over the epithelium
  • Presence of goblet cells produce mucus

  
• Pseudostratified epithelium


  Made up of one layer of cells, but as cells do not have the same shape, the epithelium appears to have more than one layer. All cells rest on the basement membrane.

  
• Stratified squamous epithelium


  Made up of several layers of squamous cells, with the upper layer continuously removed by friction and replaced by the germinal layer below
• Stratified cuboidal epithelium


  Made up of several layers of cuboidal cells

  
• Stratified columnar epithelium


  Made up of several layers of columnar cells

  
• Stratified transitional epithelium


  Consists of 3-4 layers of cells, all of similar size and shape except at the apical side where the cells are more flattened. All cells are able to modify their shape when placed under different conditions.
• Plant tissues


  • Simple tissues made up of one type of cell
  • Complex tissues made up of several types of cells
• Simple plant tissues


  • Parenchyma
  • Collenchyma
  • Sclerenchyma

  
• Complex plant tissues


  • Xylem
  • Phloem

  
• Ground tissue system


  Performs storage, support and photosynthetic functions, made up of parenchyma, collenchyma and sclerenchyma

  
• Parenchyma


  Cells have a thin primary cell wall made up of cellulose, pectins and hemicelluloses, and a middle lamella composed of calcium and magnesium pectates that cement adjacent cells. Cells have a large central vacuole with cytoplasm at the periphery. Shape is variable.

  
• Parenchyma


  • May be tightly packed as in palisade mesophyll but usually contains prominent intercellular air spaces
  • Air spaces are particularly large in spongy mesophyll and aerenchyma

  
• Functions of parenchyma


  • Packing tissue
  • Support
  • Gaseous exchange
  • Metabolism including photosynthesis
  • Water and mineral salt transport
  • Storage
• Collenchyma


  Cells have a definite shape with unevenly thickened walls with extra deposition of cellulose and pectins at their corners, allowing them to stretch and accommodate growth

  
• Collenchyma


  • Intercellular spaces are very small or absent due to thickening of the cell wall
  • Usually polygonal and elongated (up to 1mm long) with slightly tapering ends

  
• Function of collenchyma


  • Provides mechanical support to organs in which it occurs, primarily dicot stems, petioles and the midrib of leaves
  • Provides support whilst enabling stretching/movement and growth of adjacent growing regions
• Collenchyma


  • Alive at maturity, flexible but strong and provides mechanical support to organs in which it occurs, primarily dicot stems, petioles (leaf stalks) and the midrib of leaves
  • Very important supportive role in young growing organs and herbaceous (non-woody) plants (assists turgid parenchyma)
  • Provides support whilst enabling stretching/movement and growth of adjacent growing regions
  • May develop chloroplasts and contribute to photosynthesis (rare)

  
• Sclerenchyma


  Two main types: Fibres and Sclereids
• Sclerenchyma


  • Principal characteristic is a deposition of a thick lignified secondary cell wall (i.e. deposited after elongation has finished)
  • Lignification prevents further growth – hence sclerenchyma is generally found around tissues that have stopped elongating
  • Lignin has high tensile strength and high compressional strength

  
• Sclerenchyma cells


  • Undergo apoptosis (programmed cell death) at maturity and therefore are dead with no protoplasm when they attain their functional role
  • Main function is purely to support plant structures and provide mechanical strength
• Fibres


  Elongated (usually 1-3 mm long) and needle-like, with interlocking/overlapping tapering edges
• Sclereids


  Roughly spherical but other variable forms are known – e.g. oval, cylindrical, stellate, branched
• Xylem


  A complex tissue made up of both living and dead cells that has two main functions: Conduction of water and dissolved minerals, and Support