Unit 2: Cells

^^Part 1: Cells^^

  • cells are the structural units of all living things
  • the human body has 50 to 100 trillion cells

==Overview of the Cellular Basis of Life==

  • %%The Cell Theory:%%
  1. a cell is the basic structural and functional unit of living organisms
  2. the activity of an organism depends on the collective activities of its cells
  3. according to the principle of complementarity, the biochemical activities of cells are dictated by their structure (anatomy) which determines their function (physiology)
  4. continuity of life has a cellular basis
  • %%Most cells are composed of four elements:%%
  1. carbon
  2. hydrogen
  3. oxygen
  4. nitrogen
  • Cells are about 60% water

==Anatomy of a Generalized Cell==

  • %%In general, a cell has three main regions or parts:%%
  1. nucleus
  2. cytoplasm
  3. plasma membrane

%%The Nucleus%%

  • control center of the cell
  • contains genetic material known as deoxyribonucleic acid, or DNA
    • DNA is needed for building proteins
    • DNA is necessary for cell reproduction
  • Nuclear envelope (membrane)
    • consists of a double membrane that bounds the nucleus
    • contains nuclear pores that allow for exchange of material with the rest of the cells
    • encloses the jellylike fluid called cytoplasm
  • Nucleolus
    • nucleus contains cone or more dark-staining nucleoli
    • sites of ribosome assembly
    • ribosomes migrate into the cytoplasm through nuclear pores to serve as the site of protein synthesis
  • Chromatin
    • composed of DNA wound around histones (proteins)
    • scattered throughout the nucleus and present when the cell is not dividing
    • condenses to form dense, rodlike bodies called chromosomes when the cell divides

%%The Plasma Membrane%%

  • transparent barrier for cell contents
  • contains cell contents
  • separates cell contents from surrounding environment
  • ^^fluid mosaic model constructed of:^^
    • two layers of phospholipids arranged “tail to tail”
    • cholesterol and proteins scattered among the phospholipids
    • sugar groups may be attached to the phospholipids, forming glycolipids
  • ^^phospholipid arrangement in the plasma membrane^^
    • hydrophilic (“water loving”) polar “heads” are oriented on the inner and outer surfaces of the membrane
    • hydrophobic (“water fearing”) non polar “tails” from the center (interior) of the membrane
    • this interior makes the plasma membrane relatively impermeable to most water-soluble molecules
  • ^^role of proteins^^
    • responsible for specialized membrane functions:
    • enzymes
    • receptors for hormones or other chemical messengers
    • transport as channels or carriers
  • ^^role of sugars^^
    • glycoproteins are branched sugars attached to proteins that abut the extracellular space
    • glycocalyx is the fuzzy, sticky, sugar-rich area on the cell’s surface
  • ^^cell membrane junctions^^
    • cells are bound together in 3 ways:
    • \       
      1. glycoproteins in the glycocalyx act as an adhesive or cellular glue
    • \       
      1. wavy contours in a tongue-and-grove function
    • \       
      1. special cell membrane junctions are formed, which vary structurally depending on their roles
  • ^^main types of cell junctions^^
    • tight junctions
    • impermeable junctions
    • bind cells together into leak proof sheets
    • plasma membranes fuse like a zipper to prevent substances from passing through extracellular space between cells
  • ^^desmosomes^^
    • anchoring junctions, like rivets, that prevent cells from being pulled apart as a result of mechanical stress
    • created by button like thickenings of adjacent plasma membranes
  • ^^gap junctions (communicating junctions)^^
    • allow communication between cells
    • hollow cylinders of proteins (connexons) span the width of the abutting membranes
    • molecules can travel directly from one cell to the next through these channels