1.2 Cell organisation

•  The cell theory states that all organisms are composed of cells; the cell is the

  basic unit of life.

   Organisms can be unicellular, such as amoeba and bacteria, or multicellular such as

  plants and animals.

   New cells arise from pre-existing cells; specialised cells arise from undifferentiated

  stem cells.

   Advances in microscopy have allowed us to understand the ultrastructure of cells.
• Eukaryotic cells have a nucleus and membrane bound organelles. Eukaryotic cells include

  plant and animal cells.
• Plant cells have additional organelles and structures e.g. chloroplasts for photosynthesis

  and cellulose cell walls for support and to maintain turgor pressure.
• Nucleus

  Contains DNA which codes for or controls protein synthesis.

  DNA replication occurs here. Transcription produces mRNA

  templates.
• Nuclear pores Allow the transport of mRNA and ribosomes out of the

  nucleus.
• Nuclear envelope

  or double

  membrane

  Separates the contents of the nucleus form the cytoplasm.
• Nucleolus Produces rRNA, tRNA and ribosomes.
• Chromatin Condenses before cell division to form chromosomes.
• Rough endoplasmic

  reticulum

  Packaging and storing proteins. Producing transport vesicles

  which merge to form the Golgi body.
• Smooth

  endoplasmic

  reticulum

  Produce, package and transport steroids and lipids.
• Golgi

  body/apparatus

  Packaging proteins for secretion from the cell. Modification

  of proteins e.g. by adding carbohydrate chains to form

  glycoproteins. Producing lysosomes and digestive enzymes

  (tertiary structure).
• Lysosomes

  Contain powerful digestive enzymes to break down worn out

  organelles or cells. Phagocytes use lysosomes to digest

  engulfed bacteria.
• Centrioles Form the spindle during cell division. They are not present in

  higher plant cells.
• Mitochondria ATP synthesis by aerobic respiration.
• Chloroplasts Contain photosynthetic pigments which trap light energy for

  photosynthesis.
• Vacuole Contains cell sap and stores solutes such as glucose. Swells

  due to osmosis for turgidity.
• Ribosomes Protein synthesis. Primary protein structure is formed at the

  ribosome.
• Plasmodesmata Connects cells via cytoplasm filled canals, which pass through

  cell walls. Allows transport via the symplastic pathway.
• Cell wall

  Mechanical strength due to the high tensile strength of

  cellulose microfibrils. Transport of solutes via the apoplastic

  pathway. Cell to cell communication via the plasmodesmata.
• Mitochondria and chloroplasts have many similar features.

  Similarities include:

   double membrane

   highly folded inner membranes

   circle of DNA for self-replication

   ribosomes

   produce ATP
• Mitochondria and chloroplasts have many similar features.

  Differences include:

   Mitochondria have cristae, but chloroplasts have thylakoid membranes.

   Chloroplasts contain photosynthetic pigments to absorb light energy, mitochondria

  do not.

   Mitochondria have an inner matrix, but chloroplasts have a stroma.
• Bacteria do not have membrane bound organelles in their cells – no nucleus, rough

  endoplasmic reticulum, Golgi apparatus, mitochondria or chloroplasts; these cells are

  prokaryotic cells.
• Ribosomes are produced in the nucleolus; they leave the nucleus via the nuclear

  pores and take up their positions on the rough endoplasmic reticulum (ER).
• The nuclear pores allow mRNA molecules (formed from DNA templates by

  transcription) to leave the nucleus. The mRNA molecules attach to the ribosomes

  on the rough ER.
• Protein synthesis takes place at the ribosome. The mRNA molecule contains the

  code for the primary structure of a protein; the order of amino acids in a

  polypeptide chain.
• The rough ER transports the polypeptides via transport vesicles, which merge with

  the Golgi body.
• The polypeptides are modified in the Golgi body and converted to their tertiary

  structure e.g. enzymes.
• Secretion: enzymes are packaged into secretory vesicles and transported to the cell

  membrane. The secretory vesicles merge with the cell membrane and release the enzymes by

  exocytosis.
• Prokaryotic cells

  • Small cells 0.2-2 μm
  • Ribosomes smaller and free in cytoplasm
  • No membrane bound organelles
  • DNA free in cytoplasm DNA contained within the nucleus
  • No nuclear envelope (double membrane)
  • Plasmids present
  • No mitochondria, uses a mesosome (a

  folded region of the cell membrane) for

  aerobic respiration
• • Eukaryotic cells Larger cells 5-100 μm
  • Ribosomes larger and bound to the rough endoplasmic reticulum
  • Membrane bound organelles are present
  • DNA contained within the nucleus
  • Nucleus has a double membrane
  • No plasmids
  • Cell wall (when present) is composed of cellulose
  • Mitochondria are used for aerobic

  respiration. There is no mesosome.
• Viruses do not fit the cell theory; they have no cell membrane, no cytoplasm, no

  organelles and no chromosomes.
• Viruses can only reproduce with the help of a host cell.
• Viruses are composed of a protein coat or capsid which surrounds DNA, RNA or simply a

  few genes
• Viruses

  are extremely small; they can only be viewed using an electron microscope.
• Outside a

  living cell a virus exists as an inert viron. When they invade the cell they are able to take

  over the cell’s metabolism and reproduce within the cell.
• Atoms to systems:

   Atoms are arranged into molecules.

   Molecules form cells.

   Cells work together to form tissues.

   Tissues form organs and organs form systems.
• A tissue is a group of similar cells working together to perform a

  particular function.
• Epithelial tissue – forms a continuous

  layer, covering or lining the

  internal or external surfaces

  of the body. Epithelia have no

  blood vessels, but may have

  nerve endings. The cells sit on

  a basement membrane, made

  of collagen and protein. The often have a

  protective or secretory

  function.
• Cuboidal epithelium lines the kidney tubules and the small

  intestine. Cuboidal epithelium cells are cube shaped.
• Ciliated epithelium is composed of cells which transport

  substances like mucus in the bronchi and ova in the fallopian

  tubes/oviducts. The cilia move and sweep substances along.

  These cells are columnar (they look like columns).