General Cell Structure

• Eukaryotic cells

  Plant and animal cells
• Differences between eukaryotic and prokaryotic cells

  • Prokaryotic cells are much smaller than eukaryotic cells
  • Eukaryotic cells contain membrane bound-organelles and a nucleus containing genetic material, while prokaryotes do not
• Plasmids


  Small, circular loops of DNA found free in the cytoplasm and separate from the main DNA, carry genes that provide genetic advantages e.g. antibiotic resistance
• Micrometre (µm)


  1 × 10-6 metres
• Nanometre (nm)


  1 x 10-9 metres
• Components of both plant and animal cells


  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
• Additional cell components found in plant cells

  • Chloroplasts
  • Permanent vacuole
  • Cell wall
• Function of the nucleus (other than storing genetic information)


  Controls cellular activities
• Structure of the cytoplasm

  Fluid component of the cell, contains organelles, enzymes and dissolved ions and nutrients
• Function of the cytoplasm

  Site of cellular reactions
• Function of the cell membrane

  Controls the entry and exit of materials into and out of the cell
• Function of the mitochondria

  Site of later stages of aerobic respiration in which ATP is produced
• Function of the ribosomes

  Joins amino acids in a specific order during translation for the synthesis of proteins
• Function of the plant cell wall


  Provides strength, Prevents the cell bursting when water enters by osmosis
• Contents of the permanent vacuole

  Cell sap
• Function of the permanent vacuole

  Supports the cell, maintaining its turgidity
• Function of chloroplasts

  Site of photosynthesis
• Adaptations of sperm cells in animals

  • Haploid nucleus contains genetic information
  • Tail enables movement
  • Mitochondria provide energy for tail movement
  • Acrosome contains enzymes that digest the egg cell membrane
• Adaptations of nerve cells in animals

  • Long axon allows electrical impulses to be transmitted all over the body from the central nervous system
  • Dendrites from the cell body connect to and receive impulses from other nerve cells, muscles and glands
• Adaptations of muscle cells in animals

  • Arrangement of protein filaments allows them to slide over each other to produce muscle contraction
  • Mitochondria to provide energy for muscle contraction
  • Merged cells in skeletal muscle allow muscle fibre contraction in unison
• Adaptations of root hair cells in plants

  • Large surface area to absorb nutrients and water from surrounding soil
  • Thin walls that do not restrict water absorption
• Adaptations of xylem cells in plants

  • No upper or lower margins between cells to provide a continuous route for water to flow
  • Thick, woody side walls strengthen their structure and prevent collapse
• Adaptations of phloem cells in plants

  • Sieve plates let dissolved amino acids and sugars be transported up and down the stem
  • Companion cells provide energy needed for active transport of substances along the phloem
• Cell differentiation


  The process by which cells become specialised
• Importance of cell differentiation

  Allows production of different tissues and organs that perform various vital functions in the human body
• Timing of cell differentiation in animals

  Early in their life cycle
• Timing of cell differentiation in plants

  Throughout their entire life cycle
• Purpose of cell division in mature animals


  Repair and replacement of cells
• Changes during cell differentiation


  Becomes specialised through acquisition of different sub-cellular structures to enable a specific function to be performed by the cell
• Magnification


  The number of times bigger an image appears compared to the size of the real object
• Cell division in mature animals

  Repair and replacement of cells
• Cell differentiation

  Cells become specialised through acquisition of different sub-cellular structures to enable a specific function to be performed
• How a light microscope works

  Passes a beam of light through a specimen which travels through the eyepiece lens, allowing the specimen to be observed
• Advantages of light microscopes

  • Cheap
  • Easy to use
  • Portable
  • Observe both dead and living specimens
• Disadvantage of light microscopes

  Limited resolution
• How an electron microscope works

  It uses a beam of electrons which are focused using magnets. The electrons hit a fluorescent screen which emits visible light, producing an image
• Advantage of electron microscopes

  Greater magnification and resolution
• Electron microscopes have greater magnification and resolution because they use a beam of electrons which has a shorter wavelength than photons of light
• How electron microscopes have enabled scientists to develop their understanding of cells

  • Allow small sub-cellular structures to be observed in detail
  • Enable scientists to develop more accurate explanations about how cell structure relates to function
• Disadvantages of electron microscopes

  • Expensive
  • Large so less portable
  • Require training to use
  • Only dead specimens can be observed