A level Biology cell structure

• Cell is the basic unit of life
• Resolution


  The ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the details that can be seen
• Magnification


  The number of times larger an image of an object is than the real size of the object
• Light microscope

  • Source of radiation is light
  • Specimen can be living
  • Maximum resolution of 200 nm
  • Lenses are glass
  • Mechanism is light passing through specimen
  • Produces coloured images

  
• Scanning electron microscope (SEM)

  • Source of radiation is electron
  • Specimen must be non-living/dead
  • Maximum resolution of 0.5 nm
  • Lenses are magnets
  • Mechanism is electron irradiating surface of specimen
  • Produces black and white images

  
• Transmission electron microscope (TEM)

  • Source of radiation is electron
  • Specimen must be non-living/dead
  • Maximum resolution of 0.5 nm
  • Lenses are magnets
  • Mechanism is electron passing through specimen
  • Produces black and white images

  
• Shorter wavelength

  Greater energy
• Longer wavelength

  Lesser energy
• Limit of resolution is approximately half the wavelength
• Transmission electron microscope has higher magnification (without loss of detail), higher resolution, and can see organelles such as SER, ribosomes, etc. compared to light microscope

  
• Scanning electron micrograph shows surface/contour/3D views, while transmission electron micrograph shows 2D/flat views and internal cell contents

  
• Calculating magnification

  Observed size of image / Actual size = Magnification
• Calculating actual size from magnification

  Observed size of image / Magnification = Actual size
• Eyepiece graticule and stage micrometer scale can be used to make measurements and convert between units (mm, μm, nm)

  
• Cells use ATP from respiration for energy-requiring processes
• Organelles and cell structures in eukaryotic cells

  • Cell surface membrane
  • Nucleus, nuclear envelope and nucleolus
  • Rough and smooth endoplasmic reticulum
  • Golgi body
  • Mitochondria and chloroplast
  • Ribosomes
  • Lysosomes
  • Centrioles, microtubules, cilia and microvilli
  • Cell wall and plasmodesmata
  • Large permanent vacuole and tonoplast
• Cell surface membrane

  Partially permeable, controls exchange of materials between cell and environment, 7 nm thick
• Nucleus

  Largest organelle (5-10 μm), contains chromatin, controls cell activities
• Nuclear envelope

  Double membrane, contains nuclear pores, controls exchange between nucleus and cytoplasm
• Nucleolus

  Manufactures rRNA and ribosomes
• Endoplasmic reticulum (ER)

  Has cisternae
• Cell surface membrane

  Partially permeable, controls exchange of materials between cell and the environment, 7 nm thick
• Nucleus

  Largest organelle (~ 5 – 10 µm), contains chromatin, controls cell activities
• Nucleolus

  Manufactures rRNA and ribosome
• Rough endoplasmic reticulum (RER)

  Contains ribosomes, transports proteins
• Smooth endoplasmic reticulum (SER)

  Lipid and steroid synthesis, calcium ion storage, drug metabolism (liver)
• Ribosomes

  Size: 20 nm (70S) / 25 nm (80S), made of rRNA and protein, free in cytoplasm or attached to RER, protein synthesis
• Prokaryotes have chloroplasts and mitochondria
• Mitochondrion

  Size: ~ 1 µm, double membrane-bound, inner membrane forms cristae, aerobic respiration (produce ATP), lipid synthesis
• Chloroplast

  Size: 3 – 10 µm, double membrane-bound, photosynthesis
• Endosymbiont theory explains the origin of chloroplasts and mitochondria in eukaryotic cells
• Golgi apparatus

  Stack of flattened sacs (cisternae), collects and processes molecules, transports molecules via Golgi vesicles, makes lysosomes, glycosylation, synthesises new cell walls during plant cell division, secretes mucin
• Lysosome

  Single membrane sac, contains and separates hydrolytic enzymes from the cytoplasm, breakdown of unwanted substances
• Vacuoles, phagosomes, phagolysosomes, endocytosis, and exocytosis are not covered in this learning outcome
• Large permanent vacuole

  Single membrane (tonoplast) bound sac, contains pigments, enzymes, sugars and other organic compounds, mineral salts, oxygen, carbon dioxide, regulates cell's osmotic properties, supports through turgidity, lysosomal activity (digestion)
• Cell wall

  Rigid structure surrounding cell surface membrane, freely permeable, may be strengthened via lignification, provides mechanical strength and support, prevents cell bursting from osmosis, gives shape to cell
• Plasmodesmata

  Link neighbouring cells
• Microtubule

  Size: 25 nm diameter, long, rigid, hollow tube made of proteins (tubulins), mechanical support, forms cytoskeleton, involved in movement of cilia and flagella, intracellular transport system, involved in nuclear division (spindle), part of centriole
• Centriole (and centrosome)


  Hollow cylinder, size: ~ 500 nm long, contain 9 triplets of microtubules, basal bodies for cilia and flagella, two centrioles lie at right angles within a region called the centrosome which is the Microtubule Organising Centre (MTOC), spindle assembly
• Spindles are still produced during cell division despite the absence of centriole/centrosome