Cell structure

• To prepare a temporary mount of tissue for a light microscope:

  • Obtain a thin section of tissue using an ultratome or by maceration
  • Place plant tissue in a drop of water
  • Stain tissue on a slide to make structures visible
  • Add a coverslip using a mounted needle at 45° to avoid trapping air bubbles
• Light microscopes work by:

  • Lenses focusing rays of light and magnifying the view of a thin slice of specimen
  • Different structures absorbing different amounts and wavelengths of light
  • Reflected light being transmitted to the observer via the objective lens and eyepiece
• Transmission electron microscopes (TEM) work by:

  • Passing a high energy beam of electrons through a thin slice of specimen
  • More dense structures appearing darker since they absorb more electrons
  • Focusing the image onto a fluorescent screen or photographic plate using magnetic lenses
• Scanning electron microscopes (SEM) work by:

  • Focusing a beam of electrons onto a specimen’s surface using electromagnetic lenses
  • Reflected electrons hitting a collecting device and being amplified to produce an image on a photographic plate
• Laser scanning confocal microscopes work by:

  • Focusing a laser beam onto a small area on a sample’s surface using objective lenses
  • Fluorophores in the sample emitting photons
  • Photomultiplier tube amplifying the signal onto a detector. An image is produced pixel by pixel in the correct order
• The field of view in microscopy should be recorded by:

  • Drawing a diagram with a sharp pencil
  • Not using sketchy lines or shading
  • Including a scale bar
  • Annotating visible structures
• Equation to calculate the actual size of a structure from microscopy:

  • Actual size = image size / magnification
• Magnification: factor by which the image is larger than the actual specimen

  Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another
• Samples need to be stained for light microscopes because:

  • Coloured dye binds to the structures
  • Facilitates absorption of wavelengths of light to produce an image
  • Provides differential staining for contrast between heavily & lightly stained areas to distinguish structures
• Magnification and resolution of a compound light microscope:

  • Magnification: x 2000
  • Resolution: 200 nm
• Magnification and resolution of a TEM:

  • Magnification: x 500 000
  • Resolution: 0.5 nm
• Magnification and resolution of an SEM:

  • Magnification: x 500 000
  • Resolution: 3 - 10 nm
• Using an eyepiece graticule and stage micrometer to measure the size of a structure involves:

  • Placing the micrometer on the stage to calibrate the eyepiece graticule
  • Lining up scales on the graticule and micrometer
  • Counting how many graticule divisions are in 100μm on the micrometer
  • Calculating the length of 1 eyepiece division and using calibrated values to calculate the actual length of structures
• Structure of the nucleus:

  • Surrounded by a nuclear envelope, a semipermeable double membrane
  • Nuclear pores allow substances to enter/exit
  • Dense nucleolus made of RNA & proteins assembles ribosomes
• Function of the nucleus:

  • Contains DNA coiled around chromatin into chromosomes
  • Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication
• Structure and function of the endoplasmic reticulum (ER):

  • Cisternae: network of tubules & flattened sacs extending from cell membrane & connecting to nuclear envelope
  • Rough ER: many ribosomes attached for protein synthesis & transport
  • Smooth ER: lipid synthesis
• Structure and function of the Golgi apparatus:

  • Planar stack of membrane-bound, flattened sacs, cis face aligns with rER
  • Molecules are processed in cisternae
  • Vesicles bud off trans face via exocytosis
• Structure and function of the Golgi apparatus:

  • Planar stack of membrane-bound, flattened sacs
  • Cis face aligns with rER
  • Molecules are processed in cisternae
  • Vesicles bud off trans face via exocytosis
  • Modifies & packages proteins for export
  • Synthesises glycoproteins
• Structure and function of ribosomes:

  • Formed of protein & rRNA
  • Have large subunit which joins amino acids & small subunit with mRNA binding site
• Relationship between organelles involved in the production and secretion of proteins:

  • Ribosomes that synthesise proteins are attached to the rER
  • Golgi apparatus, which modifies proteins for secretion, aligns with the rER
• Structure of a mitochondrion:

  • Surrounded by double membrane
  • Folded inner membrane forms cristae: site of electron transport chain
  • Fluid matrix contains mitochondrial DNA, respiratory enzymes, lipids, proteins
• Structure of a chloroplast:

  • Vesicular plastid with double membrane
  • Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll
  • Intergranal lamellae: tubes attach thylakoids in adjacent grana
  • Stroma: fluid-filled matrix
• Function of mitochondria and chloroplasts:

  • Mitochondria: site of aerobic respiration to produce ATP
  • Chloroplasts: site of photosynthesis to convert solar energy to chemical energy
• Structure and function of a lysosome:

  • Sac surrounded by single membrane embedded H+ pump maintains acidic conditions
  • Contains digestive hydrolase enzymes
  • Glycoprotein coat protects cell interior
  • Digests contents of phagosome
  • Exocytosis of digestive enzymes
• Structure and function of a plant cell wall:

  • Made of cellulose microfibrils for mechanical support
  • Plasmodesmata form part of apoplast pathway to allow molecules to pass between cells
  • Middle lamella separates adjacent cell walls
• Bacterial and fungal cell walls composition:

  • Bacteria: peptidoglycan (murein)
  • Fungi: chitin
• Structure and function of centrioles:

  • Spherical group of 9 microtubules arranged in triples
  • Located in centrosomes
  • Migrate to opposite poles of cell during prophase & spindle fibres form between them
• Structure and function of the cell-surface plasma membrane:

  • 'Fluid mosaic' phospholipid bilayer with extrinsic & intrinsic proteins embedded
  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling / cell recognition
• Role of cholesterol, glycoproteins & glycolipids in the cell-surface membrane:

  • Cholesterol: steroid molecule connects phospholipids & reduces fluidity
  • Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together
  • Glycolipids: cell signalling & cell recognition
• Structure and function of flagella:

  • Hollow helical tube made of the protein flagellin
  • Rotates to propel (usually unicellular) organism
• Structure and function of cilia:

  • Hairlike protrusions on eukaryotic cells
  • Move back and forth rhythmically to sweep foreign substances e.g. dust or pathogens away / to enable the cell to move
• Importance of the cytoskeleton:

  • Provides mechanical strength
  • Aids transport within cells
  • Enables cell movement
• Comparison between eukaryotic and prokaryotic cells:

  • Both have: cell membrane, cytoplasm, ribosomes
• Contrast between eukaryotic and prokaryotic cells:

  • Prokaryotic cells are small, always unicellular, no membrane-bound organelles & no nucleus, circular DNA not associated with proteins, small ribosomes (70S), binary fission for reproduction, made of murein
  • Eukaryotic cells are larger, often multicellular, always have organelles & nucleus, linear chromosomes associated with histones, larger ribosomes (80S), undergo mitosis & meiosis for reproduction, have cellulose cell wall (plants)/ chitin (fungi), no capsule, no plasmids, always have cytoskeleton
• Structure of a vacuole: A membrane-bound sac that contains cell sap.
• Function of a vacuole: stores water and keeps the cell turgid
• Plasmid: A small circular DNA molecule that is found in bacteria.
• Pili: on bacterial cells, helps them to stick together
• What type of image do SEM’s produce?

  High-resolution 3D images.
• What type of image do TEM’s produce?

  2D High-resolution images