Topic 2

• Cell membrane function:

  • Selectively permeable barrier
  • controls passage of substances in and out the cell
  • barrier between internal and external cell environments
• Nucleus Structure: Nuclear pores, nucleolus, DNA and nuclear envelope

  
• Nucleus Function:

  • Site of transcription & premRNA splicing - mRNA production
  • site of DNA replication
  • nucleolus makes ribosomes
  • nuclear pore allows movement of substances to/from cytoplasm
• Mitochondria Function:

  • Site of aerobic respiration
  • produces ATP
• Chloroplast structure:

  • Thylakoid membranes stacked to form grana, linked by lamellae
  • stroma contains enzymes
  • contains starch granules, small circular DNA and 70S ribosomes

  
• Chloroplast function: Chlorophyll absorbs light for photosynthesis to produce organic molecules (glucose)
• Organisms containing chloroplasts: Plants and algae
• Golgi apparatus stucture:

  • Fluid-filled, membrane-bound sacs (horseshoe shaped)
  • vesicles at edge

  
• Lysosome structure: Type of Golgi vesicle containing digestive enzyme

  
• Lysosome function:

  • Contains digestive enzymes
  • e.g lysozymes to hydrolyse pathogens/cell waste products
• Rough endoplasmic reticulum function:

  • Site of protein synthesis
  • folds polypeptides to secondary & tertiary structures
  • packaging into vesicles to transport to Golgi
• Smooth endoplasmic reticulum function: Synthesises and processes lipids
• Ribosome function: Site of translation in protein synthesis.
• Rough endoplasmic reticulum structure:

  • System of membranes with bound ribosomes
  • continuous with nucleus

  
• Smooth endoplasmic reticulum structure: System of membranes with no bound ribosomes

  
• Cell wall structure:

  • In plant, fungal and bacterial cells
  • plants - made of microfibrils of cellulose
  • fungi - made of chitin
  • bacteria - murein

  
• Contrast prokaryotic & eukaryotic cells:

  • Prokaryotic cells are smaller
  • prokaryotes have no membrane bound organelles
  • prokaryotes have smaller 70S ribosomes
  • prokaryotes have no nucleus - circular DNA not associated with histones
  • prokaryotic cell wall made of murein instead of cellulose/chitin
• Occasional features of prokaryotes:

  • Plasmids - loops of DNA capsule surrounding
  • cell wall - helps agglutination + adds protection
  • flagella for movement
• Cell vacuole function:

  • Makes cells turgid - structural support
  • temporary store of sugars, amino acids
  • coloured pigments attract pollinators
• Protein carriers:

  • Bind with a molecule, e.g. glucose, which causes a change in the shape of the protein
  • this change in shape enables the molecule to be released to the other side of the membrane

  
• Features of viruses:

  • Non living and acellular
  • contain genetic material, capsid and attachment proteins
  • some (HIV) contain a lipid envelope + enzymes (reverse transcriptase)
• 3 types of microscopes:

  • Optical (light) microscopes
  • Scanning electron microscopes (SEM)
  • Transmission electron microscopes (TEM)
• Magnification: How many times larger the image is compared to the object
• Optical microscopes:

  • Beam of light used to create image
  • glass lens used for focusing
  • 2D coloured image produced
• Evaluate optical microscopes:

  • Poorer resolution as long wavelength of light - small organelles not visible
  • lower magnification
  • can view living samples
  • simple staining method
  • vaccum not required
• Transmission electron microscopes:

  • Beam of electrons passes through the sample used to create an image
  • focused using electromagnets
  • 2D, black & white image produced
  • can see internal ultrastructure of cell
  • structures absorb electrons and appear dark
• Evaluation TEMs:

  • Highest resolving power
  • high magnification
  • extremely thin specimens required
  • complex staining method
  • specimen must be dead
  • vaccum required
• Scanning electron microscopes:

  • Beam of electrons pass across sample used to create image
  • focused using electromagnets
  • 3D, black and white image produced
  • electrons scattered across specimen producing image
• Evaluation SEM:

  • High resolving power
  • high magnification
  • thick specimens usable
  • complex staining method
  • specimen must be dead
  • vaccum required
• Scanning electron microscopes:

  • Beam of electrons pass across sample used to create image
  • focused using electromagnets
  • 3D, black and white image produced
  • electrons scattered across specimen producing image
• Why calibrate eyepiece graticule?

  • Calibration of the eyepiece is required each time the objective lens is changed
  • Calibrate to work out the distance between each division at that magnification
• Purpose of cell fractionation:

  • Break open cells & remove cell debris
  • so organelles can be studied
• Homogenisation conditions:

  • Cold reduces enzyme activity preventing organelle digestion
  • Isotonic prevents movement of water by osmosis - no bursting / shrivelling of organelles
  • Buffered resists pH changes preventing organelle + enzyme damage
• Ultracentrifugation:

  • Homogenate solution filtered to remove cell debris
  • solution placed in a centrifuge which spins at a low speed initially
  • then increasingly faster speeds to separate organelles according to their density
• Differential centrifugation:

  • Supernatant first out (spun at lowest speed) is most dense = nuclei
  • spun at higher speeds
  • chloroplasts -> mitochondria -> lysosomes -> RER/SER -> ribosomes (least dense)
• Binary Fission:

  • Involves circular DNA & plasmids replicating
  • cytokinesis creates two daughter nuclei
  • each daughter cell has one copy of circular DNA and a variable number of plasmids
• Cell cycle:

  1. Interphase (G1, S, G2)
  2. Nuclear division - mitosis or meiosis
  3. Cytokinesis
• Interphase:

  • Longest stage in the cell cycle
  • when DNA replicates (S-phase) and organelles duplicate while cell grows (G1&G2-phase)
  • DNA replicates and appears as two sister chromatids held by centromere
• Mitosis:

  • One round of cell division
  • two diploid, genetically identical daughter cells
  • growth and repair (e.g. clonal expansion)
  • comprised of prophase, metaphase, anaphase and telophase
• Prophase:

  • Chromosomes condense and become visible
  • nuclear envelope disintegrates
  • in animals - centrioles separate & spindle fibre structure forms