Topic 2

• Cell membrane function


  Selectively permeable barrier controls passage of substances in and out the cell
• Cell membrane structure


  Phospholipid bilayer with embedded intrinsic & extrinsic proteins

  
• Nucleus structure


  Nuclear pores, nucleolus, DNA and nuclear envelope
• Nucleus function


  Site of transcription & pre-mRNA splicing - mRNA production, site of DNA replication, nucleolus makes ribosomes, nuclear pore allows movement of substances to/from cytoplasm

  
• Mitochondria structure


  Double membrane with inner membrane folded into cristae, 70S ribosomes in matrix, small, circular DNA, enzymes in matrix

  
• 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)
• Golgi apparatus structure


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


  Modifies proteins received from RER, packages them into vesicles to transport to cell membrane for exocytosis, makes lysosomes
• Lysosome structure


  Type of Golgi vesicle containing digestive enzymes
• 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
• Cell wall function


  Provides structural strength, rigidity and support to cell, helps resist osmotic pressures
• Ribosome structure


  Small and large subunit made of protein and rRNA, free floating in cytoplasm & bound to RER, 70S in prokaryotes, mitochondria and chloroplasts, 80S in eukaryotes
• 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
• Cell vacuole structure


  Fluid-filled, surrounded by a single membrane called a tonoplast
• 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
• Protein channels


  Tubes filled with water enabling water-soluble ions to pass through the membrane, selective, channel proteins only open in the presence of certain ions when they bind to the protein
• 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, calculated by equation
• Resolution


  The minimum distance between two objects in which they can still be viewed as separate, determined by wavelength of light (for optical microscopes) or electrons (for electron microcopes)
• 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
• Evaluate 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
• Evaluate SEMs


  • High resolving power, high magnification, thick specimens usable, complex staining method, specimen must be dead, vaccum required
• 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


  Process by which cells are broken open so organelles are free to be separated, done using homogeniser (blender)
• 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