Topic 2 - Cells

• Types of cells

  • Eukaryotic
  • Prokaryotic
• Eukaryotic cells are larger than prokaryotic cells, and their DNA is contained in a nucleus
• Levels of biological organisation

  • Organ systems
  • Organs
  • Tissues
  • Specialised cells
• Eukaryotic cell


  Cells that contain a nucleus and membrane-bound organelles
• Organ system


  A group of organs working together to perform a function
• Organ


  A group of tissues working together to perform a function
• Tissue


  A group of specialised cells with similar structures and functions
• Animals, plants and fungi are all eukaryotic organisms
• Organelles in eukaryotic cells

  • Cell-surface membrane
  • Nucleus
  • Mitochondria
  • Chloroplasts
  • Golgi apparatus and Golgi vesicles
  • Lysosomes
  • Ribosomes
  • Rough endoplasmic reticulum and smooth endoplasmic reticulum
  • Cell wall
  • Cell vacuole

  
• Cell-surface membrane


  Phospholipid bilayer with molecules embedded within and attached on the outside

  
• Nucleus


  Contains the genetic code for each cell, site of DNA replication and transcription
• Nucleus


  • Nuclear envelope- double membrane
  • Nuclear pores
  • Nucleoplasm- granular, jelly-like material
  • Chromosomes- protein-bound, linear DNA
  • Nucleolus- site of rRNA production and ribosome synthesis
• Mitochondria


  Double membrane, inner membrane called the cristae, fluid centre called the mitochondrial matrix, contains 70S ribosomes and circular DNA
• Chloroplasts


  Surrounded by a double membrane, contains thylakoids (folded membranes embedded with pigment), fluid-filled stroma contains enzymes for photosynthesis, contains 70S ribosomes and circular DNA
• Golgi apparatus and Golgi vesicles

  Folded membranes making cisternae, secretory vesicles pinch off from the cisternae
• Golgi apparatus and Golgi vesicles

  • Add carbohydrates to proteins to form glycoproteins
  • Produce secretory enzymes
  • Secrete carbohydrates
  • Transport, modify and store lipids
  • Form lysosomes
  • Molecules are 'labelled' to reach their destination
  • Finished products are transported to the cell surface in Golgi vesicles where they fuse with the membrane and the contents are released via exocytosis

  
• Lysosomes


  Bags of digestive enzymes, can contain 50 different enzymes
• Lysosomes


  • Hydrolyse pathogens in phagosomes
  • Completely break down dead cells (autolysis)
  • Exocytosis- release enzymes outside of the cell to destroy material
  • Digest worn-out organelles for reuse of materials

  
• Ribosomes


  Small granules, made up of two sub-units of protein and rRNA, 80S-large ribosome found in eukaryotic cells (25nm), 70S -smaller ribosome found in prokaryotic cells, mitochondria and chloroplasts
• Endoplasmic reticulum


  Rough and Smooth ER both have folded membranes called cisternae, Rough ER have ribosomes on the cisternae

  
• Endoplasmic reticulum


  • RER –Protein synthesis
  • SER –Synthesises and stores lipids and carbohydrates
• Cell wall


  In plants and fungal cells, made of microfibrils of the cellulose polymer (plants) or chitin, a nitrogen-containing polysaccharide (fungi)

  
• Cell vacuole


  Filled with fluid surrounded by a single membrane called a tonoplast
• Cell vacuole


  • Makes cells turgid and therefore provides support
  • Temporary store of sugars and amino acids
  • The pigments are responsible for coloured petals which attract pollinators
• Key differences between prokaryotic and eukaryotic cells

  • Prokaryotic cells are much smaller
  • Prokaryotic cells have no membrane bound-organelles whereas eukaryotic cells do
  • Prokaryotic cells have smaller ribosomes
  • Prokaryotic cells have no nucleus whereas eukaryotic cells do
  • Prokaryotic cells have a cell wall made of murein, whereas eukaryotic cells are made of cellulose or chitin
• Prokaryotic cells


  Cells that are much smaller, have no membrane bound-organelles, have smaller ribosomes, have no nucleus, and have a cell wall made of murein
• Murein


  A glycoprotein that makes up the cell wall of prokaryotic cells
• Additional features of prokaryotic cells

  • Plasmids- rings of DNA containing genes linked to survival such as antibiotic resistance
  • Capsule- provides protection from other cells and helps bacteria agglutinate (stick together)
  • Flagella- used for locomotion
• Viruses


  Non-living and acellular, smaller than bacteria, only contain genetic material, a capsid and attachment proteins
• HIV is surrounded by a further lipid envelope which has attachment proteins on the outside to enable the virus to identify which host cells to enter
• Methods of studying cells

  • Microscopes
  • Cell fractionation
  • Ultracentrifugation
• Microscope


  Instrument used to view small objects by magnifying and resolving them
• Magnification


  How many times larger the image is compared to the object
• Resolution


  The minimum distance between two objects in which they can still be viewed as separate
• Types of microscopes


  • Optical (light) microscope
  • Transmission electron microscope
  • Scanning electron microscope
• Optical (light) microscope


  • A beam of light is condensed to create the image
  • A glass lens is used to condense the beam of light
  • Poorer resolution due to light having a longer wavelength
  • Lower magnification
  • Coloured images
  • Can view living samples
• Electron microscope (scanning or transmission)

  • A beam of electrons is condensed to create the image
  • Electromagnets are used to condense the beam
  • Higher resolving power as electrons have a shorter wavelength
  • Higher magnification
  • Black and white images
  • The sample must be in a vacuum, and therefore non-living
• Transmission Electron Microscopes (TEM)

  Extremely thin specimens are stained and placed in a vacuum, an electron gun produces a beam of electrons that passes through the specimen, some parts absorb the electrons and appear dark, the image produced is 2D and shows detailed images of the internal structure of cells
• Scanning Electron Microscope (SEM)

  The specimens do not need to be thin, as the electrons are not transmitted through, instead, the electrons are beamed onto the surface and the electrons are scattered in different ways depending on the contours, this produces a 3D image
• Magnification formula


  Image size = Actual size x Magnification