2.1

• Cell

  The basic unit of all living organisms
• Complex organisms

  • Cells are organised into tissues
  • Tissues into organs
  • Organs into organ systems
• Eukaryotic cells

  Contain a nucleus and membrane-bound organelles
• Ultrastructure

  Detailed structure of a cell that can be observed using a microscope
• Nucleus

  • Surrounded by a double membrane called the nuclear envelope
  • Contains chromatin
  • Contains a nucleolus which is the site of ribosome production
• Rough Endoplasmic Reticulum (RER)

  • A series of flattened sacs enclosed by a membrane with ribosomes on the surface
  • Folds and processes proteins made on the ribosomes
• Smooth Endoplasmic Reticulum (SER)

  • A system of membrane-bound sacs
  • Synthesises and processes lipids and steroids
• Golgi Apparatus

  • A series of fluid-filled, flattened and curved sacs called cisternae, with vesicles surrounding the edges
  • Sorts, processes and packages proteins and lipids
  • Produces lysosomes
• Mitochondria

  • Usually oval-shaped and bound by a double membrane called the envelope
  • The inner membrane is folded to form projections called cristae
  • Contains a matrix with the enzymes needed for cellular respiration
• Centrioles

  • Hollow cylinders containing a ring of microtubules arranged at right angles to each other
  • Involved in cell division
• 80S Ribosomes

  • Composed of a large (60S) subunit and a small (40S) subunit
  • The site of protein synthesis
• Lysosomes

  • Vesicles, containing digestive enzymes, bound by a single membrane
• Prokaryotic cells (e.g. bacteria)

  • Cell wall - rigid outer covering made of peptidoglycan, provides strength and support
  • Slime capsule - protective slimy layer, helps retain moisture and adhere to surfaces
  • Plasmid - circular piece of DNA
  • Flagellum - tail-like structure that rotates to move the cell
  • Pili - hair-like structures that attach to other bacterial cells and allow plasmids to move from cell to cell
  • 70S Ribosomes - composed of a large (50S) subunit and a small (30S) subunit, the site of protein synthesis
  • Mesosomes - infoldings of the inner membrane which contain enzymes required for respiration
• Gram positive bacteria

  Have a cell wall comprised of a thick layer of peptidoglycan, and an inner plasma membrane
• Gram negative bacteria

  Have a thin layer of peptidoglycan with an outer lipopolysaccharide membrane
• Magnification

  How much bigger the image is compared to the original object
• Resolution

  How far apart two points can be before they are seen as one
• Optical microscope

  • A beam of light through the object, objective lens and eyepiece lens magnifies the sample
  • Magnification = 1500X
  • Resolution = 200nm
• Scanning electron microscope

  • A beam of electrons scans back and forth over the surface of the sample, producing a 3D image
  • Magnification = 50,000X
  • Resolution = 0.1nm
• Transmission electron microscope

  • A beam of electrons is transmitted through the object, producing a 2D image
  • Magnification = 100,000x-500,000x
  • Resolution = 0.1nm
• Ultrastructure

  Structures only visible through a transmission electron microscope
• Artefacts

  Things that are observed due to preparative or investigative techniques (e.g. bits of stain)
• Staining

  Required for both light and electron microscopes to provide contrast between organelles and cytoplasm, allowing structures to be observed
• Advantages of electron microscopes

  • Higher resolution and magnification
• Disadvantages of electron microscopes

  • Sample must be placed in a vacuum, so you can't magnify living things
  • Very expensive and not portable
  • Produces only black and white images
• State the principle of cell theory

  Cells are fundamental structural units in all living organisms. They determine the function and organisation of biological systems
• What is the nucleoid?

  Irregulary-shaped region of cytoplasm where loop of bacterial DNA is located
• Relationship between a system and specialised cells

  Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems
• Nucleoid

  Irregularly-shaped region of cytoplasm where loop of bacterial DNA is located
• Plasmid

  1 or more rings of DNA found in some bacterial cells. Contains non-essential genes. Can be exchanged between bacterial cells via conjugation.
• Bacterial cell wall and slime capsule

  • Peptidoglycan (murein) cell wall provides mechanical strength. May be coated by slime layer to prevent dessication, adhere cells & provide nutrients.
• Gram staining process:

  1. Stain culture with crystal violet. Remove & rinse with water.

  2. Add iodine solution & remove after 1 min.

  3. Add alcohol.

  4. Counterstain with red safranin for 1 min.

  5. Dry and examine sample under microscope.
• Gram positive cell walls

  • Thick peptidoglycan layer (insoluble in alcohol). Purple under microscope when stained.
• Gram negative cell walls

  • Thin peptidoglycan layer with outer lipopolysaccharide membrane (alcohol-soluble). Red under microscope when stained.
• Why do Gram positive and Gram negative bacteria respond differently to certain antibiotics?

  Antibiotics that inhibit peptidoglycan formation e.g. penicillin work on Gram positive bacteria. Since Gram negative bacteria have an outer membrane & a much thinner peptidoglycan layer, they are not affected.
• Ribosomes in eukaryotic and prokaryotic cells

  • Formed of protein & rRNA. Have large subunit which joins amino acids & small subunit with mRNA binding site.
• Structure of the nucleus

  • Surrounded by nuclear envelope, a semi-permeable 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 extends from cell membrane & connects to nuclear envelope.
  • Rough ER: many ribosomes attached for protein synthesis & transport.
  • Smooth ER: lipid synthesis.
• 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.