AQA Combined Bio paper 1

• Organisms can be Prokaryotes or Eukaryotes
• Types of cells


  • Prokaryotic cells
  • Eukaryotic cells
• Prokaryotic cells


  • Have a rigid cell wall
  • Have a single circular strand of DNA that floats freely in the cytoplasm
  • May also contain one or more small rings of DNA called plasmids
• Eukaryotic cells


  • Include plant and animal cells
  • Have a true nucleus that contains the genetic material
• Subcellular structures in cells


  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
• Plant cells


  • Have a rigid cell wall made of cellulose
  • Have a permanent vacuole containing a weak solution of sugar and salts
  • Have chloroplasts where photosynthesis occurs
• Bacterial cells are much smaller than plant and animal cells
• Bacterial cells do not have a true nucleus or mitochondria
• To estimate the area of a subcellular structure, use the appropriate geometric formula (e.g. area = length x width for a rectangle)
• Microscopy


  The study of cells and subcellular structures using microscopes
• Cells are studied using microscopes
• Light microscopes


  • Use visible light and lenses to form an image of a specimen
  • Have lower resolution than electron microscopes
• Electron microscopes


  • Use a beam of electrons instead of light to form an image
  • Have higher resolution than light microscopes
• Microscopes allow us to see smaller details and subcellular structures
• Calculating magnification


  Image size / Real size
• Preparing slides involves staining, cutting, and adding coverslips
• Using a light microscope


  1. Clip slide onto stage

  2. Select lowest power objective lens

  3. Use coarse adjustment to focus

  4. Use fine adjustment to focus

  5. Switch to higher power lens and refocus
• Drawing observations


  Draw at least half the space, use pencil, no colour or shading, draw subcellular structures in proportion, include scale and magnification
• Adding stain to a sample can highlight objects in the cell
• Cell differentiation


  The process by which a cell changes to become specialised for a particular function
• Most cells lose the ability to differentiate after they become specialised, but plant cells retain this ability
• Examples of specialised cells


  • Sperm cells
  • Nerve cells
  • Muscle cells
  • Root hair cells
  • Phloem and xylem cells
• Sperm cells


  • Long streamlined head
  • Many mitochondria for energy
  • Enzymes to digest egg cell membrane
• Nerve cells


  • Long to cover distance
  • Branched connections to form a network
• Muscle cells


  • Long to allow contraction
  • Many mitochondria for energy
• Root hair cells


  • Long "hairs" to increase surface area for absorption
  • Absorb water and minerals from soil
• Phloem and xylem cells


  • Long and joined end-to-end to form tubes
  • Phloem cells have few subcellular structures, xylem cells are hollow to allow flow
• Phloem tubes transport food substances from leaves to growing parts of the plant
• A feature of a phloem cell

  It is long and joined end-to-end to form tubes to allow transport of substances
• Root hair cells have a large surface area to absorb water and minerals from the soil
• Chromosomes


  Contain genetic information in the form of genes
• Chromosomes


  • Each chromosome contains a set of genes
  • Body cells normally have two copies of each chromosome, one from each parent
• Cell cycle


  The series of stages a cell goes through to divide and produce new cells
• Stages of the cell cycle


  1. Growth and DNA replication

  2. Mitosis
• Mitosis


  1. Chromosomes line up at the centre

  2. Chromosomes are pulled apart to opposite ends

  3. Nuclei and cytoplasm divide to form two new identical daughter cells
• Mitosis allows multicellular organisms to grow, develop, and repair damaged cells
• Percentage of cells undergoing mitosis

  Can be calculated by counting the number of cells in mitosis and dividing by the total number of cells
• Stem cells can differentiate into any cell type
• Adult stem cells can only differentiate into certain cell types, not all
• Stem cells from embryos and bone marrow can be grown in a lab and made to differentiate into specialised cells for use in medicine