Topic 1: Cell biology

• Eukaryotic cells

  Cells with genetic material (DNA) enclosed in a nucleus
• Components of eukaryotic cells

  • Nucleus
  • Genetic material (DNA)
  • Cell membrane
  • Ribosomes
  • Cytoplasm
• Prokaryotic cells have no nucleus
• Functions of sub-cellular structures in animal and plant cells

  • Cell membrane - Controls movement of substances in and out of cell
  • Cytoplasm - Gel-like substance where chemical reactions take place
  • Nucleus - Contains genetic material (DNA) and controls activities of cell
  • Mitochondria - Site of aerobic respiration and releases energy from glucose
  • Ribosomes - Site of protein synthesis
• Root hair cell

  • Long projection to increase surface area for absorption of water by osmosis
  • Many mitochondria to increase rate of respiration to release energy for active transport of mineral ions
• Phloem cell

  • No nucleus to maximise space for movement of dissolved sugars
  • Pores in end walls so dissolved sugars can move from cell to cell
• Cell differentiation

  Cell acquires different sub-cellular structures to enable it to carry out a certain function, so it has become a specialised cell
• Most animal cells differentiate at an early stage (embryo development)
• In mature animals, cell division mainly restricted to repair and replacement (such as new blood or skin cells)
• Many plant cell types retain the ability to differentiate throughout life
• Magnification

  Number of times bigger image is compared to real object
• Resolution

  Shortest distance between 2 points that can be seen as separate points (level of detail)
• Magnification formula

  Magnification = image size / actual size of object
• Unit conversions

  • 1 centimetre = 10000 micrometres
  • 1 millimetre = 1000 micrometres
• Bacterial cell division

  • Simple cell division (binary fission) - genetic material (loop of DNA and plasmids) replicates, then cell divides
  • Once every 20 minutes if enough nutrients and suitable temperature
• Growing bacteria

  In a nutrient broth solution or as colonies on an agar gel plate
• Preparing an uncontaminated bacterial culture using aseptic technique

  1. Sterilise petri dishes, culture media (such as agar) and surfaces to kill unwanted bacteria

  2. Sterilise inoculating loop by passing it through a flame to kill unwanted bacteria

  3. Use this to spread bacteria onto agar, minimise lifting lid of Petri dish

  4. Secure lid of Petri dish with adhesive tape to prevent microorganisms from air entering

  5. Store plates upside down so condensation doesn't drip onto agar, disrupting colonies

  6. Incubate at 25 degrees celsius to allow growth of bacteria, preventing growth of pathogens
• Chromosomes are arranged in pairs in the nucleus of a body cell. In humans, there are 23 pairs of chromosomes, 46 total (except in gametes)
• Importance of cell division by mitosis

  • Parent cell divides to produce 2 genetically identical cells for growth and development of multicellular organisms
  • Replacing (worn out or damaged) cells to repair damaged tissues
• Cell cycle

  Series of stages in which a cell divides (DNA is doubled then divided into 2 identical cells)
• Stem cells

  Undifferentiated cells which can give rise to many more cells of the same type, and differentiate into certain other cells
• Issues associated with the use of stem cells in medicine

  • Transfer of viral infection
  • Risk of rejection by immune system so have to take immunosuppressant drugs
  • Ethical and religious objections such as potential life destroyed and embryo cannot give consent
• Diffusion

  Net movement or spreading out of particles from an area of higher concentration to an area of lower concentration (down a concentration gradient)
• Why single-celled organisms can transport enough molecules in and out by diffusion

  Have a (relatively) large surface area to volume ratio (and short diffusion distance)
• How the effectiveness of an exchange surface is increased

  • Increased surface area of cell membrane
  • Thin wall or membrane to provide a short diffusion path
  • (In animals) efficient blood supply
  • (In animals, for gaseous exchange) being ventilated
• How the small intestine is adapted for exchanging materials

  • Internal surface covered in many projections (villi) to increase surface area to maximise diffusion
  • Wall of villi are thin and only one cell thick so short diffusion distance
  • Good blood supply to villi so moves food molecules away to maintain a steep concentration gradient
  • Very long to increase time for absorption
  • Many mitochondria in cells to increase respiration to transfer energy for active transport
• How gills in fish are adapted for gas exchange

  • Many projections to increase surface area to maximise diffusion
  • Very thin so short diffusion distance
  • Good blood supply to remove oxygen quickly to maintain a steep concentration gradient
• Osmosis

  Net movement of water particles from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) through a partially permeable membrane
• Active transport

  • Movement of substances from a dilute to a more concentrated solution (against a concentration gradient)
  • Active process so requires energy from respiration