Biology papaer 1

• Prokaryotic cell


  Differs from an animal cell
• Prokaryotic cell


  • Genetic material is not enclosed in a nucleus
  • Genetic material is a single DNA loop and may have one or more small rings of DNA called plasmids
  • Does not contain mitochondria or chloroplasts
• Eukaryotic cell

  • Plant, animal and fungal cells are all eukaryotic
  • Bacterial cells are prokaryotic
• Describe the structure of a typical bacterial cell

  1. Cytoplasm

  2. Cell wall provides structural support (not made of cellulose)

  3. Chromosomal DNA (not found within a nucleus, usually one long looped chromosome)

  4. Plasmid DNA (small, commonly circular, section of DNA that can replicate independently)

  5. Flagella (tail-like structures that rotate to help some bacteria move)
• Sub-cellular structures in an animal cell

  • Nucleus (controls cell activities and contains genetic material)
  • Cytoplasm (where most chemical reactions take place)
  • Cell membrane (controls passage of substances in and out)
  • Mitochondria (where aerobic respiration takes place)
  • Ribosomes (where proteins are synthesised)
• Additional sub-cellular structures in plant cells


  • Cell wall (made of cellulose, strengthens the cell)
  • Permanent vacuole (filled with cell sap, supports the plant)
  • Chloroplasts (absorb light to make food by photosynthesis)
• Measurement units to describe the size of cells

  • centimetre (cm)
  • millimetre (mm)
  • micrometre (μm)
  • nanometre (nm)
• Typical plant cell size

  0.1mm in diameter
• Typical animal cell size

  0.02mm in diameter
• Prokaryotic cell size


  Often about 0.002mm long
• It is not possible to see cells as separate objects using the naked eye
• Resolution


  The ability to see two or more objects as separate objects
• Not all plant cells have chloroplasts, e.g. root cells do not receive light
• The light microscope was developed in the late 16th century and gave a greater resolution than the human eye
• Some sub-cellular structures are even smaller than the resolution achieved by a light microscope and cannot be seen using this method
• In 1933, scientists first used an electron microscope
• Electron microscope


  Passes electrons, rather than light, through the specimen and can give much better resolution
• What can be seen with an electron microscope


  • The structures inside mitochondria and chloroplasts
  • Ribosomes and their role in making proteins
• Required practical: Use a light microscope to observe, draw and label a selection of plant and animal cells


  1. Place a tissue sample on a microscope slide

  2. Add a few drops of a suitable stain

  3. Lower a coverslip onto the tissue

  4. Place the slide on the microscope stage and focus on the cells using low power

  5. Change to high power and refocus

  6. Draw any types of cells that can be seen

  7. Add a scale line to the diagram
• Magnification


  How many times larger the image is than the real object
• Magnification is not the same as resolution. Light microscopes can be built to magnify more and more, but after a certain point the images do not get any clearer.
• Arrange these structures in order of size with the largest first

  • liver cell
  • nucleus
  • bacterium
  • ribosome
• The nucleus of a cell contains chromosomes made of DNA
• Each chromosome carries hundreds to thousands of genes
• In body cells, the chromosomes are found in pairs, with one chromosome coming from each parent
• Different species have different numbers of pairs of chromosomes, e.g. humans have 23 pairs and dogs have 39 pairs
• The cell cycle and mitosis


  1. Cells go through a series of changes involving growth and division

  2. DNA is copied and new chromosomes are made

  3. Each cell grows and makes new sub-cellular structures

  4. Chromosomes line up along the centre of the cell, divide and the copies move to opposite poles

  5. Each daughter cell has the same number of chromosomes and contains the same genes as the parent cell
• Importance of mitosis


  • Makes new cells for growth and development of multicellular organisms
  • Makes new cells for repairing damaged tissues
  • Makes new cells for asexual reproduction
• Undifferentiated cells


  Cells that have not yet become specialised
• Stem cells

  Undifferentiated cells that can divide to make different types of cells
• Where stem cells are found

  • Human embryos
  • Umbilical cord of a newborn baby
  • Some organs and tissues
• Embryonic stem cells


  Stem cells from human embryos that can make all types of cells
• Adult stem cells

  Stem cells found in some organs and tissues that can only make certain types of cells and have limited capacity to divide
• Stem cells may be very useful in treating conditions where cells are damaged or not working properly, such as in diabetes and paralysis
• Prokaryotic cells are much simpler in structure than eukaryotic cells


  Scientists think prokaryotic cells developed before eukaryotic cells
• Plasmids


  Small, commonly circular, sections of DNA that can replicate independently and allow genes to be inserted into bacteria in genetic engineering
• Diffusion

  The net movement of particles from an area of higher concentration to an area of lower concentration until they are evenly spread out
• Examples of diffusion in living organisms

  • Oxygen and carbon dioxide diffusing during gas exchange in lungs, gills and plant leaves
  • Urea diffusing from cells into the blood plasma for excretion by the kidney
  • Digested food molecules from the small intestine diffusing into the blood
• Factors affecting the rate of diffusion

  • Concentration gradient
  • Temperature
  • Surface area of the membrane
• A single-celled organism has a large surface area to volume ratio, allowing enough molecules to diffuse into and out of the cell to meet its needs