Cells

• This chapter covers specification points 1.1.1 to 1.1.11. It covers the structures of animal, plant and bacterial cells; using a light microscope; magnification and size; resolution and the electron microscope; movement of substances into and out of cells by diffusion, stem cells and specialisation of cells in multi-celled organisms.
• In Double Award Science, it covers specification points 1.1.1 to 1.1.5 and covers the structures of animal, plant and bacterial cells, and the specialisation of cells in multi-celled organisms.
• Cells


  Living organisms are made up of microscopic units called cells
• Parts of an animal cell

  • cell membrane
  • cytoplasm
  • chloroplast
  • nuclear membrane
  • nucleus
  • mitochondria
  • vacuole
• Parts of a plant cell

  • cell wall
  • cell membrane
  • cytoplasm
  • mitochondria
  • nucleus
  • chloroplast
  • large permanent vacuole
• Animal cells


  Surrounded by a selectively permeable cell membrane that forms a boundary to the cell and controls what enters or leaves; cytoplasm is where chemical reactions take place; nucleus contains chromosomes with genetic information
• Mitochondria


  Structures in the cytoplasm within which the chemical reactions of cell respiration take place
• Plant cells


  Have a cellulose cell wall, a large permanent vacuole, and chloroplasts that contain chlorophyll and help the plant make food during photosynthesis
• Bacterial cells


  Have a cell membrane surrounding cytoplasm but no nucleus; genetic material is in the form of a circular chromosome and plasmids within the cytoplasm; have a cell wall but it is not made of cellulose
• Making temporary slides containing onion skin cells

  1. Peel thin, transparent layer of epidermis cells from onion leaf

  2. Place epidermis cells on microscope slide with water, iodine solution or methylene blue

  3. Lower coverslip onto slide using mounted needle or forceps
• Light microscope


  • Start with low power objective lens to see wider field of view and find what you're looking for
  • Use higher power objective lenses to see more detail, but be careful as they are very close to the slide
• Drawing biological specimens


  Use pencil with firm, continuous lines; have same proportions and faithful representation; label using separate ruled lines
• Magnification


  The number of times the length of the image (in a photograph or drawing) is larger than the actual length of the cell or specimen
• Length of image


  Actual length = Magnification
• The length of a cell is most appropriately measured in micrometres, but measurements from photographs or drawings will be in millimetres, so you need to convert millimetres to micrometres by multiplying by 1000.
• To calculate actual length, divide the length of the image by the magnification. To calculate magnification, divide the length of the image by the actual length.
• Scale bar


  A line drawn on or near an image that has a label showing the actual length before magnification, allowing the magnification to be calculated
• Resolution


  The ability of a microscope to let us see detail, not just the magnification
• Light microscopes can resolve details 0.2μm apart, requiring about x1500 magnification. Electron microscopes have much greater resolution, able to show details about 0.2nm apart, requiring magnifications up to x500,000.
• Magnification


  Continually increasing the magnification of an image beyond a certain point does not increase the detail that you can see
• Resolution


  The ability of a microscope to let us see detail
• The best light microscopes can resolve details which are 0.2μm apart and require a magnification of about x1500 so that our eyes can see this detail
• Electron microscopes have much greater resolution, being able to show details that are about - um apart
• The highest-powered electron microscopes can even show the detail of large molecules such as proteins
• Cell structures observed under microscopes

  • Cell membrane
  • Nucleus
  • Cytoplasm
• Selectively permeable


  The cell membrane prevents the passage of some molecules while allowing others to pass through
• Diffusion


  1. Random movement of a substance from where it is in a high concentration to where the concentration is lower

  2. Concentration gradient affects rate of diffusion

  3. Temperature affects rate of diffusion

  4. Surface area affects rate of diffusion
• Diffusion is particularly important in gas exchange, the movement of oxygen and carbon dioxide between the air and living organisms
• As cell size increases


  Surface area to volume ratio decreases
• There is a limit on the size of single-celled organisms due to the need for a large enough surface area to volume ratio to efficiently exchange substances
• Multi-celled organisms


  • Develop special gas exchange organs to increase surface area
  • Develop transport systems like circulatory system to move substances around body
• Tissues


  Groups of cells with the same specialised structure and function
• Organs


  Structures made of several types of tissue that carry out a particular function
• Organ systems


  Organs which operate together to carry out a particular function
• Examples of organ systems in humans

  • Nervous system
  • Reproductive system
  • Excretory system
• Stem cells


  Unspecialised cells that can divide to produce more stem cells and differentiate into a wide variety of specialised cell types
• Embryonic stem cells can be collected from embryos not used in fertility treatments or from the umbilical cord and placenta
• Adult animals have most stem cells permanently differentiated into specialised cells, but some stem cells can be collected from tissues like bone marrow
• In plants, stem cells are found in the apical growing points or meristems at the end of shoots and roots
• Cloning plants


  Techniques that can produce large numbers of genetically identical plants in a short time