Cells

• Muscle cells


  Specialised to contract quickly to move bones (striated muscle) or simply to squeeze (smooth muscle, e.g. found in blood vessels so blood pressure can be varied), therefore causing movement
• Muscle cells


  • Special proteins (myosin and actin) slide over each other, causing the muscle to contract
  • Lots of mitochondria to provide energy from respiration for contraction
  • They can store a chemical called glycogen that is used in respiration by mitochondria
• Root hair cells


  Specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of roots
• Root hair cells


  • Have a large surface area due to root hairs, meaning more water can move in
  • The large permanent vacuole affects the speed of movement of water from the soil to the cell
  • Mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cell
• Xylem cells


  Specialised to transport water and mineral ions up the plant from the roots to the shoots
• Xylem cells


  • Upon formation, a chemical called lignin is deposited which causes the cells to die. They become hollow and are joined end-to-end to form a continuous tube so water and mineral ions can move through
  • Lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water
• Phloem cells


  Specialised to carry the products of photosynthesis (food) to all parts of the plants
• Phloem cells


  • Cell walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell
  • Despite losing many sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells
• Cell differentiation


  To become specialised and be suited to its role, stem cells must undergo differentiation to form specialised cells. This involves some of their genes being switched on or off to produce different proteins, allowing the cell to acquire different sub-cellular substances for it to carry out a specific function.
• In animals, almost all cells differentiate at an early stage and then lose this ability. Most specialised cells can make more of the same cell by undergoing mitosis (the process that involves a cell dividing to produce 2 identical cells). Others such as red blood cells (which lose the nucleus) cannot divide and are replaced by adult stem cells (which retain their ability to undergo division).
• In mature animals, cell division only happens to repair or replace damaged cells.
• In plants, many types of cells retain the ability to differentiate throughout life. They only differentiate when they reach their final position in the plant, but they can still re-differentiate when the plant is moved to another position.
• Light microscope


  Has two lenses, an objective and eyepiece. The objective lens produces a magnified image, which is then magnified and directed into the eye by the eyepiece lens. It is usually illuminated from underneath.
• Light microscope


  • They have, approximately, a maximum magnification of x2000 and a resolving power (this affects resolution: the ability to distinguish between two points) of 200nm (the lower the RP, the more detail is seen)
  • Used to view tissues, cells and large sub-cellular structures
• Electron microscope


  Enables scientists to view deep inside sub-cellular structures, such as mitochondria, ribosomes, chloroplasts and plasmids.
• Electron microscope


  • Electrons, as opposed to light, are used to form an image because the electrons have a much smaller wavelength than that of light waves
  • There are two types: a scanning electron microscope that create 3D images (at a slightly lower magnification) and a transmission electron microscope which creates 2D images detailing organelles
  • They have a magnification of up to x2,000,000 and resolving power of 10nm (SEM) and 0.2nm (TEM)
• Calculating magnification of a light microscope

  Magnification of the eyepiece lens x magnification of the objective lens
• Calculating size of an object

  Size of image/magnification = size of object
• Standard form


  A way to represent very large or small numbers by multiplying a certain number by a power of 10
• The number which is being multiplied by a power of 10 needs to be between 1 and 10 to be able to compare the size of numbers while using standard form
• Nutrient broth solution


  Involves making a suspension of bacteria to be grown and mixing with sterile nutrient broth (the culture medium), stoppering the flask with cotton wool to prevent air from contaminating and shaking regularly to provide oxygen for the growing bacteria.
• Agar gel plate


  The agar acts as the culture medium, and bacteria grown on it form colonies on the surface.
• Making an agar gel plate


  1. Hot sterilised agar jelly is poured into a sterilised Petri dish, which is left to cool and set

  2. Wire loops called inoculating loops are dipped in a solution of the microorganism and spread over the agar evenly

  3. A lid is taped on and the plate is incubated for a few days so the microorganisms can grow (stored upside down)
• Petri dishes and culture media must be sterilised before use, often done by an autoclave (an oven) or UV light. This is to prevent contamination with other microorganisms.
• Inoculating loops must be stored by passing them through a flame to sterilise them.
• The lid of the Petri dish should be sealed (but not completely with tape) to stop airborne microorganisms from contaminating the culture, but it should not be sealed all the way around as this would be harmful for anaerobic bacteria growing due to no oxygen entering.
• The Petri dish should be stored upside down to prevent condensation from the lid landing on the agar surface and disrupting growth.
• The culture should be incubated at 25 degrees, not 37 degrees (human body temperature), as this would be more likely to allow bacteria that could be harmful to humans to grow, as this is their optimum temperature. At lower temperatures, colonies of such bacteria would not be able to grow.
• Binary fission


  The process where one bacterial cell splits into two identical daughter cells.
• Calculating the number of bacteria in a population after a certain time

  Bacteria at beginning x 2^(time/mean division time) = bacteria at end
• The number of bacteria at the end of the growth period can be very large, so it is common for it to be left in standard form.
• Testing the effects of different antibiotics on bacterial growth

  1. Soak paper discs in different antibiotics and place on an agar plate evenly spread with bacteria. One disc should be a control, soaked in sterile water.

  2. Leave the plate at 25 degrees for 2 days.

  3. The zone of inhibition (the clear area around the discs where bacteria have died) can be measured - the bigger it is, the more effective the antibiotic is.
• Chromosomes


  Contain coils of DNA. A gene is a short section of DNA that codes for a protein and as a result controls a characteristic.
• There are 23 pairs of chromosomes in each cell of the body, as you inherit one from your mother and one from your father-resulting in 46 chromosomes in total in each cell.
• Sex cells (gametes) have half the number of chromosomes, resulting in 23 chromosomes in total in each gamete cell.
• The cell cycle


  1. Stage 1 (Interphase): The cell grows, organelles grow and increase in number, protein synthesis occurs, DNA is replicated

  2. Stage 2 (Mitosis): The chromosomes line up at the equator of the cell and cell fibres pull each chromosome of the 'X' to either side of the cell

  3. Stage 3 (Cytokinesis): Two identical daughter cells form when the cytoplasm and cell membranes divide
• Cell division by mitosis in multicellular organisms is important in their growth and development, and when replacing damaged cells. Mitosis is also a vital part of asexual reproduction, as this type of reproduction only involves one organism, so to produce offspring it simply replicates its own cells.
• Stem cells


  Undifferentiated cells that can undergo division to produce more stem cells, of which some will differentiate to have distinct functions.
• Types of stem cells

  • Embryonic stem cells
  • Adult stem cells
  • Meristematic cells in plants
• Embryonic stem cells


  Form when an egg and sperm cell fuse to form a zygote. They can differentiate into any type of cell in the body.