B1

• Organelles

  Structures in a cell that have different functions
• Cell structures in eukaryotic cells

  • Cell membrane
  • Cytoplasm
  • Nucleus containing DNA
• In bacterial cells, the size or area of sub-cellular structures can be calculated by finding a shape that resembles it and applying the rules used for that shape
• Specialised cells in animals

  • Sperm cells - specialised to carry the male's DNA to the egg cell for successful reproduction, streamlined head and long tail to aid swimming
• Cells are extremely small, and orders of magnitude are used to understand the size differences
• Subcellular structures and their functions in animal and plant cells

  • Nucleus - Contains DNA coding for a particular protein needed to build new cells
  • Cytoplasm - Liquid substance in which chemical reactions occur
  • Cell membrane - Controls what enters and leaves the cell
  • Mitochondria - Where aerobic respiration reactions occur, providing energy for the cell
  • Ribosomes - Where protein synthesis occurs
  • Chloroplasts - Where photosynthesis takes place, providing food for the plant
  • Permanent vacuole - Contains cell sap, improves cell's rigidity
  • Cell wall - Made from cellulose, provides strength to the cell
• Cell structures in bacterial cells

  • Cell wall
  • Cell membrane
  • Cytoplasm
  • Single circular strand of DNA and plasmids
• Cell Specialisation

  1. Cells specialise by undergoing differentiation, gaining new sub-cellular structures suited to their role

  2. Cells can differentiate once early on or have the ability to differentiate their whole life (stem cells)

  3. In animals, most cells only differentiate once, but in plants many cells retain the ability
• Nerve cells

  • The axon is long, enabling the impulses to be carried along long distances
  • Having lots of extensions from the cell body (called dendrites) means branched connections can form with other nerve cells
  • The nerve endings have many mitochondria which supply the energy to make special transmitter chemicals called neurotransmitters. These allow the impulse to be passed from one cell to another
• Specialised cells in animals

  • Sperm cells: specialised to carry the male’s DNA to the egg cell (ovum) for successful reproduction
  • Nerve cells: specialised to transmit electrical signals quickly from one place in the body to another
  • Muscle cells: specialised to contract quickly to move bones (striated muscle) or simply to squeeze (smooth muscle)
• 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
• Specialised cells in plants

  • 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
  • Xylem cells: specialised to transport water and mineral ions up the plant from the roots to the shoots
  • Phloem cells: specialised to carry the products of photosynthesis (food) to all parts of the plants
• Cell Differentiation

  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
• 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

  • 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
• 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. Others such as red blood cells cannot divide and are replaced by adult stem cells which retain their ability to undergo differentiation
• Sperm cells

  • Streamlined head and long tail to aid swimming
  • Many mitochondria (where respiration happens) which supply the energy to allow the cell to move
  • The acrosome (top of the head) has digestive enzymes which break down the outer layers of membrane of the egg cell
• 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
• 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 moved to another position
• Culturing Microorganisms

  Scientists grow microorganisms in the lab using nutrients in a culture medium containing carbohydrates, minerals, proteins, and vitamins
• The first cells of a cork were observed by Robert Hooke in 1665 using a light microscope
• Red blood cells lose their nucleus and cannot divide; they are replaced by adult stem cells which retain their ability to undergo differentiation
• In the 1930s, the electron microscope was developed, enabling scientists to view deep inside sub-cellular structures such as mitochondria, ribosomes, chloroplasts, and plasmids
• Making an agar gel plate


  Hot sterilised agar jelly is poured into a sterilised Petri dish, left to cool and set, inoculating loops are used to spread the microorganism solution evenly, and the plate is incubated
• Common calculations in microscopy include magnification of a light microscope and size of an object
• In mature animals, cell division mostly happens to repair or replace damaged cells
• Ways to grow microorganisms in the lab

  • In nutrient broth solution
  • On an agar gel plate
• Cell division


  Involves a cell dividing to produce 2 identical cells
• Standard form can be useful when working with very large or small numbers in microscopy
• 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
  • Usually illuminated from underneath
  • Has a maximum magnification of x2000 and a resolving power of 200nm
  • Used to view tissues, cells, and large sub-cellular structures
• Microscopy

  Extremely small structures such as cells cannot be seen without microscopes, which enlarge the image
• Electron microscope


  • Uses electrons to form an image due to their smaller wavelength than light waves
  • Two types: scanning electron microscope for 3D images and transmission electron microscope for 2D images detailing organelles
  • Has a magnification of up to x2,000,000 and resolving power of 10nm (SEM) and 0.2nm (TEM)
• Testing the effects of antibiotics on bacterial growth

  Soak paper discs in different antibiotics, place on an agar plate with bacteria, measure the size of the clear area around the discs to determine effectiveness
• If bacteria have a supply of nutrients and a suitable temperature, they can multiply by binary fission as fast as every 20 minutes
• Inoculating loops must be sterilised by passing them through a flame to kill unwanted microorganisms
• Petri dishes and culture media must be sterilised before use, often done by an autoclave (an oven) or UV light
• The Petri dish should be stored upside down to prevent condensation from the lid disrupting growth
• Measuring zone of inhibition

  Leave the plate at 25 degrees for 2 days, measure the zone of inhibition to determine effectiveness of the antibiotic
• Investigation steps

  • Soak paper discs in different types/concentrations of antibiotics and place on agar plate evenly spread with bacteria. One disc should be a control, soaked in sterile water. 2. If bacteria are resistant to the antibiotic, they will not die, leaving an inhibition zone
• Making the 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)