Gcse biology single science ocr

• Organisms can be Eukaryotes or Prokaryotes
• Eukaryotes include animal and plant cells
• Prokaryotes (e.g. bacteria) are smaller and simpler cells called prokaryotic cells
• Structures Within Eukaryotic Cells
• NUCLEUS:

  • Contains DNA (genetic material) in the form of chromosomes that controls the cell's activities
• CYTOPLASM:

  • Gel-like substance where most of the chemical reactions happen
  • Provides support for the cell
• MITOCHONDRIA:

  • Site of cellular respiration and contain the enzymes needed for the reactions involved
• RIBOSOMES:

  • Where proteins are made in the cell
  • Contain a substance called chlorophyll
• CELL MEMBRANE:

  • Holds the cell together and controls what goes in and out
  • Provides a selective barrier
  • Contains receptor molecules for cell communication
• Structures Within Prokaryotic Cells
• CHROMOSOMAL DNA:

  • One long circular chromosome that controls the cell's activities and replication
  • Floats free in the cytoplasm (not in a nucleus)
• PLASMIDS:

  • Small loops of extra DNA that aren't part of the chromosome
  • Contain genes for things like drug resistance and can be passed between bacteria
• CELL MEMBRANE:

  • Controls what goes in and out
  • Supported by a cell wall
• Microscopes
• Microscopes use lenses to magnify images and increase resolution
• Light microscopes were invented in the 1590s and allow us to see things like nuclei and chloroplasts
• Electron microscopes were invented in the 1930s and allow us to see much smaller things in more detail, like the internal structure of mitochondria
• Transmission electron microscopes (TEMs) have higher magnification and resolution than light microscopes but are not portable, expensive, and require a complicated process to prepare specimens
• Main parts of a light microscope:

  • Eyepiece lens: looked through to see the image and magnifies it
  • Objective lens: magnifies the image, usually three different lenses (e.g. x4, x10, x40)
  • Stage: supports the slide
  • Clip: holds the slide in place
  • Handle: used for carrying the microscope
  • Lamp: shines light through the slide for better visibility
  • Focusing knobs: move the stage up and down to bring the image into focus
• Specimens need to be prepared before investigation:

  1. Specimen needs to let light through, so if it's thick, take a thin slice
  2. Place a drop of water or mountant on a clean slide to secure the specimen
  3. Use tweezers to place the specimen on the slide
  4. Add a drop of stain if needed to enhance visibility
  5. Place a cover slip over the specimen, ensuring no air bubbles are trapped
• Steps for viewing a prepared specimen:

  1. Clip the slide onto the stage
  2. Select the lowest-powered objective lens
  3. Use the coarse adjustment knob to bring the specimen into focus
  4. Adjust focus with the fine adjustment knob for a clear image
  5. Swap to a higher-powered objective lens if greater magnification is needed
• Magnification is how many times bigger the image is than its real size
• To calculate the total magnification of an image viewed through a microscope:

  • Use the formula: total magnification = eyepiece lens magnification x objective lens magnification
• DNA contains all of an organism's genetic material and is arranged into chromosomes
• Chromosomes are long molecules of coiled up DNA, divided into short sections called genes
• DNA is a double helix, with two complementary base pairs made up of nucleotides joined together in a long chain
• DNA has four different bases: A (adenine), C (cytosine), G (guanine), and T (thymine)
• In DNA, A always pairs up with T, and C always pairs up with G, known as complementary base-pairing
• Each DNA nucleotide contains a sugar, a phosphate group, and a base (A, C, G, or T)
• The base is attached to the sugar in a DNA nucleotide
• DNA is a polymer, composed of long chains of nucleotide monomers