AQA Biology

• Cell


  The smallest unit of life that can replicate
• Eukaryotes


  • Include animal and plant cells, which are multicellular organisms
  • When their cells replicate they are growing or replacing dead cells
• Bacterial cells


  • Are prokaryotes, which are unicellular organisms
  • When they replicate (asexual reproduction occurs) by binary fission, unicellular organisms only reproduce by cell division
• Features common to both animal and plant cells

  • Mitochondria
  • Cytoplasm
  • Cell membrane
  • Nucleus
  • Ribosomes
• Mitochondria


  The power house of the cell, the site of aerobic respiration where energy is released for the cell to function
• Cytoplasm


  A jelly-like substance in which all the sub-cellular structures sit, where majority of chemical reactions occur, containing enzymes that control reactions
• Cell membrane


  A partially permeable membrane which controls what enters and leaves the cell
• Nucleus


  Contains genetic information or DNA that controls the cell
• Ribosomes


  Carry out protein synthesis
• Features only found in plant cells


  • Chloroplasts
  • Permanent vacuole
  • Cell wall
• Chloroplasts


  The site for photosynthesis, containing chlorophyll which helps absorb sunlight and converts it into glucose
• Permanent vacuole


  Contains cell sap, such as salt, sugar and water, responsible for the shape of a cell
• Cell wall


  Contains cellulose, which is really strong and helps support the cell and controls the passage of other molecules
• Bacterial cell


  • Is a prokaryote, therefore lacks a nucleus and other membrane-bound organelles
  • Is unicellular and reproduces during cell division
  • Does not contain mitochondria, chloroplasts, vacuole or a nucleus
  • Contains ribosomes, cell wall, cytoplasm, plasmids, flagellum
• Flagellum


  A long tail protruding from the end of a bacterial cell, helps to propel the cell and move it effectively
• Plasmids


  Circular rings of DNA that carry genes with advantages such as antibiotic resistance
• Prokaryotes


  • Unicellular organisms such as bacterial cells, lack a nucleus and have DNA free in the cytoplasm
  • Include plasmids which are small rings of DNA that carry genes with genetic advantages
• Eukaryotes


  • Complex cells found in multicellular organisms, contain a nucleus and other membrane-bound organelles
  • Are 10 - 100 micrometres in size
• Main subcellular structures in animal cells

  • Nucleus
  • Cell membranes
  • Mitochondria
  • Ribosomes
  • Cytoplasm
• Some cellular structures can only be seen when viewed with an electron microscope
• Additional structures found in plant cells

  • Cell wall made of cellulose
  • Permanent vacuole filled with cell sap
  • Chloroplasts (in leaf and stem cells)
• The plant cell shown contains chloroplasts, so it would be found in the leaves of a plant
• Magnification


  How many times bigger the image of a specimen observed is compared to the actual (real-life) size of the specimen
• Resolution


  • The ability to distinguish between two separate points
  • If two separate points cannot be resolved, they will be observed as one point
  • The resolution of a light microscope is limited by the wavelength of light
  • Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have a much smaller wavelength than visible light
• Comparison of light microscopes and electron microscopes


  • Light microscopes are used for specimens above 200 nm, shine light through the specimen
  • Electron microscopes are used for specimens above 0.5 nm, fire a beam of electrons at the specimen
• Light microscopes

  • Useful for looking at whole cells, small plant and animal organisms, tissues within organs
  • Specimens can be living or dead
• Electron microscopes


  • Useful for looking at organelles, viruses and DNA, as well as whole cells in more detail
  • Specimens must be dead, but can provide a snapshot in time of what is occurring in a cell
• Comparison of light and electron microscopes


  • Light microscopes - magnification up to 2000x, resolution limited by wavelength of light
  • Electron microscopes - magnification up to 1,000,000x, resolution not limited by wavelength
• Preparing a microscope slide

  1. Specimens must be prepared on a microscope slide to be observed

  2. Stains are used to highlight structures within cells - methylene blue for cheek cells, iodine for onion cells
• Using a microscope

  1. Always hold the microscope by the arm when moving it

  2. Always start observation with the lowest-powered objective lens
• Specialised cells


  Cells that have a particular structure and composition of subcellular structures, enabling them to perform specific functions
• Nerve cell (neurone)


  • Has a cell body where most cellular structures are located and protein synthesis occurs
  • Has extensions (dendrites and axons) that allow communication with other cells
  • Axon is covered in a fatty sheath that speeds up nerve impulses
• Muscle cells


  • Contain layers of protein filaments that can slide over each other, causing contraction
  • Have a high density of mitochondria to provide energy for contraction
  • Skeletal muscle cells fuse together during development to form multinucleated cells
• Dendrites


  Receive signals
• Axons


  Transmit signals
• Axons are covered with a fatty sheath which speeds up nerve impulses
• Axons can be up to 1m long in some animals
• Muscle cells


  • Contain layers of fibres which allow them to contract
  • Skeletal, smooth and cardiac (heart) muscle types
• Muscle contraction


  Layers of protein filaments slide over each other
• Muscle cells


  • Have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction
  • Skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison