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• Eukaryotic cells


  Plant and animal cells, have a cell membrane, cytoplasm and genetic material enclosed in a nucleus
• Prokaryotic cells


  Bacterial cells, smaller, have cytoplasm and membrane surrounded by a cell wall, genetic material not enclosed in a nucleus, single DNA loop
• Additional structures in prokaryotic cells

  • One or more small rings of DNA called plasmids
• Structures in animal cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
• Additional structures in plant and algal cells

  • Cell wall
  • Chloroplasts
  • Ribosomes
  • Permanent vacuole
• Cell specialisation

  A cell is specialised to carry out a particular function
• Cell differentiation


  Where a cell becomes specialised to carry out a particular job
• Most animal cells differentiate at an early stage, plant cells retain ability to differentiate throughout life</b>
• Mature animals restrict differentiation to repair and replacement
• Chromosomes


  Made of DNA molecules, found in pairs in body cells
• Cell cycle


  1. Growth and increase in subcellular structures

  2. DNA replication

  3. Mitosis - chromosomes pulled to each end, nucleus divides, cytoplasm and cell membrane divide
• Stem cell

  Undifferentiated cell capable of giving rise to many more cells of the same type
• Sources of stem cells

  • Human embryos
  • Bone marrow
• Stem cells from human embryos can be cloned and made to differentiate into most types of human cells
• Stem cells from bone marrow can form types of cells e.g. blood cells
• Plant meristems can differentiate into any type of plant cell throughout life
• Stem cells can help heal paralysis
• Light microscope

  Uses light and lenses to form an image, allows seeing individual cells and large subcellular structures
• Electron microscope


  Uses electrons to form an image, allows seeing smaller subcellular structures in more detail, has higher magnification and resolving power than light microscope
• Magnification


  Size of image / size of real object
• In therapeutic cloning, an embryo is produced with the same genes as the parent, stem cells from the embryo are not rejected by the patient's body so they may be used for medical treatment
• Use of stem cells has potential risks such as risk of viral infection, and some people have ethical/religious objections
• Plant stem cells from meristems can be used to produce clones of plants quickly and economically
• Rare species can be cloned to prevent extinction, disease resistant strains can be cloned
• Some are against the use of stem cells because they feel it's a potential human life, people destroy unused embryos
• Diffusion


  The spreading out of particles from an area of high concentration to an area of low concentration
• Substances transported in and out of cells by diffusion include oxygen, carbon dioxide, and the waste product urea
• Factors affecting rate of diffusion

  • Concentration gradient
  • Temperature
  • Surface area of membrane
• Bigger the concentration gradient, the faster the diffusion rate
• Higher temperature gives faster diffusion rate as particles have more energy
• Larger cell membrane gives faster diffusion rate as more particles can pass through
• A single-celled organism has a relatively large surface area to volume ratio, allowing sufficient transport of molecules into and out of the cell to meet its needs
• Osmosis


  The movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane
• Active transport

  Moves substances from a more dilute solution to a more concentrated solution (against concentration gradient), requires energy from respiration
• Active transport allows mineral ions to be absorbed into plant root hairs from dilute solutions in the soil, and sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration
• Exchange surfaces in multicellular organisms

  • Larger surface area compared to volume
  • Membrane is thin to provide short diffusion path
  • Efficient blood supply
  • Well ventilated for gas exchange
• Single-celled organisms have a large surface area to volume ratio, allowing sufficient substances to diffuse from their outer surface to supply their entire volume
• The small intestine has a large surface area, thin walls, and good blood supply to assist absorption of food
• Lungs contain alveoli with large surface area, thin walls, and good blood supply to facilitate gas exchange
• Fish gills have gill fragments and lamellae to increase surface area and minimise diffusion distance for gas exchange