Cells and Microorganisms

• Prokaryotic and eukaryotic cells have many features in common
• Features common to prokaryotic and eukaryotic cells

  • Both have a phospholipid membrane
  • Chromosomes made of DNA
  • Similar biochemistry pathways
  • Same genetic code
• This is a reflection on their common evolutionary past
• There was a single common ancestor of all life forms
• Asexual Reproduction


  Organisms are capable of reproducing without another partner
• Types of asexual reproduction


  • Binary Fission
  • Mitosis
• New Cells from Old

  • New cells arise from pre-existing cells
  • The ability to replicate is a requirement for life
  • All cells in your body can be traced back to a single cell – the zygote
  • Multicellular organisms need to be continuously replacing damaged and worn out cells, and each cell must receive an exact copy of the genetic information
• Cell Division


  • Permits genetic information to be transmitted from generation to generation
  • Provides a way for multicellular organisms to grow and develop from a fertilised ovum
  • Makes new cells available in multicellular organisms to replace dead or damaged cells
  • Allows unicellular organisms to reproduce
• Cell Division


  1. DNA replication (before cell division)

  2. The amount of DNA in a cell always doubles before cell division – each cell will receive an exact copy of the genetic information

  3. This process occurs all the time in your body – 1 billion cells divide every second
• Binary Fission


  • A form of asexual reproduction where the parent cell divides into two approximately equal parts (two genetically identical daughter cells)
  • Each new generation receives genetic material from the previous generation
• Prokaryotes


  • Have one circular chromosome and no nucleus
  • The circular chromosome is replicated prior to cell division so each daughter cell can receive a copy
  • The chromosome is attached to the cell membrane by proteins
  • The cell begins to grow and the cell membrane becomes larger, separating the chromosomes
  • This results in the chromosomes moving to opposite ends of the cell
  • The cell membrane pinches at the equator (creating a septum) and two new daughter cells form
• Mitotic Division


  • Gives rise to two, genetically identical, daughter cells
  • Enables the new zygote cell to grow into a multicellular organism
  • Used for repair and replacement of old and damaged cells
  • Used to reproduce asexually in eukaryotic organisms
• Phases of Mitosis


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Interphase


  1. Before mitosis, the DNA in the nucleus of the cell is present in long strands called chromatin

  2. DNA replication occurs during interphase

  3. After interphase, there are two sets of chromatin

  4. At the beginning of prophase, the DNA begins to condense and the chromosomes become visible

  5. 2 identical chromatids are joined at the centromere
• Prophase and Metaphase


  1. Prophase: At the beginning of prophase, the DNA begins to condense and the chromosomes become visible

  2. 2 identical chromatids are joined at the centromere

  3. Condensation is necessary so that DNA can easily move around the cell

  4. The nuclear membrane breaks down

  5. Spindle fibres, made of microtubules, begin to assemble and form the spindle apparatus

  6. Centrioles move towards the poles of the cell

  7. Metaphase: The microtubules extend from the poles towards the cell's equator

  8. The chromosomes align along the equator

  9. This imaginary line in the middle of the cell is called the metaphase plate
• Anaphase and Telophase


  1. Anaphase: The chromosomes separate

  2. The pairs of sister chromatids of each chromosome are drawn apart by the spindle fibres

  3. These move to opposable poles

  4. Each chromatid is now an identical daughter chromosome

  5. Telophase: After the daughter cells have moved to opposite poles, new nuclei can then form

  6. The chromosomes begin to de-condense into chromatin

  7. The chromosomes are no longer visible discrete structures
• Phases of Mitosis


  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Cytokinesis


  • During mitosis, the cytoplasm divides
  • The cytoskeleton constricts the cell membrane forming a cleavage furrow
  • Through cytokinesis, the cell pinches to form two genetically identical daughter cells
  • Organelles are distributed between the two cells
• Differences between Binary Fission and Mitosis

  • Binary Fission: Produces 2 new genetically identical daughter cells, DNA replicated prior to division, Occurs in Prokaryotes, No spindle apparatus formed, DNA attaches to membrane, Ribosomes are replicated prior to division, Used for asexual reproduction
  • Mitosis: Produces 2 new genetically identical daughter cells, DNA replicated prior to division, Occurs in Eukaryotes, Spindle apparatus present, DNA attaches to spindle apparatus, Organelles are replicated in interphase, Used for asexual reproduction, growth and repair
• Energy


  • All cells require an input of energy for processes such as movement, synthesis of materials and maintenance of a stable internal environment
  • Energy cannot be created or destroyed
  • Energy transformations occur in a cell
  • Life stops without a continued input of energy
• Cell processes requiring energy

  • DNA synthesis
  • Synthesis of proteins, carbohydrates and lipids
  • Active transport of substances across the cell membrane
  • Certain enzyme-dependent reactions
  • Contraction of muscles
• Transformation of Energy


  • Chemical bonds hold atoms together – energy is needed to break these bonds
  • When a chemical reaction occurs, some chemical bonds are broken and new ones will form as atoms combine again
  • The breakdown of glucose in the presence of oxygen releases energy
  • This is because the energy stored within the reactants (glucose and oxygen) is greater than the energy stored in the products (carbon dioxide and water)
• Aerobic Respiration


  • When oxygen is present, cells breakdown glucose via aerobic respiration
  • The energy is released as ATP (energy carrier)
  • Energy is then released and used to do work in the cell
• Autotrophs


  • Able to make their energy-rich compounds from simple inorganic substances
  • Most autotrophs use sunlight as a physical form of energy for these reactions
  • This process is called photosynthesis and occurs in photosynthetic autotrophs
  • Solar energy is converted to chemical energy in organic molecules that are then used by the cells
• Photosynthesis


  • Converts inorganic simple molecules into complex organic molecules
  • These organic molecules can be stored or transformed for use in various energy dependent processes
• Heterotrophs


  • Cannot produce their own organic compounds and rely on other organisms for nutrients
  • Obtain energy and nutritional requirements through feeding on other organisms
• Cell Requirements


  • Autotrophs: Produce their own energy-rich organic compounds, mostly via photosynthesis, do not need an input of glucose into the cell
  • Heterotrophs: Need an input of organic compounds they are unable to produce themselves, also need inorganic inputs containing essential elements like nitrogen, sulphur and phosphorus
• Cell Outputs


  • Autotrophs: Net output of oxygen, produce ethanol and carbon dioxide
  • Heterotrophs: Net output of carbon dioxide, produce lactic acid
  • Animals: Produce urea as a metabolic waste product
• Cell Size


  • Cells and their subunits are so small – it is important to appreciate their relative size
  • Cells are relatively large, following in decreasing order: Organelles, Bacteria, Viruses, Membranes, Molecules
  • Surface area to volume ratio limits cell size
  • In a cell, the rate of heat and waste production, and the rate of resource consumption, depend on the volume of the cell
  • If a cell has a greater surface area per unit volume, it is able to move more material in and out of the cell
  • As a cell grows, its volume increases at a faster rate to the volume, therefore the surface area to volume ratio decreases as a cell becomes larger
  • Cells are limited in their size so they can efficiently carry out the functions of life
  • Large animals do not have large cells, instead are made of many more cells
  • Cells that are larger in size have modifications that allow them to function efficiently, such as changes in shape or folds in the membrane to increase surface area
• Cell Membrane


  • Cell boundary that separates the cytoplasm from the extracellular fluid surrounding the cell
  • Separates one cell from another
  • Cell is a complex, selective structure (semi-permeable): Only allows certain substances to enter and leave the cell – selective, Is fundamental to regulating the flow of macromolecules into and out of the cell
  • Provides an abundant surface for chemical reactions to occur on, Surface area is increased in some cells through the presence of microvilli (extensions of the cell membrane)
• The Cell Membrane


  • The phospholipids form a bilayer with hydrophobic tails that point inwards, The polar phosphate heads face outwards creating a hydrophilic surface on both sides of the membrane
  • The model used to describe the membrane is the fluid mosaic model: Fluid - the fatty acid chains do not form strong bonds so the membrane is flexible/fluid, Mosaic - the membrane is embedded with functional proteins
• Membrane Proteins


  • Integral proteins: have a hydrophobic region in the centre that will interact with the phospholipid bilayer interior and hydrophobic on either side exposed to the aqueous solutions
  • Peripheral proteins: are not embedded in the membrane but remain bound to the surface of the membrane
• Passive Transport


  Occurs when there are concentration differences of a substance inside and outside of the cell, Movement occurs from an area of high concentration to an area of low concentration, i.e. down the concentration gradient, Does not require energy
• Diffusion


  • All particles have kinetic energy and are in constant random motion, This random motion means molecules will tend to spread out until they fill all the space available, At equilibrium, the particles do not stop moving, but diffusion is no longer occurring
  • The overall movement of a substance in a fluid from a region of high concentration of the substance towards regions of lower concentration of the substance, In living systems, this often involves a membrane, Can also occur in non-living systems
• Facilitated Diffusion


  Does not require an input of energy as molecules or ions move down the concentration gradient, Uses channel proteins to facilitate the movement of molecules across the membrane
• Osmosis


  The net movement of solvent across a semi-permeable membrane from a region of higher solvent concentration (lower solute concentration) to a region of lower solvent concentration (higher solute concentration)
• Equilibrium


  The particles will continue to spread apart until the substance is equally dispersed throughout the container and there is no net movement of particles as there is no concentration gradient
• Diffusion


  The overall movement of a substance in a fluid from a region of high concentration of the substance towards regions of lower concentration of the substance
• In living systems, diffusion often involves a membrane
• Diffusion can also occur in non-living systems (i.e. diffusion of perfume throughout a room)