2.1.6

• Cell cycle


  Series of events during which the cell duplicates its contents and splits in two
• Cell cycle


  1. Copying the chromosomes

  2. Checking the genetic information

  3. Increasing in size

  4. Producing new organelles

  5. Storing energy for another division
• Mitosis


  • A small part of the cell cycle
  • Produces two genetically identical cells
• Mitosis


  • Used for: growth of the organism
  • Repair of tissues
  • Replacement of old cells
  • Asexual reproduction
• Mitosis


  Four stages
• Meiosis


  • An alternative form of cell division
  • Produces four cells that are not genetically identical
  • Produces gametes
  • Produces haploid cells (contain half the normal number of chromosomes)
• Meiosis


  1. Prophase 1

  2. Metaphase 1

  3. Anaphase 1

  4. Telophase 1

  5. Prophase 2

  6. Metaphase 2

  7. Anaphase 2

  8. Telophase 2
• Genetic variation can be created by meiosis
• Genetic variation in meiosis

  • Chromatid crossover
  • Random orientation of bivalents on the equator
  • Random orientation of sister chromatids on the equator
• Erythrocytes (red blood cells)

  • Carry oxygen in the blood
  • Small and flexible to fit through tiny capillaries
  • Full of haemoglobin to bind to the oxygen
  • No nucleus to allow more space for haemoglobin
  • Biconcave shape to provide a large surface area to take up oxygen quickly
• Neutrophils


  • Engulf and digest foreign matter or old cells
  • Flexible shape to enable movement through tissues
  • Lobed nucleus to help movement through membranes
  • Many ribosomes to manufacture digestive enzymes
  • Many lysosomes to hold digestive enzymes
  • Many mitochondria to release the energy needed for activity
  • Well-developed cytoskeleton to enable movement
  • Membrane-bound receptors to recognise materials that needs to be destroyed
• Sperm


  • Carry the paternal chromosomes to the egg
  • Tail (flagellum) to enable rapid movement
  • Acrosome to help digest egg surface
  • Small to make movement easier
  • Many mitochondria to release the energy needed for rapid movement
• Epithelial cells


  • Act as surfaces
  • Often flat (squamous) to cover a large area
  • Often thin (squamous) to provide a short diffusion distance
  • May be ciliated to move mucus
  • May be cuboid to provide a barrier
  • Many glycolipids and glycoproteins in cell-surface membrane to hold cells together or for cell signalling
• Palisade cells


  • Elongate to fit many chloroplasts into the space
  • Contain many chloroplasts to absorb as much light as possible
  • Show cytoplasmic streaming to move the chloroplasts around
  • Contain starch grains to store products of photosynthesis
• Root hair cells


  • Long extension (hair) to increase surface area
  • Active pumps in cell-surface membrane to absorb mineral ions by active transport
  • Thin cell wall to reduce barrier to movement of ions and water
• Guard cells


  • Active pumps in cell-surface membrane to move mineral ions in and out of cell to alter the water potential
  • Unevenly thickened wall to cause the cell to change shape as it becomes more turgid
  • Large vacuole to take up water and expand to open the stoma
• Tissue


  A collection of cells that work together to perform a particular function
• Organ


  A collection of tissues working together to perform a common function
• Organ system


  Made up of two or more organs working together to perform a life function
• Stem cells


  • Cells that are not specialised or differentiated
  • Maintain the capacity to undergo mitosis and differentiate into a range of cell types
• Differentiation


  The ability of a cell to specialise to form a particular type of cell
• Stem cell differentiation


  1. Stem cells divide to produce new cells

  2. New cells then differentiate to become specialised to their function
• Stem cells


  • Have the ability to use all their genes
  • Differentiation occurs by switching on or off appropriate genes
• Production of blood cells

  Stem cells in the bone marrow divide and differentiate to form both red and white blood cells
• Erythrocytes


  • Their haemoglobin molecules are synthesised during development before the other organelles are lost
• Neutrophils


  • The most common type of phagocyte used to ingest and destroy bacteria
• Production of xylem vessels and phloem sieve tubes

  1. New cells are produced by mitosis in the meristem

  2. Cells are expanded by the uptake of water and the development of a vacuole

  3. Cells then differentiate into xylem and phloem
• Xylem


  • Lignin is deposited in their cell walls to strengthen and waterproof the wall
  • The cells die and the contents are removed as the end walls break down, forming continuous columns of cells
  • The lignification is incomplete in some places, forming bordered pits
• Phloem


  • Sieve tube elements lose their nucleus and most of their organelles
  • Sieve tube elements have numerous sieve pores to form sieve plates between the elements
  • Companion cells retain their organelles and can carry out metabolism to obtain and use ATP to actively load sugars into the sieve tubes
  • Sieve tube elements and companion cells are linked by numerous plasmodesmata
• Sources of stem cells

  • Embryonic stem cells
  • Blood from the umbilical cord
  • Adult stem cells found in developed tissues such as bone marrow
  • Induced pluripotent stem cells (iPS cells)
• Uses of stem cells in research and medicine

  • Treating diseases of the blood
  • Repairing the spinal cord
  • Treating type 1 diabetes
  • Producing new light-sensitive cells in the retina
  • Treating neurological conditions
  • Treating other conditions such as arthritis, stroke, burns, blindness, deafness and heart disease
• Developmental biology using stem cells

  1. Studying how differentiation occurs

  2. Studying what happens when differentiation goes wrong

  3. Finding ways to re-enable differentiation and growth in adult cells to help tissue repair and regrowth