cell biology

• Chromosomes


  DNA molecules combined with proteins that contain genes which provide the instructions for protein synthesis
• Gene


  A section of DNA which codes for a protein
• The nucleus of human body cells contains 46 chromosomes
• The nucleus of gametes contains 23 chromosomes
• Arrangement of chromosomes in body cell nucleus

  Arranged in pairs - 46 chromosomes form 23 pairs
• Cell cycle


  1. Replication of DNA and synthesis of organelles

  2. Mitosis

  3. Division of cell
• Mitosis


  • Produces identical cells which all have the same genetic information
  • Produces additional cells for growth and repair
• First stage of cell cycle

  Longest stage - cells grow and increase in mass, replicate DNA and synthesise more organelles (eg. mitochondria, ribosomes)
• Second stage of cell cycle

  Each chromosome in a pair is pulled to opposite poles of the cell, then the nucleus divides
• Third stage of cell cycle

  The cytoplasm and cell membrane divide - two identical daughter cells are produced
• Stem cell


  An unspecialised cell which is capable of differentiating into other cell types and of self-renewal
• Function of embryonic stem cells

  • Can replicate themselves and differentiate into many other types of cells
  • May be able to treat conditions such as paralysis and diabetes by dividing to replace damaged cells
• Function of adult stem cells in bone marrow

  • Can differentiate into several cell types to replace dead or damaged tissues, e.g. form different blood cell types
• Function of stem cells in plant meristems


  • Retain the ability to differentiate into any type of plant cell throughout their lifespan - they can differentiate into any cell which is required by the plant
• Therapeutic cloning


  A cloning method where an embryo is produced with the same genetic makeup as the patient, so the stem cells will not be rejected by the patient's immune system and can be used to treat certain medical conditions
• Advantages of cloning plants using meristem stem cells

  • Can prevent rare plants from becoming extinct
  • Can produce large numbers of plants with a favourable characteristic
  • Can produce identical plants for research
• Issues associated with the use of stem cells

  • Many embryonic stem cells are sourced from aborted embryos - some people have an ethical/religious objection
  • Development of stem cell therapies is slow, expensive and difficult
  • Adult stem cells infected with viruses could transfer infections to patients
  • If donor stem cells do not have a similar genetic makeup to the patient, an immune response could be triggered
• what is diffusion?


  The movement of particles from an area of high concentration to an area of low concentration.
• what is osmosis?


  The movement of water molecules from an area of high concentration to an area of low concentration across a semi-permeable membrane.
• how does temperature affect diffusion?

  increases the rate of diffusion
• The larger the surface area of the membrane that a substance is diffusing through (e.g. the membrane around a cell), the faster the rate of diffusion
• how does a larger concentration gradient affect the rate of diffusion?

  Increases
• how are root hair cells adapted?


  they are specialised to absorb more water and increase efficiency of diffusion. They have no chloroplasts as they are located underground so there is no light for photosynthesis. Larger surface area due to long projections so plant can absorb more water and minerals.
• what is a xylem cell specialised for and how are they specialised?

  Transport water and minerals; they are made up of a series of connected dead xylem cells and the end walls of the dead cells are broken to allow water to move through, the cell walls contain lignin which strengthens the cell .
• why are phloem cells specialised and how?

  to transport nutrients to parts of the plants where they are needed. they are made up of columns of living cells, the end walls contain small holes to allow food products to move up and down the vessels throughout the plant
• what is resolution?

  the ability to distinguish between (tell apart) two or more objects that are close together.
• what is magnification?


  how many times larger an image seen through a microscope is compared to the real object.
• magnification equation

  • Magnification = image size ÷ actual size
• what is the job of the objective lens?

  Magnification
• light microscope - uses light rays , 1500 X magnification , 200nm resolution , small and portable , £100

  
• electron microscopes

  • uses electron beams
  • 2000000 times magnification
  • resolution of 0.2nm
  • very large , not portable
  • over £100,000
  • can study sub-cellular structures
  • first used in 1933
• what is active transport?


  Active transport is the process where molecules move from a less concentrated solution to a more concentrated solution. As the net movement of molecules is against the concentration gradient, energy is required. This energy is provided by respiration
• describe the stages of the cell cycle

  1. Initial growth stage: Extra ribosomes, mitochondria and other sub-cellular structures are produced. The cell’s chromosomes are replicated so that there are two sets of the cell’s chromosomes.
  2. Mitosis: The two sets of chromosomes are pulled to opposite ends of the cell. Then, the nucleus divides into two.
  3. Cell division: The cytoplasm and cell membranes divide to create two identical daughter cells.
• what is mitosis used for?

  asexual reproduction , cell growth and repair
• what are chromosomes?

  made from molecules of DNA and can be found in the nucleus of a cell. Each chromosome carries a large number of genes which produce proteins. Chromosomes are typically found in pairs in body cells.
• what happens during mitosis

  1. The two sets of chromosomes are separated by being pulled to opposite ends of cell.
  2. The nucleus divides in two.
  3. The cytoplasm and cell membranes divide.
  4. Two identical cells are created.
• Small organisms normally have higher surface area to volume ratios than large organisms. This means that diffusion can happen through a large area in small organisms. This makes diffusion useful for transporting molecules through a small organism.
• Multicellular organisms need a transport system with specialised surfaces to exchange molecules as their cells are too far from the external environment.
• exchange surface adaptations

  thin membrane so distance for diffusion is shorter

  large surface to allow more of a substance to diffuse at the same time.

  Where substances are exchanged through blood in animals, exchange surfaces are densely packed with blood vessels: These blood vessels replenish the blood supply to maintain a high concentration gradient by bringing in new blood as diffusion starts to even out the concentrations.
• what can cultures of microorganisms be used for

  to investigate the effects of antibiotics and disinfectants, which both kill bacteria.