Cells Exam qs

• why should you not press to hard on cover slip?

  to avoid damaging/breaking any chromosomes or cells
• risk assessment of mitosis practical?

  • hazard: scalpel
  • risk: cutting yourself
  • prevention: cut away from your body and cut on a white tile to prevent slipping
  • hazard: HCL acid
  • risk: irritant
  • prevention: wear goggles and wash hands if any spills on you
• how do you calculate mitotic index?

  The mitotic index is calculated by dividing the number of cells undergoing mitosis by the total number of cells observed.
• when counting the cells to calculate mitotic index, what should you do to ensure your count is accurate?

  examine a large number of cells to ensure a representative sample
• What is a tumour?

  a mass of undifferentiated/abnormal cells that have undergone uncontrollable cell division
• What is a malignant tumour?

  a mass of undifferentiated cells that have undergone uncontrollable cell division and can metastasise
• A pathogen is a microorganism that causes disease.
• A vaccine is an inactivated/attenuated form of a pathogen that causes an immune response, and little to no symptoms of the disease.
• Antigens are specific proteins on the surface of a pathogen that causes an immune response.
• describe phagocytosis?

  Phagocyte attracted to pathogen by chemicals; its receptors bind to antigens on pathogen

  Engulf / ingest bacteria into a phagasome

  Lysosome fuses with phagasome; empties lysozymes.

  Bacteria digested / hydrolysed into cell debris
• Describe T-helper cell recognition

  1. Phagocyte engulfs and hydrolyses pathogen (phagocytosis)
  2. phagocyte presents an antigen on it’s cell surface membrane and becomes an antigen-presenting cell
  3. A specific T-helper cell with complementary receptor binds to the presented antigen
  4. This activates the T-helper cell and it undergoes mitosis to form many identical copies of itself (clonal selection).
  5. These cloned T-helper cells become memory T cells, stimulate phagocytosis, activate cytotoxic T cells and B cells.
• an antibody is a specific protein secreted by a plasma cell 
• Monoclonal antibodies are antibodies with the same tertiary structure produced from one clone of B cells.
• Method for finding antibodies present in blood using ELISA test

  1. Apply blood serum to well in a plate (plate already has bound antigens)
  2. Wash (removes any unbound antibodies)
  3. Add 2nd antibody with enzyme attached (binds to antibody if present in blood)
  4. Wash (removes unbound 2nd antibody)
  5. Colourless substrate added which binds to enzyme. Enzyme catalyses it forming a coloured product.

  Concentration of protein present indicated by intensity of colour
• The washing steps are important in the ELISA test to make sure unbound antibodies aren't left in the well which could affect the results by creating a false positive.
• Structure of HIV?

  • lipid envelope
  • attachment proteins
  • reverse transcriptase (enzymes)
  • RNA (genetic material)
  • capsid
• Advantages of light microscopes?

  • low cost
  • small and portable
  • simple sample preparation
  • specimen doesn’t have to be dead
  • natural colour can be seen
• Disadvantages of light microscopes?

  • Limited resolution and magnification means that smaller organelles can’t be seen.
• Advantages of TEMs?

  • High resolution imaging means that internal structures of organelles can be seen.
• Disadvantages of TEMs?

  • High cost.
  • They can only be used with very thin specimens
  • They cannot be used to observe live specimens, as there is a vacuum 
  • lengthy, complex sample prep means that artefacts can be introduced
  • They do not produce a colour or 3D image
• Advantages of SEMs?

  • They can be used on thick specimens
  • They produce 3D images
• Disadvantages of SEMs?

  • They give lower resolution images (less detail) than TEMs
  • They cannot be used to observe live specimens
  • They do not produce a colour image
• why do we cut at the growing root?

  where mitosis occurs
• why do we use the mounted needle to lower the cover slip?

  To prevent air bubbles from forming between the slide and the cover slip.
• why do we press down on the cover slip?

  to make the root tip thinner so light can easily pass through.
• Vaccination immune response

  Vaccine administered, containing antigen from a particular pathogen

  Antigen is presented by phagocyte

  Specific helper T cell detects antigen and stimulates specific B cell

  B cell divides and forms plasma cells

  Plasma cells produce antibody 

  Memory cells form

  If pathogen is encountered then these memory cells quickly divide and produce antibodies

  Antibodies destroy pathogen, often before the host experiences any symptoms
• How is actual size of cells measured?

  • By using an eyepiece graticule
  • Calibrated using a stage micrometer
• Co-transport of glucose and sodium ions in the ileum

  1. Sodium ions are actively transported out of the epithelial cell into the blood via sodium-potassium pumps
  2. This reduces the sodium ion concentration in the epithelial cell
  3. Sodium ions can then diffuse down their concentration gradient from the lumen into the epithelial cell
  4. This movement is facilitated by a co-transporter protein that moves glucose against its concentration gradient into the cell along sodium ions
  5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood.
• Why are the cells placed in an ice-cold, isotonic, buffered solution?

  • ice-cold: reduces the activity of enzymes that break down organelles
  • isotonic: the same water potential to prevent cell lysis
  • buffered: maintains a constant pH so enzymes do not denature.
• Explain the arrangement of phospholipids in the cell-surface membrane (2)

  • bilayer
  • hydrophobic fatty acid tails point away OR hydrophilic phosphate heads point to water
• Name and describe five ways substances can move across the cell-surface membrane into a cell (5)

  • simple diffusion of small/non-polar molecules down a concentration gradient
  • facilitated diffusion down a concentration gradient via protein carrier/channel
  • osmosis of water down a water potential gradient
  • active transport against a concentration gradient via carrier protein using ATP
  • co-transport of 2 different substances using a carrier protein
• Compare and contrast the processes by which water and inorganic ions enter cells (3)

  • Comparison: both move down a concentration gradient
  • Comparison: both move through protein channels in membrane
  • Contrast: ions can move against a concentration gradient by active transport
• The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)

  • phospholipid bilayer allows diffusion of non-polar/lipid-soluble substances
  • phospholipid bilayer prevents diffusion of polar/charged/lipid-insoluble substances
  • carrier proteins allow active transport
  • channel/carrier proteins allow facilitated diffusion
  • number of channel/carrier proteins determine how much movement
• Describe how HIV is replicated

  • attachment protein attach to receptors on helper T cells
  • RNA enters cell
  • reverse transcriptase converts RNA to DNA
  • viral protein produced
  • virus particles assembled and released from cell
• Give two types of cell, other than pathogens, that can stimulate an immune response

  • cancerous/abnormal cells
  • cells from other organisms/transplanted cells
  • cells infected by virus
• What is the role of the disulphide bridge in forming the quaternary structure of an antibody? (1)

  • joins 2 different polypeptides
• Describe and explain the role of antibodies in stimulating phagocytosis (2)

  • bind to antigen
  • causes agglutination
• When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism. Describe how (5)

  • vaccine contains antigen from a pathogen
  • phagocytes presents antigen on its surface
  • T cell with complementary receptor protein binds to antigen
  • T cell stimulates B cell
  • with complementary antibody on its surface
  • B cell secretes large amounts of antibody
  • B cell divides to form clone all producing the same antibody
• Describe the differences between active and passive immunity (5)

  • active involves memory cells, passive does not
  • active involves production of antibody by plasma cells / memory cells
  • passive involves antibody introduced into body from outside source
  • active is long term, because antibody is produced in response to antigen
  • passive short term, because antibody is broken down
  • active can take time to develop / work, passive is fast acting
• Explain why phospholipids can form a bilayer but triglycerides cannot. (3)

  • phospholipid both hydrophobic and hydrophilic
  • triglycerides only hydrophobic
  • hydrophilic / phosphate group attracts water (to either side of bilayer)