OCR A Level Biology Module 1-4

• why do specimens need to be thin when observing them under a light microscope?

  to allow light to pass through
• identify and explain the starch test

  add iodine dissolved in potassium iodide solution to the sample- positive test = blue-black- negative test = orange-brown
• why is the structure of the plasma membrane often referred to as the fluid mosaic model?

  - the continuous bilayer is 'fluid' as the phospholipid molecules are constantly moving- protein molecules are scattered through the bilayer like a mosaic
• why is cell signalling important?

  so that cells can communicate with each other
• how do cells communicate?

  - one cell releases a messenger molecule by exocytosis - the molecule travels in the blood to another cell- messenger molecule is detected by another cell as it binds to the receptor on the plasma membrane
• why is cell signalling specific?

  different receptors are specific to different molecules
• what does cell signalling initiate?

  a physiological response
• what is the approximate size of eukaryotic cells?

  10-100 µm
• state the defining features of eukaryotic cells

  - nucleus- membrane-bound organelles
• describe the process of protein synthesis

  - in the nucleus, DNA coding is copied onto a molecule of mRNA- in ribosomes in the RER, amino acids are assembled into polypeptide chains with mRNA and tRNA- the polypeptide chain is folded and modified in the RER and transported to the Golgi body in labelled vesicles- Golgi body modifies and packages the protein and transports it to the plasma membrane for secretion- vesicle fuses with the plasma membrane and is secreted
• what is the function of the cytoskeleton?

  it allows eukaryotic and prokaryotic cells to move, retain a structured shape and transport organelles
• what is the function of microfilaments?

  responsible for cell movement and contract to aid with cell division
• what is the function of microtubules?

  - form part of the spindle fibre during cell division- scaffold-like structures that determine the shape of the cell
• what is the function of intermediate fibres?

  to give mechanical strength
• how does blood circulation help to maintain a concentration gradient of oxygen?

  blood is constantly taking away oxygen and bringing in more carbon dioxide
• what happens during inspiration?

  - diaphragm contracts (flattens) which presses down on the digestive organs and allows room for the lungs to fill with air - intercostal muscles contract which moves the ribs up and out- volume of thorax increases and therefore pressure in the lungs decreases which forces air to rush in
• what happens during expiration?

  - diaphragm relaxes (domes) and pushes against the lungs - intercostal muscles relax which moves the ribs down and outwards- volume of the thorax decreases and therefore pressure in the lungs increases and forces air out
• is expiration active or passive?

  both; normal expiration is passive as inspiration muscles relax, however forced breathing can be active as the abdominal muscles depress the rib cage to forcefully decrease the volume of the thorax
• why is cartilage essential in the trachea?

  it acts as a support column because its 'C' shape allows the pipe to stay open and not collapse under high pressure
• what are the inner tissues of the trachea?

  glandularconnectiveelastic fibressmooth muscleciliated epithelium
• what is the lumen in the trachea formed from?

  the ciliated epithelium
• which tissues are present in arteries and veins?

  endotheliumconnective tissue wallssmooth muscle
• what are the functions of the smooth muscle and elastic fibres in the trachea?

  to allow the lumen to dilate and narrow:- larger volumes of air can enter the lungs during exercise - trachea can constrict when there are toxic substances in the air
• why is it important that the smooth muscle and elastic fibres work together in the trachea?

  - muscles cannot reverse contraction on their own - when muscles contract, elastic fibres deform and stretch, they then recoil when energy has stopped being applied to contraction
• why are ciliated epithelial and goblet cells important in the trachea?

  - goblet cells secrete mucus which traps dust and microorganisms in the inhaled air, preventing them from reaching the alveoli- ciliated cells beat the mucus away from the lungs which prevents infection
• what happens to haemoglobin after the first oxygen molecule has bound to it?

  it undergoes a conformational change to make it easier for the other oxygen molecules to bind to it
• what is the name of the compound that forms one oxygen ha bound to haemoglobin?

  oxyhaemoglobin
• what is the function of haemoglobin?

  to transport oxygen around the body
• in which type of cell is haemoglobin found?

  red blood cell
• where does haemoglobin bind to oxygen?

  in the lungs
• why does a graph of an oxygen dissociation curve plateau at around 8 kPa partial pressure?

  haemoglobin molecules become the limiting factor; there could be an oxygen excess
• how is oxygen released from haemoglobin?

  in respiring tissues there is a low partial pressure of oxygen and a high concentration of carbon dioxide which lowers the pH and brings about a conformational change in the haemoglobin molecule, allowing it to release the oxygen - which means that there is a decreased % saturation of oxygen
• which ions does carbonic acid dissociate into? (H₂CO₃)

  H+ and HCO3-
• which ions are responsible for lowering the pH of a solution?

  H+
• which ions are responsible for increasing the pH of a solution?

  OH-
• why do different organisms have different haemoglobin activities?

  depending on the organism's needs and environment, the structure of haemoglobin will vary and affect the oxygen affinity
• what is meant by partial pressure of oxygen?

  the measure of oxygen concentration
• how does oxyhaemoglobin form?

  4 oxygen molecules will load onto a haemoglobin molecule when there is a high partial pressure - this is known as a high affinity for oxygen
• explain the shape of the oxygen dissociation graph

  "S" shaped:- at a low partial pressure, haemoglobin has a low affinity for oxygen because it is hard for the O2 molecule to reach the haem group in the centre of the blood cell- when partial pressure rises, the diffusion gradient of oxygen into haemoglobin steeply rises, when one oxygen molecule has bound to a haem group the haemoglobin undergoes a conformational change to make it easier for the second and third oxygen molecules to associate- when the haemoglobin molecule starts to become saturated it is harder for the fourth oxygen molecules to bind which is why the graph begins to level off- it is difficult to achieve 100% saturation even at high partial pressures
• explain the Bohr effect

  when tissues respire they produce CO2 which raises the partial pressure of carbon dioxide and makes the blood more acidic and causes oxygen to unload more readily as haemoglobin can take up the H+ ions to form haemoglobonic acid so that acidity does not affect the cell's activity