Topic 3 - Biology Paper 1

• Exchange surface


  Specialised surface that arises as the size of the organism, and its surface area to volume ratio increases
• Features of an efficient exchange surface

  • Large surface area
  • Thin to ensure short distance for substances to cross
  • Good blood supply/ventilation to maintain a steep gradient
• Ventilation and gas exchange in fish

  1. Fish opens mouth

  2. Lowers floor of buccal cavity

  3. Water flows in

  4. Fish closes mouth

  5. Buccal cavity floor raises, increasing pressure

  6. Water forced over gill filaments
• Gills of bony fish

  • Four pairs of gills, each supported by an arch
  • Multiple projections called gill filaments with lamellae
  • Blood and water flow in counter current direction
• Fish cannot survive very long out of water as the gill filaments stick together without water flow
• Ventilation and gas exchange in terrestrial insects

  1. Oxygen transported directly to tissues via spiracles, trachea and tracheoles

  2. Gases move by diffusion, mass transport, and volume changes
• Gas exchange in plants

  • Leaves have many small holes called stomata
  • Air spaces allow gases to move around the leaf and contact photosynthesising cells
• Mammalian gaseous exchange system

  • Lungs are a pair of lobed structures with a large surface area in the chest cavity that can inflate
  • Surrounded by rib cage and diaphragm to enable ventilation

  
• Structures and functions of mammalian gaseous exchange system

  • Cartilage - supports trachea and bronchi
  • Ciliated epithelium - moves mucus
  • Goblet cells - secrete mucus
  • Smooth muscle - controls airway diameter
  • Elastic fibres - control air flow

  
• Inspiration


  1. External intercostal muscles contract, diaphragm contracts

  2. Increases volume of thorax, decreases pressure, air forced into lungs
• Expiration


  1. Internal intercostal muscles contract, diaphragm relaxes

  2. Decreases volume of thorax, increases pressure, air forced out of lungs
• Spirometer


  Device used to measure lung volumes and breathing rate

  
• Lung volumes measured by spirometer


  • Vital capacity
  • Tidal volume
  • Residual volume
  • Inspiratory reserve volume
  • Expiratory reserve volume

  
• Digestion


  Hydrolysis of large biological molecules into smaller molecules that can be absorbed
• Enzymes that digest carbohydrates

  • Amylases
  • Maltases
  • Sucrases
  • Lactases
• Lipid digestion


  • Lipases hydrolyse ester bonds
  • Bile salts emulsify lipids to increase surface area
• Enzymes that digest proteins

  • Endopeptidases
  • Exopeptidases
  • Dipeptidases
• Absorption of digestion products

  1. Amino acids absorbed by facilitated diffusion with Na+ co-transport

  2. Monoglycerides and fatty acids diffuse into epithelial cells, reformed into triglycerides, transported in lymph
• Haemoglobin

  • Water soluble globular protein with haem groups that can bind 4 oxygen molecules
  • Affinity for oxygen varies with partial pressure, high in lungs (loading), low in respiring tissues (unloading)

  
• Dissociation curves illustrate the changes in haemoglobin-oxygen affinity

  
• Haemoglobin

  Globular protein which consists of two beta polypeptide chains and two alpha helices. Each molecule forms a complex containing a haem group.
• Haemoglobin

  • Carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group
  • Each molecule can carry four oxygen molecules

  
• Partial pressure of oxygen

  Affects the affinity of oxygen for haemoglobin

  
• As partial pressure increases

  The affinity of haemoglobin for oxygen increases

  
• Loading

  Oxygen binds to haemoglobin in the lungs

  
• Unloading

  Oxygen is released from haemoglobin in respiring tissues

  
• Dissociation curves

  Illustrate the change in haemoglobin saturation as partial pressure changes

  
• Haemoglobin saturation

  Affected by its affinity for oxygen

  
• High partial pressure

  Haemoglobin has high affinity for oxygen and is highly saturated

  
• Low partial pressure

  Haemoglobin has low affinity for oxygen and is less saturated

  
• Binding of first oxygen molecule

  Increases the affinity of haemoglobin for oxygen

  
• Positive cooperativity


  The change in shape of haemoglobin after binding the first oxygen molecule makes it easier for the other oxygen molecules to bind

  
• Fetal haemoglobin


  Has a higher affinity for oxygen compared to adult haemoglobin

  
• Presence of carbon dioxide

  Decreases the affinity of haemoglobin for oxygen

  
• Bohr effect


  Carbon dioxide creates slightly acidic conditions which change the shape of the haemoglobin protein, thus making it easier for the oxygen to be released

  
• In large organisms the surface area to volume ratio is not large enough for diffusion alone to supply substances like oxygen, glucose and other molecules to cells where they are needed
• Common features of a circulatory system

  • Suitable medium (blood)
  • Means of moving the medium (heart)
  • Mechanism to control flow around the body (valves)
  • Close system of vessels
• Closed double circulatory system

  The heart at the centre has two pumps - one pumps blood to the lungs to be oxygenated, the other pumps oxygenated blood around the body
• Atrium


  Thin walled and elastic, can stretch when filled with blood
• Ventricle


  Thick muscular wall to pump blood around the body or to the lungs