Topic 3

• Gas exchange


  • Organisms need to get oxygen and glucose to their cells and get rid of waste products (carbon dioxide and water)
  • Organisms need to transport heat to maintain body temperature
• Single-celled organisms


  • Have a large surface area
  • Have a thin surface
  • Have a short diffusion pathway
  • Do not need transport systems
  • Absorb gases by diffusion
• Multicellular organisms


  • Have a small surface area to volume ratio
  • Cannot get substances by diffusion as there is less space for diffusion to take place
  • Have a large diffusion pathway
  • Need mass transport systems (e.g. the circulatory system) and exchange organs (the lungs/intestine)
• Adaptations to control body temperature

  • Organisms with smaller surface area to volume ratios lose less heat
  • Smaller organisms have higher metabolic rates to generate more heat energy
  • Organisms adapted to hot climates have features to increase surface area
  • Organisms with high surface area to volume ratio tend to lose more water by evaporation
  • Animals adapted to cold climates have thick layers of insulating fat and fur, and may hibernate
• Adaptations of exchange surfaces

  • Extremely thin (sometimes just one cell thick)
  • Large surface area to volume ratio
  • Features to maximise the concentration gradient of gases
• Gas exchange in fish

  1. Water (containing oxygen) enters through mouth and passes over gills

  2. Gill filaments are covered in lamellae to increase surface area

  3. Lamellae have thin walls to reduce diffusion distance

  4. Filled with capillaries

  5. Blood in capillaries flows in opposite direction to water (counter-current system)
• Gas exchange in insects

  1. Air enters through pores called spiracles

  2. Air moves down trachea which branches into tracheoles

  3. Tracheole walls are thin and porous to speed up diffusion

  4. Rhythmic abdominal movements push air in and out of spiracles
• Gas exchange in plants

  1. Gases exchange through stomata

  2. Guard cells control opening and closing of stomata

  3. Air spaces in spongy mesophyll layer allow gas circulation

  4. Oxygen and carbon dioxide diffuse from air spaces into plant cells
• Adaptations to prevent water loss in insects

  • Closing spiracles when dehydrated
  • Spiracles surrounded by hairs to trap water vapour
  • Covered by waterproof, waxy cuticle
• Adaptations to prevent water loss in plants

  • Closing stomata at night
  • Sunken stomata
  • Hairs around stomata
  • Curled leaves
  • Fewer stomata
  • Thicker cuticle
• Respiratory system


  1. Air enters through nose and mouth

  2. Travels down trachea

  3. Trachea branches into bronchi

  4. Bronchi divide into bronchioles

  5. Bronchioles lead to alveoli

  6. Oxygen diffuses from alveoli to bloodstream

  7. Carbon dioxide diffuses from bloodstream to alveoli
• Inhalation


  1. Diaphragm contracts and flattens

  2. Rib cage moves up and out

  3. Increases volume of thorax

  4. Reduces pressure

  5. Air sucked into lungs
• Exhalation


  1. Diaphragm relaxes and forms dome shape

  2. Rib cage moves down and in

  3. Reduces volume of thorax

  4. Increases pressure

  5. Air pushed out of lungs
• Adaptations of alveoli


  • Large surface area
  • Good blood supply
  • Short diffusion distance
  • Moist surfaces
  • Steep concentration gradient
• Pathway of oxygen molecule

  1. Passes through alveolar epithelium

  2. Passes through capillary endothelium

  3. Binds to haemoglobin in red blood cell
• Investigating lung function


  1. Using a spirometer

  2. Measures tidal volume

  3. Measures forced vital capacity (FVC)

  4. Can calculate breathing rate, oxygen consumption, forced expiratory volume (FEV1)
• Lung diseases like TB, fibrosis, emphysema and asthma involve impaired gas exchange
• Tuberculosis


  • Caused by Mycobacterium tuberculosis bacteria
  • Immune cells build wall around bacteria forming tubercles
  • Death of tissue and fibrosis reduces tidal volume
  • Ventilation rate increases to compensate
• Fibrosis


  • Formation of scar tissue
  • Scar tissue is thicker and less elastic
  • Reduces tidal volume and FVC
  • Increases diffusion distance, reducing gas exchange efficiency
  • Ventilation rate increases to compensate
• Emphysema


  • Smoking/pollution causes inflammation of alveoli
  • Elastase enzyme digests elastin, alveoli lose elasticity
  • Alveoli cannot recoil properly to push air out
  • Degradation of alveoli walls reduces gas exchange efficiency
  • Ventilation rate increases to compensate
• Asthma


  • Inflammation of airways due to allergic reaction
  • Contraction of smooth muscle constricts airways, reducing FEV1
  • Drugs can relax smooth muscle to relieve symptoms
• Digestion


  Breaking down large food molecules into smaller molecules that can be absorbed by cells
• Digestion process


  1. Large food molecules cannot fit through cell membranes

  2. Digestive enzymes break bonds in macromolecules

  3. Hydrolysis reaction uses water to break bonds

  4. Monomers transported into ileum and diffuse into bloodstream
• Digestion and absorption of carbohydrates

  1. Amylase breaks down starch into maltose

  2. Disaccharidases break down disaccharides into monosaccharides

  3. Monosaccharides absorbed by epithelial cells
• Digestion and absorption of lipids

  1. Lipase hydrolyses lipids into monoglycerides and fatty acids

  2. Bile emulsifies lipids

  3. Micelles transport digested lipids to epithelium

  4. Monoglycerides and fatty acids diffuse through epithelium
• Digestion and absorption of proteins

  1. Proteases break down proteins into amino acids

  2. Endopeptidases break peptide bonds in middle of chain

  3. Exopeptidases break peptide bonds at ends of chain

  4. Amino acids absorbed by cotransport with sodium
• Haemoglobin


  Protein in red blood cells that transports oxygen by binding to it
• Haemoglobin


  • Quaternary structure with 4 polypeptide chains
  • Each chain has a haem group containing iron that binds oxygen
• Haemoglobin saturation and dissociation curves

  Show how much oxygen haemoglobin binds at different oxygen partial pressures
• Higher CO2 concentration


  Lowers haemoglobin's affinity for oxygen (Bohr effect)
• Mass transport systems


  Deliver oxygen and glucose, remove waste, in multicellular organisms
• Arteries


  • Thick muscular walls with elastic tissue
  • Folded endothelium
  • Small lumen for high pressure
• Veins


  • Large lumen and thin walls
  • Valves prevent backflow
  • Muscle contraction helps flow
• Capillaries


  • Connect arteries and veins
  • Pores allow exchange of substances
  • One cell thick walls
  • Large surface area of capillary beds
• Tissue fluid


  Fluid that bathes cells, made of substances that move out of capillaries
• Formation of tissue fluid

  1. High hydrostatic pressure forces fluid out at arteriole end

  2. Low water potential draws fluid back in at venule end

  3. Excess drained by lymphatic system
• Heart structure


  Four chambers divided into two sides
• Tissue fluid


  Smaller substances pushed out of the capillary bed
• Formation of tissue fluid

  1. At arteriole end, hydrostatic pressure in capillary exceeds tissue fluid pressure, forcing fluid and dissolved substances out

  2. At venule end, less water in capillary due to movement out, so water moves back in by osmosis

  3. Excess tissue fluid drains into lymphatic system and returns to bloodstream
• Heart


  • Made up of four chambers divided into two sides
  • Left side has thicker wall to pump blood around body, right side just pumps to lungs
  • Left side carries oxygenated blood, right side carries deoxygenated blood