Biology Topic 4- exchange and transport

• Single-celled organisms


  • Do not have specialised transport systems
  • Substances can enter the cell by passive transport as the diffusion distances are short
• Multicellular organisms


  • Require specialised transport systems and gas exchange surfaces
  • Diffusion distance is greater
  • Metabolic rate is higher
  • Surface area : volume ratio is smaller
• Mass transport system


  • Vessels
  • Directional movement
  • Transport medium
  • Maintenance of speed
• Efficient exchange surface


  • Large surface area
  • Thin
  • Steep concentration gradient
• Cell membrane


  • Partially permeable
  • Composed of a sea of phospholipids with protein molecules between
  • Contains transport proteins, receptor proteins, enzymes, structural and recognition proteins
  • Main function is controlling the movement of substances in and out of the cell/organelle
  • Contains receptors for other molecules such as hormones
  • Enables adjacent cells to stick together
  • Fluid mosaic model
• Movement of molecules through cell membranes


  1. Diffusion

  2. Facilitated diffusion

  3. Osmosis

  4. Active transport

  5. Endocytosis/Exocytosis
• Diffusion


  Passive movement of small, non-polar, lipid-soluble molecules from an area of high concentration to an area of low concentration
• Factors affecting rate of gas exchange by diffusion


  • Surface area increases
  • Diffusion distance decreases
  • Diffusion gradient becomes steeper
• Osmosis


  Movement of water molecules from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane
• Turgor pressure


  • Inward pressure exerted by the plant cell wall on the protoplasm as the protoplasm expands and pushes out
  • Generated because water moves in by osmosis, causing the protoplasm to swell and push against the cell wall, generating hydrostatic pressure
  • Prevents water moving into a cell
• In plants, water potential (ψ) = osmotic pressure (π) + turgor pressure (P)
• Mammalian gas exchange system


  • Lungs
  • Boyle's Law (volume inversely proportional to pressure)
  • Inhalation by contraction of intercostal muscles and diaphragm
  • Exhalation by relaxation of intercostal muscles and diaphragm
  • Oxygen moves into capillaries from alveoli via diffusion
  • Alveoli provide large surface area and short diffusion distance
• Insect gas exchange system


  • Spiracles (openings) that can be opened and closed by sphincters
  • Trachea (tubes) lined with rings of chitin to prevent collapse
  • Tracheoles (small tubes) where gas exchange occurs
  • Active insects remove water buildup in tracheoles
  • Some active insects ventilate respiratory system mechanically and/or use air reserves
• Fish gas exchange system


  • Use Boyle's Law to continuously pump water over gills
  • Gills made of filaments covered in folds called lamellae
  • Continuous water movement keeps gills spread out
  • Mouth floor opens, operculum (gill flap) closes, mouth floor raised to increase pressure and force water over gills
  • Countercurrent exchange system maintains maximum concentration gradient between water and blood
• Plant leaf structure


  • Waxy cuticle
  • Upper epidermis
  • Palisade mesophyll layer
  • Spongy mesophyll layer
  • Lower epidermis, guard cells, stomata
• Stomata opening and closing


  1. Ions (mainly K+) move into guard cells by active transport

  2. Water moves in by osmosis due to decreased water potential

  3. Guard cells swell and stomata open
• Lenticels


  Areas of loosely arranged cells that act as pores to allow gas exchange in lignified (woody) plants
• Stomata opening


  1. Ions (mainly K+) move into the guard cells by active transport

  2. Water moves in by osmosis because water potential is decreased

  3. Guard cell becomes turgid

  4. Guard cell swells

  5. Stomata opens
• Lenticels


  Areas of loosely arranged cells which act as a pore to allow gas exchange in lignified (woody) plants
• Structure of the heart


  • Four chambers - right and left atria, right and left ventricles
  • Four main blood vessels - pulmonary vein, aorta, vena cava, pulmonary artery
  • Atrioventricular valves - mitral or tricuspid/bicuspid - prevent backflow of blood from ventricles to atria
  • Semilunar valves - pulmonary/aortic - separate arteries from ventricles
  • Tendinous chords/valve tendons - prevent atrioventricular valves turning inside out
  • Septum - muscle and connective tissue - prevents oxygenated/deoxygenated blood mixing
  • Coronary arteries - wrapped around the heart to supply blood to cardiac muscle
  • Cardiac muscle - thicker on the LHS because higher pressure is needed

  
• Circulatory systems


  • Can be open or closed, where the blood is confined to blood vessels only
  • Can be single, where the blood is only pumped once around the whole system, or double, where the blood is pumped twice

  
• Advantages of a double circulatory system

  • Concentration gradient is maintained, as oxygenated and deoxygenated blood do not mix
  • Blood pressure to the body tissues is higher
  • Blood pressure to the lungs is lower, which avoids damaging the capillaries in the lungs and increases time for gas exchange
  • Organisms can develop larger bodies
• Myogenic


  The heart's ability to initiate its own contraction
• Cardiac cycle


  1. Atrial systole - atria contract, atrioventricular valves open, blood flows into ventricles

  2. Ventricular systole - ventricles contract, atrioventricular valves close, semilunar valves open, blood leaves ventricles

  3. Cardiac diastole - atria and ventricles relax, pressure inside heart decreases, semilunar valves close

  
• Functions of blood


  • Transport
  • Defence against pathogens
  • Formation of lymph and tissue fluid
• Components of blood


  • Plasma
  • Erythrocytes (red blood cells)
  • Leukocytes (white blood cells)
  • Platelets
• Thrombosis


  Blood clotting, prevents blood loss when a blood vessel is damaged, prevents the entry of disease-causing microorganisms, and provides a framework for repair

  
• Blood clotting process


  1. Platelets attach to exposed collagen fibres

  2. Thromboplastin is released from platelets and triggers conversion of inactive prothrombin to active thrombin

  3. Thrombin catalyses conversion of soluble fibrinogen into insoluble fibrin

  4. Fibrin forms a network trapping platelets, red blood cells and debris to form a blood clot
• Atherosclerosis


  Hardening of arteries caused by the build-up of fibrous plaque called an atheroma

  
• Atheroma formation


  1. Endothelium lining the arteries is damaged

  2. Increases risk of blood clotting and leads to inflammatory response, causing white blood cells to move to the site

  3. Over time, white blood cells, cholesterol, calcium salts and fibres build up and harden, leading to plaque (atheroma) formation

  4. Build-up of fibrous plaque leads to narrowing of the artery and restricts blood flow, increasing blood pressure which damages the endothelial lining

  
• Risk factors for atherosclerosis


  • Genetics
  • Age
  • Diet
  • Gender
  • High blood pressure
  • High cholesterol levels
  • Smoking
  • Physical inactivity
  • Obesity
• Haemoglobin


  Water soluble globular protein which carries oxygen in the blood, as oxygen can bind to the haem (Fe2+) group
• Oxygen affinity of haemoglobin

  Increases as partial pressure of oxygen increases, and decreases as partial pressure of oxygen decreases
• Dissociation curves


  Illustrate the change in haemoglobin saturation as partial pressure changes, have a sigmoid shape due to cooperative binding of oxygen to haemoglobin
• Foetal haemoglobin


  Has a higher affinity for oxygen compared to adult haemoglobin, important for countercurrent exchange system in the placenta
• Myoglobin


  Respiratory pigment used for oxygen storage, has a higher affinity for oxygen than haemoglobin
• Bohr effect


  In the presence of carbon dioxide, the affinity of haemoglobin for oxygen decreases, causing oxygen to dissociate and be used in respiring tissues
• Countercurrent exchange system


  Important because maternal and foetal blood run through the placenta - the difference in affinity is needed so that when oxygen dissociates from maternal haemoglobin it can bind to foetal haemoglobin
• Myoglobin


  Another respiratory pigment that is used for storage, has a higher affinity for oxygen than haemoglobin and acts as a storage molecule for oxygen, only made up of one subunit
• Affinity of haemoglobin for oxygen


  Affected by the partial pressure of carbon dioxide, in the presence of carbon dioxide the affinity decreases, causing oxygen to dissociate from haemoglobin and be used in respiring tissues - this is known as the Bohr effect