2.3 Transport

• Circulatory systems


  • Open (e.g. insects)
  • Closed (e.g. fish and mammals where blood is confined to blood vessels only)
• Closed circulatory systems


  • Single (heart with two chambers, blood passes through heart once per circuit)
  • Double (heart with four chambers, blood passes through heart twice per circuit)
• Arteries


  Adapted to carry blood away from heart, thick walled, contain elastic tissue and smooth muscle, lined with smooth endothelium
• Arterioles


  Branch off arteries, have thinner and less muscular walls, feed blood into capillaries
• Capillaries


  Smallest blood vessels, site of metabolic exchange, only one cell thick
• Venules


  Larger than capillaries but smaller than veins
• Veins


  Carry blood from body to heart, contain wide lumen, thin walled, contain valves, little elastic tissue or smooth muscle
• Tissue fluid


  Liquid containing dissolved oxygen and nutrients, enables exchange of substances between blood and cells
• Hydrostatic pressure


  Pressure created when blood is pumped along arteries, forces blood fluid out of capillaries
• Water potential


  Tissue fluid has less negative water potential than blood, so water moves from tissue fluid to blood by osmosis
• Lymphatic system


  Carries lymph fluid, similar to tissue fluid but contains less oxygen and nutrients, filters out bacteria and foreign material
• Myogenic


  Heart can initiate its own contraction
• Sinoatrial node


  Pacemaker of the heart, initiates wave of electrical stimulation causing atria to contract
• Atrioventricular node


  Passes electrical excitation from atria to ventricles
• Cardiac cycle


  1. Atrial systole (atria contract, AV valves open, blood flows into ventricles)

  2. Ventricular systole (ventricles contract, AV valves close, semilunar valves open, blood leaves ventricles)

  3. Cardiac diastole (atria and ventricles relax, semilunar valves close, blood drawn from arteries and veins)
• Haemoglobin


  Water soluble globular protein with two beta polypeptide chains and a haem group, carries oxygen in the blood
• Oxygen affinity of haemoglobin


  Increases with higher partial pressure of oxygen (in lungs), decreases with lower partial pressure (in respiring tissues)
• Dissociation curves


  Illustrate change in haemoglobin saturation as partial pressure changes
• Fetal haemoglobin


  Has higher affinity for oxygen than adult haemoglobin, to absorb oxygen at low partial pressures in placenta
• Bohr effect


  Affinity of haemoglobin for oxygen decreases in presence of carbon dioxide released by respiring cells
• Vascular bundle in roots


  • Xylem vessels arranged in X shape for mechanical support
  • Surrounded by endodermis which supplies xylem with water
  • Inner layer of pericycle meristem cells
• Vascular bundle in stems


  • Xylem on inside in non-woody plants for support and flexibility
  • Phloem on outside
  • Layer of cambium meristem cells between xylem and phloem
• Vascular bundle in leaves


  • Forms midrib and veins
  • Dicotyledonous leaves have network of veins for transport and support
• Xylem vessels


  Transport water and minerals, provide structural support, made of dead tissue with open ends and lignin-thickened walls
• Phloem vessels


  Tubes made of living cells, involved in translocation of nutrients to storage and growing parts
• Meristem cells


  Cells located in between xylem and phloem, involved in the production of new xylem and phloem tissue
• Vascular bundle in the leaf


  • Form the midrib and veins of a leaf
  • Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards, involved in transport and support
• Xylem vessels


  • Transport water and minerals, provide structural support
  • Long cylinders made of dead tissue with open ends, can form a continuous column
  • Contain pits which enable water to move sideways between the vessels
  • Thickened with lignin in spiral patterns to enable the plant to remain flexible
• Phloem vessels


  • Tubes made of living cells
  • Involved in translocation, the movement of nutrients to storage organs and growing parts of the plant
  • Consist of sieve tube elements and companion cells
  • Sieve tube elements form a tube to transport sugars such as sucrose
  • Companion cells are involved in ATP production for active processes such as loading sucrose into sieve tubes
  • Cytoplasm of sieve tube elements and companion cells is linked through plasmodesmata
• Transpiration


  1. Plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves

  2. Carbon dioxide enters, while water and oxygen exit through a leaf's stomata
• Transpiration stream


  The movement of water up the stem that enables processes such as photosynthesis, growth and elongation, and supplies the plant with required minerals while enabling temperature control
• Transpiration


  1. Involves osmosis, where water moves from the xylem to the mesophyll cells

  2. Involves evaporation from the surface of mesophyll cells into intercellular spaces and diffusion of water vapour down a water vapour potential gradient out of the stomata
• Potometer


  Device used to investigate the rate of transpiration by measuring the movement of the meniscus as water lost by the leaf is replaced
• Factors affecting rate of transpiration


  • Number of leaves
  • Number/size or position of stomata
  • Presence of waxy cuticle
  • Amount of light
  • Temperature
  • Humidity
  • Air movement
  • Water availability
• Xerophytes


  • Plants adapted to living in dry conditions
  • Have smaller leaves to reduce surface area for water loss
  • Have densely packed mesophyll and thick waxy cuticle to prevent water loss via evaporation
  • Respond to low water availability by closing stomata to prevent water loss
  • Contain hairs and pits to trap moist air, reducing water vapour potential
  • Roll leaves to reduce exposure of lower epidermis to atmosphere, trapping air
• Water movement in the root

  1. Water enters through root hair cells and moves into the xylem tissue in the centre of the root

  2. Movement occurs due to a water potential gradient, as water potential is higher in the soil than inside the root hair cells

  3. Root hair cells provide a large surface area for water movement

  4. Minerals are absorbed through root hair cells by active transport
• Symplast pathway


  Water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata
• Apoplast pathway


  Water moves through the water-filled spaces between cellulose molecules in the cell walls, without passing through any plasma membranes
• Casparian strip


  A layer of suberin in the endodermis that cannot be penetrated by water, forcing water to enter the symplast pathway to cross the endodermis
• Water movement in the xylem up the stem


  1. Water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient

  2. Root pressure, where the endodermis moves minerals into the xylem by active transport, drives water into the xylem by osmosis, pushing it upwards

  3. Surface tension and cohesion of water molecules (tension-cohesion theory) and capillary action maintain the flow of water