3.3 Organisms exchange substances w/ their environment

• Risk factor


  An aspect of a person's lifestyle or substances in a person's body / environment that have been shown to be linked to an increased rate of disease
• Risk factors for cardiovascular disease

  • Age
  • Diet high in salt or saturated fat
  • Smoking
  • Lack of exercise
  • Genes
• The principles of analysis, interpretation and evaluation of data covered in '3.2 gas exchange' also apply here.
• Oxygen binds to haemoglobin.

  
• Oxygen is needed for aerobic respiration.
• It is the organism's haemoglobin that has a high / low affinity for oxygen, not the organism itself.

  
• Binding of the first oxygen changes the tertiary / quaternary structure of haemoglobin.

  
• Valves


  Open and close to prevent backflow of blood
• Muscle contraction in arteriole walls

  Narrows the lumen of arterioles, reducing blood flow to capillaries
• Pressure drops from arteries, to arterioles, to capillaries, to venules, to veins. The vena cava has the lowest blood pressure.
• Water (and some dissolved substances) are forced out of capillaries, resulting in tissue fluid.
• Precautions when performing a dissection

  • Cover any cuts with a waterproof dressing
  • When using a scalpel, cut away from body onto a hard surface
  • When using a scalpel, use a sharp blade
  • When using a scalpel, carry with blade protected / pointing down
  • Wear disposable gloves and disinfect hands / wash with soap
  • Disinfect surfaces / equipment
  • Safe disposal - put gloves / paper towels / organ in a separate bag / bin to dispose
  • If poisonous chemicals / toxins involved, work in a well ventilated environment
• Ethical consideration when dissecting animals

  Morally wrong to kill animals just for dissection, so use animals for dissection that have already been killed (humanely) for meat
• Preparing a temporary mount of a piece of plant tissue for observation with an optical microscope

  1. Add a drop of water to glass slide

  2. Obtain a thin section of specimen and place on slide

  3. Stain (eg. with iodine / potassium iodide to view starch)

  4. Lower coverslip at angle using mounted needle without trapping air bubbles
• Rules of scientific drawing

  • Look similar to specimen / image, draw all parts to same scale / relative size
  • No sketching / shading - only clear, continuous lines
  • Include a magnification scale (eg. x 400)
  • Label with straight, uncrossed lines
• Xylem tissue

  Transports water (and mineral ions) through the stem, up the plant to leaves of plants

  
• Xylem tissue

  • Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
  • Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
  • Thick cell walls with lignin → provides support / withstand tension / prevents water loss
  • Pits in side walls → allow lateral water movements
• Cohesion-tension theory of water transport in the xylem

  1. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

  2. Reducing water potential of mesophyll cells

  3. So water drawn out of xylem down a water potential gradient

  4. Creating tension ('negative pressure' or 'pull') in xylem

  5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column

  6. Water also adheres (sticks to) to walls of xylem

  7. Water enters roots via osmosis

  
• Setting up a potometer

  1. Cut a shoot underwater at a slant → prevent air entering xylem

  2. Assemble potometer with capillary tube end submerged in a beaker of water

  3. Insert shoot underwater

  4. Ensure apparatus is watertight / airtight

  5. Dry leaves and allow time for shoot to acclimatise

  6. Shut tap to reservoir

  7. Form an air bubble - quickly remove end of capillary tube from water

  
• Using a potometer to measure the rate of transpiration

  1. Record position of air bubble

  2. Record distance moved in a certain amount of time (eg. 1 minute)

  3. Calculate volume of water uptake in a given time: Use radius of capillary tube to calculate cross-sectional area of water (πr^2), Multiply this by distance moved by bubble

  4. Calculate rate of water uptake - divide volume by time taken

  
• Using a potometer to investigate the effect of a named environmental variable on the rate of transpiration

  1. Carry out the above, change one variable at a time (wind, humidity, light or temperature)

  2. Eg. set up a fan OR spray water in a plastic bag and wrap around the plant OR change distance of a light source OR change temperature of room

  3. Keep all other variables constant

  
• Rate of water uptake might not be same as rate of transpiration
• Rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
• How different environmental variables affect transpiration rate

  • Light intensity increases rate of transpiration
  • Temperature increases rate of transpiration
  • Wind intensity increases rate of transpiration
  • Humidity decreases rate of transpiration
• Phloem tissue


  Transports organic substances eg. sucrose in plants
• Phloem tissue


  • Sieve tube elements: No nucleus / few organelles → maximise space for / easier flow of organic substances, End walls between cells perforated (sieve plate)
  • Companion cells: Many mitochondria → high rate of respiration to make ATP for active transport of solutes
• Translocation


  Movement of assimilates / solutes such as sucrose from source cells (where made, eg. leaves) to sink cells (where used / stored, eg. roots) by mass flow
• Mass flow hypothesis for translocation in plants

  1. At source, sucrose is actively transported into phloem sieve tubes / cells by companion cells

  2. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis

  3. This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient

  4. So mass flow occurs - movement from source to sink

  5. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
• Using tracer experiments to investigate transport in plants

  1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C

  2. Radioactive carbon incorporated into organic substances during photosynthesis

  3. These move around plant by translocation

  4. Movement tracked using autoradiography or a Geiger counter

  
• Using ringing experiments to investigate transport in plants

  1. Remove / kill phloem eg. remove a ring of bark

  2. Bulge forms on source side of ring

  3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem

  4. Tissues below ring die as cannot get organic substances
• Transpiration is the loss of water vapour from leaves. The transpiration stream is the constant movement of water through the plant.
• Leaves lose water by transpiration.
• The mass flow hypothesis involves phloem and sucrose whereas the cohesion-tension hypothesis involves xylem and water.

  
• Sucrose moves by mass flow down a hydrostatic pressure gradient.
• Sucrose moves by active transport or facilitated diffusion into sink cells, not by simple diffusion.
• Gas exchange in lungs

  1. Oxygen diffuses from alveolar air space into blood down its concentration gradient,
  2. Across alveolar epithelium then across capillary endothelium