BIO 104 Handout: Little Green Man

This is the handout that accompanied the Wed. April 24th Biology 104 lecture

     A visit from the little green humanoid from Mars.*

A) The green humanoid comes down to earth - what does the green humanoid
   actually look like?
   a)  Green = chlorophyll = autotroph = photosynthesis.         
   b)  Can he (she/it) actually support energy needs by photosynthesis?
       For human energy needs it is - 2000 cal/day for maintenance (not growth)

     For reference: The surface area of a human - 6' high, 180 lbs = 2 m^2

B) What do green plants yield in photosynthesis? (Gain in dry weight = photosynthetic product)
     1. Green plants fix 0.5 To 2.0 G/m^2 of leaf/hr in full sun
     2. Assume a 12 hr full sun day - yield = 24 g/m^2/day

C) A 2 m^2 photosynthesizing human would yield 48 grams of photosynthetic
   product/day. Assume 180 grams of glucose = 673 calories.  Therefore,
   48 grams of glucose = ~ 180 cal/day

Problem:  photosynthesis then would serve only 10% of daily needs.

Solution: increase surface area 10-fold!

D) still not enough photosynthetic product.  Why? Because:   
      Area increases as diameter^2 - but - volume increases as diameter^3

      and...   energy requirement is largely a volume function.

Problem:  We need a maximum area increase, with a minimum volume increase

Solution: Thin flat sheets to increase total surface to 20 m^2 (10 x 2 m^2)

E) Problem: still not making enough. Why? Raw material requirements:
   a)  CO2 intake
     1)  a) 673 cal = 1 mole glucose (180 g)  = 6 moles CO2/180 g glucose
         b) 2,000 cal = 3 moles glucose       = 18 mols CO2

     2)  a) 1 mole CO2 = 22.4 L pure CO2
              b) 18 mole CO2 = 400 L pure CO2  but
         c) air = 0.03% CO2
         d) 400 L pure CO2 = 400/.03 = 13,000 L of air needed
         e) a need of 13,000 l of air!

The average daily volume of air inhaled by a human is 15,000 L over 24 hours

Remember that the CO2 required by "green humanoid" is in light (12 hrs)

If the humanoid were to inhale 13,000 L air in 12 hrs - it would need to breathe
twice as fast.  This would increase workload which would increase calorie requirements.

Solution:  perforate the thin plates, and let CO2 diffuse in directly from air. STOMA!

F) new problem: H20 diffuses through the holes and evaporates from the surface

   solution:  waterproof the skin to prevent evaporation from surface (CUTICLE!)


G) new problem:
     Water still diffuses through the open stoma.
     Water loss is 50-250 ml/m^2/hr.
     The Green humanoid with 20 m^2 of surface area would lose 1-5 L H20/hr
     in the sun. Therefore, he would need 120 L of H20/day

 solution: stay near water or sink pipes to the water supply in the soil = roots

H) new problem: The transport of H20 requires a "lift" of 20-120 L of H20/day -
           inordinate work load for a heart.

  solution: discard "heart" & use a passive transport system - osmosis,
  evaporation, adhesion, cohesion, transpiration, xylem - to transport water. 

Therefore:

A tree is a tree because that is the only structural functional system which
will permit an autotroph to become large and survive.  The observed nature of
a tree is not "accidental" or one out of a thousand possible ways a tree to be.
Any major deviations tried by a large autotroph will be eliminated by death or
Competition.

*Based on a lecture by J.A. Chiscon - Purdue University