|Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; firstname.lastname@example.org; http://www.employees.csbsju.edu/ssaupe/|
Gas Exchange in Plants & Animals
I. The importance of gas exchange
Recall the equation for photosynthesis & respiration:
(CH2O) n + O2 �� CO2 + H2O
II. Primer on molecular movement
A. Diffusion vs. Bulk Flow
B. Fick's Law
Take-home-lessons: This equation tells us that for a given molecule diffusion rate is:
III. Biological Implications of Fick's Law:
A large surface area (A) for gas exchange is required
There are various solutions to this problem. The key feature is increase the total surface area for gas exchange (oxygen uptake, carbon dioxide loss). This is another good example of surface-to-volume ratios. As we learned, to increase surface area for a particular volume, a filament or flat are the best shapes to be � so, how do organisms accomplish this:
IV. Biological Implications of Fick's Law:
There must be a short diffusion distance (D)
There must be a short diffusion distance between the environment and inside the organism. Fick's Law tells us that diffusion rate is inversely related to distance � the greater the distance, the slower the rate of diffusion. In fact, diffusion is painfully slow over long distances. But, how much slower? As an example, the time it would take glucose to cross a typical membrane is 2.5 seconds. However, the time it would take glucose to diffuse one meter is 32 years!
Take Home Lesson: No individual gas absorbing surface is more than a few cells thick (e.g., gills, lungs, sea cucumbers, hydra � tubes with a central cavity bathed in fluid, sponges � lots of chambers; leaves are flat, thin)
V. Biological Implications of Fick's Law:
A Large Surface Area Provides a Large Area for Desiccation
A paradox: in order to exchange gases for metabolism, animals and plants need a large surface area, BUT, that means that the area through which water is lost is also increased. This problem is exacerbated by the diffusion tendencies of water, carbon dioxide, and oxygen. Diffusion is inversely related to molecular weight � this makes senses, football lineman can�t run as fast as a wide receiver. Thus, the tendency of water [MW = 18] to evaporate from a plant is much greater than the tendency of carbon dioxide [MW = 44] to diffuse into the leaf for photosynthesis.
VI. Biological Implications of Fick's Law:
There must be a way to get gases to the absorbing surface.
This can be a particular concern in species that stick the gas absorbing surfaces deep within the organism. Remember that the two major ways of moving any molecules, including gases, are diffusion and bulk flow. Ultimately organisms use one or a combination of these methods to get the gases to the absorbing surface.
Diffusion is the primarily method of moving gas to the absorbing tissue in plants and some animals. Similarly, oxygen reaches the lungs of slugs and spiders, and enters the tracheae of insects via diffusion.
Bulk flow may play a role in some of these cases. For example, studies on hawk moths have shown that when wing muscles beat that alternately contract the tracheae which will cause pressure changes resulting in the increased uptake of gas by bulk flow. Gas moves through some plants via bulk flow, also. Imagine a steady wind blowing across the upper surface of a leaf; this will essentially generate a slight negative pressure compare to the lower side of the leaf resulting in gas molecules being �sucked� through the leaf.
Bulk Flow Methods
Most organisms rely on bulk flow for moving gases to their respiratory surfaces. These mechanisms are �active� and require cellular energy (ATP). There are several methods:
fish move water across gills; in mouth � out gill
Some fish (tuna) that swim fast and have large energy requirements swim with mouth open to force water across gills.
2. Positive Pressure breathing
Frogs - push air down throat, lower throat - air enters - raise up to push down throat
3. Negative Pressure Breathing
diaphragm expands � chest cavity volume increases � pressure drops � air enters by bulk flow due to pressure gradient (difference between inside and outside)
controlled by pH � medullary respiratory center; acidic pH stimulates
4. Additional Concerns: Keeping the lung inflated (maintaining pressure)
Lungs in chest cavity, surrounded by two linings (pleura) � one around lung, other lines chest cavity
Negative pressure (-5 mm Hg) in pleural space; therefore normally little space between two layers
Pressure in lung greater than chest cavity; therefore � lung inflated
Sucking chest wound � damage to outer layer, air enters, pressure increases � lung collapses
"Growth spurt� � most common in young males, tall & thin � vesicle from lung blebs into cavity, punctures lining � pressure change � collapse lung
5. Birds � require huge oxygen demands, especially during flight. Respiratory system more efficient because: (a) less dead space with shorter trachea; (b) gas exchange occurs during inhalation and exhalation (due to flow through system that is unidirectional and continuous. Works because chest and air sac don�t compress at same time. Similar to a bagpipe; (c) blood flows perpendicular to lung cells and is more efficient that capillary web around animals
6. Additional concerns:
Protect the surface from dust and other particles � mucus
What enables the lung to expand and contract during breathing(elastic)? Answer: the surface tension of water. Pressure of breathing expands lung � surface tension contracts and returns to original position. Mechanism is similar to formation of a raindrop. But, there is a problem � the surface tension of the lung would be theoretically so large that the lung wouldn�t be able to expand. Solution: the lung excretes surfactants that help to cut the surface tension some � or else be too hard to breathe. Problem with premature births � surfactants aren�t produced until about 32-34 weeks. Lungs too rigid = respiratory distress syndrome.
VII. Biological Implications of Fick's Law:
Mechanism to maintain a large concentration (ΔC)
Organisms have evolved to maximize the concentration gradient from inside to outside the absorbing surface that maximizes diffusion rates.
A. General � Solubility
B. Partial Pressure Primer
C. Counter-current mechanisms � Gills
D. Ventilation � mechanism to bring fresh, high concentration air to absorbing surface
A. Oxygen low solubility in water
C. Saturation curve - oxygen saturation (%) vs. PO2 (mm Hg)
D. Factors that affect oxygen/hemoglobin binding
IX. Carbon Dioxide Transport
Table 1. Forms in which carbon dioxide is transported
Dissolved in plasma (as CO2)
7 - 8
Bound to hemoglobin
Bicarbonate in plasma
B. Carbon dioxide and water
C. Carbonic anhydrase - catalyzes formation of bicarbonate, fast enzyme, reversible; changes partial pressure of carbon dioxide to load/unload from blood stream
X. Gas Exchange in Plants
Gas exchange occurs primarily through stomata (pores in leaf) that are opened/closed by specialized epidermal cells called guard cells. Some gas exchange exchange occurs through other structures (e.g., lenticels in bark).
B. Guard cell structure
Guard cell Function
Guard cells open the stoma because of the osmotic entry of entry of water into the GC. In turn, this increases the turgidity (water pressure) in the GC and causes them to elongate. The radial orientation of cellulose microfibrils prevents increase in girth. Since GC are attached at the ends and because the inner wall is thicker, the guard cells belly out with the outer wall moving more and pulling open the guard cell. Guard cell closure essential involves reversing this process.
Water entry into the guard cells is controlled by increasing the solute concentration (osmotic concentration) in the guard cells. This occurs by: (a) transporting potassium (and chloride) ions into the guard cells from surrounding (subsidiary) cells. This process is mediated by a proton pump; and (b) by sugars produced during photosynthesis or from starch breakdown (recall that guard cells are the only epidermal cells with chloroplasts). Thus, we can summarize the mechanics of GC action as follows:
stoma closed (GC flaccid) → add solute → lower water potential → water uptake (osmosis) → increase pressure → stoma open (GC turgid)
Environmental Control of GC Action
As a consequence of needing to keep open the stomata for photosynthesis, plants loose water (transpiration). Thus, water loss is a �necessary evil� of photosynthesis. However, plants tightly regulate the two processes � or in other words, they must compromise between the amount of photosynthesis and the amount of transpiration. This is called the photosynthesis/transpiration compromise. Guard cells control this compromise.
Guard cells are very responsive to their environment, especially any factors that impact the photosynthesis/transpiration compromise. Thus, we expect any factor important in photosynthesis to exert regulatory control on GC. And, we also expect water, a major player in photosynthesis, to have the final word on control since if a plant dries out too much it's as good as dead! Controls of GC action:
D. Mechanism of Guard Cell Action. discussed in class
Last updated: February 01, 2008 � Copyright by SG Saupe