Lecture 2: Circulation and Gas Exchange I (Chapter 42)
Keywords
cellular respiration
Diffusion of gases
Speed of diffusion
Effect of size on oxygen supply
Gas exchange structures
Gills, lungs
Gastrovascular cavity
Surface area
Themes that we’ll often come across:
Organisms have similar functional needs, but have developed diverse ways of meeting them
Organisms must obey physical laws
Understanding how an organism works involves biochemistry, cell biology, physiology, ecology and evolution
Cellular respiration
A type of controlled combustion:
Reduced carbon (e.g., glucose) + O2
-------> CO2 + H2O
Organismal respiration -- a simple view
The simple view is true for animals, but there are differences
Need to consider the problem of how gases get into and out of an animal
Differences are observed in types of respiratory surfaces
This will be the focus of today’s lecture
Major causes for these differences
:
Being big vs. small
Breathing air vs. water
Why does size matter in respiration?
Rate of diffusion of gases (e.g., oxygen)
How fast is diffusion of oxygen?
1 micron (µm) in 10-4 seconds
One millionth of a meter in one tenth of a millisecond
How does diffusion work?
Consider a point source of a diffusing substance
Each molecule will travel randomly (brownian motion)
Over time particles will become separated
But particles don’t just move away from the original point source
They are travelling randomly
Thus...
It takes a long time for molecules to diffuse over long distances
"Speed" of oxygen diffusion in liquid
1 µm in 10-4 seconds
1000 µm (1 mm) in 100 seconds
Thus diffusion can supply oxygen only over very short distances
Examples where oxygen diffuses only short distances
Vertebrate lung
Very small organisms
Vertebrate Lung
Very Small Organisms
Rate of oxygen entry is influenced by external concentration
How small does an organism have to be to rely on diffusion alone?
Consider a spherical sea creature 1 mm wide
oxygen concentration in normal seawater is sufficient to support low rates of respiration
Predicted that oxygen concentration only needs to be 71% of normal levels
How about a
spherical sea creature 1 cm wide?
How about a
spherical sea creature 1 cm wide?
The oxygen concentration in the water would need to be 71 times normal levels to support a low metabolic rate
Relationship between surface area and volume changes as a function of size
Another example of scaling
Example of organism relying solely on surface: the Protist Paramecium
Paramecium is a freshwater ciliate
Other small organisms that use their surfaces only include: bacteria, microalgae, yeasts
What do you do if you want to be bigger than 1 mm?
Adaptations to enhance gas exchange:
Circulatory systems and/or increased surface area
Example of increased surface area: Green Hydra (several mm long)
The jelly fish Aurelia
Complex gastrovascular cavity that circulates fluid
Gas exchange structures
Surface only (very small organisms ¾ 1 mm)
Gastrovascular cavity (hydra, jellyfish, also flatworms)
Gills, tracheal systems, lungs
Mixture of the above
Gills (definition)
Appendages around which the medium (usually water) passes.
often richly supplied with blood vessels
Found in many types of invertebrates and vertebrates
Gills vs. Lungs
Fish Gill