B is for Biology

Talking Bio with UCSB Professor Alice Alldredge

For this edition of Curioser and Curioser, Martha Sadler sent
questions via email to UCSB professor Alice Alldredge (pictured), who is the chair of
the Department of Ecology, Evolution, and Marine
Biology
and teaches the animal kingdom portion of the
university’s introductory biology series. The professor, a marine
biologist who personally studies zooplankton (also pictured below), explained
that “these were odd questions” but managed to entertain our
queries successfully. So here we go.

1) Is there any sense to the notion that animals are
mutated and evolved forms of plant, that they retain some plant
characteristics?

Animals and plants form two major kingdoms in the classification
of organisms. Both kingdoms arose from one-celled protists. Keep in
mind that the ability to photosynthesize does not define whether
something is a plant or not. Many algae, which are protists, can
photosynthesize and are not classified as plants. The protists that are ancestral to
plants had the ability to photosynthesize while those that are
directly ancestral to animals did not. However, plants and animals
do share a protistan ancestor from very long ago. Thus they share
many cellular characteristics and actually have some common
genes.

But animals are not mutated or evolved forms of plants.

2) Is it ever hard to draw the line between plant and
animal?

No, it is not difficult to draw the line.

Plants are defined as organisms that: 1) Develop from embryos
that are protected by the tissue of the parent plant (for this
reason plants are sometimes called embryophytes);

2) Use chlorophylls a and b for photosynthesis;

and 3) Use starch as a photosynthetic storage product. (Note
that some algae have the second characteristics, but not the first
one, so are not plants.)

Animals are defined as being:

1) multicellular, with tissues usually differentiated into organ
systems or cell layers;

2) receive their nutrition from organic molecules they obtain
from the environment (like protein, carbohydrates, fats) .They
cannot make organic molecules from inorganic ones the way plants do
but instead ingest these organic molecules by eating other
organisms or absorbing them through their body walls;

and 3) Show some capability of locomotion and sensory response
at some point in their life cycle. With only very few exceptions,
they have some kind of a nervous system that relays signals
throughout the body electrochemically.

3) Would life on another planet necessarily be
cellular?

Well, in that carbon-based life as we know it must depend on the
interactions of chemical molecules, and it helps to have those
molecules held together in some kind of a concentrated environment
for them to work together, then yes, I think life on other planets
is very likely to be cellular. But a cell might need to be very
broadly defined: a sack holding chemicals together. Having never
been to another planet where there is life, this is an educated
guess!

4) Is it tricky to draw the line between lifeform and
non-lifeform?

No. Living organisms are cellular and contain genetic material
known as nucleic acids DNA, RNA, etc. They have the important
characteristic of being able to reproduce and pass information to
the next generation. Viruses are tiny infectious particles
consisting of a core of nucleic acid (genetic material) surrounded
by a coat of protein that can reproduce within the cells of living
organisms. They are not considered living themselves because they
are not cellular.

5) Plants are responsive to their environment, right?
Leaves seek sun, roots seek water; plants heal their wounds. Is
there any possibility they are operating on a pleasure principle
when they reach for the sun? Or a pain principle, or that they
experience longing or desire or will or something analogous to
dreams? Or is it all purely mechanical? (Good luck with this weird
question.)

“Feeling” implies a nervous system that can respond to the
environment. Plants don’t experience pleasure or pain because they
have no nervous system to detect these. Certainly they grow toward
light or moisture, but this is purely mechanical. Words like
longing, desire, will, and dreaming again imply a complex nervous
system.

6) How did life begin?

The first cells probably assembled from organic molecules.
Assemblages of “protobionts” which are assemblages of non-living
organic polymers have been created by scientists in the laboratory.
Some of these assemblages resemble living cells. For instance, some
divide when they get too big. Others produce electrical potential
across their surfaces and adsorb materials from their surroundings.
These are things cells can do. These give us clues. The first cells
were probably simple bags of chemicals surrounded by a porous
organic membrane.

How organic molecules got concentrated enough to form
protobionts is controversial. Some scientists think this could have
occurred in hydrothermal vents in the deep ocean floor. Others
think organic molecules formed on the earths surface in its early
history and accumulated in shallow seas.

7) Was the first lifeform a plant?

No, certainly not! Plants are highly complex organisms that
evolved long after the first cells. The first living things were
very simple cells that lacked a nucleus and probably absorbed
organic molecules from the soup around them. They could not
photosynthesize.

To carry on the conversation, email the professor at
alldredg@lifesci.ucsb.edu
or add your comments below.

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