Botany/Plant cells discussion

The questions posed in Chapter 2. Plant cells are discussed further on this page. Remember, some questions are intended to be thought-provoking and more than one answer may be "correct". Students and contributors are encouraged to expand upon the discussions here.

1. Can you think of reasons why macroscopic organisms are multicellular? 

We know that building any structure from small, similar or identical units will confer advantages on that structure over a similar structure molded in one piece, despite the fact that the former approach is likely to be more time consuming to undertake. Consider building a house out of clay (mud), something that is done in many parts of the world where the climate is dry. Typically the mud is attached to other structural elements (called wattle) of sticks or even grasses. In time the mud dries and hardens, forming a shelter that lasts, so long as it is not permitted to get too wet for too long. Yet, such structures have also been built for centuries in very wet places like London and New York. However, the problem of the dried clay returning to soft mud in a heavy downpour was solved by dividing the clay into small units that could be fired in a kiln to a dryness and hardness that completely resisted rewetting. We call these units bricks. Using bricks (small units) for construction of walls provided many benefits to the resulting structure: the units could receive special treatment so the resulting structure could be made stronger (and thus taller) and very resistant to weathering. Constructing from small units meant materials could be made elsewhere, then transported and assembled on the site. Structures could be designed to be identical to other structures, for more efficiency in construction and use.

Although multicellular organisms are not exactly like brick houses, think of how the advantages of building structures out of bricks can be advantageous properties for an organism as well.

We believe that life arose as contained chemical reactions in very small structures that reproduced. It is hard to imagine that anything else would be regarded as "living". Chemical reactions in an open environment, although more or less identical with the reactions carried out within a cell, simply lack permanence or continuity: the reactions run their course until reactants are used up. Our definition of life requires control over such chemical reactions, and the complexity of reactions involved in living systems requires boundaries that separate the life reactions from the outside environment. However, with a membrane (a separating boundary) the area enclosed cannot efficiently increase in size indefinitely. The larger the volume inside (the bigger the "cell"), the further from the membrane reactions near the middle become. Since reactions inside require access to substances in the environment outside and perhaps removal of useless or harmful substances from inside to outside, a living cell cannot just grow to a large size.

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