MadSci Network: Cell Biology
Query:

Re: What is a protist? How is it like an animal and plant cell combined?

Area: Cell Biology
Posted By: Matthew Barchok, High School Senior, none
Date: Tue Oct 7 21:41:29 1997
Area of science: Cell Biology
ID: 875885372.Cb
Message:

Hi -

It seems you have suddenly found out that there are more forms of life than just animals and plants! There are actually five main groups, called KINGDOMS in which organisms are classified. The kingdoms are as follows: Animalia, Plantae, Fungae, Protista, and Monera.

Monera represent the simplest organisms. Their cells are little more than blobs of protein, lipids, and DNA, surrounded by a membrane. Sometimes, they have little protein hairs, called flagella, which make them move. All monera are unicellular, that is, they are made of one cell. Some monera can make energy from light, but most just absorb energy from whatever is around them. No moneran has internal structures called ORGANELLES. Moneran DNA is a round ring. All other organsims have DNA in chromosomes, which have ends to the DNA strands.

All other kingdoms have some specialized structures in their cells, called ORGANELLES. Each organelle preforms a specific function within a cell, just like each organ in our body does one particular thing. For instance, MITOCHONDRIA produce energy, and LYSOSOMES degrade proteins and other cellular components so they can be recycled.

Plantae is easy to recognize. Plants have square cells surrounded by thick cell walls. All plants contain PLASTIDS, a certain type of organelle. Some plastids store starch, others hold pigments, but the most important, the CHLOROPLAST, contains chlorophyll. Chlorophyll allows a plant to capture sunlight - the beginning of a process that creates energy for the plant. Most plants contain chlorophyll, although some plants have evolved into parasites that don't need it. An important characteristic of plants is that they don't use thier own energy to move around. All plants are multicellular. Plant cell division is a much more complex process than cell division in monera.

Fungae is a surprizingly complex kingdom. No fungus makes its own energy. Fungi simply absorb whatever nutrients are nearby. A fungus cell is usually long and thin, with many branches. Most fungi have cell walls like plants, but they don't have plastids. Fungus organelles are much like animal organelles. Most fungi are multicellular, but some, like yeast, exist mostly in colonies of individual cells. Fungi, like plants, don't use their own energy to move around. In addition, fungus cell division is slightly different from that of plants.

Animalia is yet another kingdom. All animals are multicellular. At some stage in their life cycle, they all move under their own energy. All use motion to capture food, and no animal makes its own food. Since animals move, they cannot have rigid cell walls. Animals, like fungi, have complex organelles, and do not have plastids. Animal cell division is much different from plant cell division. In animal cell division, threads of protein actually guide the chromosomes to the daughter cells that arise from the single parent cell, as happens during mitosis.

Protista is a group made up of whatever doesn't quite fit into the others. Some are multicellular, although most are unicellular. Some move under their own energy, others don't. Some have cell walls. All have organelles, and some have what might be considered plastids. Some divide like plants, some divide like animals, some divide like bacteria, and some, like paramecia, can and sometimes do divide in their own unique way.

Don't let your confusion stop you from enjoying science. Even biologists get confused about the kingdom protista. There have been many debates over whether an organsim is in this kingdom or that one. In fact, there is a debate currently about the kingdom monera. It seems some monera have what appears to be a membrane surrounding their DNA. This internal structure is not allowed by the current definition of monera. Many propose that these organisms have their own kingdom - Archaea. Others propose that the definition of monera be revised.

Remember, science is not KNOWING what and why, but the effort to find out.

Matthew Barchok, Mad Scientist.
P.S. I was just as confused when I found out about protista.


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