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Organisms consist almost entirely of highly organized matter, and rely on multiple processes to ensure they remain that way. Bacteria, protists, and other unicellular organisms only need to regulate processes within a single cell. In contrast, multicellular organisms must coordinate the activities of different organs, each made up of thousands to trillions of cells.

Consider the two organisms in the pair of videos below. One video shows a single celled protist (Paramecium, left) the other a small multicellular animal called a rotifer (right). These organisms are similar sizes, and both organisms catch similar food from which they extract nutrients. Unlike the unicellular Paramecium, a rotifer's body has about 1000 cells. The Paramecium sweeps prey into a long channel called the oral groove. Prey are digested in a vacuole and the raw materials directly distributed to the rest of the cell. Being multicellular, rotifers are more complex. They have a spinning corona that sweeps prey into a mouth with jaw-like grinding structures. Food passes through an esophagus to a stomach and short intestine for processing. Nutrients move through the body via a primitive circulatory system. They also have eyes, and a simple brain and nervous system that regulates their digestion and other physiological processes.

Video A.


Video B.


The Challenges of Just Staying Alive

Regardless of whether they are unicellular or multicellular, plant, animal, fungus, or bacterial, all organisms face similar challenges to survival. If they do not solve these challenges, they will die.

Challenge 1: coordinate actions of organs and tissues to maintain homeostatic set-points

Even simple organisms have hundreds of setpoints they must monitor and maintain. There is a set-point for every essential ion or molecule, and even for intracellular water levels. The number of setpoints goes up as organisms become larger and more complex.

Challenge 2: Obtain energy, raw materials, and essential nutrients from the environment

Cells and organisms cannot manufacture everything they need. They must take up a variety of materials from their environment that include:

  • Water
  • Complex molecules like phosphate (PO4-3))
  • Essential ions like sodium (Na+), potassium (K+) and calcium (Ca+2).

Plants use their roots and leaves to extract these and other materials from their environment. Then they rely on photosynthesis to manufacture sugars, amino acids, and lipids. Animals cannot manufacture these macronutrients. Instead, we have digestive systems that reduce complex foods into amino acids, simple monosaccharides, and lipids that can be transported to cells and tissues.

Challenge 3: Distribute materials within the body

Most multicellular organisms are too large for materials to move by diffusion alone. They need a dedicated internal transport system. The larger and more complex an organism is, the more complex that transport system will be.

Challenge 4: Transport and eliminate wastes safely

The chemical processes of life produce waste materials. Some can be recycled for other purposes, but others must be discarded. Once more, the larger and more complex an organism is, the more complex its waste disposal processes will be.

Challenge 5: Sense and respond to the internal and external environment

The environment outside of an organism is not stable. Organisms must be able to sense changes in temperature, light, presence of dangerous conditions, and innumerable other stimuli, then respond to them appropriately. Just as important, they must know what is occurring inside their body. Otherwise they cannot adapt to internal changes.

Challenge 6: Support the body and move when needed

Though it is not always easy to detect, almost every organism moves. Most animals move around as part of their daily existence. Plants move too; if you watch a time-lapse video of a sunflower, you will see it turn to follow the sun. Movement requires two things: structures to support the organism and some process to generate the force needed for movement.

Challenge 7. Defend against predators, parasites, and pathogens

Every organism’s body contains useful molecules that others could use to survive, grow, and reproduce. Heterotrophs are organisms that survive by consuming other species for their resources. The most obvious example of a heterotroph is a carnivorous predator like a tiger that kills and eats other animals. While you may not have thought of it this way, the same thing happens when herbivores eat plants. Parasites and pathogenic micro-organisms consume their hosts too. For example, athlete’s foot is a parasitic skin fungus that digests skin cells, and uses the proteins to grow and reproduce. Similarly, a dog tick uses nutrients from the blood it sucks to produce eggs.

Challenge 8. Grow and reproduce successfully

Every species of organism must reproduce; otherwise that species will disappear. Branches and Leaves related to ecology and evolutionary biology will explore the different strategies that have evolved to maximize reproduction. For now, you should know that organisms invest a great deal of energy and material resources into reproducing successfully.

Which Organ Systems Help Organisms Meet These Challenges?

ALL of them. The goal of every organism is to maintain homeostasis, that is, to keep internal composition at optimum setpoints, and to meet the challenges of life successfully. These are just a few examples of these interactions:

  • The digestive, circulatory, and respiratory systems cooperate to acquire energy and raw materials, and distribute them to cells and tissues.
  • The digestive, urinary, circulatory, and respiratory systems cooperate to eliminate solid, liquid, and gaseous wastes.
  • The nervous, endocrine, and circulatory systems provide communication and coordination.
  • The integumentary, immune, digestive, circulatory, and lymphatic systems all protect the body from pathogenic organisms.

You should keep in mind that the challenges of life described here are are not set in stone. Other books and authors may word or organize them differently. Just memorizing the list of challenges is not very helpful, especially if you are not studying all of the different organ systems. Instead try to understand WHY these challenges are so important to life.

What Will You Learn About From This Branch?

Suggestions for Studying and Mastering This Branch

After you have read the various Leaves attached to this Branch, come back here and use the Check My Learning questions to see how well you can connect the various pieces you have learned together into a complete picture.

Page last modified on Tuesday 23 of February, 2016 23:52:05 EST

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