The diversity of single-celled organisms is astounding. These differences are apparent in both the structure and
function of the organisms. For starters, single-celled organisms range greatly in size, from less than 1 micron
(one-millionth of a meter) in diameter for the smallest bacteria, to more than 100 microns for some protozoans.
The manner in which single-celled organisms acquire food varies greatly too. Some, like the amoeba, go in
search of food, crawling on pseudopods (temporary bulges in the cell membrane) toward prey that they
eventually engulf and digest internally. Others, including all of the different species of algae, make their own
food by harnessing the sun's energy just as plants do. Structures called chloroplasts inside the algae contain the
pigment chlorophyll that allows them to use the sun's energy to make carbohydrates from carbon dioxide and
water.
Several types of algae are so versatile that, depending on light conditions, they change the mode by which they
obtain food. When sunlight is available, Euglena can be seen gravitating toward the light, which they sense
through a photoreceptor, or eyespot, at one end of the cell. If the light is too bright, the organisms will swim
away toward more optimal light conditions. If no sunlight is available, however, Euglena have the ability to
transition entirely to absorbing nutrients from the water. They may acquire their food this way indefinitely;
assuming sufficient nutrients are available and light is not.
Single-celled organisms reproduce in some interesting and surprisingly efficient ways. While some rely on
hosts or a series of host species to complete their complex life cycles, many reproduce by simply dividing in
two. In fact, the biological success and remarkable diversity of single-celled organisms is due, at least in part, to
their rapid rate of cell division. Growing under optimal conditions, the common bacterium Escherichia coli, for
example, can double its population size in just 20 minutes. (Contrast that with humans, who take on average 20
years to produce a single offspring.) Within weeks, a population of E. coli bacteria can virtually reinvent itself.
Given that the time required for one individual to pass its genes to the next generation is one of the most
important factors in the evolutionary process, single-celled organisms are on the evolutionary fast track!
In recent decades, this phenomenon has pitted doctors and researchers against quickly evolving bacteria in an
anxious battle over antibiotic resistance. Experts now estimate that as many as two dozen types of bacteria have
evolved resistance to most, if not all, antibiotics available today. A battle that we had been winning handily for
50 years, thanks to advances in medicine, appears now to be an even match.
Name ___________________________________________________________________ Date ____________
Asexual and Sexual Reproduction
HR __________
DO NOW:
•What is the result of reproduction? ____________________________________________________________
•Why do organisms bother to reproduce? Why don't they just live forever? _____________________________
•What would eventually happen to a species if every member suddenly lost its ability to reproduce? _________
_________________________________________________________________________________________
Single-Celled Organisms
•What type of reproduction -- asexual or sexual -- do most single-celled organisms use? ___________________
•What must a single-celled organism do before it can reproduce? _____________________________________
•When a single-celled organism reproduces, what is the result? _______________________________________
•In what ways, if any, does a single-celled organism differ from its parent? _____________________________
Floral Arrangements
•What type of reproduction -- asexual or sexual -- do most plants use? _________________________________
•What nonliving force do plants rely on most often for pollination? ___________________________________
•What are some of the ways in which plants encourage or trick animals into carrying their pollen to other
plants? ___________________________________________________________________________________
•What proportion of each parent plant's genetic material does each offspring plant have? __________________
The Mating Game
Describe the reproductive strategies of two different organisms.
•What are some of the things that animals can't do when they're focusing so much time and energy on finding or
attracting a mate? __________________________________________________________________________
•What proportion of each parent's genetic material would the offspring of any of these species have? ________
Asexual Reproducers
•What type of reproduction -- asexual or sexual -- do the whiptail lizards in the video use? ________________
•How many parents do whiptail lizards have? ____________________________
•How do young whiptail lizards differ from their parents, if at all? ____________________________________
•How much of their parent's genetic material do whiptail lizards have? ________________________________
The Red Queen
•What are the differences between the two species of minnows featured in the video? ____________________
•Which species -- the asexual or the sexual reproducers -- tends to be more heavily parasitized by the worm that
causes black-spot disease? ___________________________________________________________________
•How are the sexual reproducers able to evolve defenses against parasites more quickly and more effectively
than their asexual counterparts? _______________________________________________________________
Ask students to consider under what conditions each type of reproduction -- asexual and sexual -- might be the
most effective strategy for passing on one's genes and avoiding extinction.