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6.2 Unicellular Organisms
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unicellular organisms and made of only one cell and perform the same life
processes as multicellular organisms
Nutrition
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Most unicellular organisms obtain nutrients by eating other organisms
Some plant-like protists and some bacteria can make their own food using
photosynthesis (e.g. Euglena can eat other organisms and make its own food
using photosynthesis)
Paramecia are found in water and are animal-like (find food to eat). They eat
bacteria and algae cells by beating many cilia within their oral groove to create a
current that draws food into a cavity. Once the cavity is filled with food, the food
is enclosed in a vacuole where it is slowly digested. Paramecia removes wastes
by moving vacuoles filled with waste toward their anal pore and then using
exocytosis to expel the wastes.
Amoeba is another animal-like protest that needs to find food. It uses
phagocytosis (a special endocytosis) to feed on other organisms. It pushes its
cytoplasm in a way that creates pseudopods (fake feet) that extend around the
food and form a vacuole where the food is slowly digested. Wastes are released
by exocytosis.
Gas Exchange
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Oxygen diffuses across cell membrane into the cell
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Carbon dioxide diffuses out of the cell across cell membrane
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Some unicellular organisms, such as yeast, can survive without oxygen.
Responding to the Environment
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Unicellular organisms use special ways to sense their environment and respond
to it.
Some bacteria can detect chemicals, such as sugar, in their environment and
move toward them
Photosynthetic protists, like Euglena, can detect light using special sensors (eye
spot) and move toward the light
Movement and Locomotion
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Unicellular organisms can use movement or locomotion to move toward or away
from things such as food, light, and predators
Movement — a change in the shape or figure of all or part of an organism
— usually achieved using pseudopods
— e.g. amoeba and white blood cells use pseudopods to obtain food
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Locomotion — movement that takes an object form one place in its environment
to another
— usually achieved using cilia or flagella creating currents in the
surrounding environment
NOTE: Some unicellular organisms are good for humans (e.g. bacteria living in your
digestive system helps you to digest your food). Some unicellular organisms are bad
for humans (e.g. bacteria such as streptococcus will give you strep throat)