Cells function differently in unicellular and multicellular organisms, but in every organism, each cell has specialized cell structures, or organelles, of which there are many. These organelles are responsible for a variety of cellular functions, such as obtaining nutrients, producing energy, and making proteins. Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function. 

Unicellular organisms include bacteria, protists, and yeast. For example, a paramecium is a slipper-shaped, unicellular organism found in pond water. It takes in food from the water and digests it in organelles known as food vacuoles. Nutrients from the food travel through the cytoplasm to the surrounding organelles, helping to keep the cell, and thus the organism, functioning.  

Multicellular organisms are composed of more than one cell, with groups of cells differentiating to take on specialized functions. In humans, cells differentiate early in development to become nerve cells, skin cells, muscle cells, blood cells, and other types of cells. One can easily observe the differences in these cells under a microscope. Their structure is related to their function, meaning each type of cell takes on a particular form in order to best serve its purpose. Nerve cells have appendages called dendrites and axons that connect with other nerve cells to move muscles, send signals to glands, or register sensory stimuli.  Outer skin cells form flattened stacks that protect the body from the environment. Muscle cells are slender fibers that bundle together for muscle contraction. 

The cells of multicellular organisms may also look different according to the organelles needed inside of the cell. For example, muscle cells have more mitochondria than most other cells so that they can readily produce energy for movement; cells of the pancreas need to produce many proteins and have more ribosomes and rough endoplasmic reticula to meet this demand. Although all cells have organelles in common, the number and types of organelles present reveal how the cell functions. 

 

Unicellular vs. Multicellular

There are many types of unicellular organisms in the world, including protists like this one, which feed mainly on diatoms, amoebas, bacteria, and algae.

axon
Noun

long projection from the neuronal body that conducts electrical impulses away from the neuron.

cell differentiation
Noun

development of cells into a specific type of cells.

dendrite
Noun

branch that conducts electrical impulses toward the neuron.

endoplasmic reticulum
Noun

organelle that transports proteins.

enzyme
Noun

proteins produced in living cells that act as catalysts to accelerate the vital processes of an organism.

food vacuole
Noun

cell structure that digests food using enzymes.

golgi body
Noun

organelle that packages proteins.

mitochondria
Plural Noun

(singular: mitochondrion) structure (organelle) in the cytoplasm of most cells in which nutrients (sugars, fatty acids, and amino acids) are broken down in the presence of oxygen and converted to energy in the form of ATP.

multicellular
Adjective

composed of more than one cell.

organelle
Noun

specialized part of a cell that performs a specific function.

paramecium
Noun

slipper-shaped protist found in pond water.

protist
Noun

type of microscopic organism (not an animal, plant, or fungus). 

unicellular
Adjective

having one cell.