Single-celled and

Multi-celled Organisms

Living Organisms

Living organisms are divided into two types:

This division is based on internal complexity. The following pages provide graphic roadmaps to the organization of both of these cell types.

prokaryotic (single celled) and

eukaryotic (multi celled)

Eukaryotic Organisms

The cells of protozoa, higher plants and animals are highly structured.

These cells tend to be larger than the cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size.

Plant and Animal Cells

Plant and Animal Cells

Here are representative drawings of plant and animal cells showing their respective organelles. Click on a cell to explore them further.

plant cell animal cell

Prokaryotic Organisms These cells are simple in

structure, with no recognizable organelles.

They have an outer cell

wall that gives them shape. Just under the rigid cell wall is the more fluid cell membrane. The cytoplasm enclosed within the cell membrane does not exhibit much structure when viewed by electron microscopy.

bacterial cell

Plant Cell Organelles

Nucleus

Endoplasmic Reticulum (ER)

Cytosol/Cytoplasm

Golgi Apparatus

Mitochondria

Lysosome

Vacuole

Cell Membrane/Cell Wall

Chloroplast

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Animal Cell Organelles

Nucleus

Endoplasmic Reticulum (ER)

Cytosol/Cytoplasm

Golgi Apparatus

Mitochondria

Lysosome

Vacuole

Cell Membrane

Centrosome/Centrioles

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The Nucleus The nucleus is the most obvious organelle in any eukaryotic cell. It is a membrane-bound organelle and is surrounded by a double membrane. It communicates with the surrounding cytoplasm via numerous nuclear pores.

Within the nucleus is the DNA responsible for providing the cell with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off - that's why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell.

Plant Cell Animal Cell

Endoplasmic Reticulum Throughout the eukaryotic cell is a vast amount of membrane called the endoplasmic reticulum (ER). The ER membrane is a continuation of the outer nuclear membrane and its function suggests just how complex and organized the eukaryotic cell really is. Some areas of the endoplasmic reticulum look "smooth" (smooth ER) and some appear "rough" (rough ER).

Plant Cell Animal Cell

Smooth and Rough ER

Smooth ER is important in the synthesis of lipids and membrane proteins.

Rough ER is important in the synthesis of other proteins.

The rough ER appears rough due to the presence of ribosomes.

Plant Cell Animal Cell

Golgi Apparatus The Golgi apparatus is a membrane-bound structure with a single membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell.

Plant Cell Animal Cell

Vacuole A vacuole is a membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products. In animal cells, vacuoles are generally small. Vacuoles tend to be large in plant cells and play a role in turgor pressure. When a plant is well-watered, water collects in cell vacuoles producing rigidity in the plant. Without sufficient water, pressure in the vacuole is reduced and the plant wilts.

Plant Cell Animal Cell

Cytosol/Cytoplasm The cytosol is the "soup" within which all the other cell organelles reside and where most of the cellular metabolism occurs. Though mostly water, the cytosol is full of proteins that control cell metabolism. Cytoplasm is a collective term for the cytosol plus the organelles suspended within the cytosol.

Plant Cell Animal Cell

Cell Membrane/Cell Wall

Every cell is enclosed in a membrane. The membrane is a double layer of lipids (lipid bilayer) but is made quite complex by the presence of numerous proteins that are important to cell activity. These proteins include receptors, pores, and enzymes. The membrane is responsible for the controlled entry and exit of ions like sodium (Na) potassium (K), calcium (Ca++). Prokaryotic cells and plant cells both have a rigid cell wall made up of polysaccharides. The cell wall provides and maintains the shape of these cells and serves as a protective barrier.

Plant Cell Animal Cell

Cell Membrane

Every cell is enclosed in a membrane. The membrane is a double layer of lipids (lipid bilayer) but is made quite complex by the presence of numerous proteins that are important to cell activity. These proteins include receptors, pores, and enzymes. The membrane is responsible for the controlled entry and exit of ions like sodium (Na) potassium (K), calcium (Ca++).

Animal Cell

Lysosome Lysosomes (common in animal cells but rare in plant cells) contain enzymes necessary for intracellular digestion.

Plant Cell Animal Cell

In white blood cells that eat bacteria, lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria.

Uncontrolled release of lysosome contents into the cytoplasm can also cause cell death.

Mitochondria Mitochondria provide the energy a cell needs to move, divide, contract - in short, they are the power centers of the cell. Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. It is on these organelles that food (sugar) is combined with oxygen to produce ATP - the primary energy source for the cell.

Plant Cell Animal Cell

Chloroplast

Chloroplasts are specialized organelles found in all higher plant cells. These organelles contain the plant cell's chlorophyll, hence provide the green color. They have a double outer membrane.

Plant Cell Animal Cell

Centrosome/Centrioles The centrosome is an area in the cell where microtubles are produced.

Plant Cell Animal Cell

During animal cell division, the centrosome divides and the centrioles replicate (make new copies). The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells.

Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of microtubules.

Bacterial Cell

Nucleoid

Ribosomes

Cell wall

Plasma membrane

Capsule

Flagellum

Pilus

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Nucleoid

DNA in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior

Bacterial

Cell

Ribosomes

Ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs.

Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger RNA into the production of peptide sequences (proteins).

Bacterial

Cell

Cell Wall

Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell.

The three primary shapes in bacteria are coccus (spherical), bacillus (rod-shaped) and spirillum (spiral).

Mycoplasma are bacteria that have no cell wall and therefore have no definite shape.

Bacterial

Cell

Plasma Membrane

This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells.

There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.

Bacterial

Cell

Capsule

This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.

Bacterial

Cell

Flagella

The purpose of flagella (sing., flagellum) is motility.

Flagella are long appendages which rotate by means of a "motor" located just under the cytoplasmic membrane.

Bacteria may have one, a few, or many flagella in different positions on the cell.

Bacterial

Cell

Pilli

These are hollow, hair-like structures made of protein allow bacteria to attach to other cells.

A specialized pilus, the sex pilus, allows the transfer from one bacterial cell to another.

Pili (sing., pilus) are also called fimbriae (sing., fimbria).

Bacterial

Cell