A cell is the basic structural and functional unit of life, but it is far more organized than a simple bag of chemicals. Inside many cells are specialized structures called organelles, each adapted to perform particular jobs. Some make proteins, some produce usable energy, some store or transport materials, and others control what enters and leaves the cell.
Understanding organelles is essential to understanding how cells grow, respond to their surroundings, maintain themselves, and reproduce. The exact collection of organelles depends on the type of cell. Animal and plant cells, for example, share many structures but also have important differences.
What are cell organelles?
Cell organelles are specialized structures within cells that carry out specific functions. The word organelle means “little organ,” reflecting the idea that these structures perform specialized roles within a cell much as organs perform specialized roles within a body.
Organelles are particularly prominent in eukaryotic cells, including animal, plant, fungal, and protist cells. Many eukaryotic organelles are enclosed by membranes. Others, such as ribosomes, are not membrane-bound but are still considered essential cellular structures.
Prokaryotic cells, such as bacteria and archaea, do not contain a membrane-bound nucleus or most of the membrane-bound organelles found in eukaryotic cells. Their DNA occupies a region called the nucleoid, and their internal organization is simpler, although prokaryotic cells are still highly organized and contain structures specialized for particular functions.
The nucleus: the cell’s DNA-containing control center
The nucleus is a prominent organelle in most eukaryotic cells. It contains most of the cell’s DNA, organized into chromosomes, and helps regulate gene expression and cellular activity.
The nucleus is surrounded by the nuclear envelope, a double membrane containing openings called nuclear pores. These pores regulate the movement of proteins, RNA, and other molecules between the nucleus and the cytoplasm.
Inside the nucleus is the nucleolus, a dense region where components of ribosomes are produced and assembled. The resulting ribosomal subunits leave the nucleus through nuclear pores and later participate in protein synthesis in the cytoplasm.
The nucleus does not independently “run” every cellular process. Rather, DNA in the nucleus provides genetic information that cells use to produce RNA and proteins. Those molecules help carry out and regulate much of the cell’s activity.
Ribosomes: the machines that build proteins
Ribosomes are cellular structures that assemble proteins from amino acids. They do this by reading information carried by messenger RNA, or mRNA.
Ribosomes are made from ribosomal RNA (rRNA) and proteins. Unlike the nucleus, endoplasmic reticulum, mitochondria, and other membrane-bound organelles, ribosomes do not have a surrounding membrane.
Ribosomes can be found free in the cytoplasm or attached to the surface of the rough endoplasmic reticulum. Free ribosomes generally make proteins that function within the cell, while ribosomes attached to the rough endoplasmic reticulum commonly make proteins destined for secretion, membranes, or certain cellular compartments.
The endoplasmic reticulum: a network for production and processing
The endoplasmic reticulum (ER) is a connected network of membranes involved in producing, processing, and transporting molecules.
There are two major forms.
Rough endoplasmic reticulum
The rough ER is covered with ribosomes, giving it a rough appearance under a microscope. It is a major site of production and initial processing for proteins that will be secreted from the cell, inserted into cellular membranes, or delivered to certain organelles.
Proteins entering the rough ER can be folded and chemically modified as they begin their journey to their final destinations.
Smooth endoplasmic reticulum
The smooth ER lacks ribosomes on its surface. Its functions vary among cell types but include lipid production, chemical processing, and storage or regulation of calcium ions.
In certain cells, such as liver cells, smooth ER contributes to the processing and detoxification of various substances. In muscle cells, a specialized form called the sarcoplasmic reticulum plays a major role in storing and releasing calcium needed for muscle contraction.
The Golgi apparatus: sorting and shipping cellular products
The Golgi apparatus consists of flattened, membrane-bound sacs. It receives molecules, particularly proteins and lipids, from the endoplasmic reticulum and modifies, sorts, and packages them.
Molecules leaving the Golgi apparatus can be directed to different destinations. Some may be incorporated into the plasma membrane, some may be secreted outside the cell, and others may be delivered to lysosomes or other cellular compartments.
The Golgi apparatus is therefore not simply a storage site. It functions as a processing and distribution center for many of the molecules produced inside the cell.
Mitochondria: converting energy into a usable form
Mitochondria are organelles that play a central role in cellular energy metabolism. They use energy from molecules such as glucose and other nutrients to produce ATP (adenosine triphosphate), a major energy-carrying molecule used to power cellular processes.
Mitochondria have two membranes. The inner membrane is extensively folded into structures called cristae, which increase the membrane area available for important reactions involved in ATP production.
Mitochondria also contain their own DNA and ribosomes. This is one reason they are thought to have evolved from bacteria that entered into a long-term symbiotic relationship with an ancestral eukaryotic cell. This idea is known as the endosymbiotic theory.
Mitochondria are not found only in animal cells. They are also present in plant and other eukaryotic cells, where they perform essential roles in energy metabolism.
Lysosomes: breaking down and recycling materials
Lysosomes are membrane-bound compartments containing enzymes that break down a wide range of biological materials. They help digest worn-out cellular components, large molecules, and material taken into the cell.
Their enzymes work effectively in the acidic environment inside the lysosome. By breaking materials into smaller components, lysosomes allow useful building blocks to be reused.
Lysosomes are particularly important for cellular recycling. A related process called autophagy allows cells to deliver damaged or unnecessary components to lysosomes for degradation and reuse of their constituent molecules.
Peroxisomes: specialized chemical processing
Peroxisomes are small membrane-bound organelles involved in several metabolic reactions. They help break down certain fatty acids and participate in reactions that produce or consume hydrogen peroxide, a chemically reactive substance.
Because hydrogen peroxide can damage cellular components, peroxisomes contain enzymes that help control it. Their functions vary among cell types, and they are particularly important in tissues with substantial metabolic activity.
Vacuoles: storage and cellular balance
Vacuoles are membrane-bound compartments that store substances and help regulate the internal environment of cells. Their size and importance vary considerably between organisms.
Plant cells often contain a large central vacuole that can occupy much of the cell’s interior. It stores water, ions, pigments, metabolites, and other substances. By accumulating water, the central vacuole also contributes to turgor pressure, which helps support plant tissues.
Animal cells can contain smaller vacuoles and other membrane-bound storage compartments, although they generally do not have the enormous central vacuole characteristic of many plant cells.
Chloroplasts: capturing light energy in plant cells
Chloroplasts are organelles found in plants and certain algae. They carry out photosynthesis, the process through which light energy is used to help convert carbon dioxide and water into energy-rich organic molecules, with oxygen released as a byproduct of the overall process.
Chloroplasts contain the green pigment chlorophyll, which absorbs light energy. Like mitochondria, chloroplasts have two membranes and contain their own DNA and ribosomes. Their evolutionary history is also explained by the endosymbiotic theory, which proposes that chloroplasts originated from photosynthetic bacteria that became incorporated into ancestral eukaryotic cells.
Inside a chloroplast are membrane-bound structures called thylakoids, often arranged into stacks called grana. The light-dependent reactions of photosynthesis occur in the thylakoid membranes, while other reactions involved in carbon fixation occur in the surrounding fluid, called the stroma.
The plasma membrane: controlling the cell’s boundary
The plasma membrane forms the outer boundary of the cell. It separates the cell’s internal environment from its surroundings while allowing controlled exchange of substances.
The membrane consists primarily of a phospholipid bilayer containing proteins, cholesterol, and other molecules. Its structure is often described as a fluid mosaic because many of its components can move within the membrane.
Membrane proteins perform numerous functions, including transporting substances, receiving chemical signals, anchoring cellular structures, and helping cells recognize one another.
The plasma membrane is selectively permeable, meaning that different substances cross it in different ways. Small or nonpolar molecules may cross relatively easily, while ions and many larger or polar molecules generally require specialized transport proteins.
The cytoplasm and cytoskeleton: organizing the cell’s interior
The cytoplasm refers broadly to the material inside the plasma membrane and outside the nucleus in eukaryotic cells. It includes the cytosol, organelles, and other cellular structures.
The cytosol is the fluid portion of the cell in which many chemical reactions occur. It contains water, ions, proteins, metabolites, and numerous other dissolved substances.
Running through the cytoplasm is the cytoskeleton, a dynamic network of protein filaments that helps maintain cell shape, organize internal components, enable movement, and assist in transporting materials within the cell.
The three major components of the cytoskeleton are microfilaments, intermediate filaments, and microtubules. They differ in structure and function but work together to give cells mechanical support and internal organization.
Centrosomes and centrioles in animal cells
The centrosome is an important region involved in organizing microtubules. In many animal cells, it contains a pair of structures called centrioles.
Centrosomes are especially important during cell division because they help organize the microtubules that form the mitotic spindle. The spindle helps separate duplicated chromosomes into daughter cells.
Centrioles also contribute to the formation of structures such as cilia and flagella in certain animal cells.
Cilia and flagella: moving cells or moving substances
Cilia and flagella are hair-like structures built largely from microtubules. They can move cells or move fluid and particles across a cell’s surface.
Cilia are generally shorter and more numerous. For example, cilia lining parts of the respiratory tract help move mucus and trapped particles.
Flagella are generally longer and fewer in number. A familiar example is the flagellum of a sperm cell, which helps propel the cell forward.
Although both structures can produce movement, their precise organization and motion vary among organisms and cell types.
Plant cells have structures that animal cells lack
Plant and animal cells share many organelles because both are eukaryotic. Plant cells, however, have several distinctive features.
A cell wall surrounds the plasma membrane of plant cells. It is composed primarily of cellulose and provides structural support and protection. Because the cell wall is rigid, it also helps prevent excessive expansion when water enters the cell.
Plant cells typically contain chloroplasts, which enable photosynthesis, and a large central vacuole, which contributes to storage and structural support.
Animal cells lack a cellulose cell wall and chloroplasts. Their shape is instead strongly influenced by the plasma membrane, cytoskeleton, and surrounding extracellular environment.
How organelles work together
Organelles do not function as isolated units. Cell survival depends on coordinated activity among many structures.
For example, a protein destined to be secreted may begin when DNA in the nucleus provides the information needed to make an mRNA molecule. A ribosome associated with the rough ER then uses that mRNA to assemble the protein. The protein enters the ER, where it begins folding and processing, and is transported to the Golgi apparatus. The Golgi modifies and sorts it before directing it toward the plasma membrane for secretion.
Energy for these processes depends heavily on mitochondria, which produce ATP. Lysosomes recycle materials, peroxisomes carry out specialized metabolic reactions, and the cytoskeleton helps organize and transport components throughout the cell.
This cooperation is what makes the cell an integrated system rather than a collection of unrelated parts.
Membrane-bound versus non-membrane-bound organelles
One useful way to organize organelles is by whether they are surrounded by a membrane.
Membrane-bound organelles include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, peroxisomes, vacuoles, and chloroplasts. Their membranes create distinct internal environments in which particular chemical reactions can occur.
Non-membrane-bound structures include ribosomes and major components of the cytoskeleton. They still perform highly specialized functions but are not enclosed within their own lipid membrane.
This distinction matters because compartmentalization allows eukaryotic cells to perform different, sometimes incompatible, chemical processes simultaneously.
