What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Cell Parts: Organelles and Their Functions

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

A cell is the smallest unit of life that can carry out all the processes needed to stay alive and reproduce. Its parts fall into two broad groups: the plasma membrane and cytoplasm that hold everything together, and the organelles, which are specialized internal structures that each perform a defined job.

Understanding cell parts matters because almost every question in biology eventually comes back to them. When a tissue repairs itself, when a plant grows toward light, when an immune cell engulfs a bacterium, the explanation lives at the level of organelles. The same vocabulary also anchors medicine, agriculture and biotechnology, from mitochondrial genome assembly in human sequencing projects [1] to the plastid division machinery that controls how many chloroplasts a crop plant builds [2].

This guide defines each organelle by its membrane status and primary function, then compares plant, animal and bacterial cells side by side. It is written for students meeting the material for the first time, for researchers who need a clean refresher, and for anyone planning a life science career who wants the core map in one place.

The Basic Architecture of a Cell

Every cell shares a small set of structural features. The plasma membrane is a phospholipid bilayer that separates the inside of the cell from the outside and controls what crosses that boundary. The cytoplasm is everything inside the membrane, and the cytosol is the fluid portion of the cytoplasm, the water-based solution where many reactions occur. Cells are highly compartmentalized entities built from both membranous and nonmembranous structures [3].

The word organelle means "little organ." Organelles are the functional compartments of the cell, and the most useful way to classify them is by membrane status.

Membrane-bound organelles are wrapped in at least one lipid bilayer. The nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes and endosomes belong here [3].
Nonmembranous structures are not enclosed by a lipid bilayer. Ribosomes and the cytoskeleton belong here, along with structures such as centrioles.

Membrane status is not a trivial detail. A lipid bilayer lets an organelle maintain its own internal chemistry, including its own pH and ion gradients. Compartment-specific membrane potentials, pH gradients and ion dynamics influence how a cell behaves, including how stem cells choose what to become [4]. The lysosome, for example, keeps an acidic interior that shapes metabolic programming during cell transitions [4].

The Nucleus: Control Center and Genome Vault

The nucleus is the largest membrane-bound organelle in most animal and plant cells. It is enclosed by a double membrane called the nuclear envelope, which is perforated by nuclear pores that regulate traffic between the nucleus and the cytoplasm.

Its primary function is to store the cell's DNA and to host transcription, the process of copying DNA into RNA. In eukaryotes, most of the genome is kept in the nucleus, and the nuclear environment helps organize chromatin, the packaged form of DNA. Nuclear ionic and mechano-osmotic conditions can shape chromatin organization and transcriptional competence, which is one way the physical state of the nucleus feeds back into gene activity [4].

Inside the nucleus, a region called the nucleolus assembles ribosomal subunits. This connects the nucleus directly to protein production, because ribosomes built in the nucleolus are exported to the cytoplasm to translate messenger RNA.

Mitochondria: Energy Conversion and Signaling

Mitochondria are double-membrane organelles. The inner membrane is folded into cristae, which increase surface area, and the space between the two membranes is the intermembrane space. Mitochondria are the cell organelles that produce most of the chemical energy required to power the cell's biochemical reactions [1].

Their primary function is oxidative metabolism. Mitochondria convert nutrients into usable energy currency and also participate in calcium handling, signaling and programmed cell death. They carry their own small genome, separate from the nuclear genome. The origin of that genome is linked to the endosymbiosis of a prokaryotic cell by the host cell, and mitochondrial genomes encode independent genomic information [1]. This is why mitochondrial genetics is treated as its own field, and why assembling a human mitochondrial genome is a distinct bioinformatics task with dedicated tools [1].

Mitochondrial function depends on coordinated expression of both mitochondrial and nuclear genomes [5]. Mitochondria also vary in membrane potential, and differences in mitochondrial membrane potential and inheritance influence lineage commitment in stem cells [4]. In tissue-level studies, fasting-induced organelle remodeling has been linked to altered mitochondrial membrane potential measured in living animals, which shows that organelle architecture is a structural readout of how a tissue adapts [6].

Endoplasmic Reticulum: Rough and Smooth

The endoplasmic reticulum (ER) is a membrane-bound network of tubules and flattened sacs that spreads through the cytoplasm. It comes in two functionally distinct regions.

Rough Endoplasmic Reticulum

The rough ER is studded with ribosomes on its cytoplasmic surface, which gives it a bumpy appearance under the microscope. Its primary function is to fold, modify and quality-check proteins destined for secretion or for membranes. Newly made proteins enter the ER lumen, the internal space, where folding and initial glycosylation occur. The ER also handles ER-calcium, and ER-calcium handling plus ER-mitochondrial coupling influence lineage commitment in stem cells [4]. The ER physically and functionally connects to other compartments, and stress sensed at the ER triggers adaptive responses that adjust protein biosynthetic rates [3].

Smooth Endoplasmic Reticulum

The smooth ER lacks ribosomes. Its primary functions are lipid synthesis, detoxification of certain molecules, and calcium storage. In muscle cells, a specialized smooth ER called the sarcoplasmic reticulum stores and releases the calcium that drives contraction. In liver cells, smooth ER enzymes help process drugs and metabolic byproducts.

Golgi Apparatus: Sorting and Shipping

The Golgi apparatus is a membrane-bound stack of flattened sacs called cisternae. It has a receiving face (cis) and a shipping face (trans).

Its primary function is to modify, sort and package proteins and lipids. Proteins made in the rough ER travel to the Golgi, where they receive further carbohydrate modifications and are addressed to their destinations. The Golgi is a central node in the secretory pathway, and it is one of the organelles whose structural state changes measurably when a cell is under stress [3]. Because the Golgi sits between the ER and the plasma membrane, disruptions in cell polarity and barrier function register here as part of a broader organelle stress response [3].

Lysosome: Digestion and Recycling

The lysosome is a single-membrane organelle with an acidic interior. Its primary function is to break down macromolecules, worn-out organelles and material taken in from outside the cell.

Lysosomes receive cargo from phagocytosis, endocytosis and autophagy. In primary human monocytes, transmission electron microscopy shows that cells internalize polystyrene micro- and nanoplastics through phagocytosis, followed by phagosome maturation and fusion with lysosomes [7]. That same exposure remodels the autophagy-lysosomal system, with increased lysosomal abundance, an elevated LC3-II/I ratio, and accumulation of autophagy-related vesicular structures [7]. The LC3-II/I ratio is a standard readout of autophagosome formation.

Lysosomal pH is not just a digestive detail. It influences metabolic programming that supports cell transitions, including stem cell fate decisions [4]. Lysosomes are a defining feature of animal cells and are generally not listed among the standard organelles of plant cells.

Peroxisome: Oxidative Chemistry in a Small Package

The peroxisome is a single-membrane organelle, usually small and roughly spherical. Its primary function is to carry out oxidative reactions, including the breakdown of very long chain fatty acids and the neutralization of hydrogen peroxide by catalase.

Peroxisomes are metabolically connected to mitochondria, and comparisons of mitochondrial, peroxisomal and plastid proliferation reveal shared division factors among these organelles [2]. That shared machinery is a reminder that organelles are not isolated islands. They divide, grow and are inherited through processes that overlap at the molecular level.

Ribosome: The Protein Factory

The ribosome is a nonmembranous organelle. It is a complex of ribosomal RNA and proteins, built from a large subunit and a small subunit.

Its primary function is translation, the process of reading messenger RNA and assembling a chain of amino acids into a protein. Ribosomes are found free in the cytosol and bound to the rough ER. Free ribosomes typically make proteins that stay in the cytosol, while ER-bound ribosomes make proteins destined for secretion or membranes. Because ribosomes are not membrane-bound, they are present in every living cell type, including bacteria. The 16S ribosomal RNA gene is so universally useful that it anchors modern environmental sequencing databases, which now combine bacterial, archaeal and organelle 16S rRNA sequences to identify both prokaryotes and eukaryotes in a single amplicon analysis [8].

Cytoskeleton: Internal Scaffolding and Transport Rails

The cytoskeleton is a nonmembranous network of protein filaments. It has three main classes: actin filaments (microfilaments), intermediate filaments and microtubules.

Its primary functions are to give the cell shape, to anchor organelles in place, to drive cell movement, and to serve as tracks for intracellular transport. Motor proteins walk along microtubules carrying vesicles between compartments. The cytoskeleton also drives cell division, and in plant cells, cytosolic proteins such as ARC5/DRP5B act in concert with internal ring structures to position and drive organelle division [2].

The cytoskeleton is dynamic. Filaments assemble and disassemble in response to signals, which is how a cell changes shape, migrates or divides. In neurons, mitochondrial trafficking along the cytoskeleton supports synaptic transmission and axonal transport, and defects in that transport contribute to neurological disease phenotypes [5].

How Plant, Animal and Bacterial Cells Differ

Prokaryotes lack membrane-bound organelles. Bacteria keep their DNA in a nucleoid region rather than a nucleus, and they carry out energy metabolism at the plasma membrane rather than inside mitochondria. They do have ribosomes, and bacterial ribosomes differ enough from eukaryotic ribosomes that many antibiotics target them selectively. Prokaryotic cells also typically have a cell wall outside the plasma membrane.

Plant cells add three structures that animal cells do not have: a cell wall, chloroplasts and a large central vacuole. The cell wall is a rigid layer outside the plasma membrane that provides structural support and limits water uptake. Chloroplasts are plastids that carry out photosynthesis. Plastids retain the imprint of their cyanobacterial ancestry and divide primarily through binary fission using a division apparatus assembled from both prokaryotic ancestral and eukaryotic host components [2]. The large central vacuole stores water, ions and pigments and helps maintain turgor pressure.

Animal cells have centrioles and lysosomes. Centrioles are nonmembranous cylindrical structures that organize microtubules, including the spindle that separates chromosomes during division. Lysosomes handle digestion and recycling, and they are a standard feature of animal cells.

Comparison Table: Organelles Across Cell Types

Organelle	Membrane status	Primary function	Plant	Animal	Bacteria
Nucleus	Double membrane	Stores DNA, hosts transcription	Yes	Yes	No
Mitochondria	Double membrane	Produces most cellular chemical energy	Yes	Yes	No
Rough ER	Single membrane	Folds and modifies proteins	Yes	Yes	No
Smooth ER	Single membrane	Makes lipids, stores calcium, detoxifies	Yes	Yes	No
Golgi apparatus	Single membrane	Modifies, sorts and packages cargo	Yes	Yes	No
Lysosome	Single membrane	Digests and recycles material	Rare	Yes	No
Peroxisome	Single membrane	Oxidative reactions, fatty acid breakdown	Yes	Yes	No
Ribosome	Nonmembranous	Translates mRNA into protein	Yes	Yes	Yes
Cytoskeleton	Nonmembranous	Shape, movement, transport, division	Yes	Yes	Yes
Chloroplast	Double membrane	Photosynthesis	Yes	No	No
Cell wall	Nonmembranous (outside membrane)	Structural support, shape	Yes	No	Yes
Large central vacuole	Single membrane	Stores water, ions, pigments	Yes	No	No
Centrioles	Nonmembranous	Organize microtubules and spindle	No	Yes	No

How Organelles Are Studied in Practice

Organelle biology is observed with a small set of standard methods, and knowing them helps you read primary literature.

Transmission electron microscopy (TEM) resolves internal ultrastructure. It is how researchers visualize phagosome maturation and lysosome fusion directly [7]. It also reveals mitochondrial structural damage and cristae disruption in tissue models [9].

Fluorescence imaging and organelle-specific dyes report on membrane potential, pH and ion concentrations. Mitochondrial membrane potential is a common readout, and it has been measured in living animals to link fasting-induced organelle remodeling to metabolic state [6].

Spatial organellomics is a newer approach that combines automated segmentation with machine learning to classify and spatially map cell states from signatures across multiple organelles at once. Applied to liver and pancreas, it distinguished broad cellular classes, and in liver it showed that hepatocytes formed intermixed communities within canonical zones rather than cleanly separating by position [6]. This matters because it treats multi-organelle architecture as a structural readout of tissue adaptation.

Biochemical fractionation and Western blotting separate organelles by density and detect specific proteins. The LC3-II/I ratio measured this way is a standard marker of autophagosome formation [7].

Sequencing now reaches organelles directly. Organellar 16S rRNA sequences are amplified alongside prokaryote 16S rRNA, and databases that combine bacterial, archaeal and organelle sequences allow identification of both prokaryotes and eukaryotes from one environmental sample [8]. Mitochondrial genomes are assembled with dedicated bioinformatics tools because they are small, high-copy and inherited separately from the nuclear genome [1].

Comparative and Clinical Relevance

Organelle function scales up to whole-tissue behavior. When salivary gland epithelial cells lose polarity and barrier function, the change is sensed by subcellular organelles as a condition of stress, which triggers a response organized into three sequential components: sensors detect the damage, a signaling cascade transmits the alarm, and effectors execute the adaptive action [3]. One such effector mechanism is the integrated stress response, which adjusts protein biosynthetic rates to redirect resources and restore homeostasis [3].

Mitochondrial quality control is another practical example. In a rat model of D-galactose-induced kidney aging, mitochondrial structural damage was associated with impaired respiratory chain activity, altered PINK1/Parkin expression, downregulation of mitochondrial genes, increased oxidative stress and enhanced apoptosis, and nano-encapsulated chrysin significantly restored mitochondrial function and structure [9]. PINK1 and Parkin are proteins involved in mitophagy, the selective removal of damaged mitochondria.

Organelle diversity also appears in unexpected places. Warnowiid dinoflagellates build a camera eye-like structure called an ocelloid, and single-cell transcriptome sequencing across 12 additional species supports the finding that the plastid inside the ocelloid complex has likely lost photosystem II while retaining photosystem I as a putative light-sensing mechanism [10]. That is a case where a plastid, normally a photosynthetic organelle, has been repurposed for sensing.

Quick Review: Seven Points Worth Memorizing

Membrane-bound organelles include the nucleus, mitochondria, ER, Golgi, lysosomes and peroxisomes. Ribosomes and the cytoskeleton are nonmembranous [3].
Mitochondria produce most of the cell's chemical energy and carry their own genome, inherited from a prokaryotic endosymbiont [1].
The rough ER folds proteins and the smooth ER makes lipids, stores calcium and detoxifies.
The Golgi modifies, sorts and packages cargo moving through the secretory pathway.
Prokaryotes lack membrane-bound organelles. Plant cells add a cell wall, chloroplasts and a large central vacuole. Animal cells have centrioles and lysosomes.
Lysosomal pH and mitochondrial membrane potential are functional states, not fixed properties, and they influence cell fate [4].
Multi-organelle architecture can be mapped spatially and used as a readout of tissue state [6].

Common Mistakes and Limitations

Treating all organelles as membrane-bound. Ribosomes and the cytoskeleton are not enclosed by membranes. This distinction drives how they are purified, imaged and studied.

Assuming plant cells have no mitochondria. Plant cells have mitochondria and chloroplasts. Both are double-membrane organelles, and both divide using machinery with shared components [2].

Confusing the Golgi with the ER. The ER is the entry point for the secretory pathway. The Golgi is the sorting and packaging station downstream of it.

Reading one organelle in isolation. Organelles are coupled. ER-mitochondrial coupling, shared division factors and multi-organelle signatures all show that structure and function are coordinated across compartments [4][2][6].

Overgeneralizing from a single model. Organelle abundance and behavior vary by cell type, tissue and nutritional state. Spatial mapping in liver showed that position within a zone did not fully explain organelle-defined hepatocyte categories [6].

Assuming a static picture. Organelles divide, fuse, move and are recycled. Mitochondrial dynamics, trafficking and quality control are active processes, and defects in them produce measurable phenotypes [5].

Individual cells and tissues vary, and any specific experimental or clinical question needs expert interpretation rather than a general guide alone.

Frequently Asked Questions

What are the main cell parts?

The main cell parts are the plasma membrane, cytoplasm, and organelles. The core organelles are the nucleus, mitochondria, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, ribosomes and the cytoskeleton.

Which organelles are membrane-bound?

The nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and peroxisomes are membrane-bound [3]. Ribosomes and the cytoskeleton are nonmembranous.

Do bacteria have organelles?

Bacteria lack membrane-bound organelles. They have ribosomes and a cytoskeleton-like machinery but no nucleus, mitochondria or ER, and they carry out energy metabolism at the plasma membrane.

What do plant cells have that animal cells do not?

Plant cells have a cell wall, chloroplasts and a large central vacuole. Animal cells instead have centrioles and lysosomes.

What is the difference between rough and smooth ER?

Rough ER has ribosomes on its surface and folds proteins. Smooth ER lacks ribosomes and makes lipids, stores calcium and detoxifies molecules.

Why do mitochondria have their own DNA?

Mitochondria carry a separate genome whose origin is linked to the endosymbiosis of a prokaryotic cell by the host cell [1]. Their function depends on coordinated expression of both mitochondrial and nuclear genomes [5].