Animal Cell: Organelles, Functions, and How to Remember Them

Every part of you, (your skin, your heartbeat, the thought you are having right now) is the work of animal cells. A single one is far too small to see, yet inside it sits a busy set of compartments, each doing one job, all working at once.

Learn what these compartments are and what they do, and the rest of biology starts to make sense. This article walks through every organelle, shows how they work as a team, and gives you simple ways to remember them.

Every organ and system in an animal is built from tiny units called cells. Animals and plants are made of a more advanced cell type that keeps its parts in separate compartments. These compartments are called membrane-bound organelles, and they include the nucleus (plural: nuclei), mitochondria, and others. Because their cells store the genetic material inside a true nucleus, animals and plants are called eukaryotes.

The essential structural components in the animal cell include the nucleus, lysosome, mitochondria, vacuoles, Golgi bodies, ribosomes, endoplasmic reticulum, etc. Cell membrane surrounds these cell organelles and the organelles work together to keep the cell active by performing their functions.

Organelles of an Animal Cell

Animal cells are eukaryotic cells bound by cell membranes instead of rigid cell walls and have membrane-bound cell organelles. Most animal cells are visible only under the microscope due to their size (ranging from 1 µm to a few mm). The largest known animal cell is the yolk of an ostrich egg, which can be 130 to 170 mm across before the shell and white are added.

Figure: Different structural components of animal cells

The shape of animal cells is diverse. An animal cell can be oval, round, rod-shaped, concave, curved, or rectangular. The membrane-bound cell organelle- nucleus encloses the genetic material (DNA).

Why animal cells have no cell wall (and why shape matters)

A plant cell sits inside a stiff cell wall that fixes its shape, like juice inside a rigid box. An animal cell has no wall. Only the thin, flexible cell membrane holds it in.

This one difference explains a lot:

• Animal cells can take many shapes. A nerve cell is long and thread-like to carry signals over distance. A red blood cell is a flat disc so it can bend through narrow vessels. A muscle cell is long so it can shorten and pull.
• Without a wall to hold them up, animal cells build shape from the inside using a protein scaffold called the cytoskeleton (covered below under microtubules and microfilaments).
• A wall-free surface lets animal cells move, change shape, and take in large particles, things a walled plant cell cannot do easily.

So when someone asks "what shape is an animal cell?", the honest answer is: it depends on the job. Shape follows function.

The following are the cellular structure of animal cells with their functions:

Cell membrane

The outermost layer of animal cells is the cell membrane. The cell membrane function increases in animal cells due to the lack of cell walls. The cell membrane helps transport the material in and out of the cell and provides a protective covering for the cell organelles.

It is also called the plasma membrane and comprises fatty acid-based lipids and proteins. The cell membrane is composed of two types of lipids; sterols and phospholipids. These lipids are amphiphilic (attract to water and organic solvents), making them a suitable cell membrane building block. The proteins in the cell membrane are also of two types; extrinsic and intrinsic proteins.

The extrinsic proteins loosely bind to the electrically charged phosphoryl group and intrinsic proteins. The intrinsic proteins embed inside the phospholipid bilayer. See more: Cell Membrane: Structure, Function, and the Three Models

Cytoplasm

The gel-like substance inside the cell membrane is called cytoplasm. All the cell organelles lie inside the cytoplasm. The cytoplasm is made of water, dissolved salts, and organic molecules such as proteins and sugars. Inside the cytoplasm, cellular growth, expansion, and activity by cell organelles occur.

It has three parts; cytosol, cell organelles, and cell inclusions. The cytosol is the liquid part of the cytoplasm. The cell organelles are membrane-bound, like the nucleus, mitochondria, or non-membrane-bound ribosomes or centrioles. The cell inclusions are the nutrients or pigments dissolved in the cytoplasm.

Nucleus

The nucleus is a membrane-bound organelle that keeps the DNA separated from the rest of the cell. The nuclear membrane has double layer with pores that help interact with the cytoplasm.

The membrane also binds the nucleolus and DNA, the cell’s genetic material. The nucleolus is the spherical structure inside the nucleus that helps to produce and assemble ribosomes. Here the ribosomal RNA also gets produced, which is transported to the cell’s cytoplasm for protein synthesis.

Figure: Nucleus

DNA (deoxyribonucleic acid) is the hereditary material of an animal cell. Its structure is a double helix, and the DNA forms by deoxyribose sugar and nucleotides. DNA tightly bind to histone protein and forms a complex known as chromatin. The chromatin condenses to form chromosomes.

Mitochondria

Mitochondria are the cellular organelles bound by membranes. They are also known as the cell’s powerhouse because mitochondria produce the chemical energy required for the proper functioning of the cell. Different metabolic processes like the Krebs cycle, urea cycle, heme biosynthesis, cardiolipin synthesis, quinone, and steroid biosynthesis occur inside the mitochondria. See this article: Mitochondria: Structure, Function, Location for detailed information.

Figure: Structure of mitochondria of animal cell

Oxidative phosphorylation coupled with the Kreb cycle helps in the production of the energy currency of the cell ATP. ATP is the small molecule that stores the chemical energy required for proper cell functioning.

Ribosomes

Figure: Ribosome

The ribosome is the non-membrane-bound cell organelle responsible for protein synthesis. It comprises proteins and RNA molecules. DNA codes for a specific protein, and the mRNA carries the code to ribosome. The ribosome reads the code carried by mRNA and joins amino acids together in the right order. This chain of linked amino acids is a polypeptide. The polypeptide chains fold to produce specific proteins.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a group of flattened sacs that forms a network. The endoplasmic reticulum is composed of a network of membranes called cisternae. It processes and transports proteins. It also stores calcium and region for lipid metabolism.

Figure: Smooth and rough endoplasmic reticulum

ER is of two types; smooth and rough. Rough ER has ribosomes around it, and smooth ER has no ribosomes. The smooth ER synthesizes lipids, removes toxic substances, and acts as storage. The rough ER plays a vital role in protein synthesis.

Golgi Bodies

Golgi bodies, apparatus, or complexes consist of membrane-bound sacs named after the discoverer Camillo Golgi. It receives proteins from ER and packages proteins into vesicles. Unlike ER, the Golgi complex does not interconnect. It also helps in the packaging and processing of lipid molecules.

Centrioles

Figure: Centriole

Centrioles are cylindrical structures usually occurring as pairs. These are located near the nuclear envelope. These organize the assembly of microtubules, a cytoskeleton, during the division of cells. They also decide the location of the nucleus and other cellular organelles in the cell.

Microtubules

Microtubules are hollow tubes built from a protein called tubulin (made of alpha and beta tubulin units). They are the thickest fibers of the cytoskeleton, the internal scaffold that gives an animal cell its shape.

What microtubules do:

• Hold the cell's shape. Because there is no cell wall, this internal scaffold does the job a wall would do in a plant.
• Act as tracks. Tiny motor proteins walk along microtubules carrying vesicles and organelles from one part of the cell to another, like rails moving cargo across a warehouse.
• Build the spindle during cell division. When a cell divides, microtubules form the spindle that pulls the copied chromosomes to opposite ends. This is why microtubules are central to mitosis.

Microtubules are not fixed. They constantly grow and shrink, which lets the cell rebuild its scaffold quickly when it changes shape or divides.

Microfilaments

Microfilaments or actin filaments are proteinous structures inside the cell. These are structural components of the cytoskeleton and provide shape and movement to the cells. Microfilaments consists of a group of proteins called actin. When associated with myosin filaments, microfilaments help in cell contraction and movement.

Peroxisomes

Peroxisomes are membrane-bound organelles. These structures contain enzymes that help detoxify alcohol, break down fats, and form bile acid.

In doing so they produce hydrogen peroxide, a toxic byproduct, which the peroxisome then breaks down with its own enzyme (catalase). This make-and-destroy handling of hydrogen peroxide is what gives the peroxisome its name.

Lysosomes

Figure: Lysosome

Lysosomes are a group of vesicles bound by a membrane that contains digestive enzymes. These enzymes break down large molecules like organelles, carbohydrates, lipids, and proteins into small parts so cells can reuse them. These enzymes also help to protect cells against invading bacteria and viruses.

Vacuoles

Vacuoles are another type of vesicle that are membrane-bound. Unlike plant vacuoles, animal vacuoles are not well-developed and numerous. But their function is the same as plant vacuoles, which remove waste from the cell. See more about vacuoles.

How the organelles work together

Organelles are easier to remember when you see them as a team, not a list. Follow one job, making and shipping a protein, and most of the cell falls into place:

1. The nucleus holds the DNA. The instructions for a protein are copied from DNA into mRNA.
2. The mRNA leaves the nucleus and reaches a ribosome. Ribosomes on the rough endoplasmic reticulum read the message and build the protein.
3. The rough ER folds and checks the protein, then packs it into a small bubble called a vesicle.
4. The vesicle carries the protein to the Golgi apparatus, which finishes it, labels it, and sorts it for its final address.
5. The Golgi ships the protein in another vesicle to where it is needed, often the cell membrane, which releases it out of the cell.

All the while, mitochondria supply the ATP energy that powers each step, and lysosomes stand by to break down anything faulty. This assembly line, nucleus to ER to Golgi to membrane, is called the secretory pathway. If you can retell it in your own words, you understand the cell.

Types of Animal Cell

Animal cells form different tissues and organs of the human body. Based on the organs/tissue formed by the cells, they are of the following types:

1. Skin cells: Skin cells are the cells that form the epidermal tissue, which protects the body from the external environment. The skin cells are of four different types; keratinocytes, melanocytes, Langerhans, and Merkel cells.
2. Nerve cells: These are the cells that form the communication channel of the nervous system. The nerve cells have a cell body inside: the nucleus, an axon (major branching fiber), and multiple dendrites (minor branching fiber).
3. Gamete cells: The gamete cell is the reproductive cells in the body. These cells are usually haploid. There are two types of gametes; female or ova or egg cells and male or sperm cells. When the two gametes join at fertilization, they form a diploid cell that develops into an embryo.
4. Muscle cells: Muscle cells or myocytes form the tissues of skeletal muscles and the heart. Muscle cells are of three types in the human body: cardiac muscle cells- found in the heart; smooth muscle cells- which form the muscular layer of vessels; and skeletal muscle cells- found in muscle cells connecting the skeleton.
5. Blood cells: Blood consists of different cells, namely white blood cells, red blood cells (erythrocytes), and platelets (thrombocytes). White blood cells are of different types: lymphocytes, monocytes, eosinophils, basophils, macrophages, and neutrophils.
6. Fat cells: Fat cells or adipocytes are special cells that can store large globules of fats. These are found underneath the skin, around the internal organs, and between the muscles.
7. Stem cells: These are particular types of cells that can develop into any kind of cells in the body, from the brain to muscle cells. These cells help in repairing damaged cells or tissues. These cells are used as therapy for severe diseases like Alzheimer’s.

How to Remember

The cell is a factory. This one analogy anchors almost every organelle:

Powerhouse of the cell = mitochondria. The oldest biology mnemonic, and it still works: power → power station → mitochondria.

Rough vs smooth ER: Rough has Ribosomes (both start with R). Rough = protein; smooth = lipids and detox.

Lyso = loosen/break. A Lysosome Loosens and breaks big molecules apart. (From Greek lysis, to loosen.)

Key exam facts

Where students get confused

Rough ER vs smooth ER. Rough ER has ribosomes stuck on it and makes proteins. Smooth ER has none and makes lipids and handles detox. Trick: Rough has Ribosomes.

Free ribosomes vs bound ribosomes. Ribosomes floating in the cytoplasm make proteins for use inside the cell. Ribosomes bound to the rough ER make proteins for export or for the membrane. Same ribosome, different address.

Cytoplasm vs cytosol. Cytoplasm is everything inside the membrane except the nucleus, organelles included. Cytosol is only the fluid part, without the organelles. Cytosol is a part of the cytoplasm.

Lysosome vs peroxisome. Both break things down. Lysosomes use digestive enzymes to break large molecules and worn-out parts. Peroxisomes mainly break down fats and neutralize toxic substances, producing and then destroying hydrogen peroxide. Clue: peroxisome contains "peroxi."

Centriole vs centromere. Sound alike, unrelated. Centrioles are cylinders that organize microtubules during division. The centromere is the pinched point on a chromosome where the two copies are joined. One is an organelle; the other is a part of a chromosome.

Chromatin vs chromosome. Same DNA, different packing. Chromatin is the loose, uncoiled form during normal cell life. Chromosomes are the tightly coiled form seen during cell division.

References

1. Urry LA, Cain ML, Wasserman SA, Minorsky PV, Reece JB. Campbell Biology. 12th ed. Pearson; 2021.
2. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. W. W. Norton; 2022.
3. Lodish H, Berk A, Kaiser CA, et al. Molecular Cell Biology. 9th ed. W. H. Freeman; 2021.
4. Cooper GM. The Cell: A Molecular Approach. 8th ed. Oxford University Press; 2019.
5. National Center for Biotechnology Information. Organelles of the Eukaryotic Cell. In: Molecular Cell Biology. Bookshelf. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21743/