Animal Cell Structure, Diagram and Organelles – Definition, Types, Functions

Introduction

An animal cell is the basic structural and functional unit of animal life. Understanding its organization helps students understand how tissues, organs, and whole organisms work. In this article, you will learn what defines an animal cell, its size and shape, how it differs from a plant cell, and the structure and functions of its 16 major components.

What is an Animal Cell?

An animal cell is a eukaryotic cell that lacks a cell wall and is enclosed by the plasma membrane. Its organelles, including the nucleus, are enclosed within this membrane.

Animals represent a large and diverse group of living organisms, making up about three-quarters of all described species on Earth. Their ability to move, respond to stimuli, adapt to environmental changes, and show diverse modes of feeding, defense, and reproduction is supported by specialized cells.

Unlike plants, animals cannot manufacture their own food by photosynthesis and therefore depend directly or indirectly on plants.

All living things are made of cells. Some organisms are unicellular, made of a single cell, while others are multicellular, made of many cells. A cell is the smallest microscopic structural-functional unit of life. Cells that constitute animals are called animal cells, and those that constitute plants are called plant cells.

Most plant cells, bacteria, and fungi are surrounded by a rigid cell wall that gives shape and rigidity. Animal cells lack this rigid wall. This allows them to develop a great diversity of cell types, tissues, and organs. Specialized cells such as nerve cells and muscle cells enable movement and rapid communication, functions that plant cells cannot perform in the same way.

The coordinated work of all animal cells enables movement, reproduction, response to stimuli, digestion and absorption of food, and maintenance of homeostasis.

Animal Cell Size and Shape

Animal cells show wide variation in shape and size.

• Size range: Most animal cells range from about 10 to 30 micrometers in diameter, though the range extends from a few micrometers to millimeters.
• Largest animal cell: The ostrich egg, from Struthio camelus, before incubation, is often cited as the largest single animal cell. It can be about 13 cm (about 5 inches) in diameter and weigh 1.2 to 1.4 kg. It is essentially a single cell with a large yolk reserve.
• Smallest animal cells: Among the smallest are red blood cells, about 7.5 micrometers in diameter, and sperm cells. Neurons have a cell body that is typically 10 to 100 micrometers, but they can be the longest cells due to their axons.

Animal cells are generally smaller than plant cells and are irregular in shape because they lack a cell wall. They can be round, oval, flattened, rod-shaped, spherical, concave, or irregular. Most are microscopic and can only be studied in detail under a microscope.

Despite differences in shape and wall structure, animal cells share many organelles with plant cells because both evolved from a common eukaryotic ancestor.

As eukaryotic cells, animal cells have a membrane-bound nucleus that encloses genetic material in the form of DNA, and a variety of membrane-bound organelles in the cytoplasm that perform specific functions.

Overview of Animal Cell Structure

The animal cell is made of several structural organelles enclosed by the plasma membrane. Each organelle has a specific role, and their coordinated activity maintains normal body mechanisms.

The cytoplasm consists of cytosol, a gel-like aqueous fluid, plus all organelles except the nucleus. Cytosol itself contains water, ions, proteins, and metabolites.

List of Animal Cell Organelles

The major components commonly described in animal cells are:

• Plasma membrane (Cell membrane)
• Nucleus
• Cytoplasm
• Mitochondria
• Ribosomes
• Endoplasmic Reticulum (ER)
• Golgi apparatus (Golgi complex)
• Lysosomes
• Cytoskeleton
• Microtubules
• Centrioles
• Peroxisomes
• Cilia and Flagella
• Endosome
• Vacuoles
• Microvilli

Plasma Membrane (Cell Membrane)

Definition: A thin, semi-permeable membrane that surrounds the animal cell and separates its interior from the external environment.

Structure:

• It is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates, described by the fluid mosaic model.
• Lipids form the basic barrier.
• Proteins function as transporters, receptors, and adhesion molecules.
• Glycoproteins and glycolipids on the outer surface help in cell recognition.
• All living cells have a plasma membrane.

Functions:

• Encloses and protects cell contents.
• Regulates passage of molecules into and out of the cell, maintaining homeostasis.
• Protein components transport materials by passive and active mechanisms.
• Lipids and proteins allow cell communication and cell-to-cell recognition via sugar chains.

Nucleus

Definition: A spherical organelle, usually located centrally, surrounded by a double-layered nuclear envelope that separates it from the cytoplasm.

Structure:

• The double membrane is continuous with the endoplasmic reticulum network and contains nuclear pores that allow transport of large molecules like RNA and proteins.
• Nucleoplasm suspends chromatin and the nucleolus.
• Nucleolus is involved in rRNA synthesis and ribosome subunit assembly.
• Chromatin consists of DNA wrapped around histones to form nucleosomes.
• Most animal cells have one nucleus. Multinucleated cells exist, for example skeletal muscle fibers. Some cells lose their nuclei after maturation, such as mature mammalian red blood cells.

Functions:

• Controls cell growth, metabolism, and reproduction.
• Stores hereditary information in genes.
• DNA codes for proteins via its amino acid sequences.
• Site of transcription, where mRNA is formed from DNA. The mRNA is then transported through nuclear pores to the cytoplasm.

Cytoplasm

Definition: The entire region between plasma membrane and nucleus that contains cytosol and organelles.

It contains mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, cytoskeletal filaments, vesicles, and other inclusions. It is the site of many metabolic reactions.

Mitochondria

Definition: Membrane-bound organelles in the cytoplasm of all eukaryotic cells, often called the powerhouse of the cell.

Structure:

• Rod-shaped, oval, or spherical, typically 0.5 to 10 micrometers.
• Two membranes: a smooth outer membrane permeable to small molecules, and a folded inner membrane that forms cristae.
• Matrix is the central gel-like space containing mitochondrial DNA (mtDNA), ribosomes, and enzymes for the tricarboxylic acid (TCA) or Krebs cycle.

Functions:

• Generates adenosine triphosphate (ATP) by converting nutrients and oxygen into energy through the electron transport chain (ETC) and chemiosmotic coupling or oxidative phosphorylation.
• Stores calcium for cell signaling, generates heat, and regulates cell growth and death.
• Inner membrane houses ETC proteins that carry out sequential oxidation-reduction reactions to phosphorylate ADP to ATP. This powers muscle contraction and brain function.
• Human mtDNA contains 37 genes, 13 of which encode major ETC components. Most other mitochondrial proteins are encoded by nuclear DNA.
• mtDNA is more vulnerable to mutations because it has limited DNA repair mechanisms. Reactive oxygen species (ROS), also called free radicals, are produced in mitochondria and normally neutralized by antioxidant proteins, but excess ROS can damage mtDNA.
• Alcohol metabolism can increase ROS and damage mtDNA.
• In most animals, mtDNA is maternally inherited because the egg contributes most cytoplasm, while paternal mitochondria are usually degraded.
• Accumulation of mutated mtDNA is linked to aging, certain cancers, and mitochondrial disorders including some neurodegenerative conditions. Mitochondria also play a central role in apoptosis or programmed cell death. Failure of apoptosis can contribute to cancer development.

Ribosomes

Definition: Small granules made of ribosomal RNA (rRNA) and proteins, responsible for protein synthesis.

Structure:

• Composed of about 60% rRNA and 40% protein in many descriptions, with roughly equal proportions in eukaryotic ribosomes.
• Present free in cytoplasm or bound to rough ER.
• Each eukaryotic ribosome is 80S, made of a 40S small subunit and 60S large subunit. A single active animal cell can contain around 10 million ribosomes.

Functions:

• Site of translation, where mRNA code is read to assemble amino acids via transfer RNA (tRNA).
• rRNA catalyzes peptidyl transferase activity to form peptide bonds between amino acids. Newly formed proteins detach and move to other parts of the cell for use.

Endoplasmic Reticulum (ER)

Structure: A continuous network of flattened interconnected sacs and tubules extending from the nuclear envelope into the cytoplasm. The lumen inside is called the cisternal space.

Functions:

• Manufacturing, processing, and transport of proteins and lipids.
• Directly connected to the nuclear envelope, providing a passage between nucleus and cytoplasm.
• Contains more than half of total membrane in many animal cells, providing a large surface area for chemical reactions and lipid synthesis.

Types of ER:

1. Rough Endoplasmic Reticulum (Rough ER): Surface covered with ribosomes, giving a rough appearance. Ribosomes synthesize proteins with a signal sequence that directs them to ER for processing. Rough ER transports proteins and lipids to Golgi or plasma membrane.
2. Smooth Endoplasmic Reticulum (Smooth ER): Lacks ribosomes. Synthesizes lipids including cholesterol and phospholipids for new membranes and steroid hormones from cholesterol in certain cells. Involved in detoxification of drugs and toxic chemicals in liver cells.
3. Sarcoplasmic reticulum: A specialized smooth ER in muscle cells that regulates calcium ion concentration in cytoplasm to control contraction.

Golgi Apparatus (Golgi Complex)

Structure: Membrane-bound organelle found near ER and nucleus. Supported by cytoplasmic microtubules and a protein matrix. Made of flattened stacked pouches called cisternae. Typically 4 to 10 cisternae per stack in animal cells, though some protists can have up to 60. Has three regions: cis face nearest ER, medial region, and trans face farthest from ER. Animal cells usually have 1 to 2 Golgi stacks, while plant cells can have hundreds.

Functions:

• Modifies, sorts, and packs proteins and lipids into Golgi vesicles for delivery to target sites.
• Vesicles from ER fuse with the cis-Golgi network via a vesicular-tubular cluster. Cargo moves from cis to trans while being modified.
• Modifications include cleaving oligosaccharide chains, attaching sugar moieties, adding fatty acids or phosphate groups by phosphorylation, and removing mannose residues in cis and medial cisternae and adding galactose in trans cisternae.
• Sorting occurs in the trans-Golgi network, and modified proteins and lipids are packed into vesicles that go to lysosomes or to the plasma membrane for exocytosis, often assisted by receptor-ligand interactions.

Lysosomes

Also known as cell vesicles, discovered by Christian René de Duve in the 1950s.

Structure: Round organelles, found in almost all eukaryotic cells, surrounded by a single membrane. They are very acidic inside.

Functions:

• Site for intracellular digestion, excretion, and renewal.
• Break down macromolecules from outside and inside the cell into simpler units that are transported to cytoplasm via proton pumps for reuse.
• Targets include worn-out organelles, waste products, microorganisms, and cell debris.
• Contains hydrolytic enzymes or acid hydrolases that break proteins into amino acids, carbohydrates into simple sugars, and lipids into fatty acids.
• Enzymes are active only at acidic pH inside lysosome, protecting the cell from self-digestion if leakage occurs because cytoplasmic pH is neutral to slightly alkaline.

Cytoskeleton

Structure: A fibrous network in cytoplasm formed by proteins made of long chains of amino acids. Composed of three types of filaments: actin filaments (microfilaments), microtubules, and intermediate filaments. Associated proteins include septin, which helps assemble filaments, and spectrin, which links plasma membrane to cytoskeleton.

Functions:

• Organizes cell components and maintains cell shape.
• Provides uniform movement of cell and organelles.
• Enables cell elasticity to endure physical tension.
• Actin filaments form a meshwork running parallel, important for cell shape, movement, adherence to substrates, and cleavage during mitotic cell division.
• Microtubules are long filaments that help move daughter chromosomes during mitosis.
• Intermediate filaments are more stable and form the true internal scaffold, holding nucleus in position.

Microtubules

Structure: Long, straight, hollow cylinders constructed from 13 to 15 protofilaments made of tubulin, a globular protein found only in eukaryotic cells. Found throughout cytoplasm.

Functions:

• Transport organelles such as mitochondria and vesicles, for example along axons from neuron cell body to axon tips.
• Provide structural support and help position Golgi bodies.
• Provide rigid and organized component of cytoskeleton to maintain cell shape.
• Main elements of cilia and flagella.
• Form spindle fibers that attach to chromosomes during mitotic division.

Centrioles

Distinctly found in animal cells and some lower plants, capable of duplication.

Structure: Small structure made of 9 sets of triplet microtubules arranged in a cylinder. Triplet microtubules are held by proteins and surrounded by pericentriolar matrix containing molecules that nucleate new microtubules. Each microtubule is made of tubulin subunits forming hollow tubes. Found in centrosome.

Functions:

• Anchor and organize microtubules.
• Glycoprotein-linked signals help target proteins to specific locations.
• In mitosis, each centriole replicates to form two centrosomes. Microtubules between centrosomes push centriole pairs to opposite ends of cell. Microtubules then extend to seek chromosomes and bind at centromere. Depolymerization helps move chromosomes apart.

Peroxisomes

Tiny bodies in cytoplasm.

Structure: Spherical, single membrane-bound, among the most common microbodies in cytoplasm.

Functions:

• Lipid metabolism, including breakdown of very long-chain fatty acids by beta-oxidation.
• Chemical detoxification. Move hydrogen atoms to oxygen to produce hydrogen peroxide, then break down hydrogen peroxide using catalase, neutralizing poisons such as alcohol.
• Important in reactive oxygen species metabolism.

Cilia and Flagella

Locomotive projections on cell surface.

Structure: Made of microtubule bundles. Most motile cilia and flagella have a 9+2 arrangement: nine doublet microtubules around two central microtubules. Partial and complete microtubules extend along projection. Motor protein dynein links doublets, generating bending. Whole assembly is enclosed by plasma membrane.

Functions:

• Flagella allow sperm cells to swim to ova for fertilization.
• In single cells, enable swimming.
• Cilia move fluids and particles over cell surfaces, such as moving mucus in respiratory epithelium.
• In nostrils and airways, help clear surface particles.

Endosome

Vesicles formed by endocytosis in cytoplasm.

Structure: Membranous organelles derived from invagination of plasma membrane.

Functions:

• Sorting compartment for material internalized from extracellular fluid.
• Primary role in intracellular transport and removing material via endocytic processes, including recycling to membrane or delivery to lysosomes. Participates in phagocytosis and exocytosis pathways.

Vacuoles

Fluid-filled sacs enclosed by membrane.

Structure: Membrane-bound sacs in cytoplasm. In animal cells, vacuoles are generally small, numerous, and temporary. The term tonoplast is primarily used for the membrane of the large central vacuole in plant cells, not typical for animal cells.

Functions:

• Store food, water, ions, and waste materials temporarily.
• Regulate pH and help in exocytosis of waste.
• Remove toxic substances and misfolded proteins as part of cellular protection.
• Can change shape and size according to cell needs. In some animal cells, they play roles in endocytosis and autophagy.

Microvilli

Surface protrusions, not membrane-bound organelles in classical sense but specialized membrane specializations.

Structure: Formed from bundled actin filaments with accessory proteins projecting from cell surface.

Functions:

• In small intestine, greatly increase surface area for absorption of digested food and water.
• In inner ear, stereocilia, which are related structures, help detect sound waves and convert them to electrical signals to brain.
• Help anchor sperm to egg during fertilization.
• In white blood cells, act as anchors that allow rolling and attachment along blood vessel walls to reach sites of infection.

Key Differences Between Animal and Plant Cells