What Is A Cell? Definition, Structure, Types, Functions, Biology

image of a cell
Picture of a Eukaryote cell

Cells are a vital part of existence as cellular structures form the basic foundations of life. The cell is a structural, functional, and biological unit in every plant, animal, and human body.

Every cell is bound by a cell membrane that contains the different parts of a cell. They are incredibly small – approximately 0.001 mm wide at a height between 150 mm and 0.0001 m.

This article will discuss the types of cells, functions of cells, cell structure, and more.

Table of Contents

Definition of a cell

The definition of a cell is the smallest living unit that makes up all organisms. A cell is a mass of cytoplasm contained by a cell membrane. This contains a number of cell parts and organelles that carry out various functions necessary for organisms to survive.

Cell types

There are two types of cells: eukaryotic and prokaryotic. Eukaryotic cells contain a nucleus, and prokaryotic cells do not have a nucleus. Prokaryotes are single celled organisms, while eukaryotes may consist of single cells and multicellular organisms.

Prokaryotic cells

In cell biology prokaryotes are typically bacteria and archaea. Prokaryote cells are one of the first living organisms on Earth, and possess essential physiologic processes like cellular signaling.

The cellular components are less complicated than eukaryotic cells as they have no nucleus or membrane-bound organelles. DNA in prokaryotic cells contains a single circular chromosome containing cytoplasm. The cytoplasmic region is known as the nuclease.

Most procaryotes can grow from 2.5 m diameter to 0.5 m diameter. The basic functions of prokaryotes are carried out by diffusion.

Eukaryotic cells

Eukaryotic cells are cells with nuclei surrounded by a membrane containing membrane-bound organelles. These organelles have different functions that help the eukaryotic cell operate. Eukaryotic organisms include protozoans, algae, fungi, plants, and animals.

These cells have a size of 10 m to 100 m and divide through meiosis or mitosis.

Differences between Prokaryotic and Eukaryotic cells

  • Prokaryotic cells lack a nucleus while Eukaryotic cells have nuclei.
  • Prokaryotic cells divide by binary fission while Eukaryotic cells divide by meiosis and mitosis.
  • Prokaryotic cell structures are simple while the structure of Eukaryotic cells is complex with many organelles and parts to support the cell.
  • The cell wall of prokaryotic cells is made up of peptidoglycan while that of Eukaryotic cells is made up of glycoproteins and lipids, cellulose, chitin, or lignin.

Similarities between Prokaryotic and Eukaryotic cells

  • Both Prokaryotic and Eukaryotic cells have cell membranes
  • Prokaryotes and Eukaryotes both have DNA
  • Prokaryotic cells and some Eukaryotic cells have cell walls
  • Both prokaryotes and eukaryotes have ribosomes involved in the production of proteins.
  • Both prokaryotic and eukaryotic cells have cytoplasm

Cell Structure

The structure of a cell differs slightly between plants and animals. All plant cells and animal cells structures have a membrane that contains organelles which help the cell operate as a functional unit. These organelles all perform a specific task and the study of these structures is known as cell biology.

Structure of an animal cell

The structure of an animal cells consist of the cell membrane, the cytoplasm, and organelles. The organelles are bounded by a membrane that separates them from the cytoplasm and they function distinctly.

Cell membrane

The animal cell membrane is extremely thin measuring about 7.5-10 nm and has three layers known as a trilaminar appearance. The cell membrane has two layers of phospholipid molecules, which are all oriented with their water-soluble (hydrophilic) ends toward the outside and their fat-soluble portions(hydrophobic) toward the inside of the fluidlike membrane.

diagram of a cell membrane

The cell membrane is made up of lipids and glycoproteins which function mainly as receptors for molecules that control the cell. They also help with the transportation of substances in and out of the cell controlling homeostasis.

The cell membrane helps to protect the organelles located within the cell.


The cell nucleus is the largest organelle of the cell. The primary function of the nucleus is to regulate all activities within the cell.

It is a spherically structured organelle found at the cell center. The membrane surrounding the nucleus is called the nuclear membrane or nuclear envelope. The nucleus contains and holds other cell organelles like the nucleolus, chromatins, and nucleosomes. These parts are critical for determining genetic information such as genotype and phenotype.

The organelles within the nucleus control the exchange of molecules and substances such as messenger RNA (mRNA), ATP, ribosomes, nucleotides, and some proteins.


The cytoplasm of an animal cell is the fluid material found between the nucleus and the cell surface membrane.

The main functions of the cytoplasm are to support the organelles in it and serve as a medium for metabolism. The aqueous solution these membrane bound organelles float in is called cytosol. There are several key differences between cytosol and cytoplasm.


The Mitochondria are the center for aerobic respiration, hence it is commonly called the powerhouse of the cell as it produces the energy needed for metabolism. These organelles are located in the cytoplasm of the cell.

Mitochondria convert nutrients and oxygen to produce energy in the form of Adenosine Tri-phosphate (ATP).

The outer membrane of the mitochondria is permeable. This allows for the transportation of small molecules and also serves as a special channel for the transportation of large molecules. They also help with the synthesis of lipids or fat molecules.

Endoplasmic reticulum

The endoplasmic reticulum is a thin continuous folded membranous organelle found in the cytoplasm. There are two types of endoplasmic reticulum – smooth endoplasmic reticulum and rough endoplasmic reticulum.

The primary functions of these organelles are to synthesize and transport lipids (fat molecules) and proteins.


Ribosomes are small organelles found on the endoplasmic reticulum. They can be broken down in to ribosomal RNA and ribosomal proteins.

Ribosomes exist to help with the synthesis of proteins and act as a site for genetic coding into proteins.

diagram of golgi apparatus

Golgi apparatus (Golgi complex)

The Golgi apparatus is found in the cytoplasm of the animal cell and is supported by cytoplasmic microtubules. The primary function of the Golgi apparatus is to transport and modify lipids and proteins into Golgi vesicles.


Lysosomes are subcellular acidic organelles that are also known as cell vesicles. Lysosomes have digestive enzymes that breakdown proteins, lipids, and carbohydrates into small molecules such as amino acids.


The vacuole is a fluid-filled organelle located in the cytoplasm of an animal cell. Its primary function is to store food, water, and carbohydrates for the cell to use. Vacuoles also help with the removal of harmful toxins by regulating ion movement.

Contractile Vacuole

A contractile vacuole is a sub-cellular organelle involved in osmoregulation. This is the process in which the vacuole expands and contracts to pump water out of a cell.

Osmoregulation is an important part of osmosis and is guided by the osmotic pressure within the cell at any given time.

picture of cytoskeleton through a microscope
Cytoskeleton through microscope


The cytoskeleton functions as an internal framework of the cell. It is made of actin filaments (microfilaments), microtubules, and intermediate filaments.

The cytoskeleton helps to create a cell network by organizing the different organelles and cell components.


Plasmids are small extrachromosomal DNA molecules found within cells that can replicate independently because they are not attached to chromosomal DNA.

The primary function of plasmids is to transport antibiotic-resistant genes throughout the host body. They also produce toxins to help defend the host cell from potentially harmful organisms.


Centrosomes are cellular organelles that function as the primary microtubule-organizing centers in animal cells.

The centrosome is composed of two perpendicular centrioles, a daughter centriole, and a mother centriole, which are attached by interconnecting fibers.

Structure of a plant cell

The structure of plant cells can be broken down into the cell wall, cell surface membrane, and organelles within the cytoplasm. These parts all work together to help the cell function.

picture of a cell wall through a microscope
Cell wall through microscope

Cell Wall

A plant cell wall is a rigid structure that surrounds the cell membrane. It is made up of cellulose which is a polysaccharide. The primary purpose of the cell wall is to give rigidity to plants and protect cell organelles. The plant cell wall also allows free movement of molecules and ions through to the cell surface membrane.

Plant plasma membrane (cell surface membrane)

The plasma membrane is partially permeable and very thin, measuring 7 nm. Its primary function is to protect organelles and control the exchange between the cell and its environment.

picture of a chloroplast
Chloroplast through microscope


Chloroplasts are large organelles found in the leaves and green parts of plants. Chloroplasts exist to absorb sunlight during photosynthesis which helps plant cells generate the energy they need to survive.


A thylakoid is a sheet-like membrane-bound organelle in chloroplasts and that is the site of light-dependent photosynthesis reactions. Thylakoids are made up of a thylakoid membrane that surrounds a thylakoid lumen. It is also the location where chlorophyll is found in plant cells.

Central Vacuole

A central vacuole is a large vacuole in plant cells that is filled with fluids and molecules. Its primary function is to maintain the turgor pressure in plant cells which gives the plant rigidity.

Other parts of plant cells

Plant cells also contain numerous other components and organelles including cytoplasm, mitochondria, nucleus, vacuoles, endoplasmic reticulum, ribosomes, and Golgi bodies.

Similarities and differences between plant and animal cell structures

Similarities in plant and animal cells

Plant cells and animal cells contain a number of similar properties and organelles including a nucleus, golgi apparatus, membrane, endoplasmic reticulum, ribosomes, mitochondria, nucleolus, and cytoplasm.

Differences in plant cells and animal cells

There are several key differences between plant and animal cells. Plant cells are rigid, contain chloroplasts, have a cell wall, and have plastids.

Animal cells have centrioles and lysosomes which are not present in plant cells.

cell cycle diagram
Cell cycle diagram

Cell cycle

Cell cycles relate to the sequence in which a cell grows and divides via binary fission. The cell cycle involves the duplication and replication of DNA which causes splitting of a mother cell and produces two daughter cells.

The eukaryotic cell cycle includes two primary phases – Interphase and the M Phase (mitotic phase). These can be broken down further into G1, S stage, G2, Mitosis, and Cytokinesis.


During the interphase of the cell cycle, the cell grows to its normal size after cell division and then continues to carry out its normal functions until a signal to divide is received.

Throughout the interphase process the cell is actively synthesizing ribonucleic acid (RNA) as well as a number of proteins. In the last stage of interphase, the cell replicates its DNA and organelles to prepare for the M Phase.

M Phase


There are four main stages in mitosis – Prophase, Metaphase, Anaphase and Telophase; some scientists may add another stage before Metaphase known as the Prometaphase.

During mitosis, the cell separates the duplicate copies of organelles and chromosomes, known as sister chromatids, into opposite halves of the cell.


Cytokinesis is the process in which a cell divides into two new daughter cells. This is the final step in the cell cycle.


Meiosis is a type of cell division that occurs as a part of the cell cycle for sex cells. During meiosis a cell divides twice(compared to once in mitosis) to produce four daughter cells.

This process is broken down into the phases of Meiosis I and Meiosis II.

Functions of Cells

Cells serve as structural and functional building blocks for all living organisms. There are many processes that take place inside the cell and outside the cell that contribute to keeping organisms alive and healthy.


The endocytosis process involves the transportation of large molecules into the cell by engulfing them with the cell membrane. Molecules without charge, water, and other small molecules can cross the cell membrane through the process of facilitated diffusion.

However, large molecules are surrounded by an area of the cell membrane and then cut off to create a membranous vesicle. This process is known as endocytosis and is considered a form active transport.

Endocytosis can be broken down into two types; phagocytosis and pinocytosis.


Exocytosis is a process by which cells move materials that are within the cell, outside, and into the extracellular fluid. The process of exocytosis occurs as a result of a vesicle fusing with the plasma membrane which allows its content to be released outside the cell.

This is the process by which cells move large waste molecules outside of the cell to maintain homeostasis.

Differences and similarities of Endocytosis and Exocytosis

The primary difference between endocytosis and exocytosis is that in endocytosis, molecules are transported from outside of the cell into the interior of the cell. Whereas, in exocytosis, molecules and substances are transported from inside the cell to outside the cell.

The key similarities between endocytosis and exocytosis are that both mechanisms are a form of active transportbecause they require energy. They also both utilize vesicle pores to transport molecules.


Plasmolysis is the process in which the protoplasm of a plant cell shrinks away from its cell wall.

Plasmolysis occurs due to water molecules diffusing to their concentration gradient, causing plants to lose water.


Cells are extremely complicated organisms that serve as the fundamental building blocks of life on Earth. Scientists are still discovering new pieces of information about this subject in biology and will likely continue to do so as technology improves.