Cell Structure

Cells are the basic units of all life. They make up every organ of every animal, plant, fungus, and bacteriaCells in a body are like the building blocks of a house. They also have a specific basic structure that is shared by most cells. Cells usually consist of:

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Table of contents

    • The cell membrane - this is a lipid bilayer that marks the limits of the cell. Within it, we can find the other two basic components of the cell: the DNA and the cytoplasm. All cells have a cell or plasma membrane.
    • DNA - the DNA contains the instructions so that the cell can function. The genetic material can be protected within the nucleus (eukaryotic cells) or floating in the cytoplasm (prokaryotic cells). Most cells have DNA, but red blood cells, for example, don't.
    • Cytoplasm - the cytoplasm is the viscous substance within the plasma membrane in which the other components of a cell (the DNA/nucleus and other organelles) are floating.

    Prokaryotic and eukaryotic cell structures

    The definition of prokaryote roughly translates from Greek as: 'without kernel' meaning 'without nucleus'. Hence, prokaryotes never have a nucleus. Prokaryotes are usually unicellular, which means that bacteria, for example, are only made up of one single cell. There are, however, exceptions to that rule where the organism is unicellular but has a nucleus, so it's a eukaryote. Yeast is one example.

    On the other hand, eukaryote in Greek translates to “true nucleus”. This means that all eukaryotes have a nucleus. Except for yeast, eukaryotes are multicellular as they can be made up of millions of cells. Humans, for example, are eukaryotes, and so are plants and animals. In terms of cell structure, eukaryotes and prokaryotes share some traits but are different in others. The following table shows the similarities and differences while also giving us a general overview of the cell structures we will be discussing in this article.

    Table 1. Features of prokaryotic and eukaryotic cells.

    Prokaryotic cells

    Eukaryotic cells
    Size1-2 μmUp to 100 μm
    CompartmentalizationNoMembranes that separate different organelles of the cell
    DNACircular, in the cytoplasm, no histonesLinear, in the nucleus, packed with histones
    Cell membraneLipid bilayerLipid bilayer
    Cell wallYesYes
    Endoplasmic reticulumNoYes
    Golgi apparatusNoYes
    Lysosomes & PeroxisomesNoYes

    Cell Structure cell structure of a prokarytoic cells example StudySmarterFig. 1 - An example of prokaryotic cells

    Cell structure, eukaryotic cell, StudySmarterFig. 2 - An animal cell

    Human Cell Structure and Function

    The structure of a human cell, as for any cell, is tightly linked to its function. Overall, all cells have the same basic functions: they give structure to the organs or organisms they are part of, they turn food into usable nutrients and energy and carry out specialised functions. It is for those specialised functions that human (and other animal cells) have distinct shapes and adaptations.

    For example, many neurons have an elongated section (axon) sheathed in myelin to facilitate the transmission of action potentials.

    Structures within a cell

    Organelles are structures within a cell that are surrounded by a membrane and carry out different functions for the cell. For example, mitochondria are in charge of generating energy for the cell, while the Golgi apparatus is involved in sorting proteins, among other functions.

    There are many cell organelles, the presence and abundance of each organelle will depend on whether an organism is prokaryotic or eukaryotic, and the cell type and function.

    Cell membrane

    Both eukaryotic and prokaryotic cells contain cell membranes that are made up of a phospholipid bilayer (as seen below). The phospholipids (red in the figure) are made up of heads and tails. Heads are hydrophilic (water-loving) and face into the extracellular medium, while the tails are hydrophobic (do not like water) and face inwards.

    The cell membrane separates the cellular contents from the surrounding medium. The cell membrane is a single membrane.

    Cell structure cell membrane structure phospholidpid bilayer StudySmarterFig. 3 - Phospholipid bilayer of the plasma membrane

    If there are two lipid bilayers on the membrane, we call this a double membrane (Figure 4).

    Most organelles have single membranes, except the nucleus and the mitochondria, which have double membranes. In addition, cell membranes have different proteins and sugar-bound proteins (glycoproteins) embedded in the phospholipid bilayer. These membrane-bound proteins have different functions, for example, facilitating communication with other cells (cell signalling) or allowing specific substances to enter or leave the cell.

    Cell signalling: Transport of information from the cell's surface to the nucleus. This allows communication between the cells and the cell and its environment.

    Cell Structure phospholipid bilayer monolayer double layer StudySmarterFig. 4 - Structural differences between single and double membranes

    Regardless of the structural differences, these membranes provide compartmentalization, separating the individual contents that these membranes surround. One good way to understand compartmentalization is to imagine walls of a house that separate the interior of the house from the external environment.

    Cytosol (matrix)

    The cytosol is a jelly-like liquid within the cell and supports the function of all the cells' organelles. When you refer to the whole contents of the cell, including the organelles, you would call it the cytoplasm. The cytosol consists of water and molecules such as ions, proteins, and enzymes (proteins that catalyse a chemical reaction). Various processes take place in the cytosol, such as the translation of RNA into proteins, also known as protein synthesis.


    Though flagella can both be found in prokaryotic and eukaryotic cells, they have a different molecular build. They are, however, used for the same purpose: motility.

    Cell Structure sperm cell example of eukaryotic flagellum StudySmarterFig. 5 - A sperm cell. The long appendage is an example of a eukaryotic flagellum.

    Flagella in eukaryotes are made up of microtubules that have tubulin - a structural protein. These types of flagella will use ATP to move forwards and backwards in a sweeping/whip-like motion. They can be easily confused with cilia as they resemble them in structure and motion. An example of the flagellum is one on the sperm cell.

    Flagella in prokaryotes, also often called "the hook" is enclosed by the cell's membrane, it contains protein flagellin. Different from the eukaryotic flagellum, the movement of this type of flagellum is more like a propeller - it will move in clockwise and anti-clockwise motions. In addition, the ATP is not used for the motion; the motion is generated with a proton-motive (movement of protons down the electrochemical gradient) force or the difference in ion gradients.


    Ribosomes are small protein-RNA complexes. You can either find them in the cytosol, mitochondria or membrane-bound (rough endoplasmic reticulum). Their main function is to produce proteins during translation. The ribosomes of prokaryotes and eukaryotes have different sizes, with prokaryotes having smaller 70S ribosomes and eukaryotes having 80S.

    Cell structure, ribosome during transcription, StudySmarterFig. 6 - Ribosome during transcription

    70S and 80S refer to the ribosome sedimentation coefficient, an indicator of the sizes of ribosomes.

    Eukaryotic cell structure

    Eukaryotic cell structure is much more complex than prokaryotic. Prokaryotes are also single-celled, so they can not "create" specialized structures. For example, in the human body, eukaryotic cells form tissues, organs and organ systems (e.g. cardiovascular system).

    Here are some structures unique to eukaryotic cells.

    Nucleus and nucleolus

    The nucleus contains most of a cell's genetic material and has its own double membrane called the nuclear membrane. The nuclear membrane is covered in ribosomes and has nuclear pores throughout. The biggest part of the eukaryotic cell's genetic material is stored in the nucleus (different in prokaryotic cells) as chromatin. Chromatin is a structure where special proteins called histones package the long DNA strands to fit inside the nucleus. Inside the nucleus is another structure called the nucleolus that synthesizes rRNA and assembles ribosomal subunits, which are both needed for protein synthesis.

    Cell structure Nucleus nucleolus StudySmarterFig. 7 - Structure of the nucleus


    Mitochondria are often referred to as energy-producing cell's powerhouses and for a good reason - they make ATP which is essential for the cell to carry out its functions.

    Cell structure Mitochondria ATP Granules StudySmarterFig. 8 - Structure of the mitochondrion

    They are also one of the few cell organelles that have their own genetic material, mitochondrial DNA. Chloroplasts in plants are another example of an organelle with its own DNA.

    Mitochondria have a double membrane just like the nucleus, but without any pores or ribosomes attached. Mitochondria produce a molecule called ATP which is the organism’s energy source. ATP is essential for all organ systems to function. For example, all our muscle movements require ATP.

    Endoplasmic reticulum (ER)

    There are two types of endoplasmic reticulum - the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

    Cell Structure endomembrane eukaryotic cell system endoplasmic reticulum rough smooth StudySmarterFig. 9 - The endomembrane system of the eukaryotic cell

    The RER is a channel system that is directly connected to the nucleus. It is responsible for the synthesis of all proteins as well as the packaging of these proteins into vesicles that are then transported to the Golgi apparatus for further processing. For proteins to be synthesised, ribosomes are needed. These are directly attached to the RER, giving it a rough appearance.

    In contrast, the SER synthesises different fats and stores calcium. The SER does not have any ribosomes and therefore has a smoother appearance.

    Golgi apparatus

    The Golgi apparatus is a vesicle system that bends around the RER on one side (also known as the cis side), the other side (trans side) faces towards the inside of the cell membrane. The Golgi apparatus receives the vesicles from the ER, processes the proteins and packages the processed proteins to be transported out of the cell for other uses. Furthermore, it synthesises lysosomes by loading them with enzymes. In plants, the Golgi apparatus also synthesises cellulose cell walls.

    Cell structure Golgi apparatus transport vesicle cisternae StudySmarterFig. 10 - Structure of the Golgi apparatus


    Lysosomes are membrane-bound organelles that are packed with specific digestive enzymes called lysozymes. Lysosomes break down all unwanted macromolecules (i.e. big molecules made up of a lot of parts) they are then recycled into new molecules. For example, a large protein would be broken down into its amino acids, and those can later be reassembled into a new protein.


    The cytoskeleton is like the bones of cells. It gives the cell its shape and keeps it from folding in on itself. All cells have a cytoskeleton, which is made up of different protein filaments: big microtubules, intermediate filaments, and actin filaments which are the smallest part of the cytoskeleton. The cytoskeleton is found in the cytoplasm near the cell membrane of a cell.

    Plant cell structure

    Plant cells are eukaryotic cells just like animal cells, but plant cells have specific organelles that are not found in animal cells. Plant cells, however, still have a nucleus, mitochondria, a cell membrane, Golgi apparatus, endoplasmic reticulum, ribosomes, cytosol, lysosomes and a cytoskeleton. They also have a central vacuole, chloroplasts, and a cell wall.

    Cell Structure illustration of a plant cell StudySmarterFig. 11 - Structure of the plant cell


    Vacuoles are large, permanent vacuoles mostly found in plant cells. A vacuole of a plant is a compartment that is filled with isotonic cell sap. It stores fluid that maintains turgor pressure and contains enzymes that digest chloroplasts in mesophyll cells.

    Animal cells also have vacuoles but they are much smaller and have a different function - they help sequester waste material.


    Chloroplasts are organelles present in leaf mesophyll cells. Like mitochondria, they have their own DNA, termed chloroplast DNA. Chloroplasts are where photosynthesis takes place within the cell. They contain chlorophyll, which is

    a pigment responsible for the green colour that is typically associated with leaves.

    Cell Structure chloroplast structure StudySmarterFig. 12 - Structure of a chloroplast

    There is a whole article dedicated to the humble chloroplast, go have a look!

    Cell wall

    The cell wall surrounds the cell membrane and, in plants, is made of a very sturdy material called cellulose. It protects the cells from bursting at high water potentials, makes it more rigid and gives plant cells a distinctive shape.

    It is important to note that many prokaryotes also have a cell wall; however, the prokaryotic cell wall is made of a different substance called peptidoglycan (murein). And so do fungi! But theirs is made of chitin.

    Prokaryotic cell structure

    Prokaryotes are much simpler in structure and function than eukaryotes. Here are some of the features of these types of cells.


    Plasmids are DNA rings that are commonly found in prokaryotic cells. In bacteria, these rings of DNA are separate from the rest of the chromosomal DNA. They can be transferred into other bacteria to share genetic information. Plasmids are often where the genetic advantages of bacteria originate, such as antibiotic resistance.

    Antibiotic resistance means that the bacteria will be resistant to the antibiotics. Even if one bacterium with this genetical advantage survives, it will divide at a high speed. This is why it is essential for people taking antibiotics to finish their course and also only take antibiotics when required.

    Vaccines are another good way to lower the risk of antibiotic resistance in the population. If a lower number of people are infected, a lower number will need to take antibiotics to combat the disease and thus a decreased use of antibiotics!


    A capsule is usually found in bacteria. Its sticky outer layer prevents the cell from drying out and helps bacteria, for example, stick together and stick to surfaces. It is made up of polysaccharides (sugars).

    Cell Structure - Key takeaways

    • Cells are the smallest unit of life; they have a specific structure made up of a membrane, cytoplasm and different organelles.
    • Eukaryotic cells have a nucleus.
    • Prokaryotic cells have circular DNA that is in the cytoplasm. They do not have a nucleus.
    • Plant cells and some prokaryotes have a cell wall.
    • Both eukaryotic and prokaryotic cells can have a flagellum.
    Frequently Asked Questions about Cell Structure

    What is cell structure?

    Cell structure includes all the structures that make up a cell: the cell surface membrane and sometimes cell wall, the organelles and the cytoplasm. Different cell types have different structures: Prokaryotes vary from eukaryotes. Plant cells have different structures than animal cells. And specified cells may have more or fewer organelles depending on the function of the cell. 

    Which structure provides the most energy?

    Though energy itself cannot be produced, energy-rich molecules can. This is the case with ATP, and it is mainly produced in the mitochondria. The process is called aerobic respiration. 

    What cell structures are found only in the eukaryotic cell?

    Mitochondria, Golgi apparatus, nucleus, chloroplasts (only plant cells), lysosome, peroxisome and vacuoles.

    What is the structure and function of the cell membrane?

    The cell membrane is made of a phospholipid bilayer, Carbohydrates and Proteins. It closes off the cell to the extracellular space. It also transports material in and out of the cell. Receptor proteins in the cell membrane are needed for communication between cells. 

    What structures are found in both plant and animal cells?

    Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Cytoskeleton, Plasma membrane and Ribosomes are found in both plant and animal cells. Vacuoles can both be present in animal cells and plant cells. However, they are much smaller in animal cells and can be more than one, whereas a plant cell usually only has one big vacuole. Lysosomes and Flagella are usually not found in Plant cells.

    Test your knowledge with multiple choice flashcards

    All eukaryotes have one nucleus each.

    Plasmids are _____ that are commonly found in ______ cells

    Prokaryotic cells have one chromosome.


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