Animal Cell Structure and Organelles with their functions

Table of Contents

Animal Cell Definition

An animal cell is a eukaryotic cell that serves as the basic structural and functional unit of tissues and organs in the animal. However, the animal cell is microscopic and can only be seen with the help of a microscope.

Animal cell as eukaryotic cells has membrane-bound organelles in the cytoplasm which is enveloped by a plasma membrane, unlike the prokaryotic cells. However, prokaryotic cells are smaller than eukaryotic cells.

The animal cell has a membrane-bound nucleus which possesses the presence of DNA in the nucleus. The cellular parts and membrane-bound organelles of an animal cell carry out specific functions needed for the cell to work properly.

All organisms are made up of cells as cells are the building block of life. The internal structure of an animal cell is different from a plant cell. Although the animal cell shares some common cellular organelles with plant cells as they both evolved from eukaryotic cells.

However, the animal cell is generally smaller than the cells of a plant. The shape and size of an animal cell vary in size.

They can range in size from a few microscopic microns to a few millimeters. Some animal cells can be oval, flat, or rod-shaped. Others too can be curved, concave, spherical, and rectangular. However, the cell of an animal has an irregular shape due to the absence of a cell wall.

The ostrich egg is the largest known animal cell, weighing about 1.4 kg and stretching over 5.1 inches.

Structure of an Animal Cell

Animal cell structures are best described with the aid of a well-labeled diagram showing the different animal cell parts. The functions of the animal cell parts would be stated with their description.

Labeled Animal Cell Diagram showing the organelles
Labeled Animal Cell Diagram showing the organelles

The simple animal cell diagram shown above was viewed with a light microscope.  When the animal cell is viewed with an electron microscope, more features are often seen and this is called the ultrastructure of an animal cell.

Ultrastructure (viewed with an Electron microscope) of a labeled animal cell diagram showing the organelles
Ultrastructure (viewed with an Electron microscope) of a labeled animal cell diagram showing the organelles.
Picture of a typical labeled simple animal cell diagram showing the organelles and other parts as seen with a light microscope
Picture of a typical labeled simple animal cell diagram showing the organelles and other parts as seen with a light microscope

Animal Cell Parts

  1. The cell membrane (or the plasma membrane)
  2. Nucleus
  3. Cytoplasm
  4. Mitochondria
  5. Smooth and Rough Endoplasmic reticulum (ER)
  6. Ribosomes
  7. Golgi apparatus or Golgi complex
  8. Lysosomes
  9. Peroxisomes
  10. Vacuole
  11. Nucleo pore
  12. Microvilli
  13. Microtubules
  14. Centrioles
  15. Flagella and Cilia
  16. Cytoskeleton
  17. Endosome

Animal Cell Structures and Functions

The parts of 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.

The cell membrane (or the plasma membrane)

Structure of the cell membrane

The animal cell membrane is extremely thin measuring about 7.5-10 nm which when magnified can be seen to have three layers (trilaminar appearance). Additionally, It is a thin semipermeable layer that consists of protein and fats. It is made up of lipids and glycoprotein which makes a semipermeable barrier between the cell and its environment.

The fluid-mosaic model is the recent concept accepted describing the plasma membrane structure. A plasma membrane appears as two dark lines by electronic microscopy. Each of the lines is approximately 3nm thick at each side of the light zone.

fluid mosaic 300
A Diagram illustrating the Fluid-mosaic model of a plasma membrane

It creates an image as a result of a phospholipid bilayer, 2 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 membrane. An important feature of the phospholipid bilayer is that it is fluidlike.

This feature gives the membrane flexibility and allows the phospholipid molecules to move freely sideways within their own monolayer.

However, cholesterol molecules are interspersed in the lipid portion of the bilayer. Making the membrane even less permeable to water-soluble ions and molecules. Thus decreasing membrane flexibility.

The Function of the Cell membrane

  1. The cell membrane is made up of lipids and glycoproteins which function mainly as receptors for molecules that control the cell.
  2. The lipids and glycoproteins also help in the transportation of substances into the cell and out of the cell.
  3. The cell membrane, therefore, controls homeostasis.
  4. The cell membrane also encloses and protects the cell’s organelles as an animal cell has no cell wall.


Structure of the Animal cell nucleus

The cell nucleus is the largest organelle of the cell.

It is a spherical structured organelle found mostly at the cell center.

The nucleus with the help of the filaments and microtubules is held together to the cytoplasm

It is a double-layered membrane which is a continuous channel of membranous from the endoplasmic reticulum network.

This 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. Hence, within the nucleus is the nucleolus which helps in making ribosomes.

The nuclear envelope contains less cholesterol compared to the cell membrane and pores that allow and control the exchange of molecules and substances such as messenger RNA (mRNA), ATP, ribosomes, nucleotides, and some proteins.

The nucleus by containing the DNA material of the cell ultimately controls all activities of the cell and also plays an essential role in reproduction and heredity.

Moreso, the nucleus contains chromosomes that code the DNA of the cell (the genetic material of the cell).

During the metaphase stage of mitosis or meiosis in the cell cycle, the nucleus first divides before cytokinesis (a division of the cytoplasm).

The functions of the animal cell nucleus

  1. The main role of the nucleus is to control and regulate all activities of the cell.
  2. It regulates cell growth activities and maintains cell metabolism.
  3. The nucleus plays an essential role in reproduction and heredity.
  4. Also, the nucleus contains the DNA material of the cell, hence it carries the gene that has the cell hereditary information. Hence it is the information center.
  5. The genetic material and chromosomal DNA are made up of genetic code which makes up their proteins amino acid sequence for cell use.
  6. The nucleus is the site for transcription (formation of mRNA from DNA) and transportation of the mRNA to the nuclear envelope.


Structure of the Cytoplasm

The cytoplasm of an animal cell is the fluid material found between the nucleus and the cell surface membrane. The consistency of the cytoplasm varies from aqueous (watery) to jelly-like.

Also, it can be described as a gel-like substance that contains all the organelles of the cell, enclosed within the plasma membrane.

However, the substance found within the nucleus of the cell, contained by the nuclear membrane is called the nucleoplasm.

The Function of the cytoplasm

  1. The main function of the cytoplasm is to support the organelles in it.
  2. It is also a medium for metabolism.


Structure of a Mitochondria

These organelles are located in the cytoplasm of the cell and are scattered in the cytoplasm.

However, the number of mitochondria in each cell varies. Their number depends solely on the function of the cell it performs.

The Mitochondrion is the center for aerobic respiration, hence it is commonly called the powerhouse of the cell as it produces the energy needed for metabolism.

A structure of a typical mitochondria
A Structure of a Typical Mitochondria
Photo credit: Image from Integrated Principles of Zoology (Fifteenth Edition) by Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson. Pg 42

The mitochondria are variable in shapes and can be capsule-like, rod-shaped, oval, spherical, or sausage-shaped.

They have double membranes (they are made up of two membranes)- outer and inner membrane. The membranes of the mitochondria bend into folds known as Cristae. Also, there is a mitochondrial gel-matric in the central mass.

The function of the Mitochondria

  1. The main function of the mitochondria is to generate energy for the cell to perform its function and also to release excess energy from the cell.
  2. They are the powerhouse of the cell as they convert nutrients and oxygen to produce energy in form of Adenosine Tri-phosphate (ATP).
  3. Also, the mitochondria help in the synthesis of lipids.
  4. They store calcium which helps in cell signaling activity, mediating cellular growth or death, and generating cellular and mechanical heat.
  5. The inner membrane of the mitochondria is less permeable and allows only very small molecules into the mitochondrial gel-matrix in the central mass. However, the gel matrix is made up of the mitochondrial DNA (mtDNA) and enzymes for the Tricarboxylic Acid (TCA) cycle or Kreb’s Cycle.
  6. The outer membrane of the mitochondria is permeable which allows transportation of small molecules and also serves as a special channel for the transportation of large molecules.
  7. The mitochondrial DNA (mtDNA) is very vulnerable to mutation as they don’t have a large DNA repair mechanism which is a common element seen in other nuclear DNA. However, mtDNA is inherited by most organisms from the mother. This is because the egg from the mother donates most of the cytoplasm to the embryo whereas the mitochondria inherited from the father’s sperm are destroyed. Hence such mitochondrial diseases due to mutations like Parkinson’s disease and Alzheimer’s disease are transmitted into the embryo and inherited from the maternal and paternal DNA or maternal mtDNA.
  8. Furthermore, Mitochondria can play a role in programmed cell death (apoptosis) as mutations in the mtDNA can inhibit cell death which can cause cancer development. When the mutated mtDNA accumulates over time, it has been linked to aging and the development of certain diseases and cancer.

Endoplasmic reticulum (ER)

Structure of the Endoplasmic reticulum

The endoplasmic reticulum is an extended system of flattened compartments forming sacs that spread throughout the cell connecting from the cytoplasm to the nucleus of the cell.

It is a thin continuous folded membranous organelle seen in the cytoplasm.

The membrane of the ER is continuous with the outer membrane of the nuclear envelope.

Thus there are membranous spaces within its membranes which are called cristae spaces. The membrane folding is however called cristae.

There are two types of ER: Rough endoplasmic reticulum (it is also called ergastoplasm and has ribosomes on it) and Smooth endoplasmic reticulum (without ribosomes on it).

The ER breaks off and joined together to form stacks of the Golgi bodies.

Types of Endoplasmic Reticulum

The two types of ER are based on the variation in their physical and functional characteristics.

Smooth Endoplasmic Reticulum

The Smooth endoplasmic reticulum does not have ribosomes on it. Although the smooth ER lies adjacent to the Rough ER, its function is different from the rough ER. Its main function is to synthesize lipids (cholesterol and phospholipids) which are utilized for producing new cellular membranes.

Also, they are involved in the synthesis of steroid hormones from cholesterol for some kind of cell type. Also, the smooth ER helps in the detoxication of the liver after the intake of drugs or toxic substances. There is a specialized type of Smooth Endoplasmic reticulum also that regulates the concentration of calcium ions in the muscle cell cytoplasm. This type of smooth ER is called the Sarcoplasmic reticulum.

Rough Endoplasmic Reticulum

The Rough ER is called rough because its surface is covered with ribosomes which gives it a rough appearance, unlike the smooth ER. However, the ribosomes on the rough ER synthesis protein have a signaling sequence thus directing them to the ER for processing. Then the rough endoplasmic reticulum transports the lipids and proteins into the cristae through the cell. They are however inserted into the plasma membrane or sent into the Golgi bodies.

diagram of an endoplasmic reticulum
Diagram Showing the Endoplasmic reticulum.

The function of the Endoplasmic reticulum

  1. The ER generally helps in the manufacturing, processing, and transportation of proteins for the cell to utilize both inside the cell and outside.
  2. Therefore, it is a site for lipid synthesis because it contains the enzymes for virtually all cell lipid synthesis.
  3. The rough ER helps in the synthesis of proteins.
  4. The smooth endoplasmic reticulum plays a role in lipid biosynthesis, carbohydrate metabolism, and detoxification of toxic compounds.
  5. Also, the endoplasmic reticulum has a large surface area where chemical reactions take place because it has more than half the membranous cell content.


Structure of the Ribosomes

Ribosomes are tiny organelles that measure about 25 nm in size often seen as tiny dots located mostly on the endoplasmic reticulum (making the Endoplasmic reticulum rough hence the name rough endoplasmic reticulum).

They are made up of ribosomal RNA(r RNA) and ribosomal proteins. Each ribosome has two subunits (large and small subunits with their own distinct shape) in the animal cell. They make up 40% proteins and 60% cytoplasmic- granules.

Living cells contain ribosomes. Some may be bound to the endoplasmic reticulum whereas some may circulate freely in the cytoplasm. However, a single replicated cell has about 10 million ribosomes.

The function of the Ribosomes

  1. The function of ribosomes is the synthesis of proteins as they are made of RNA (ribonucleic acid) and protein. The proteins made by the ribosomes on the rough ER enter the sacs and are modified and transported to where they are needed.
  2. The ribosomal subunits serve as a site for genetic coding into proteins.
  3. Also, it is a site too for protein synthesis.

Golgi apparatus or Golgi complex

Structure of the Golgi apparatus

The Golgi apparatus is found in the cytoplasm of the animal cell next to the ER and near the nucleus. However, they are supported together by cytoplasmic microtubules and are held by a protein matrix.

The Golgi body consists of a stack of membranous cisternae that functions mainly in modifying and packaging polypeptides produced by ribosomes on the rough endoplasmic reticulum.

The cisternae of the Golgi bodies can be 4-10 in number in an animal cell. Although it is more in a single-celled organism.

The cisternae have 3 main compartments: Cis ( the Cisternae nearest to the endoplasmic reticulum), Medial ( the central layers of cisternae), and the Trans ( the cisternae farthest from the Endoplasmic reticulum).

diagram of a golgi complex
Diagram of a Golgi complex

There are few Golgi bodies in an animal cell (1-2) whereas plants have about a hundred of them.

Golgi bodies do not produce proteins but they modify the proteins by adding carbohydrates to them forming glycoproteins.

Thus when modified, the glycoproteins are transported by Golgi vesicles which bud off from one end of the Golgi bodies. The Golgi body together with the vesicles being formed are called the Golgi apparatus or Golgi complex.

Functions of the Golgi apparatus (Golgi bodies or Golgi complex)

  1. The main function of the Golgi apparatus is to transport, modify and pack proteins and lipids into Golgi vesicles so as to deliver them to their target sites.
  2. The Cis and Trans Golgi network are in charge of sorting the proteins and lipids received by the Golgi bodies at the cis face and released by the trans face.
  3. The removal of mannose moieties occurs in the cis and medial cisternae.
  4. Also, the addition of galactose occurs in the Trans cisternae.
  5. The sorting of modified proteins and lipids takes place in the trans- Golgi network and are packed into the trans vesicle which delivers them to the cell membrane or lysosomes for exocytosis.


Structure of Lysosome

Lysosomes are round subcellular, membrane-bound, acidic organelles that contain digestive enzymes. They are also called cell vesicles.

Function of Lysosomes

  1. It contains digestive enzymes called hydrolytic enzymes(or acid hydrolases) which help to breakdown various particles such as proteins, lipids, carbohydrates into small molecules as amino acids, fatty acid, and simple sugars that can be utilized by the cell.
  2. Lysosome acts as a site for digestion of cell nutrients, excretion, and cell rejuvenation.
  3. They break down macromolecules such as old cells, microorganisms, cell waste products, and cell debris from outside the cell into simpler elements. Then these elements are transported into the cytoplasm through a proton pump to build new cell materials.
  4. However, the enzymes are only active when inside the acidic lysosome and their acidity helps to protect the cell from degrading itself even when there is lysosomal leakage. Additionally, the cell ph is usually neutral to slightly alkaline.


Structure of Peroxisomes

Peroxisomes are tiny, spherical-shaped, membrane-bound organelles scattered in the cytoplasm.

The peroxisomes are formed by free ribosomes in the cytoplasm through the synthesis of peroxin proteins and the incorporation of these peroxin proteins into existing peroxisomes.

Once formed, the growing peroxisome then divides by a process similar to binary fission to multiply.

However, peroxisomes are the most common microbodies in the cytoplasm of the animal cell.

Function of Peroxisomes

  1. Peroxisomes produce hydrogen peroxide which is highly reactive and helps in breaking down molecules and invading microorganisms in the cell.
  2. They help in lipid metabolism.
  3. They also help in chemical detoxication by moving hydrogen atoms from several oxygen molecules to produce hydrogen peroxide. This neutralizes poison in the body.


Structure of the vacuole

The vacuole is a fluid-filled organelle in the cell. Thus, It is a membrane-bound sac seen within the cytoplasm of the animal cell

The single membrane surrounding the vacuole sac resembles the cell membrane and is known as a tonoplast.

Functions of the Vacuole

  1. The main function of the vacuole in an animal cell is to store water, food, and carbohydrates in form of sugar and waste products.
  2. The tonoplast of the vacuole act as a regulator that regulates the movement of ions around the cell and isolates materials that may be toxic or harmful to the cell.
  3. However, vacuole gets rid of harmful toxins by bringing them into the cell and converting them into safe compounds.
  4. They take in and also get rid of waste products. Sometimes the waste product can be water.
  5. They maintain the balance of water outside and inside the animal cell.
  6. Vacuoles remove poorly folded protein from the cell.
  7. Additionally, they also have the ability to change shape and size to provide necessary roles that suit the cell.


Structure of the Nucleopore

The nucleopores are those tiny holes in the nuclear membrane.

Function of Nucleopore

  1. The primary function of the nucleopore is the transportation of proteins and nucleic acids into and out of the nucleus of the cell.


Structure of the Microvilli

Microvilli are surface protrusions that are found on egg cell surfaces, white blood cells, and in the intestinal lining.

Accessory proteins come together on the surface of the cell membrane to form microvilli. Hence, they are formed from accessory proteins of the actin filaments.

Functions of Microvilli

  1. In the ear, some microvilli are found which help in the detection of sounds and transmit sound waves through an electric signal to the brain.
  2. Microvilli allow the white blood cells to move freely to attach to possible pathogens in the circulatory system.
  3. They help in the absorption of digested food and water in the small intestine by increasing the surface area of the intestine.
  4. Also, they aid easy fertilization by anchoring the sperm to the egg.


Structure of the Microtubules

Microtubules are long straight hollow cylindrical filaments found only in eukaryotic cells. The microtubules are found in the cytoplasm of the animal cell.

They are formed from 13-15 sub filaments (protofilament) strands of a special globular protein. This globular protein is called tubulin.

Functions of the Microtubules

  1. The Microtubules are actually the main elements that make up the cilia and flagella (the locomotive projections of an animal cell).
  2. They transport some organelles such as the vesicles and mitochondria from the neuron cell body to the axon tip and back to the cell body.
  3. They give the rigid component of the cytoskeleton of the cell that enables the cell to take up a specific shape.
  4. Microtubules give structural support to the Golgi bodies by holding them within the gel matrix of the cytoplasm.
  5. During mitotic cell division, they form the spindle fibers of the cell chromosome.


Structure of centrioles

The centriole is a small structure that consists of 9 sets of microtubules which are placed in a group of three, bonded together by a protein that gives it its shape. Actually, they are distinctly seen in an animal cell that has the ability to replicate.

They are seen in the centrosome, holding microtubules within the cell. As triplets, they stand strong together and have been observed to be in structures like flagella and cilia.

The triplet microtubules are surrounded by a pericentriolar which contains molecules that build up the microtubules.

Function of centrioles

  1. The main function of this organelle is to assist in organizing the cell division process.
  2. Centriole microtubules assist and permit the transportation to any cell location, of substances linked together with a glycoprotein.
  3. It contains the factors needed to create more tubules and anchors the microtubules that extend from it.

Flagella and Cilia

Structure of the Flagella and Cilia

Flagella and cilia are locomotive projections seen on the cell surface. Hence, they are made of filament strands that have partial and complete microtubules that extend the projections.

However, the complete microtubules usually extend to the tip of the cilium whereas the partial microtubules do not extend to the tip of the cilium. These microtubules have motor proteins that make a link between the complete microtubule and the partial microtubule. The motor proteins are called dynein.

This whole collection is combined together as an extension on the cell membrane of the cell.

Functions of Flagella and Cilia

  1. Flagella and Cilia aids in the locomotion of the cell itself or in the movement of other substances or objects past the cell.
  2. The flagella on sperm cells allow them to swim to the ova for fertilization.
  3. Also, in the lining of the fallopian tubes, the cilia aid fertilization by moving the egg towards the uterus.
  4. Cilia aids in the removal of contaminants or surface particles on the epithelial lining of the nostrils and also move mucus over the surface of the cell. Moreso, the lining of the trachea and the nasopharynx are covered in cilia.


Structure of Cytoskeleton

The cytoskeleton appears like a fibrous network that is formed from different proteins of long chains of amino acids. These proteins are seen in the eukaryotic cell cytoplasm.

However, the cytoskeleton is made up of three types of tiny filaments such as Actin filaments (Microfilaments), Microtubules, and Intermediate filaments. Thus, the cytoskeleton is the internal framework of the animal cell.

The function of the Cytoskeleton

  1. Its function is to maintain the shape of the cell and create a network organizing the cell components.
  2. The cytoskeleton also plays a role in the movement of some cell organelles in the cytoplasm and also in the movement of the cell itself. As the Actin filaments change consistently helping the cell to move.
  3. However, the intermediate filaments form the true skeleton of the cell and hold the cell nucleus within the cell in its rightful position.
  4. The microtubules assist in mitosis by moving daughter chromosomes to new forming daughter cells.
  5. The actin filaments also mediate some cell activities like, during mitosis, the ability to substrates and cleavage mechanisms.


Structure of the Endosome

Endosomes are vesicles bound by membranes. They are formed by a mechanism of endocytosis and are formed in the cytoplasm of the cell.

Functions of Endosome

  1. The primary function of the endosome involves the folding in of the cell membrane (plasma membrane) to allow diffusion of molecules through the extracellular fluids.
  2. Also through endocytic processes, they remove waste materials from the cell. Endocytic processes such as phagocytosis and exocytosis.

Types of Animal cell

There are several animal cell types that are designed for a specific function. However, the common animal cell types are Nerve cells, blood cells, muscle cells, skin cells, fat cells (Adipose tissue), bone cells, brain cells, sex cells (Spermatocytes and oocytes), stem cells, and lung cells. We will be looking at a few of these animal cell types.

Nerve cells

The nerve cells electrochemically send information between sensory receptors and the central nervous system in the animal. They are also called neurons which are cells of the nervous tissue. Typical examples are the glial cells, Schwann cells, and others.

Blood cells

Blood cells are the cellular element of the organism’s blood. The blood cell’s main function is to carry oxygen to the tissues of the body and collect carbon dioxide. Also, they carry enzymes, hormones, and vitamins to different parts of the body for utilization. Blood cells examples are Leukocytes, platelet, and erythrocytes.

Muscle cells

Muscle cells are shaped differently, help the body to function properly, and make up the muscular tissues. They comprise skeletal, cardiac, and smooth types of muscle cells. A typical example of muscle cells is Myocyte, Tendon cells, Cardiac muscle cells, and Myosatellite cells.

Skin cells

The tissue that covers humans and animal’s body contains skin cells. Hence skin cells make up the epithelial or skin tissue. These cells are very good at preventing water loss and creating a barrier to the external environment. Typical examples of skin cells are Melanocytes, Merkel cells, Langerhans cells, and Keratinocytes.

Fat cells

These are the cells of the adipose tissues. Fat cells give triglycerides to fuel the majority of the internal work in the body as well as physical activity. The fat layer under the skin keeps the body warm. An example of fat cells is the Adipocytes.

Sex cells

These cells are involved in sexual reproduction. They are the only haploid cells as other animal cells are diploid and are called somatic cells. For males, the sperm cell is the functional sex cell whereas, in females, the egg cell is the sex cell.

Stem cells

These cells are totipotent, meaning that they can develop into any animal cell type.

Plant cell vs Animal cell

The animal cell and plant cell have common organelles as well as differences in different parts. However, animal cells and plant cells are eukaryotic cells. Hence they have a defined nucleus and other membrane-bound organelles in their cell.

The differences between the plant cell and animal cell are the existence of chloroplast, cell wall, and large vacuole in plant cells which are not present in animal cells while animal cells have lysosomes and centrioles which are absent in plant cells. Generally, plant cells can be larger than animal cells.

Furthermore, Plant and fungi cells have a cell wall whereas animal cells do not have a cell wall. Rather multicellular animals have other structures like the cartilage and skeleton that provide the support their tissues and organs need.

Also, animal cells lack chloroplasts that are found in plant cells because animals don’t manufacture their own food as plants do (through a process of photosynthesis).

Additionally, another difference between the plant and animal cells is the central vacuole. Plant cells tend to have a large central vacuole, unlike animal cells that have a small vacuole.

However, even with their differences, plant and animal cells still share similar organelles. Such organelles are:

  1. Mitochondria
  2. Cell membrane
  3. Nucleus
  4. Golgi apparatus
  5. Endoplasmic reticulum
  6. Peroxisome
  7. Cytosol
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