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Animal Cell: Its Parts and Structure with Functions

What is an Animal Cell

A cell is the structural and functional unit of life. It is also the smallest and the most fundamental biological unit of living organisms. An animal cell is thus defined as the basic structural and functional unit of life in organisms of the kingdom Animalia.

They are eukaryotic cells that have their genetic material enclosed within a membrane with specialized structures called organelles that carry out different functions.

Structure and Characteristics of an Animal Cell

The shape of a typical animal cell varies widely from being flat, oval to rod-shaped, while others assume more intriguing shapes such as curved, spherical, concave, and rectangular. A typical animal cell ranges in size from 10 to 30 micrometers.

Most of the animal cells being microscopic, a typical cross-sectional view shows many different parts that perform distinct functions and together keep the cell functional; they are known as organelles. An animal cell contains some unique organelles that are absent in all plant cells.

Different Parts of an Animal Cell

Among all other intracellular organelles, Centrioles, Centrosomes, and Lysosomes are present only in an animal cell. On the contrary, Cell wall, Chloroplast, and Vacuole are completely absent in an animal cell but are present in a plant cell. Also, cilia and flagella together are the only extracellular organelle of an animal cell.

Animal Cell Diagram

1) Centrioles

Paired cylindrical tube-like organelles composed of short lengths of microtubules arranged in the form of an open-ended cylinder about 500nm long and 200nm in diameter. Centrioles are found as single structures in cilia and flagella of animal cells and also in some lower plant cells.

Functions

  • Producing asters during interphase
  • Forming spindle fibers during cell division
  • Helping in cell movement

2) Centrosomes

They are known as the microtubule organizing center of the animal cells. The centrosome is made up of two perpendicular centrioles, a daughter centriole, and a mother centriole, linked together by interconnecting fibers.

Centrosome in the animal cells is very much like DNA. During cell division, one centrosome from each parent cell is transferred to each daughter cell.

Functions

  • Assisting in cell division during mitosis
  • Contributing to the efficiency of assembling the mitotic spindle fibers
  • Stimulating the changes in the shape of the cell membrane by phagocytosis
  • Organizing the microtubules and ensuring that the centrosomes are distributed to each daughter cell
  • Maintaining the chromosome number during cell division
  • Directing the movement of microtubules and cytoskeletal structures, thereby facilitating changes in the shapes of the membranes of the animal cell

3) Lysosomes

Small membrane-bound sphere-shaped sacs made up of lipid bilayer and are filled with hydrolytic enzymes that can break down many types of biomolecules. The size of lysosomes varies among different cell types, with the largest ones measuring more at than 1.2 μm. Lysosomes are made from the Golgi apparatus by budding, and the hydrolytic enzymes within them are obtained from the endoplasmic reticulum.

Sometimes, the digestive enzymes end up damaging the lysosomes themselves, and this can ultimately cause the destruction of the cell (autolysis). Hence, lysosomes are also known as ‘Suicidal Bags’ of the cell.

Functions

  • Digesting complex molecules such as carbohydrates, lipids, proteins, and nucleic acids, which the cell recycles for other uses
  • Digesting the food molecules that enter the cell into smaller pieces if a vesicle that brings particles into the cell (endocytic vesicles) fuses with them
  • Destructing the improperly functioning organelle (autophagy)
  • Removing cellular and extracellular waste products

4) Cell Membrane

Also known as the plasma membrane, it is the outermost membrane of an animal cell having a thickness of 5-10 nm, thus physically separating the intracellular components from the extracellular environment around it.

The cell membrane consists of a lipid bilayer, including cholesterols and glycolipids, in combination with membrane proteins arranged in a specific pattern. Proteins make up about half of the cell membrane. The composition of the membrane is not always constant and keeps changing during different stages of cell development.

Functions

  • Protecting the integrity of the cell from the outside environment
  • Allowing selective entry and exit of substances in and out of the cell
  • Maintaining the shape of the cell
  • Providing mechanical support to the cell
  • Helping to maintain growth and turgidity of the cell
  • Helping in cell signaling and communication

5) Endoplasmic Reticulum

It is the largest, complex membrane-bound tubular organelle exclusively present in the cytoplasm of the eukaryotic cell. The endoplasmic reticulum (ER) is a highly dynamic organelle whose membranes are continuous from the nuclear envelope to the cell membrane. Enclosed in the membrane of the endoplasmic reticulum is the lumen that is separated from the cytoplasm of the cell.

There are two types of ER: i) rough endoplasmic reticulum (RER) and ii) smooth endoplasmic reticulum (SER). The surface of RER is studded with ribosomes, which gives a rough appearance to it, while in SER, the tubules are devoid of ribosomes, thus providing a smooth shape.

Functions

  • Helping in protein synthesis (RER)
  • Aiding in the synthesis of essential lipids such as phospholipids and cholesterol (SER)
  • Helping in the production and secretion of steroid hormones (SER)
  • Performing the metabolism of carbohydrates (SER)
  • Helping in protein folding and maturation (RER)
  • Helping in protein sorting (RER)

6) Golgi Apparatus

It is typically comprised of a series of five to eight cup-shaped, membrane-covered sacs called cisternae. Cisternae are a flattened membrane disk-shaped, stacked pouches that make up the Golgi apparatus. Animal cells generally contain between ten and twenty Golgi stacks per cell which is usually present close to the cell nucleus.

The Golgi complex works closely with the rough ER. When the ER makes a protein, a transition vesicle is also made. It drifts through the cytoplasm to the Golgi apparatus, where it gets absorbed. After processing the molecules, the Golgi secretes the vesicle into the cytoplasm, known as secretory vesicle, which releases the protein molecule outside the cell.

Functions

  • Processing, packaging and transporting or secretion of the proteins generated in the endoplasmic reticulum thus acts as a post office inside the cell
  • Performing modifications such as phosphorylation and glycosylation of proteins
  • Performing breakdown of proteins and thus forming small active fragments
  • Helping in the absorption of substances

7) Microfilaments or Actin filament

They are polymers of actin protein that is part of a cell’s cytoskeleton. The cytoskeleton is the network of protein filaments that extends throughout the cell, giving the cell structure and keeping the organelles in place. Microfilaments are the thinnest filaments of the cytoskeleton, having a diameter of about 6 to 7 nanometers.

Functions

  • Helping in the contraction of muscles
  • Aiding in cell movement
  • Allowing transport of nutrients, waste products, and cell organelles from one part of the cell to another (cytoplasmic streaming)
  • Helping in cell division

8) Microtubules

Hollow tubes composed of polymers of the protein tubulin. Microtubules are highly dynamic structures that are always in a stage of assembly and disassembly via the addition and removal of the dimers. They are the largest and thickest component of the cytoskeleton, measuring about 24 nanometers in thickness that are forming cell structures called centrioles and asters in animal cells.

Functions

  • Forming the component of cilia and flagella that helps in cell movement
  • Playing a key role in forming the mitotic spindle fibers that help in cell division
  • Helping in the movement of organelles except for the nucleus throughout the cell
  • Helping in communication and intracellular transport within the cell

9) Intermediate Filaments

Cytoskeletal structural components having a diameter of 8-10 nm that are intermediate in size compared to microfilaments and microtubules. Microtubules, together with microfilaments and intermediate filaments, complete the cytoskeleton of the cell.

Functions

  • Providing structural and mechanical support to the cell membrane
  • Maintaining cell shape

10) Mitochondria

Commonly known as the ‘Powerhouse of the cell,’ mitochondria (singular: mitochondrion) are a double-membrane-bound organelle found in most eukaryotic organisms with a length of about 1 – 10 microns. They can be spherical or rod-shaped.

The outer membrane of mitochondria surrounds the entire organelle, and the inner one has many inward protrusions called cristae that enclose the inward compartment known as the mitochondrial matrix, the two membranes being separated by a region known as intermembrane space.

Functions

  • Regulating the metabolic activity of the cell
  • Promoting the growth of new cells and in cell multiplication
  • Aiding in various cellular activities like cellular differentiation, cell signaling, cell senescence, controlling the cell cycle and also in cell growth
  • Maintaining an adequate concentration of calcium ions within the compartments of the cell
  • Playing an essential role in apoptosis or programmed cell death

11) Nucleus

Spherical double membrane-bound cell organelle that contains the genetic material of the cell. They are the largest and most prominent of all cell organelle. A nucleus has four main parts:

Nuclear membrane or Nuclear envelope – Double-membrane structure that separates cell cytoplasm from the nuclear content

Chromatin threads or Chromosomes – The genetic material of the cell that participates in cell division

Nuclear sap or Nucleoplasm – A clear transparent liquid that contains chromosomes

NucleolusA membrane-less organelle that manufactures the protein-synthesizing structure, the ribosome

Functions

  • Controlling the functioning of the whole cell
  • Storing of cell’s hereditary material
  • Controlling cell growth and reproduction
  • Acting as the home of some of the vital cellular processes and coordinating them
  • Creating two identical copies of the host or duplicating one’s genetic material or DNA (replication)
  • Creating other forms of genetic material from DNA that acts as instructions to the rest of the cell (transcription)

12) Peroxisomes

They are single membrane-bound small cell vesicles, ranging in diameter from 0.1 to 1 mm that is found in all eukaryotic cells. Peroxisomes vary in shape, size, and number depending upon the energy requirements of the cell.

Peroxisomes exist either in the form of a network of interconnected tubules (peroxisome reticulum) or as individual micro peroxisomes. They are a heterogeneous group of organelles that are distinguished from other organelles by the presence of marker proteins and enzymes.

Functions

  • Helping in the detoxification of reactive oxygen species
  • Performing synthesis of some lipids, such as glycerolipids of plasmalogens.
  • Helping in the breakdown of purines, polyamines, and amino acids
  • Forming bile acids and cholesterol
  • Helping in lipid metabolism involving β–oxidation of fatty acids,

13) Ribosome

It is a complex molecular machine largely made up of specialized RNA known as ribosomal RNA (rRNA), as well as dozens of distinct proteins, thus also known as a ribonucleoprotein.  Ribosomes are composed of two subunits; the smaller subunit binds to mRNA and decodes its information, whereas the larger subunit adds up amino acids during protein synthesis.

Functions

  • Assembling amino acid to form proteins that are essential to carry out cellular functions
  • Helping the DNA to produce mRNA from DNA by a process transcription
  • Acting as catalysts in the process of synthesizing proteins from amino acids by a process called translation

14) Cytoplasm

They are jelly-like substance, found between the cell membrane and nucleus.  They are mainly composed of water, organic, and inorganic compounds. The cytoplasm is the region of the cell where all the cell organelles are embedded. Apart from cell organelles, the cytoplasm contains cytosol – a gel-like substance, and numerous small cytoplasmic inclusions. The cytoplasm is composed of about 80% water and is usually colorless.

Functions

  • Acting as the site of various cellular activities and chemical reactions such as the pathway of glycolysis and cell division or cytokinesis
  • Providing the raw materials necessary for the various chemical reactions within the cell
  • Maintaining turgidity thus helping to keep cell shape
  • Keeping the cell organelles in position

15) Cilia and Flagella

Both are extracellular, fine, hair-like extensions formed from specialized groupings of microtubules called basal bodies that are connected to the plasma membrane of the cell and arranged in a specific pattern. If the protrusions are short and numerous, they are termed cilia, whereas when they are longer and less numerous, they are termed flagella.

Functions

  • Helping in locomotion (cilia and flagella)
  • Helping to circulate food in the gastrovascular cavity (flagella)
  • Helping in the internal transport of several organs such as the passage of excretory substances in the kidney (cilia)
  • Helping to perceive a number of sensations (cilia and flagella)
  • Creating a current in water for quick dispersal of carbon dioxide and replenishment of oxygen (cilia and flagella)
  • Helping to move the mucus trapped dust particles in the nasal cavity, trachea, and bronchioles towards pharynx for elimination (cilia)

Most Commonly Known Types of Animal Cells

1) Skin Cells

Mostly consist of two types of cells – keratinocytes and melanocytes. Keratinocytes make up around 90% of all skin cells and produce a protein called ‘keratin.’ The keratin in skin cells helps to provide an active layer of protection for the body. Keratin also makes hair and nails. The rest are melanin-producing cells located in the bottom layer of the skin epidermis.

2) Muscle Cells

Long tubular cells responsible for moving an organism’s limbs and organs. Muscle cells can be either skeletal muscle cells, cardiac muscle cells, or smooth muscle cells.

3) Blood Cells

Found mainly in the blood, they can be divided into two types – red and white blood cells. Red blood cells makeup around 99.9% of all blood cells and are responsible for delivering oxygen from the lungs to the rest of the body. White blood cells are a vital part of an animal’s immune system that helps to battle infections by killing off germs present inside the body.

4) Nerve Cells

Also called neurons, they are the main cells of the nervous system. They receive, carry and deliver signals throughout the body

5) Fat Cells

Also known as adipocytes or lipocytes, they are used to store fats and other lipids as energy reserves. There are two common types of fat cells in animals – white fat cells and brown fat cells.

Article was last reviewed on Monday, February 10, 2020

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