Animal Cell

All living organisms are made up of cells and it is the smallest unit of life. It helps in carrying out the functions such as respiration, nutrition, digestion, excretion etc so it is called the structural and functional unit of life. It is normally microscopic and consists of cytoplasm and a nucleus enclosed in a membrane.

In biological terms, an animal cell is a typical eukaryotic cell with a membrane-bound nucleus with DNA present inside the nucleus. It comprises other cellular structures and organelles which helps in carrying out some specific functions required for the proper functioning of the cell. Even though plant cells are eukaryotic the difference can be easily identified as the animal cells lack chloroplasts through which photosynthesis is carried out.

The article deals with the discussion of the animal cell structure, which comprises the discussion of the animal cell organelle. The functions of the specific organelle is laso mentioned in the article. 

Functions of Animal Cells

A cell carries out all the processes of the body which includes producing energy and storing it, making proteins which are molecules that have roles in metabolism, transportation of other molecules and DNA replication. 

For instance, the heart has cardiac muscles that beat in unison, digestive tract cells have cilia which are finger-like projections that help in increasing the surface area for the absorption of nutrients during the digestion process. 

Multiple cells will form the tissues that are organized as a group of cells which helps in carrying out a function. And the same way a group of similar tissues will form the organs of the body such as the lungs, heart, and brain etc. organs work together to form organ systems like the circulatory system, nervous system, and digestive system. Depending on the species, the organ systems vary accordingly.

Structure of Animal Cells

Animal cells have different parts which contain many types of specialized organelles that help in carrying out various functions of the body. Every animal cell does not have all types of organelles, but commonly animal cells contain most of the organelles mentioned below. All the major organelles are mentioned below. 

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Nucleus

The nucleus is the specialized organelle that plays the role of the information and administrative centre of the cell. The nucleus has two important functions, which are storing the hereditary material of the cell or DNA and coordinating the activities of the cell; it includes protein synthesis, growth, intermediary metabolism and reproduction of the cells. Eukaryotes that have a nucleus will be seen only in the cells of advanced organisms. Usually, there will be only one nucleus per cell but slime moulds and a siphonal group of algae are some of the exceptions. Bacteria and cyanobacteria that are one-celled and called prokaryotes do not have a nucleus. In such organisms, the information and administrative role functions will be carried out throughout the cytoplasm.

The nucleus will be sphere-shaped and it occupies nearly 10% of the volume of a cell which will make it the prominent feature of a cell. Chromatin will be present in most of the nuclear material, which is the unstructured form of the DNA of a cell and helps in organizing it to form chromosomes during cell division or mitosis. Inside the nucleus will be present the nucleolus which is an organelle to synthesize protein-producing macromolecular assemblies that are called ribosomes.

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The nuclear envelope, which is a double-layered membrane, separates the contents of the nucleus from the cellular cytoplasm. The nuclear envelope will be riddled with holes which are called nuclear pores to allow the specific sizes and types of molecules to pass back and forth between the nucleus and the cytoplasm. Following are the parts of the nucleus: 

• Chromatin or Chromosomes

The nucleus of every cell will be packed with nearly 6 feet of DNA, divided into 46 individual molecules, one for each chromosome and each one will be about 1.5 inches long. In order for the functioning of the DNA, it is combined with proteins and organized into a compact structure and a dense string-like fibre called chromatin/ chromosomes.

Every DNA strand will enfold to form groups of small protein molecules called histones forming a series of bead-like structures called nucleosomes that are connected by the DNA filament. When seen under the microscope, the chromatin will have an appearance like beads on a string.

Chromatins are of two types, they are Euchromatin and Heterochromatin. Euchromatin is genetically active and will be involved in transcribing the RNA to produce proteins. These proteins will be used in the growth and functioning of the cell. Where Heterochromatin contains inactive DNA and it is the most condensed portion of the DNA since it is not being used. While interphase happens, when a cell is busy in its normal function, the chromatin will be dispersed throughout the nucleus and it will appear like a triangle of fibres and so the euchromatin is exposed and it will be made available for the transcription process. Hence, throughout the entire life of a cell, the chromatin fibres will take on different forms inside the nucleus.

Secondly, when the cell enters the metaphase and when it prepares to divide again the chromatin changes. At first, all the strands of chromatin duplicate themselves through the DNA replication process. Then, they will be compressed to a great extent more than interphase, a 10,000 fold compaction into specialized structures for reproduction purposes and they are termed as chromosomes.

Ribosomes

Every living organism contains ribosomes which are tiny organelles composed of nearly 60% of rRNA (ribosomal RNA) and 40% of proteins. Ribosomes are not bound by any membranes and are much smaller than the other organelles. Some of the cell types would hold a few million ribosomes, but several thousand are very typical. In order to view the organelles an electron microscope is needed.

Mostly, the ribosomes are found to be bound to the endoplasmic reticulum and the nuclear envelope. It is also freely scattered throughout the entire cytoplasm and it will depend on the cell whether it is a plant or animal or bacterial cell. Here the organelles will play the role of protein production machinery for the cell. It will be consequently most abundant in the cells like brain cells and pancreas that are active in protein synthesis.

The rRNA in ribosomes is organized into four strands in eukaryotes and three strands in prokaryotes. Eukaryotic ribosomes will be produced and assembled in the nucleolus. The ribosomal proteins enter the nucleolus and combine with four rRNA strands to create the two ribosomal subunits which will be a small one and a large one that will make up the full ribosome as mentioned above in the picture.

The ribosome units will leave the nucleus all the way through the nuclear pores and unite in the cytoplasm for photosynthesis purposes. When protein production does not take place, the two subunits of a ribosome will get separated. In addition to rRNA, the protein synthesis will require two more RNA molecules; they are the mRNA (messenger RNA) which provides the instruction template from the cellular DNA for building a specific protein and tRNA (transfer RNA) that brings the protein building blocks like amino acids to the ribosome.

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The Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is an arrangement of compacted sacs that extends throughout the cytoplasm in both animal and plant cells. The sacs and tubules are interconnected to each other by a single membrane so that the organelle has only one large and complexly arranged internal space called the lumen. Generally called the endoplasmic reticulum cisternal space, the lumen often takes up more than 10% of the total cell volume.

The endoplasmic reticulum membrane allows the molecules to transfer between the lumen and cytoplasm and as it is connected to the double-layered nuclear envelope, it provides a pipeline between the nucleus and the cytoplasm.

The ER manufactures processes and transports a huge variety of biochemical compounds for inside and outside use of the cell. Many of the proteins found in the cisternal space of the ER will be present there only transiently as they pass on to other locations.

However, the other proteins are targeted to remain constantly in the lumen and are known as endoplasmic reticulum resident proteins. These proteins are necessary for the endoplasmic reticulum to carry out its normal functions. It contains a specialized retention signal that consists of a specific sequence of amino acids to enable them to be retained by the organelle.

There are two kinds of ER morphologies, they are rough and smooth. The surface of the rough ER will be covered with ribosomes which will give a bouncy appearance when viewed under the microscope. It is mainly involved in the production and processing of proteins that will be exported from the cell or secreted from the cell. The smooth ER will mostly be involved in the production of lipids or fats, detoxifying the drugs and poisons and as building blocks for carbohydrate metabolism.

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Vesicles

Vesicles are transient structures that will be formed in the process of secretion of molecules from or into the cell and it helps in transporting the substances in the cell. They are formed by the pinching of the cell membrane of the endoplasmic reticulum or in case of any extracellular particle that gets surrounded by the cell membrane. Vesicle formation will involve a set of proteins that form the shape of the vesicle and these proteins help to engulf the materials that are needed to be transported within the vesicles.

A cell comprises many organelles that function in an organized way to carry out the metabolic process and among these are the vesicles that are a tiny Intra or extracellular structure which is enclosed by a lipid membrane. Vesicles can fuse with both the cell membrane as well as the organelle membranes as they are enclosed by a lipid bilayer and because of this, they can move in & out of the cell and between the organelles like Endoplasmic Reticulum and the Golgi bodies. Vesicles are of different types, these are mentioned below:

• Vacuoles: Vacuoles are tiny lipid enclosed structures that contain water usually and will be seen in plants and certain bacteria in common.

• Lysosomes: These are a type of vesicles that will be involved in cellular digestion.

• Peroxisomes: Like that of lysosomes, peroxisomes are also specialized vesicles containing hydrogen peroxide.

• Transport Vesicles: These are tiny sacs that are enclosed by a lipid bilayer and are involved in the transport of materials to and from the cell and also in between the organelles.

• Secretory Vesicles: This carries substances out of the cell and is usually generated from the Golgi apparatus.

• Synaptic Vesicles: These are specialized types of vesicles that will be found in the neurons that store and transport neurotransmitter molecules.

• Extracellular Vesicles: They are used for transport into the cell and will be found outside the cell. They are commonly seen in both eukaryotic and prokaryotic cells.

• Gas Vesicles: They will be found in bacteria and will provide buoyancy to the cell.

The Golgi Apparatus

The Golgi Apparatus (GA) is also called a Golgi complex or golgi body and will be found in both animal and plant cells. It is typically comprised of a series of five to eight cup-shaped membrane-covered sacs that are called cisternae. It will resemble a stack of deflated balloons. 60 cisternae will combine to make up the Golgi apparatus in some unicellular flagellates. In a cell, the number of Golgi bodies will vary according to their function. Usually, there will be 10- 20 Golgi stacks per cell in an animal cell which will be linked into a single complex by tubular connections between cisternae and it will be located close to the cell nucleus.

The GA is considered as the distribution and shipping department for the cell’s chemical products. It helps in modifying the lipids and proteins which have been built in the endoplasmic reticulum and prepares them for transport to other locations of the cell.

Lipids and proteins that are built in the rough and smooth endoplasmic reticulum bud off in the tiny bubble-like vesicles which move through the cytoplasm until they reach the Golgi complex. The Golgi membranes and Vesicles fuse together and release their internally stored molecules into the organelle.

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Mitochondria

Mitochondria are rod-shaped organelles that are considered as the power generators of the cell for converting oxygen and nutrition into Adenosine Tri-Phosphate (ATP). ATP is the chemical energy of the cell which powers the metabolic activities of the cell this process is called aerobic respiration and this is the reason for animals to breathe oxygen. 

Mitochondria enable the cells to produce 15 times more ATP and complex animals like human beings will need a large amount of energy in order to survive. The number of mitochondria depends upon the metabolic requirements of a cell. It may range from a single large mitochondrion to thousands of organelles. Most of the eukaryotes, including animals, plants, fungi, and protists will be large enough when viewed through a light microscope.

Mitochondria are usually oblong organelles ranging between 1- 10 micrometres in length and occurs in numbers that are directly linked with the level of metabolic activities in the cell. Mitochondria are organized into lengthy travelling chains which will be packed tightly into stable groups or appear in many other formations based on the particular needs of the cell and also the microtubular network characteristics.

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Cytosol

The cytosol is a liquid found inside the cells and it is a water-based solution in which proteins, organelles, and other cell structures float. It has proteins, mRNA, ribosomes, sugars, ions, amino acids, messenger molecules etc.

At first, it was thought to be a simple solution but as of now, scientists have increasingly discovered that it can have a structure and organization. Certain species use cytoplasm organization in order to direct the growth of embryos from the fertilized egg cell. In those species, the messenger molecules will be distributed throughout the cytoplasm of the egg cell. 

Membrane-bound organelles float in the cytosol. This serves as the medium for the intracellular process and it contains proper ions, proteins and other ingredients for cytosolic activities.

Cytoskeleton

The cytoskeleton is a network of tubules and filaments that extends throughout a cell through the cytoplasm. The cytoskeleton gives the shape for the cell, teachers and organizes the organelles and also plays an important role in cell division, transport, and cell signaling. All the cells have cytoskeletons. The eukaryotic cytoskeletons contain three types of filaments, they are as follows:

• Microfilaments: These are also called actin filaments as they are composed of the protein actin.

• Intermediate filaments: These are about 8- 12 nm wide in length and are called intermediate because they are in between the size of microtubules and microfilaments. They are made of proteins such as desmin, vimentin, lamin, and keratin.

• Microtubules: They are about 23 nm in size which is the largest of the cytoskeleton fibres. They form structures like flagella called tails that push a cell forward.

Cell Membrane

The cell membrane is also called a plasma membrane which contains a double layer of proteins and lipids that surrounds a cell and separates the cytoplasm from its surroundings. It allows only certain molecules to enter and exit so it is called selectively permeable. This is because it has control over the amount of some substances that enter and exit out of the cell.

It gives the structure to the cell and regulates the particles that enter and leave the cell. Oxygen, which is required to carry out metabolic functions such as cellular respiration and carbon dioxide can be entered and left easily through the membrane. In the cell membrane, phospholipids are an important component. The properties of this component allow them to form a double-layered membrane spontaneously. The technical term for this component is a phospholipid bilayer. The eukaryotic cells except the bacteria and archaea have a nucleus that is surrounded by a phospholipid bilayer membrane.

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It can be concluded that we have learnt in detail about the animal cell structure and function. We have also discussed the various cell organelles.