Diagram of An Animal cell, Practice Problems and FAQs

You know that the life of organisms starts from a single cell. For example, the life of a mango tree starts from a single celled zygote formed by the fusion of male and female gametes. It will be present inside the mango seed which grows into a big mango tree under favourable conditions. 

                              Fig: Structure of a mango

Now consider our life. Once we were also a single celled zygote formed by the fusion of sperm and ovum. Later it underwent many divisions and grew into a fully developed foetus inside the mother’s uterus. 

                   Fig: A fully developed foetus

Since cell division is initiated from a single cell, all the cells of the organism will be of similar type. Right? But the cells may be different according to their functions in different parts of the body. So the cells will undergo differentiation and will become different types of cells like epithelial cells, nerve cells, blood cells etc., in animals.

It is clear that all the functions of the animal body are controlled by the cellular functions. It is done by different structures present inside the cells called the cell organelles. Even though the shape and function of cells differs in the animal body, they all have a basic structure. Then how should it be?

The easiest way to observe animal cells is to take a sample of your cheek cell and watch it through a microscope. You can see the membrane of the cell and the granulated cytoplasm. The dense and dark spot inside the cell will be the nucleus. All the other organelles are not visible due to their small size. The size of organelles are close to the limit of resolution of the compound light microscope we use. In this article we are going to discuss the diagram of an animal cell in depth.

                             Fig: Cheek cells under the microscope

Table of contents

• Animal cell
• Diagram of an animal cell 
• Parts of animal cell
• Practice Problems
• FAQs

Animal cell

Animal cells are eukaryotic cells which means they have a true nucleus. Compared to the plant cells, animal cells are smaller. They lack cell walls, hence they have an erratic structure. But they have many cell organelles similar to plant cells, because they both originated from the same eukaryotic cells. Animal cells have a nucleus with a double membrane and many other membrane bound cell organelles. These cell organelles carry out all the proper functions of a cell. 

                                          Fig: Structure of a eukaryotic cell 

Diagram of an animal cell 

Most of the animal cells contain the plasma membrane or cell membrane enclosing the cytoplasm inside the cell. We can observe many cell organelles in the cytoplasm of an animal cell. Some of them have membranes around them and some others do not have a membrane. Hence they can be divided into membrane bound cell organelles and non membranous cell organelles. 

Membrane bound cell organelles

Membrane bound cell organelles are as follows:

• Nucleus 
• Endoplasmic reticulum
• Golgi apparatus
• Lysosome
• Mitochondria
• Microbodies

Non membranous cell organelles

Non membranous cell organelles are as follows: 

• Ribosomes
• Cytoskeleton
• Centrioles 

                                                         Fig: Animal cell

Endomembrane system 

Among the membrane bound cell organelles there is a system of organelles which can function in close coordination with each other. They are together known as the endomembrane system. It includes the nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes and vesicles. 

Parts of an animal cell 

Now we will discuss the structure and function of the plasma membrane, cytoplasm and all the organelles present in an animal cell. 

Plasma membrane or cell membrane

The outermost cover of the animal cell is called a plasma membrane or cell membrane. The membrane is selectively permeable and it is made up of two asymmetric layers of phospholipids known as lipid bilayers. The lipids have a hydrophilic head and a hydrophobic tail. Hence the lipid bilayer is amphipathic. Other than lipids it is composed of carbohydrates (glycolipids and glycoproteins), membrane proteins and water. The function of cell membrane is to help in the transport of materials from and into the cells. 

                       Fig: Structure of the cell membrane

Cytoplasm 

The major volume of the cell is occupied by the semi-fluid matrix called cytoplasm. It is the area where major cellular activities are carried out. It consists of proteins, lipids and inorganic salts. Vacuoles are generally absent in the cytoplasm of animal cells, but can be found in very few numbers.

                                                                Fig: Cytoplasm 

Membrane bound cell organelles

Some of the cell organelles of animal cells are surrounded by a membrane which is similar to the structure of a cell membrane. But they have a different protein and phospholipid composition. Membrane bound cell organelles can increase the efficiency of chemical reactions. The membrane of these organelles also protects them from the harmful enzymes released by other cell organelles. So the functions of the membrane bound organelles will be less affected. So let us discuss more about the membrane bound cell organelles of animal cells.

Nucleus

Nucleus is the place where all the hereditary information of an animal is stored. It is the control centre of the cell, which regulates all the functions and division of cells. Hence it is known as the brain of the cell. It was first described by Robert Brown. Nucleus is enclosed in a double membrane called a nuclear envelope. The outer membrane is continuous with the endoplasmic reticulum. Nuclear envelope has some pores called nuclear pores which help to transport the substances from the nucleus and into the nucleus. 

The nucleus contains a protoplasm which is known as nucleoplasm. It is composed of chromatin fibres and nucleolus. Nucleolus is the dense spherical structure in a nucleoplasm. Chromatin fibres have DNA and proteins which condense to form chromosomes which carry genes. Nucleolus is responsible for the synthesis of RNA. 

                                Fig: Structure of the nucleus

Endoplasmic reticulum (ER)

The interconnected network of membranous cisternae is the endoplasmic reticulum. It is a sac-like structure which is seen in continuation with the nuclear membrane. ER is the organelle where synthesis of lipid and storage of calcium takes place. There are two types of endoplasmic reticulum based on the presence or absence of ribosomes. They are as follows:

Rough endoplasmic reticulum (RER)

Presence of ribosomes on the surface makes it rough or granular. It functions mainly in the synthesis of proteins.

Smooth endoplasmic reticulum (SER)

Ribosomes are absent on the surface of SER. It is mainly involved in the synthesis of fats and lipids. It also helps in detoxification in liver cells. 

                                                   Fig.: Endoplasmic reticulum

Golgi Apparatus

The organelle with sac-like, smooth and flat layers seen near the nucleus is the Golgi apparatus. It was first discovered in the nerve cell of an owl through the metallic impregnation method. It was done by Camillo Golgi, hence the organelle is named the Golgi apparatus. It is a single membrane bound cell organelle. The cisternae, which are the folds and the vesicles coming and going out from the cisternae, can be together called the Golgi apparatus. Vesicles are also considered as a cell organelle. They have a structure with fluid enclosed in a lipid bilayer. The packaging, processing and the transport of proteins and lipids is the major function of the Golgi apparatus.

                                                           Fig: Golgi apparatus

Lysosome

When the vesicles bud off from the Golgi apparatus, some will get involved in the intracellular digestion and they are called lysosomes. It contains hydrolytic enzymes. It is a single membrane bound cell organelle. They can repair the cell membrane and also can breakdown the macromolecules. They are also involved in the digestion of foreign substances like bacteria, viruses and other pathogens. When a cell is starved or damaged, lysosomes will help to digest the cellular contents. They are known as the suicidal bags of the cell. 

                                                                         Fig: Lysosome 

Mitochondria

The circular or rod-shaped organelle which has a double membrane is called mitochondria. It was first discovered in the striated muscles of insects by Kolliker and it was named by Carl Benda. It is the powerhouse of a cell, because it helps in the generation of ATP during aerobic respiration. It is an organelle which has its own RNA, DNA and ribosomes. Hence it is known as the semi-autonomous cell organelle.

                  Fig: Structure of the mitochondria

Microbodies

The membrane bound vesicles which contain various enzymes are called microbodies or cytosomes. The three types of microbodies such as spherosomes, glyoxysomes and peroxisomes are found in cells. But out of these three only two are present in animal cells such as peroxisomes and glyoxysomes. They help in the metabolic activities of the cell with the help of their enzymes. Peroxisomes mainly help in the breakdown of long chain fatty acids and glyoxysomes help in the conversion of fats into carbohydrates. 

                                               Fig: Structure of peroxisomes

Non-membranous cell organelles

Those organisms which are not surrounded by any kind of membrane are called non membranous cell organelles.

Ribosomes

The smallest organelle which lacks a membrane is the ribosome. It is also known as the ribonucleoprotein particle. Ribosomes in animal cells were discovered by Robert E Palade. It can be seen as scattered organelles in the cytoplasm or attached to the surface of RER. Their function is to synthesise proteins. In eukaryotic cells, the ribosomes are of the 80S type. Hence they are composed of the 60 S and 40 S subunits.

                       Fig: Structure of ribosome

Cytoskeleton

The three structures such as the microtubules, microfilaments and intermediate filaments are collectively called the cytoskeleton. They are the solid protein filaments which are capable of contraction and they are made up of a protein called actin. So the function of a cytoskeleton is to maintain the shape of the cell and internal organisation. They also help in the division and movement of the cells.

                                                   Fig: Structure of cytoskeleton

Microtubules

They are the hollow tubes made up of alpha and beta tubulin. 

                                                           Fig: Structure of microtubules 

Microfilaments

They are the smallest filaments of the cytoskeleton which are made up of intertwined strands of actin. They help in the movement of cells, division of cells and muscle construction. 

                                                         Fig: Structure of microfilaments

Intermediate filaments 

They are made up of keratin-like proteins and they are the tough cytoskeleton which provides strength to cells. They are present in the cell-to-cell junctions and are not directly involved in the cell movements. 

                                              Fig: Intermediate filaments

Centrioles and centrosome

The non membrane bound organelles located near the nucleus are called centrosomes. They are devoid of DNA. They have a structure of dense centres and scattered tubules. During the prophase stage of the cell cycle, centrosomes duplicate and they will help to form the spindle fibres. The pulling of the chromosomes to both ends of the cells during cell division is done by the spindle fibres. 

Diplosomes

They are a pair of centrioles seen in a cell during the non-dividing phase. Diplosomes are arranged perpendicularly inside the centrospheres or kinoplasm, which is a specialised cytoplasm. So the centriole and centrospheres are together called the centrosome.

                                                 Fig: Centrosome

Centrioles

They are made up of nine peripheral fibrils or tubules. Each fibril is made up of three microtubules such as C, B and A from outside to inside. There are no tubules on the centre, hence it is known as a 9+0 arrangement. Here the centre possesses a rod-like proteinaceous mass called hub. Each triplet sends a fine fibril of proteinaceous material towards the hub called spoke.

                                                     Fig: Structure of centriole

Cell organelles present outside the cell membrane

Cilia and flagella are the cell organelles mainly seen outside the cell membrane.

Cilia and Flagella

Apart from the cell organelles inside the cells, there will be some hair like outgrowths from the cell membrane. These are normally seen covered by a plasma membrane. They are the cilia and flagella, which are mainly the locomotory structures. But it also has sensory functions in animals. Cilia will be more in number and shorter than the flagella,. Cilia can be seen on the epithelial cells of several internal organs like lungs, trachea, digestive system etc. Sperm is an example for the cells with flagella in animals. 

                                               Fig: Examples of cilia and flagella in animals

Parts of cilia and Flagella

Both cilia and flagella consist of four parts such as the basal body, rootlets, basal plate and shaft. 

Basal body 

It is the starting point for the production of cilia and flagella. It lies normally in the outer part of the cytoplasm below the plasma membrane.

Rootlets

These are the fibrillar outgrowths arising from the basal body. They are made up of microfilaments.

Basal plate

It is an area of high density that lies above the basal body.

Shaft

The core of the shaft is called an axoneme and it is covered by a plasma membrane. The structure of axoneme is composed of nine microtubule doublets that are arranged in a circle. They are arranged around two central tubules. Hence it shows a 9+2 arrangement of microtubules. 

                                                       Fig: 9+2 arrangement of microtubules

Practice Problems

Q 1. Which of the following statements are incorrect?

I) Endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi complex, ribosomes and vesicles. 
II) The outer membrane of the nucleus is continuous with the endoplasmic reticulum. 
III)The packaging, processing and the transport of proteins and lipids is the major function of the Golgi apparatus.
IV) Mitochondria is known as the suicidal bags of the cell. \
V) Diplosomes are a pair of centrioles seen in a cell during the non-dividing phase. 

a. I, II and III
b. I and IV
c. Only IV
d. I and III

Answer: Among the membrane bound cell organelles there is a system of organelles which can function in close coordination with each other. They are together known as the endomembrane system. It includes the nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes and vesicles. The smallest organelle which lacks a membrane is the ribosome. Nucleus is enclosed in a double membrane called a nuclear envelope. The endoplasmic reticulum extends from the outer membrane of the nucleus. The organelle with sac-like, smooth and flat layers seen near the nucleus is the Golgi apparatus. The packaging, processing and the transport of proteins and lipids is the major function of the Golgi apparatus. Lysosome is a single membrane bound cell organelle. When a cell is starved or damaged, lysosomes will help to digest the cellular contents. The suicidal bags of the cell is called lysosomes. Diplosomes are a pair of centrioles seen in a cell during the non-dividing phase. Hence the correct option is b.

Q 2. Assertion: Mitochondria has its own genetic materials.

Reason: Mitochondria is known as the semi-autonomous cell organelle.

• Both, A and R, are true and R is the correct explanation of A

1. Both, A and R, are true but R is not the correct explanation of A
2. A is true but R is false
3. Both A and R are false

Answer: Mitochondria is a circular or rod-shaped organelle which has a double membranous structure. It is the powerhouse of a cell, because it helps in the generation of ATP during aerobic respiration. It is an organelle which has its own RNA, DNA and ribosomes. It is able to synthesise a part of its proteins. Hence it is known as the semi-autonomous cell organelle. So here the assertion and reason are true and reason is the correct explanation of the assertion. Hence the correct option is a.

Q 3. How is the endoplasmic reticulum classified?

Answer: The interconnected network of membranous cisternae is the endoplasmic reticulum. It is a sac-like structure which is seen in continuation with the nuclear membrane. ER is the organelle where synthesis of lipids, proteins and storage of calcium takes place. There are two types of endoplasmic reticulum based on the presence or absence of ribosomes. They are as follows:

• Rough endoplasmic reticulum (RER)

Presence of ribosomes on the surface makes it rough or granular. It functions in the synthesis of proteins.

• Smooth endoplasmic reticulum (SER)

Ribosomes are absent on the surface. It is mainly involved in the synthesis of fats and helps in detoxification in liver cells.

Q 4. What is an endomembrane system?
Answer: Among the membrane bound cell organelles there is a system of organelles which can function in close coordination with each other. They are together known as the endomembrane system. It includes the nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes and vesicles. 

FAQs

Q 1. Which animal cell lacks a nucleus?
Answer: In the eukaryotic cell types, the only cell which lacks a nucleus is the red blood cells (RBCs). It lacks mitochondria also. Initial stages of RBCs like erythroblast have a nucleus, but during maturation, it loses the nucleus. Since they lack a nucleus, they will be flexible and this will help them to pass through blood vessels and capillaries.

Q 2. What is the lifespan of animal cells?
Answer: The life span of the cells in the human or animal body vary from a few minutes to many years. For certain intestinal cells the survival time is just a few minutes, it is about 4 months for red blood cells and many years for some nerve cells or neurons.

Q 3. Why do animal cells do not possess a cell wall like plant cells?
Answer: The cell walls give rigid structure and support to the cells. Animals can move and they are not static in one position, hence their cells should also be flexible. If cell walls are present, then they will lack this flexibility. But plants need a cell wall for the structure and support since they are not moving from one place to another on their own.

Q 4. Which cell in the human body is the largest?
Answer: The largest cells in the human body are egg cells or ova. They have a diameter of about 0.1 millimetres and are 20 times bigger than sperm cells.

YOUTUBE LINK: https://www.youtube.com/watch?v=Lq0wyc1wlmw

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