Animal Cell: Shape and Size, Structure and its Types

Butterfly. Horses, bats, fish, and humans are all very different with respect to their external appearance,but did you know that they have many similarities at the most basic level of organisation? Can you tell what level are we talking about? Yes, you are right, we are talking about the cellular level. All animals, whether small or big ,are all made of the same kind of cells known as animal cells. 

We are aware of the fact that the cell is the structural and fundamental unit of life. It is also the smallest and the most basic biological unit of living organisms. Animal cells are much different from that of plant cells as they lack some of the specialised cellular structures that are unique to plant cells. Nevertheless, they also possess some cellular structures that are unique to animals. Sound interesting right? Come let’s learn more about animal cells.

Table of Contents

• Shape and size
• Structure of animal cell
• Types of animal cell
• Practice problems
• FAQs

Shape and Size

Animal cells range in size from a few microscopic microns to a few millimetres. The largest known animal cell is the ostrich egg, which can range over 5.1 inches in diameter and weighs about 1.4 kilograms. The longest cell in the human body is the neuron which can be as long as a metre in length and the smallest is the red blood cell which is around 7 microns in diameter.

The shape of animal cells also vary based on the function they perform. For example, the red blood cells have a biconcave shape (due to loss of their nucleus at maturity) to increase the surface area for carrying oxygen. The white blood cells have no definite shape which helps them to easily reach the tissues to eliminate the invading foreign particles.

Structure of Animal cell

An animal cell is eukaryotic in nature with a membrane-bound nucleus. Furthermore, these cells exhibit the presence of DNA inside the nucleus. They also comprise other membrane-bound organelles and cellular structures which carry out specific functions necessary for a cell to function properly.

The animal cell contains various organelles which are crucial to the proper functioning of a cell. These include 

Cell Membrane

The plasma membrane is the selectively permeable, outermost cover that encloses the cell contents and is composed of phospholipids present in two layers, which are arranged asymmetrically. They are also called amphipathic, i.e. they contain both hydrophilic and hydrophobic regions. The chemical composition of the plasma membrane contains lipids, carbohydrates, proteins and water. The cell membrane helps in the entry and exit of substances from the cell.

Endoplasmic Reticulum

The ER or endoplasmic reticulum is made up of an interconnected network of membranous cisternae. It is well developed in metabolically active cells. It is of two types- Smooth ER and Rough ER. SER helps in the detoxification of cells. RER has ribosomes attached to its surface and plays a role in the synthesis of proteins. ER is also the location of lipid synthesis and calcium storage.

Golgi Apparatus

Golgi Apparatus was first discovered in the nerve cell of an owl by Camillo Golgi with the help of the metallic impregnation method. The main function of the Golgi apparatus is to package, process and help in the transport and release of proteins and lipid molecules.

Lysosome

Lysosomes are formed by budding off vesicles from the Golgi apparatus. They are involved in intracellular digestion and contain hydrolytic enzymes. They are involved in the repair of the cell membrane, breakdown of various macromolecules and intracellular scavenging, and finally, the digestion of foreign substances like bacteria, viruses, and other pathogens. Lysosomes help in digesting the cellular contents when the cell is starved or damaged.

Vacuoles

Vacuoles are small single membrane-bound organelles found in the cytoplasm of animal cells. The membrane covering the vacuole is called a tonoplast. In animal cells, vacuoles are either absent or very few in number and smaller in size as compared to plant cells. The vacuoles in animal cells help in the excretion of materials useless to the cell.

Mitochondria

Mitochondria are also known as Power Houses of the Cell as they are associated with ATP generation during aerobic respiration. It was discovered by Kolliker in striated muscles of insects, and Carl Benda gave the present name. It contains its RNA, DNA and ribosomes. In addition, mitochondrial DNA can produce some of its proteins and make up 1% of the total DNA of the cell.

Ribosomes

Robert E Palade discovered the ribosomes in the animal cell. They are the smallest, membraneless organelle and are also called ribonucleoprotein particles. Ribosomes are either scattered in the cytoplasm or remain attached to the surface of the rough endoplasmic reticulum. Their function is to help in protein synthesis. Eukaryotic ribosomes found in animal cells are the 80S type, composed of 60S and 40 S subunits.

Microbodies

Many membrane-bound vesicles are called microbodies containing various enzymes present in both plant and animal cells. These are also known as cytostomes. They include sphaerosome, peroxisomes and glyoxysomes. They actively take part in the metabolic activities of the cells via their enzymes.

Cytoskeleton

The three principal types of protein filaments (which comprise cytoskeleton in animal cells) are microtubules, microfilaments and intermediate filaments. Microfilaments are solid, made up of a protein called actin and are capable of contraction. Microtubules are made up of alpha and beta-tubulin. Intermediate filaments are tough and made of keratin-like protein. They generally form a basket around the nucleus and are present in the cell-to-cell junctions. 

Centrioles and Centrosome

Centrosomes are non-membrane bound organelles without DNA which are involved in cell division. Centrosomes are absent in plant cells.They duplicate themselves in the prophase of the cell cycle and help in the formation of spindle fibres during cell division to pull the chromosomes apart. 

In a non-dividing phase cell there is a pair of centrioles called diplosomes. They are inside a specialised cytoplasm called centrospheres or kinoplasm. The complex form of centriole and centrospheres is called the centrosome. The centrioles are arranged perpendicularly to each other. 

Cilia and Flagella

These are a hair-like outgrowth of the cell membrane. Their core is called the axoneme, which is covered by a plasma membrane. The shaft or axoneme consists of nine microtubule doublets arranged in a circle around two central tubules. This is a 9+2 pattern of microtubules. The cilia are shorter and fewer in number and are present all over the periphery of the cell. The flagella are generally longer and fewer in number. Both of these organelles help in locomotion of the cell.

Nucleus

The nucleus is the storehouse of hereditary information in an eukaryotic cell. Robert Brown first described the nucleus. It is a double membrane-covered protoplasmic body that contains hereditary information in the form of DNA. The nucleoplasm of the nucleus contains the nucleolus and the chromatin fibres which are composed of DNA and proteins. These are the strands which condense to form chromosomes which contain genes. Genes are the units of heredity.

The nucleolus helps in RNA synthesis. The nuclear membrane possesses pores which help in transport of substances in and out of the nucleus. The nucleus is the control centre of the cell and regulates all its functions and cellular division.

Animal Cell Types

There are numerous types of animal cells, each designed to serve specific functions. The most common types of animal cells are:

• Skin Cells - Melanocytes, keratinocytes, Merkel cells and Langerhans cells
• Muscle Cells - Myocyte, Myosatellite cells, Tendon cells, Cardiac muscle cells
• Blood Cells - Leukocytes, erythrocytes, platelet
• Nerve Cells - Schwann cell, glial cells etc
• Fat Cells - Adipocytes
• Reproductive cells - Sperms and ovum

Practice problems of Animal Cell

Question 1.Which of the following can be considered to be the function of a centrosome?

a. Formation of spindle fibres
b. Osmoregulation
c. Secretion
d. Protein synthesis

Solution:  The complex form of centriole and centrospheres is called the centrosome. Centrosome helps in the formation of spindle fibres. Thus, the correct answer is option (a).

Question 2. Lysosomes are known as “suicidal bags” because

a. They are parasitic in nature
b. they can digest foreign particles
c. they possess hydrolytic enzymes to destroy cellular organelles
d. they possess catalytic enzymes

Solution: Lysosomes are formed by budding off vesicles from the Golgi apparatus. They are involved in intracellular digestion and contain the enzymes needed to digest all types of macromolecules. Whenever there is any damage to the cell, the hydrolytic enzymes of the lysosome disperse in the whole cytoplasm and destroy the cellular organelles. Thus these are known as ‘suicidal bags’. Thus, the correct option is (c).

Question 3. Which of the following cell organelles does not contain DNA?

(a) Nucleus
(b) Lysosomes
(c) Mitochondria
(d) both (b) and (c) are correct

Solution: Nucleus,chloroplast and mitochondria contain double stranded DNA but the lysosome has no DNA inside it. Thus, the correct option is (b).

Question 4. Which of the following cellular structures regulates the entry and exit of molecules to and from the cell?

(a) Lysosomes
(b) Golgi bodies
(c) Cell membrane
(d) Mitochondria

Solution:  The cell membrane is selectively permeable and controls the entry and exit of substances to and from the cell.Thus, the correct option is (c).

FAQs of Animal Cell

Question 1. Why can’t animal cells prepare their own food?

Answer: Animal cells do not bear the chlorophyll containing organelles known as chloroplasts. The chlorophyll pigment is responsible for trapping the light energy which is used to prepare food from carbon dioxide and water in plant cells, by the process of photosynthesis.

Question 2. Which cell organelle in the human sperm cell helps in motility?

Answer: The flagella present in the tail region of the human sperm helps in its motility.

Question 3. Which organelle is known as the brain of the cell?

Answer: Nucleus is like the brain of the cell which is used to control most of its functions due to the presence of genetic information in the chromosomes. 

Question 4. How do plant cells form spindle fibres in the absence of centrioles?

Answer: Plant cells have a microtubule organising centre near the nucleus which helps in setting up the spindle fibres in the absence of the centrioles.

Related Topics to Animal Cell in Biology

• Differences between Prokaryotic and Eukaryotic Cell Difference
• Cell
• Eukaryotic cells 
• Mitochondria
• What is a Cell
• Prokaryotic Cells
• Plant Cell
• Cell Envelopes and Cell Membranes
• Types of Cells
• Endomembrane System

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