Animal cell

Introduction 

As we know, every living organism comprises the microscopic unit, i.e. cells. These cells vary in their shapes, sizes, function and even differ in different organelles as per the organism. The animal cell consists of the well-developed nucleus as in other eukaryotic cells. The cells range in between 1 to 100 micrometres. We can easily differentiate between a plant and animal cell model, as the cells of animals lack cell walls and chloroplast, which is a characteristic feature of the plant cell. Due to the absence of this cell wall, the cells of animals develop great diversity. The diversity in muscle cells provides movement to the animal. Other organisms like protozoan too locomote; however, they locomote by the means of cilia or flagella instead of muscle cells. 

Functions of Animal Cell

The basic functions of the eukaryotic animal cells are as follows:

• Cells join to form tissues to carry out specific functions
• The animal cells lack cell walls due to which they occupy different shapes and possess different functions. For instance, a neuron and the sperm cell are completely different in shape and function as well. The nerve cells send and receive signals to and from different parts of the body
• The cell organelles produce energy in the form of ATP, which is necessary for every metabolic activity in animals
• The cardiac cells of the heart are specially designated for a rhythmic beating
• The digestive tracts consist of muscles with cilia for better mobility and increasing surface area for the absorption of nutrients

Structure of Animal Cell

The eukaryotic animal cell possesses a well-developed nucleus and all other organelles. However, plastids and cell walls in animal cells are absent. The structures are as follows:

•     Cell Membrane

Cells of animals lack cell walls, but they possess a protective membrane called the cell membrane. It is also called plasma or cytoplasmic membrane. This membrane is thin, elastic, and semi-permeable. Thus, it only allows certain molecules to pass through it. The cell membrane possesses several pores which allow the passage of water, oxygen, CO2, and other nutrients. The cell membrane comprises lipids, protein, carbohydrates and water. 

The cell membrane acts as the protective covering of the cells in animals. They provide a definite shape and mechanical support to it. As they are semi-permeable, they regulate the materials exchange within and outside of the cell. It also maintains the homeostatic balance. 

•     Cytoplasm

The cell is filled with a matrix inside it called cytoplasm or cytosol. Numerous organelles remain embedded or free-floating in it. 90% of it is water and contains enzymes, proteins, RNAs, vitamins, nucleotides, sugar components, and other organic and inorganic compounds. It acts as the storehouse of different chemical compounds necessary for life. 

•     Mitochondria 

Mitochondria is a rod-like cytoplasmic organelle. They are present throughout it. We have often heard that mitochondria are the powerhouse of the cell. It is because it acts as the energy-transporting system of the cell. The energy production is in the form of ATP (adenosine triphosphate). This energy is further utilised for every catabolic and anabolic activity of the body.

•     Ribosomes 

Ribosomes are small granular bodies scattered in the cytoplasm. They are associated with protein formation, therefore, called the protein factory of the cell. They are composed of RNA and proteins. They also synthesise haemoglobin.

•     Lysosomes 

The lysosomes are small spherical pouch-like structures present within the cytoplasm. These are rarely present in plant cells but are an important organelle of an animal cell. The liver, spleen, thyroid gland, and brain have large numbers of them. Lysosomes possess digestive enzymes and play an important role in intracellular digestion. They are phagocytic in nature and engulf the pathogens. They also remove dead cells through phagocytosis. In some conditions, the lysosomes start digesting their own cell content. Therefore, they are also called “suicidal bags” of the cell.

•     Endoplasmic Reticulum

The endoplasmic reticulum (ER) is the network of tube-like structures arising from the outer membrane of the nucleus. They are of two types: smooth ER and rough ER. When ribosomes are present on the surface of ER, it is referred to as rough ER. On the other hand, when the surface lacks ribosomes, it is smooth ER. 

They transfer the contents from the nucleus to the cytoplasm and vice versa. They also play a vital role in the synthesis of lipoproteins, steroids, and glycogen.

•     Golgi Apparatus

Golgi apparatus or Golgi body is present in both plants and animal cells. They comprise 5 to 8 cup-shaped sacs, i.e., cisternae. They resemble somewhat like a bundle of deflated balloons. The Golgi bodies act as the transporting medium for the cell’s chemical products. They transport lipids and proteins from ER to other parts of the cells after their modifications. The Golgi bodies are also in association with sensory activities. 

•     Nucleus 

The nucleus is the specialised organelle acting as the control centre of the cell. They play an important role in the control and coordination of various activities of the cell. It is spherical in shape and contributes 10% of the cell contents. It comprises a double phospholipid membrane with nuclear pores. This membrane separates the nucleus from the outer cytoplasmic contents. The nucleus contains Chromatin, the unstructured form of DNA. Moreover, another structure, the nucleolus, is present, which is responsible for the synthesis of ribosomes. 

•     Centrosomes 

Centrosomes are present near the nucleus and can only be seen in an animal cell model. They act as the primary microtubule-organising centres and regulate cell motility, adhesion, and polarity in interphase. It consists of two microtubule rings, i.e., centrioles.

•     Chromosomes 

The nucleus consists of thread-like fibres called chromosomes. During the non-dividing phase, the chromosomes are present in the nucleus as Chromatin. These chromosomes consist of genes that contain DNA in the form of nodes. Thus, they are the hereditary materials of the cell. In human beings, there are 23 pairs of chromosomes, out of which one pair is a sex chromosome. 

Difference between Plant and Animal Cell

Conclusion 

Animal cells possess several structures that are similar to plant cells. However, it lacks cell walls and plastids, which are present in plants. Moreover, the cells of animals possess centrosomes. The cells are irregular and vary in different shapes due to the absence of a cell wall. The main function of the cell is the production and storage of energy in the form of ATP. Mitochondria act as the site for the production of energy.