Animal cell: Definition, structure, and properties

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Introduction

Cells are considered the primary unit of life in all organisms. An organism consists of one or more cells. The cells are arranged one on top of the other to form a tissue which further forms an organism. The human body is made up of billions of cells. The following article is about a detailed discussion of an animal cell labeled with each organelle and its functions.

The cell is considered to be the smallest unit in all organisms. A cell is so small that it is invisible to the naked eye and observed with a microscope. At present small parts of the cell can be better observed by an electron microscope. Unicellular organisms are those whose bodies are made up of single cells such as bacteria, protozoa, etc. On the other hand, multicellular organisms are those organisms whose bodies are made up of many cells like humans. According to the kingdom of the organism, cells are two types. Such as plant cells and animal cells (1) & (5).

Animal cell

Biological activities such as respiration, nutrition, excretion, reproduction, growth, etc. occur in every cell. So the cells are also called the functional unit of the animals and plants.  Animal cells are eukaryotic cells that have a well-organized nucleus (nuclear membrane, nucleoplasm, nucleolus, and nuclear reticulum) and cell membranes containing cellular organelles. Mycoplasma gallicepticum is the smallest animal cell and is about 10 micrometers. Ostrich eggs are the largest animal cells about 6 inches or 15 cm in diameter (3).

Animal cell properties

1. The nucleus of an animal cell is well organized and is located in the center of the cell.

2. Animal cells do not have cell walls like plant cells.

3. The primordial utricle is never formed in animal cells.

4. Microvilli are present in animal cells.

5. Cellular connections in animal cells are maintained through desmosomes. The co-organization of the plasma membranes of two adjacent animal cells that helps maintain the cytoplasmic link between the two cells is called desmosome. These are made up of mucopolysaccharides and proteins.

6. The cellular organelle centrosome is present in animal cells.

7. Plastid organelles are not seen in animal cells.

8. There are few numbers of vacuoles in animal cells and they are small in size. Many times there is no vacuole.

9. In the process of pinocytosis pinocytic vesicles are formed in animal cells. These vesicles are called pinosomes.

10. The cell organelle lysosome is present in animal cells.

11. In the process of phagocytosis, the cell membrane of the animal cell folds inwards towards the cell and accepts the solid material inside the cell (1) & (4).

Structure of animal cell

Like plant cells, the structure of animal cells is also quite complex. An animal cell consists mainly of two parts the cell membrane and protoplasm. The structure and function of different parts of animal cells are discussed below.

Cell membrane

Definition

The protoplasm of every animal cell that is covered from the outside by a fine elastic and semipermeable membrane is called the cell membrane or plasma membrane.

Structure

The cell membrane of animal cells is made up of two phospholipid layers called a lipid bilayer. Globular protein molecules are dispersed in the lipid bilayer according to the mosaic. These proteins also move through the liquid lipid layer.

There are two types of proteins in the cell membrane of animal cells. These are internal proteins and external proteins. Internal proteins are a type of protein that partially or completely breaks down two lipid layers. On the other hand, proteins that are loosely attached to the outer or inner lipid layer are external proteins.

Example

An example of a cell membrane visible to the necked eye is a thin membrane attached to a boiled egg.

Function

The function of cell membranes of animal cells is as follows.

  1. The cell membrane of animal cells helps the entry of liquid food into the cell through the pinocytic vesicle by the process of pinocytosis.
  2. The main function of the cell membrane is to protect the protoplasm inside the cell.
  3. The cell membrane produces cell organelles and protects them (2).

Protoplasm

It is the semi-liquid, colloidal, and jelly-type living part of the cell membrane. The protoplasm of an animal cell has two parts as a nucleus and cytoplasm. The nucleus and cytoplasm are discussed in detail below (2).

Nucleus

The largest, spherical, membranous, chromatin-containing part of the protoplasm of the eukaryotic cell that controls all the physiological functions of the cell is called the nucleus. The nucleus of animal cells is usually round or oval. The nucleus of animal cells consists mainly of four parts.

Nuclear membrane

The nuclear membrane is the living two-layer covering that surrounds the nucleus. This membrane is made up of proteins and fats. The nuclear membrane has numerous holes. These are nuclear holes.

Nucleoplasm

The semi-liquid, transparent substance of the nucleus is called nucleoplasm. Nucleoplasm is more granular and dense jelly-type than the cytoplasm.

Nucleolus

The large spherical part of the nucleus is called the nucleolus. A nucleus contains a single nucleolus.

Nuclear reticulum

The fine thread-type part of the nucleoplasm is called the nuclear reticulum. During cell division, the separation of water makes the nuclear reticulum clearer and thicker.

Function

  1. The nucleus is the brain of a cell because it controls the physiological functions of the cell.
  2. It plays an active role in the formation of chromosomes during cell division.
  3. The nucleolus present in the nucleus is the production center of the ribosome.
  4. It participates in protein synthesis and RNA formation.
  5. Chromosomes located in the nucleus take part in DNA synthesis (5).

Cytoplasm

Another part of the protoplasm of animal cells in the cytoplasm. A living, colorless, translucent, semi-liquid, granular part of protoplasm is called cytoplasm. Dead cells do not have cytoplasm. The cytoplasm has two parts called the cytosol or hyloplasm and the other is the cell organelle. The cytoplasm contains 75% water and 25% solid organic and inorganic substances.

Cytosol or hyaloplasm

The cytosol or hyaloplasm is a homogeneous, opaque, colloidal cytoplasmic organ. Various inorganic objects are floating in the hyaloplasm or cytosol. The transparent, grain-less outer layer of the cytoplasm (adjacent to the cell membrane) is called the ectoplasm. And the granular, opaque inner part of the cytoplasm is called the endoplasm.

Cell organelle

Besides the cytosol, the cytoplasm of animal cells contains various cellular organelles such as mitochondria, centrosomes, Golgi bodies, ribosomes, endoplasmic reticulum, lysosome, and vacuoles. Cell organelles located in the cytoplasm of animal cells are described below.

1. Mitochondria

They are scattered, rod or spiral-shaped double-membrane organelles in the cytoplasm of animal cells. Mitochondria are covered by two single membranes.

Function

  1. Mitochondria help in the cellular respiration of the animal.
  2. The function of mitochondria is to control the reaction of the Krebs cycle in the process of aerobic respiration.
  3. It synthesizes energy-filled ATP molecules.
  4. Mitochondria release the energy needed to regulate the various biological functions of the cell. That is why it is called the powerhouse of the cell.
  5. The haem part of the hemoglobin is synthesized in mitochondria.
  6. It regulates the beta-oxidation of fatty acids.
  7. Mitochondria help to store calcium in the cytoplasm of animal cells (1) & (2).

2. Centrosome

In the cytoplasm of an animal cell, a tiny, star-shaped bright organelle located near the nucleus that helps in the formation of spindle fibers during cell division is called the centrosome. The centrosome has four main parts. These are centriole, pericentriolar material, centrosphere, and astral rays. The centriole is two hollow pip-type parts at right angles to the center of the centrosome. The two centrioles together are called diplosomes. The fine fibers of the reticulated tubulin protein that surrounded the two centrioles are called pericentriolar material. The cytoplasm located around the centriole is called the centrosphere. The microtubules are formed from the centriole in the cell. These microtubules are arranged in the form of rays from the centrosome to the outside. These are called astral rays.

Function

  1. Centrosomes help in cell division through the formation of spindle fibers.
  2. The function of the centrosome is to assist in the marginal movement of the chromosome.
  3. The flagella and cilia are formed in the cell by the centrosome (2).

3. Golgi body or Golgi apparatus

The single membrane cells located near the nucleus that are involved in cellular secretion are the Golgi bodies or apparatus.

Properties

  1. Golgi bodies or apparatus are round or cylindrical in shape.
  2. These are densely located near the nucleus of the cell.
  3. The Golgi bodies or apparatus are living cellular organelles made up of many small vacuoles and flattened sacs.
  4. The Golgi bodies are located just behind the endoplasmic reticulum that surrounds the nucleus of the cell.

Function

  1. The Golgi body is the secretory organelle of the cell. It secretes enzymes, hormones, and mucus.
  2. It helps in the formation of acrosomes of the mature sperm.
  3. The Golgi bodies or apparatus act as a storehouse of food.
  4. The Golgi body transmits various interstitial substances such as proteins, enzymes, etc. to the lysosomes or to the cell membrane or outside the cell (2) & (3).

4. Ribosome

Location

The ribosome is located on the endoplasmic reticulum and nucleus membrane of animal cells.

Properties

  1. The ribosome is a type of membrane-less circular cell organelle.
  2. It consists of a combination of proteins and ribonucleic acid.
  3. The ribosomes of eukaryotic cells are of the 80s type.
  4. When the ribosome is located single, it is called menosome.
  5. During protein formation, multiple ribosomes combine with RNA fibers to form polyribosomes or polysomes.
  6. The lower units of the ribosome are connected to each other. Magnesium ions help in this attachment.

Function

The ribosome is the main center of protein synthesis in animal cells. So the ribosome is called the protein factory of the cell (2) & (4).

5. Endoplasmic reticulum

It consists of three parts. These are cisternae, vesicles, and tubules. The cisternae is a branchless, long, flattened sac-type part. The vesicle is the part of the endoplasmic reticulum that is like a fine round or oval hole. A tubule is a branched and tube-shaped structure.

Location

The endoplasmic reticulum of animal cells is located on the cytosol or hyaloplasm of the cell. It extends from the cell membrane to the nuclear membrane.

Properties

  1. The endoplasmic reticulum is a single membrane duct or sac.
  2. It is a reticular cell organelle.
  3. The endoplasmic reticulum has two types. If there is a ribosome on the endoplasmic reticulum’s body it is called a rough endoplasmic reticulum. And if there is no ribosome it is called the smooth endoplasmic reticulum.

Function

  1. The endoplasmic reticulum directly or indirectly participates in the formation of cell organelles,
  2. Rough endoplasmic reticulum helps fold in newly formed proteins.
  3. The endoplasmic reticulum helps in the production of SER lipids, steroids, and hormones.
  4. It increases the effective area inside the cell and gives mechanical strength to the cell.
  5. The endoplasmic reticulum isolates the various chemical reactions of the cytoplasm (2) & (5).

6. Lysosome

A single-membraned organelle scattered in the cytoplasm of an animal cell is called a lysosome. Lysosomes are found in animal cells. The lysosomes look like round sacs. The internal structure of the lysosome is indefinite and variable. It often contains granular substances and undigested substances.

Function

  1. When a living cell is damaged for some reason, its function is lost. In this condition the lysosomes of the corresponding cell rupture. As a result, hydrolysis enzymes are released and destroy all the cells. It is called a ‘suicide sac’ because the lysosomes rupture and destroy the cell itself.
  2. The main function of lysosomes is to help the digestion of cellular substances (1) & (2).

7. Vacuole

Animal cells often do not have a vacuole. Some animal cells have vacuoles. The number of vacuoles in animal cells is less and the size of vacuoles is smaller.

Function

  1. In some cases the vacuole stores food.
  2. Vacuoles help remove excretory substances from the contractile holes and excretory holes of animal cells (5).

Written By: Manisha Bharati

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