Cell - Structure and Function

A cell is the smallest and fundamental unit of life, responsible for all the functions of life. It is the basic biological, structural, and functional components of all living things. Every cell is made up of cytoplasm that is enclosed in a membrane and includes many small molecules of nutrients and metabolites, as well as many macromolecules, including proteins, DNA, and RNA.

Characteristics of Cells

Given below are the following characteristics of a cell:

Cells help in the formation of the structure of an organism.
A cell is a membrane-bound unit that has other cell organelles, which perform different functions.
In eukaryotic cells, the nucleus contains most of the genetic material, but its position may vary within the cell.”
Lysosomes (mainly in animal cells) help in intracellular digestion
Cytoplasm is present in the interior of a cell, in which all cell organelles are present.
It was first discovered by Robert Hooke, and the study of cells is known as cell biology.
The organisms can be classified into two types based on the number of cells, i.e., unicellular and multicellular organisms.
The size and shape of the cell are determined by the function of that cell.
The cell is a combination of different cell organelles, which perform different functions.

Structure of a Cell

The cell structure consists of various components and cell organelles with distinct functions.

Structure of the cell	Functions
Cell Wall	The cell wall is a characteristic feature of a plant cell. It comprises the outermost layer of the cell. The cell wall is made up of cellulose, hemicellulose, and pectin. It protects the plasma membrane. It is a rigid and immovable structure enclosing the cell membrane. It protects the cells from mechanical shocks and damage while providing them with form and support. Animal cells do not have a cell wall.
Cell Membrane	A cell membrane surrounds the cell. The cell membrane divides cells into the internal and external components of the cell. It protects a cell's integrity and regulates the flow of materials into and out of the cell. Two layers of phospholipid molecules form the cell membrane. The cell membrane proteins provide structural support, facilitate material transport, and act as receptor sites for substances and transport substances across the membrane.
Cytoplasm	The gel-like substance that fills a cell is called cytoplasm. Cytoplasm functions as the site where various cellular processes occur, including metabolism, protein synthesis, and intracellular transport. The cytoplasm of a cell performs every task necessary for cell division, growth, and replication. Diffusion moves materials within the cytoplasm.
Nucleus and Nucleolus	The nucleus is a crucial organelle found in eukaryotic cells, serving as the control centre of the cell. It contains the genetic material (DNA) organised into chromosomes. The nuclear membrane surrounding the cell acts as a barrier, separating the nucleus from other cell parts. Within the nucleus, the nucleolus stands out as a distinct region responsible for ribosome assembly. It contains ribosomal RNA (rRNA) and combines with proteins to form ribosomes, essential for protein synthesis. Together, the nucleus and nucleolus play vital roles in controlling cellular activities, maintaining genetic integrity, and regulating gene expression.

Discovery of the Cell

Robert Hooke observed a cell for the first time in 1665. He was observing a dried piece of cork with a compound microscope. During the observation, he noticed various little chambers which he named the ‘cells’.

Early microscopes had limited magnification, so detailed structures of cells could not be clearly observed.
After some time, another scientist, Anton van Leeuwenhoek observed living cells (like bacteria and protozoa) in water.
During his observation, he saw that cells were showing some movements. Hence, he stated that cells are living.
Magnification technology progressed enough to find cells, as microscopes experienced continuous improvement over time.
Robert Hooke is usually acknowledged for making this discovery, which established cell biology as a field of study.
He could see pores when looking through the microscope at a piece of cork. At the time, this was remarkable because it was thought that nobody else had seen these.
Matthias Schleiden and Theodor Schwann both investigated plant and animal cells to support their respective theories. They found that the two varieties of cells differed significantly from one another. This promoted the concept that cells were essential to both plants and animals.

Cell Theory

Cell theory was proposed by Schleiden (1838) and Schwann (1839). The cell theory postulates are given below:

All living things are composed of cells.
Cells serve as the fundamental building blocks for the growth of tissues and organs.
Cells can only develop from other cells, which is the third and possibly most important characteristic of the theory.

Every living being starts with a single cell, which splits through the process of mitosis to generate multiple identical cells. These cells can specialise to create various tissues and organs. This method is used to create a complete complex organism. A unicellular organism divides into two identical daughter cells. This method is known as asexual reproduction.

Types of Cells

There are two main types of cells: eukaryotic cells and prokaryotic cells. Prokaryotic cells lacks a true nucleus. Eukaryotic Cells are those that contain true nuclei.

Cell Organelles

Organelles are cell components that have been modified and/or specialised to perform one or more important functions. Cell organelles are found in eukaryotic cells; however, bacterial organelles are more basic and not membrane-bound. A cell contains different kinds of organelles.

Cell Organelles	Structure	Function
Chromosomes	Thread-like structures made of DNA and protein	Carries genetic information
Endoplasmic Reticulum	Network of membranous tubules and sacs; rough ER has ribosomes attached, while smooth ER does not.	Involved in protein and lipid synthesis, as well as the transportation of molecules within the cell.
Golgi Bodies	Stack of flattened membrane-bound sacs (cisternae) with associated vesicles	Modifies, sorts, and packages proteins and lipids for transportation or secretion.
Ribosomes	Made of RNA and protein; can be free in the cytoplasm or attached to the ER.	Site of protein synthesis.
Mitochondria	Double membrane-bound organelle with inner cristae folds; contains mitochondrial DNA (mtDNA).	Generates ATP through cellular respiration; known as the "powerhouse" of the cell
Chloroplast	Double membrane-bound organelles with internal thylakoid membranes	Site of photosynthesis in plant cells; it contains chlorophyll for capturing light energy.
Vacuoles	Membrane-bound sacs filled with fluid or sap; larger in plant cells.	Store water, nutrients, and waste; maintain turgor pressure in plant cells.
Lysosomes	Membrane-bound vesicles containing digestive enzymes.	Carries digestive enzymes for breaking down waste materials and foreign invaders.

Prokaryotic VS Eukaryotic

The major difference between these two is that Prokaryotic Cells do not have any membrane-bound organelles. Also, the true nucleus is absent in prokaryotes. Whereas Eukaryotic Cells contain all the membrane-bound organelles, including the true nucleus.

Parameters	Prokaryotic cell	Eukaryotic cell
Development	Less developed	More developed
Nucleus	Absent	Present
Genetic Material	Nucleoid containing DNA	True nucleus containing DNA
Organelles	Absent	Membrane-bound organelles present
Size	0.1 to 5.0 μm	10-100 μm
Reproduction	Binary fission, fragmentation, conjugation	Sexual and asexual reproduction
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Functions of a Cell

The following are some of the functions of a cell:

Structure and Support: Cells provide the structural basis of all organisms.
Growth: Cells are responsible for the growth of the organism.
Transport: Cells transport the nutrients that are used in the different chemical processes that take place inside them. As a result of these processes, a waste product is produced. Cells then work to get rid of this waste. In this manner, small molecules like oxygen, carbon dioxide, and ethanol pass through the cell membrane by diffusion. This method is known as passive transport. On the other hand, the larger molecules, like proteins and polysaccharides, go in and out of the cell via active transport.
Energy Production: Organisms need energy to perform different chemical reactions. In plants, the energy comes from the process of photosynthesis, while in animals, the energy comes via respiration.
Metabolism: The cell is responsible for metabolism, which includes all the chemical reactions that take place inside an organism to keep it alive.
Reproduction: A cell helps in reproduction through the processes of mitosis and meiosis.

Cytoplasm – Structure and Function
Animal Cell
Plant Cell
Diagram of an Animal Cell