What is a Cell?
A cell is the smallest, basic membrane-bound unit that contains the fundamental component of life. Cells are the structural, functional, and biological units of all living beings.
Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. In the cytoplasm, several biomolecules are present such as proteins, nucleic acids and lipids. Cellular structures called cell organelles are also suspended in the cytoplasm.
The study of cells from their basic structure to every cell organelle’s functions is called Cell Biology. All organisms are made up of cells. They may be made up of a single cell called unicellular, or many cells called multicellular.
Mycoplasmas are the smallest known cells. A cell can replicate itself independently. Therefore, they are known as the building blocks of life. They provide structure to the body and convert the nutrients taken from the food into energy.
Cells are complex, and their components perform various functions in an organism. They are of different shapes and sizes.
Cells are the lowest level of organization in every life form. From organism to organism, the count of cells can vary. Humans have several cells compared to bacteria.
Cells contain several cell organelles that perform specialized functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells.
Discovery of Cells
- Robert Hooke discovered the cell in 1665.
- Robert Hooke observed a piece of bottle cork under a compound microscope and noticed minuscule structures that reminded him of small rooms. Consequently, he named these “rooms” as cells. However, his compound microscope had limited magnification, and hence, he could not see any details in the structure. Owing to this limitation, Hooke concluded that these were non-living entities.
- After that, Anton Van Leeuwenhoek observed cells under another compound microscope with higher magnification. This time, he had noted that the cells exhibited some form of movement. Therefore Leeuwenhoek concluded that these microscopic entities were “alive.” Eventually, after a host of other observations, these entities were named animalcules.
- In 1883, Robert Brown, a Scottish botanist, provided the first insights into the cell structure. He was able to describe the nucleus present in the cells of orchids.
Characteristics of Cells
Following are the various essential characteristics of cells:
- Cells provide structure and support to the body of an organism.
- The cell interior is organized into different individual organelles surrounded by a separate membrane.
- The nucleus holds genetic information necessary for reproduction and cell growth.
- Every cell has one nucleus and membrane-bound organelles in the cytoplasm.
- Mitochondria, a double membrane-bound organelle, are mainly responsible for the energy transactions vital for its survival.
- The endoplasmic reticulum plays a significant role in the cell’s internal organization by synthesizing selective molecules and processing, directing and sorting them to their appropriate locations.
Types of Cells
Cells are of two types, such as:
- Prokaryotic Cells: Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes, including cell signaling. They are simpler and smaller than eukaryotic cells and lack a nucleus and other membrane-bound organelles.
The DNA of a prokaryotic cell consists of a single circular chromosome in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter. A prokaryotic cell has three regions:
- Enclosing the cell is the cell envelope consisting of a plasma membrane covered by a cell wall that may be further covered by a third layer called a capsule for some bacteria. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions, such as bacteria and archaea, which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates its interior from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria and acts as an additional barrier against exterior forces.
- It also prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment. Inside the cell is the cytoplasmic region that contains the DNA, ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm.
- Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular.
- Eukaryotic Cells
Eukaryotic cells are thought to be the most modern major cell type. All multicellular organisms are eukaryotes. Eukaryotic cells seem to have “learned” to work together to create multicellular organisms.
Eukaryotic cells usually have more than one chromosome, which contains large amounts of genetic information. Within a multicellular organism’s body, different genes within these chromosomes may be switched “on” and “off,” allowing cells with different traits and performing different functions within the same organism.
The main distinguishing feature of eukaryotes as compared to prokaryotes is membrane-bound compartments in which specific activities occur.
Eukaryotic cells also have one or more internal membranes, which led scientists to conclude that eukaryotic cells likely evolved when prokaryote types began living in other cells’ symbiotic relationships. Organelles with interior membranes found in eukaryotic cells typically include:
- For animal cells: Mitochondria, which liberate the energy from sugar and turn it into ATP efficiently.
- Mitochondria even have their DNA, separate from the cells’ nuclear DNA, which supports the theory that they used to be independent bacteria.
- For plant cells: Chloroplasts, which perform photosynthesis, making ATP and sugar from sunlight and air.
- Chloroplasts also have their DNA, suggesting that they may have originated as photosynthetic bacteria.
- Nucleus: In eukaryotic cells, the nucleus contains the essential DNA blueprints and operating instructions.
- The nuclear envelope is thought to provide an extra layer of protection for the DNA against toxins or invaders, which might damage it.
- It is unknown whether the nucleus might also have been an endosymbiotic prokaryote at one time or whether its membrane evolved as an extra layer of protection for the cell’s DNA.
- Golgi apparatus: This internal membrane complex can be thought of as the endoplasmic reticulum. It receives proteins from the ER, packages and “labels” them by attaching sugars as needed, and then sends them to their final destinations.
- Endoplasmic reticulum: This complex internal membrane is a major site of protein creation for cells. The evolutionary origin of the endoplasmic reticulum is not known.
Many eukaryotic cells can create temporary internal membrane “sacs,” called “vacuoles,” to store waste or package important materials. Some cells have special vacuoles called “lysosomes”, which are full of corrosive substances and digestive enzymes.
Cells dump their “trash” into lysosomes, where the harsh environment breaks them down into simpler components that can be re-used!
Structure of Cell
The cell structure comprises individual components with specific functions essential to carry out life’s processes. These components are the cell wall, cell membrane, cytoplasm, nucleus, and cell organelles.
- Cell Membrane
The cell membrane supports and protects the cell. It controls the movement of substances in and out of the cells. It separates the cell from the external environment. The cell membrane is present in all the cells.
The cell membrane is the outer covering of a cell within which all other organelles, such as the cytoplasm and nucleus, are enclosed. It is also referred to as the plasma membrane.
By structure, it is a porous membrane (with pores) that permits selective substances in and out of the cell. Besides this, the cell membrane also protects the cellular component from damage and leakage.
It forms the wall-like structure between two cells as well as between the cell and its surroundings.
Plants are immobile, so their cell structures are well-adapted to protect them from external factors. The cell wall helps to reinforce this function.
- Cell Wall
The cell wall is the most prominent part of the plant’s cell structure. It is made up of cellulose, hemicellulose and pectin.
The cell wall is present exclusively in plant cells. It protects the plasma membrane and other cellular components. The cell wall is also the outermost layer of plant cells.
It is a rigid and stiff structure surrounding the cell membrane. It provides shape and support to the cells and protects them from mechanical shocks and injuries.
The cytoplasm is a thick, clear, jelly-like substance present inside the cell membrane.
Most of the chemical reactions within a cell take place in this cytoplasm.
The cell organelles such as endoplasmic reticulum, vacuoles, mitochondria, ribosomes are suspended in this cytoplasm.
The nucleus contains the hereditary material of the cell, the DNA.
It sends signals to the cells to grow, mature, divide and die.
The nucleus is surrounded by the nuclear envelope that separates the DNA from the rest of the cell.
The nucleus protects the DNA and is an integral component of a plant’s cell structure.
Functions of Cell
A cell performs these major functions essential for the growth and development of an organism.
- Energy Production
Cells require energy to carry out various chemical processes. This energy is produced by the cells through a process called photosynthesis in plants and respiration in animals.
- Provides Support and Structure
All the organisms are made up of cells. They form the structural basis of all the organisms. The cell wall and the cell membrane are the main components that function to provide support and structure to the organism.
- Facilitate Growth Mitosis
In the process of mitosis, the parent cell divides into the daughter cells. Thus, the cells multiply and facilitate the growth of an organism.
- Helps in Reproduction
A cell helps in reproduction through the processes called mitosis and meiosis.
- Mitosis is termed as the asexual reproduction where the parent cell divides to form daughter cells.
- Meiosis causes the daughter cells to be genetically different from the parent cells.
Therefore, we can understand why cells are known as the structural and functional unit of life. This is because they are responsible for providing structure to the organisms and performing several functions necessary to carry out life’s processes.
- Allows Transport of Substances
Various nutrients are imported by the cells to carry out various chemical processes going on inside the cells. The waste produced by the chemical processes is eliminated from the cells by active and passive transport.
Small molecules such as oxygen, carbon dioxide, and ethanol diffuse across the cell membrane along the concentration gradient. This is known as passive transport.
The larger molecules diffuse across the cell membrane through active transport, where the cells require a lot of energy to transport the substances.
Cell Theory was proposed by the German scientists Theodor Schwann, Matthias Schleiden, and Rudolf Virchow. The cell theory states that:
- A cell is the basic unit of life.
- All living species on Earth are composed of cells.
- All cells arise from pre-existing cells.
A modern version of the cell theory was formulated, and it contains the following postulates:
- Energy flows within the cells.
- The chemical composition of all the cells is the same.
- Genetic information is passed on from one cell to the other.
Cells are composed of various cell organelles that perform certain specific functions to carry out life’s processes. The different cell organelles, along with their principal functions, are as follows:
- Nucleolus: The nucleolus is the site of ribosome synthesis. Also, it is involved in controlling cellular activities and cellular reproduction.
- Nuclear membrane: The nuclear membrane protects the nucleus by forming a boundary between the nucleus and other cell organelles.
- Chromosomes: Chromosomes play a crucial role in determining the sex of an individual. Each human cell contains 23 pairs of chromosomes.
- Endoplasmic reticulum: The endoplasmic reticulum is involved in the transportation of substances throughout the cell. It plays a primary role in the metabolism of carbohydrates, synthesis of lipids, steroids and proteins.
- Golgi Bodies: Golgi bodies are called the cell’s post office as it is involved in the transportation of materials within the cell.
- Ribosome: Ribosomes are the protein synthesizers of the cell.
- Mitochondria: The mitochondrion is called “the powerhouse of the cell” because it produces ATP (the cell’s energy currency).
- Lysosomes: Lysosomes protect the cell by engulfing the foreign bodies entering the cell and helps in cell renewal. Therefore, it is known as the cell’s suicide bags.
- Chloroplast: Chloroplasts are the primary organelles for photosynthesis. It contains the pigment chlorophyll.
- Vacuoles: Vacuoles store food, water, and other waste materials in the cell.