Biology

Structure of Cell

“A cell is defined as the smallest, basic unit of life that is responsible for all of life’s
processes.”

Cells are the structural, functional, and biological units of all living beings. A cell can replicate
itself independently. Hence, they are known as the building blocks of life.

Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also
present in the cytoplasm are several biomolecules like proteins, nucleic acids and lipids.
Moreover, cellular structures called cell organelles are suspended in the cytoplasm.

What is a Cell?
A cell is the structural and fundamental unit of life. The study of cells from its basic structure
to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first
Biologist who discovered cells.

All organisms are made up of cells. They may be made up of a single cell (unicellular), or many
cells (multicellular). Mycoplasmas are the smallest known cells. Cells are the building blocks
of all living beings. 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, pretty much like bricks of the buildings. Our body is made up of
cells of different shapes and sizes.

Cells are the lowest level of organisation in every life form. From organism to organism, the
count of cells may vary. Humans have more number of cells compared to that of bacteria.

Cells comprise several cell organelles that perform specialised 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
Discovery of cells is one of the remarkable advancements in the field of science. It helps us
know that all the organisms are made up of cells, and these cells help in carrying out various
life processes. The structure and functions of cells helped us to understand life in a better
way.

Who discovered 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.

Later 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
(motility). As a result, Leeuwenhoek concluded that these microscopic entities were “alive.”
Eventually, after a host of other observations, these entities were named as animalcules.

In 1883, Robert Brown, a Scottish botanist, provided the very 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 organised into different individual organelles surrounded by a separate
membrane.
The nucleus (major organelle) 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 is mainly responsible for the energy
transactions vital for the survival of the cell.
Lysosomes digest unwanted materials in the cell.
Endoplasmic reticulum plays a significant role in the internal organisation of the cell by
synthesising selective molecules and processing, directing and sorting them to their
appropriate locations.

Also Read: Nucleus

Types of Cells
Cells are similar to factories with different labourers and departments that work towards a
common objective. Various types of cells perform different functions. Based on cellular
structure, there are two types of cells:

Prokaryotes
Eukaryotes

Explore: Difference Between Prokaryotic and Eukaryotic Cells

Prokaryotic Cells
Main article: Prokaryotic Cells

1. Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a
region within the cell where the genetic material is freely suspended. This region is called
the nucleoid.
2. They all are single-celled microorganisms. Examples include archaea, bacteria, and
cyanobacteria.
3. The cell size ranges from 0.1 to 0.5 µm in diameter.
4. The hereditary material can either be DNA or RNA.
5. Prokaryotes generally reproduce by binary fission, a form of asexual reproduction. They
are also known to use conjugation – which is often seen as the prokaryotic equivalent to
sexual reproduction (however, it is NOT sexual reproduction).

Eukaryotic Cells

Main article: Eukaryotic Cells

1. Eukaryotic cells are characterised by a true nucleus.

2. The size of the cells ranges between 10–100 µm in diameter.
3. This broad category involves plants, fungi, protozoans, and animals.
4. The plasma membrane is responsible for monitoring the transport of nutrients and
electrolytes in and out of the cells. It is also responsible for cell to cell communication.
5. They reproduce sexually as well as asexually.
6. There are some contrasting features between plant and animal cells. For eg., the plant
cell contains chloroplast, central vacuoles, and other plastids, whereas the animal cells
do not.

Cell Structure
The cell structure comprises individual components with specific functions essential to carry
out life’s processes. These components include- cell wall, cell membrane, cytoplasm,
nucleus, and cell organelles. Read on to explore more insights on cell structure and function.

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) which permits the movement of
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.

Cytoplasm
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.

Nucleus

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.

Cell Organelles

Cells are composed of various cell organelles that perform certain specific functions to carry
out life’s processes. The different cell organelles, along with its principal functions, are as
follows:
 Cell Organelles and their Functions

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 cells
contain 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 synthesisers of the cell.

Mitochondria

The mitochondrion is called “the powerhouse of the cell.” It is called so because it

produces ATP – the cell’s energy currency.

Lysosomes

Lysosomes protect the cell by engulfing the foreign bodies entering the cell and help in
cell renewal. Therefore, they are known as the cell’s suicide bags.
 Chloroplast

Chloroplasts are the primary organelles for photosynthesis. It contains the pigment called
chlorophyll.

Vacuoles

Vacuoles store food, water, and other waste materials in the cell.

Cell Theory
Cell Theory was proposed by the German scientists, Theodor Schwann, Matthias Schleiden,
and Rudolf Virchow. The cell theory states that:

All living species on Earth are composed of cells.

A cell is the basic unit of life.
All cells arise from pre-existing cells.

A modern version of the cell theory was eventually formulated, and it contains the following
postulates:

Energy flows within the cells.

Genetic information is passed on from one cell to the other.
The chemical composition of all the cells is the same.

Functions of Cell
A cell performs major functions essential for the growth and development of an organism.
Important functions of cell are as follows:

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. For eg., the skin is made up of a large number of cells.
Xylem present in the vascular plants is made of cells that provide structural support to the
plants.

Facilitate Growth Mitosis

In the process of mitosis, the parent cell divides into the daughter cells. Thus, the cells
multiply and facilitate the growth in an organism.
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.

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.

Aids in Reproduction

A cell aids 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.

Thus, 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 perform several
functions necessary for carrying out life’s processes.

Cellular Respiration

Cellular respiration is one of the most elegant, majestic, and fascinating metabolic pathways on earth.
At the same time, it’s also one of the most complicated. When I learned about it for the first time, I felt
like I had tripped and fallen into a can of organic-chemistry-flavored alphabet soup!

Luckily, cellular respiration is not so scary once you get to know it. Let's start by looking at cellular
respiration at a high level, walking through the four major stages and tracing how they connect up to
one another.

Steps of cellular respiration

Overview of the steps of cellular respiration.

1. Glycolysis. Six-carbon glucose is converted into two pyruvates (three carbons each). ATP and
NADH are made. These reactions take place in the cytosol.
2. Pyruvate oxidation. Pyruvate travels into the mitochondrial matrix and is converted to a two-
carbon molecule bound to coenzyme A, called acetyl CoA. Carbon dioxide is released and NADH
is made.
3. Citric acid cycle. The acetyl CoA combines with a four-carbon molecule and goes through a cycle
of reactions, ultimately regenerating the four-carbon starting molecule. ATP (or, in some cases,
GTP), NADH, and FADH_2 are made, and carbon dioxide is released. These reactions take place in
the mitochondrial matrix.
 4. Oxidative phosphorylation. The NADH and FADH_2 produced in other steps deposit their
electrons in the electron transport chain in the inner mitochondrial membrane. As electrons
move down the chain, energy is released and used to pump protons out of the matrix and into the
intermembrane space, forming a gradient. The protons flow back into the matrix through an
enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen
accepts electrons and takes up protons to form water.

During cellular respiration, a glucose molecule is gradually broken down into carbon dioxide and
water. Along the way, some ATP is produced directly in the reactions that transform glucose. Much
more ATP, however, is produced later in a process called oxidative phosphorylation. Oxidative
phosphorylation is powered by the movement of electrons through the electron transport chain, a
series of proteins embedded in the inner membrane of the mitochondrion.

These electrons come originally from glucose and are shuttled to the electron transport chain by
electron carriers and , which become and when they gain electrons. To be clear, this is what's
happening in the diagram above when it says or . The molecule isn't appearing from scratch, it's just
being converted to its electron-carrying form:

To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as
ATP and in one of your body's cells, let’s walk step by step through the four stages of cellular
respiration.

1. Glycolysis. In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical

transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon
organic molecule. In these reactions, ATP is made, and is converted to .
2. Pyruvate oxidation. Each pyruvate from glycolysis goes into the mitochondrial matrix—the
innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule
bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and is generated.
3. Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and
goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP,
, and are produced, and carbon dioxide is released.
4. Oxidative phosphorylation. The and made in other steps deposit their electrons in the electron
transport chain, turning back into their "empty" forms ( and ). As electrons move down the chain,
energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow
back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the
electron transport chain, oxygen accepts electrons and takes up protons to form water.

Glycolysis can take place without oxygen in a process called fermentation. The other three stages of
cellular respiration—pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation—require
oxygen in order to occur. Only oxidative phosphorylation uses oxygen directly, but the other two
stages can't run without oxidative phosphorylation.

Photosynthesis

Photosynthesis, the process by which green plants and certain other organisms transform light
energy into chemical energy. During photosynthesis in green plants, light energy is captured and used
to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life

on Earth. If photosynthesis ceased, there would soon be little food or other organic matter on Earth.
Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of
gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic
bacteria, which can utilize the chemical energy of certain inorganic compounds and thus are not
dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the
fossil fuels (i.e., coal, oil, and gas) that power industrial society. In past ages, green plants and small
organisms that fed on plants increased faster than they were consumed, and their remains were
deposited in Earth’s crust by sedimentation and other geological processes. There, protected from
oxidation, these organic remains were slowly converted to fossil fuels. These fuels not only provide
much of the energy used in factories, homes, and transportation but also serve as the raw material for
plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries
the excess of photosynthetic production accumulated over millions of years. Consequently, the
carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over
millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in
Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected
to have major implications on Earth’s climate.

Requirements for food, materials, and energy in a world where human population is rapidly growing
have created a need to increase both the amount of photosynthesis and the efficiency of converting
photosynthetic output into products useful to people. One response to those needs—the so-called
Green Revolution, begun in the mid-20th century—achieved enormous improvements in agricultural
yield through the use of chemical fertilizers, pest and plant-disease control, plant breeding, and
mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas
of the world despite rapid population growth, but it did not eliminate widespread malnutrition.
Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to
decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of
agricultural production, which required ever-increasing inputs of fertilizers and pesticides and
constant development of new plant varieties, also became problematic for farmers in many countries

A second agricultural revolution, based on plant genetic engineering, was forecast to lead to increases
in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists
have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA) with the
aim of achieving improvements in disease and drought resistance, product yield and quality, frost
hardiness, and other desirable properties. However, such traits are inherently complex, and the
process of making changes to crop plants through genetic engineering has turned out to be more
complicated than anticipated. In the future such genetic engineering may result in improvements in
the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate
that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are
able to convert light energy into chemical energy. The emerald green sea slug (Elysia chlorotica), for
example, acquires genes and chloroplasts from Vaucheria litorea, an alga it consumes, giving it a
limited ability to produce chlorophyll. When enough chloroplasts are assimilated, the slug may forgo
the ingestion of food. The pea aphid (Acyrthosiphon pisum) can harness light to manufacture the
energy-rich compound adenosine triphosphate (ATP); this ability has been linked to the aphid’s
manufacture of carotenoid pigments.

Cell Division

Cell division is the process by which a parent cell divides into two daughter cells.[1] Cell division usually
occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before
dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis),
producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid
gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in
the diploid parent cell to one of each type in the daughter cells.[2] Mitosis is a part of the cell cycle, in
which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically
identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the
nucleus) is preceded by the S stage of interphase (during which the DNA replication occurs) and is
followed by telophase and cytokinesis; which divides the cytoplasm, organelles, and cell membrane of one
cell into two new cells containing roughly equal shares of these cellular components. The different stages
of mitosis all together define the M phase of an animal cell cycle—the division of the mother cell into two
genetically identical daughter cells.[3] To ensure proper progression through the cell cycle, DNA damage is
detected and repaired at various checkpoints throughout the cycle. These checkpoints can halt
progression through the cell cycle by inhibiting certain cyclin-CDK complexes. Meiosis undergoes two
divisions resulting in four haploid daughter cells. Homologous chromosomes are separated in the first
division of meiosis, such that each daughter cell has one copy of each chromosome. These chromosomes
have already been replicated and have two sister chromatids which are then separated during the second
division of meiosis. [4] Both of these cell division cycles are used in the process of sexual reproduction at
some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.