Cell definition: The most basic unit of all living things is the cell, which has a plasma membrane, cytoplasm, and hereditary material. They come in two primary types: Prokaryotic, which does not have a nuclear membrane, and eukaryotic, which has a nuclear membrane. These are mostly designed and optimised for the organism’s life-supporting roles and tasks. Cells is the topic of the chapter Cells: The Unit of Life in Biology.

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This Story also Contains

1. What is Cell?
2. Discovery of Cells
3. The Cell Theory
4. Human Cell Structure
5. Types of Cells
6. Differences Between Prokaryotic cells and Eukaryotic cells
7. Cell Structure and Functions
8. Cell Organelles
9. Cell Division (Mitosis)
10. Meiosis:
11. Cell Metabolism
12. Cellular Respiration
13. Photosynthesis (Plant Cells)
14. Specialised Cells and Their Functions
15. Differences Between Plant and Animal Cells
16. Recommended video for "Cells: The Building Blocks of Life"

What is Cell?

• A cell is the most fundamental structure in life and is regarded as the smallest of all forms of life.

• Cells can live independently, as do the bacteria or as part of a large profit-forming organism like plants and animals.

• Prokaryotes and eukaryote cells also have relative similarities, such as the plasma membrane and cytoplasm, and then contain genetic material in DNA.

• Unlike eukaryotic cells, prokaryotic ones are comparatively simpler and do not possess a nucleus, for example, bacterial cells. These are found freely in the cytoplasm of the cellular structure.

• Animal, plant, fungi, and protist cells are larger, are known as eukaryotic cells and have a nucleus where the DNA is stored. Eukaryotic cells also have other distinctive structures called organelles, like Mitochondria- used to produce energy and Endoplasmic Reticulum- used in the synthesis of protein products.

• As can be seen, the functions of the cell are numerous and they include.

• Cells take the nutrients into the systems and transform them into energy, incorporate proteins, duplicate DNA and other structures, and respond to signals from the environment.

Discovery of Cells

The discovery of cells goes back to the 17th century- specifically in 1665—when Robert Hooke observed cell walls in a piece of cork through an improved illuminated microscope. He called them ‘cells’ because the bodies resembled human cells or rooms in a Monastery.

Then Anton van Leeuwenhoek further enhanced the compound microscope and managed to observe living cells which he referred to as “animalcules”, which paved the way for cell theory set up in the 19th century by Schleiden, Schwann, and Virchow and changed the face of Biology. These discoveries shaped the future formation of cell theory. This theory holds that all living beings are made up of cells, and that cell is the building block of the structure and function of all living beings.

Who discovered Cells?

• English scientist, Robert Hooke was the first to observe cells in 1665 however, he referred to them as ‘ Elemental Particles’.

• Looking at cork through the lens, he distinguished small, something like cubic shapes which he called “cells. ”

• He described living cells such as bacteria and protozoa through a microscope during the late seventeenth century and called them animalcules.

• The last three components were established in the 19th century with contributions by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.

• Schleiden and Schwann suggested that all plant and animal life is made up of cells, and further, Virchow made a synthesis by stating that all cells come from other pre-existing cells.

The Cell Theory

The Cell Theory is one of the primary concepts in contemporary biosciences that helps explain the nature of all living structures.

Key Principles of Cell Theory

• All living organisms are composed of one or more cells: This principle states that the cell is the most globally important structure in kingdoms of life, for example, plants, animals, fungi, and microorganisms.

• The cell is the basic unit of structure and function in all living organisms: cells are a fundamental unit of organisms, cells provide organisms with the ability to capture energy, carry out chemical reactions, grow, and reproduce.

• All cells arise from pre-existing cells through the process of cell division: This principle is in a way related to the fact of ongoing living as new cells come into being through the division of the old ones, and the genetic material is passed from one generation to another.

Contributions of Scientists (Schleiden, Schwann, Virchow)

• Matthias Schleiden (1838): Schleiden from Germany stated that every plant tissue is made of cells through his work showed the major structures of the plant tissues and the fact that cells made them .

• Theodor Schwann (1839): another German physiologist who made similar observations as Schleiden but for animals, showing also that animal tissues were made up of what he called cells.

• Rudolf Virchow (1855): One is the famous German physician who first described the idea that “Omnis cellula e cellula,” or “ all cells arise from pre-existing cells “ which gave the cell division the credit for growth mechanisms in living organisms.

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• Plasma Membrane
• Cytoplasm

Human Cell Structure

The overview of human cell structure is described below-

Human cells are the basic structural and functional units of the body wherein all life processes are carried out. Every cell is surrounded by a cell membrane, which helps protect the cell and determines what goes in and out. Inside it is the cytoplasm, the organelle-containing contents of the cell, the nucleus, functioning as the control centre of a cell, contains genetic information (DNA) and regulates cell functions. Other organelles within the cell would assist in the generation of energy, synthesis of proteins, breakdown of wastes, and structural support. Collectively, the cell would perhaps grow, respond to environmental stimuli, reproduce, and provide an appropriate background for tissues and organs known to exist in the human body.

Types of Cells

The major types of cells are; prokaryotic and eukaryotic. Prokaryotic cells are primitive and small and do not contain a true nucleus or membrane-bound organelles, DNA is in a region referred to as the nucleoid; and belongs to the kingdoms of bacteria and archaea. In contrast to this, Eukaryotic cells have true nuclei and the organelles are enclosed by the membranes. They are found in plants, animals, fungi, and protists. The human cells are the most specialised ones with all the required organelles except chloroplast.

Prokaryotic Cells

• Do not possess a proper nucleus; their DNA is not protected by membranes but is diffused in space, known as the nucleoid.

• Without small membrane-bound organelles, most of their functions are performed either in the cytoplasm or on the cell membrane.

• Normally, it is approximately 0.2 to 0.5 micrometres in size and has fewer organelles than eukaryotic cells.

• Both prokaryotic and eukaryotic cells contain a cell wall consisting of peptidoglycan in bacteria and pseudo peptidoglycan in archaea to provide the cell shape and rigidity.

• Prokaryotic cells have ribosomes for building proteins, but the ones present here are not as large as those in eukaryotic cells (70S).

• Many have other appendages, including flagella for motility and pili for mating, and can also contain smaller circles of DNA called plasmids.

Examples:

• Bacteria: Examples are Escherichia coli (E. coli), bacteria that are normal inhabitants of the human intestine and animals; and Streptococcus pneumoniae that causes pneumonia.

• Archaea: For instance, there are organisms such as Halobacteria in high-salt environments and methanogens in low-oxygen environments that produce methane and feed in the stomachs of ruminant animals.

Eukaryotic Cells:

• Holds a true nuclear envelope or double membrane which contains the outlined hereditary material or DNA in the form of chromosomes.

• A variety of membrane-bound organelles where each structure has a distinct function such as the mitochondria for the production of energy, the endoplasmic reticulum specialises in the synthesis of proteins and lipids, and the Golgi apparatus is involved in the modification and packaging of proteins.

• They are usually much bigger and structurally more diverse than the prokaryotic cells, ranging in size from 10 to 100 micrometres in diameter. Human cells are the eukaryotic ones.

• Contains microtubules and microfilaments, which serve as network support, involved in the cell division process and transport of materials within the cell.

• In plants, eukaryotic cells possess a rigid cell wall made up of cellulose, which contains chloroplasts to carry out photosynthesis, and large central vacuoles that are involved in storage and also in maintaining cell turgidity.

Examples

• Plants: For instance, the oak tree is a very large and structurally highly differentiated organism, consisting of many specialized cells, the main functions of which are photosynthesis, structural support, and transport of nutrients.

• Animals: For instance, humans have specialized cells like neuron cells for energizing the transfer of nerve impulses and muscle cells for moving our body.

Differences Between Prokaryotic cells and Eukaryotic cells

Cell Structure and Functions

The cell structure consists of the cell covering, or ‘skin’ which modulates the access of materials into and out of the cell; and organelles—the nucleus, mitochondria, endoplasmic reticulum, and Golgi body—which perform certain cell functions such as the direction of the synthesis of genetic information and generation of energy. These are located in the cytoplasm, and they help in intracellular transport and metabolic reactions, hence playing a central role in cellular functions of life.

There various cell components and their functions discussed under this heading. The various parts of a cell are described below-

Cell Membrane

Structure

• Made up of two phospholipid layers with proteins integrated in between them.

• Some may be trans-membrane proteins which are fixed throughout the cell membrane, there are also proteins that are fixed on the surface of the cell membrane.

Function:

• Controls movement of smaller particles, including ions and molecules, into the cell as well as out of the cell through a selective membrane.

• Helps the cells to communicate with the adjacent cells through the receptor and signalling molecules.

• Helps in the structural framework of the cell and also protects the genetic material.

Cell Wall

• It occurs in plants, fungi, and some bacteria.

• Cell wall structure is made up of cellulose in plants and chitin in fungi.

Function :

• This gives the cell mechanical strength so that does not collapse under osmotic pressure.

• Provides a shield from mechanical stress as well as potential pathogens.

• Concerns the cell’s shape and allows cells of a tissue to adhere and communicate with each other.

Cytoplasm

Components

• It has cellular organelles like mitochondria, ER, Golgi apparatus, lysosomes, and other cytoskeleton components including microtubules and microfilaments.

• Located in a solution called the cytoplasm in which they are partially dissolved and consist of water, dissolved ions, and organic species.

Functions

• It plays a role as the place of metabolic processes such as glycolysis, synthesized proteins, and lipid metabolism.

• Situation and regulation of shape and support of the cell with the help of cytoskeleton.

• An endomembrane system that transports organelles and vesicles within the cell; it uses motor proteins and tracks the cytoskeleton.

Nucleus

Structure

• One of the most important part of a cell, this is enclosed by a nuclear envelope which is two membranes with nuclear pore complexes to regulate the traffic of molecules.

• Comprises chromatin DNA assisted by histone proteins and coils during the cell division process into a chromosome.

Function

• Contributes to retaining the cell’s DNA and other proteins which are associated with DNA.

• An enzyme that binds to DNA and RNA polymerase in order to determine the levels of genes in the body of an organism.

• Supports cell division functions such as the synthesis of the DNA and the distribution of cell contents.

Cell Organelles

They are specialised subunits present within a cell enclosed by its membrane, carrying out a specific function. They are present within the cytoplasm and perform cellular activities.

Mitochondria:

Structure

• That is a place within the cell where the process of oxidations takes place for the production of ATP molecules.

Function

• Achieves ATP generation by the process of electron transport chain as well as oxidative phosphorylation.

• Involved in calcium mobilization and in programmed cell death also known as apoptosis.

Chloroplasts:

Structure:

• Both the outer and inner membranes on the thylakoid membranes exist and are organized in structures called grana.

• Made up of Different chlorophyll pigments that aid in the absorption of light energy.

Function:

• Part of the plant cell that light energy transforms to chemical energy in glucose through the Calvin cycle and the light-dependent reaction.

• Is capable of forming highly condensed complex organic compounds such as glucose and releasing oxygen at the same time.

Endoplasmic Reticulum:

Smooth ER

• Lack of ribosome on the outer membrane of the organelle.

• Involved in lion synthesis, lion metabolism, and the process of detoxification of drugs and poison.

• Releases and stores the calcium ions.

Rough ER

• It is surrounded by the ribosomes which are grafted on its surface.

• It is involved in the synthesis of proteins, folding of the protein and modifying it by for instance adding new amino acid residues.

• Transports newly synthesized proteins to other organelles or the cell membrane.

Golgi Apparatus

Structure

• It is made of flattened ovoidal to discoidal membranous units called cisternae arranged in parallel series.

• Having a cis face where on the receiving part and a trans face is on the shipping part.

Function

• Affects, classifies, and packages the proteins and lipids manufactured in the endoplasmic reticulum.

• It synthesises lysosomes by forming a membrane-bound structure with hydrolytic enzymes.

• Transports proteins in secretory vesicles to different locations in the cell or to be released out of the cell.

Lysosomes:

Structure

• It is a membrane-bound vesicles that contain hydrolytic enzymes held in lysosomes such as acid hydrolases.

• It is synthesized at the Golgi apparatus by vesicular budding mechanisms.

Function

• Versus cellular waste, damaged organelles, and engulfed particles through hydrolysis, it digests and recycles.

• This saccades in cellular renewal, autophagy, which is also known as self-eating, and programmed cell death, also known as apoptosis.

• It helps in maintaining cellular homeostasis by controlling the intracellular degradation procedure.

Ribosomes:

Structure

• Ribosomes comprise two subunits; a Large subunit and a Small subunit, both have ribosomal RNA (rRNA) and proteins.

• In eukaryotic cells, these subunits are synthesized in the nucleus in an organelle called nucleolus and then transported to the cytoplasm for assembly.

Function

• Hence, the synthesis of proteins is carried out by molecular machines called ribosomes.

• It converts mRNA into amino acid sequences and synthesizes proteins required for different cellular activities.

Vacuoles:

Structure

• Vacuoles are those structures present in the cell that have the membrane surrounding it.

• While in plant cells the vacuoles are giant and occupy most of the volume of the cells, in animal cells the vacuoles are more numerous but relatively small.

Function

• Some of their functions include food reserves, the storage of various products, and also helping to sustain turgor pressure, which is an important feature of plant cells in relation to the rigidity and support of the plant body.

• They also participate in intracellular digestion and in the liberation of the waste products of the cell.

Cytoskeleton:

Components

• Microfilaments: Ffilaments of small diameter; composed of actin.

• Intermediate Filaments: Structural strands resembling ropes that help to give support.

• Microtubules: Structures that are formed by tubulin proteins, in other words, small tubes that are empty or contain a few molecules.

Functions

Proteins of the cytoskeleton are involved in providing the cells mechanical strength, maintaining the shape of the cell, supporting movements of the cell, and being involved in transport within the cell also playing a role in the division of the cell.

Centrioles:

Structure

• Centrioles are barrel-shaped organelles made up of nine batches of triplet microtubules coiled in a circular fashion.

• They are usually present in twos in the centrosome though sometimes one of the pair can be absent.

Function

• Centrioles are involved structural organization of the mitotic spindle for chromosome separation during mitosis and meiosis.

Nucleolus

• The nucleolus is another prominent nuclear structure and it is generally spherical in shape and densely stained.

• This is the place where ribosomal RNA (rRNA) is produced, along with the individual ribosomal subunits, and is directly involved in the translation process.

Nuclear membrane

• The nuclear membrane or nuclear envelope is a double-layered lipid layer that surrounds the nucleus.

• It surrounds the genes and keeps control of moving particles and molecules in and out of the nuclear membrane through nuclear pore complexes.

Chromosome

Chromosomes are thread-like structures made of DNA and protein (histones). It contains genes and is located in the nucleus of the eukaryotic cells.

Cell Division (Mitosis)

The stages of mitosis are explained below-

Stages

• Prophase: Chromosomes start condensing and the spindle fibers start forming and the nuclear membrane soon breaks.

• Metaphase: They also known as chromosomes align themselves along the equatorial plate.

• Anaphase: Two identical chromatids which are a part of the same chromosome move towards different poles.

• Telophase: Chromosomes return to a less condensed state the nuclear envelope re-emerges and cytokinesis begins.

Significance:

• It helps maintain the genetic stability of the cell by forming two daughter cells out of one parent cell.

• Required in the synthesis of tissues, in growth and development, for repair of body tissues, and in asexual reproduction in the single-celled organism.

Meiosis:

The stages of meiosis are explained below-

Stages

• Prophase I: These similar chromosomes pair up in the process known as crossing over.

• Metaphase I: The homologous pairs lie at the center of the cell also known as the equator.

• Anaphase I: Sister chromatids break and the chromosomes of the same kind migrate to the poles of the cell.

• Telophase I: Chromosomes begin to reduplicate while at the same time getting less compact, the nuclear envelope starts to emerge, and cytokinesis takes place.

• After, it is succeeded by prophase II, metaphase II, anaphase II, and telophase II leading to the formation of four haploid daughter cells.

Significance:

• Forms functional gametes like the sperm and eggs that have a chromosome number half that of the parent cell.

• Serves to prevent intra-specific maturity in sexual / rewriting: It helps check on alleles and genes in sexually reproducing organisms.

Cell Metabolism

• Anabolism: Mechanistic operations that involve assembling larger biomolecules starting from simpler units, being energy-demanding.

• Catabolism: The type of metabolism that is associated with the breakdown and destruction of the biochemical molecules, which involves the liberation of energy.

Cellular Respiration

• In glycolysis, glucose is metabolized, through a series of reactions, into two molecules of pyruvic acid.

• The citric acid cycle converts pyruvate into carbon dioxide and forms ATP and some of the electron carriers.

• ATP synthesis in the electron transport chain occurs by chemiosmosis where oxidative phosphorylation takes place.

• Serves as a source of ‘energy of life’ (ATP) for any ongoing cellular activity.

• It is the main metabolic route in aerobic creatures/organisms.

Photosynthesis (Plant Cells)

Photosynthesis is described below-

Structure:

Photosynthesis light-independent reactions, referred to as the Calvin cycle, take place at the stroma where ATP and NADPH are used to incorporate CO2 into glucose.

Function:

• Has as a side end the evolution of oxygen.

• Supplies energy in the form of molecules that are used for plant growth and development.

Cell Communication and Signalling

This includes direct physical contact with adjacent cells, through the use of ligand-containing molecules, and through the use of electrical connections known as gap junctions.

Types of Signaling

• Autocrine: A cell releases substances that have an effect on the same cell.

• Paracrine: Info transfers occur at the local level.

• Endocrine: You get signals through the bloodstream and these are carried over long distances to target cells.

Importance in Multicellular Organisms

• Known to control cell division rates, differentiation, and immune reactions in addition to the maintenance of tissues.

• Ensures that the cells are well coordinated within tissues as well as organs.

Specialised Cells and Their Functions

Some specialised cells and their functions are discussed below-

Red Blood Cells

• Carry oxygen from the lungs to the tissues and carbon dioxide, from the tissues to the lungs.

• Enclosing cells with hemoglobin, a substance that grips and transports oxygen.

• Lack of a nucleus in order to provide the maximum amount of space for the transportation of oxygen.

Additional Information:

• Biconcave shape facilitates diffusion due to the larger surface area available for the exchange.

• Because of the flexible cell membrane, red blood cells can pass unnoticed through the narrow capillaries.

• Has a short life span of approximately 120 days and thus requires to be produced continually in the bone marrow.

White Blood Cells

• Would function as some category of the immune system that responds to disease-causing pathogens and any materials that the body does not recognize.

• Enclose and kill bacteria, viruses, and other pathogens of various diseases by the process known as phagocytosis.

• Generate anthologies for the proper functioning of the immune responses and cytokines for the proper regulation of inflammation.

Additional Information:

• It is important to notice that various kinds of white blood cells carry out distinct tasks – for instance, neutrophils are characterized by attacking bacteria whereas lymphocytes are immune.

• Neutrophil Chemotaxis; which in general means that neutrophils migrate to sites of infection or inflammation in response to chemical attractants.

• They are involved in adaptive immunity since they provide specific identification and memory of particular pathogens.

Muscle Cells

• Pump blood to offer bodies the required force and movements as concerns locomotion, posture, and organ functions.

• Some of the types of blood-borne tissues are as follows: Skeletal muscle cells – these cells result in voluntary movements in the muscular tissues.

• Cardiac and smooth – these cells bring about involuntary movements such as heartbeats and digestion of food.

Additional Information:

• High level of organization of the broad networks of actin and myosin filaments into sarcomeres, the fundamental contractile subunits.

• Mitochondria generate ATP which is used in the contraction of muscles.

• They can increase or hypertrophy in size in response to new loading or exercise or physiological demands and can also increase in number as in hyperplasia.

Nerve Cells

• Convey messages in the form of electrical impulses (nerve impulses) to form a network through which signals are sent to various parts of the body to cause sensation, muscle movement, and thought processes.

• Contains a cell body (surrounding the nucleus), dendrites (the part used to get input signals from other neurons), and axons (the part used to send output signals to other neurons or target cells).

• To derive action potentials to presynaptic neurons or to release neurotransmitters at synapses to signal adjacent neurons or effector cells.

Additional Information:

• Some axons are myelinated by oligodendrocytes mainly in the CNS and by Schwann cells in the PNS to enhance signal transmission rate.

• Dendritic spines enhance the contact area for acceptor PSM and for synaptic plasticity.

• Frequently show special features such as nodes of Ranvier and synaptic terminals for quick communication.

Plant Cells (Guard Cells, Root Hair Cells)

• Guard Cells:

Function: Participate in controlling the opening and closing of stomata to help in the regulation of gas exchange and water evaporation leading to Transpiration and photosynthesis rates.

Additional Information: Control the size of the stomatal pore by altering turgor pressure in the guard cells with the help of an osmotic uptake or release of ions and water.

• Root Hair Cells:

Function: Improve the plants’ water and nutrient intake from the soil to increase growth and nutrient assimilation.

Additional Information: Develop tubular protrusions from the root epidermal layer to enhance the extent of the absorbing surface and enhance the root’s interaction with the soil.

Differences Between Plant and Animal Cells

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