Cells: The Building Blocks of Life

What is a 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 bacteria or as part of a large profit-forming organism like plants and animals. 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.

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.

Discovery of Cells

The discovery of cells includes:

Robert Hooke (1665)

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.

Anton van Leeuwenhoek

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.

Contributions of Schleiden, Schwann, Virchow

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.

Cell Theory and Its Principles

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 are:

• 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.

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. In contrast, eukaryotic cells have true nuclei and the organelles are enclosed by the membranes.

Prokaryotic Cells

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

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

• 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 inhabitants of the human intestine and animals and Streptococcus pneumoniae that causes pneumonia.

• Archaea: Organisms such as Halobacteria in high-salt environments and methanogens in low-oxygen environments.

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 for modification and packaging of proteins.

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

• 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 and large central vacuoles are involved in storage and also in maintaining cell turgidity.

Examples

• Plants: Oak tree is a very large and structurally highly differentiated organism, consisting of many specialized cells performing 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

The difference between prokaryotic and eukaryotic cells is discussed in the table given below:

Human Cell Structure

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. Other organelles within the cell 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.

Plasma Membrane

• 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.

Cytoplasm

• 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

• One of the most important parts of a cell, 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 and Their Functions

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

Mitochondria s 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

The stages of mitosis are explained below-

Mitosis (Stages + Significance)

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 (Stages + Significance)

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 and Catabolism

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

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, Paracrine, Endocrine)

• 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.

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

Cells NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Cell theory

• Discovery of Cell

Practice Questions for NEET

Q1. Omnis cellula e cellula is the concept of______given by______

1. Cell division; Rudolf Virchow

2. Cell respiration; Meredith Grey

3. Cell death; Harper Avery

4. Cell maturation; Leeuwenhoek

Correct answer: 1) Cell division; Rudolf Virchow

Explanation:

In 1838, German botanist Matthias Schleiden observed that all plants are composed of cells, and in 1839, German zoologist Theodor Schwann extended this idea to animals, declaring that they too are made up of cells. These foundational observations led to the formulation of the Cell Theory, which was later refined by German physician Rudolf Virchow, who introduced the famous dictum Omnis cellula e cellula ("new cells arise from pre-existing cells"). The Cell Theory states: (1) All living organisms are made up of one or more cells, (2) Cells are the basic unit of structure and function in organisms, and (3) New cells arise only from pre-existing cells, as they are self-reproducing. This theory remains a cornerstone of modern biology.

Hence the correct answer is option 1) Cell division; Rudolf Virchow

Q2. Which one of the following does not differ in E.coli and Chlamydomonas

1. Ribosomes

2. Chromosomes

3. Cell wall

4. Cell membrane

Correct answer: 4) Cell membrane

Explanation:

It is semi-permeable and interacts with the outside world. It is PLP (protein lipid-protein). Cell membranes are protine - lipid-protein bilayers which are almost structurally similar in eukaryotes and prokaryotes. The plasma membrane, also known as plasmalemma, regulates the entry and exit of substances, maintaining the cell's internal environment. It consists of a phospholipid bilayer with embedded proteins, allowing selective permeability. Membrane proteins serve various functions, including transport, signalling, and structural support. Despite differences between eukaryotic and prokaryotic cells, the fundamental structure of the plasma membrane is conserved across both, with similar lipid-protein interactions.

Hence, the correct answer is option 4 - Cell membrane

Q3. During development, unspecified cells become cells having unique functions. This process is called :

1. Evolution

2. Differentiation

3. Translation

4. Replication

Correct answer: 2) Differentiation

Explanation:

Differentiation is the biological process by which unspecialized cells develop into specialized cells with distinct structures and functions. This process enables cells to perform specific roles in the body, leading to the formation of specialized tissues and organs. For example, stem cells differentiate into muscle cells for movement, nerve cells for signal transmission, and red blood cells for oxygen transport. Differentiation is crucial for growth, development, and tissue repair, and is controlled by gene expression, signaling pathways, and environmental factors. As a result, cells, tissues, and organs acquire unique features that enable them to efficiently perform their specific functions.

Hence, the correct answer is option (2) differentiation.

Also Read:

• MCQs on Cells: The Building Blocks of Life

• Plasma Membrane

• Cytoplasm

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