The cell is the basic structural and functional unit of living beings. Every living organism, whether simple or complex, is made up of one or more cells.
The study of cells is called cytology.
Most organisms, including humans, begin life as a single cell called a zygote. This single cell divides again and again to form tissues, organs and complete body systems.
Table of Contents
Historical Discoveries Related To Cells
The discovery of cells was not made in one day. It developed slowly with the improvement of microscopes.
Robert Hooke – 1665
Robert Hooke first observed tiny box-like structures in a thin slice of cork under a microscope.
He called these structures cells.
The word cell comes from the Latin word cellula, meaning “little room”.
Hooke’s observations were published in his famous book Micrographia.
Anton Van Leeuwenhoek – 1674
Anton van Leeuwenhoek observed living cells in pond water with his improved microscope.
He is known as the Father of Microbiology.
Robert Brown – 1831
Robert Brown discovered the nucleus of the cell.
The nucleus later became known as the control centre of the cell.
Cell Theory
The cell theory was developed by Schleiden, Schwann and Rudolf Virchow.
The main points of cell theory are:
- All living organisms are made up of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing living cells.
Virchow gave the famous statement:
Omnis cellula e cellula
It means every cell arises from a pre-existing cell.
Exception Of Cell Theory
Viruses are an exception to cell theory because they are acellular. They do not have a true cellular structure.
Types Of Organisms Based On Number Of Cells
Living organisms can be divided into two main groups on the basis of cell number.
Unicellular Organisms
Unicellular organisms are made up of only one cell.
Examples:
- Amoeba
- Paramecium
- Bacteria
- Chlamydomonas
In Amoeba, a single cell performs all life functions such as nutrition, movement, respiration and reproduction.
Multicellular Organisms
Multicellular organisms are made up of many cells.
Examples:
- Plants
- Animals
- Some fungi
In these organisms, different cells perform different functions.
Shape And Size Of Cells
Cells are not always of the same shape and size. Their shape and size depend on their function.
Important Examples Of Cell Size
| Type | Example |
|---|---|
| Smallest cell | Mycoplasma |
| Longest animal cell | Nerve cell or neuron |
| Largest isolated single cell | Ostrich egg |
Mycoplasma is about 0.3 micrometre in length. It is also known as PPLO, or Pleuro Pneumonia Like Organism.
Important Examples Of Cell Shape
| Cell Type | Shape |
|---|---|
| Red Blood Cells | Circular and biconcave |
| White Blood Cells | Amoeboid |
| Guard Cells | Bean-shaped |
Red blood cells are biconcave because this shape helps in efficient transport of oxygen.
White blood cells have an amoeboid shape because they need to move and engulf foreign particles.
Types Of Cells
Cells are broadly divided into two types:
- Prokaryotic cells
- Eukaryotic cells
Prokaryotic Cells
Prokaryotic cells are simple cells.
They do not have a membrane-bound nucleus. Instead, they have an undefined nuclear region called the nucleoid.
Features Of Prokaryotic Cells
- No membrane-bound nucleus
- Nucleoid is present
- Membrane-bound organelles are absent
- Ribosomes are 70S
- Example: Bacterial cell
In 70S ribosomes, the subunits are 30S + 50S.
Eukaryotic Cells
Eukaryotic cells are more advanced and complex.
They have a proper membrane-bound nucleus and membrane-bound organelles.
Features Of Eukaryotic Cells
- Membrane-bound nucleus is present
- Organelles like ER, Golgi complex and mitochondria are present
- Ribosomes are 80S
- Examples: Plant cells and animal cells
In 80S ribosomes, the subunits are 40S + 60S.
Prokaryotic Cells Vs Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent | Present |
| Nuclear region | Nucleoid | True nucleus |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S | 80S |
| Example | Bacteria | Plant and animal cells |
Cell Structure
A cell is mainly composed of four components:
- Cell wall
- Cell membrane
- Protoplasm
- Cell organelles
Cell Wall
The cell wall is a non-living, rigid structure present outside the plasma membrane in plant cells.
It provides shape, protection and mechanical support.
Functions Of Cell Wall
- Gives shape to the cell
- Protects the cell from mechanical damage
- Protects the cell from infection
Composition Of Cell Wall In Different Organisms
| Organism | Cell Wall Composition |
|---|---|
| Plants | Cellulose, hemicellulose, pectin and proteins |
| Algae | Cellulose, galactans, mannans and minerals |
| Fungi | Chitin |
| Bacteria | Peptidoglycan |
Layers Of Cell Wall
Primary Wall
The primary wall is present in young plant cells. It becomes less prominent as the cell matures.
Secondary Wall
The secondary wall is formed on the inner side of the primary wall.
Middle Lamella
The middle lamella is the outermost layer.
It is made up of calcium and magnesium pectate.
It helps neighbouring plant cells remain attached to each other.
Cell Membrane Or Plasma Membrane
The cell membrane is a selectively permeable membrane.
It is made up of:
- Lipid bilayer
- Proteins
- Carbohydrates
The structure of the cell membrane is explained by the Fluid Mosaic Model, proposed by Singer and Nicolson in 1972.
Functions Of Cell Membrane
- Maintains the shape of the cell
- Controls movement of materials in and out of the cell
- Allows diffusion of oxygen and carbon dioxide
- Maintains internal balance of the cell
Transportation Through Cell Membrane
Substances move through the cell membrane by different processes.
The two main types are:
- Active transport
- Passive transport
Active Transport
Active transport requires energy in the form of ATP.
It moves molecules against the concentration gradient.
This means molecules move from a region of lower concentration to a region of higher concentration.
Example Of Active Transport
- Sodium-potassium pump, also written as Na⁺-K⁺ pump
Passive Transport
Passive transport does not require energy.
It takes place along the concentration gradient.
This means molecules move from a region of higher concentration to a region of lower concentration.
Diffusion
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Example:
- Movement of oxygen and carbon dioxide through the cell membrane
When oxygen enters the blood from the lungs, diffusion plays an important role.
Osmosis
Osmosis is the movement of solvent molecules, mainly water, from a region of higher solvent concentration to lower solvent concentration through a semi-permeable membrane.
Osmosis is very important in plant cells and animal cells.
Types Of Solutions
Cells behave differently in different types of solutions.
Hypotonic Solution
A hypotonic solution has lower solute concentration than the cell.
Water moves into the cell.
The cell swells and becomes turgid.
Isotonic Solution
An isotonic solution has equal solute concentration inside and outside the cell.
There is no net movement of water.
The cell remains the same size.
Example:
- 0.9% NaCl solution is isotonic to red blood cells.
Hypertonic Solution
A hypertonic solution has higher solute concentration than the cell.
Water moves out of the cell.
The cell shrinks.
In plant cells, this condition may cause plasmolysis.
Plasmolysis
When a living plant cell loses water through osmosis, the cell contents shrink away from the cell wall.
This process is called plasmolysis.
Protoplasm
Protoplasm is the living content of the cell surrounded by the plasma membrane.
The term protoplasm was coined by Purkinje in 1839.
It is a jelly-like substance present inside the cell.
Formula Of Protoplasm
Protoplasm = Cytoplasm + Nucleoplasm
Cytoplasm
Cytoplasm is a complex, viscous and fluid-like substance present between the plasma membrane and the nucleus.
It contains cell organelles and supports many cellular activities.
Cell Organelles
Cell organelles are specialized structures present inside the cell.
Each organelle performs a specific function.
Important cell organelles include:
- Endoplasmic reticulum
- Golgi bodies
- Lysosomes
- Vacuoles
- Mitochondria
- Plastids
- Ribosomes
- Centrosome and centrioles
- Nucleus
Endomembrane System
The endomembrane system includes organelles that work together inside the cell.
It includes:
- Endoplasmic reticulum
- Golgi complex
- Lysosomes
- Vacuoles
Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is a network of tubular structures scattered in the cytoplasm.
It acts like an internal supporting framework of the cell.
Functions Of Endoplasmic Reticulum
- Synthesizes proteins and lipids
- Helps in transportation of materials inside the cell
- Helps in membrane biogenesis
Types Of Endoplasmic Reticulum
Rough Endoplasmic Reticulum
Rough ER has ribosomes on its surface.
It helps in protein synthesis.
Smooth Endoplasmic Reticulum
Smooth ER does not have ribosomes on its surface.
It helps in the synthesis of:
- Lipids
- Steroid hormones in animal cells
In liver cells of vertebrates, smooth ER helps in detoxifying poisons and drugs.
Membrane Biogenesis
Some proteins and lipids made in the ER help in building the cell membrane.
This process is called membrane biogenesis.
Golgi Bodies Or Golgi Apparatus
Golgi bodies were discovered by Camillo Golgi.
They consist of flattened, disc-shaped sacs called cisternae.
They also contain tubules, vesicles and vacuoles.
Functions Of Golgi Bodies
- Package proteins and lipids
- Help in formation of glycoproteins and glycolipids
- Synthesize and secrete enzymes and hormones
- Help in formation of acrosome of sperm
In plant cells, Golgi bodies are called dictyosomes.
Lysosomes
Lysosomes are known as the suicidal bags of the cell.
They are formed by the packaging process in the Golgi complex.
They contain nearly 40 types of hydrolytic enzymes, such as lipases and proteases.
These enzymes work best in acidic pH.
Functions Of Lysosomes
- Intracellular digestion
- Destruction of foreign substances
- Digestion of damaged organelles
They are called suicidal bags because they can digest worn-out cell parts and, in some situations, may digest the cell’s own components.
Vacuoles
Vacuoles are storage structures in the cell.
They contain:
- Water
- Excretory products
- Food materials
- Pigments
They are surrounded by a single membrane called the tonoplast.
Functions Of Vacuoles
- Store water
- Store food
- Store pigments
- Store waste products
Plant cells usually have a large central vacuole.
Animal cells may have many small vacuoles.
Mitochondria
Mitochondria are known as the powerhouse of the cell.
They produce energy in the form of ATP, or Adenosine Triphosphate.
Structure Of Mitochondria
- Sausage-shaped organelle
- Double-walled structure
- Inner membrane folded into cristae
- Has its own DNA
- Has some RNA and 70S ribosomes
The folds of the inner membrane are called cristae. These folds increase surface area for energy production.
Function Of Mitochondria
Mitochondria help in cellular respiration.
The simplified process is:
Glucose → Pyruvic acid → CO₂ + H₂O + 38 ATP
Mitochondria can synthesize some of their own proteins because they have their own DNA and ribosomes.
Plastids
Plastids are found in plant cells and euglenoids.
They contain specific pigments and perform different functions.
There are three main types of plastids:
- Chloroplast
- Chromoplast
- Leucoplast
Chloroplast
Chloroplasts contain chlorophyll and carotenoid pigments.
They are double-membrane organelles.
They are mainly found in mesophyll cells of leaves.
Chloroplasts have their own DNA and ribosomes.
Function Of Chloroplast
Chloroplasts help in photosynthesis.
They trap sunlight and help plants prepare food.
Chromoplast
Chromoplasts contain coloured pigments.
Examples:
- Carotene – orange-red pigment
- Xanthophyll – yellow pigment
The red colour of tomato is due to lycopene, a carotenoid pigment.
Leucoplast
Leucoplasts are colourless plastids.
They store food materials.
Types Of Leucoplasts
| Type | Function |
|---|---|
| Amyloplast | Stores starch |
| Elaioplast | Stores oils and fats |
| Aleuroplast or Proteinoplast | Stores proteins |
Ribosomes
Ribosomes were discovered by George Palade in 1953.
They are made up of RNA and proteins.
They are not surrounded by any membrane.
Function Of Ribosomes
Ribosomes are the sites of protein synthesis.
They join amino acids to form proteins.
Types Of Ribosomes
| Ribosome Type | Found In |
|---|---|
| 70S | Prokaryotes, mitochondria and chloroplasts |
| 80S | Eukaryotic cytoplasm |
Centrosome And Centrioles
Centrosomes and centrioles are present only in animal cells.
They help in cell division.
They form spindle fibres with the help of asters.
Nucleus
The nucleus was discovered by Robert Brown.
It is known as the control centre or brain of the cell.
It controls cellular activities such as growth, metabolism, protein synthesis and cell division.
Structure Of Nucleus
The nucleus has four main parts:
- Nuclear membrane
- Nucleoplasm
- Nucleolus
- Chromatin
Nuclear Membrane
The nuclear membrane is a double-layered structure made of proteins and lipids.
It encloses the nucleus.
Nucleoplasm
Nucleoplasm is the protoplasm present inside the nucleus.
It is slightly acidic.
It is also called karyoplasm.
Nucleolus
The nucleolus is a spherical structure.
It synthesizes rRNA and helps in ribosome formation.
Chromatin
Chromatin condenses during cell division to form chromosomes.
Chromosomes And Genes
Chromosomes are thread-like or rod-shaped structures made up of DNA and proteins called histones.
Genes are the fundamental units of heredity.
Each chromosome contains two chromatids and carries genes.
Nucleic Acids
Chromosomes are made up of proteins and nucleic acids.
There are two main types of nucleic acids:
- DNA – Deoxyribonucleic Acid
- RNA – Ribonucleic Acid
Nucleic acids are long-chain polymers of nucleotides. That is why they are also called polynucleotides.
Chemical Composition Of Nucleic Acids
Complete hydrolysis of DNA or RNA gives:
- Pentose sugar
- Phosphoric acid
- Nitrogenous base
Pentose Sugar In DNA And RNA
| Nucleic Acid | Sugar |
|---|---|
| RNA | Beta-D-ribose sugar |
| DNA | Beta-D-2-deoxyribose sugar |
Nitrogenous Bases
| DNA | RNA |
|---|---|
| Adenine | Adenine |
| Guanine | Guanine |
| Cytosine | Cytosine |
| Thymine | Uracil |
DNA contains A, G, C and T.
RNA contains A, G, C and U.
Structure Of Nucleic Acid
A nucleotide is made up of:
Sugar + Base + Phosphate
A nucleoside is made up of:
Sugar + Base
Difference Between Nucleotide And Nucleoside
| Feature | Nucleotide | Nucleoside |
|---|---|---|
| Components | Sugar + Base + Phosphate | Sugar + Base |
| Phosphate group | Present | Absent |
DNA Structure
Watson and Crick proposed the double helix structure of DNA.
DNA has two strands wound around each other.
The two strands are held together by hydrogen bonds.
RNA Structure
RNA is generally single-stranded.
It may fold to form a secondary structure.
Types Of RNA
- mRNA – Messenger RNA
- rRNA – Ribosomal RNA
- tRNA – Transfer RNA
Transcription
Transcription is the process by which DNA forms RNA.
This is an important step in gene expression.
DNA Fingerprinting
DNA fingerprinting is based on the fact that the sequence of bases in DNA is unique for every individual.
This unique pattern is called DNA fingerprinting.
Applications Of DNA Fingerprinting
- Identification of criminals in forensic labs
- Determination of paternity
- Identification of dead bodies
Plant Cell Vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present | Absent |
| Shape | Rigid shape | Usually irregular shape |
| Chloroplasts | Present | Absent |
| Nutrition | Autotrophic | Heterotrophic |
| Vacuoles | Large central vacuole | Numerous small vacuoles |
| Centrosome | Absent | Present |
| Lysosome | Generally absent | Present |
Cell Division And Cell Cycle
The cell cycle is the period from the beginning of one cell division to the beginning of the next cell division.
The cell cycle has two main phases:
- Interphase
- M-phase
Interphase
Interphase is the preparatory phase of the cell cycle.
It is often called the resting phase, but the cell is actually very active during this stage.
Interphase accounts for more than 95% of the cell cycle.
Events During Interphase
- Cell grows
- Organelles increase
- DNA replicates
Phases Of Interphase
Interphase is divided into:
- G₁ phase
- S phase
- G₂ phase
DNA replication occurs during the S phase.
M-Phase
M-phase is the division phase.
It accounts for less than 5% of the cell cycle.
M-phase includes:
- Karyokinesis – division of nucleus
- Cytokinesis – division of cytoplasm
Stages Of M-Phase Or Karyokinesis
Karyokinesis takes place in four main stages:
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
Prophase is the first and longest phase.
It lasts for about 50 minutes.
Events In Prophase
- Chromosomal material condenses
- Nucleolus disappears
- Nuclear membrane disappears
- Spindle fibres are formed
Spindle fibres are formed with the help of calmodulin, a calcium-binding protein.
Metaphase
Metaphase is a short phase.
It lasts for about 2 to 10 minutes.
Events In Metaphase
- Chromosomes align at the equatorial plane
- Metaphase plate is formed
Metaphase is the best stage to study chromosome morphology.
Anaphase
Anaphase lasts for about 2 to 3 minutes.
Events In Anaphase
- Sister chromatids separate
- Chromatids move towards opposite poles
Telophase
Telophase is the final stage of karyokinesis.
Events In Telophase
- Nuclear membrane reforms
- Spindle fibres disappear
- Organelles like Golgi bodies and ER reappear
Types Of Cell Division
There are two main types of cell division:
- Mitosis
- Meiosis
Mitosis Vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Occurs in | Somatic cells | Germ cells |
| Ploidy | Haploid or diploid cells | Diploid cells |
| Number of divisions | One | Two |
| Number of cells formed | Two | Four |
Mitosis
Mitosis occurs in somatic cells.
It produces two daughter cells.
The daughter cells are genetically similar to the parent cell.
Mitosis is important for:
- Growth
- Repair
- Replacement of damaged cells
Meiosis
Meiosis occurs in germ cells.
It involves two divisions and produces four cells.
Meiosis is important for the formation of gametes.
It helps maintain chromosome number across generations.
Key Takeaways
- The cell is the basic structural and functional unit of life.
- Cell theory explains that all living organisms are made up of cells and cells arise from pre-existing cells.
- Prokaryotic cells lack a true nucleus, while eukaryotic cells have a membrane-bound nucleus.
- Cell membrane is selectively permeable and controls the movement of substances.
- Mitochondria produce ATP and are called the powerhouse of the cell.
- Ribosomes help in protein synthesis.
- Nucleus controls all major activities of the cell.
- DNA and RNA are nucleic acids responsible for heredity and gene expression.
- Cell cycle has two main phases: interphase and M-phase.
- Mitosis produces two cells, while meiosis produces four cells.
FAQs On Cell And Cell Cycle
What is a cell?
A cell is the basic structural and functional unit of living organisms. All life processes occur inside cells.
Who discovered the cell?
Robert Hooke discovered the cell in 1665 while observing cork under a microscope.
Who discovered living cells?
Anton van Leeuwenhoek discovered living cells in pond water in 1674.
Who discovered the nucleus?
Robert Brown discovered the nucleus in 1831.
What is cell theory?
Cell theory states that all living organisms are made up of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
Why are viruses an exception to cell theory?
Viruses are an exception because they are acellular. They do not have a proper cellular structure.
What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells do not have a membrane-bound nucleus, while eukaryotic cells have a true membrane-bound nucleus.
What is the function of the cell membrane?
The cell membrane controls the movement of materials into and out of the cell.
What is osmosis?
Osmosis is the movement of water molecules through a semi-permeable membrane from higher solvent concentration to lower solvent concentration.
What is plasmolysis?
Plasmolysis is the shrinkage of cell contents away from the cell wall when a plant cell loses water through osmosis.
Why are mitochondria called the powerhouse of the cell?
Mitochondria produce ATP, which provides energy for cellular activities.
Why are lysosomes called suicidal bags?
Lysosomes contain digestive enzymes that can break down foreign particles, damaged organelles and sometimes the cell’s own components.
What is DNA fingerprinting?
DNA fingerprinting is the identification of an individual based on the unique sequence of bases in their DNA.
What happens during interphase?
During interphase, the cell grows, organelles increase and DNA replicates.
What is the difference between mitosis and meiosis?
Mitosis forms two daughter cells, while meiosis forms four daughter cells. Mitosis occurs in somatic cells, while meiosis occurs in germ cells.
Last Moment Exam Cheat Sheet – Cell & Cell Cycle
- Cell was first discovered by Robert Hooke.
- Living cells were first observed by Anton van Leeuwenhoek.
- Nucleus was discovered by Robert Brown.
- Cell theory was proposed by Schleiden and Schwann.
- “Omnis cellula e cellula” was stated by Virchow.
- Smallest living cell is Mycoplasma.
- Ribosomes are the sites of protein synthesis.
- Mitochondria are the powerhouse of the cell.
- Lysosomes are called suicidal bags.
- Smooth ER synthesizes lipids and steroid hormones.
- Rough ER helps in protein synthesis.
- DNA replication occurs in S phase of interphase.
- Metaphase is the best stage to study chromosome morphology.
- Genes are segments of DNA.
- Nucleotide = Sugar + Base + Phosphate.
- Nucleoside = Sugar + Base.