WHAT IS A CELL?
A cell is the basic structural and functional unit of life.
All living organisms are made up of one or more cells.
Cells perform all the essential life processes such as nutrition, respiration, excretion, growth and reproduction.
Cells are of different shapes and sizes according to their functions.
DISCOVERY OF CELL
Robert Hooke (1665) observed a thin slice of cork under a microscope and saw small box-like structures. He named them “cells”.
Anton van Leeuwenhoek observed living cells such as bacteria and protozoa using a microscope.
Schleiden and Schwann stated that all plants and animals are made up of cells.
Rudolf Virchow proposed that new cells arise from pre-existing cells.
CELL THEORY
The cell theory states that:
All living organisms are composed of one or more cells.
The cell is the basic unit of structure and function in living organisms.
New cells are formed from pre-existing cells.
TYPES OF CELLS
PROKARYOTIC CELLS
Prokaryotic cells are generally small and simple cells.
They do not have a well-defined nucleus.
Their genetic material is present in a region called the nucleoid.
Membrane-bound organelles are absent.
Example: Bacteria.
EUKARYOTIC CELLS
Eukaryotic cells are larger and more complex.
They have a well-defined nucleus surrounded by a nuclear membrane.
They contain several membrane-bound organelles.
Examples: Plant cells, animal cells, fungal cells and protist cells.
STRUCTURE OF A CELL
A cell contains several structures called cell organelles. Each organelle performs a specific function.
PLANT CELL
A typical plant cell contains:
Cell wall
Cell membrane
Cytoplasm
Nucleus
Nucleolus
Chloroplast
Large central vacuole
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Ribosomes
ANIMAL CELL
A typical animal cell contains:
Cell membrane
Cytoplasm
Nucleus
Nucleolus
Mitochondria
Endoplasmic reticulum
Ribosomes
Golgi apparatus
Lysosomes
MAJOR CELL ORGANELLES AND THEIR FUNCTIONS
PLASMA MEMBRANE
The plasma membrane controls the movement of substances into and out of the cell.
It is selectively permeable.
CELL WALL
The cell wall provides support, shape and protection to plant cells.
NUCLEUS
The nucleus contains genetic material and controls the activities of the cell.
NUCLEOLUS
The nucleolus helps in the formation of ribosomes.
RIBOSOMES
Ribosomes are the site of protein synthesis.
ENDOPLASMIC RETICULUM
The endoplasmic reticulum is a network of membrane-bound tubes and sacs present in the cytoplasm.
It helps in the synthesis and processing of proteins and lipids.
GOLGI APPARATUS
The Golgi apparatus modifies, sorts and packages proteins and lipids.
It helps in secretion and transport of materials.
It also forms vesicles for transport.
MITOCHONDRIA
Mitochondria are the sites of cellular respiration.
They release energy in the form of ATP.
They have their own DNA and ribosomes.
They are found in large numbers in actively working cells.
CHLOROPLAST
Chloroplasts are the sites of photosynthesis.
They contain chlorophyll, which helps plants absorb light energy.
LYSOSOMES
Lysosomes are membrane-bound vesicles containing digestive enzymes.
They help in breaking down unwanted materials, waste products and cellular components.
They are sometimes known as the “suicidal bags” of the cell.
VACUOLES
Vacuoles are membrane-bound sacs for storage.
They store water, food and other substances.
Plant cells generally contain a large central vacuole.
LEVELS OF ORGANISATION
Living organisms are organised into different levels:
Cell → Tissue → Organ → Organ System → Organism
CELL
A cell is the basic unit of life.
TISSUE
A tissue is a group of similar cells performing a common function.
ORGAN
An organ is made up of different tissues working together.
ORGAN SYSTEM
An organ system is a group of organs working together to perform specific functions.
ORGANISM
An organism is a complete living individual.
PLANT CELL AND ANIMAL CELL
CELL WALL
Plant cell: Present, made of cellulose.
Animal cell: Absent.
PLASMA MEMBRANE
Plant cell: Present.
Animal cell: Present.
CHLOROPLAST
Plant cell: Present.
Animal cell: Absent.
VACUOLE
Plant cell: Large central vacuole.
Animal cell: Small and temporary vacuoles.
SHAPE
Plant cell: Usually more regular or rigid.
Animal cell: Usually irregular or flexible.
PLASTIDS
Plant cell: Present, including chloroplast, chromoplast and leucoplast.
Animal cell: Absent.
CENTROSOME
Plant cell: Usually absent.
Animal cell: Present.
DIFFUSION
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.
It occurs until the particles become evenly distributed.
Example: The spreading of perfume or ink in water.
Diffusion does not require cellular energy.
OSMOSIS
Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
Osmosis is important for maintaining the water balance of cells.
OSMOSIS IN A PLANT CELL
When a plant cell is placed in a hypotonic solution, water enters the cell and the cell becomes turgid.
When a plant cell is placed in a hypertonic solution, water moves out of the cell and the cell becomes plasmolysed.
HYPOTONIC, ISOTONIC AND HYPERTONIC SOLUTIONS
HYPOTONIC SOLUTION
A hypotonic solution has a lower concentration of dissolved substances compared with the cell contents.
Water enters the cell.
Plant cells become turgid.
ISOTONIC SOLUTION
An isotonic solution has the same concentration of dissolved substances as the cell contents.
There is no overall movement of water.
There is no significant change in cell size.
HYPERTONIC SOLUTION
A hypertonic solution has a higher concentration of dissolved substances compared with the cell contents.
Water moves out of the cell.
Plant cells shrink.
PLASMOLYSIS
When a plant cell loses a large amount of water, the cell membrane and cytoplasm shrink away from the cell wall.
This process is called plasmolysis.
Plasmolysis occurs when a plant cell is placed in a concentrated solution.
CELL WALL
The cell wall is a rigid outer covering present outside the plasma membrane of plant cells.
It is mainly made of cellulose.
The cell wall:
Provides mechanical support and protection.
Helps maintain the shape of the cell.
Prevents excessive expansion of the cell when water enters.
Provides rigidity to plant cells.
CELL MEMBRANE OR PLASMA MEMBRANE
The cell membrane is a thin, flexible and selectively permeable boundary of the cell.
It separates the contents of the cell from the external environment.
It controls the movement of substances into and out of the cell.
The cell membrane helps maintain the internal environment of the cell.
NUCLEUS
The nucleus is a membrane-bound organelle present in eukaryotic cells.
It contains genetic material in the form of DNA.
The nucleus controls the activities of the cell.
It helps in cell division.
The nucleus contains a dense body called the nucleolus.
MAIN PARTS OF THE NUCLEUS
Nuclear membrane: Surrounds the nucleus.
Nucleoplasm: Fluid present inside the nucleus.
Nucleolus: Helps in the formation of ribosomes.
Chromatin: Contains the genetic material.
Nuclear pores: Allow transport of substances between the nucleus and cytoplasm.
ENDOPLASMIC RETICULUM
The endoplasmic reticulum (ER) is a network of membrane-bound tubes and sacs present in the cytoplasm.
There are two main types of ER.
ROUGH ENDOPLASMIC RETICULUM
Rough ER has ribosomes attached to its surface.
It is involved in protein synthesis and processing.
SMOOTH ENDOPLASMIC RETICULUM
Smooth ER does not have ribosomes.
It is involved in lipid synthesis and detoxification and performs other cellular functions.
GOLGI APPARATUS
The Golgi apparatus consists of stacks of flattened membrane-bound sacs.
Its functions include:
Modification of proteins and lipids.
Sorting and packaging of materials.
Formation of secretory vesicles.
Transport of materials.
Secretion of substances.
MITOCHONDRIA
Mitochondria are membrane-bound organelles known as the sites of cellular respiration.
They release energy in the form of ATP.
A mitochondrion has:
Outer membrane
Inner membrane
Cristae
Matrix
The inner membrane is folded to form cristae.
Mitochondria contain their own DNA and ribosomes.
They are present in both plant and animal cells.
RIBOSOMES
Ribosomes are small structures responsible for protein synthesis.
They are found in the cytoplasm and are also associated with the rough endoplasmic reticulum.
Ribosomes help in the formation of proteins.
LYSOSOMES
Lysosomes are membrane-bound vesicles containing digestive enzymes.
They help in:
Breaking down unwanted materials.
Digesting waste products.
Destroying damaged cellular components.
Recycling cellular materials.
They are commonly called the “suicidal bags” of the cell.
VACUOLES
Vacuoles are membrane-bound sacs used mainly for storage.
They can store:
Water
Food
Waste materials
Other substances
Plant cells usually contain a large central vacuole.
The large vacuole helps maintain the internal pressure and supports the shape of the plant cell.
PLASTIDS
Plastids are membrane-bound organelles found mainly in plant cells.
There are three major types of plastids:
CHLOROPLASTS
Chloroplasts contain chlorophyll.
They are the sites of photosynthesis.
They help plants capture light energy.
CHROMOPLASTS
Chromoplasts contain pigments other than chlorophyll.
They provide colour to flowers and fruits.
LEUCOPLASTS
Leucoplasts are colourless plastids.
They are involved mainly in the storage of food materials such as starch, oils and proteins.
CELL DIVISION
Cell division is the process by which a parent cell divides to form new daughter cells.
Cell division helps in:
Growth
Repair
Replacement of damaged cells
Reproduction
There are two major types of cell division:
Mitosis and Meiosis.
MITOSIS
Mitosis is an equational division that produces genetically identical daughter cells.
It occurs mainly in somatic cells, or body cells.
Mitosis is involved in growth, repair and replacement of cells.
The chromosome number remains the same.
A diploid cell with 2n chromosomes produces daughter cells with 2n chromosomes.
STAGES OF MITOSIS
INTERPHASE
The cell prepares for division.
Chromatin replicates and the cell prepares for division.
PROPHASE
Chromosomes become condensed and visible.
The nuclear membrane begins to disappear.
METAPHASE
Chromosomes line up at the equatorial plane of the cell.
ANAPHASE
Sister chromatids separate and move towards opposite poles.
TELOPHASE
Nuclear membranes form around the daughter chromosomes.
Two daughter nuclei are formed.
MEIOSIS
Meiosis is a reduction division that produces genetically different daughter cells.
It occurs in germ cells involved in reproduction.
Meiosis helps in the formation of gametes.
The chromosome number is reduced by half.
A diploid cell with 2n chromosomes produces cells with n chromosomes.
Meiosis consists of two successive divisions:
Meiosis I and Meiosis II.
MITOSIS AND MEIOSIS
NUMBER OF DIVISIONS
Mitosis: One division.
Meiosis: Two divisions.
NUMBER OF DAUGHTER CELLS
Mitosis: Produces two daughter cells.
Meiosis: Produces four daughter cells.
GENETIC MAKEUP
Mitosis: Daughter cells are genetically identical.
Meiosis: Daughter cells are genetically different.
CHROMOSOME NUMBER
Mitosis: Same as the parent cell, such as 2n → 2n.
Meiosis: Reduced by half, such as 2n → n.
FUNCTION
Mitosis: Growth, repair and replacement.
Meiosis: Formation of gametes.
GENETIC VARIATION
Mitosis: No major genetic variation is produced.
Meiosis: Produces genetic variation.
BINARY FISSION IN BACTERIA
Binary fission is a type of asexual reproduction in prokaryotic cells.
A single cell divides into two daughter cells.
The process involves:
Parent cell → DNA replication → Cell elongation → Division into two daughter cells
Binary fission is a simple and quick process.
Example: Bacteria.
SIGNIFICANCE OF CELL DIVISION
Cell division is important because it:
Helps in growth and development.
Repairs damaged or dead cells.
Replaces worn-out cells.
Produces gametes for reproduction.
Maintains genetic stability through mitosis.
Creates genetic variation through meiosis.
CELL CYCLE
The cell cycle is a series of events that a cell undergoes from one division to the next.
It consists of two main phases:
INTERPHASE
The cell grows and prepares for division.
Interphase includes:
G₁ phase: Cell growth.
S phase: DNA replication.
G₂ phase: Further growth and preparation for cell division.
M PHASE
The M phase includes:
Mitosis: Nuclear division.
Cytokinesis: Division of the cytoplasm.
CYTOKINESIS
Cytokinesis is the division of the cytoplasm after nuclear division.
It results in the formation of separate daughter cells.
CYTOKINESIS IN ANIMAL CELLS
A cleavage furrow forms and gradually divides the cell into two daughter cells.
CYTOKINESIS IN PLANT CELLS
A cell plate forms between the two daughter nuclei and develops into a new cell wall.
IMPORTANCE OF CELLS
Cells are the basic unit of life.
They perform essential life processes such as:
Nutrition
Respiration
Excretion
Growth
Reproduction
Cells allow growth and development of organisms.
They help in the repair and replacement of damaged cells.
Specialised cells perform specialised functions.
Different types of cells work together to form tissues and organs.
SPECIALISED CELLS
Different cells are specialised to perform specific functions.
MUSCLE CELL
Helps in contraction and movement.
NERVE CELL
Helps in the transmission of nerve impulses.
RED BLOOD CELL
Helps in the transport of oxygen.
ROOT HAIR CELL
Helps in the absorption of water and minerals from the soil.
KEY TERMS AND DEFINITIONS
- Cell: The basic structural and functional unit of life.
- Cell wall: A rigid outer covering present in plant cells.
- Plasma membrane: A selectively permeable membrane surrounding the cell.
- Cytoplasm: Jelly-like substance where organelles are present.
- Nucleus: Contains genetic material and controls cell activities.
- Organelle: A specialised structure that performs a specific function inside a cell.
- Mitochondria: Sites of cellular respiration and ATP production.
- Chloroplast: Site of photosynthesis in plant cells.
- Vacuole: Storage structure containing water, food, waste and other materials.
- Cell division: The process by which a parent cell produces daughter cells.
- Mitosis: Cell division producing genetically identical daughter cells.
- Meiosis: Reduction division producing genetically different cells.
- Diffusion: Movement of particles from a region of higher concentration to a region of lower concentration.
- Osmosis: Movement of water molecules through a selectively permeable membrane from higher water concentration to lower water concentration.
- Plasmolysis: Shrinkage of the cell contents away from the cell wall due to loss of water.
PROKARYOTIC AND EUKARYOTIC CELLS
SIZE
Prokaryotic cell: Usually small, approximately 1–10 μm.
Eukaryotic cell: Usually larger, approximately 10–100 μm.
NUCLEUS
Prokaryotic cell: No well-defined nucleus.
Eukaryotic cell: Well-defined nucleus surrounded by a nuclear membrane.
DNA
Prokaryotic cell: Genetic material is present in the nucleoid region.
Eukaryotic cell: DNA is present inside the nucleus.
MEMBRANE-BOUND ORGANELLES
Prokaryotic cell: Absent.
Eukaryotic cell: Present, such as mitochondria, ER and Golgi apparatus.
RIBOSOMES
Prokaryotic cell: 70S ribosomes.
Eukaryotic cell: 80S ribosomes.
EXAMPLES
Prokaryotic cells: Bacteria and archaea.
Eukaryotic cells: Plant cells, animal cells, fungal cells and protist cells.
DIFFUSION, OSMOSIS AND ACTIVE TRANSPORT
DIFFUSION
Movement of particles from a region of higher concentration to a region of lower concentration.
No energy is required.
OSMOSIS
Movement of water molecules through a selectively permeable membrane from higher water concentration to lower water concentration.
No cellular energy is required.
ACTIVE TRANSPORT
Movement of substances against their concentration gradient.
It requires energy in the form of ATP.
It occurs through specific transport proteins or carriers in the cell membrane.
CELL MEMBRANE TRANSPORT
SIMPLE DIFFUSION
Small molecules move directly through the cell membrane from higher concentration to lower concentration.
FACILITATED DIFFUSION
Substances move through specific membrane proteins from higher concentration to lower concentration.
ACTIVE TRANSPORT
Substances move against the concentration gradient.
Energy from ATP is required.
CELL AS THE BASIC UNIT OF LIFE
All living organisms are made up of one or more cells.
Cells perform all essential life processes.
New cells arise from pre-existing cells.
Cells contain specific structures that perform specialised functions.
Cells are the structural and functional units of life.
SPECIALISED CELLS AND THEIR FUNCTIONS
ROOT HAIR CELL
Root hair cells increase the surface area for absorption of water and minerals from the soil.
MUSCLE CELL
Muscle cells help in contraction and movement.
NERVE CELL
Nerve cells transmit nerve impulses.
RED BLOOD CELL
Red blood cells transport oxygen.
GUARD CELL
Guard cells control the opening and closing of stomata and help regulate gas exchange and water loss.
MICROSCOPIC DIAGRAMS
PLASMOLYSIS
In a normal plant cell, the cell membrane and cytoplasm are closely associated with the cell wall.
In a plasmolysed cell, water is lost and the cell contents shrink away from the cell wall.
DEPLASMOLYSIS
When water enters a plasmolysed plant cell, the cell contents expand and the cell returns towards its normal condition.
TURGID AND FLACCID CELLS
A turgid cell contains sufficient water and becomes firm.
A flaccid cell has less internal water pressure and becomes less firm.
CELL THEORY
The cell theory was developed through the work of scientists including Matthias Schleiden, Theodor Schwann and Rudolf Virchow.
Matthias Schleiden (1838): Stated that plants are made up of cells.
Theodor Schwann (1839): Stated that animals are made up of cells.
Rudolf Virchow (1855): Proposed that new cells arise from pre-existing cells.
MAIN POINTS OF CELL THEORY
All living organisms are made up of one or more cells.
The cell is the basic unit of structure and function of life.
New cells arise from pre-existing cells through cell division.
Cell theory explains the unity and diversity of life.
CELL ORGANELLES AND THEIR FUNCTIONS
NUCLEUS
Controls cell activities and contains genetic material.
NUCLEOLUS
Helps in ribosome formation.
ROUGH ER
Helps in protein synthesis and processing.
SMOOTH ER
Helps in lipid synthesis and detoxification.
GOLGI APPARATUS
Modifies, sorts and packages proteins and lipids.
MITOCHONDRIA
Responsible for cellular respiration and ATP production.
LYSOSOMES
Contain digestive enzymes and help break down unwanted materials.
RIBOSOMES
Responsible for protein synthesis.
CHLOROPLASTS
Responsible for photosynthesis.
VACUOLES
Store water, food and other materials.
CHLOROPLAST AND PHOTOSYNTHESIS
Chloroplasts are organelles found mainly in plant cells.
They contain chlorophyll, the green pigment that absorbs light energy.
Chloroplasts are the sites of photosynthesis.
The main parts of the chloroplast include:
Outer membrane
Inner membrane
Stroma
Granum
Thylakoid
Starch grain
FUNCTIONS OF CHLOROPLAST
Chloroplasts contain chlorophyll.
They are the sites of photosynthesis.
They help in the conversion of light energy into chemical energy.
MITOCHONDRIA AND CELLULAR RESPIRATION
Mitochondria are the sites of cellular respiration.
They release energy in the form of ATP.
They are known as the powerhouse of the cell.
A mitochondrion contains:
Outer membrane
Inner membrane
Cristae
Matrix
The inner membrane is folded into cristae, increasing the surface area for cellular respiration.
Mitochondria contain their own DNA and ribosomes.
ROUGH AND SMOOTH ENDOPLASMIC RETICULUM
ROUGH ER
Rough ER has ribosomes attached to its surface.
Functions:
Protein synthesis.
Protein processing.
Transport of proteins.
SMOOTH ER
Smooth ER does not have ribosomes.
Functions:
Lipid synthesis.
Detoxification.
Other cellular functions.
GOLGI APPARATUS
The Golgi apparatus consists of flattened membrane-bound sacs called cisternae.
Functions:
Modifies proteins and lipids.
Sorts and packages materials.
Forms secretory vesicles.
Helps transport substances.
Plays a role in secretion of materials.
PLANT CELL: STRUCTURE AND FUNCTION
A typical plant cell contains:
Cell wall: Provides protection and shape.
Cell membrane: Controls movement of substances.
Large central vacuole: Stores water, food, waste and other materials.
Chloroplast: Performs photosynthesis.
Mitochondria: Carry out cellular respiration.
Nucleus: Controls cell activities and contains genetic material.
Rough ER: Helps in protein synthesis.
Smooth ER: Helps in lipid synthesis.
Golgi apparatus: Modifies and packages materials.
Ribosomes: Carry out protein synthesis.
Cytoplasm: Contains cell organelles and provides the site for many cellular activities.
ANIMAL CELL: STRUCTURE AND FUNCTION
A typical animal cell contains:
Cell membrane: Controls movement of substances.
Cytoplasm: Jelly-like medium containing organelles.
Nucleus: Controls cell activities and contains genetic material.
Mitochondria: Responsible for cellular respiration.
Rough ER: Involved in protein synthesis.
Smooth ER: Involved in lipid synthesis.
Golgi apparatus: Modifies and packages materials.
Ribosomes: Responsible for protein synthesis.
Lysosomes: Contain digestive enzymes.
Centrosome: Involved in cell division.
PLANT CELL AND ANIMAL CELL: KEY DIFFERENCES
CELL WALL
Plant cell: Present.
Animal cell: Absent.
SHAPE
Plant cell: Usually fixed and regular.
Animal cell: Usually irregular and flexible.
VACUOLE
Plant cell: One large central vacuole.
Animal cell: Small and temporary vacuoles.
PLASTIDS
Plant cell: Present.
Animal cell: Absent.
CHLOROPLAST
Plant cell: Present in photosynthetic cells.
Animal cell: Absent.
CENTROSOME
Plant cell: Usually absent.
Animal cell: Present.
LYSOSOMES
Plant cell: Usually less prominent.
Animal cell: Present.
PLANT CELL WALL
The plant cell wall is made mainly of cellulose.
It consists of different layers including:
Middle lamella
Primary cell wall
Secondary cell wall
FUNCTIONS OF CELL WALL
Provides shape and mechanical strength.
Protects the cell.
Prevents excessive expansion of the cell.
Provides support.
Helps maintain the structure of plant cells.
TYPES OF PLASTIDS
CHLOROPLAST
Contains chlorophyll.
Responsible for photosynthesis.
CHROMOPLAST
Contains pigments other than chlorophyll.
Provides colour to flowers and fruits.
LEUCOPLAST
Colourless plastid.
Involved mainly in storage of food materials such as starch, oils and proteins.
VACUOLES
Vacuoles are storage structures surrounded by a membrane.
They store:
Water
Food
Waste products
Other substances
In plant cells, the large central vacuole helps maintain the internal pressure of the cell.
It also contributes to maintaining the shape of the plant cell.
IMPORTANT POINTS
- Plant cells have a cell wall, chloroplasts and a large central vacuole.
- Animal cells do not have a cell wall or chloroplasts.
- The nucleus controls cell activities and contains genetic material.
- Mitochondria are involved in cellular respiration and ATP production.
- Ribosomes are the sites of protein synthesis.
- Chloroplasts are the sites of photosynthesis.
- Golgi apparatus modifies, sorts and packages proteins and lipids.
- Endoplasmic reticulum helps in protein and lipid synthesis.
- Lysosomes contain digestive enzymes.
- Vacuoles help in storage.
- Diffusion involves movement from higher concentration to lower concentration.
- Osmosis involves movement of water through a selectively permeable membrane.
- Active transport requires energy and can move substances against their concentration gradient.
- Mitosis produces genetically identical daughter cells and is important for growth and repair.
- Meiosis produces genetically different cells and is important for the formation of gametes.
- Binary fission is a simple form of asexual reproduction in bacteria.
- The cell is the basic structural and functional unit of life.