What is a Cell?
A cell is the smallest structural and functional unit and biological unit of the body.
It is also called the building block of life .
Cells are very small so cannot see in naked eye . microscope needed to see that cell .
All living things are made of cell
VARIOUS ORGANELLES IN CYTOPLASM OF CELL
1-Nucleus
2- Mitochondria
3- ribosomes
4- endoplasmic reticulum
5- Golgi apparatus
6- Lysosomes
7- Microfilaments and microtubules.
All these organelles perform specific function necessary for the cell survival.
Cell can be prokaryotic and eukaryotic
In prokaryote nucleus is absent
In prokaryotes, other membrane-bound organelles are missing” means: Prokaryotic cells do not have organelles surrounded by membranes.
They do not have:
Nucleus
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Lysosomes
They have only simple structures, such as:
Cell membrane
Cell wall
Cytoplasm
Ribosomes (not membrane-bound)
Example of prokaryotes:
- Bacteria
Eukaryotic cells have membrane-bound organelles.
They have:
Nucleus (surrounded by nuclear membrane)
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles (large in plant cells)
Chloroplasts (in plant cells)
Examples of eukaryotes:
Human cells
Plant cells
Animal cells
Fungi
Plasma Membrane (Cell Membrane) –
What is the plasma membrane?
The plasma membrane is a thin, flexible covering that surrounds the cell.
It separates the inside of the cell from the outside environment.
It is present in all cells (prokaryotic and eukaryotic).
Structure of plasma membrane (simple)
Very thin and delicate
Made of lipids (fats) and proteins
Described as fluid mosaic model
Fluid → membrane is flexible
Mosaic → proteins are scattered like tiles
Main components:
Phospholipid bilayer
Two layers of phospholipids
Head → likes water (hydrophilic)
Tail → hates water (hydrophobic)
Proteins
Embedded in the lipid layer
Help in transport and communication
Carbohydrates (small amount)
Present on the outer surface
Help in cell recognition
Functions of plasma membrane
Protection
Protects the cell contents
Selective permeability
Allows some substances to pass
Prevents harmful substances
Transport of substances
Diffusion
Osmosis
Facilitated diffusion
Active transport
Maintains shape of cell
Gives support and flexibility
Cell communication
Receptors on membrane receive signals
Cell recognition
Helps cells identify each other
NUCLEUS
The nucleus is the control center of the cell. It is usually the largest organelle in a eukaryotic cell.
1. Location and Structure
The nucleus is found inside the cell.
It is surrounded by a double membrane called the nuclear membrane (nuclear envelope).
The nuclear membrane has nuclear pores that allow materials to move in and out.
2. Parts of the Nucleus
a) Nuclear membrane
Double-layered covering.
Protects the nucleus.
Controls movement of substances.
b) Nucleoplasm
Jelly-like fluid inside the nucleus.
Contains chromosomes and nucleolus.
c) Chromatin
Thin, thread-like material.
Made of DNA and proteins.
Carries genes that control cell activities.
During cell division, chromatin becomes chromosomes.
d) Nucleolus
Dense, round body inside the nucleus.
Helps in making ribosomes.
3. Functions of the Nucleus
Controls all activities of the cell.
Stores genetic material (DNA).
Helps in cell growth and reproduction.
Plays an important role in cell division.
4. Cells Without Nucleus
Mature red blood cells (RBCs) do not have a nucleus.
Prokaryotic cells do not have a true nucleus.
Mitochondria –
Mitochondria are called the powerhouse of the cell because they produce energy needed for all cell activities.
1. What mitochondria do
They make energy (ATP) from food.
This energy is used for growth, movement, repair, and other life processes.
2. Structure of Mitochondria.
Double membrane organelle.
Outer membrane: smooth
Inner membrane: folded; folds are called cristae
The inner space is called the matrix.
The folds (cristae) increase surface area to produce more energy.
3. Functions of Mitochondria
Produce ATP (energy).
Help in cellular respiration.
Regulate cell death (apoptosis).
Important for metabolism.
4. Special Features
Mitochondria have their own DNA.
They can divide on their own.
More mitochondria are present in active cells (e.g., muscle cells).
5. Where mitochondria are absent
Red blood cells (RBCs) do not have mitochondria.
RIBOSOMES
Ribosomes are very tiny parts of the cell.
They are found floating in the cytoplasm or attached to the endoplasmic reticulum.
Ribosomes are made of RNA and protein.
Their main job is to make proteins.
They read the message carried by mRNA (messenger RNA).
This process of making protein is called translation.
Ribosomes are very small in size, but they are very important for the cell.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a network of membranes present inside the cell.
It helps in transporting materials within the cell and in making proteins and lipids.
Rough Endoplasmic Reticulum (RER)
Has ribosomes attached on its surface.
Looks rough under the microscope.
Helps in protein synthesis.
Proteins made here are sent to the Golgi apparatus.
Found in cells that make a lot of proteins.
Smooth Endoplasmic Reticulum (SER)
No ribosomes on its surface.
Looks smooth.
Helps in:
Making lipids and steroids
Detoxification of drugs and poisons
Calcium storage (important in muscle cells)
Golgi Apparatus
Looks like a stack of flattened sacs.
Modifies, packs, and sends proteins and lipids.
Works like a post office of the cell.
Lysosomes
Known as the suicide bags of the cell.
Contain digestive enzymes.
Break down:
Waste materials
Old organelles
Germs and bacteria
Microfilaments
Very thin protein fibers.
Help in:
Maintaining cell shape
Cell movement
9. Microfilaments and Microtubules
Microfilaments
Thin protein fibers
Functions:
Maintain cell shape
Help in cell movement
Microtubules
Thick tubular structures
Functions:
Form spindle fibers in cell division
Help in movement of cilia and flagella
Maintain cell structure
Cell Division
What is Cell Division?
Cell division is the process by which one cell divides to form new cells.
Why cell division is important
Growth of the body
Repair of damaged tissues
Replacement of old cells
Formation of reproductive cells
Types of Cell Division
There are two main types:
Mitosis
Meiosis
1. MITOSIS
Definition
Mitosis is the division of a body (somatic) cell into two identical daughter cells.
Where it occurs
Skin cells
Bone marrow
Intestinal lining
Importance of Mitosis
Growth of body
Healing of wounds
Replacement of dead cells
Maintains chromosome number
Stages of Mitosis
Before Mitosis – Interphase
(Not a true stage but very important)
Cell grows
DNA duplicates
Cell prepares for division
1. Prophase
Chromosomes become visible
Nuclear membrane disappears
Spindle fibers form
Preparation stage
2. Metaphase
Chromosomes line up at the center of the cell
Spindle fibers attach to chromosomes
Middle stage
3. Anaphase
Sister chromatids separate
Move to opposite sides of the cell
Away stage
4. Telophase
Nuclear membrane reforms
Chromosomes become thin again
Two nuclei formed
Two nuclei stage
Cytokinesis
Cytoplasm divides
Two identical daughter cells formed
Result of Mitosis
2 identical cells
Same chromosome number as parent cell
2. MEIOSIS
Definition
Meiosis is the division of reproductive cells to form gametes (sperm and ovum).
Where it occurs
Testes (male)
Ovaries (female)
Importance of Meiosis
Formation of sperm and ovum
Reduces chromosome number to half
Creates genetic variation
Stages of Meiosis
Meiosis occurs in two divisions:
Meiosis I
Meiosis II
MEIOSIS I (Reduction Division)
Prophase I
Homologous chromosomes pair
Crossing over occurs (exchange of genetic material)
Variation stage
Metaphase I
Paired chromosomes align at center
Anaphase I
Homologous chromosomes separate
Chromosome number reduced to half
Telophase I
Two daughter cells formed
Each has half the chromosomes
MEIOSIS II (Like Mitosis)
Prophase II
Spindle fibers form
Metaphase II
Chromosomes align at center
Anaphase II
Sister chromatids separate
Telophase II
Four daughter cells formed
Result of Meiosis
4 non-identical cells
Half the chromosome number
Difference Between Mitosis and Meiosis
Mitosis
Meiosis
Occurs in body cells
Occurs in reproductive cells
One division
Two divisions
Produces 2 cells
Produces 4 cells
Cells are identical
Cells are different
Chromosome number same
Chromosome number half
. Crossing Over (Cell Division)
What is Crossing Over?
Crossing over is the exchange of genetic material between homologous chromosomes during meiosis.
In simple words:
Crossing over means swapping of DNA parts between chromosomes.
When does Crossing Over occur?
During Prophase I of Meiosis
How does Crossing Over occur? (Simple Steps)
Homologous chromosomes come close together (pairing)
Chromatids overlap and form chiasmata
Exchange of gene segments takes place
Chromosomes separate with new genetic combination
Importance of Crossing Over
Produces genetic variation
Makes each individual genetically different
Important for evolution
Ensures proper separation of chromosomes
Example
Differences in height, eye color, skin color among siblings
2. Mutation
What is Mutation?
A mutation is a sudden and permanent change in DNA or gene structure.
In simple words:
Mutation means a change in genetic material.
Types of Mutation
1. Gene Mutation
Change in a single gene
Example: Sickle cell anemia
2. Chromosomal Mutation
Change in structure or number of chromosomes
Example: Down syndrome (extra chromosome 21)
Causes of Mutation
Radiation (X-rays, UV rays)
Chemicals
Viruses
Errors during DNA replication
Effects of Mutation
Harmful Effects
Genetic diseases
Cancer
Birth defects
Beneficial Effects
New traits
Evolution
Adaptation to environment
Difference Between Crossing Over and Mutation
Crossing Over
Mutation
Occurs in meiosis
Can occur anytime
Exchange of genes
Change in gene
Normal process
May be harmful
Causes variation
Causes variation or disease
Transport Across Cell Membrane
Cells need to take in useful substances and remove waste.
Movement of substances across the plasma membrane occurs by passive and active transport.
I. Passive Transport
Definition:
Passive transport is the movement of substances across the cell membrane without using energy (ATP).
Substances move from higher concentration to lower concentration.
Types of Passive Transport
1. Diffusion
Definition
Diffusion is the movement of molecules from an area of high concentration to low concentration until equilibrium is reached.
Characteristics
No energy required
Occurs directly through cell membrane
Depends on concentration gradient
Substances moved
Oxygen
Carbon dioxide
Fat-soluble substances
Example
Oxygen enters blood from lungs
Carbon dioxide leaves blood to lungs
2. Facilitated Diffusion
Definition
Facilitated diffusion is the movement of substances from high to low concentration with the help of carrier proteins or channels.
Characteristics
No energy required
Requires transport proteins
Faster than simple diffusion
Substances moved
Glucose
Amino acids
Ions (Na⁺, K⁺)
Example
Glucose entering cells with help of carrier protein
3. Osmosis
Definition
Osmosis is the movement of water molecules across a semi-permeable membrane from high water concentration to low water concentration.
Occurs through
Cell membrane
Types of Osmotic Solutions
a) Isotonic Solution
Same concentration inside and outside cell
Cell size remains normal
b) Hypotonic Solution
Outside solution has less solute
Water enters cell
Cell swells or bursts
c) Hypertonic Solution
Outside solution has more solute
Water leaves cell
Cell shrinks
Example
RBC swelling in distilled water
II. Active Transport
Definition
Active transport is the movement of substances across the membrane against the concentration gradient (from low to high concentration) using energy (ATP).
Characteristics
Requires energy
Uses carrier proteins
Highly selective
Maintains proper concentration of ions
Types of Active Transport
1. Primary Active Transport
Energy used directly from ATP
Example: Sodium-Potassium pump
2. Secondary Active Transport
Uses energy stored in ion gradient
Example: Glucose absorption in intestine
Examples of Active Transport
Sodium and potassium movement in nerve cells
Absorption of glucose in intestine
Reabsorption in kidney tubules
Difference Between Passive and Active Transport
Passive Transport
Active Transport
No energy needed
Energy required
High → Low
Low → High
Simple process
Complex process
Diffusion, osmosis
Ion pumps
Quick Revision Table
Type
Energy
Direction
Example
Diffusion
No
High → Low
O₂
Facilitated diffusion
No
High → Low
Glucose
Osmosis
No
Water movement
RBC
Active transport
Yes
Low → High