CELL THE UNIT OF LIFE

● A cell is the fundamental structural and functional unit of all living

organisms, capable of independent existence and performing
essential life functions. Cells are classified into two types:
1. Unicellular Organisms: Composed of a single cell that can live
independently and perform all life functions.
2. Multicellular Organisms: Composed of multiple cells with specialized
roles.

Introduction to Cell Theory

● Cell Theory describes the fundamental properties of cells, formulated
by scientists Matthias Schleiden, Theodor Schwann, and later
expanded by Rudolf Virchow.
● The key principles of cell theory are:
● All living organisms are composed of cells and products of cells*–
This means that all plants and animals are built from cells, which act
as the basic structural units.
● All cells arise from pre-existing cells – First explained by Rudolf
Virchow in 1855, this principle asserts that new cells form only
through the division of existing cells, captured by the phrase "Omnis
cellula e cellula."

OVERVIEW OF CELL
● - Cells are the basic units of structure and function in all organisms.
● Plant and Animal Cells: Plant cells have a distinct cell wall
surrounding the cell membrane, while animal cells only have a cell
membrane.
 ● Nucleus: Both types of cells contain a nucleus, which houses genetic
material (DNA) within chromosomes.

Cell Types
● Eukaryotic Cells: These have a membrane-bound nucleus and other
organelles like mitochondria, endoplasmic reticulum, and Golgi
apparatus. Examples include plant, animal, and fungi cells.
● Prokaryotic Cells - These lack a membrane-bound nucleus and
organelles. Their genetic material is not enclosed within a nuclear
envelope. Examples include bacteria and archaea.
● The cytoplasm is a semi-fluid matrix where most cellular activities
occur.
● -Organelles, each with specific functions, are either membrane-bound
(e.g., mitochondria) or non-membrane-bound (e.g., ribosomes).

Prokaryotic Cells
● Prokaryotic Cells** are simple, single-celled organisms that lack a
membrane-bound nucleus and other membrane-bound organelles.
They include bacteria, blue-green algae, mycoplasma, and PPLO
(Pleuro Pneumonia-Like Organisms).

*Key Features of Prokaryotic Cells:

● Cell Wall:** Surrounds the plasma membrane and provides

structural support and shape. Most prokaryotes, except mycoplasma,
have a rigid cell wall.
● Genetic Material:** DNA is organized in a single, circular
chromosome that is not enclosed within a nuclear membrane.
 Additional small DNA molecules, called **plasmids**, may also be
present, often providing advantages like antibiotic resistance.
● Cytoplasm:** Contains ribosomes (70S type) for protein synthesis,
and in some cases, inclusions for storage.

Cell Envelope and Modifications:

● The **cell envelope** is a multi-layered structure, usually composed
of the glycocalyx **cell wall**, and **plasma membrane**.
● The glycocalyx may be a **slime layer** (loose) or a **capsule**
(thick and protective).

Special Structures:
● Mesosomes:** Infoldings of the plasma membrane that assist in
cellular functions such as DNA replication, cell wall formation, and
respiration.
● Flagella, Pili, and Fimbriae:** Prokaryotes may have structures like
flagella for movement, pili for gene transfer, and fimbriae for
attachment to surfaces.

*Types of Prokaryotic Cells:**

● Prokaryotic cells vary in shape, including **bacillus** (rod-shaped),
**coccus** (spherical), **vibrio** (comma-shaped), and **spirillum**
(spiral-shaped).

Eukaryotic Cells
● Eukaryotic Cells are complex, membrane-bound cells that make up
plants, animals, fungi, and protists. They have a true nucleus,
 membrane-bound organelles, and a cytoskeleton, which allows for
compartmentalized cellular functions and efficient organization.

Cell Membrane
● Structure:** Composed mainly of a **phospholipid bilayer** with
embedded proteins, carbohydrates, and cholesterol, forming a
selectively permeable barrier.
● Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972),
describes the membrane as a flexible layer where proteins and lipids
can move laterally.
● Function:** Regulates the transport of molecules, maintains cellular
integrity, and allows for communication with the environment.

Cell Wall (in Plants, Fungi, and Some Protists)

● Composition:** Made of cellulose in plants, chitin in fungi, and various
polysaccharides in algae.
● Function:** Provides rigidity, protection, shape, and prevents
excessive water uptake.
● Middle Lamella:** A layer mainly of calcium pectate that glues
neighboring plant cells together.

Endomembrane System
● Endomembrane System** includes a series of connected organelles
that function together to synthesize, modify, package, and transport
proteins and lipids.

Endoplasmic Reticulum (ER):

● Rough ER (RER):** Studded with ribosomes, it synthesizes proteins
for secretion and membrane insertion.
● Smooth ER (SER):** Lacks ribosomes; involved in lipid synthesis,
detoxification, and calcium storage.

Golgi Apparatus:
 ● Structure:** Flattened, membrane-bound sacs called cisternae.
● Function:** Modifies proteins and lipids received from the ER,
packages them into vesicles, and directs them to various
destinations.

Lysosomes:**
● Structure:** Membrane-bound vesicles filled with hydrolytic
enzymes.
● Function:** Digest macromolecules, damaged organelles, and
foreign particles; involved in autophagy and cellular defense.

Vacuoles:**
● Plant Cells:** Large central vacuole maintains cell turgidity and
stores nutrients and waste.
● Animal Cells:** Smaller, more numerous vacuoles; in some protists,
contractile vacuoles assist in osmoregulation.

Mitochondria
● Structure:** Double-membrane organelle; the inner membrane has
folds called cristae to increase surface area for ATP production.
● Function:** Site of aerobic respiration; produces ATP, known as the
cell’s energy currency.
● Special Features:** Contains its own DNA and ribosomes, enabling it
to produce some of its proteins independently.
 Plastids (in Plant Cells and Algae)
● Plastids are specialized organelles in plants and some protists for
storage and photosynthesis.
● Types of Plastids:**
● Chloroplasts:** Contain chlorophyll; involved in photosynthesis.
● Chromoplasts:** Store pigments, giving color to fruits and flowers.
● Leucoplasts:** Store starch, oils, and proteins; include amyloplasts
(starch storage) and elaioplasts (lipid storage).
● Chloroplast Structure:** Double membrane with inner membrane
structures called thylakoids arranged in stacks (grana) where
photosynthesis occurs.

Ribosomes
● Structure:** Composed of RNA and proteins, not membrane-bound.
● Type:** 80S ribosomes in eukaryotes, with two subunits (60S and
40S).
● Function:** Site of protein synthesis; found free in the cytoplasm or
attached to the ER.

Cytoskeleton
● Components:**
● Microtubules:** Thick, hollow tubes that provide structural support
and form tracks for organelle movement.
● Microfilaments:** Thin strands of actin involved in cell movement
and shape maintenance.
● Intermediate Filaments:** Provide mechanical support and help
maintain cell integrity.
 ● Function:** Provides structural support, maintains cell shape, aids in
movement, and assists in cell division.

Cilia and Flagella

● Structure:** Hair-like structures with a 9+2 arrangement of
microtubules.
● Function:**
● Cilia:** Short, numerous, move in a coordinated fashion to propel
fluids or move cells.
● Flagella:** Longer, typically one or two per cell, used for cell
movement.
● Basal Bodies:** Cilia and flagella originate from basal bodies, similar
to centrioles.

Centrosome and Centrioles (in Animal Cells)

● Structure:** Centrosome contains a pair of centrioles arranged
perpendicularly.
● Function:** Organizes microtubules; forms the spindle apparatus
during cell division.

Nucleus
● Structure:** Enclosed by a nuclear envelope with pores that allow
material exchange.
● Nucleolus:** Dense region within the nucleus responsible for
ribosomal RNA (rRNA) synthesis.
● Chromatin:** DNA and proteins that condense into chromosomes
during cell division.
 ● Function:** Controls cellular activities, stores genetic information, and
directs protein synthesis.

Microbodies
● Structure:** Small, membrane-bound organelles containing enzymes.
● Types:**
● Peroxisomes:** Contain enzymes for fatty acid metabolism and
detoxification.
● Glyoxysomes:** Specialized peroxisomes found in plant seeds,
converting fats into sugars during germination.

Summary of Key Differences Between Plant and

Animal Cells
● Plant Cells:** Have a cell wall, chloroplasts, large central vacuole;
lack centrioles.
● Animal Cells:** Lack a cell wall and chloroplasts, have centrioles and
small vacuoles.