3. 1. Structure of Prokaryotic Cells:
Cell Envelope:
Prokaryotes have a cell envelope composed of
three layers:
Cell Wall: Provides shape, protection, and
prevents the cell from bursting due to
osmotic pressure
Cell Membrane: A lipid bilayer that
controls the movement of substances in
and out of the cell
Capsule (optional): A slimy outer layer
that provides protection and assists in
attaching to surfaces
4. Cytoplasm: The cytoplasm contains a gel-like substance called the cytosol, where
various metabolic reactions occur
Ribosomes: Small structures responsible for protein synthesis. Prokaryotes have
smaller ribosomes compared to eukaryotes
Nucleoid Region: This is the region where the prokaryotic DNA is located. It is not
enclosed within a nuclear membrane
Plasmids (optional): Small circular DNA molecules that exist separately from the main
chromosome
Pili/Fimbriae: Thin, hair-like appendages that help bacteria adhere to surfaces,
facilitate conjugation (transfer of genetic material), or aid in motility
Contd…
5. Flagella: Long, whip-like appendages that allow bacteria to move in liquid
environments
Endospores: Tough, dormant structures formed by some bacteria to withstand
harsh conditions and facilitate survival
Inclusions: Storage structures within the cytoplasm that accumulate nutrients or
reserve materials, such as glycogen granules or lipid droplets
Gas Vesicles: Gas-filled structures found in certain aquatic bacteria, enabling
them to float or regulate buoyancy
Contd…
6. 2. Structure of Eukaryotic Cells:
• Eukaryotic cells are more complex and have a true
nucleus and membrane-bound organelles that
perform specific functions
• Here are the key features of eukaryotic cell
ultrastructure:
Nucleus:
• The nucleus contains the genetic material (DNA)
organized into chromosomes
• It is surrounded by a double membrane called the
nuclear envelope, which has nuclear pores for
communication with the cytoplasm
7. Cell Membrane: Like prokaryotes, eukaryotic cells have a cell membrane that
separates the cell from its environment and regulates the movement of substances
Cytoplasm: The cytoplasm is a gel-like substance where various cellular activities
occur
Endoplasmic Reticulum (ER): A network of interconnected membranous tubules
involved in protein synthesis and lipid metabolism. There are two types: rough ER
(with ribosomes attached) and smooth ER (without ribosomes)
Golgi Apparatus: Stacked membranous sacs responsible for processing, modifying,
and sorting proteins and lipids synthesized in the ER
Mitochondria: Known as the powerhouse of the cell, mitochondria produce energy
through cellular respiration and have their own DNA
Contd…
8. Lysosomes: Vesicles containing enzymes that break down waste materials, cellular
debris, and foreign substances
Vacuoles: Larger in plant cells, vacuoles store water, nutrients, and waste products.
Animal cells have smaller and fewer vacuoles
Chloroplasts (in plant cells): These organelles contain chlorophyll and perform
photosynthesis, converting sunlight into chemical energy
Cytoskeleton: A network of protein filaments (microtubules, microfilaments, and
intermediate filaments) that provides structural support, helps in cell movement, and
facilitates the transport of organelles
Centrioles: Paired cylindrical structures involved in cell division and organization of the
cytoskeleton. Found in animal cells and some protists
Contd…
9. Cilia: Short, hair-like structures projecting from the cell surface involved in movement or
the movement of surrounding fluids. Found in various eukaryotic cells
Cell Junctions: Specialized structures that connect cells together, such as tight junctions,
gap junctions, and desmosomes, facilitating cell adhesion and communication
Microvilli: Finger-like projections on the surface of some cells, particularly in the epithelial
lining of organs, increasing surface area for absorption
Nucleolus: A distinct region within the nucleus where ribosomal RNA (rRNA) synthesis
and ribosome assembly take place
Peroxisomes: Membrane-bound organelles involved in various metabolic processes,
including the breakdown of fatty acids and detoxification of harmful substances
Contd…
10. Glyoxysomes: Specialized peroxisomes found in plant cells involved in the
conversion of stored lipids into carbohydrates during germination
Contractile Vacuole: A specialized vacuole in certain protists responsible for
regulating water balance and expelling excess water from the cell
Nematocysts: Stinging organelles found in cnidarians (such as jellyfish and corals)
used for defense and capturing prey
Secondary Lysosomes: Formed by the fusion of lysosomes with other vesicles or
endocytic compartments for further breakdown and recycling of cellular
components
Contd…
