CELLS
What is a Cell?
A cell is the basic structural and functional unit of all known living organisms.
It is the smallest unit of life that is responsible for all life processes (movement, respiration, sensitivity,
growth, reproduction, excretion, nutrition).
It has a membrane-bound nucleus and organelles that regulate cellular functions.
Most cells contain a protective outer cell membrane, a jelly-like cytoplasm, and genetic material (DNA).
Main types of cells
Prokaryotes: simple single-celled organisms that do not have a membrane- bound nucleus (such as
bacteria and archaea).
Eukaryotes: more complex cells that contain a true nucleus and specialized internal structures called
organelles (such as plant, animal, and fungal cells).
Eukaryotic Organelles (Structure, Function, and Location)
Eukaryotic cells contain various organelles that work together to maintain homeostasis, synthesize proteins, generate
energy, and manage waste.
1. Nucleus
Structure: The largest organelle, surrounded by a double membrane called the nuclear envelope
(perforated with nuclear pores). It contains chromatin (DNA wrapped around histone proteins) and a dense,
spherical region called the nucleolus.
Function: Stores the cell's genetic material (DNA), controls cellular activities by regulating transcription
(gene expression), and the nucleolus is responsible for producing ribosomal RNA (rRNA) and assembling
ribosomes.
Location: Typically found centrally within both plant and animal cells.
2. Ribosomes
Structure: Small, non-membrane-bound organelles composed of two subunits (large and small) made of
rRNA and protein. Eukaryotic ribosomes are the 80S type (larger than prokaryotic 70S ribosomes).
Function: The site of protein synthesis (translation), translating messenger RNA (mRNA) into
polypeptide chains.
, Location: Found either free-floating in the cytoplasm or bound to the exterior of the Rough Endoplasmic
Reticulum.
3. Rough Endoplasmic Reticulum (RER)
Structure: A series of flattened membrane-bound sacs called cisternae, continuous with the outer nuclear
membrane, studded with 80S ribosomes on its outer surface.
Function: Folds and processes proteins that have been made at the ribosomes; transports these proteins in
vesicles to the Golgi apparatus.
Location: Extends throughout the cytoplasm, closely associated with the nucleus.
4. Smooth Endoplasmic Reticulum (SER)
Structure: A network of tubular membrane-bound cisternae, lacking ribosomes on its surface.
Function: Synthesizes, stores, and processes lipids, carbohydrates, and steroid hormones.
Location: Distributed throughout the cytoplasm, continuous with the RER.
5. Golgi Apparatus
Structure: A stack of curved, membrane-bound, flattened cisternae (similar to the RER, but not continuous
with it), accompanied by small vesicles budding off the edges.
Function: Modifies proteins received from the RER (e.g., adding carbohydrate chains to form
glycoproteins), packages them into secretory vesicles for transport to the cell membrane for exocytosis,
and produces lysosomes.
Location: Typically situated in the cytoplasm near the endoplasmic reticulum and close to the cell
membrane.
6. Mitochondria
Structure: Oval-shaped organelles bound by a double membrane. The inner membrane is folded into
projections called cristae, enclosing a fluid-filled matrix containing mitochondrial DNA, 70S ribosomes,
and enzymes.
Function: The site of aerobic respiration, where ATP (adenosine triphosphate) is synthesized to provide
energy for cellular processes.
Location: Dispersed throughout the cytoplasm, particularly concentrated in regions with high metabolic
activity (e.g., muscle cells, sperm tails).
7. Chloroplasts (Plant Cells Only)
Structure: Biconvex, double-membrane-bound organelles containing a fluid-filled stroma. Inside are
stacked, membrane-bound discs called thylakoids (stacks of thylakoids are called grana), linked by
lamellae. They contain chlorophyll pigments.
A thylakoid is a membrane-bound, coin-shaped sac or compartment inside a plant or algae chloroplast
where the light-dependent reactions of photosynthesis take place.
Key Structure of the thylakoid:
Thylakoid Membrane: The outer layer that holds chlorophyll (the green pigment that absorbs sunlight)
and protein complexes like Photosystem I and Photosystem II.
Thylakoid Lumen: The hollow, fluid-filled space inside the thylakoid sac where protons accumulate to
drive ATP (energy) synthesis.
Granum (plural: Grana): A neat stack of individual thylakoid discs that resembles a stack of coins.
Stroma Lamellae: Thin, tube-like membrane extensions that connect separate grana stacks
across the chloroplast.
What is a Cell?
A cell is the basic structural and functional unit of all known living organisms.
It is the smallest unit of life that is responsible for all life processes (movement, respiration, sensitivity,
growth, reproduction, excretion, nutrition).
It has a membrane-bound nucleus and organelles that regulate cellular functions.
Most cells contain a protective outer cell membrane, a jelly-like cytoplasm, and genetic material (DNA).
Main types of cells
Prokaryotes: simple single-celled organisms that do not have a membrane- bound nucleus (such as
bacteria and archaea).
Eukaryotes: more complex cells that contain a true nucleus and specialized internal structures called
organelles (such as plant, animal, and fungal cells).
Eukaryotic Organelles (Structure, Function, and Location)
Eukaryotic cells contain various organelles that work together to maintain homeostasis, synthesize proteins, generate
energy, and manage waste.
1. Nucleus
Structure: The largest organelle, surrounded by a double membrane called the nuclear envelope
(perforated with nuclear pores). It contains chromatin (DNA wrapped around histone proteins) and a dense,
spherical region called the nucleolus.
Function: Stores the cell's genetic material (DNA), controls cellular activities by regulating transcription
(gene expression), and the nucleolus is responsible for producing ribosomal RNA (rRNA) and assembling
ribosomes.
Location: Typically found centrally within both plant and animal cells.
2. Ribosomes
Structure: Small, non-membrane-bound organelles composed of two subunits (large and small) made of
rRNA and protein. Eukaryotic ribosomes are the 80S type (larger than prokaryotic 70S ribosomes).
Function: The site of protein synthesis (translation), translating messenger RNA (mRNA) into
polypeptide chains.
, Location: Found either free-floating in the cytoplasm or bound to the exterior of the Rough Endoplasmic
Reticulum.
3. Rough Endoplasmic Reticulum (RER)
Structure: A series of flattened membrane-bound sacs called cisternae, continuous with the outer nuclear
membrane, studded with 80S ribosomes on its outer surface.
Function: Folds and processes proteins that have been made at the ribosomes; transports these proteins in
vesicles to the Golgi apparatus.
Location: Extends throughout the cytoplasm, closely associated with the nucleus.
4. Smooth Endoplasmic Reticulum (SER)
Structure: A network of tubular membrane-bound cisternae, lacking ribosomes on its surface.
Function: Synthesizes, stores, and processes lipids, carbohydrates, and steroid hormones.
Location: Distributed throughout the cytoplasm, continuous with the RER.
5. Golgi Apparatus
Structure: A stack of curved, membrane-bound, flattened cisternae (similar to the RER, but not continuous
with it), accompanied by small vesicles budding off the edges.
Function: Modifies proteins received from the RER (e.g., adding carbohydrate chains to form
glycoproteins), packages them into secretory vesicles for transport to the cell membrane for exocytosis,
and produces lysosomes.
Location: Typically situated in the cytoplasm near the endoplasmic reticulum and close to the cell
membrane.
6. Mitochondria
Structure: Oval-shaped organelles bound by a double membrane. The inner membrane is folded into
projections called cristae, enclosing a fluid-filled matrix containing mitochondrial DNA, 70S ribosomes,
and enzymes.
Function: The site of aerobic respiration, where ATP (adenosine triphosphate) is synthesized to provide
energy for cellular processes.
Location: Dispersed throughout the cytoplasm, particularly concentrated in regions with high metabolic
activity (e.g., muscle cells, sperm tails).
7. Chloroplasts (Plant Cells Only)
Structure: Biconvex, double-membrane-bound organelles containing a fluid-filled stroma. Inside are
stacked, membrane-bound discs called thylakoids (stacks of thylakoids are called grana), linked by
lamellae. They contain chlorophyll pigments.
A thylakoid is a membrane-bound, coin-shaped sac or compartment inside a plant or algae chloroplast
where the light-dependent reactions of photosynthesis take place.
Key Structure of the thylakoid:
Thylakoid Membrane: The outer layer that holds chlorophyll (the green pigment that absorbs sunlight)
and protein complexes like Photosystem I and Photosystem II.
Thylakoid Lumen: The hollow, fluid-filled space inside the thylakoid sac where protons accumulate to
drive ATP (energy) synthesis.
Granum (plural: Grana): A neat stack of individual thylakoid discs that resembles a stack of coins.
Stroma Lamellae: Thin, tube-like membrane extensions that connect separate grana stacks
across the chloroplast.
