Cells are the smallest units of life, and they contain specialized structures known as organelles that perform unique and essential roles to keep the cell functioning efficiently and responding to its environment.
Eukaryotic and Prokaryotic Cells
All living organisms are made of cells, which fall into two main categories: eukaryotic and prokaryotic.
Eukaryotic Cells
Eukaryotic cells are characterized by having a true nucleus enclosed within a nuclear membrane and many membrane-bound organelles. These cells are typically found in animals, plants, fungi, and protists. The compartmentalization provided by internal membranes allows these cells to perform more complex functions. Each organelle serves a specific role, such as energy production, protein synthesis, and waste disposal.
Key characteristics include:
Nucleus containing DNA
Organelles like mitochondria, endoplasmic reticulum, Golgi apparatus
Larger cell size (10–100 micrometers)
Linear DNA organized into chromosomes
Prokaryotic Cells
Prokaryotic cells, found in bacteria and archaea, are more primitive and lack internal membrane-bound structures. They do not have a nucleus; instead, their genetic material resides in a region called the nucleoid, which is not enclosed by a membrane. These cells are generally much smaller (1–5 micrometers) and simpler in structure.
Key characteristics include:
No nucleus; DNA is circular and floats freely in the cytoplasm
No membrane-bound organelles
Ribosomes are present but smaller than in eukaryotes
Have a cell wall made of peptidoglycan (in bacteria)
May have structures like flagella for movement
Plasma Membrane
The plasma membrane, also known as the cell membrane, is the outermost boundary of animal cells and lies just beneath the cell wall in plant cells. It plays a key role in maintaining cellular homeostasis by regulating the movement of substances in and out of the cell.
Image Courtesy of Wikimedia Commons
Structure:
Composed of a phospholipid bilayer: Each phospholipid molecule has a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.
The hydrophilic heads face outward towards the water inside and outside the cell, while the hydrophobic tails face each other in the middle of the bilayer, forming a semi-permeable barrier.
Other components:
Proteins embedded within the membrane serve as transport channels, signal receptors, and structural anchors.
Cholesterol molecules (in animal cells) help stabilize membrane fluidity.
Carbohydrate chains are attached to proteins and lipids, forming glycoproteins and glycolipids, which are important for cell recognition.
The membrane is described by the fluid mosaic model, where lipids and proteins can move laterally, allowing flexibility and functionality.
Nucleus
The nucleus is the most prominent organelle in eukaryotic cells and is often centrally located.
Functions:
Stores the cell's genetic information in the form of DNA.
Controls cell activities by regulating gene expression and protein synthesis.
Plays a key role in cell division and reproduction.
Structure:
Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for the movement of materials like RNA and proteins.
Inside is the nucleoplasm, which contains chromatin (DNA and proteins).
The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and combined with proteins to form ribosomal subunits.
Ribosomes
Ribosomes are the molecular machines responsible for protein synthesis, also known as translation.
Structure:
Made up of two subunits composed of rRNA and proteins.
Do not have membranes and are found in both prokaryotic and eukaryotic cells.
Types:
Free ribosomes: Float freely in the cytoplasm and produce proteins that function within the cell itself.
Bound ribosomes: Attached to the rough endoplasmic reticulum and produce proteins destined for membranes, lysosomes, or export outside the cell.
Ribosomes read messenger RNA (mRNA) sequences and link together amino acids using peptide bonds, forming polypeptide chains that eventually fold into functional proteins.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is an extensive membrane system that plays a role in the synthesis of biomolecules and the transport of cellular products.
Rough Endoplasmic Reticulum (RER)
Covered in ribosomes, giving it a "rough" appearance.
Involved in the synthesis of secretory proteins, membrane proteins, and lysosomal enzymes.
As newly synthesized proteins enter the RER lumen, they undergo folding and post-translational modifications.
These proteins are packaged into transport vesicles that bud off and are sent to the Golgi apparatus.
Smooth Endoplasmic Reticulum (SER)
Lacks ribosomes and appears smooth.
Performs lipid synthesis (including phospholipids and steroids).
Participates in detoxification of drugs and poisons, especially in liver cells.
Regulates calcium ion storage, important in muscle cell function.
Helps with carbohydrate metabolism, such as the breakdown of glycogen.
Golgi Apparatus
The Golgi apparatus, also called the Golgi complex, is a series of flattened membrane sacs called cisternae.
Image courtesy of WikiMedia Commons-en.svg).
Functions:
Receives proteins and lipids from the ER through the cis face.
Modifies proteins by adding carbohydrate groups (forming glycoproteins) or lipid groups (forming lipoproteins).
Sorts and packages proteins and lipids into vesicles for delivery.
Sends modified molecules out through the trans face to their final destinations.
Plays a role in lysosome formation by producing vesicles containing digestive enzymes.
The Golgi apparatus is crucial for the post-translational processing and targeted delivery of cellular materials.
Mitochondria
Mitochondria are known as the powerhouses of the cell because they generate ATP, the main energy currency of the cell, through cellular respiration.
Structure:
Enclosed by a double membrane.
Outer membrane is smooth.
Inner membrane is highly folded into structures called cristae, which increase the surface area for reactions.
The inner space is filled with a gel-like matrix containing enzymes for the Krebs cycle.
Functions:
Perform aerobic respiration:
Glucose + Oxygen → Carbon dioxide + Water + ATPMost ATP is produced during the electron transport chain on the inner membrane.
Contain their own DNA and ribosomes, allowing them to replicate and synthesize some proteins independently.
Their origin is explained by the endosymbiotic theory, which proposes that mitochondria evolved from engulfed prokaryotic cells.
Lysosomes
Lysosomes are membrane-bound vesicles that contain powerful hydrolytic enzymes.
Functions:
Break down macromolecules, worn-out organelles, and ingested particles.
Participate in phagocytosis, where the cell engulfs food or pathogens.
Carry out autophagy, recycling the cell's own components.
Involved in apoptosis or programmed cell death, by releasing enzymes that break down the cell from within.
The inside of a lysosome is acidic, which helps activate the enzymes. They are especially abundant in immune cells like macrophages.
Vacuoles
Vacuoles are large, fluid-filled sacs that function in storage, support, and waste disposal.
In Plant Cells:
A single, large central vacuole holds water, ions, enzymes, and waste.
Maintains turgor pressure to support the plant structure.
Stores compounds like toxins or pigments.
In Animal Cells:
Smaller and more numerous.
Used for storing food, water, and waste.
In Protists:
Contractile vacuoles pump out excess water to prevent bursting in freshwater environments.
Chloroplasts
Chloroplasts are organelles found in plant cells and some protists that perform photosynthesis.
Structure:
Bounded by a double membrane.
Internal fluid called the stroma houses enzymes for the Calvin cycle.
Contain thylakoids, membrane-bound sacs stacked into grana.
Chlorophyll embedded in thylakoid membranes captures light energy.
Function:
Convert light energy, carbon dioxide, and water into glucose and oxygen.
Light + CO2 + H2O → C6H12O6 + O2Have their own DNA and ribosomes, supporting the endosymbiotic theory.
Centrioles
Centrioles are cylindrical structures composed of microtubules found in animal cells.
Functions:
Play a central role in organizing the mitotic spindle during cell division.
Help in the formation of cilia and flagella.
Located in the centrosome, which is the microtubule-organizing center of the cell.
Plant cells generally lack centrioles but still form spindle fibers using other structures.
Structural Differences: Plant vs. Animal Cells
Understanding the differences between plant and animal cells is critical for identifying cell types.
Unique to Plant Cells:
Cell wall made of cellulose for protection and support
Chloroplasts for photosynthesis
Large central vacuole for water and ion storage
Unique to Animal Cells:
Centrioles involved in cell division
Smaller, numerous vacuoles
More prominent lysosomes
Prokaryotic Cells: A Minimalist Structure
Prokaryotes do not have the complex internal organization of eukaryotes. They lack all membrane-bound organelles, including the nucleus, mitochondria, ER, and Golgi apparatus. The only subcellular components they possess are:
Plasma membrane
Cell wall (in most species)
Ribosomes
DNA in the nucleoid
Flagella for motility (in some)
Despite this simplicity, prokaryotes perform all essential life processes such as growth, reproduction, and metabolism.
FAQ
Compartmentalization allows eukaryotic cells to carry out complex and simultaneous biochemical processes by segregating incompatible reactions into distinct organelles. Each organelle provides a specialized environment tailored to its function, increasing efficiency and regulation. For example:
The nucleus protects DNA and regulates gene expression in a controlled space.
Lysosomes maintain an acidic pH optimal for digestion without damaging the rest of the cell.
The ER and Golgi coordinate protein modification and trafficking.
Mitochondria and chloroplasts generate energy in isolated environments.
This spatial separation enables higher-order functions, greater metabolic complexity, and precise internal control in eukaryotic cells.
Lysosomes rely on hydrolytic enzymes to break down cellular waste and macromolecules. Mutations in genes encoding these enzymes can impair their function, leading to lysosomal storage disorders where substrates accumulate inside cells. This buildup causes cellular dysfunction and tissue damage over time. Examples include:
Tay-Sachs disease: Deficiency in hexosaminidase A leads to GM2 ganglioside accumulation in neurons, causing neurological decline.
Gaucher disease: Lack of glucocerebrosidase results in lipid accumulation, affecting liver, spleen, and bone.
Pompe disease: Glycogen buildup in muscle cells leads to muscle weakness and cardiac issues.
Early diagnosis and enzyme replacement therapies are critical.
Mitochondria and chloroplasts contain their own circular DNA and prokaryote-like ribosomes because they are believed to have evolved from free-living prokaryotic organisms through endosymbiosis. This theory suggests:
An ancestral eukaryote engulfed aerobic bacteria (mitochondria) and photosynthetic bacteria (chloroplasts).
Instead of digesting them, a symbiotic relationship formed.
Over time, many genes transferred to the host nucleus, but some remained to allow organelle-specific protein synthesis.
Their independent DNA allows them to produce certain proteins and enzymes locally, reducing reliance on the nuclear genome and allowing rapid response to energy needs.
Proteins contain specific sequences called signal peptides or targeting sequences that act like cellular zip codes. These sequences guide proteins to their correct destination within or outside the cell. The process involves:
Cytosolic ribosomes synthesizing proteins without signal sequences for use in the cytoplasm.
Ribosomes on the rough ER producing proteins with ER signal sequences that direct them into the ER lumen.
From the ER, proteins are packaged into vesicles and sent to the Golgi apparatus.
The Golgi tags and sorts proteins using molecular markers for delivery to locations such as lysosomes, the plasma membrane, or secretion outside the cell.
Transport vesicles are small, membrane-bound sacs that shuttle molecules between organelles and to the cell membrane. They are essential for maintaining a connected and dynamic internal network. Functions include:
Carrying proteins and lipids from the ER to the Golgi for further processing.
Delivering modified products from the Golgi to their final destinations (e.g., lysosomes, membrane, extracellular space).
Maintaining membrane composition by recycling components between compartments.
Preventing cross-contamination of organelle contents by enclosing cargo in a separate lipid bilayer.
This vesicle trafficking ensures coordination between organelles, efficient distribution of cellular materials, and proper compartmental function.
Practice Questions
Describe how the structure of the mitochondrion is related to its function in eukaryotic cells.
The mitochondrion's structure is directly linked to its function of producing ATP through cellular respiration. It has a double membrane: the outer membrane encloses the organelle, while the inner membrane folds into cristae that increase surface area for the electron transport chain. These cristae maximize ATP production by providing more space for proteins involved in oxidative phosphorylation. Inside the inner membrane is the matrix, where the Krebs cycle occurs. The mitochondrion also contains its own DNA and ribosomes, enabling it to produce some of its own proteins necessary for energy conversion, supporting its semi-autonomous function.
Compare the roles of the rough endoplasmic reticulum and the Golgi apparatus in protein processing and transport.
The rough endoplasmic reticulum (RER) is covered with ribosomes and plays a key role in synthesizing proteins that are destined for export, membranes, or lysosomes. Once synthesized, these proteins enter the RER lumen, where they are folded and undergo modifications. They are then packaged into vesicles and sent to the Golgi apparatus. The Golgi further modifies these proteins, adding carbohydrate or lipid groups, and sorts them for their final destinations. The Golgi's cis face receives vesicles from the ER, and its trans face sends out the modified proteins in vesicles, ensuring accurate protein processing and cellular distribution.
