Prokaryotic vs Eukaryotic Cells

• Prokaryotes and eukaryotes are the two fundamental types of cells
• Prokaryotes include Bacteria and Archaea; eukaryotes comprise protists, fungi, plants, and animals
• The primary distinction lies in their cellular organization: eukaryotes have a nucleus and other membrane-bound organelles, while prokaryotes do not

Prokaryotic Cell Structure

• Prokaryotic cells are generally smaller (0.5-5 μm) than eukaryotic cells (10-100 μm)
• Prokaryotes lack a true nucleus; their DNA is concentrated in a nucleoid region without a membrane
• The prokaryotic cytoplasm contains ribosomes, which are responsible for protein synthesis
• The cell is enclosed by a plasma membrane, a phospholipid bilayer with embedded proteins
• Most prokaryotes have a cell wall outside the plasma membrane, providing shape and protection; bacterial cell walls contain peptidoglycan
• Some prokaryotes have a capsule, a sticky outer layer for adherence and protection
• Flagella are used by some prokaryotes for movement; they are structurally different from eukaryotic flagella
• Pili (fimbriae) are appendages that help prokaryotes attach to surfaces

Eukaryotic Cell Structure

• Eukaryotic cells are characterized by the presence of a nucleus, which houses the cell's DNA
• The nucleus is surrounded by a nuclear envelope, a double membrane with pores for transport
• The cytoplasm of eukaryotes contains a variety of membrane-bound organelles, each with specific functions
• The endomembrane system includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane
• The endoplasmic reticulum (ER) is involved in protein and lipid synthesis; rough ER has ribosomes, while smooth ER does not
• The Golgi apparatus modifies, sorts, and packages proteins and lipids
• Lysosomes contain enzymes for intracellular digestion
• Vacuoles have various functions, including storage, waste disposal, and maintaining cell turgor
• Mitochondria are responsible for cellular respiration, generating ATP
• Chloroplasts (in plant cells and algae) are the sites of photosynthesis
• The cytoskeleton provides structural support and facilitates cell movement; it includes microtubules, microfilaments, and intermediate filaments
• Eukaryotic cells may have flagella or cilia for movement; these are structurally more complex than prokaryotic flagella
• Plant cells have a cell wall made of cellulose, providing support and protection

Nucleus

• Eukaryotic cells have a defined nucleus which contains most of the cell’s genes
• The nuclear envelope is a double membrane enclosing the nucleus; it contains pores that regulate movement of molecules
• Chromosomes are structures carrying genetic information; each chromosome contains one long DNA molecule associated with proteins
• Chromatin is the complex of DNA and proteins making up chromosomes
• The nucleolus is a region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosomes are assembled

Ribosomes

• Ribosomes are responsible for protein synthesis
• Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes
• Ribosomes are found in two locations: free in the cytosol and bound to the endoplasmic reticulum or nuclear envelope

Endoplasmic Reticulum (ER)

• The ER is an extensive network of membranes
• Smooth ER lacks ribosomes; it synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons
• Rough ER has ribosomes on its surface; it produces proteins and membranes

Golgi Apparatus

• The Golgi apparatus modifies products of the ER, manufactures certain macromolecules, and sorts and packages materials into transport vesicles
• It consists of flattened membranous sacs called cisternae

Lysosomes

• Lysosomes are membranous sacs of hydrolytic enzymes that can digest macromolecules
• They break down cellular waste products and debris

Vacuoles

• Vacuoles are large vesicles derived from the ER and Golgi apparatus
• They perform a variety of functions, including storage, waste disposal, protection, and growth
• Plant cells often have a large central vacuole that stores water and helps maintain cell turgor

Mitochondria

• Mitochondria are the sites of cellular respiration
• They have a double membrane: an outer membrane and an inner membrane with folds called cristae
• The inner membrane creates two compartments: the intermembrane space and the mitochondrial matrix
• The matrix contains enzymes, DNA, and ribosomes

Chloroplasts

• Chloroplasts are found in plants and algae and are the sites of photosynthesis
• They contain chlorophyll, which captures light energy
• Chloroplasts have a double membrane and also contain thylakoids (membranous sacs) and stroma (internal fluid)

Cytoskeleton

• The cytoskeleton is a network of fibers extending throughout the cytoplasm
• It provides structural support and facilitates cell motility and regulation
• It is composed of microtubules, microfilaments, and intermediate filaments
• Microtubules are hollow rods that shape and support the cell and serve as tracks for motor proteins
• Microfilaments are thin, solid rods that function in muscle contraction, cell motility, and cell division
• Intermediate filaments are fibers that provide structural support and anchor organelles

Cell Wall

• Plant cells have a cell wall made of cellulose
• The cell wall provides support, protection, and shape to the cell
• Bacteria have cell walls containing peptidoglycan, while archaea have cell walls composed of other materials

Extracellular Matrix (ECM)

• Animal cells lack cell walls but are covered by an extracellular matrix (ECM)
• The ECM is made up of glycoproteins and other macromolecules
• It provides support, adhesion, movement, and regulation

Intercellular Junctions

• Neighboring cells in tissues often adhere, interact, and communicate through direct physical contact
• Examples include: Plasmodesmata (plants), tight junctions, desmosomes, and gap junctions (animals)