Cell Components

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, and they are classified into two main types: prokaryotic and eukaryotic. Understanding their differences is essential for studying cell biology.

• Prokaryotic Cells: Smaller, simpler, lack membrane-bound organelles, and do not have a nucleus. Their DNA is located in a region called the nucleoid.

• Eukaryotic Cells: Larger, more complex, contain membrane-bound organelles, and have a true nucleus that houses their DNA.

Prokaryotic Cell Structure

Prokaryotic cells, such as bacteria, have a simple structure but are highly efficient. Their main features include:

• Nucleoid: Region where the cell's DNA is located (not enclosed by a membrane).

• Ribosomes: Sites of protein synthesis.

• Plasma Membrane: Selectively permeable barrier that encloses the cytoplasm.

• Cell Wall: Rigid structure providing support and protection.

• Capsule: Outer jelly-like coating for protection and adhesion.

• Fimbriae: Hair-like structures for attachment to surfaces.

• Flagella: Long, whip-like appendages for movement.

Eukaryotic Cell Structure

Eukaryotic cells are more complex and compartmentalized, allowing for specialized functions. They include animal, plant, fungal, and protist cells.

• Nucleus: Contains the cell's DNA and is surrounded by a double membrane (nuclear envelope).

• Nucleolus: Dense region within the nucleus; site of rRNA synthesis and ribosome assembly.

• Chromatin: DNA and associated proteins, organized for efficient packaging within the nucleus.

• Ribosomes: Free in cytosol or bound to the endoplasmic reticulum (ER); synthesize proteins.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down macromolecules and damaged organelles.

• Vacuoles: Large vesicles for storage and transport; central vacuole in plants maintains turgor pressure.

• Mitochondria: Sites of cellular respiration; generate ATP from nutrients.

• Chloroplasts (plants and algae): Sites of photosynthesis; convert solar energy to chemical energy.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for structural support and transport.

• Plasma Membrane: Controls movement of substances in and out of the cell.

• Cell Wall (plants, fungi, some protists): Provides structural support and protection.

• Plasmodesmata (plants): Channels between adjacent plant cells for communication.

The Nucleus and Ribosomes

Nucleus

The nucleus is the genetic control center of eukaryotic cells.

• Nuclear Envelope: Double membrane with pores for molecular exchange.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Chromatin: DNA-protein complex for efficient DNA packaging.

Ribosomes

Ribosomes are molecular machines that synthesize proteins using instructions from DNA (via mRNA).

• Free Ribosomes: Float in the cytosol; make proteins for use within the cell.

• Bound Ribosomes: Attached to the rough ER; make proteins for secretion or for use in membranes.

The Endomembrane System

Components and Functions

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Nuclear Envelope

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis)

• Golgi Apparatus: Modifies and sorts proteins and lipids

• Lysosomes: Digestion of macromolecules

• Vesicles: Transport materials between organelles

• Plasma Membrane: Exocytosis and endocytosis

Endoplasmic Reticulum (ER)

The ER is an extensive network of membranes, continuous with the nuclear envelope.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs.

Protein Production and Transport

Proteins synthesized in the rough ER are packaged into vesicles and sent to the Golgi apparatus for further modification and sorting.

Golgi Apparatus

The Golgi apparatus consists of flattened membranous sacs (cisternae) and functions as the cell's shipping and receiving center.

• Receives vesicles from the ER

• Modifies proteins and lipids

• Sorts and packages materials for secretion or delivery to other organelles

Lysosomes

Lysosomes are membrane-bound sacs containing hydrolytic enzymes for intracellular digestion.

• Break down macromolecules, old organelles, and foreign substances

• Play a key role in autophagy and phagocytosis

Vacuoles

Vacuoles are large vesicles with diverse functions, especially prominent in plant cells.

• Central Vacuole: Stores water, ions, and nutrients; maintains turgor pressure in plant cells.

• Food Vacuoles: Formed by phagocytosis in some protists and animal cells.

Mitochondria and Chloroplasts

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration.

• Double-membrane structure: smooth outer membrane, highly folded inner membrane (cristae)

• Matrix: contains enzymes, mitochondrial DNA, and ribosomes

Chloroplasts

Chloroplasts are found in plants and algae and are the sites of photosynthesis.

• Double-membrane structure

• Thylakoids: flattened sacs where light reactions occur

• Granum: stack of thylakoids

• Stroma: fluid surrounding thylakoids, contains DNA and enzymes

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Both organelles have their own DNA and ribosomes

• They replicate independently of the cell

• Similar in size and structure to certain prokaryotes

Cytoskeleton and Cell Movement

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, maintains cell shape, and facilitates movement.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

• Intermediate Filaments: Rope-like fibers for mechanical strength.

• Microfilaments: Thin filaments of actin; involved in cell movement and muscle contraction.

Cilia and Flagella

Cilia and flagella are motile appendages made of microtubules, enabling movement of cells or movement of substances across cell surfaces.

• Cilia: Short, numerous, move substances past the cell

• Flagella: Longer, usually one or a few per cell, propel the cell

Extracellular Structures and Cell Junctions

Extracellular Matrix (ECM)

The ECM is a network of glycoproteins, polysaccharides, and proteoglycans outside animal cells, providing structural support and mediating cell signaling.

• Collagen: Main structural protein in connective tissues

• Integrins: Transmembrane proteins connecting ECM to cytoskeleton

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells and facilitate communication.

Plant Cell Walls

Plant cell walls are rigid structures outside the plasma membrane, composed mainly of cellulose, providing support and protection.

• Plasmodesmata: Channels that connect the cytoplasm of adjacent plant cells, allowing for transport and communication.

Summary Table: Prokaryotic vs. Eukaryotic Cells