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 true nucleus. Their DNA is located in a region called the nucleoid.

• Eukaryotic Cells: Larger, more complex, contain membrane-bound organelles, and have a nucleus that houses their DNA. Found in animals, plants, fungi, and protists.

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: Provides structural support and protection; composed of peptidoglycan in bacteria.

• Capsule: Outer jelly-like layer for protection and adherence (in some bacteria).

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

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

Eukaryotic Cell Structure

Eukaryotic cells are compartmentalized into organelles, each with specialized functions. They are found in animals, plants, fungi, and protists.

• 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 wrapped around proteins (histones), organizing genetic material.

• Ribosomes: Protein synthesis; can be free in cytosol or bound to the endoplasmic reticulum (ER).

• 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: Site of cellular respiration; produces ATP.

• Chloroplasts (plants and algae): Site of photosynthesis.

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

• Plasma Membrane: Controls entry and exit of substances.

• Cell Wall (plants, fungi, some protists): Provides structural support and protection; made of cellulose in plants.

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

The Nucleus and Ribosomes

Nucleus

The nucleus is the genetic control center of the eukaryotic cell.

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

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

• Chromatin: DNA-protein complex that condenses to form chromosomes during cell division.

Ribosomes

Ribosomes are molecular machines that synthesize proteins using mRNA as a template.

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

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

The Endomembrane System

Overview

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

• Includes: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium ions.

Protein Production in the ER

Proteins synthesized by ribosomes on the rough ER are often destined for secretion or for use in membranes.

• Proteins enter the ER lumen, are folded and modified (e.g., glycosylation), and then packaged into vesicles for transport to the Golgi apparatus.

Golgi Apparatus

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

• Modifies, sorts, and packages proteins and lipids received from the ER.

• Produces lysosomes and secretory vesicles.

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: Maintains cell turgor, stores nutrients and waste products 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.

• Enclosed by two membranes: smooth outer membrane and highly folded inner membrane (cristae).

• Matrix contains enzymes, mitochondrial DNA, and ribosomes.

Chloroplasts

Chloroplasts are the sites of photosynthesis in plants and algae.

• Enclosed by double membranes.

• Contain thylakoids (flattened sacs), grana (stacks of thylakoids), and stroma (fluid containing DNA, ribosomes, and enzymes).

Endosymbiotic Theory

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

• Evidence includes their own DNA, double membranes, and ribosomes similar to those of 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: Provide 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 fluid or cells over their surface.

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

• Both have a "9+2" arrangement of microtubules.

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 the cytoskeleton.

Cell Junctions

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

• Tight Junctions: Seal neighboring cells to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together using intermediate filaments.

• Gap Junctions: Provide cytoplasmic channels for communication and exchange of materials.

• Plasmodesmata (plants): Channels through cell walls connecting the cytoplasm of adjacent cells.

Plant Cell Walls

Plant cell walls are rigid structures outside the plasma membrane, composed mainly of cellulose, providing protection, shape, and preventing excessive water uptake.

• Cellulose: A polysaccharide and the main component of plant cell walls.

• Plasmodesmata: Allow direct communication between plant cells.

Summary Table: Prokaryotic vs. Eukaryotic Cells