Inside the Cell

Cell Theory and Universal Cell Features

Cell theory states that all living organisms are composed of cells, which are the fundamental units of life. Despite their diversity, all cells share several essential components:

• Proteins: Perform most cellular functions, including catalysis, structure, and signaling.

• Nucleic acids: Store, transmit, and process genetic information.

• Carbohydrates: Provide energy, carbon, structural support, and cellular identity.

• Plasma membrane: Acts as a selectively permeable barrier, regulating entry and exit of substances.

Cell Types and Domains of Life

Cells are classified based on morphology and evolutionary history:

• Eukaryotes: Possess a membrane-bound nucleus.

• Prokaryotes: Lack a membrane-bound nucleus.

Phylogenetically, life is divided into three domains:

• Bacteria (prokaryotic)

• Archaea (prokaryotic)

• Eukarya (eukaryotic)

Prokaryotic Cell Structures

General Structure and Components

Prokaryotic cells exhibit a variety of structures and functions. Key features include:

• At least one chromosome

• Numerous ribosomes for protein synthesis

• Distinct phospholipid compositions in membranes (fatty acids in bacteria, branched isoprenoids in archaea)

• Cytoplasm: All contents within the cell membrane

Chromosome Organization and Plasmids

The chromosome is the most prominent structure in prokaryotic cells, typically a single, circular DNA molecule associated with proteins. Plasmids are smaller, circular, supercoiled DNA molecules that carry additional genes.

Ribosomes and Protein Synthesis

Prokaryotic ribosomes are macromolecular machines composed of RNA and protein, responsible for synthesizing proteins. While similar in size and function across bacteria and archaea, their primary structures differ.

Cytoskeleton and Cell Shape

Long, thin protein filaments form the cytoskeleton, essential for cell division and maintaining cell shape in prokaryotes.

Internal Membrane Complexes in Photosynthetic Species

Photosynthetic prokaryotes possess internal membrane systems that convert sunlight into chemical energy. These membranes are infoldings of the plasma membrane and contain enzymes and pigments necessary for photosynthesis.

Organelles for Specialized Functions

Some bacteria have internal, membrane-bound compartments (organelles) for specialized tasks, such as storing ions, orienting with magnetite crystals, or concentrating enzymes.

Cell Wall Structure

The cell wall acts as a protective exoskeleton, providing shape and rigidity. In bacteria, the primary component is peptidoglycan, a polysaccharide. Some bacteria also have an outer membrane made of glycolipids.

External Structures: Flagella and Fimbriae

Prokaryotes interact with their environment using external appendages:

• Flagella: Long filaments that rotate to propel the cell.

• Fimbriae: Needlelike projections that facilitate attachment to surfaces or other cells.

Eukaryotic Cell Structures and Functions

Overview and Size Comparison

Eukaryotic cells, which include protists, fungi, plants, and animals, are generally larger and more complex than prokaryotic cells. They may be unicellular or multicellular.

Benefits of Organelles

Organelles compartmentalize the cell, allowing separation of incompatible reactions and increasing efficiency. The cytosol is the fluid portion outside organelles.

The Nucleus

The nucleus is a large, membrane-bound compartment containing the cell's genetic material. It is surrounded by a double-membrane nuclear envelope with pore-like openings and contains the nucleolus, where ribosomal RNA is synthesized and ribosome subunits are assembled.

Ribosomes

Ribosomes are molecular machines that synthesize proteins. They lack membranes and can be free in the cytosol or attached to the endoplasmic reticulum.

Endoplasmic Reticulum (ER)

The ER is a membrane-enclosed factory with two distinct regions:

• Rough ER (RER): Studded with ribosomes, synthesizes proteins for secretion, membrane insertion, or organelle targeting.

• Smooth ER (SER): Lacks ribosomes, catalyzes reactions involving lipids, detoxifies molecules, and stores calcium ions.

Golgi Apparatus

The Golgi apparatus processes, sorts, and ships proteins from the RER. It consists of stacked, flat membranous sacs (cisternae) with distinct cis and trans faces.

Lysosomes

Lysosomes are recycling centers found in animal cells, containing enzymes (acid hydrolases) that hydrolyze macromolecules. They maintain a low internal pH for optimal enzyme activity.

Vacuoles

Vacuoles are prominent organelles in plant and fungal cells, serving as storage centers for water, ions, proteins, pigments, and defensive compounds.

Peroxisomes

Peroxisomes are globular organelles that catalyze reduction-oxidation reactions, often detoxifying hydrogen peroxide.

Mitochondria

Mitochondria supply ATP to cells and have a double membrane. The inner membrane is folded into cristae, enclosing the mitochondrial matrix. Mitochondria possess their own DNA and ribosomes, and can divide independently.

Chloroplasts

Chloroplasts are found in plant and algal cells, where photosynthesis occurs. They have three membranes, with thylakoids arranged in stacks (grana) and surrounded by stroma. Chloroplasts also contain their own DNA and ribosomes.

Cell Wall and Extracellular Matrix

Fungi, algae, and plants have a stiff cell wall outside the plasma membrane for structural support. Animal cells lack a cell wall but are supported by the extracellular matrix (ECM), a mixture of proteins and polysaccharides.

Cell Structure and Function Correlation

The structure of each cell component correlates with its function. For example, fat cells store lipids, cardiac muscle cells generate force, and leaf cells contain chloroplasts for photosynthesis.

Nuclear Transport

Nuclear Envelope and Pore Complexes

The nuclear envelope separates the nucleus from the cytosol and is perforated with nuclear pore complexes, which regulate traffic between the nucleus and cytoplasm.

Nuclear Localization Signals

Proteins destined for the nucleus contain a nuclear localization signal (NLS), a 17-amino-acid sequence that acts as a molecular zip code for transport through the nuclear pore complex.

The Endomembrane System: Manufacturing, Shipping, and Recycling

Secretory Pathway Hypothesis

Proteins synthesized in the RER are transported through the endomembrane system, passing through the Golgi apparatus and being sorted for their final destinations.

Pulse–Chase Experiment

The pulse–chase experiment tracks protein movement through the endomembrane system, confirming the existence of the secretory pathway.

Signal Hypothesis and Protein Targeting

Proteins bound for the endomembrane system have an ER signal sequence, which directs them to the RER. The signal recognition particle (SRP) binds the sequence and guides the ribosome to the ER membrane.

Protein Sorting in the Golgi Apparatus

Proteins are modified and sorted in the Golgi apparatus, receiving molecular tags for their final destinations. Transport vesicles carry proteins to organelles or the plasma membrane for secretion.

Lysosomal Recycling Pathways

Cells recycle materials via lysosomes through three pathways: receptor-mediated endocytosis, phagocytosis, and autophagy.

The Dynamic Cytoskeleton

Cytoskeletal Elements

The cytoskeleton is a network of protein fibers that maintains cell shape, organizes cellular contents, and enables movement. It consists of three major types:

• Actin filaments (microfilaments): Smallest, made of actin, involved in movement and cell shape.

• Intermediate filaments: Provide structural support, include keratins and nuclear lamins.

• Microtubules: Largest, made of tubulin dimers, involved in chromosome separation and vesicle transport.

Actin Filament Function

Actin filaments interact with myosin motor proteins to produce movement, including cytokinesis, cytoplasmic streaming, and cell crawling.

Microtubules and Centrosomes

Microtubules originate from the microtubule organizing center (MTOC), called the centrosome in animal cells, which contains centrioles. Microtubules are essential for chromosome separation during cell division.

Microtubule-Based Vesicle Transport

Vesicles move along microtubule tracks, facilitated by motor proteins such as kinesin, which convert ATP into mechanical energy.

Flagella and Cilia

Flagella and cilia are cellular appendages that enable movement. Prokaryotic flagella rotate, while eukaryotic flagella and cilia have a "9 + 2" microtubule arrangement and move via bending.

Cell-to-Cell Junctions

Plant Cell Junctions: Plasmodesmata

Plasmodesmata are channels between plant cells that allow communication and transport of materials.

Animal Cell Junctions

Three types of junctions are common in animal epithelial tissues:

• Tight junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together into strong sheets.

• Gap junctions: Provide cytoplasmic channels for communication.

Summary Table: Eukaryotic Cell Components

The following table summarizes the main components of eukaryotic cells, their structure, and function: