Cell Biology: Prokaryotes vs. Eukaryotes, Organelles, Cytoskeleton, and Cell Junctions

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Prokaryotes vs. Eukaryotes; Animal vs. Plant Cells

Overview of Cell Types

Cells are classified as either prokaryotic or eukaryotic, and eukaryotic cells are further divided into animal and plant cells. Understanding the differences in their structures and organelles is fundamental in cell biology.

Prokaryotic cells lack a nucleus and membrane-bound organelles.
Eukaryotic cells possess a nucleus and various membrane-bound organelles.
Plant cells have unique structures such as cell walls and chloroplasts, while animal cells do not.

Key Comparisons

Feature	Prokaryotes	Eukaryotes	Animal Cells	Plant Cells
Nucleus	No	Yes	Yes	Yes
Ribosomes	Yes	Yes	Yes	Yes
Endoplasmic Reticulum	No	Yes	Yes	Yes
Cell Wall	Yes (bacteria)	No (animals), Yes (plants)	No	Yes
Chloroplasts	No	No (animals), Yes (plants)	No	Yes

Example: The cell wall is present in plant cells and most prokaryotes, but absent in animal cells.

Nucleus and Ribosomes

Genetic Information and Protein Synthesis

The nucleus is the site of transcription in eukaryotic cells, where DNA is converted into RNA. Ribosomes are responsible for translation, the process of synthesizing proteins from RNA.

Transcription occurs in the nucleus.
Translation occurs in the cytoplasm, where ribosomes are located.
Ribosomes can be free in the cytoplasm or bound to the endoplasmic reticulum.
The nuclear envelope consists of two lipid bilayers.

Example: A free ribosome can become a bound ribosome by attaching to the rough endoplasmic reticulum.

Endomembrane System

Structure and Function of Organelles

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins. Key components include the endoplasmic reticulum (ER), Golgi apparatus, vesicles, and lysosomes.

Smooth ER: Involved in lipid biosynthesis and drug detoxification.
Rough ER: Studded with ribosomes; synthesizes secreted and membrane-bound proteins.
Golgi apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
Transport vesicles: Move materials between organelles.

Organelle	Main Function
Smooth ER	Lipid biosynthesis, detoxification
Rough ER	Protein synthesis
Golgi apparatus	Protein modification and sorting
Lysosome	Digestion of macromolecules

Example: Insulin is processed through the rough ER, smooth ER, transport vesicles, Golgi apparatus, and finally secreted via the cell membrane.

Additional info: The cis face of the Golgi apparatus receives vesicles from the ER, while the trans face releases processed vesicles.

Cytoskeleton

Cell Structure and Movement

The cytoskeleton is a network of protein fibers that provides structural support, enables cell movement, and organizes organelles within the cell. The main types of cytoskeletal fibers are microfilaments, microtubules, and intermediate filaments.

Microfilaments: Composed of actin; involved in cell movement and shape.
Microtubules: Hollow tubes; facilitate intracellular transport and cell division.
Intermediate filaments: Provide mechanical strength.
Motor proteins (e.g., myosin, dynein, kinesin): Move along cytoskeletal fibers to transport cellular cargo.

Fiber Type	Main Function
Microfilaments	Cell movement, shape
Microtubules	Transport, division
Intermediate filaments	Strength, stability

Example: Kartagener's syndrome is caused by defects in microtubules, leading to respiratory issues and infertility.

Additional info: Microfilaments and microtubules can rapidly assemble and disassemble, allowing dynamic changes in cell shape and movement.

Endosymbiosis Theory

Origin of Mitochondria and Chloroplasts

The endosymbiosis theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Several lines of evidence support this theory.

Both organelles have two membranes.
Both contain their own DNA and ribosomes.
Both can replicate independently within the cell.
Not all evidence supports the theory; for example, both organelles do not allow a cell to use photosynthesis.

Example: Mitochondria perform cellular respiration, while chloroplasts carry out photosynthesis in plant cells.

Cell Junctions

Connecting and Communicating Between Cells

Cell junctions are specialized structures that connect adjacent cells and facilitate communication and transport. They are essential for maintaining tissue integrity and function.

Gap junctions: Allow direct communication between cells via channels.
Desmosomes: Provide strong adhesion between cells.
Tight junctions: Prevent leakage of substances between cells.

Junction Type	Main Function
Gap junctions	Communication
Desmosomes	Adhesion
Tight junctions	Barrier

Example: Tight junctions in bladder cells prevent urine from leaking out of the organ.

Cell Biology of the 7 Proteins

Protein Localization and Function

Proteins have specific cellular localizations and functions, which are critical for cell biology. The following are examples of important proteins and their locations:

OPN1LW: Embedded in the membranes of cone photoreceptor cells in the retina.
TAS2R38: A G-protein coupled receptor (GPCR) located on taste receptor cell membranes.
CFTR: Located in the cell membrane of lung epithelial cells; facilitates chloride ion movement.
TYR (Tyrosinase): A membrane glycoprotein enzyme involved in pigmentation; found in melanocytes.
FUT1: Involved in blood cell development; located in precursor cell membranes.
ABO: Glycosyltransferase enzyme encoded by the ABO gene; located in the Golgi apparatus.
HBB: Hemoglobin; located in the cytoplasm of red blood cells.

Protein	Location	Function
OPN1LW	Retina (cone cells)	Photoreception
TAS2R38	Taste receptor cells	GPCR signaling
CFTR	Lung epithelial cells	Ion transport
TYR	Melanocytes	Pigmentation
FUT1	Blood cell precursors	Antigen biosynthesis
ABO	Golgi apparatus	Glycosylation
HBB	Red blood cells	Oxygen transport

Example: Tyrosinase is a membrane glycoprotein enzyme consisting of 529 amino acids, essential for melanin production.

Additional info: Transmembrane proteins span the lipid bilayer and are involved in signaling and transport.