Cell Biology Fundamentals: Structure, Function, and Classification of Cells

What is a Cell?

Basic Unit of Life

Cells are the fundamental structural and functional units of all living organisms. They carry out essential life processes and are the smallest entities considered alive.

• All cells have a cell membrane that encloses their contents and regulates the movement of substances in and out.

• Cells can grow, divide, move, and replicate (on their own, except for some exceptions like viruses).

• All chemical reactions needed for life (metabolism) occur within the cell.

• Cells maintain a high surface area to volume ratio, which facilitates efficient exchange of materials with the environment.

Viruses: Are They Alive?

Characteristics of Viruses

Viruses are not considered living cells because they lack key features of cellular life.

• No metabolism: Viruses do not carry out metabolic processes independently.

• Cannot grow, divide, or replicate on their own: They require a host cell to reproduce.

• Not classified as cells.

Universal Features of All Cells

Common Cellular Components

Despite diversity, all cells share several fundamental structures and molecules:

• Cell membrane

• DNA (nucleic acid)

• RNA (messenger RNA, transfer RNA, ribosomal RNA)

• Proteins

• Cytosol/cytoplasm

• Ribosomes

• Carbohydrates

• Lipids (phospholipids, triglycerides, cholesterol)

Classification of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural features:

• Prokaryotic cells: Lack membrane-bound organelles, including a nucleus.

• Eukaryotic cells: Contain membrane-bound organelles (nucleus, mitochondria, Golgi apparatus, etc.).

Types of Eukaryotic Cells: Plant vs. Animal

Shared Features

Both plant and animal eukaryotic cells have several organelles in common:

• Mitochondria

• Golgi apparatus

• Lysosomes

• Nucleus

• Cell membrane

• Endoplasmic reticulum (rough and smooth)

Unique Features

• Plant cells: Have chloroplasts, cell walls, and large central vacuoles.

• Animal cells: Have centrosomes, rounded shape, cilia, and flagella.

Endomembrane System

Coordination of Gene Expression and Protein Processing

The endomembrane system consists of organelles that work together to synthesize, modify, and transport proteins and lipids.

• Gene expression: DNA → mRNA (transcription) → polypeptide (translation) → protein (folding, modification, shipping).

• Transcription: Occurs in the nucleus, where DNA is used to make mRNA.

• Translation: mRNA is translated into polypeptides by ribosomes.

• Protein processing: Proteins are modified in the rough ER and Golgi apparatus, then packaged into vesicles for transport.

Phagocytosis and Pinocytosis

Cellular Uptake Mechanisms

• Phagocytosis: The process by which cells engulf large particles or "food" by surrounding them with a vacuole.

• Pinocytosis: The uptake of liquid into the cell by forming small vesicles.

Vacuoles

Types and Functions

• Food vacuoles: Contain particles for digestion.

• Contractile vacuoles: Expel excess water (common in protists).

• Central vacuoles: Store water and organic compounds (in plants).

Autophagy

Cellular Self-Degradation

• Cells break down their own components for recycling.

• Involves the peroxisome and lysosome, as well as mitochondria.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as independent prokaryotic cells that were engulfed by ancestral eukaryotic cells.

• Both organelles have their own DNA and double membranes.

• They replicate independently within the cell.

• They have their own ribosomes and can synthesize some of their own proteins.

Cellular Respiration

Energy Production

Cells convert glucose and oxygen into carbon dioxide, water, and energy (ATP) through cellular respiration.

Equation:

$C_6H_{12}O_6 \; (\text{glucose}) + 6\; O_2 \rightarrow 6\; CO_2 + 6\; H_2O + \text{ATP}$

Peroxisomes and Smooth ER

Detoxification and Lipid Metabolism

• Peroxisomes: Break down waste, especially hydrogen peroxide (H2O2), using catalase.

• Smooth ER: Also involved in detoxification and lipid synthesis.

Cell Junctions

Types and Functions

• Plant cells: Plasmodesmata (channels between cells).

• Animal cells: Desmosomes (muscle, elasticity, tension), tight junctions (skin, impermeable), gap junctions (pores/channels for communication).

Cytoskeleton

Structure and Function

The cytoskeleton maintains cell shape, enables movement, and assists in cell division.

• Microtubules: Tubulin, cell division, movement of organelles, cilia/flagella.

• Actin microfilaments: Muscle contraction, cell shape, movement.

• Intermediate filaments: Keratin, structural support, organelle positioning.

Fluid Mosaic Model

Membrane Structure

The cell membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Proteins serve as channels, receptors, enzymes, and structural components.

• Cholesterol modulates membrane fluidity.

• Phospholipids create a barrier, with hydrophilic heads and hydrophobic tails.

Membrane Transport

Passive vs. Active Transport

• Passive transport: Movement down a concentration gradient (high to low), includes diffusion and facilitated diffusion (via proteins).

• Active transport: Requires energy (ATP), moves substances against a concentration gradient (low to high).

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Cell size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Key terms and processes are defined to ensure the notes are self-contained for study purposes.