Cell Basics

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life and can be classified as either prokaryotic or eukaryotic based on their structural characteristics.

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Their DNA is located in a region called the nucleoid.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess various membrane-bound organelles.

• Compartmentalization in eukaryotic cells allows for specialized environments and processes within the cell, increasing efficiency and complexity.

• Example: Bacteria are prokaryotes; plants and animals are eukaryotes.

Nucleus & Ribosomes

Nuclear Envelope and Nucleus

The nucleus is the control center of the eukaryotic cell, containing most of the cell's genetic material.

• Nuclear envelope: A double membrane that surrounds the nucleus, separating it from the cytoplasm.

• Function: Protects DNA and regulates the passage of materials in and out of the nucleus through nuclear pores.

• Protein synthesis control: The nucleus contains DNA, which is transcribed into mRNA. mRNA exits the nucleus and is translated by ribosomes in the cytoplasm.

Ribosomes

• Structure: Composed of rRNA and proteins; can be free in the cytoplasm or bound to the endoplasmic reticulum (ER).

• Function: Sites of protein synthesis.

• Free ribosomes: Synthesize proteins that function in the cytosol.

• Bound ribosomes: Attached to the rough ER; synthesize proteins for membranes, organelles, or export.

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.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER has ribosomes and synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Lysosomes: Contain digestive enzymes for intracellular digestion; break down waste and cellular debris.

• Vacuoles: Storage organelles; types include food vacuoles, contractile vacuoles (pump excess water), and central vacuoles (in plants, store nutrients and waste products).

Comparison of Smooth and Rough ER

• Rough ER: Studded with ribosomes; synthesizes proteins.

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

Golgi Apparatus

• Cis face: Receives vesicles from the ER.

• Trans face: Ships modified products to their destinations.

Lysosomes

• Function: Intracellular digestion via hydrolytic enzymes.

• Examples of intracellular digestion: Phagocytosis (engulfing particles), autophagy (recycling cell components), and apoptosis (programmed cell death).

• Defective lysosomes: Can cause storage diseases (e.g., Tay-Sachs disease).

Mitochondria & Chloroplasts

Energy Conversion

Mitochondria and chloroplasts are organelles involved in energy transformation.

• Mitochondria: Sites of cellular respiration; convert glucose and oxygen into ATP (energy currency of the cell).

• Chloroplasts: Found in plants and algae; sites of photosynthesis, converting solar energy into chemical energy (glucose).

Mitochondrial Structure and Function

• Double membrane: Outer and inner membranes; inner membrane folds into cristae to increase surface area.

• Matrix: Innermost compartment containing enzymes for the Krebs cycle.

Chloroplast Structure and Function

• Three functional compartments: Intermembrane space, stroma (fluid), and thylakoid space (inside thylakoids).

• Thylakoids: Flattened sacs where the light reactions of photosynthesis occur.

• Stroma: Site of the Calvin cycle (sugar synthesis).

Endosymbiotic Theory

• Evidence: Mitochondria and chloroplasts have their own DNA, double membranes, and ribosomes, suggesting they originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, organization, and movement for the cell.

• Microtubules: Hollow tubes; maintain cell shape, facilitate cell division, and serve as tracks for organelle movement.

• Microfilaments: Thin rods; involved in cell movement and muscle contraction.

• Intermediate filaments: Provide mechanical support and maintain cell shape.

Cell Surfaces & Junctions

Plant Cell Walls

Plant cells are surrounded by a rigid cell wall that provides structural support and protection.

• Primary cell wall: Thin and flexible; formed first.

• Middle lamella: Layer rich in pectins; glues adjacent plant cells together.

• Secondary cell wall: Thicker and stronger; formed after the primary wall in some cells.

Extracellular Matrix (ECM) in Animal Cells

• ECM: Network of glycoproteins (e.g., collagen) and carbohydrates outside animal cells; provides structural support, regulates cell behavior, and facilitates communication.

Cell Junctions

• Plasmodesmata (plants): Channels that connect plant cells, allowing transport of substances.

• Tight junctions (animals): Seal cells together to prevent leakage.

• Desmosomes (animals): Anchor cells together for strength.

• Gap junctions (animals): Allow communication between cells.