Cell Theory

Foundations of Modern Biology

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life. It provides the framework for understanding the structure and function of all living organisms.

• All organisms are composed of cells.

• All cells come only from preexisting cells.

• Cells are the smallest structural and functional unit of organisms.

Key Point: The cell theory emphasizes the continuity of life and the central role of cells in biological organization.

Cell Size and Diffusion

Surface Area to Volume Ratio

Cells are generally small to maximize the efficiency of diffusion, which is essential for transporting materials in and out of the cell. The surface area to volume ratio decreases as a cell grows larger, limiting the rate at which substances can diffuse.

• Surface Area: The total area of the cell membrane available for exchange.

• Volume: The internal space of the cell where metabolic processes occur.

• As cell size increases, volume grows faster than surface area.

Formula:

Example: Smaller cells have a higher surface area to volume ratio, allowing for more efficient diffusion of nutrients and waste products.

Types of Cells: Prokaryotic and Eukaryotic

Prokaryotic Cells

Prokaryotic cells are the simplest and most ancient forms of life, including bacteria and archaea. They lack membrane-bound organelles and a true nucleus.

• Domains: Bacteria and Archaea

• Key Features: No nucleus, no membrane-bound organelles, DNA in a nucleoid region, cell wall (often peptidoglycan in bacteria), ribosomes present

• Shapes: Rod-shaped (bacilli), spherical (cocci), spiral (spirilla)

Eukaryotic Cells

Eukaryotic cells are more complex and are found in animals, plants, fungi, and protists. They possess a true nucleus and various membrane-bound organelles.

• Domain: Eukarya

• Key Features: Nucleus, membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus), multiple linear chromosomes

• Both prokaryotic and eukaryotic cells have: Cell membrane, cytoplasm, ribosomes, DNA

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Origin of Organelles

Endosymbiotic Theory

The endosymbiotic theory explains the origin of certain organelles in eukaryotic cells, such as mitochondria and chloroplasts. It proposes that these organelles originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Mitochondria and chloroplasts have their own DNA and ribosomes, supporting the endosymbiotic origin.

• Engulfment of aerobic bacteria led to mitochondria; engulfment of photosynthetic bacteria led to chloroplasts.

Example: Mitochondria in animal cells and chloroplasts in plant cells are believed to have evolved from symbiotic relationships with prokaryotic organisms.

Structure of Animal and Plant Cells

Basic Structure

Both animal and plant cells share several common features, but plant cells have additional structures that support their unique functions.

• Common features: Plasma membrane, cytoplasm, nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, cytoskeleton

• Plant-specific structures: Cell wall, central vacuole, chloroplasts, plastids

Cytoskeleton, Cilia, and Flagella

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and facilitates movement. Cilia and flagella are extensions of the plasma membrane involved in cell movement.

• Cytoskeleton: Composed of microtubules, microfilaments, and intermediate filaments

• Cilia and Flagella: Arise from basal bodies (derived from centrioles/centrosome), important for movement and cell division

Nucleus

The nucleus is the control center of the eukaryotic cell, containing genetic material and coordinating cellular activities.

• Nuclear envelope: Double membrane with nuclear pores

• Chromatin: DNA and associated proteins (histones)

• Nucleolus: Site of ribosome assembly

Endomembrane System

Organization and Function

The endomembrane system is a group of interconnected or related membranes that compartmentalize the cell, allowing specialized functions to occur in distinct regions.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, plasma membrane

• Function: Synthesis, modification, packaging, and transport of proteins and lipids

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for membranes or secretion

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

Golgi Apparatus

• Function: Sorts, chemically modifies, and packages molecules for transport

• Modifies proteins: Adds carbohydrate groups to form glycoproteins

• Synthesizes polysaccharides: Used in plant cell walls

• Packages molecules: Into vesicles for transport within the cell or for export

Lysosomes and Peroxisomes

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules (mainly in animal cells)

• Peroxisomes: Use oxygen to break down fatty acids and detoxify harmful substances (present in both plant and animal cells)

Mitochondria and Chloroplasts

Mitochondria

Mitochondria are the sites of cellular respiration, producing ATP for cellular energy. They have their own DNA and ribosomes, and are surrounded by a double membrane.

• Inner membrane: Highly folded into cristae, increasing surface area for ATP production

• Matrix: Contains enzymes for breaking down carbohydrates

Chloroplasts and Plastids (Plant Cells)

Chloroplasts are specialized plastids found in plant cells, responsible for photosynthesis. Plastids are large organelles that store various substances, such as pigments or starch.

• Chloroplasts: Contain chlorophyll, have a double membrane, and internal thylakoid membranes

• Other plastids: Store pigments (chromoplasts) or starch (amyloplasts)

Specialized Structures in Plant Cells

Cell Wall

The cell wall is a rigid structure outside the plasma membrane, providing support, protection, and preventing over-expansion when water enters the cell.

Central Vacuole

The central vacuole occupies much of the plant cell's volume and serves multiple functions:

• Maintains water balance (turgor pressure)

• Stores hazardous wastes, nutrients, or pigments

Summary Table: Key Differences Between Animal and Plant Cells

Additional info: The notes reference the "Central Dogma of Molecular Biology," which describes the flow of genetic information from DNA to RNA to protein, but details are not provided in the original content.