A - Unity and Diversity

Cell Theory and Exceptions

Cell theory is a fundamental principle in biology that states all living organisms are composed of cells, cells are the basic unit of life, and all cells arise from pre-existing cells. This theory forms the foundation of our understanding of life at the microscopic level.

Note

The cell theory was developed through the work of several scientists, including Robert Hooke, Matthias Schleiden, and Theodor Schwann.

However, there are some exceptions to the cell theory:

1. Viruses: These are non-cellular entities that can replicate only inside host cells. They possess genetic material but lack cellular structures.
2. Prions: Infectious proteins that cause diseases like Creutzfeldt-Jakob disease. They have no genetic material or cellular structure.
3. Syncytia: Multinucleated cells formed by the fusion of multiple cells, such as muscle fibers.

Example

The human placenta contains a syncytiotrophoblast layer, which is a large multinucleated cell formed by the fusion of multiple cells.

Functions of Life in Unicellular Organisms

Unicellular organisms, despite their simplicity, perform all essential life functions within a single cell. These functions include:

1. Nutrition: Obtaining and processing nutrients
2. Excretion: Removing waste products
3. Respiration: Generating energy through cellular respiration
4. Growth and reproduction: Increasing in size and producing offspring
5. Movement: Locomotion or internal movement of cellular components
6. Sensitivity: Responding to environmental stimuli

Example

Paramecium, a unicellular protist, exhibits all these functions. It uses cilia for movement and feeding, has contractile vacuoles for excretion, and reproduces through binary fission.

Surface Area to Volume Ratio and Cell Size Limitations

The surface area to volume ratio (SA:V) is crucial in determining cell size limitations. As a cell grows, its volume increases faster than its surface area, leading to a decrease in the SA:V ratio.

$$\text{SA:V ratio} = \frac{\text{Surface Area}}{\text{Volume}}$$

This ratio is important because:

1. Nutrients and oxygen enter the cell through its surface
2. Waste products must be removed through the surface
3. Cell signaling occurs at the cell surface

Note

A higher SA:V ratio allows for more efficient exchange of materials with the environment.

As cells grow larger, they face challenges:

1. Reduced efficiency in material exchange
2. Increased diffusion distances within the cell
3. Difficulty maintaining cellular processes

To overcome these limitations, cells have evolved strategies such as:

• Flattened or elongated shapes to increase surface area
• Internal membrane systems (e.g., endoplasmic reticulum) to increase functional surface area
• Specialized transport mechanisms

Common Mistake

Students often assume that larger cells are always more efficient, but in reality, smaller cells generally have higher SA:V ratios and are more efficient in terms of material exchange.

Properties of Multicellular Organisms

Multicellular organisms possess several unique properties that distinguish them from unicellular life forms:

1. Cell specialization: Different cell types perform specific functions
2. Tissue formation: Groups of similar cells work together
3. Organ systems: Tissues combine to form organs with specific roles
4. Intercellular communication: Cells coordinate activities through signaling molecules
5. Complex development: Multicellular organisms undergo intricate developmental processes

Example

In humans, specialized cells like neurons form tissues (nervous tissue), which in turn form organs (brain) that are part of organ systems (nervous system).

Cell Differentiation and Specialization

Cell differentiation is the process by which cells become specialized for specific functions. This process involves:

1. Selective gene expression: Activating or repressing specific genes
2. Morphological changes: Alterations in cell shape and structure
3. Biochemical modifications: Changes in cellular components and metabolic pathways

Note

Cell differentiation is crucial for the development of complex multicellular organisms.

Examples of specialized cells include:

• Neurons: Specialized for transmitting electrical signals
• Muscle cells: Adapted for contraction
• Red blood cells: Optimized for oxygen transport

Gene Expression in Differentiation

Gene expression plays a pivotal role in cell differentiation. Key aspects include:

1. Transcription factors: Proteins that regulate gene expression
2. Epigenetic modifications: Chemical changes to DNA or histones that affect gene expression without altering the DNA sequence
3. RNA interference: Small RNA molecules that can silence specific genes

Example

During the differentiation of a red blood cell, the gene for hemoglobin is highly expressed, while genes for other cell types are repressed.

Stem Cells and Their Importance

Stem cells are undifferentiated cells capable of self-renewal and differentiation into various cell types. They are classified as:

1. Totipotent: Can form all cell types, including extraembryonic tissues
2. Pluripotent: Can form all cell types of the embryo proper
3. Multipotent: Can form multiple cell types within a specific lineage
4. Unipotent: Can form only one cell type

Note

Embryonic stem cells are pluripotent, while adult stem cells are typically multipotent or unipotent.

Importance of stem cells:

• Tissue repair and regeneration
• Potential therapeutic applications in regenerative medicine
• Study of developmental processes and disease mechanisms

Tip

Understanding stem cell biology is crucial for developing new treatments for diseases like Parkinson's and spinal cord injuries.

Ultrastructure of Prokaryotic and Eukaryotic Cells

Prokaryotic cells (bacteria and archaea) have a simpler structure compared to eukaryotic cells:

1. No membrane-bound nucleus
2. Circular DNA in the nucleoid region
3. No membrane-bound organelles
4. Cell wall (in most cases)
5. Ribosomes (smaller than eukaryotic ribosomes)

Eukaryotic cells (found in protists, fungi, plants, and animals) have:

1. Membrane-bound nucleus
2. Linear DNA organized into chromosomes
3. Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus)
4. Larger ribosomes
5. Cytoskeleton

Example

A typical animal cell contains a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and various other organelles, while a bacterial cell like E. coli has a nucleoid region, ribosomes, and a cell wall but lacks membrane-bound organelles.

Electron Microscopy and Cell Structure

Electron microscopy has revolutionized our understanding of cell structure by providing much higher resolution than light microscopy. Two main types are used:

1. Transmission Electron Microscopy (TEM): Provides detailed images of internal cell structures
2. Scanning Electron Microscopy (SEM): Gives 3D images of cell surfaces

Key structures revealed by electron microscopy:

• Fine structure of organelles (e.g., cristae in mitochondria)
• Membrane systems (e.g., thylakoids in chloroplasts)
• Cytoskeletal elements (microtubules, microfilaments)
• Cell surface features (microvilli, flagella)

Note

Electron microscopy requires special sample preparation, including fixation, dehydration, and staining or coating with heavy metals.

Classification of Organisms into Three Domains

Modern biological classification recognizes three domains of life:

1. Bacteria: Prokaryotic cells with peptidoglycan cell walls
2. Archaea: Prokaryotic cells with unique cell wall composition and membrane lipids
3. Eukarya: All organisms with eukaryotic cells

This system, proposed by Carl Woese, is based on genetic and biochemical differences between organisms.

Common Mistake

Students often confuse archaea with bacteria, but they are distinct domains with significant genetic and biochemical differences.

Hierarchical Classification System for Eukaryotes

The hierarchical classification system for eukaryotes includes the following levels, from broadest to most specific:

1. Domain
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

Tip

A mnemonic device to remember this order is "Dear King Philip Came Over For Good Soup".

Example classification for humans:

• Domain: Eukarya
• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Primates
• Family: Hominidae
• Genus: Homo
• Species: Homo sapiens

Natural Classification Based on Evolutionary Relationships

Natural classification aims to group organisms based on their evolutionary relationships, reflecting their common ancestry. This approach uses:

1. Morphological similarities
2. Genetic similarities
3. Biochemical similarities
4. Fossil evidence

Modern classification methods include:

• Phylogenetic analysis: Constructing evolutionary trees based on shared derived characteristics
• Molecular clock: Estimating divergence times using genetic differences
• Comparative genomics: Analyzing whole genomes to infer evolutionary relationships

Example

The close evolutionary relationship between humans and chimpanzees is reflected in their high genetic similarity (about 98% of DNA sequences are identical) and shared morphological features.

Features of Major Plant and Animal Phyla

Major plant phyla:

1. Bryophyta (mosses): Non-vascular, no true roots or leaves
2. Pteridophyta (ferns): Vascular, with roots and leaves, reproduce by spores
3. Gymnospermae: Vascular, seeds not enclosed in fruits (e.g., conifers)
4. Angiospermae: Vascular, seeds enclosed in fruits, flowers for reproduction

Major animal phyla:

1. Porifera (sponges): Simple, sessile, no true tissues
2. Cnidaria (jellyfish, corals): Radial symmetry, two tissue layers
3. Platyhelminthes (flatworms): Bilateral symmetry, three tissue layers
4. Annelida (segmented worms): Segmented body, closed circulatory system
5. Mollusca (snails, octopuses): Soft body, often with shell
6. Arthropoda (insects, crustaceans): Jointed appendages, exoskeleton
7. Echinodermata (starfish, sea urchins): Radial symmetry as adults, unique water vascular system
8. Chordata (vertebrates): Notochord, dorsal hollow nerve cord, pharyngeal slits

Note

These phyla represent major evolutionary innovations and adaptations to different environments.

Construction and Use of Dichotomous Keys

Dichotomous keys are tools used for identifying organisms based on a series of paired, mutually exclusive choices. They are constructed by:

1. Identifying distinctive characteristics of the organisms
2. Arranging these characteristics in a logical sequence
3. Creating a series of paired statements or questions

To use a dichotomous key:

1. Start at the first pair of statements
2. Choose the statement that best fits the organism
3. Follow the directions to the next pair of statements
4. Continue until the organism is identified

Example

A simple dichotomous key for identifying common trees:

1a. Leaves needle-like → Go to 2 1b. Leaves broad and flat → Go to 3

2a. Needles in clusters of 2-5 → Pine 2b. Needles single, flat → Fir

3a. Leaves opposite → Maple 3b. Leaves alternate → Oak

Tip

When constructing a dichotomous key, ensure that each pair of choices is mutually exclusive and covers all possibilities within that group.

Membrane Structure and Function

Biological membranes are composed of a phospholipid bilayer with embedded proteins. Key components include:

1. Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails
2. Membrane proteins: Integral (spanning the membrane) and peripheral (associated with the surface)
3. Cholesterol: Regulates membrane fluidity in animal cells
4. Glycolipids and glycoproteins: Involved in cell recognition and signaling

Functions of membranes:

• Selective permeability: Controlling the movement of substances in and out of cells
• Cell signaling: Receptors in the membrane respond to external signals
• Cell recognition: Surface molecules allow cells to identify each other
• Compartmentalization: Separating cellular processes in eukaryotic organelles

Note

The fluid mosaic model, proposed by Singer and Nicolson, describes the dynamic nature of biological membranes.

Types of Membrane Transport

Membrane transport can be classified into two main categories:

1. Passive transport: Movement of substances down their concentration gradient without energy input
   • Simple diffusion: Direct movement through the phospholipid bilayer
   • Facilitated diffusion: Movement through protein channels or carriers
2. Active transport: Movement of substances against their concentration gradient, requiring energy
   • Primary active transport: Directly uses ATP (e.g., sodium-potassium pump)
   • Secondary active transport: Uses energy stored in electrochemical gradients (e.g., glucose-sodium cotransport)

Other transport mechanisms:

• Osmosis: Diffusion of water across a selectively permeable membrane
• Endocytosis: Uptake of materials by engulfing them in membrane vesicles
• Exocytosis: Release of materials from vesicles to the extracellular space

Example

The sodium-potassium pump uses ATP to move 3 sodium ions out of the cell and 2 potassium ions into the cell, maintaining crucial ion gradients across the cell membrane.

Origin of Cells and Endosymbiotic Theory

The origin of cells is a fundamental question in biology. Key ideas include:

1. Abiogenesis: The hypothesis that life arose from non-living matter
2. RNA world hypothesis: Suggests that self-replicating RNA molecules preceded DNA-based life
3. Protocells: Simple membrane-enclosed structures that may have been precursors to modern cells

The endosymbiotic theory, proposed by Lynn Margulis, explains the origin of eukaryotic cells. It suggests that:

1. Mitochondria evolved from aerobic bacteria engulfed by larger prokaryotic cells
2. Chloroplasts evolved from photosynthetic bacteria (like cyanobacteria) engulfed by early eukaryotes

Evidence supporting this theory includes:

• Mitochondria and chloroplasts have their own DNA
• These organelles reproduce by binary fission, similar to bacteria
• They have ribosomes similar in size to bacterial ribosomes

Note

The endosymbiotic theory is now widely accepted and explains the presence of DNA in mitochondria and chloroplasts.

Cell Division by Mitosis and Meiosis

Mitosis is the process of cell division that produces two genetically identical daughter cells. Stages of mitosis:

1. Prophase: Chromosomes condense, nuclear envelope breaks down
2. Metaphase: Chromosomes align at the cell's equator
3. Anaphase: Sister chromatids separate and move to opposite poles
4. Telophase: Nuclear envelopes reform, chromosomes decondense

Meiosis is a specialized form of cell division that produces gametes with half the chromosome number of the parent cell. It involves two rounds of division:

Meiosis I:

1. Prophase I: Homologous chromosomes pair and exchange genetic material (crossing over)
2. Metaphase I: Homologous pairs align at the equator
3. Anaphase I: Homologous chromosomes separate
4. Telophase I: Two haploid cells form

Meiosis II: Similar to mitosis, separating sister chromatids

Note

Meiosis results in genetic variation through crossing over and independent assortment of chromosomes.

Cell Cycle Control

The cell cycle is regulated by a complex system of checkpoints and regulatory proteins. Key components include:

1. Cyclins: Proteins that fluctuate in concentration during the cell cycle
2. Cyclin-dependent kinases (CDKs): Enzymes activated by cyclins to phosphorylate target proteins
3. Checkpoints: Points in the cell cycle where progress is halted if conditions are not suitable

Major checkpoints:

• G1 checkpoint: Ensures the cell is ready to enter S phase
• G2 checkpoint: Ensures DNA replication is complete and the cell is ready for mitosis
• Metaphase checkpoint: Ensures all chromosomes are properly attached to the spindle

Example

The p53 protein, often called the "guardian of the genome," can halt the cell cycle if DNA damage is detected, allowing time for repair or triggering apoptosis if the damage is severe.

Cancer and Uncontrolled Cell Division

Cancer is characterized by uncontrolled cell division and the ability to invade other tissues. Key aspects include:

1. Mutations in proto-oncogenes: Genes that promote cell division when activated
2. Mutations in tumor suppressor genes: Genes that normally inhibit excessive cell division
3. Genomic instability: Increased mutation rate in cancer cells
4. Angiogen