Evolutionary Origin of Cells and Their General Features

Chapter Overview

• This chapter discusses the evolutionary origin of cells and their structural and functional characteristics

Key Concepts

• Origin of Living Cells on Earth

• Microscopy

• Overview of Cell Structure and Function

• The Cytosol

• The Nucleus and Endomembrane System

• Semiautonomous Organelles

• Protein Sorting to Organelles

• Systems Biology of Cells: A Summary

Cell Biology

• Definition: Study of individual cells and their interactions with each other

• Topics in cell biology include cancer, cell signaling, and cellular processes.

Cell Theory

• Credited to Schleiden and Schwann, with contributions from Virchow.

  • Postulates of Cell Theory:

    • All living organisms are composed of one or more cells.

    • Cells are the smallest units of life.

    • New cells arise only from pre-existing cells through cell division.

Origin of Life

• Four overlapping stages:

  1. Nucleotides and amino acids produced prior to the existence of cells.

  2. Nucleotides and amino acids polymerized to form RNA, DNA, and proteins.

  3. Polymers became enclosed in membranes.

  4. These polymers acquired cellular properties.

Stage 1: Origin of Organic Molecules

• Primitive Earth conditions were conducive to spontaneous formation of organic molecules:

  • Little free oxygen gas.

  • Formation of a prebiotic soup.

  • Several hypotheses on the origin of organic molecules include:

    • Reducing Atmosphere Hypothesis:

      • The atmosphere was rich in water vapor, hydrogen, methane, and ammonia.

      • Stanley Miller simulated this atmosphere, producing amino acids and sugars.

    • Extraterrestrial Hypothesis:

      • Suggests meteoritic delivery of organic molecules, including amino acids to Earth.

    • Deep-Sea Vent Hypothesis:

      • Suggests that complex biological molecules may have formed in the unique environment of hydrothermal vents.

Stages of Organic Polymers Formation

• Stage 2: Evidence shows that nucleic acids and polypeptides can form on clay surfaces and in aqueous solutions.

• Stage 3: Protobionts formed with boundaries (lipid bilayers) maintained distinct internal environments.

• Stage 4: The RNA world hypothesis supports that RNA was the first macromolecule, capable of both storing information and self-replication.

Chemical Selection

• Chemical selection occurs when certain RNA molecules replicate faster due to beneficial mutations.

• Hypothetical two-step process:

  1. Mutation allows RNA to self-replicate faster.

  2. Second mutation enhances synthesis of ribonucleotides independently from prebiotic synthesis.

Advantages of DNA/RNA/Protein World

• The modern cellular world relies on DNA for stable information storage and proteins for catalysis.

• DNA helps reduce errors in genetic material due to lower mutation rates.

Microscopy

• Microscopes enable visualization of cells and their structures.

• Key parameters in microscopy include:

  • Resolution: Distinct observation of adjacent objects.

  • Contrast: Differences in structure visualization.

  • Magnification: Size of image compared to actual size.

Types of Microscopes

• Light Microscope: Utilizes light, resolution of 0.2 micrometer.

• Electron Microscope: Uses electron beams, achieving resolution of 2 nanometers (100x better).

• Types of light microscopy:

  • Standard (bright field), phase contrast, and differential interference contrast.

Overview of Cell Structure and Function

• Determined by:

  • Matter, energy, organization, and protein interactions creating intricate structures.

• Life categorized into:

  • Prokaryotes: Simple structure, no nucleus.

  • Eukaryotes: Complex structure, DNA in membrane-bound nucleus.

Prokaryotic Cells

• Two Types:

  • Bacteria: Common, varied, mostly harmless.

  • Archaea: Found in extreme environments, less common.

Typical Bacterial Cell Structure

• Components: Plasma membrane, cytoplasm, nucleoid (DNA), ribosomes for protein synthesis, cell wall for support, and glycocalyx for protection.

Eukaryotic Cells

• Structure: DNA in nucleus, multiple organelles with varied functions, ranging from energy production to protein synthesis.

• Key organelles include mitochondria, rough/smooth ER, Golgi apparatus, lysosomes, and more.

Cell Morphology and Characteristics

• Variations in size and shape among eukaryotic cells.

• Cells can display different morphologies despite having identical genomes.

Cytosol and Metabolism

• Cytosol: Region outside organelles, central for metabolic activities.

• Metabolism: Sum of all enzymatic reactions, including catabolism (breakdown) and anabolism (synthesis).

Cytoskeleton

• Comprised of microtubules, intermediate filaments, and actin filaments aiding in cell shape, strength, and intracellular movement.

Motor Proteins and Movement

• Use ATP for movement along cytoskeletal filaments.

• Three types of movements facilitated:

  1. Cargo transport.

  2. Filament movement.

  3. Filament bending through motor protein action.

Flagella and Cilia

• Structures responsible for cellular movement:

  • Flagella: Longer, often singular.

  • Cilia: Shorter, more numerous, cover cell surfaces.

The Nucleus and Endomembrane System

• Encloses the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

• Nuclear Envelope: Double-membrane structure containing nuclear pores for material transport.

Endoplasmic Reticulum (ER)

• Types: Rough ER (ribosome-studded, protein synthesis) and Smooth ER (lipid synthesis, detoxification).

Golgi Apparatus

• Processes and sorts proteins and lipids, involved in glycosylation and proteolysis.

• Proteins travel from ER to Golgi for modification before secretion.

Lysosomes and Vacuoles

• Lysosomes: Contain enzymes for degradation (acid hydrolases).

• Vacuoles: Storage, particularly in plant cells (central vacuole).

Peroxisomes and Their Function

• Involved in breaking down harmful substances like hydrogen peroxide.

Plasma Membrane

• Functions as a barrier between cell and environment, regulating entry and exit of substances.

• Roles: Membrane transport, cell signaling, and adhesion.

Semiautonomous Organelles

• Mitochondria and Chloroplasts: Can grow and divide but rely on cellular components for functionality.

Mitochondria and ATP Production

• Main role in ATP synthesis, involvement in various metabolic processes.

Chloroplasts and Photosynthesis

• Capture light energy for organic molecule synthesis, essential in plants and algae.

Genomes of Chloroplasts and Mitochondria

• Both organelles have their own DNA, replicating autonomously.

Endosymbiosis Theory

• Suggests that mitochondria and chloroplasts originated from symbiotic relationships with bacteria.

Protein Sorting in Eukaryotic Cells

• Proteins sorted to their destinations via amino acid sequences that act as signals.

• Cotranslational Sorting: Occurs concurrently with translation for proteins synthesized into the ER.

Systems Biology of Cells

• Systems biology examines how cellular complexity arises through interactions among cellular components.

• Eukaryotic cells consist of interconnected parts: nucleus, cytosol, endomembrane system, and organelles.

Comparison of Cell Structures

Summary

• The cell is the fundamental unit of life.

• Understanding cell structure and function is essential for studying biological processes, disease mechanisms, and the evolution of life forms.