Evolutionary Origin of Cells and Their General Features

Chapter 4: Evolutionary Origin of Cells and Their General Features

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 Theory

• Cell theory is credited to Schleiden and Schwann with contributions from Virchow.

• Key points 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 the process of cell division.

Microscopy

• Microscope: A magnification tool that enables visualization of cell structure.

• Micrograph: An image taken with a microscope.

• Important parameters in microscopy include:

  • Resolution: Ability to observe two adjacent objects as distinct.

  • Contrast: Difference in appearance between structures, can be enhanced by special dyes.

  • Magnification: The ratio of the size of the image produced to the actual size.

Types of Microscopes

1. Light Microscope:

   • Utilizes light for illumination.

   • Resolution is about 0.2 micrometers.

2. Electron Microscope:

   • Uses a beam of electrons for illumination.

   • Superior resolution of 2 nanometers (approximately 100 times better than light microscopes).

Overview of Cell Structure and Function

• Two categories of life:

  1. Prokaryotes:

  • Simple cell structure, lack a nucleus.

  • 2. Eukaryotes:

  • More complex cells with DNA enclosed within membrane-bound nuclei, possessing internal membranes forming organelles.

Prokaryotic Cells

• There are two main categories of prokaryotes:

  1. Bacteria:

  • Size: typically 1 to 10 micrometers in diameter.

  • Abundant in the environment and human bodies; majority are non-harmful but some cause disease.

  1. Archaea:

  • Similar size range as bacteria, less common, often found in extreme environments.

Structure of a Typical Bacterial Cell

• Plasma Membrane: Double layer of phospholipids and proteins, serves as a barrier.

• Cytoplasm: Everything contained within the plasma membrane.

• Nucleoid Region: Location of DNA.

• Ribosomes: Sites of protein synthesis.

• Cell Wall: Offers support and protection.

• Glycocalyx: Gelatinous layer for protection and evasion of the immune system.

• Appendages:

  • Pili: Attachment structures.

  • Flagella: Aids in movement.

Eukaryotic Cells

• Nucleus: Contains DNA, is surrounded by a double membrane.

• Organelles: Compartmentalized structures with unique functions.

Animal Cell Components

• Centrosome: Site of microtubule growth.

• Nuclear Pore: Regulates passage into and out of the nucleus.

• Rough Endoplasmic Reticulum (Rough ER): Site for protein sorting and secretion.

• Smooth Endoplasmic Reticulum (Smooth ER): Site of detoxification and lipid synthesis, calcium storage.

• Mitochondrion: Site of ATP synthesis.

• Peroxisome: Breaks down hydrogen peroxide and other harmful molecules.

• Golgi Apparatus: Modifies, sorts, and secretes lipids and proteins.

• Lysosome: Degrades macromolecules.

• Cytoskeleton: Composed of protein filaments that provide shape and aid in movement.

• Plasma Membrane: Controls movement of substances in and out of the cell.

Plant Cell Components

• Central Vacuole: Storage and regulation of cell volume.

• Chloroplast: Site of photosynthesis, contains thylakoid membranes.

• Similar organelles as animal cells with additional structures such as cell walls.

Droplet Organelles

• Cells can also compartmentalize through liquid-liquid phase separation, known as droplet organelles, example being the nucleolus, where rRNA processing occurs.

The Proteome Determines Cell Characteristics

• Same DNA across different cell types but differing proteomes lead to different cell characteristics.

• Proteome variations depend on:

  • Which proteins are expressed.

  • Levels of expression.

  • Protein subtypes and post-translational modifications.

  • Differences between healthy and cancerous cells.

Cell Surface Area and Volume

• As cell size increases, the surface area-to-volume ratio decreases, affecting the cell's ability to absorb nutrients and export wastes; a higher ratio is beneficial.

Cytosol

• The region outside cell organelles and inside the plasma membrane, involved in various metabolic pathways in eukaryotic cells.

Metabolism

• Metabolism encompasses all chemical reactions within cells.

  • Catabolism: Breakdown of molecules.

  • Anabolism: Synthesis of cellular molecules and macromolecules.

Cytoskeleton

• Composed of three types of protein filaments:

  1. Microtubules: Hollow structures important for cell shape, chromosome movement; dynamic instability allows phase changes between growth and shortening.

  2. Intermediate Filaments: Provide tensile strength and help maintain cell shape, composed of keratins and nuclear lamins.

  3. Actin Filaments (Microfilaments): Support cell shape and strength; dynamic structures engaged in various cellular activities.

Flagella and Cilia

• Flagella: Usually longer, number present varies; facilitate movement.

• Cilia: Usually shorter, often cover the cell surface; both involve microtubules arranged in a 9+2 structure with motion facilitated by the motor protein dynein.

Nucleus and Endomembrane System

• Comprises membranes surrounding the nucleus, endoplasmic reticulum, golgi apparatus, lysosomes, and vacuoles, including plasma membrane.

Nuclear Envelope

• Double-membrane structure enclosing the nucleus with nuclear pores that regulate material passage.

Chromosomes in the Nucleus

• Composed of DNA and proteins called chromatin, organized within the nuclear matrix.

Ribosome Assembly

• Occurs within the nucleolus where ribosomal proteins and RNA are assembled.

Endoplasmic Reticulum (ER)

• Network of membranes, forms flattened tubules called cisternae, enclosing the ER lumen.

  • Rough ER: Studded with ribosomes for protein synthesis, sorting, and glycosylation.

  • Smooth ER: Engaged in detoxification, lipid synthesis, and calcium ion storage.

Golgi Apparatus

• A stack of flattened compartments that modifies, sorts, and packages materials for secretion or use in the cell.

• Two models of transport:

  • Vesicular Transport: Vesicles bud and fuse between compartments.

  • Cisternal Maturation: Compartments mature from cis to trans.

Lysosomes

• Contain acid hydrolases for breaking down organic molecules; involved in autophagy—recycling worn-out organelles.

Vacuoles

• Variability in function across cell types; essential for storage and regulation of organelle volume.

  • Types include central vacuoles in plants, contractile vacuoles in protists, and phagocytic vacuoles in white blood cells.

Peroxisomes

• Catalyze reactions for breaking down molecules, producing hydrogen peroxide, which is subsequently broken down by catalase.

Plasma Membrane Functions

• Serves as a barrier between the cell and extracellular environment.

  • Involved in selective transport, cell signaling, and cell adhesion.

Semiautonomous Organelles

• Includes mitochondria and chloroplasts, capable of growth and division but depend on the rest of the cell for certain components.

Mitochondria and Chloroplasts

• Mitochondria are involved in ATP production and other metabolic functions, featuring inner membranes folded into cristae.

• Chloroplasts conduct photosynthesis, with specific structures including thylakoid membranes and granum stacks.

Endosymbiosis Theory

• Proposes that mitochondria and chloroplasts originated from an endosymbiotic relationship with ancestral prokaryotic cells, providing a basis for the evolution of eukaryotic cells.

Systems Biology of Cells

• Systems biology examines how complex interactions of cellular components give rise to emergent properties of life.

• Eukaryotic cells consist of interconnected parts: nucleus, cytosol, endomembrane system, and semiautonomous organelles, working together to perform complex functions efficiently.

Comparative Cell Complexity