Prokaryotes, Protists, and the Evolution of Eukaryotes: Structure, Function, and Diversity

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Introduction to Prokaryotes

Prokaryotic and Eukaryotic Cells

All living organisms are classified into two broad cell types: prokaryotic and eukaryotic cells. These categories represent the most fundamental division in cellular life.

Prokaryotic Cells: Lack a membrane-bound nucleus and other membrane-bound organelles. Includes both Bacteria and Archaea.
Eukaryotic Cells: Possess a true nucleus and various membrane-bound organelles. Includes Protists, Fungi, Plants, and Animals.

Example: Bacteria and Archaea are prokaryotes, while plants and animals are eukaryotes.

Labeled diagram of a bacterial cell showing nucleoid, ribosomes, cell wall, capsule, and flagellum

Features of Bacterial Cells

Bacteria are the most abundant and diverse organisms on Earth. Their cellular structure is distinct from eukaryotes in several ways:

DNA: Circular DNA located in a region called the nucleoid.
Ribosomes: Small (70S) ribosomes for protein synthesis.
Cell Division: Divide by binary fission, a simple form of asexual reproduction.
Cell Wall: Most have a cell wall composed of peptidoglycan (in Bacteria).
Motility: Many possess flagella for movement.

Diagram of a bacterial cell highlighting chromosome in nucleoid and ribosome

Comparison of Prokaryotic and Eukaryotic Cells

Key differences between prokaryotic and eukaryotic cells include:

Cell Size: Prokaryotes are generally smaller (1–10 μm) than eukaryotes (10–100 μm).
Nucleus: Prokaryotes lack a nucleus; eukaryotes have a true nucleus.
Organelles: Prokaryotes lack membrane-bound organelles; eukaryotes possess them.
Ribosome Size: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes.
Genetic Material: Prokaryotes have circular DNA; eukaryotes have linear DNA.

Table comparing domains of life: Bacteria, Archaea, Eukarya by cell type, nucleus, and organelles

Introduction to Archaea

Characteristics of Archaea

Archaea (singular: archaeon) are one of the three domains of life. Like bacteria, they have a prokaryotic cell structure but differ in several key aspects:

Cell Structure: Prokaryotic, but with unique ribosomal RNA (rRNA) sequences.
Cell Wall: Lack peptidoglycan (unlike bacteria).
Habitats: Many are extremophiles, thriving in extreme environments (e.g., high temperature, salinity, or acidity), but some live in moderate environments.

Diagram showing Bacteria and Archaea as prokaryotes, with examples of extremophiles

Prokaryotic Metabolism

Nutritional Factors of Microbial Growth

Microbes are classified based on three key nutritional factors:

Energy Source: Where the organism gets its energy (light or chemicals).
Electron Source: The original molecule supplying electrons to the electron transport chain (ETC).
Carbon Source: The original carbon-based molecule used to build cell components.

Diagram summarizing the three nutritional factors: energy, electron, and carbon source

Energy Source: Phototrophs vs. Chemotrophs

Phototrophs: Obtain energy from sunlight.
Chemotrophs: Obtain energy from chemical compounds (organic or inorganic).

Cartoon comparing phototrophs (sunlight) and chemotrophs (chemicals)

Electron Source: Lithotrophs vs. Organotrophs

Lithotrophs: Use electrons from reduced inorganic molecules (e.g., H2O, Fe2+).
Organotrophs: Use electrons from organic molecules (e.g., glucose).

Cartoon comparing lithotrophs (inorganic electron source) and organotrophs (organic electron source)

Carbon Source: Autotrophs vs. Heterotrophs

Autotrophs: Use carbon fixation (e.g., from CO2) to make their own food and cell components.
Heterotrophs: Obtain carbon by consuming organic molecules.

Cartoon comparing autotrophs (fix CO2) and heterotrophs (consume organics)

Nutritional Diversity Among Microbes

Microbes can be classified by combinations of their energy, electron, and carbon sources. For example:

Photoautotrophs: Use light for energy and CO2 for carbon (e.g., cyanobacteria).
Chemoheterotrophs: Use chemicals for energy and organic molecules for carbon (e.g., humans).
Photoheterotrophs: Use light for energy and organic molecules for carbon.
Chemoautotrophs: Use chemicals for energy and CO2 for carbon.

Table summarizing energy, electron, and carbon sources for different nutritional types

Introduction to Protists

What is a Protist?

Protists are a diverse, paraphyletic group of eukaryotic organisms that are not classified as plants, animals, or fungi. Most eukaryotes are protists, and they likely represent the first eukaryotic lineages.

Protists are not a true monophyletic group but are grouped for convenience.
All protists have a membrane-bound nucleus and other eukaryotic organelles.
They can be unicellular or multicellular, and exhibit a wide range of morphologies and lifestyles.

Diversity of Protist Structure & Function

Protists are highly diverse in structure and function:

Most are unicellular and live in moist or aquatic environments.
They may be heterotrophic, photosynthetic, or mixotrophic (combining both modes).
Some can engulf nutrients via phagocytosis.

Evolution of Protists

Primary and Secondary Endosymbiosis

Endosymbiosis is a symbiotic relationship where one organism lives inside another. This process played a key role in the evolution of eukaryotes:

Primary Endosymbiosis: A host cell engulfs a prokaryotic cell (e.g., origin of mitochondria from an aerobic bacterium).
Secondary Endosymbiosis: A host cell engulfs a eukaryotic cell (e.g., red or green algae), leading to organelles with multiple membranes.

Advantage: The engulfed cell provided the host with ATP or photosynthetic capability, increasing survival and energy efficiency.

Protist Life Cycles

General Features

Protists exhibit a wide variety of life cycles, which may be sexual, asexual, or both. Some involve alternation between haploid and diploid forms, and may require multiple hosts.

Plasmodium Life Cycle (Malaria)

Plasmodium is a protist that causes malaria. Its life cycle involves both humans and mosquitoes, with sexual and asexual stages:

Infected mosquito injects sporozoites into human blood.
Sporozoites infect liver cells, become merozoites, and reproduce asexually.
Merozoites infect red blood cells, causing symptoms.
Some merozoites become gametocytes, which are taken up by another mosquito.
Sexual reproduction occurs in the mosquito, completing the cycle.

Laminaria Life Cycle: Alternation of Generations

Some protists, like Laminaria (a brown alga), exhibit alternation of generations:

Sporophyte (2n): Multicellular diploid form produces haploid spores by meiosis.
Gametophyte (n): Multicellular haploid form produces gametes by mitosis.
Fertilization produces a diploid zygote, which grows into a new sporophyte.

Paramecium Life Cycle: Conjugation & Asexual Reproduction

Paramecium exchanges DNA sexually via conjugation and reproduces asexually by binary fission:

Conjugation involves exchange and fusion of micronuclei, resulting in genetic recombination.
Binary fission increases population size.

Land Plants: Evolution and Adaptations

Origin and Major Groups

Land plants evolved from freshwater green algae and developed adaptations for terrestrial life:

Nonvascular plants (Bryophytes): Gametophyte-dominant, lack lignified vascular tissue.
Seedless vascular plants: Sporophyte-dominant, possess vascular tissue (xylem and phloem).
Seed plants: Include gymnosperms (naked seeds) and angiosperms (seeds in fruits/flowers).

Adaptations for Terrestrial Life

Cuticle: Waxy covering to prevent water loss.
Stomata: Pores for gas exchange, regulated by guard cells.
Vascular Tissue: Xylem (water/mineral transport) and phloem (nutrient transport).
Lignin: Provides structural support for vertical growth.

Alternation of Generations in Plants

Gametophyte: Haploid, produces gametes by mitosis.
Sporophyte: Diploid, produces spores by meiosis.
Homospory: One type of spore.
Heterospory: Microspores (male) and megaspores (female).

Animals: Structure, Development, and Diversity

General Features of Animals

Multicellular, heterotrophic eukaryotes.
Lack cell walls; have extracellular matrix for support.
Motile at some stage; most reproduce sexually.
Development involves cleavage, blastula, and gastrulation.

Animal Development and Body Plans

Cleavage: Rapid mitotic divisions of the zygote.
Blastula: Hollow ball of cells.
Gastrulation: Formation of germ layers (ectoderm, mesoderm, endoderm).
Protostomes: Mouth develops from blastopore.
Deuterostomes: Anus develops from blastopore.
Coelom: Body cavity derived from mesoderm.

Major Animal Groups

Lophotrochozoans: Includes flatworms, mollusks, annelids.
Ecdysozoans: Includes arthropods (insects, crustaceans) and nematodes; characterized by molting (ecdysis).
Echinoderms: Deuterostomes with water vascular system (e.g., starfish).
Chordates: Defined by notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail.

Vertebrate Evolution

Jawless fish: Hagfish, lampreys.
Gnathostomes: Jawed vertebrates (fish, amphibians, reptiles, mammals).
Tetrapods: Four-limbed vertebrates (amphibians, reptiles, mammals).
Amniotes: Tetrapods with amniotic eggs (reptiles, birds, mammals).

Primates and Human Evolution

Primates: Mammals with large brains, opposable thumbs.
Hominids: Great apes, including humans.
Homo: Genus including modern humans and extinct relatives.