Prokaryote Characteristics

Prokaryotic Structures and Functions

• Some prokaryotes have fimbriae, which allow them to stick to their substrate or other individuals in a colony.
• Pili (or sex pili) are longer than fimbriae and enable prokaryotes to exchange DNA.

Locomotion

• Many bacteria exhibit taxis, the ability to move toward or away from a stimulus.
• Chemotaxis is the movement toward or away from a chemical stimulus.
• Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends.
• Bacterial flagella are composed of a motor, hook, and filament.

Internal Organization

• Prokaryotic cells usually lack complex compartmentalization.
• Some prokaryotes have in-foldings of the plasma membrane that perform metabolic functions.
• Most of the genome consists of a circular chromosome located in the nucleoid region.
• Some species of bacteria also have smaller rings of DNA called plasmids.

Prokaryotic Reproduction

• Prokaryotes reproduce by binary fission, which takes around 1-3 hours.
• They have short generation times, which contributes to their genetic diversity.
• Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries.
• Genetic diversity in prokaryotes is also contributed by rapid reproduction, mutation, and genetic recombination.
• Mutation rates during binary fission are low, but rapid reproduction allows mutations to accumulate rapidly in a population.

Genetic Recombination

• Transformation is the process where a prokaryotic cell takes up and incorporates foreign DNA from the surrounding environment.
• Transduction is the movement of genes between bacteria by bacteriophages (viruses that infect bacteria).
• Horizontal gene transfer is the movement of genes among individuals from different species.

Conjugation

• Conjugation is the process where genetic material is transferred between prokaryotic cells.
• During conjugation, a donor cell attaches to a recipient by a pilus, pulls it closer, and transfers DNA.
• A piece of DNA called the F factor is required for the production of pili.

Metabolism and Nutrition

• Prokaryotes can be categorized by how they obtain energy and carbon.
• Phototrophs obtain energy from light, while chemotrophs obtain energy from chemicals.
• Autotrophs require CO2 as a carbon source, and heterotrophs require an organic nutrient to make organic compounds.
• Energy and carbon sources are combined to give four major modes of nutrition.

Nitrogen Metabolism

• Nitrogen is essential for the production of amino acids and nucleic acids.
• In nitrogen fixation, some prokaryotes convert atmospheric nitrogen (N2) to ammonia (NH3).
• In the cyanobacterium Anabaena, photosynthetic cells and nitrogen-fixing cells called heterocysts exchange metabolic products.

Prokaryotic Metabolism

• Prokaryotic metabolism varies with respect to O2.
• Obligate aerobes require O2 for cellular respiration, while obligate anaerobes are poisoned by O2 and use fermentation or anaerobic respiration.
• Facultative anaerobes can survive with or without O2.

Prokaryotic Diversity

• Domain Archaea shares certain traits with bacteria and other traits with eukaryotes.
• Some archaea live in extreme environments and are called extremophiles.
• Extreme halophiles live in highly saline environments, while extreme thermophiles thrive in very hot environments.
• Methanogens live in swamps and marshes and produce methane as a waste product.

Domain Bacteria

• Proteobacteria include photoautotrophs, chemoautotrophs, and heterotrophs.
• Some proteobacteria are anaerobic, while others are aerobic.
• Chlamydia are parasites that live only within animal cells.
• Spirochetes are helical heterotrophs, which can be parasitic.
• Cyanobacteria are photoautotrophs that generate O2.

Gram-Positive Bacteria

• Actinomycetes decompose organic matter.
• Bacillus anthracis causes anthrax, while Clostridium botulinum causes botulism.
• Some Staphylococcus and Streptococcus can be pathogenic.
• Mycoplasms are the smallest known cells.

Ecological Importance

• Prokaryotes play a major role in the recycling of chemical elements between the living and nonliving components of ecosystems.
• Chemoheterotrophic prokaryotes function as decomposers, breaking down dead organisms and waste products.
• Prokaryotes can increase the availability of nitrogen, phosphorus, and potassium for plant growth.

Symbiosis

• Symbiosis is an ecological relationship in which two species live in close contact: a larger host and smaller symbiont.

Fungal Characteristics

• Fungi are diverse and widespread, with approximately 100,000 described species and an estimated 1.5 million total species
• They are essential for terrestrial ecosystems, breaking down organic material and recycling vital nutrients
• Fungi are heterotrophs, absorbing nutrients from outside their bodies using enzymes to break down complex molecules
• They exhibit diverse lifestyles, including decomposers, parasites, and mutualists

Morphology

• Fungi grow as either filaments (hyphae) or yeasts, with some species capable of growing as both
• Multicellular fungi have a morphology that enhances their ability to absorb nutrients
• Fungi consist of mycelia, networks of branched hyphae adapted for absorption
• A mycelium's structure maximizes its surface area-to-volume ratio
• Fungal cell walls contain chitin
• Most fungi have hyphae divided into cells by septa, with pores allowing cell-to-cell movement of organelles
• Coenocytic fungi lack septa and have a continuous cytoplasmic mass with hundreds or thousands of nuclei

Specialized Hyphae

• Some fungi have hyphae specialized to capture prey
• Some unique fungi have specialized hyphae called haustoria, allowing them to penetrate the tissues of their host
• Most vascular plants have mycorrhizae, mutually beneficial relationships between fungi and plant roots
• Mycorrhizal fungi deliver phosphate ions and minerals to plants
• Types of mycorrhizal fungi include ectomycorrhizal fungi (do not penetrate root cells) and arbuscular mycorrhizal fungi (penetrate root cells)

Phylum Basidiomycota

• Includes mushrooms, puffballs, and shelf fungi, mycorrhizae, and plant parasites
• Defined by a clublike structure called a basidium, a transient diploid stage in the life cycle
• Also called club fungi
• Many basidiomycetes are decomposers of wood
• Life cycle usually includes a long-lived dikaryotic mycelium that reproduces sexually by producing elaborate fruiting bodies called basidiocarps
• Mushrooms are examples of basidiocarps
• Basidia in a basidiocarp are sources of sexual spores called basidiospores

Conidia

• Not formed inside sporangia, but produced asexually at the tips of specialized hyphae called conidiophores

Ecological Roles of Fungi

• Decomposition: fungi are efficient decomposers of organic material, including cellulose and lignin
• Recycling: fungi perform essential recycling of chemical elements between the living and nonliving world
• Bioremediation: fungi are used in bioremediation projects