Microbes, Plants and Animals Flashcards
Major Episodes in the History of Life: Earth was formed about 4.6 billion years ago.
Prokaryotes, having cells that lack true nuclei;
- evolved by about 3.5 billion years ago,
- began oxygen production about 2.7 billion years ago as a result of photosynthesis by autotrophic prokaryotes,
- lived alone for about 1.7 billion years, and
- continue in great abundance today.
Major Episodes in the History of Life (Cont.)
The Cambrian explosion, about 541 million years ago, resulted in the evolution of all major animal body plans and all the major groups.
About 500 million years ago plants, fungi, and insects began to colonize the land.
At the end of the Mesozoic, 65 million years ago, flowering plants, birds, and mammals, including primates, began to dominate the landscape.
The origin of modern humans, Homo sapiens, occurred roughly 195,000 years ago.
Formation of Pre-Cells
A key step in the origin of life would have been the isolation of a collection of organic molecules within a membrane.
Researchers have demonstrated that pre-cells could have formed spontaneously from fatty acids.
Prokaryotes
- lived and evolved all alone on Earth for about 2 billion years,
- are found wherever there is life,
- have a collective biomass that is at least ten times that of all eukaryotes,
- thrive in habitats too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote, and
- cause about half of all human diseases.
However, prokaryotes also form our microbiota, the community of microorganisms that live in and on our bodies, which help to
- supply essential vitamins,
- allow us to extract nutrition from food molecules that we cannot otherwise digest,
- decompose dead skin cells, and
- guard against disease-causing intruders.
Prokaryotes also help decompose dead organisms and other waste materials, returning vital chemical elements such as nitrogen to the environment.
Prokaryotic Nutrition
The prokaryotic pathways that transform energy and matter are far more diverse than those of eukaryotes.
- Some species harvest energy from inorganic substances such as ammonia (NH3) and hydrogen sulfide (H2S).
- The metabolic talents of prokaryotes make them excellent symbiotic partners with animals, plants, and fungi.
In addition to photosynthesis, many cyanobacteria are also capable of nitrogen fixation, the process of converting atmospheric nitrogen (N2) into a form usable by plants.
- Symbiosis with cyanobacteria gives plants such as the water fern Azolla an advantage in nitrogen-poor environments.
- This tiny, floating plant has been used to boost rice production for more than a thousand years.
The Ecological Impact of Prokaryotes: Prokaryotes and Chemical Recycling
As a result of their nutritional diversity, prokaryotes perform a variety of ecological services that are essential to our well-being.
- Life depends on the recycling of chemical elements between the biological and physical components of ecosystems, an example of interactions within biological systems.
- Prokaryotes play essential roles in these chemical cycles.
- Prokaryotes also promote the breakdown of organic wastes and dead organisms.
The Two Main Branches of Prokaryotic Evolution: Bacteria and Archaea
By comparing diverse prokaryotes at the molecular level, biologists have identified two major branches of prokaryotic evolution:
1) Bacteria and
2) Archaea.
Thus, life is organized into three domains:
1) Bacteria and
2) Archaea.
3) Eukarya.
Archaea
Archaea are abundant in many habitats, including places where few other organisms can survive.
1) One group of Archaea, the extreme thermophiles (“heat lovers”), live in very hot water.
2) Another group is the extreme halophiles (“salt lovers”), Archaea that thrive in salty environments.
3) A third group of Archaea are methanogens, which live in anaerobic (oxygen-free) environments and give off methane as a waste product.
- They are abundant in the mud at the bottom of lakes.
- Great numbers inhabit the digestive tracts of animals.
Protists
The fossil record indicates that eukaryotes evolved from prokaryotes around 2 billion years ago.
- These primal eukaryotes were the predecessors of the great variety of modern protists and ancestral to all other eukaryotes, plants, fungi, and animals.
- The term protists is not a taxonomic category.
- Hypotheses about protist phylogeny (and thus, classification) are changing rapidly as new information causes scientists to revise their ideas.
- Thus, protist is a catch-all category that includes all eukaryotes that are not fungi, animals, or plants. Most, but not all, protists are unicellular.
Protist habitats are also diverse.
- Most protists are aquatic, living in oceans, lakes, and ponds.
- Some are found almost anywhere there is moisture, including terrestrial habitats such as damp soil and leaf litter.
- Others are symbionts that reside in the bodies of various host organisms.
- Because the classification of protists remains a work in progress, our brief survey of protists is not organized to correspond with any hypothesis about phylogeny.
Unicellular and Colonial Algae
Many unicellular algae are components of phytoplankton, the mostly microscopic photosynthetic organisms that drift near the surfaces of ponds, lakes, and oceans.
- Each dinoflagellate species has a characteristic shape reinforced by external plates made of cellulose.
- Diatoms have glassy cell walls containing silica, the mineral used to make glass.
- Green algae are named for their grass-green chloroplasts.
Seaweeds
- Seaweeds are large, multicellular marine algae, that grow on rocky shores, are only similar to plants because of convergent evolution, and are most closely related to unicellular algae.
- Seaweeds are classified into three different groups, based partly on the types of pigments present in their chloroplasts:
1) green algae,
2) red algae, and
3) brown algae (some of which are known as kelp).
Plant
A plant is a multicellular eukaryote that carries out photosynthesis and has a set of adaptations for living on land.
- Photosynthesis distinguishes plants from the animal and fungal kingdoms, which are also made up of eukaryotic, multicellular organisms.
- Algae lack terrestrial adaptations and thus are classified as protists rather than plants.
- Some plants live in water, but these aquatic plants evolved from terrestrial ancestors.
Terrestrial Adaptations of Plants
Living on land requires a special set of adaptations.
- Bodies that were upright in the buoyant water go limp on land and soon shrivel in the drying air.
- In addition, algae are not equipped to obtain carbon dioxide needed for photosynthesis from the air.
Roots typically have many fine branches that thread among the grains of soil, providing a large surface area that maximizes contact with mineral-bearing water in the soil—just one example of how plant organ systems exemplify the relationship between structure and function.
Most plants have symbiotic fungi associated with their roots. These root-fungus combinations, called mycorrhizae (“fungus roots”), enlarge the root’s functional surface area. Mycorrhizae are key adaptations that made it possible for plants to live on land.
Leaves are the main photosynthetic organs of most plants, utilizing
- stomata, microscopic pores found on a leaf’s surface, for the exchange of carbon dioxide and oxygen with the atmosphere,
- a waxy layer coating on the leaves and other aerial parts of most plants called the cuticle, helping the plant body retain water, and
- vascular tissue, a network of tube-shaped cells that branch throughout the plant, for the transport of vital materials between roots and shoots.
Reproductive Adaptations on Land
Adapting to land also required a new mode of reproduction.
- For the protist algae, water ensures that gametes (sperm and eggs) and developing offspring stay moist.
- Water also provides a means of dispersing the gametes and offspring.
Plants, however, must keep their gametes and developing offspring from drying out in the air and produce their gametes in a structure that allows them to develop without dehydrating.
- The egg remains within tissues of the mother plant and is fertilized there.
- In plants, but not algae, the zygote (fertilized egg) develops into an embryo while still contained within the female parent, which protects the embryo and keeps it from dehydrating.
Fungi
Fungi are eukaryotes; most are multicellular, but many have body structures and modes of reproduction unlike those of other organisms.
- A mushroom is more closely related to you than it is to any plant! Molecular studies indicate that fungi and animals arose from a common ancestor more than 1 billion years ago.
- Fungi recycle vital chemical elements back to the environment in forms other organisms can assimilate and form mycorrhizae, fungus-root associations that help plants absorb mineral and water from the soil.
