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Biological Organisms

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Biological Organisms

Biology studies organisms and their life-sustaining processes. But what exactly are living organisms? How do we distinguish living organisms like mosses and elephants from nonliving things like rocks and smartphones?

In the following, we will define biological organisms, identify their key characteristics, discuss how they are classified, and touch on how they interact with each other and their environment.

What is the meaning of biological organisms?

Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.

What are the characteristics shared by biological organisms?

Properties of life: Order

Biological organisms are organized and coordinated structures made up of one or more cells, which are tiny structures we consider as the fundamental unit of life.

Each cell is incredibly complex: at the fundamental level, it is composed of atoms. These atoms make up molecules. These molecules come together to form complex compartmentalized cell structures called organelles.

Then, in multicellular organisms, multiple cells come together to form tissues, which then form structures with specialized functions called organs, which, in turn, work together in organ systems.

Properties of life: Response to stimuli

Stimuli (singular: stimulus) are things that can elicit a response from a living organism.

Organisms can respond by moving toward the stimulus; this is called a positive response. They can also respond by moving away from the stimulus; this is called a negative response.

For example, plants exposed to light stimuli might respond by bending toward the light.

Properties of life: Reproduction

Organisms can replicate themselves by passing on their genetic information to their offspring. By passing on their genetic information, the offspring will belong to the same species and have similar traits.

Properties of life: Growth and development

Organisms grow and develop, meaning their structures and functions change over time. This change is determined by a combination of the genetic information passed on to the individual organism as well as its environment.

The organism acquires materials or energy from its environment to allow for such changes to take place.

Properties of life: Regulation

Organisms require multiple complex regulatory mechanisms to coordinate their internal processes, such as transporting nutrients and responding to stimuli.

Properties of life: Homeostasis

Homeostasis is the ability of organisms to maintain internal balance while responding to external conditions.

Organisms need to maintain homeostasis because their internal structures function optimally within a set of internal and external conditions.

For example, proteins can break down or misfold when exposed to high temperatures and pH levels. For this reason, the human body needs to maintain temperatures close to 37 °C (or 98.6 °F).

Properties of life: Energy processing

Organisms need an energy source to carry out their metabolic processes. Some organisms might produce their own food by capturing energy from the sun and converting it to chemical energy, while other organisms might obtain energy by eating other organisms.

Do all biological organisms need oxygen? What are aerobic and anaerobic biological organisms?

Considering how we often hear that we need oxygen to live, you might think that all biological organisms need oxygen. However, for the first two billion years of the Earth’s existence, the atmosphere contained no free molecular oxygen (O2).

Based on the fossil record, 3.5 billion-year-old microbial mats found in hot springs and hydrothermal vents are the earliest known organisms on Earth. These microbes were anaerobic, which means they did not require oxygen. Over time, other anaerobic organisms emerged, including cyanobacteria which took up water during photosynthesis and released oxygen as a by-product.

That means we can trace the production of the world’s first free molecular oxygen to the emergence of these photosynthetic cyanobacteria about 2.6 billion years ago. With this, oxygen slowly accumulated in the atmosphere, enabling the evolution of other more complex life forms, including aerobic organisms (including us humans) that require oxygen to live.

Classification of biological organisms

Biological organisms can be classified into three groups called domains: bacteria, archaea, and eukarya. This classification is illustrated in the phylogenetic tree.

A phylogenetic tree shows the evolutionary relationships among organisms through a diagram with branches and nodes.

The nodes represent the points in evolutionary history when an ancestor forms two new, distinct species, while the length of each branch corresponds to the amount of time that elapsed since the split.

Take some time to review the phylogenetic tree to better understand the unity and diversity of biological organisms.

The organisms that comprise bacteria and archaea are prokaryotic, meaning they are single-celled or colonial organisms that lack membrane-bound organelles. Instead of being enclosed in a nucleus, their DNA is organized into a single circular chromosome. As prokaryotes, they reproduce through fission, a process where an individual cell replicates its chromosome and splits into two distinct cells.

On the other hand, the members of domain eukarya are single-celled or multicellular organisms with eukaryotic cells, which means they have membrane-bound organelles, including a nucleus that separates their DNA from other parts of the cell. Unlike prokaryotes, eukaryotes have multiple linear chromosomes. Unlike prokaryotes, some eukaryotes can reproduce sexually.

What are the three domains of life? What are examples of biological organisms from each domain?

Now that we have cited important similarities and differences among the three domains let us take a closer look at their characteristics and cite some examples.

Domain Bacteria

Bacteria are a highly-diverse group of prokaryotic organisms that we can encounter in our everyday lives. Individual bacteria have three basic shapes:

  • Coccus: spherical

  • Bacillus: rod-like

  • Vibrio, spirillum, or spirochete: curved

Bacteria are so small that the average rod-shaped individual is about 2 micrometers long and half a micrometer wide, while the average spherical bacterium is around 1 micrometer in diameter.

Because of their size, we need to use microscopes to examine their internal and external structures.

Escherichia coli is an example of a bacillus bacteria. It is typically found in the intestines of humans and other animals. While many are harmless, some strains of E. coli are pathogenic. The consumption of water contaminated with these strains of E. coli can cause diarrhea and other gastrointestinal illnesses.

Streptococcus pneumoniae is an example of a coccus bacteria. It is one of the most common causes of bacterial pneumonia, which can affect one or more regions of the lungs.

Domain Archaea

Archaea are also prokaryotic organisms but have molecular characteristics that set them apart from bacteria. These include the following characteristics:

  • Their membrane lipids are composed of branched hydrocarbon chains attached to glycerol by ether linkages (Fig. 2).

  • Their cell walls do not have peptidoglycan, a substance typically found in bacteria cell walls.

  • Their ribosomal RNA (a molecule that forms the protein-synthesizing organelle called a ribosome) is different from those of bacteria and eukarya.

Another distinguishing feature of archaea is their ability to live in extreme environments, which can be inhospitable for other living organisms.

For example, Pyrolobus fumarii was found living in hydrothermal vents where temperatures can go up to 113 °C (235 °F), representing the upper limit of life.

On the other hand, species of Picrophilus were found growing in extremely acidic soils in Japan, where the pH can go as low as 0.

Domain Eukarya

As mentioned earlier, organisms under the domain eukarya are different from archaea and bacteria mainly due to the presence of membrane-bound organelles like the nucleus.

You might find references that identify four kingdoms under the domain eukarya, namely:

  • Plantae (or Plants)are multicellular organisms that produce their own food by photosynthesis and absorption. Their cells have cell walls and are typically organized into tissues.

    • Plants include mosses, ferns, conifers, and flowering plants.

  • Animalia (or Animals) are multicellular organisms that do not carry out photosynthesis and obtain nutrients by eating and digesting other organisms.

    • Examples of animals include sponges, insects, birds, and humans.

  • Fungi are unicellular or multicellular organisms with cell walls. Their cells are not organized into tissues. They do not undergo photosynthesis; instead, they absorb nutrients in their dissolved form from the environment.

    • Examples of fungi include yeasts, molds, mildew, and mushrooms.

  • Protista (or protists) are mostly unicellular, but some are colonial and multicellular species. They are diverse in terms of their feeding patterns, reproduction, and life cycles.

    • Examples of protists include algae, slime molds, and dinoflagellates.

It is important to note that the classification of eukaryotes has been changing in the past years due to recent findings revealing genetic and evolutionary relationships among eukaryotes.

An emerging hypothesis dissolves the kingdom Protista and divides eukaryotes into four supergroups: excavata, SAR, Archaeplastida, and unikonta. This classification was proposed because DNA evidence shows that some protists are more closely related to plants, animals, or fungi than to other protists. As such, all of these supergroups include protists.

For example, Archaeplastida includes red algae, green algae, and plants because they share a common ancestor: a cell that devoured a photosynthetic cyanobacterium. On the other hand, unikonts include animals, fungi, and some protists, which are grouped together because of their shared ancestry.

What is a biological community of interacting organisms?

Organisms interact with each other on various levels.

Individuals of a species that live together in a specific area are collectively called a population.

For example, all the pine trees in a specific forest can be considered one pine population.

When different populations of living organisms inhabit and interact in the same area, they are called a community.

For example, all trees, insects, and animals in the same forest form a forest community.

The sum of all living organisms and nonliving components of their physical environment constitute an ecosystem.

For example, the forest is an ecosystem consisting of living organisms (such as plants and animals) and nonliving things (such as water, wind, and soil).

The collection of all ecosystems on Earth is called the biosphere. The biosphere represents all the zones of life.

Biological Organisms - Key takeaways

  • Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.
  • Biological organisms share many characteristics including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.
  • Aerobic organisms require oxygen, while anaerobic organisms do not.
  • Biological organisms can be classified into three groups called domains: bacteria, archaea, and eukarya.
  • Organisms interact with each other on various levels: population, community, ecosystem, and biosphere.

References

  1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
  2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  3. Kaiser, Gary. “1.3: Classification - The Three Domain System - Biology LibreTexts.” Biology LibreTexts, 24 Dec. 2015.
  4. Encyclopedia Britannica. “Bacteria - Diversity of Structure of Bacteria.” Accessed 17 Sept. 2022.
  5. Encyclopedia Britannica. “Archaea | Definition, Characteristics, and Examples.” Accessed 17 Sept. 2022.
  6. OpenStaxCollege. “Groups of Protists – Biology.” Accessed 20 Aug. 2022.
  7. Georgia Tech Biological Sciences. "Prokaryotes: Bacteria and Archaea | Organismal Biology.” Accessed 17 Sept. 2022.
  8. Briggs, George M. “Chapter 1: Organisms – Inanimate Life.” Accessed 17 Sept. 2022.

Frequently Asked Questions about Biological Organisms

Biological organisms are individual living entities that share key characteristics or functions, including order, response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.

5 examples of biological organisms are E. coli bacteria, ferns, humans, mushrooms, and algae.

Humans are aerobic organisms, which means we need oxygen to live. 

Biological organisms are classified into three groups called domains: bacteria, archaea, and eukarya. This classification is based on their evolutionary relationships. 

A biological community of interacting organisms and their physical environment constitute ecosystem.

Final Biological Organisms Quiz

Question

______ are a diverse group of unicellular, motile, and heterotrophic eukaryotic organisms that feed on bacteria, other protozoa, organic matter, and even fungi. 

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Protozoa  

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Protozoa are also known as _____
-like protists  

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animal

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True or false: all protozoa are motile. 

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False

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_____ are protists that have the ability to change shape in order to move. 

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Amoebas

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Three modes of protozoa lomocotion:

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Pseudopod, Cilia and Flagella

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Amoeba use ______ for movement.

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Pseudopods

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In amoebas, _____ vacuoles contain engulfed food undergoing digestion. 


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Food  

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_____ protozoa use a flagellum to move.

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Flagellate

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True or false: Flagellate protozoa are divided into zooflagellates and phytoflagellates.

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True

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______ have two nuclei present: a macronucleus and a micronucleus.

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Ciliates 

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______ are nonmotile, meaning that they are not able to move by themselves.

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Sporozoa

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Most sporozoa are considered ______. 

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Parasitic

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Four subphyla of Protozoa

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sarcomastigophora, sporozoa, cnidospora, and cilosphora

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True or false: Pathogenic protozoans hey can cause different infections, such as intestinal, urogenital, and blood and tissue infections

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True

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Cryptosporidium parvum is a type of _______ protozoa parasite.

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sporozoa

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Fungi are defined as spore-producing  _______ organisms found in the Fungi kingdom. 

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eukaryotic 

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What species of fungi is known to produce mycotoxins and cause ergot poisoning? 

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Claviceps purpurea

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The kingdom fungi is classifying into ____ phyla.

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seven

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Which are the seven phyla in the kingdom Fungi? 

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  • Ascomycota

  • Basidiomycota

  • Glomeromycota

  • Microsporidia

  • Blastocladiomycota

  • Neocallimastigomycota

  • Chytridiomycota

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While animals engulf food and plants photosynthesize, fungi have: 

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 the ability to absorb externally digested nutrients.

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Which is the following is true about fungi?

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Fungi are achlorophyllous

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_____ fungi obtain their food by infecting living organisms. 

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Parasitic 

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____  obtain their food by attacking dead organic matter.

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Saprobes

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 _______ fungi may have the ability to reproduce by sexual or asexual spores.

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Multicellular 

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True or false: Yeast is a unicellular fungus and reproduce by budding or binary fission. 

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True

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The body of a fungus is called a _____ (or fruiting body), and it is used for reproduction.

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thallus  

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Most fungi are filamentous, and contain individual filaments called ____


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hyphae

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A network of hyphae is called a _____ and it is used for nutrient absorption. 

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mycelium

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Fungi in the phylum basidiomycota have have a septate mycelium and produce sexual spores called _____.

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Basidiospores

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The hallucinogenic mushroom Pcylocibe cubensis is a type of ______.

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Basidiomycete

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______ is the fusion of cytoplasm from two different hyphae, forming a dikaryon.

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Plasmogamy

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Penicillium chrysogenum -is a type of Penicillium fungus that can be used to develop the _______.


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antibiotic penicillin.

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What are anaerobic organisms?

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Anaerobic organisms are those that do not need oxygen to live.

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Do all biological organisms require oxygen to live? Explain. 

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The earliest life forms were anaerobic, meaning they do not require oxygen to live. It was not until 2.6 billion years ago when photosynthetic cyanobacteria first produced oxygen, giving rise to aerobic organisms like us humans.

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___ refers to anything that can elicit a response from a living organism.

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Stimuli

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___ is the ability of organisms to maintain internal balance while responding to external conditions.

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Homeostasis

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The organisms that comprise bacteria and archaea are _____.

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Prokaryotic

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What are prokaryotes?

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single-celled or colonial organisms that lack membrane-bound organelles

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Eukaryotic DNA is organized into ___.

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multiple, linear chromosomes

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What are the three domains of life?

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Archaea, bacteria, and eukarya

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How can we distinguish archaea from bacteria?

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Archaea are also prokaryotic organisms like bacteria, but have molecular characteristics that set them apart from bacteria. These molecular differences can be found in their cell membrane, cell wall, and ribosomal RNA. They also have the ability to live in extreme environments. 

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What are plants?

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Plants are multicellular organisms that produce their own food by photosynthesis and absorption. Their cells have cell walls and are typically organized into tissues. 

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Animals are __.

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multicellular organisms that do not carry out photosynthesis and obtain nutrients by eating and digesting other organisms

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Compare and contrast plants and fungi.

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Both plants and fungi are multicellular organisms with cell walls, but the cells in plants are typically organized into tissues, while those in fungi do not organize into true tissues. Also, while plants undergo photosynthesis, fungi do not; instead, they absorb dissolved nutrients from the environment. 

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Algae, slime molds, and dinoflagellates are examples of ____.

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protists

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Individuals of a species that live together in a specific area are collectively called a _____

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population

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When different populations of living organisms inhabit and interact in the same area, they are called a _____


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community

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The sum of all living organisms and nonliving components of their physical environment constitute an _____

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ecosystem

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Why is the classification of organisms under domain eukarya changing in the past years?

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The classification of eukaryotes has been changing in the past years due to recent findings revealing genetic and evolutionary relationships among eukaryotes. 

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What are the three basic shapes of bacteria?

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The three basic shapes of bacteria are coccus (spherical), bacillus (rod-like), and vibriospirillum, or spirochete (curved).

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