Organisms and their Classification

Organisms Organisms are living things that are made up of cells. They share 6 important characteristics. They have cellular organization, contain similar chemicals, use energy, respond to their surroundings, grow and develop, and reproduce.

Cells A cell is a basic unit of structure and function in an organism. Unicellular, or single-celled, organisms include bacteria, the most numerous organisms on Earth. Multicellular organisms are composed of many cells. The cells of organisms use energy to grow and repair injured parts.

Prokaryotic vs Eukaryotic The difference between prokaryotic and eukaryotic is considered to be the most important distinction among groups of organisms. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotic cells do not. Prokaryotes were the only form of life on Earth for millions of years until more complicated eukaryotic cells came into being.

Prokaryotes vs Eukaryotes

Classification Classification, or taxonomy, is a system of categorizing living things. There are seven divisions in the system: (1) Kingdom; (2) Phylum or Division; (3) Class; (4) Order; (5) Family; (6) Genus; (7) Species. Today, a three-domain system of classification is commonly used. The three domains are Bacteria, Archaea, and Eurarya. Within the domains are kingdoms.

Domains and Kingdoms Kingdom is the broadest division. While scientists currently disagree as to how many kingdoms there are, most are now leaning toward a six-kingdom (Animal, Plant, Protista, Archeabacteria, Eubacteria, and Fungi) system. The lowest division is species, which consists of organisms that are capable of interbreeding to produce fertile offspring. Organisms are placed into domains and kingdoms based on their cell type, their ability to make food, and the number of cells in their bodies.

Bacteria Members of the domain Bacteria are prokaryotes. Prokaryotes are single-celled organisms whose cells lack a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle. A nucleus is a dense area in a cell that contains nucleic acids – the chemical instructions that direct the cell’s activities. In prokaryotes, nucleic acids are scattered throughout the cell. Their DNA is kind of floating around in the cell – in the cytoplasm. Most bacterial cells are surrounded by a rigid cell wall that helps protect the cell. The cell wall is the target for antibiotics, as well as for carbohydrates that our immune system uses to detect infection.

Bacteria Some bacteria have flagella. A flagellum is a long, whip like structure that extends from the cell membrane and out through the cell wall. This helps the cell move. Bacteria must have a source of food and a way of breaking down the food to release its energy (respiration). Some bacteria are autotrophs and make their own food. Others are heterotrophs that obtain food by consuming autotrophs or other heterotrophs. Prokaryotic cells come in multiple shapes: cocci (round), bacilli (rods), and spirilla or spirochetes (spiral).

Prokaryotic Cell

Bacteria When bacteria have plenty of food, the right temperature, and other suitable conditions, they thrive and reproduce frequently. They can reproduce by binary fission – a process in which one cell divides to form two identical cells (asexual reproduction – one parent and identical offspring to parent). Others can reproduce by conjugation – a process in which two parents combine genetic material to produce a new organism that differs from both parents (sexual reproduction).

Bacteria Many bacteria can survive harsh conditions by forming endospores. Endospores are small, rounded, thick- walled, resting cells that form inside a bacterial cell. Some bacteria cause diseases and other harmful conditions. However – most are harmless or helpful to people. Bacteria are involved in oxygen and food production, environmental recycling and clean-up, and health maintenance/medicine production. Helpful bacteria produce foods (cheese & pickles). Others actually cause food to spoil. To combat spoilage – food can be heated to a temperature that will kill the bacteria (pasteurization).

Bacteria Heterotrophic bacteria in the soil break down materials for reuse. These bacteria are decomposers – organisms that break down large chemicals in dead organisms into small chemicals.

Archaea Members of the domain Archaea, whose name comes from the Greek word for “ancient”, can be found in some of the most extreme environments on Earth, including hot springs, very salty water, swamps, and the intestines of cows. Scientists think that the harsh conditions in which archaea live are similar to those of ancient Earth. Like bacteria, some archaea are unicellular prokaryotypes and some are autotrophs while others are heterotrophs. Although bacteria and archaea are similar in some ways, there are important differences in the structure and chemical make- up of their cells.

Eukarya Members of the domain Eukarya are eukaryotes – organisms with cells that contain nuclei. Scientists classify organisms in the domain Eukarya into one of four kingdoms: plants, animals, protists, and fungi. Plants are all multicellular eukaryotes. In general, plants are autotrophs and feed almost all of the heterotrophs on land. Animals are multicellular eukaryotes. All animals are heterotrophs. Animals have different adaptations that allow them to find food, capture it, eat it, and digest it.

Protista The protista kingdom is sometimes called the “odds and ends” kingdom because its members are so different from one another. All protists are eukaryotes that cannot be classified as animals, plants, or fungi. All live in moist surroundings. Slime molds are protists. Protists can be autotrophs or heterotrophs. Some can be both! Although many protists are unicellular, some, such as seaweeds, are multicellular. Protists can be divided into three categories: animal-like, plant-like, and fungus-like.

Animal-like Protists Animal-like protists are heterotrophs, and most are able to move from place to place to obtain food. They can also be called protozoans. Some animal-like protists survive by symbiosis – a close relationship between two species where at least one of the species benefits. Other animal-like protists survive by mutualism – in which both partners benefits. Parasites (a type of animal-like protist) feed on their hosts’ cells and body fluids.

Plant-like Protists/Fungi Plant-like protists are called algae. Algae are autotrophs. Algae can exist in a variety of colors because they contain many types of pigments – chemicals that produce color. Fungus-like protists are heterotrophs, have cell walls, and use spores to reproduce. Spores are tiny cells that are able to move at some point in their lives.

Protists

Fungi Fungi are eukaryotes that have cell walls, are heterotrophs that feed by absorbing their food, and use spores to reproduce. They need warm, moist places in which to grow. Fungi are found almost everywhere on land, but only a few live in fresh water.

Fungi A few fungi, such as yeast, are unicellular eukaryotes. Most fungi are multicellular eukaryotes. Hyphae are branching, threadlike tubes that make-up the bodies. What a fungus looks like depends on the arrangement of its hyphae. First, the fungus grows hyphae into a food source. Then digestive chemicals ooze from the hyphae into the food. The digestive chemicals break down the food into small substances that can be absorbed by the hyphae. Some fungi feed on the remains of dead organisms. Others are parasites that break down the chemicals in living organisms.

Fungi

Fungi usually reproduce by making spores. The lightweight spores are surrounded by protective covering and can be carried easily through the air or water to new sites. Spores are produced in reproductions structures called fruiting bodies. Unicellular yeasts use a form of asexual reproduction (identical to the parent) called budding. In budding, a small cell grows from the body of a large, well-fed cell. Sexual reproduction can occur when the hyphae of two fungi grow together and new genetic material is exchanged.

Important Info about Fungi They play an important roles as decomposers and recyclers on Earth. (by breaking down the chemicals in dead organisms) They provide foods for people. (yeast, mushrooms, etc.) They cause disease AND fight disease. (penicillium) They live in symbiosis with other organisms. (The hyphae of some fungi grow among the roots of plants – absorbing water and nutrients from the soil. In return, the fungus feeds on the extra food the plant makes.) A lichen consists of a fungus living in a mutualistic relationship with algae or autotrophic bacteria.

The Plant Kingdom Most plants are autotrophs. All plants are eukaryotes that contain many cells. All plants’ cells are surrounded by a cell wall. Most plants live on land. For plants to survive on land, they must have ways to obtain water and other nutrients from their surroundings, retain water, transport materials within their bodies, support their bodies, and reproduce. Plants make their own food by photosynthesis. They use carbon dioxide gas and water to make food and oxygen.

The Plant Kingdom Plant cells are organized into tissues, groups of similar cells that perform a specific function. Some plants have vascular tissue, a system of tube-like structures inside the plant through which food, minerals, and water move. Vascular tissue also provides strength, stability, and support to a plant. These plants can grow quite tall. Nonvascular plants do not have a well-developed system of tubes for transporting water and other materials. They grow low to the ground.

The Plant Kingdom All plants undergo sexual reproduction that involves fertilization. Fertilization occurs when a sperm cell unites with an egg cell. The fertilized egg is called a zygote. Plants have complex life cycles that include two different stages, the sporophyte stage and the gametophyte stage. In the sporophyte stage, the plant produces spores, tiny cells that can grow into new organisms. A spore develops into the plant’s other stage, called the gametophyte stage. In this stage, the plant produces two kinds of cells: sperm cells and egg cells.

Autotrophs vs Heterotrophs

Autotrophs Autotrophs are organisms that can produce their own food from the substances available in their surroundings using light (photosynthesis) or chemical energy (chemosynthesis). An autotroph is an organism that is able to form nutritional organic substances – ex: plants, algae, and some bacteria. They are often called “self feeders” or “primary producers” that obtain carbon from inorganic sources like carbon dioxide (CO 2 ).

Heterotrophs Heterotrophs cannot make their own food and rely on other organisms – both plants and animals – for nutrition. Heterotrophs get their reduced carbon from other organisms. Basically – heterotrophs depend on autotrophs and other heterotrophs. Herbivores, omnivores, and carnivores are all heterotrophs.

Food Chain

Autotrophs are primary producers that are placed first in the food chain. Heterotrophs that depend on autotrophs and other heterotrophs for their energy level are placed next on the food chain. Herbivores that feed on autotrophs are placed in the second trophic level. Carnivores that eat meat and omnivores that eat all types of organisms are placed next in the tropic level.