BIOLOGY 19.2 ADVENT OF MULTICELLULARITY

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Advent of Multicellularity

Many forms of Multicellularity

More than half the biomass on earth is composed of multicellular organisms; prokaryotes and some eukaryotes.

For these organisms, multicellularity has proven to be a success.

However many other organisms have found success not as individual cells but as members of coordinated groups of cells.

Groups cells that live together can have many different levels of cooperation between them.

Cell ColoniesColonies:

Occasionally, the cell walls of bacteria adhere to one another.

Some bacteria, such as cyanobacteria, form filaments, sheets or three-dimensional formations of cells.

These formations are not considered multicellular organisms if the cells do not communicate with each other and coordinate activities.

Such bacteria as these are considered colonial organisms. A colonial organism is a group of cells that are permanently associated with each other but do not communicate with one another.

Stem cell colony

Aggregations of Cells

An aggregation is a temporary collection of cells that come together for a period of time and than separate.

For example, a plasmodial slime mold is a unicellular organism that spends most of it’s life moving about and feeding as a single celled amoebas.

When starved these cells aggregate into a large group.

This web-like mass produces spores which are than dispersed to distant locations where they may find more food.

Plasmodial slime mold

True Multicellularity

A multicellular organism is an organism composed of many cells that are permanently associated with one another.

Multicellularity occurs only in eukaryotes, having eukaryotic cells with internal compartments and a nucleus.

While single cells can not grow larger than normal, multicellular organisms can be quite large, such as whales and elephants or plants such as trees.

True multicellularity occurs when individual cells are in communication with each other and coordinate their activities.

Complex Multicellularity

Complex Multicellularity:

Plants and animals have complex multicellularity.

The specialized cells of plants and animals are organized into structures called tissues and organs.

A tissue is a distinct group of cells with similar structure and function.

Muscle tissue, for example, is a specific tissue composed of many muscle cells working together.

Muscle tissue

Complex Multicellularity

Complex Multicellularity:

Different tissues may be organized into larger structures called organs.

An organ is a specialized structure with a specific function.

Various organs that work together to carry out a body function are called an organ system.

The circulatory system; which is composed of your heart, veins, blood vessels and blood within, is all part an organ system.

Kingdom ProtistaKingdom Protista:

Of the six kingdoms of organisms, the kingdom Protista is the most diverse.

Protista are defined on the basis of a single characteristic: they are eukaryotes that are NOT fungi, plants or animals.

Many are unicellular; in fact all single-celled eukaryotes (except yeasts) are protists.

Some Protists, such as certain kinds of algae, have cell specialization. Protists can vary widely in size ranging from microscopic to as large as trees.

Kingdom Protista Protists are very diverse in many ways.

While all protists have a cell membrane, some-including algae and slime molds, have strong cell walls.

Others, including diatoms and forams, produce shells of glassy silica.

Many protists move about using structures such as flagella, cilia or pseudopods.

Protists normally reproduce asexually by cell division.

When they are under stress, many protists will reproduce sexually. The most important ecologically of protists are the algae that live in the ocean and form the basis for much of the ocean’s food chain.

Kinds of Protists The following six groups

are the main groups recognized by scientists based upon physical or nutritional characteristics.

Protists that use Pseudopodia:

Amoebas are protists

that have flexible surfaces and no cell walls or flagella. These move by using extensions of cytoplasm called pseudopodia.

Kinds of Protists

Protists that use flagella:

Many protists move by the use of flagella; long tail like structures that propel them.

Kinds of Protists

Protists with Double shells:

Diatoms are photosynthetic protists with

unique double shells made of silica.

Diatoms are part of the plankton and may be found in fresh water or in salt water marine environments.

Kinds of Protists

Photosynthetic algae:

Algae are photosynthetic and are distinguishable by the kinds of chlorophyll they contain.

Many algae are multicellular and reproduce sexually.

Algae may be found in both fresh and salt water marine environments.

Kinds of Protists

Fungus-like protists:

Slime molds and water molds are often confused with fungi because they aggregate in times of stress to form spore reproducing bodies.

Slime molds are often found in fresh water, in damp soil, and on forest floors.

Kinds of Protists

Spore forming protists:

Sporozoans are nonmotile unicellular parasites that form spores.

Responsible for many significant diseases, including malaria, sporozoans have complex life cycles.

Kingdom Fungi

Kingdom Fungi:

Fungi are a very unusual and successful group of organisms.

Although most fungi are multicellular, one group, yeasts, are unicellular.

The cell walls of all fungi cells contain chitin, the same tough material that is found in a crab’s shell.

The bodies of fungi consist of long strands of cells that are connected end to end and that share cytoplasm. These slender strands of fungi are called hyphae.

Kingdom Fungi

Kingdom Fungi:

Often, these hyphae pack together to form complex reproductive structures, such as mushrooms.

The fungi reproduce by a variety of sexual and asexual methods.

Kingdom Fungi

Kingdom Fungi:

In the past, fungi were classified as part of the plant kingdom. Like plants, fungi do not move from place to place.

In addition, the general appearance of fungi is similar to many plants.

But fungi lack the green pigment chlorophyll that allows plants to carry out photosynthesis and make their own food.

Fungi must take their food from other sources thus, like animals, they are considered heterotrophs.

Kingdom Fungi

Kingdom Fungi:

Unlike most animals however, fungi do not ingest food.

Instead, fungi obtain food by secreting digestive enzymes onto whatever they are growing on.

These enzymes break down the decaying structure, such as a dead tree, into nutrients the fungi can use as food.

Kingdom Fungi

Kingdom Fungi:

Many fungi are saprophytes that live on dead organisms.

Many other are parasites that live on living organisms and cause diseases that affect plants and animals.

Kinds of Fungi

Three basic phyla of fungi

Zygomycetes:

Zygomycetes form structures for sexual reproduction called zygosporangia.

Zygomycetes include species such as Rhizopus stoloifer, common bread mold.

Kingdom Fungi

Basidiomycetes:

Basidiomycetes include fungi that make mushrooms.

Mushrooms are the sexual reproduction structures produced by basidiomycetes.

Basidiomycetes almost always reproduce sexually.

Kingdom Fungi

Ascomycetes:

Ascomycetes form sexual spores in special sac-like structures called asci.

The sexual reproduction structure formed by ascomycetes often resembles a cuplike structure called an ascocarp.