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Understanding Evolution - Your one-stop source for information on evolution

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Living things have evolved into three large clusters of closely related organisms, called "domains": Archaea, Bacteria, and Eukaryota. Archaea and Bacteria are small, relatively simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes. They are called prokaryotes.

Virtually all the life we see each day - including plants and animals - belongs to the third domain, Eukaryota. Eukaryotic cells are more complex than prokaryotes, and the DNA is linear and found within a nucleus. Eukaryotic cells boast their own personal "power plants", called mitochondria. These tiny organelles in the cell not only produce chemical energy, but also hold the key to understanding the evolution of the eukaryotic cell.

The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms. But how did the eukaryotic cell itself evolve? How did a humble bacterium make this evolutionary leap from a simple prokaryotic cell to a more complex eukaryotic cell? The answer seems to be symbiosis - in other words, teamwork.

Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union. In fact, the mitochondrion itself seems to be the "great-great-great-great-great-great-great-great-great granddaughter" of a free-living bacterium that was engulfed by another cell, perhaps as a meal, and ended up staying as a sort of permanent houseguest. The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. This kind of "internal" symbiosis - one organism taking up permanent residence inside another and eventually evolving into a single lineage - is called endosymbiosis.

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From prokaryotes to eukaryotes - Understanding Evolution

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Living things have evolved into three large clusters of closely related organisms, called "domains": Archaea, Bacteria, and Eukaryota. Archaea and Bacteria are small, relatively simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes. They are called prokaryotes.

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All living organisms can be sorted into one of two groups depending on the fundamental structure of their cells: the prokaryotes and the eukaryotes. Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus that holds genetic material as well as membrane-bound organelles.

Understanding Cells and Cell Membranes

The cell is a fundamental component of our modern definition of life and living things. Cells are regarded as the basic building blocks of life and are used in the elusive definition of what it means to be "alive."

Cells keep chemical processes tidy and compartmentalized so individual cell processes do not interfere with others and the cell can go about its business of metabolizing, reproducing, etc. To achieve this, cell components are enclosed in a membrane which serves as a barrier between the outside world and the cell's internal chemistry. The cell membrane is a selective barrier, meaning that it lets some chemicals in and others out. In so doing it maintains the chemical balance necessary for the cell to live.

The cell membrane regulates the crossing of chemicals in and out of the cell in three ways including:

Diffusion (the tendency of solute molecules to minimize concentration and thus move from an area of higher concentration towards an area of lower concentration until concentrations equalize)

Osmosis (the movement of solvent across a selective boundary in order to equalize the concentration of a solute that is unable to move across the boundary)

Selective transport (via membrane channels and membrane pumps)

Prokaryotes

Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. This means the genetic material DNA in prokaryotes is not bound within a nucleus. In addition, the DNA is less structured in prokaryotes than in eukaryotes: in prokaryotes, DNA is a single loop while in Eukaryotes DNA is organized into chromosomes. Most prokaryotes are made up of just a single cell (unicellular) but there are a few that are made of collections of cells (multicellular).

Scientists have divided the prokaryotes into two groups, the Bacteria, and the Archaea. Some bacteria, including E Coli, Salmonella, and Listeria, are found in foods and can cause disease;﻿﻿ others are actually helpful to human digestion and other functions.﻿﻿ Archaea were discovered to be a unique life form which is capable of living indefinitely in extreme environments such as hydrothermal vents or arctic ice.

A typical prokaryotic cell might contain the following parts:

Cell wall: the membrane surrounding and protecting the cell

Cytoplasm: all of the material inside a cell except the nucleus

Flagella and pili: protein-based filaments found on the outside of some prokaryotic cells

Nucleoid: a nucleus-like region of the cell where genetic material is kept

Plasmid: a small molecule of DNA that can reproduce independently

Eukaryotes

Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus (that holds DNA in the form of chromosomes) as well as membrane-bound organelles. Eukaryotic organisms may be multicellular or single-celled organisms. All animals are eukaryotes. Other eukaryotes include plants, fungi, and protists.

A typical eukaryotic cell is surrounded by a plasma membrane and contains many different structures and organelles with a variety of functions. Examples include the chromosomes (a structure of nucleic acids and protein which carry genetic information in the form of genes), and the mitochondria (often described as the "powerhouse of the cell").

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All living organisms can be sorted into one of two groups depending on the fundamental structure of their cells: the prokaryotes and the eukaryotes. Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles.

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In addition to the nucleus, eukaryotic cells may contain several other types of organelles, which may include mitochondria, chloroplasts, the endoplasmic reticulum, the Golgi apparatus, and lysosomes. Each of these organelles performs a specific function critical to the cell's survival. Moreover, nearly all eukaryotic organelles are separated from the rest of the cellular space by a membrane, in much the same way that interior walls separate the rooms in a house. The membranes that surround eukaryotic organelles are based on lipid bilayers that are similar (but not identical) to the cell's outer membrane. Together, the total area of a cell's internal membranes far exceeds that of its plasma membrane.

Like the plasma membrane, organelle membranes function to keep the inside "in" and the outside "out." This partitioning permits different kinds of biochemical reactions to take place in different organelles. Although each organelle performs a specific function in the cell, all of the cell's organelles work together in an integrated fashion to meet the overall needs of the cell. For example, biochemical reactions in a cell's mitochondria transfer energy from fatty acids and pyruvate molecules into an energy-rich molecule called adenosine triphosphate ( ATP). Subsequently, the rest of the cell's organelles use this ATP as the source of the energy they need to operate.

Because most organelles are surrounded by membranes, they are easy to visualize - with magnification. For instance, researchers can use high resolution electron microscopy to take a snapshot through a thin cross-section or slice of a cell. In this way, they can see the structural detail and key characteristics of different organelles - such as the long, thin compartments of the endoplasmic reticulum or the compacted chromatin within the nucleus. An electron micrograph therefore provides an excellent blueprint of a cell's inner structures. Other less powerful microscopy techniques coupled with organelle-specific stains have helped researchers see organelle structure more clearly, as well as the distribution of various organelles within cells. However, unlike the rooms in a house, a cell's organelles are not static. Rather, these structures are in constant motion, sometimes moving to a particular place within the cell, sometimes merging with other organelles, and sometimes growing larger or smaller. These dynamic changes in cellular structures can be observed with video microscopic techniques, which provide lower-resolution movies of whole organelles as these structures move within cells.

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In addition to the nucleus, eukaryotic cells may contain several other types of organelles, which may include mitochondria, chloroplasts, the endoplasmic reticulum, the Golgi apparatus, and lysosomes. Each of these organelles performs a specific function critical to the cell's survival.

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Eukaryote | Definition, Structure, & Facts

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Eukaryote, any cell or organism that possesses a clearly defined nucleus. The eukaryotic cell has a nuclear membrane that surrounds the nucleus, in which the well-defined chromosomes (bodies containing the hereditary material) are located. Learn more about eukaryotes in this article.

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Prokaryotic and eukaryotic cells

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Overview of eukaryotic cells and how they differ from prokaryotic cells (nucleus, organelles, and linear chromosomes).

Prokaryotic and eukaryotic cells

Biology is brought to you with support from the Amgen Foundation

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Intro to eukaryotic cells (article) | Khan Academy

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Overview of eukaryotic cells and how they differ from prokaryotic cells (nucleus, organelles, and linear chromosomes).

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The strongest thing in the worl

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