Eukaryote

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A eukaryote is an organism belonging to the eukaryota taxonomical group, and includes organisms such as animals, plants and fungi; these organisms contain eukaryotic cells with specialised and complex membrane-bound organelles. Eukaryotic cells are also larger and more complex than prokaryotic cells, so have a greater potential and diversity than prokaryotic cells. Some organelles within a eukaryotic cell, such as mitochondria and chloroplasts, are thought to be descendants of prokaryotes, which once had a symbiotic relationship with the cell, but evolved over time to become integrated within the eukaryotic cell. Eukaryotes evolved more recently than prokaryotes - prokaryotes are thought to have evolved between 3.4 to 2.7 billion years ago, whereas the earliest eukaryote fossils 2.1 billion years ago, from red algae[1].

Contents

Most eukaryotic cells contain:

Eukaryotic cells also contain a cytoskeleton, which holds the cell organelles in place, and cytosol, which is a fluid that surrounds the organelles. Eukaryotic cells are surrounded by cell membrane, which is made of a dynamic phospholipid bilayer. The membrane also contains numerous proteins, glycoproteins and cholesterol to add structural support. Unlike prokaryotes, eukaryotes contain organelles which are bound by a phospholipid membrane[2].

Eukaryotic cells also undergo cell division known as mitosis and meiosis in order to replicate and form gametes, respectively[3][4][5].

Living organisms can be sorted into two groups; these are the prokaryotes and the eukaryotes. Eukaryotes are organisms that contain cells with a membrane-bound nucleus as well as membrane-bound organelles. Examples of eukaryotes include all animals, plants, fungi and protists. Eukaryotes may variably be multicellular or unicellular. Conversely, prokaryotes contain no nucleus, no membrane-bound organelles and the majority are known to be unicellular, very rarely are they multicellular. Examples of prokaryotes are bacteria and cyanobacteria.

Labelled Eukaryotic Cell

Cell Structure

Plasma Membrane:

Plasma membrane separates the cell from the surrounding environment and controls the passage of the molecules in and out of the cell. This membrane is composed mainly of phospholipids and proteins. The plasma membrane is known as phospholipid bilayer because the phosphate group and the lipid molecules have different properties in water. The phosphate group is hydrophilic but lipids are hydrophobic; as water is found inside and outside the cell, phosphate molecules lies on the surface of the phospholipid bilayer while the lipid molecules form the internal part of the phospholipid bilayer of the membrane. The cell membrane is said to have a fluid mosaic structure in which the phospholipids and proteins are able to move around. The phospholipids are able to move around laterally and swivel in a circular motion in their places, they are also able to flip between the bilayer (but rarely do this).

Most of the organelles inside the cell are surrounded with a membrane which segregates its activity from the surrounding environment.

Cytoplasm:

The Cytoplasm is composed of the cytosol and the organelles. The cytosol is a gel-like substance. All the organelles are suspended in the cytosol.

Endoplasmic Reticulum:

There are two types of endoplasmic reticulum; rough and smooth. Rough endoplasmic reticulum, so called due to the ribosomes found on its surface, synthesises proteins, and smooth endoplasmic reticulum synthesizes lipids and steroids and metabolizes carbohydrates and steroids. The general structure is a network of sac-like structures called cisternae, held together by the cytoskeleton.

Golgi Apparatus:

The Golgi apparatus gathers simple molecules and combines them to make molecules that are more complex which then get packaged them in vesicles, and either stores them for later use or sends them out of the cell. It builds lysosomes. The structure of the Golgi apparatus can be described as a series of flattened sacs. The Golgi is polarised and it has a cis and a trans face. The cis face is nearer the endoplasmic reticulum and the trans face is near the plasma membrane.

Mitochondria:

Mitochondria are vital organelles that provide energy for the cell to carry out all its functions, for example, division. Mitochondria have double membranes, with the outer membrane fairly smooth, and the inner membrane folded into cristae, which greatly increases the surface area. It is on these cristae that sugar is combined with oxygen to produce ATP - the primary energy source for the cell.

Nucleus:

The nucleus controls all functions of the cell and contains genetic information in the form of DNA, and its associated protein, histone, which forms chromosomes. The nucleus is bounded by a double membrane, called the nuclear envelope. Within the double membrane nuclear envelope are passages which large molceules can pass through called nuclear pores. Each Nucleus contains one or more nucleolus; the site of ribosome synthesis.

Cell Wall:

Cell wall is present in fungi and plant cells, and exists around the outside of the plasma membrane. In fungi peptidoglycan forms the cell wall and in plants, the strongest component of the cell wall is a carbohydrate called cellulose. Cells walls provide structural support and have no affect on the passage of molecules and ions into and out of the cell.

This structural support comes from the cellulose-containing polysaccharides of glucose, attached by 1,4 glycosidic bonds which are covalently linked together, further stabled by hydrogen bonds. Layers of these residues can also be hydrogen bonded together which form strong form chains, or myofibrils[6].

Chloroplast:

Chloroplasts are present in all plants and algae. Like mitochondria, they have double membranes and are thought to be descended from free-living prokaryotes. The chloroplast contains thylakoids, which are chlorophyll-containing membranes. These membranes use light as the catalyst for photosynthetic reactions.

Reproduction

The cell cycle of eukaryotic cells has two major phases: the interphase and cell division.

Interphase is the longest phase, and consists of 3 stages: first gap phase (G1), synthesis (S) and second gap phase (G2). G1 consists of the newly formed cell growing, synthesising extra organelles and making molecules needed for DNA synthesis. S consists of the centrioles and DNA duplicating. And G2 consists of the cell building up its ATP reserves, in preparation for mitosis[7].

During the cell division phase, the nucleus divides in a process called mitosis and then the divided nuclei are established in separate cells in a process called cytokinesis. Each daughter cell produced will be genetically identical to that of the parent cell. The process of meiosis is a two-stage process where, in the first stage, the cell divides as it does in mitosis. Then the cell divides again (without the chromosomes replicating again, regardless of another interphase between the two stages) which is when the sister chromatids get pulled apart. The daughter cells that are the outcome of this are not identical to the parent cells and are in fact haploid of the parent cells. These cells are called gametes, and when two gamete's nuclei fuse together during reproduction, is when a diploid number in chromosomes is regained.

References

  1. http://www.wisegeek.com/what-is-a-eukaryote.htm
  2. Berg, Tymoczko and Stryer. Biochemistry (2006). Sixth edition. (page 346
  3. http://www.wisegeek.com/what-is-a-eukaryote.htm
  4. http://dictionary.reference.com/browse/eukaryote
  5. Berg, Tymoczko and Stryer. Biochemistry (2006). Sixth edition. (page 346)
  6. Alberts et al., Molecular Biology Of the cell, 5th edition, 2008, Garland Science, New York, pg 1197.
  7. Regina Bailey. Stages of Mitosis [online] Available at:http://biology.about.com/od/mitosis/ss/mitosisstep.htm [accessed 27/11/14]
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