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Cell Organization

Eukaryotic Structures

Eukaryotic cells are organized to contain several membrane-bound organelles and a nucleus that directs the functions of these organelles.

Eukaryotes are cells categorized as having a nucleus and a variety of other membrane-enclosed organelles. Eukaryotes include plants, animals, fungi, and protists. Eukaryotic cells have greater mass than prokaryotic cells and have far more internal complexity. Within each eukaryotic cell, internal membranes create multiple compartments. These compartments, called organelles, carry out specific functions through independent chemical reactions.

In eukaryotic cells, the DNA is separated from the rest of the cell in the nucleus by the nuclear membrane. This separation is important in the regulation of protein synthesis, as the cellular structures that produce proteins are found outside the nucleus. The nuclear membrane limits the type of molecules that can reach the DNA and also limits the type of molecules that can leave the nucleus and reach the protein synthesis machinery of the cell. Eukaryotic cells have more precise regulation of the production of proteins and their RNA intermediates than do prokaryotes, in which the DNA is not sequestered from the rest of the cell.

One feature of eukaryotic cells, particularly within multicellular organisms, is the variety of shapes and sizes of these cells. The shape of the cell is related to the function it performs. Nerve cells, for example, have fingerlike projections that allow them to communicate over a distance. In addition, cells aggregate to form larger structures, such as tissues and organs, that perform specific functions.

Membrane-Bound Organelles

Organelles found in eukaryotes and prokaryotes have specific cell tasks. For example, the nucleus in eukaryotes contains information in the form of DNA.
Eukaryotic cells are characterized by the presence of membrane-bound organelles. Eukaryotic cells contain organelles with either a single membrane or two membranes, as well as organelles not bound by a membrane, which are known as non-membrane bound. Every eukaryotic cell contains a nucleus that is a membrane-bound organelle that contains most of the genetic material (DNA). This structure directs a cell's growth, division, and death. This genetic material is classified as deoxyribonucleic acid (DNA), which is an organic molecule containing coded instructions for the life processes of an organism. DNA consists of nucleotides bonded together in the form of a double helix. The nuclear envelope, a double membrane that is semipermeable and surrounds a nucleus. The DNA inside the nucleus contains genetic information of the organism, except in the case of a gamete. A gamete is the sex cell of a sexually reproducing organism, with a haploid set of chromosomes. This means they contain half the genetic information of the organism. The role of the nucleus is to maintain and preserve the cell's DNA as well as provide a site for genetic transcription. This process of transcription involves transferring the genetic information in DNA to ribonucleic acid (RNA). Ribonucleic acid (RNA) is a organic molecule that carries genetic messages out of the nucleus; consists of a single strand of nucleic acids. In most organisms, RNA is involved in protein synthesis, gene regulation, and post-transcriptional modification.There are many classes of RNA. The most common are mRNA, tRNA, and rRNA. Messenger RNA (mRNA) is a molecule made from a DNA template from which a protein is translated; mRNA determines the amino acid sequence in proteins. Transfer RNA (tRNA) is the molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis. Ribosomal RNA (rRNA) is a component of ribosomes that catalyzes peptide bond formation. Once the genetic message in mRNA is created, it can leave the nucleus and be sent to the cytoplasm, composed of cytosol, an aqueous fluid that fills the fluid part of the cytoplasm in the cell, and organelles that fills the spaces around the internal cell structures and organelles.
The nucleus, an organelle enclosed by a double-layer membrane called the nuclear envelope, is found in all eukaryotic cells.
Another type of organelle enclosed in a membrane is the mitochondrion (plural, mitochondria), the organelle responsible for generating energy for the cell. The outer membrane of the mitochondrion gives the organelle its shape and structure, while the inner membrane folds back and forth within the organelle, forming an intermembrane space where the respiratory reactions of the citric acid cycle and oxidative phosphorylation take place. These reactions harness the energy from the oxidation (loss of electrons) of food molecules, usually sugars, to produce adenosine triphosphate (ATP), the chemical unit of energy for the cell. This ATP is then released for use by other cellular structures. Remarkably, mitochondria have unique DNA and enzymes when compared to the nucleus. They reproduce by dividing, just as cells do. For this reason, they are suspected to have derived from symbiotic organisms in the early evolution of eukaryotes. It is likely that an organism similar to the modern mitochondrion was engulfed by an early eukaryote. The mitochondrion provided energy for the eukaryote, which in turn provided most of the materials necessary for the mitochondrion to survive. Over time, the mitochondrion lost all functions other than providing energy but retained its own DNA. Mitochondrial DNA remains unique, separate from nuclear DNA.
The mitochondrion is the energy center of the cell, where sugars and other nutrients are oxidized, producing adenosine triphosphate (ATP), the chemical unit of energy.
Some eukaryotic cells, such as those in plants, also contain chloroplasts. The chloroplast is a membrane-bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. Chloroplasts contain stacks of disklike structures called thylakoids, which are home to light-harvesting complexes. Both chloroplasts and mitochondria are found in photosynthetic eukaryotic cells. Thus, scientists concluded both organelles to be separate structures with similar origins. Chloroplasts are thought to have been bacteria engulfed by eukaryotic cells early in evolution through a process known as endosymbiosis. During this process, bacteria and early eukaryotic cells lived in a mutually beneficial relationship. Similar to the way mitochondria were incorporated into the cell, chloroplasts lost all functions other than converting light energy into chemical energy, relying on the cell for other needs.
Chloroplasts are membrane-bound organelles found in photosynthetic eukaryotic cells. They are responsible for capturing light energy and converting it to chemical energy.

Other Organelles and Cellular Structures

Other organelles found in eukaryotic cells include the endoplasmic reticulum, ribosomes, the Golgi apparatus, and various other structures suspended in the cytosol.

Eukaryotic cells contain other organelles that are made up of membranes, which either have a single membrane (endoplasmic reticulum, Golgi body, vacuole) or double membrane (mitochondrion, chloroplast). However, there are organelles without a membrane, called non-membrane bound organelles. These organelles include ribosomes, centrosomes, centrioles, cilia/flagella, and microtubules.

The endoplasmic reticulum (ER) is a network of membranes that helps process molecules such as proteins and lipids in a cell. Specifically this organelle is a folded series of membrane spaces where cellular components, including proteins and lipids, are made. It consists of smooth endoplasmic reticulum (smooth ER), which lacks ribosomes, and rough endoplasmic reticulum (rough ER), which has attached ribosomes, hence their appearance and names. A ribosome is a small cellular structure composed of RNA and protein that constructs proteins based on the instructions provided by DNA. The Golgi apparatus is a series of membrane stacks that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their places inside or outside a cell.
The endoplasmic reticulum (ER) helps make proteins. Smooth ER does not have ribosomes, while rough ER does. Ribosomes are structures that construct the protein based on the message in messenger RNA.
The Golgi apparatus is a series of stacked membranes that helps direct molecules to their correct destinations by attaching chemical markers to them.
Cells also contain many other structures. The cells of animals include a type of organelle called a lysosome (an organelle that digests bacteria that enter a cell, eliminates toxins, and recycles worn cell materials), large protein complexes, and structures related to cell division, such as centrioles. A centriole is a structure that coordinates formation of microtubules found in pairs near the nucleus. A centrosome coordinates the formation of microtubules, which allows cell division to proceed during reproduction. The cells of plants contain a large central vacuole, a large membranous sac that plays an important role in storage and plant cell growth. These structures are all suspended in the cytosol of the cytoplasm, which is the aqueous fluid that fills the cytoplasm inside the cell. The cytoplasm is surrounded by the cell membrane, which encloses the cell and controls transport of materials into and out of the cell.
All eukaryotic cells share certain characteristics. They are surrounded by a cell membrane and have many of the same organelles. Some organelles, however, are found only in animal cells while others occur only in plant cells.

Cell Structures and Their Functions in Eukaryotic Cells

Organelle Description Animal Cells Plant Cells
Cell membrane Controls the passage of substances entering or leaving the cell
Cell wall Provides structural support for the plant cell
Central vacuole Stores water and inorganic ions inside the plant cell
Centrosome (contains a pair of centrioles) Organizes microtubules; will ultimately form the poles of the mitotic spindle during the cell cycle
Chloroplast Converts sunlight energy to chemical energy during photosynthesis in the plant cell
Cytoplasm Serves as the cell's internal environment for all of the organelles
Endoplasmic reticulum Synthesizes lipids, makes proteins by means of the ribosomes that it contains, and detoxifies drugs or poisons
Golgi apparatus Prepares the lipids and proteins made in the endoplasmic reticulum for transport
Mitochondrion Performs the Kreb's citric acid cycle and oxidative phosphorylation to produce ATP
Nucleus Directs the cell's functions and its division; contains most of the cell's DNA
Ribosome Translates messenger RNA (mRNA) into protein

Eukaryotic cells contain many structures to carry out life processes. A few are unique according to the type of organism, such as plant or animal cells.