Animal Cell Physiology and Histology (Very beautiful pictures)

Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles. Unlike the cells of the two other eukaryotic kingdoms, plants and fungi, animal cells don't have a  cell wall. This feature was lost in the distant past by the single-celled organisms that gave rise to the kingdom Animalia.

Anatomy of the animal Cell

The lack of a rigid  cell wall allowed animals to develop a greater diversity of cell  types, tissues, and organs. Specialized cells that formed nerves and muscles -- tissues impossible for plants to evolve -- gave these organisms mobility. The ability to move about by the use of specialized muscle tissues is the hallmark of the animal world. (Protozoans locomote, but by nonmuscular means, i.e. cilia, flagella, pseudopodia.)

The animal kingdom is unique amongst eukaryotic organisms because animal tissues are bound together by a triple helix of protein, called collagen. Plant and fungal cells are bound together in tissues or aggregations by other molecules, such as pectin. The fact that no other organisms utilize collagen in this manner is one of the indications that all animals arose from a common unicellular ancestor.

Animals are a large and incredibly diverse group of organisms. Making up about three-quarters of the species on Earth, they run the gamut from sponges and jellyfish to ants, whales, elephants, and -- of course -- human beings. Being mobile has given animals the flexibility to adopt many different modes of feeding, defense, and reproduction.

The earliest fossil evidence of animals dates from the Vendian Period (650 to 544 million years ago), with coelenterate-type creatures that left traces of their soft bodies in shallow-water sediments. The first mass extinction ended that period, but during the Cambrian Period which followed, an explosion of new forms began the evolutionary radiation that produced most of the major groups, or phyla, known today. Vertebrates (animals with backbones) are not known to have occurred until the Ordovician Period (505 to 438 million years ago).

Centrioles

Found only in animal cells, these paired organelles are found together near the nucleus, located at right angles to each other. Each centriole is made of nine bundles of microtubules (three per bundle) arranged in a ring. They have a role in building cilia and flagella, during which time they are referred to as basal bodies.

Centrioles also play a role in  cell division, although not as significant a role as once thought. Plant cells reproduce without centrioles and in experiments that have removed centrioles from animal cells, the cells were able to reproduce successfully without the organelles. Apparently they organize the microtubules in the mitotic spindles during mitosis and meiosis. The mitotic spindles in plant cells are less tightly organized.

These structures are self-replicating and make copies of themselves just before  cell division begins. As the  cell prepares to divide, the centrioles separate and move toward opposite poles of the cell. As they're moving apart, they radiate microtubules in a spindle-shaped formation that spans the  cell from pole to pole. The spindle fibers act as guides for the alignment of the chromosomes as they separate.

Cilia and Flagella

Cilia and flagella are made up of microtubules, which are composed of linear polymers of globular proteins called tubulin. The core (axoneme) contains two central fibers that are surrounded by an outer ring of nine double fibers and covered by the cellular membrane

  

Ultrastructure of cilia and flagella

These motile appendages are constructed by basal bodies (kinetostomes), which also function as centrioles. The basal body is located at the base of each filament, anchoring it to the  cell and controlling its movement. Cilia and flagella have the same structure. The only difference is that the flagella are longer.

For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. Protozoans belonging to the phylum Ciliophora are covered with cilia. Flagella are a characteristic of the protozoan group Mastigophora.

In multicellular organisms, cilia function to move fluid or materials past an immobile  cell as well as moving a  cell or group of cells. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs. Cilia generate water currents to carry food and oxygen past the gills of clams and transport food through the digestive systems of snails. Flagella are found primarily on gametes, but also create the water currents necessary for respiration and circulation in sponges and coelenterates.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. The ER is a continuous membrane with branching tubules and flattened sacs that extend throughout the cytoplasm. It is connected to the double-layered nuclear envelope, providing a connection between the nucleus and the cytoplasm

There are two kinds of ER, rough and smooth. Rough ER is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope. This type of ER is involved mainly with the production of proteins that will be exported, or secreted, from the cell. The ribosomes assemble amino acids into units of proteins, which are transported into the rough ER for further processing. Once inside, the proteins are folded into the correct three-dimensional conformation, as a flattened cardboard box might be opened up and folded into its proper shape in order to become a useful box. Chemicals, such as carbohydrates or sugars, are added, then the ER either transports the completed proteins to areas of the  cell where they are needed, or they are sent to the Golgi apparatus for export.

Smooth ER has a smoother appearance than rough ER when viewed through the microscope because it does not have ribosomes attached to it. This portion of the ER is involved with the production of lipids (fats), carbohydrate metabolism, and detoxification of drugs and poisons. Smooth ER is also involved with metabolizing calcium to mediate some  cell activities. In muscle cells, smooth ER releases calcium to trigger muscle contractions. Cells specializing in lipid and carbohydrate metabolism (brain, muscle) or detoxification (liver) usually have more of this type of ER.

Ribosomes

All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA

Ribosomes are scattered throughout the cytoplasm and are the protein production sites for the cell. Some of the proteins are synthesized for the cell's own use, particularly in single-celled organisms. In multicellular organisms, many of the proteins produced by a specialized cell, e.g. antibodies, will be transported and used elsewhere in the organism.

Eukaryote ribosomes are produced and assembled in the nucleolus. Three of the four strands are produced there, but one is produced outside the nucleolus and transported inside to complete the ribosome assembly. Ribosomal proteins enter the nucleolus and combine with the four strands to create the two subunits that will make up the completed ribosome. The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm. Some ribosomes will remain free-floating in the cytoplasm, creating proteins for the cell's use. Others will attach to the endoplasmic reticulum and produce the proteins that will be "exported" from the cell.

Protein synthesis requires the assistance of two other RNA molecules. Messenger RNA (mRNA) provides instructions from the cellular DNA for building a specific protein. Transfer RNA (tRNA) brings the protein building blocks, amino acids, to the ribosome. Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the Golgi apparatus. There, the proteins are completed and released inside or outside the cell.

Golgi Apparatus

The Golgi apparatus (GA), also called Golgi body or Golgi complex, is a series of five to eight cup-shaped, membrane-covered sacs that look something like a stack of deflated balloons. The GA is the distribution and shipping department for the cell's chemical products. It modifies proteins and lipids (fats) that have been built in the endoplasmic reticulum and prepares them for export as outside of the cell. The number of GAs in each  cell varies according to its function, but animal cells generally contain between ten and twenty per cell.
  

Proteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the GA. The vesicles fuse with the GA membrane and release the molecules into the organelle. Once inside, the compounds are further processed by the GA, which adds molecules or chops tiny pieces off the ends. Once completed, the product is extruded from the GA in a vesicle and directed to its final destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the cell's function in the organism. Other products are returned to the endoplasmic reticulum or become lysosomes.

Lysosomes

The main function of these microbodies is digestion. Lysosomes break down cellular waste products and debris from outside the  cell into simple compounds, which are transferred out into the cytoplasm as new cell-building materials

Like other microbodies, lysosomes are spherical organelles contained by a single layer membrane. This membrane protects the rest of the  cell from the lysosomes’ harsh digestive enzymes that would otherwise damage it.

Lysosomes originate in the Golgi apparatus, but the digestive enzymes are manufactured in the rough endoplasmic reticulum. Lysosomes are found in all eukaryotic cells, but are most numerous in disease-fighting cells, such as white blood cells.

Some human diseases are caused by lysosome enzyme disorders. Tay-sachs disease is caused by a genetic defect that prevents the formation of an essential enzyme that breaks down a complex lipid called ganglioside. An accumulation of this lipid damages the nervous system, causes mental retardation and death in early childhood. Arthritis inflammation and pain are related to the escape of lysosome enzymes.

Microfilaments

Microfilaments are solid rods made of globular proteins called actin and are common to all eukaryotic cells. Long chains of the molecules are intertwined in a helix to form individual microfilaments. These filaments are primarily structural in function and are an important component of the cytoskeleton, along with microtubules.
 

In association with myosin, microfilaments help to generate the forces used in cellular contraction and basic  cell movements. They enable a dividing  cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells. They also enable the contractions of muscle cells.

Microtubules

These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and perform a number of functions.

Microtubules form part of the cytoskeleton that gives structure and shape to a cell, serve as conveyor belts moving other organelles through the cytoplasm, are the major components of cilia and flagella, and participate in the formation of spindle fibers during  cell division (mitosis). Microtubules can function individually or join with other proteins to create larger structures (e.g. cilia). These filaments are composed of linear polymers of tubulin, which are globular proteins, and can increase or decease in length by adding or removing tubulin proteins.

Mitochondria

Mitochondria (singular, mitochondrion) are oblong shaped organelles that are found in the cytoplasm of every eukaryotic cell. They occur in varying numbers, depending on the  cell and its function.

These organelles are the power generators of the cell, converting oxygen and nutrients into ATP (adenosine triphosphate). ATP is the chemical energy "currency" of the  cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen.

The mitochondrion is different from other organelles because it has its own DNA and reproduces independently of the  cell in which it is found; an apparent case of endosymbiosis. Scientists hypothesize that millions of years ago small, free-living prokaryotes were engulfed, but not consumed, by larger prokaryotes; perhaps because they were able to resist the digestive enzymes of the engulfing organism.

The two organisms developed a symbiotic relationship over time, the larger organism providing the smaller with ample nutrients and the smaller organism providing ATP molecules to the larger one. Eventually, the larger organism developed into the eukaryotic cell, the smaller organism into the mitochondrion. Nonetheless, there are a number of prokaryotic traits that mitochondria continue to exhibit. Their DNA is circular, as it is in the prokaryotes, and their ribosomes and reproductive methods (binary fission) are more like those of the prokaryotes.

Mitochondrial DNA can be used study different aspects of inheritance. In most animal species, mitochondria are inherited through the maternal lineage. A sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When it attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism gets are from the egg its mother provided.

Unlike nuclear DNA, mitochondrial DNA doesn't get shuffled every generation, so it is presumed to change at a slower rate. That fact is being used to study human evolution and suggests that modern humans descended from a small group of hominids in Africa around 200,000 years ago.

Mitochondrial DNA is also being used in forensic science, as a tool for identifying corpses or body parts, and has been implicated in a number of genetic diseases such as Alzheimer's disease and diabetes.

Nucleus

The nucleus is a highly specialized organelle that serves as the information and administrative center of the cell. This organelle has two major functions. It stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include intermediary metabolism, growth, protein synthesis, and reproduction (cell division).

Only the cells of advanced organisms, known as eukaryotes, have a nucleus. Generally there is only one nucleus per cell, but there are exceptions such as slime molds and the Siphonales group of algae. Simpler one-celled organisms (prokaryotes), like the bacteria and cyanobacteria, don't have a nucleus. In these organisms, all the cell's information and administrative functions are dispersed throughout the cytoplasm.

The spherical nucleus occupies about 10 percent of a cell's volume, making it the cell's most prominent feature. Most of the nuclear material consists of chromatin, the unstructured form of the cell's DNA that will organize to form chromosomes during mitosis or  cell division. Also inside the nucleus is the nucleolus, an organelle that synthesizes protein-producing macromolecular assemblies called ribosomes.

A double-layered membrane, the nuclear envelope, separates contents of the nucleus from the cellular cytoplasm. The envelope is riddled with holes called nuclear pores that allow specific types and sizes of molecules to pass back and forth between the nucleus and the cytoplasm. It is also attached to a network of tubules, called the endoplasmic reticulum, where protein synthesis occurs. These tubules extend throughout the  cell and manufacture the biochemical products that a particular  cell type is genetically coded to produce.

Chromatin/Chromosomes - Packed inside the nucleus of every human  cell is nearly 6 feet of DNA, which is divided into 46 individual molecules, one for each chromosome and each about 1.5 inches long. Packing all this material into a microscopic  cell nucleus is an extraordinary feat of packaging. For DNA to function, it can't be crammed into the nucleus like a ball of string. Instead, it is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin.

Each DNA strand wraps around groups of small protein molecules called histones, forming a series of bead-like structures, called nucleosomes, connected by the DNA strand. Under the microscope, uncondensed chromatin has a "beads on a string" appearance.

The string of nucleosomes, already compacted by a factor of six, is then coiled into an even denser structure, compacting the DNA by a factor of 40. This compression and structuring of DNA serves several functions. The overall negative charge of the DNA is neutralized by the positive charge of the histone molecules, the DNA takes up much less space, and inactive DNA can be folded into inaccessible locations until it is needed.

There are two types of chromatin. Euchromatin is the genetically active portion and is involved in transcribing RNA to produce proteins used in cell function and growth. Heterochromatin contains inactive DNA and is the portion of chromatin that is most condensed, since it not being used.

Throughout the life of a cell, chromatin fibers take on different forms inside the nucleus. During interphase, when the cell is carrying out its normal functions, the chromatin is dispersed throughout the nucleus in what appears to be a tangle of fibers. This exposes the euchromatin and makes it available for the transcription process.

When the cell enters metaphase and prepares to divide, the chromatin changes dramatically. First, all the chromatin strands make copies of themselves through the process of DNA replication. Then they are compressed to an even greater degree than at interphase, a 10,000-fold compaction, into specialized structures for reproduction, termed chromosomes. As the cell divides to become two cells, the chromosomes separate, giving each cell a complete copy of the genetic information contained in the chromatin.

Nucleolus - The nucleolus is a membrane-less organelle within the nucleus that manufactures ribosomes, the cell's protein-producing structures. Through the microscope, the nucleolus looks like a large dark spot within the nucleus. A nucleus may contain up to four nucleoli, but within each species the number of nucleoli is fixed. After a cell divides, a nucleolus is formed when chromosomes are brought together into nucleolar organizing regions. During cell division, the nucleolus disappears. Some studies suggest that the nucleolus may be involved with cellular aging and, therefore, may affect the aging of an organism.

Nuclear Envelope - The nuclear envelope is a double-layered membrane that encloses the contents of the nucleus during most of the cell’s lifecycle. The space between the layers is called the perinuclear space and appears to connect with the rough endoplasmic reticulum. The envelope is perforated with tiny holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. The inner surface has a protein lining called the nuclear lamina, which binds to chromatin and other nuclear components. During mitosis, or cell division, the nuclear envelope disintegrates, but reforms as the two cells complete their formation and the chromatin begins to unravel and disperse.

Nuclear Pores - The nuclear envelope is perforated with holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. Building blocks for building DNA and RNA are allowed into the nucleus as well as molecules that provide the energy for constructing genetic material.

The pores are fully permeable to small molecules up to the size of the smallest proteins, but form a barrier keeping most large molecules out of the nucleus. Some larger proteins, such as histones, are given admittance into the nucleus. Each pore is surrounded by an elaborate protein structure called the nuclear pore complex, which probably selects large molecules for entrance into the nucleus.

Peroxisomes

Microbodies are a diverse group of organelles that are found in the cytoplasm, roughly spherical and bound by a single membrane. There are several types of microbodies but peroxisomes are the most common.

Peroxisomes function to rid the cell of toxic substances, in particular, hydrogen peroxide -- a common byproduct of cellular metabolism. These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen.

Peroxisomes are similar in appearance to lysosomes, another type of microbody, but the two have very different origins. Lysosomes are formed in the Golgi complex while peroxisomes are self-replicating. Unlike mitochondria, however, peroxisomes and lysosomes do not have their own internal DNA molecules.

Except for mature red blood cells, all human cells have peroxisomes. Since the early 1980s, a number of metabolic disorders have been found to be caused by molecular defects in the peroxisomes. Two major categories have been described so far. The first category is Disorders of Peroxisome Biogenesis (PBD) in which the organelle fails to develop normally, causing defects in numerous peroxisomal proteins. The second category includes involves defects of single peroxisomal enzymes. At present, there are no treatments for these genetic disorders, save for genetic counseling.

Plasma Membrane

All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents. The membrane has two functions. First, it is a boundary holding the cell constituents together and keeping other substances out

 Second, it is permeable, allowing nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

The membrane is made of a two molecule thick layer (bilayer) of phospholipids, an oily substance found in all cells. This layer is embedded with many diverse proteins and has carbohydrates attached to its outer surface. The lipids in the membrane can exist either in a gel-like, nearly solid, state or in a liquid-like state, which gives the lipid molecules more mobility. In living cells, the membrane seems to be in a transition between the two states, depending on physical conditions and what lipids and proteins are present in the membrane layer.

In prokaryotes and plants, the plasma membrane is the inner layer of protection. A rigid cell wall forms the outside boundary for the cell. While it has pores that allow materials to enter and leave the cell, they are not as selective about what passes through. The membrane, which lines the cell wall, provides the final filter between the cell interior and the environment.

Eukaryotic animal cells probably descended from prokaryotes that lost their cell walls. With only the flexible membrane left to enclose them, they were able to expand in size and complexity. Eukaryotic cells are generally ten times larger than prokaryotic cells and have membranes enclosing interior components, the organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials, allowing the cell to segregate its chemical functions into discrete internal compartments.

Although plants have evolved another version of the cell wall, in animal cells the plasma membrane is the only barrier between the cell and its environment.
 

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