HESI Entrance Exam Study guide
The temporal bone supports that part of the face known as the temple.
The epiphysis is filled with red bone marrow, which produces erythrocytes (red blood cells).
Foramen: A natural opening. Although a foramen is usually through bone, it can be an opening through other types of tissue, as with the foramen ovale in the heart. The plural of foramen is foramina.
the external acoustic meatus, the opening of the ear canal
The internal auditory meatus, a canal in the temporal bone of the skull
the urinary meatus, which is the opening of the urethra, situated on the glans penis in males, and in the vulva in females the superior meatus, middle meatus and inferior meatus of the nose
Large furrows (sulci) that divide the brain into lobes are often called fissures. The large furrow that divides the two hemispheres—the interhemispheric fissure—is very rarely called a “sulcus”.
•The greater trochanter: A powerful protrusion located at the proximal (near) and lateral (outside) part of the shaft of the femur;
•The lesser trochanter: A pyramidal prominence projecting from the proximal (near) and medial (inside) part of the shaft of the femur.
The greater trochanter is also called the major trochanter, the outer trochanter, and the lateral process of the femur. The lesser trochanter is alternatively called the minor trochanter, the inner trochanter and the medial process of the femur.
The trochanters are points at which hip and thigh muscles attach. The greater trochanter gives attachment to a number of muscles (including the gluteus medius and minimus, piriformis, obturator internus and externus, and gemelli muscles) while the lesser trochanter receives the insertion of several muscles (including the psoas major and iliacus [iliopsoas] muscles).
A large rounded projection for muscle attachment.
Three types of anchoring junctions are observed, and differ from one another in the cytoskeletal protein anchor as well as the transmembrane linker protein that extends through the membrane:
•The portion of the cell exposed to the lumen is called its apical surface.
•The rest of the cell (i.e., its sides and base) make up the basolateral surface.
Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface.
Tight junctions perform two vital functions:
•They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides control over what substances are allowed through.
•They block the movement of integral membrane proteins (red and green ovals) between the apical and basolateral surfaces of the cell. Thus the special functions of each surface, for example
◦receptor-mediated endocytosis at the apical surface
◦exocytosis at the basolateral surface
can be preserved.
Apocrine glands are a type of human sweat gland that are present in areas such as the axillae (armpits), areola, in the perineum (genital areas), around the belly button and in the external auditory canal(as wax-secreting glands). Specialized types of apocrine glands present on the eyelids are called Moll’s glands. Apocrine sweat glands are inactive until they are stimulated by hormonal changes in puberty.
Apocrine glands secrete a milky, viscous odourless fluid which only develops a strong odour when it comes into contact with bacteria on the skin surface. Apocrine glands secrete this fluid by a method called decapitation secretion. That is, the apical portion of the secretory cell of the gland pinches off and enters the lumen of the gland. In contrast to this mechanism of secretion, Eccrine glands secrete by a method called merocrine secretion and sebaceous glands secrete by a method called holocrine secretion.
Apocrine sweat glands are mainly thought to function as olfactory pheremones, chemicals important in attracting a potential mate.
It is sometimes equated with “loose connective tissue”. In other cases, “loose connective tissue” is considered a parent category that includes mucous connective tissue, reticular connective tissue and adipose tissue.
2 : a thin extracellular layer composed chiefly of collagen, proteoglycans, and glycoproteins (as laminin and fibronectin) that lies adjacent to the basal surface of epithelial cells or surrounds individual muscle, fat, and Schwann cells and that separates these cells from underlying or surrounding connective tissue or adjacent cells
In muscle tissue it serves as a major component of endomysium. Collagen constitutes 1% to 2% of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles. Gelatin, which is used in food and industry, is derived from collagen.
As it is well known, collagen fibers are naturally occurring proteins found exclusively in animals and they are the main proteins in the connective tissues. Collagen fiber is the most commonly found protein in mammals and it makes up 25 to 35% of the whole body protein. Since 1930s scientists have been performing active research on the conformation of the collagen monomer which is sheet like or microfibrillar. This monomeric structure of the collagen fibers was described by Fraser, Miller and Wess with close observation. The collagen molecule is also called the ‘tropocollagen’ and is a aggregate of larger collagen fibrils. Given below is a detailed description about collagen fibers and their function.
Collagen Fiber Information
Tropocollagen, is approximately 300 nm in length and 1.5 nm in diameter. It is made up of polypeptide strands known as the alpha chains and each of them is a left handed helix in conformation. There are three such left handed helices which are twisted together to form a right handed triple helix or super helix which are bonded to each other with hydrogen bonds. This entire molecule which is collectively called the collagen fiber is made up of a regular arrangement of amino acids and hence it’s called a protein. Read on for information about the formation, types and function of these collagen fibers. More on collagen injections.
Collagen Fiber Types
Collagen is found in many parts of the body but the most common is the collagen fibers connective tissue. Throughout the body there are 29 types of collagen which have been identified till date. However more than 80% of the collagen in the body are of the types I, II, III and IV, each type found in different parts of the body.
•Collagen I is found in the skin, organs, bones and tendons.
•Collagen II is found in cartilages all over the body.
•Collagen III is a major component of reticular fibers and along type I.
•Collagen IV is found in the cell basement membrane.
The most commonly found collagen type is the first one and so we have discussed the formation of the collagen type I for you to get a better idea on how it is formed.
Loose connective tissue is a mass of widely scattered cells whose matrix is a loose weave of fibers. Many of the fibers are strong protein fibers called collagen. Loose connective tissue is found beneath the skin and between organs. It is a binding and packing material whose main purpose is to provide support to hold other tissues and organs in place.
Adipose tissue consists of adipose cells in loose connective tissue. Each adipose cell stores a large droplet of fat that swells when fat is stored and shrinks when fat is used to provide energy. Adipose tissue pads and insulates the animal body.
Blood is a loose connective tissue whose matrix is a liquid called plasma. Blood consists of red blood cells, erythrocytes, white blood cells, leukocytes, and thrombocytes or platelets, which are pieces of bone marrow cell. Plasma also contains water, salts, sugars, lipids, and amino acids. Blood is approximately 55 percent plasma and 45 percent formed elements. Blood transports substances from one part of the body to another and plays an important role in the immune system of the animal.
Collagen (from the Greek kolla, meaning “glue,” and genos, meaning “descent”) is a dense connective tissue, also known as fibrous connective tissue. It has a matrix of densely packed collagen fibers. There are two types of collagen: regular and irregular. The collagen fibers of regular dense connective tissue are lined up in parallel. Tendons, which bind muscle to bone, and ligaments, which join bones together, are examples of dense regularconnective tissue. The strong covering of various organs, such as kidneys and muscle, is dense irregular connective tissue.
Cartilage (from the Latin cartilago, meaning “gristle”) is a connective tissue with an abundant number of collagen fibers in a rubbery matrix. It is both strong and flexible. Cartilage provides support and cushioning. It is found between the discs of the vertebrae in the spine, surrounding the ends of joints such as knees, and in the nose and ears.
Bone is a rigid connective tissue that has a matrix of collagen fibers embedded in calcium salts. It is the hardest tissue in the body, although it is not brittle. Most of the skeletal system is comprised of bone, which provides support for muscle attachment and protects the internal organs.
The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin family of cell adhesion molecules. They are transmembrane proteins that bridge the space between adjacent epithelial cells by way of homophilic binding of their extracellular domains to other desmosomal cadherins on the adjacent cell. Both have five extracellular domains, and have calcium-binding motifs.
The extracellular domain of the desmosome is called the Extracellular Core Domain (ECD) or the Desmoglea, and is bisected by an electron-dense midline where the desmoglein and desmocollin proteins bind to each other. These proteins can bind in a W, S, or λ manner.
On the cytoplasmic side of the plasma membrane, there are two dense structures called the Outer Dense Plaque (ODP) and the Inner Dense Plaque (IDP). These are spanned by the Desmoplakin protein. The Outer Dense Plaque is where the cytoplasmic domains of the cadherins attach to desmoplakin via plakoglobin and plakophillin. The Inner Dense Plaque is where desmoplakin attaches to the intermediate filaments of the cell.
Generally speaking, the ectoderm differentiates to form the nervous system, tooth enamel and the epidermis (the outer part of integument).
In vertebrates, the ectoderm has three parts: external ectoderm (also known as surface ectoderm), the neural crest, and neural tube. The latter two are known as neuroectoderm.
supporting and retaining the cytoplasm
being a selective barrier
The cell is separated from its environment and needs to get nutrients in and waste products out. Some molecules can cross the membrane without assistance, most cannot. Water, non-polar molecules and some small polar molecules can cross. Non-polar molecules penetrate by actually dissolving into the lipid bi-layer. Most polar compounds such as amino acids, organic acids and inorganic salts are not allowed entry, but instead must be specifically transported across the membrane by proteins.
A pair of insulin molecules bind to receptors, causing a change in the protein structure that exposes a kinase or phosphate transferring enzyme inside the cell.
The activated receptor kinase transfers a phosphate group to adjacent receptor molecules, leading to the activation of sub-cellular proteins and a cellular response to insulin.
The receptors must be integral membrane proteins that pass through the lipid bi layer. Through this system, an insulin molecule can cause a response inside the cell without passing through the lipid bi layer.
Each of your cells has a set of ‘identity tags’ on its surface, which mark it out as part of your body and no one else’s. Some are only found on cells from the same tissue or organ. These identity tag molecules are called antigens. Your set of antigens is unique, unless you have an identical twin. Your immune system recognizes invading germs because they have unfamiliar antigens on their surfaces.
Microfilaments are unusual because they vary greatly according to their location and function in the body. For example, some microfilaments form tough coverings, such as in nails, hair, and the outer layer of skin (not to mention animal claws and scales). Others are found in nerve cells, muscle cells, the heart, and internal organs. In each of these tissues, the filaments are made of different proteins.
Actin filament are made up of two chains of the protein actin twisted together. Although actin filaments are the most brittle of the cytoskeletal fibers, they are also the most versatile in terms of the shapes they can take. They can gather together into bundles, weblike networks, or even three-dimensional gels. They shorten or lengthen to allow cells to move and change shape. Together with a protein partner called myosin, actin filaments make possible the muscle contractions necessary for everything from your action on a sports field to the automatic beating of your heart.
The nucleus is the most obvious organelle in any eukaryotic cell. It is a membrane-bound organelle and is surrounded by a double membrane. It communicates with the surrounding cytosol via numerous nuclear pores.
Within the nucleus is the DNA responsible for providing the cell with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off – that’s why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. When a cell is dividing, the DNA and surrounding protein condense into chromosomes (see photo) that are visible by microscopy.
The prominent structure in the nucleus is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus to positions on the rough endoplasmic reticulum where they are critical in protein synthesis.
functions include destruction of damaged cells (which is why they are sometimes called ‘suicide bags’) & digestion of phagocytosed materials (such as bacteria) . Lysosomes, Peroxisomes, Secretory Vesicles
A variety of small membrane-bound organelles differ primarily in their contents.
Lysosomes: Lysosomes (common in animal cells but rare in plant cells) contain hydrolytic enzymes necessary for intracellular digestion. In white blood cells that eat bacteria, lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria. Uncontrolled release of lysosome contents into the cytoplasm is also a component of necrotic cell death.
Peroxisomes: This organelle is responsible for protecting the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen.
Secretory Vesicles: Cell secretions – e.g. hormones, neurotransmitters – are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release.
A typical animal cell will have on the order of 1000 to 2000 mitochondria. So the cell will have a lot of structures that are capable of producing a high amount of available energy. This ATP production by the mitochondria is done by the process of respiration, which in essence is the use of oxygen in a process which generates energy. This is a very efficient process for using food energy to make ATP. One of the benefits of “aerobic exercise” is that it improves your body’s ability to make ATP rapidly using the respiration process.
All living cells have mitochondria. Hair cells and outer skin cells are dead cells and no longer actively producing ATP, but all cells have the same structure. Some cells have more mitochondria than others. Your fat cells have many mitochondria because they store a lot of energy. Muscle cells have many mitochondria, which allows them to respond quickly to the need for doing work. Mitochondria occupy 15 to 20 percent of mammalian liver cells according to Karp.
Mitochondria are found exclusively in eukaryotic cells. These organelles are often called the “power plants” of the cell because their main job is to make energy (ATP). Mitochondria are highly unusual–they contain their own genetic material and protein-making machinery enwrapped in a double membrane. Many scientists believe mitochondria were once free-living bacteria that colonized complex cells sometime during evolution. (Source: NSF).
Ribosomes are made from complexes of RNAs and proteins. Ribosomes are divided into two subunits, one larger than the other. The smaller subunit binds to the mRNA, while the larger subunit binds to the tRNA and the amino acids. When a ribosome finishes reading a mRNA these two subunits split apart. Ribosomes have been classified as ribozymes, since the ribosomal RNA seems to be most important for the peptidyl transferase activity that links amino acids together.
Ribosomes from bacteria, archaea and eukaryotes (the three domains of life on Earth), have significantly different structure and RNA sequences. These differences in structure allow some antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. The ribosomes in the mitochondria of eukaryotic cells resemble those in bacteria, reflecting the evolutionary origin of this organelle. The word ribosome comes from ribonucleic acid and the Greek: soma (meaning body).
Centrioles are cylindrical structures that are composed of groupings of microtubules arranged in a 9 + 3 pattern. The pattern is so named because a ring of nine microtubule “triplets” are arranged at right angles to one another. Centrioles are found in animal cells and play a role in cell division. Centrioles replicate in interphase stage of mitosis and they help to organize the assembly of microtubules during cell division. Centrioles called “basal bodies” form cilia and flagella. located near the nucleus
Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a “closed” mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.
The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.
Because cytokinesis usually occurs in conjunction with mitosis, “mitosis” is often used interchangeably with “mitotic phase”. However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development. Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer.
Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.
In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization, referred to as spores. In these groups, gametes are produced by mitosis.
Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes.
Meiosis comes from the root -meio, meaning less.
•If there are many of them, they are called cilia;
•if only one, or a few, they are flagella. Flagella also tend to be longer than cilia but are otherwise similar in construction. Flagellum is relatively long and there’s typically just one Function
Cilia and flagella move liquid past the surface of the cell.
•For single cells, such as sperm, this enables them to swim.
•For cells anchored in a tissue, like the epithelial cells lining our air passages, this moves liquid over the surface of the cell (e.g., driving particle-laden mucus toward the throat).
Both cilia and flagella consist of:
•a cylindrical array of 9 filaments consisting of:
◦a complete microtubule extending into the tip of the cilium;
◦a partial microtubule that doesn’t extend as far into the tip.
◦cross-bridges of the motor protein dynein that extend from the complete microtubule of one filament to the partial microtubule of the adjacent filament.
•a pair of single microtubules running up through the center of the bundle, producing the “9+2” arrangement.
•The entire assembly is sheathed in a membrane that is an extension of the plasma membrane.
This electron micrograph (courtesy of Peter Satir) shows a cilium in cross section.
Each cilium (and flagellum) grows out from, and remains attached to, a basal body embedded in the cytoplasm. Basal bodies are identical to centrioles and are, in fact, produced by them. For example, one of the centrioles in developing sperm cells — after it has completed its role in the distribution of chromosomes during meiosis — becomes a basal body and produces the flagellum.
The Sliding-Filament Model of Bending
The bending of cilia (and flagella) has many parallels to the contraction of skeletal muscle fibers. Link to discussion of the sliding-filament model of skeletal muscle.
In the case of cilia and flagella, dynein powers the sliding of the microtubules against one another — first on one side, then on the other.
There are also enzymes on the surface for digestion. Villus capillaries collects amino acids and simple sugars taken up by the villi into the blood stream. Villus lacteals (Lymph capillary) collect absorbed chylomicrons, which are lipoproteins composed of triglycerides, cholesterol and amphipathic proteins, and are taken to the rest of the body through the Lymph fluid.
The zygote carries within its single cell continuing threads in the immemorial lifespan of the human race, as well as the mixed-and-matched microscopic material from which will stem the intricacies common to all human bodies, yet with the remarkable uniqueness of a particular person.
Size and divisions
The small intestine in an adult human measures on average about 5 meters (16 feet), with a normal range of 3 – 7 meters; it can measure around 50% longer at autopsy because of loss of smooth muscle tone after death. It is approximately 2.5-3 cm in diameter. Although the small intestine is much longer than the large intestine (typically around 3 times longer), it gets its name from its comparatively smaller diameter. Although as a simple tube the length and diameter of the small intestine would have a surface area of only about 0.5m2, the surface complexity of the inner lining of the small intestine increase its surface area by a factor of 500 to approximately 200m2, or roughly the size of a tennis court.
The small intestine is divided into three structural parts: ..
Duodenum 26 cm (9.8 in) in length
Jejunum 2.5 m (8.2 ft)
Ileum 3.5 m (11.5 ft)
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.
An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.
Transcription is the first step leading to gene expression. The stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene. If the gene transcribed encodes for a protein, the result of transcription is messenger RNA (mRNA), which will then be used to create that protein via the process of translation. Alternatively, the transcribed gene may encode for either ribosomal RNA (rRNA) or transfer RNA (tRNA), other components of the protein-assembly process, or other ribozymes.
A DNA transcription unit encoding for a protein contains not only the sequence that will eventually be directly translated into the protein (the coding sequence) but also regulatory sequences that direct and regulate the synthesis of that protein. The regulatory sequence before (upstream from) the coding sequence is called the five prime untranslated region (5’UTR), and the sequence following (downstream from) the coding sequence is called the three prime untranslated region (3’UTR).
Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA; therefore, transcription has a lower copying fidelity than DNA replication.
As in DNA replication, DNA is read from 3′ → 5′ during transcription. Meanwhile, the complementary RNA is created from the 5′ → 3′ direction. Although DNA is arranged as two antiparallel strands in a double helix, only one of the two DNA strands, called the template strand, is used for transcription. This is because RNA is only single-stranded, as opposed to double-stranded DNA. The other DNA strand is called the coding strand, because its sequence is the same as the newly created RNA transcript (except for the substitution of uracil for thymine). The use of only the 3′ → 5′ strand eliminates the need for the Okazaki fragments seen in DNA replication.
Transcription is divided into 5 stages: pre-initiation, initiation, promoter clearance, elongation and termination.
Transcription initiation is more complex in eukaryotes. Eukaryotic RNA polymerase does not directly recognize the core promoter sequences. Instead, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after certain transcription factors are attached to the promoter does the RNA polymerase bind to it. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription initiation complex. Transcription in the archaea domain is similar to transcription in eukaryotes.
Promoter clearance coincides with phosphorylation of serine 5 on the carboxy terminal domain of RNA Pol in eukaryotes, which is phosphorylated by TFIIH.
Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene.
Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure.
Transcription termination in eukaryotes is less understood but involves cleavage of the new transcript followed by template-independent addition of As at its new 3′ end, in a process called polyadenylation.
A ribosome translating a protein that is secreted into the endoplasmic reticulum. tRNAs are colored dark blue.In many instances, the entire ribosome/mRNA complex will bind to the outer membrane of the rough endoplasmic reticulum and release the nascent protein polypeptide inside for later vesicle transport and secretion outside of the cell. Many types of transcribed RNA, such as transfer RNA, ribosomal RNA, and small nuclear RNA, do not undergo translation into proteins.
Translation proceeds in four phases: activation, initiation, elongation and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation). Amino acids are brought to ribosomes and assembled into proteins.
In activation, the correct amino acid is covalently bonded to the correct transfer RNA (tRNA). The amino acid is joined by its carboxyl group to the 3′ OH of the tRNA by a peptide bond. When the tRNA has an amino acid linked to it, it is termed “charged”. Initiation involves the small subunit of the ribosome binding to 5′ end of mRNA with the help of initiation factors (IF). Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). No tRNA can recognize or bind to this codon. Instead, the stop codon induces the binding of a release factor protein that prompts the disassembly of the entire ribosome/mRNA complex.
A number of antibiotics act by inhibiting translation; these include anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin, and puromycin, among others. Prokaryotic ribosomes have a different structure from that of eukaryotic ribosomes, and thus antibiotics can specifically target bacterial infections without any detriment to a eukaryotic host’s cells.
Types of Epithelial Tissue
Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.
Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.
In humans, skin pigmentation varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habit for bacteria which number roughly a 1000 species from 19 phyla
Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: There are three bones in each middle ear called the ossicles that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments.
Immovable joints are synarthroses. In this type of joint, the bones are in very close contact and are separated only by a thin layer of fibrous connective tissue. An example of a synarthrosis is the suture in the skull between skull bones.
Slightly movable joints are called amphiarthroses. This type of joint is characterized by bones that are connected by hyaline cartilage (fibro cartilage). The ribs that connect to the sternum are an example of an amphiarthrosis joint.
Most of the joints in the adult human body are freely movable joints. This type of joint is called a diarthrosis joint. There are six types of diarthroses joints. These are:
Ball-and-Socket: The ball-shaped end of one bone fits into a cup shaped socket on the other bone allowing the widest range of motion including rotation. Examples include the shoulder and hip.
Condyloid: Oval shaped condyle fits into elliptical cavity of another allowing angular motion but not rotation. This occurs between the metacarpals (bones in the palm of the hand) and phalanges (fingers) and between the metatarsals (foot bones excluding heel) and phalanges (toes).
Saddle: This type of joint occurs when the touching surfaces of two bones have both concave and convex regions with the shapes of the two bones complementing one other and allowing a wide range of movement. The only saddle joint in the body is in the thumb.
Pivot: Rounded or conical surfaces of one bone fit into a ring of one or tendon allowing rotation. An example is the joint between the axis and atlas in the neck.
Hinge: A convex projection on one bone fits into a concave depression in another permitting only flexion and extension as in the elbow joints.
Gliding: Flat or slightly flat surfaces move against each other allowing sliding or twisting without any circular movement. This happens in the carpals in the wrist and the tarsals in the ankle.
The cranial bones makeup the protective frame of bone around the brain.
The cranial bones are:
The frontal forms part of the cranial cavity as well as the forehead, the brow ridges and the nasal cavity.
The left and right parietal forms much of the superior and lateral portions of the cranium.
The left and right temporal form the lateral walls of the cranium as well as housing the external ear.
The occipital forms the posterior and inferior portions of the cranium. Many neck muscles attach here as this is the point of articulation with the neck.
The sphenoid forms part of the eye orbit and helps to form the floor of the cranium.
The ethmoid forms the medial portions of the orbits and the roof of the nasal cavity.
The joints between bones of the skull are immovable and called sutures. The parietal bones are joined by the sagittal suture. Where the parietal bones meet the frontal is referred to as the coronal suture. The parietals and the occipital meet at the lambdoidal suture. The suture between the parietals and the temporal bone is referred to as the squamous suture. These sites are the common location of fontanelles or “soft spots” on a baby’s head.
The facial bones makeup the upper and lower jaw and other facial structures.
The facial bones are:
The mandible is the lower jawbone. It articulates with the temporal bones at the temporomandibular joints. This forms the only freely moveable joint in the head. It provides the chewing motion.
The left and right maxilla are the upper jaw bones. They form part of the nose, orbits, and roof of the mouth.
The left and right palatine form a portion of the nasal cavity and the posterior portion of the roof of the mouth.
The left and right zygomatic are the cheek bones. They form portions of the orbits as well.
The left and right nasal form the superior portion of the bridge of the nose.
The left and right lacrimal help to form the orbits.
The vomer forms part of the nasal septum (the divider between the nostrils).
The left and right inferior turbinate forms the lateral walls of the nose and increase the surface area of the nasal cavity.
The sternum is composed of three parts:
The manubrim, also called the “handle”, is located at the top of the sternum and moves slightly. It is connected to the first two ribs.
The body, also called the “blade” or the “gladiolus”, is located in the middle of the sternum and connects the third to seventh ribs directly and the eighth through tenth ribs indirectly.
The xiphoid process, also called the “tip”, is located on the bottom of the sternum. It is often cartilaginous (cartilage), but does become bony in later years.
These three segments of bone are usually fused in adults.
The sternum serves an important function in the body. The ribs are connected to it by the costal cartilage. Without the sternum, there would be a hole in the bone structure in the middle of your chest, right above your heart and lungs. The sternum protects this vital area and completes the circle of the rib cage.
These bones are divided into three categories:
The first seven bones are called the true ribs. These bones are connected to the spine (the backbone) in back. In the front, the true ribs are connected directly to the breastbone or sternum by a strips of cartilage called the costal cartilage.
The next three pairs of bones are called false ribs. These bones are slightly shorter than the true ribs and are connected to the spine in back. However, instead of being attached directly to the sternum in front, the false ribs are attached to the lowest true rib.
The last two sets of rib bones are called floating ribs. Floating ribs are smaller than both the true ribs and the false ribs. They are attached to the spine at the back, but are not connected to anything in the front.
The ribs form a kind of cage the encloses the upper body. They give the chest its familiar shape.
The ribs serve several important purposes. They protect the heart and lungs from injuries and shocks that might damage them. Ribs also protect parts of the stomach, spleen, and kidneys. The ribs help you to breathe. As you inhale, the muscles in between the ribs lift the rib cage up, allowing the lungs to expand. When you exhale, the rib cage moves down again, squeezing the air out of your lungs.
The first seven vertebrae are called the cervical vertebrae. Located at the top of the spinal column, these bones form a flexible framework for the neck and support the head. The first cervical vertebrae is called the atlas and the second is called the axis. The atlas’ shape allows the head to nod “yes” and the axis’ shape allows the head to shake “no”.
The next twelve vertebrae are called the thoracic vertebrae. These bones move with the ribs to form the rear anchor of the rib cage. Thoracic vertebrae are larger than cervical vertebrae and increase in size from top to bottom.
After the thoracic vertebrae, come the lumbar vertebrae. These five bones are the largest vertebrae in the spinal column. These vertebrae support most of the body’s weight and are attached to many of the back muscles.
The sacrum is a triangular bone located just below the lumbar vertebrae. It consists of four or five sacral vertebrae in a child, which become fused into a single bone after age 26. The sacrum forms the back wall of the pelvic girdle and moves with it.
The bottom of the spinal column is called the coccyx or tailbone. It consists of 3-5 bones that are fused together in an adult. Many muscles connect to the coccyx.
These bones compose the vertebral column, resulting in a total of 26 movable parts in an adult. In between the vertebrae are intervertebral discs made of fibrous cartilage that act as shock absorbers and allow the back to move. As a person ages, these discs compress and shrink, resulting in a distinct loss of height (generally between 0.5 and 2.0cm) between the ages of 50 and 55.
When looked at from the side, the spine forms four curves. These curves are called the cervical, thoracic, lumbar, and pelvic curves. The cervical curve is located at the top of the spine and is composed of cervical vertebrae. Next come the thoracic and lumbar curves composed of thoracic and lumbar vertebrae respectively. The final curve called the pelvic or sacral curve is formed by the sacrum and coccyx. These curves allow human beings to stand upright and help to maintain the balance of the upper body. The cervical and lumbar curves are not present in an infant. The cervical curves forms around the age of 3 months when an infant begins to hold its head up and the lumbar curve develops when a child begins to walk.
In addition to allowing humans to stand upright and maintain their balance, the vertebral column serves several other important functions. It helps to support the head and arms, while permitting freedom of movement. It also provides attachment for many muscles, the ribs, and some of the organs and protects the spinal cord, which controls most bodily functions.
The Lower Extremities
The Shoulder Girdle
The Pelvic Girdle–(the sacrum and coccyx are considered part of the vertebral column)
The arm, or brachium, is technically only the region between the shoulder and elbow. It consists of a single long bone called the humerus. The humerus is the longest bone in the upper extremity. The top, or head, is large, smooth, and rounded and fits into the scapula in the shoulder. On the bottom of the humerus, are two depressions where the humerus connects to the ulna and radius of the forearm. The radius is connected on the side away from the body (lateral side) and the ulna is connected on the side towards the body (medial side) when standing in the anatomical position. Together, the humerus and the ulna make up the elbow. The bottom of the humerus protects the ulnar nerve and is commonly known as the “funny bone” because striking the elbow on a hard surface stimulates the ulnar nerve and produces a tingling sensation.
The forearm is the region between the elbow and the wrist. It is formed by the radius on the lateral side and the ulna on the medial side when the forearm is viewed in the anatomical position. The ulna is longer than the radius and connected more firmly to the humerus. The radius, however, contributes more to the movement of the wrist and hand than the ulna. When the hand is turned over so that the palm is facing downwards, the radius crosses over the ulna. The top of each bone connects to the humerus of the arm and the bottom of each connects to the bones of the hand.
The hand consists of three parts (the wrist, palm, and five fingers) and 27 bones.
The wrist, or carpus, consists of 8 small bones called the carpal bones that are tightly bound by ligaments. These bone are arranged in two rows of four bones each. The top row (the row closest to the forearm) from the lateral (thumb) side to the medial side contains the scaphoid, lunate, triquetral, and pisiform bones. The second row from lateral to medial contains the trapezium, trapezoid, capitate, and hamate. The scaphoid and lunate connect to the bottom of the radius.
The palm or metacarpus consists of five metacarpal bones, one aligned with each of the fingers. The metacarpal bones are not named but are numbered I to V starting with the thumb. The bases of the metacarpal bones are connected to the wrist bones and the heads are connected to the bones of the fingers. The heads of the metacarpals form the knuckles of a clenched fist.
The fingers are made up of 14 bones called phalanges. A single finger bone is called a phalanx. The phalanges are arranged in three rows. The first row (the closest to the metacarpals) is called the proximal row, the second row is the middle row, and the farthest row is called the distal row. Each finger has a proximal phalanx, a middle phalanx, and a distal phalanx, except the thumb (also called the pollex) which does not have a middle phalanx. The digits are also numbered I to V starting from the thumb.
The thigh is the region between the hip and the knee and is composed of a single bone called the femur or thighbone. The femur is the longest, largest, and strongest bone in the body.
The leg is technically only the region from the knee to the ankle. It is formed by the fibula on side away from the body (lateral side) and the tibia, also called the shin bone, on the side nearest the body (medial side). The tibia connects to the femur to form the knee joint and with the talus, a foot bone, to allow the ankle to flex and extend. The tibia is larger than the fibula because it bears most of the weight, while the fibula serves as an area for muscle attachment.
The foot, or pes, contains the 26 bones of the ankle, instep, and the five toes. The ankle, or tarsus, is composed of the 7 tarsal bones which correspond to the carpals in the wrist. The largest tarsal bone is called the calcaneus or heel bone. The talus rests on top of the calcaneus and is connected to the tibia. Directly in front of the talus is the navicular bone. The remaining bones from medial to lateral are the medial, intermediate, the lateral cuneiform bones, and the cuboid bone.
The metatarsal and phalanges bones of the foot are similar in number and position to the metacarpal and phalanges bones of the hand. The five metatarsal bones are numbered I to V starting on the medial side with the big toe. The first metatarsal bone is larger than the others because it plays a major role in supporting the body’s weight. The 14 phalanges of the foot, as with the hand, are arranged in a proximal row, a middle row, and a distal row, with the big toe, or hallux, having only a proximal and distal phalanx.
The foot’s two arches are formed by the structure and arrangement of the bones and are maintained by tendons and ligaments. The arches give when weight is placed on the foot and spring back when the weight is lifted off of the foot. The arches may fall due to a weakening of the ligaments and tendons in the foot.
The patella or kneecap is a large, triangular sesamoid bone between the femur and the tibia. It is formed in response to the strain in the tendon that forms the knee. The patella protects the knee joint and strengthens the tendon that forms the knee.
The bones of the lower extremities are the heaviest, largest, and strongest bones in the body because they must bear the entire weight of the body when a person is standing in the upright position.
The clavicle, commonly called the collarbone, is a slender S-shaped bone that connects the upper arm to the trunk of the body and holds the shoulder joint away from the body to allow for greater freedom of movement. One end of the clavicle is connected to the sternum and one end is connected to the scapula.
Thescapula is a large, triangular, flat bone on the back side of the rib cage commonly called the shoulder blade. It overlays the second through seventh rib and serves as an attachment for several muscles. It has a shallow depression called the glenoid cavity that the head of the humerus (upper arm bone) fits into.
Usually, a “girdle” refers to something that encircles or is a complete ring. However, the shoulder girdle is an incomplete ring. In the front, the clavicles are separated by the sternum. In the back, there is a gap between the two scapulae.
The primary function of the pectoral girdle is to provide an attachment point for the numerous muscles that allow the shoulder and elbow joints to move. It also provides the connection between the upper extremities (the arms) and the axial skeleton.
In the back, these two bones meet on either side of the sacrum. In the front, they are connected by a muscle called the pubic symphysis (denoted in green above).
The pelvic girdle serves several important functions in the body. It supports the weight of the body from the vertebral column. It also protects and supports the lower organs, including the urinary bladder, the reproductive organs, and the developing fetus in a pregnant woman.
The pelvic girdle differs between men and woman. In a man, the pelvis is more massive and the iliac crests are closer together. In a woman, the pelvis is more delicate and the iliac crests are farther apart. These differences reflect the woman’s role in pregnancy and delivery of children. When a child is born, it must pass through its mother’s pelvis. If the opening is too small, a cesarean section may be necessary.
**The picture above is of a female pelvis. All divisions are approximations only.**
Superior – toward the head
Inferior – away from the head
Anterior – the front of the body or body part
Posterior – the back of the body or body part
Medial – toward the midline that divides left and right
Lateral – to the side away from the midline
Proximal – closer to the torso
Distal – farther away from the torso
Anatomical position – standing erect, facing the observer, arms are at the sides with palms facing forward.
The human muscular system is made up of over 600 muscles, which act in groups. Muscles, in turn, are made up of fibers and cells. Muscles are what enable you to do just about everything – from walking to lifting heavy objects to helping to pump blood throughout the body. Muscles are distinguished as either involuntary or voluntary. Involuntary muscles function within the body automatically, without you being able to control them. Voluntary muscles are the ones that are under your control.
All muscles are made up of the same type of material – a kind of an elastic tissue, akin to what rubber bands are made of. Each muscle is made up of thousands of tiny fibers. There are three kinds of muscles in the human muscular system: the skeletal muscle; the cardiac muscle; and the smooth muscle. Plus, the facial muscles and the tongue are a unique kind by themselves.
The Skeletal Muscles: These are the voluntary type of muscles in the human muscular system. This means that they can be controlled by you. For example, you cannot pick up that mug of coffee with your hand unless you want your hand to do so. They are referred sometimes as striated muscles, because the dark and light fibrous material make them seem striped. These are also known as the musculoskeletal system, or the combination of the muscles and the bones that make up the skeleton.
Generally, skeletal muscles are attached to the ends of bones, stretching all across the joint and then attached once more to another bone. Tendons, which are cords or bands of inelastic tissue, are what attach the muscles to the bones. Skeletal muscles are of different shapes and sizes, which enable them to perform a variety of tasks. The gluteus maximus, or the muscle that occurs in the buttocks, is the largest skeletal muscle in the human muscular system. Some of the other major skeletal muscles are the deltoid muscle in the shoulders, the biceps and triceps in the arm, the pectoralis in the chest, the rectus abdominus in the abdomen, the quadriceps and the hamstring muscles in the legs.
The Cardiac Muscle: The heart is made up of the cardiac muscle, which is also referred to as the myocardium. These muscles are thick and contract in order to pump out the blood and then relax in order to allow more blood in. The cardiac muscle is an involuntary muscle, or the type that works without your volition. Special type of cells in the cardiac muscle, called the pacemaker, help in controlling the heartbeat.
The Smooth Muscles: These are the involuntary muscles of the human muscular system, and they generally occur in layers or sheets, with one muscle layer behind another. These muscles are not under your control. The brain and the body control these muscles in performing their functions without any conscious volition from your part.
Some of the examples of smooth muscles are the stomach and the digestive system, which contract and relax in order to pass food through the alimentary canal of the body. The bladder is another example of smooth muscle, and so is the uterus in women. Smooth muscles also occur in the eyes, which help to keep the eyes focused. According to scientists, the eyes can move over 100,000 times in a day, making them the busiest muscles in the human muscular system.
The Facial Muscles: There are more than 30 muscles in the face. Not all of the facial muscles are attached to bones, as is the case in the other parts of the body. Many of the facial muscles are attached to the underside of facial skin. The contractions of these muscles are what give the face its various expressions, such as frowning, laughter, surprise, sadness and so on.
The Tongue: And another unique muscle is the tongue, which is free at one end and only attached on the other end. The tongue actually comprises of a group of muscles, which work in unison, enabling you to chew and swallow food, and talk.
Neurons send signals to other cells as electrochemical waves travelling along thin fibres called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal may be excited, inhibited, or otherwise modulated. Sensory neurons are activated by physical stimuli impinging on them, and send signals that inform the central nervous system of the state of the body and the external environment. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect the nervous system to muscles or other effector organs. Central neurons, which in vertebrates greatly outnumber the other types, make all of their input and output connections with other neurons. The interactions of all these types of neurons form neural circuits that generate an organism’s perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.
Nervous systems are found in most multicellular animals, but vary greatly in complexity. Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion. Placozoans and mesozoans—other simple animals that are not classified as part of the subkingdom Eumetazoa—also have no nervous system. In Radiata (radially symmetric animals such as jellyfish) the nervous system consists of a simple nerve net. Bilateria, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, spinal cord, and peripheral nerves. The size of the bilaterian nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans. Neuroscience is the study of the nervous system.
The endocrine system is made up of a series of ductless glands that produce chemicals called hormones. A number of glands that signal each other in sequence is usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. Typical endocrine glands are the pituitary, thyroid, and adrenal glands. Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.
In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.
Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.
Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.
The urinary system removes a type of waste called urea from your blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.
The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.
From the kidneys, urine travels down two thin tubes called ureters to the bladder. The ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine downward away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection can develop. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.
The bladder is a hollow muscular organ shaped like a balloon. It sits in your pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until you are ready to go to the bathroom to empty it. It swells into a round shape when it is full and gets smaller when empty. If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours.
Circular muscles called sphincters help keep urine from leaking. The sphincter muscles close tightly like a rubber band around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body.
Nerves in the bladder tell you when it is time to urinate, or empty your bladder. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The sensation to urinate becomes stronger as the bladder continues to fill and reaches its limit. At that point, nerves from the bladder send a message to the brain that the bladder is full, and your urge to empty your bladder intensifies.
When you urinate, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax. As these muscles relax, urine exits the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.
The main male sex organs are the penis and the testes which produce semen and sperm, which as part of sexual intercourse fertilize an ovum in a woman’s body and the fertilized ovum (zygote) gradually develops into a fetus, which is later born as a child
If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova are larger than sperm and have formed by the time a female is born. Approximately every month, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation.