Biology 3

The Menstrual Cycle:

Begins with FSH, which stimulates the maturation of ovum and the follicle in the ovary (FSH also stimulates maturation of sperm in males). LH then stimulates cells in the ovaries to secrete estrogen (estradiol), which signals the uterine wall to proliferate and prepare for a potential pregnancy. Just before ovulation, a surge in the level of estrogen released causes a subsequent surge in levels of LH (luteal surge), which signals ovulation and the development of the corpus luteum. The corpus lutem will begin secreting estrogen and high levels of progesterone to signal the uterine lining to prepare for implantation. If no fertilization occurs the corpus luteum degrades and the menstrual lining sloughs off. If implantation does occur, the corpus luteum continues to secrete estrogen and progesterone, maintaining pregnancy. Fertilization: Fertilization usually occurs in the fallopian tubes. Sperm and egg, traveling toward one another, generally meet here. Implantation normally occurs in the uterus, but can occur in the fallopian tubes, leading to a "tubal" or "ectopic" pregnancy.

Estrogen and Progesterone Levels:

If hormone levels go up unexpectedly, high levels could cause unexpected menstruation if— and only if—that increase was followed by a sudden decrease. It is the drop in hormone levels that causes menstruation. Unexpected menstruation of this type can sometimes be seen in elderly women given estrogen/progesterone therapy for treatment of osteoporosis. To have menstruation, or the sloughing off of the uterine lining, we must first have a build- up of that lining. Build-up and maintenance of the uterine lining is one of the normal functions associated with estrogen/progesterone (see the Hormone Chart in Biology 2). Do NOT confuse menstruation with ovulation. Ovulation requires a surge in hormone levels. Menstruation requires a chronic decline. Persistent low levels of estrogen/progesterone lead to the symptoms of menopause described below. Menopause: Know the general symptoms of menopause, including: hot flashes, vaginal dryness, and atrophy of breast tissue. All of these are the result of decreased estrogen and progesterone levels.

The Pancreas:

Is both an endocrine and an exocrine gland (exocrine because of the digestive enzymes listed below and endocrine because of insulin and glucagon). Secretes a bicarbonate rich solution, which neutralizes the stomach acid, decreasing the acidity of the intestine to pH = 6. Secretes the following digestive enzymes: trypsin, chymotrypsin, pancreatic amylase, lipase, ribonuclease, and deoxyribonuclease. All pancreatic secretions empty into the upper end of the duodenum. Just before emptying into the duodenum the pancreatic duct is joined by the bile duct.

The Gallbladder:

Stores and concentrates bile, but does NOT produce bile. Bile is produced in the liver. Bile emulsifies fats (i.e., separates fat molecules from each other, increasing the surface area available for enzymatic digestion).

Four Stomach Lining Cell Types:

1) Mucous Neck Cells: Make and secrete mucus (into gastric pits, which lead to the stomach lumen). The secreted mucous is alkaline, thus providing protection from corrosion due to the extreme acidity of the stomach. NOTE: "Goblet cells" are mucous-secreting cells found in the linings of the intestines and respiratory tract. Don't confuse them with mucous cells. 2) Chief Cells: Make and secrete the zymogen pepsinogen (into gastric pits/stomach lumen). 3) Parietal Cells: Secrete HCl (into gastric pits/stomach lumen). HCl is responsible for the extreme acidity (pH = 2) of the stomach and for the conversion of pepsinogen to pepsin, the active form of the enzyme. 4) G-cells: Make and secrete gastrin. Gastrin is released into the blood (NOT into the gastric pits/stomach lumen) where it circulates back to the parietal and chief cells stimulating them to release HCl and pepsinogen thus further enhancing digestion.

Skeletal Muscle Anatomy:

A muscle group, such as the bicep, is a bundle of many fasciculi. Each fascicle is a bundle of many long, tubular cells called muscle fibers. Around each muscle cell is a specialized cell membrane called the sarcolemma. Inside each muscle cell are many nuclei (multinucleate). Nearly the entire volume of each cell is filled with smaller round tubes called myofibrils. Myofibrils are long bundles of proteins mainly composed of actin and myosin fibers interconnected in repeating units called sarcomeres. Bundles of myofibrils are interwoven among portions of the muscle cell's endoplasmic reticulum, called the sarcoplasmic reticulum. It is the sarcoplasmic reticulum that stores and releases Ca2+ to control the contraction process (see below for more info).

Acquired Immunity:

A specific response to one particular virus, bacteria or other pathogen based upon prior exposure. There are two types of acquired immunity: humoral and cell-mediated. Humoral Immunity (a.k.a. "B-cell Immunity" or "Antibody-Mediated Immunity"): Involves B-cells (i.e., B-lymphocytes). When you see "B-cells" or "antibody," THINK of humoral immunity. B-cells develop in the bone marrow and mature either there or in lymph tissues. Each B-cell produces only one of a certain kind of protein receptor on its membrane called an antibody (a.k.a., immunoglobulin). Each antibody will recognize and bind with only one foreign particle called an antigen. If an antigen binds to a B-cell's antibody, the B-cell will undergo differentiation into a plasma cell and a memory B-cell. The plasma cells manufacture free antibodies and release them into the blood. The memory B- cells multiply and remain in the blood, preparing the body for a secondary response. Primary Response = the immune system's first exposure and reaction to a pathogen. Secondary Response = the immune system's response to that same pathogen during subsequent exposures.

Q24. As a review from Biology 2, is ACh used at the ganglia or at the effector in the sympathetic nervous system?

Acetylcholine is used at the ganglionic synapses in the sympathetic nervous system. Norepinephrine is used at the synapse with the target/effector.

The Motor Unit:

All of the cells/fibers in a skeletal muscle do not fire simultaneously during a contraction. A group of muscle cells innervated by a single motor neuron is called a "motor unit" . Motor units come in all different sizes from large to small. Delicate movements = very small motor units Gross movements = larger motor units The strength of a given contraction depends on the: 1) number of motor units being used 2) size of the motor units being used 3) frequency of action potentials (i.e., stimulation)

Antigen Presentation:

Antigen presentation is the process by which a cell that has engulfed an antigen or microbe, (via receptor-mediated endocytosis or phagocytosis) takes protein portions of that microbe or antigen and "presents" them on MHC (Major Histocompatability Complex) proteins embedded in its own cell membrane for other cells to "see." Macrophages, dendritic cells, and B-cells can all "present" antigens (Note: Other cells types can also "present" to a small extent, but they do not play a major role in immune system function)

Q33. What portion of the menstrual cycle illustrates a positive feedback mechanism?

Around day 14 of the menstrual cycle high estrogen levels stimulate a rapid increase in luteinizing hormone (LH), a good example of a positive feedback mechanism. This does not happen during other stages of the cycle where LH stimulates estrogen, and estrogen provides negative feedback to the hypothalamus inhibiting further LH secretion (i.e., classic negative feedback loop). However, as mid-cycle approaches estrogen levels provide positive feedback to the hypothalamus, stimulating it to secrete more LH, which in turn stimulates the production of more estrogen. This causes the "LH surge," which results in ovulation.

B-cells:

B-cells that bind their specific antigen engulf it via receptor-mediated endocytosis and present a portion on MCH proteins in their cell membranes. Helper T-cells recognize and bind this antigen, which causes the helper T-cell to release chemicals that stimulate other B and T cells and—most importantly—stimulates the B-cell that presented it with the antigen to divide into a plasma cell and a memory B cell as described above. In other words, it is through antigen presentation that a Helper T-cell "helps" (i.e., activates) a B-cell to perform its function. A B-cell can either bind to a free-floating antigen, or it may have an antigen "presented" to it by a macrophage.

Q11. Is bile's action an example of physical or chemical digestion?

Bile emulsifies fats, but it does not break any bonds—therefore it is an example of physical rather than chemical digestion.

Tissues of the Immune System:

Bone Marrow: All erythrocytes and leukocytes are made in the red bone marrow via hematopoiesis. Yellow bone marrow is primarily adipose tissue and does NOT produce blood cells. B lymphocytes mature in the bone marrow, while T lymphocytes migrate to the thymus to mature. Spleen: Somewhat analogous to a lymph node that filters blood instead of lymph; high concentration of leukocytes (WBCs) and platelets; storage of a considerable amount of blood that can help combat hemorrhagic shock; breaks down and recycles parts of old erythrocytes (RBCs). Thymus: Location where T-lymphocytes (a.k.a., T cells) acquire immunocompetency, differentiate, and mature. Lymph Nodes: High concentration of B- and T-lymphocytes. Filters blood for pathogens. Other Lymphatic Tissues: Lymph tissue similar to the contents of a lymph node is spread throughout the body, being particularly common in the respiratory and gastrointestinal tracks.

Q3. Digestion begins in the mouth with the physical digestion of all food types and the chemical digestion of ________ only (via the enzyme __________ ).

Carbohydrates ; alpha-amylase

Skeletal Muscle:

Characteristics: voluntary, striated, and multinucleate. Attachments: Tendons attach muscle to bone; ligaments attach bone to bone. The joints formed by most muscles and bones in the human body are examples of hinges with a poor lever system that work at a mechanical disadvantage. For example, for the biceps brachii, the resistance is six times farther from the fulcrum than the force—meaning the muscle must create a force equal to six times the weight of the object in the hand.

Q23. Following contraction, is calcium pumped into, or out of, the SR?

Contraction occurs when calcium is present. After contraction is complete, calcium must be actively transported back into the sarcoplasmic reticulum and sequestered there until the next contraction. The myofibrils are NOT located inside the sarcoplasmic reticulum. Some students seem to have developed the idea that they were, due to the use of phrases such as: "the sarcoplasmic reticulum surrounds each myofibril." This is true, an extensive network of SR does "wrap around" each myofibril, but the myofibrils are not INSIDE of the sarcoplasmic reticulum. Compare this to wrapping your gloved hand around a ball. The glove could be said to be "around" the ball, but the ball is clearly not INSIDE of the glove. It can be said that the myofibrils are inside of the sarcolemma (i.e., muscle cell membrane) and surrounded by sarcoplasm.

Q31. Describe the specific purpose of each substance secreted into the ejaculate by the seminal vesicles, prostate gland, and bulbourethral gland.

During ejaculation the first addition to the ejaculate comes from the seminal vesicles. They release the majority of the fluids that make up semen, including fructose and alkaline fluids that make the semen basic. The alkaline nature of semen helps neutralize the acidic environment of the vagina and the fructose provides nutrients for the sperm. The vas deferens continues into the prostate gland, which secretes a milky white fluid that is slightly acidic and contains proteases. The prostate gland secretions play a protective role, as sperm have been shown to have longer survival rates and better protection of their genetic material in the presence of prostate secretions as compared to without them. The vas deferens then dumps its contents into the urethra where it passes the bulbourethral glands (a.k.a., Cowper's glands). The bulbourethral glands do not add fluids to the ejaculate at this point. They secrete a fluid called pre-ejaculate that lubricates and neutralizes any acidic urine in the urethra prior to the arrival of the other semen components. The term semen refers to the entire ejaculate with all contributed fluids plus the sperm.

Q14. A disease or illness that inhibits the normal function of the large intestine will most likely result in .

Either a vitamin deficiency or problems with water balance—either too little water absorption (diarrhea), or too much water absorption (constipation). These are logical assumptions because the two primary functions of the large intestine are water absorption and vitamin absorption.

Q1. Recall that humans are heterotrophs, so they cannot ___________?

Fix carbon. The ability to "fix" carbon is the ability to capture carbon dioxide and integrate that carbon into larger macromolecules. Plants, algae and some bacteria can do this, humans and animals generally cannot. We must eat the carbon we need to build macromolecules.

Innate Immunity Inflammatory Response:

For the MCAT, we will stick to general basics. First, macrophages, mast cells and dendritic cells are residents of nearly all tissues. When damage is caused—by injury, bacterial invasion, etc.—these cells are activated to release chemicals such as histamines, leukotrienes, and prostaglandins. These chemicals increase blood flow to the injury site, creating heat and redness. These chemicals also increase the permeability of veins and lymph vessels. This causes plasma and interstitial fluid to flood the infection site, resulting in swelling (a.k.a., edema). This swelling and increase in chemicals lead to the pain associated with inflammation as well. Neutrophils are recruited via chemotaxis in very large numbers to the inflammation site.

The Immune System

Function: Protect the body from infection and disease; destroy pathogens invading the body.

The Digestive System

Function: To separate food molecules from each other (i.e., physical digestion) and break up those molecules into their monomers for absorption (i.e., chemical digestion). These monomers are then used for both energy and as a source of carbon chains and amino acids.

The Stomach:

Functions include: food storage, mixing, and the first site of protein digestion. Without an adequate stomach, food moves too fast through the digestive system, resulting in incomplete digestion and absorption.

The Musculoskeletal System

Functions: Movement, support and stabilization, generation of heat, aide to circulation, and maintenance of homeostasis.

The Integumentary System

Functions: Protection against abrasion, physical barrier to pathogens, vitamin D synthesis, insulation/cushioning (due to subcutaneous fat), prevention of water-loss, temperature regulation. o Includes the hair, nails, skin, and the oil and sweat glands located within the skin. Skin: o Epidermis: Is avascular and made up of mostly dead or dying, keratinized cells. o Dermis: Contains blood vessels, hair follicles, sebaceous glands (oil), sudoriferous glands (sweat), and nerve endings. The dermis is a connective tissue. o Thermoregulation: 1) Blood vessels closer to the surface of the skin dilate when heat needs to be released and constrict when heat needs to be retained. Blushing is the result of the dilation of these superficial blood vessels. 2) Arrector pili muscles cause erection of hair follicles (resulting in "goose bumps"), that traps an insulating layer of air next to the skin. Contraction of Arrector pili muscles can also generate a small amount of heat. 3) Subcutaneous fat provides insulation. 4) Sweating, followed by evaporation of that sweat, carries away a significant amount of heat due to the high heat of vaporization of water and its high specific heat capacity.

The Liver:

Functions: Think of the liver as the "metabolic brain" of the human body. It regulates the blood concentrations of many different solutes, plays key roles in the metabolism of proteins, fats and carbohydrates, detoxifies chemicals, recycles metabolites, and manufactures several key biomolecules. The liver has hundreds of specific functions. Here are a key few to focus on: Produces Bile (stored and concentrated in gall bladder) Filters the blood to remove toxins, drugs, metabolites, bacteria, etc. Produces blood plasma proteins, including albumin, prothrombin and fibrinogen Regulates amino acid levels in the blood. Produces cholesterol and lipoproteins and packages them for transport (LDL, HDL, etc.) Role of the Liver in Glucose Metabolism: In response to low blood glucose levels alpha cells in the pancreas secrete glucagon. Glucagon stimulates glycogenolysis in the liver—the breakdown of glycogen stored in the liver to form free glucose for release into the blood. In contrast, high blood glucose levels stimulate beta cells in the pancreas to secrete insulin. Insulin stimulates glycogenesis in the liver—the synthesis of glycogen for storage in the liver. Insulin also stimulates the uptake of glucose from the blood into the cells. Finally, the liver also makes glucose out of lactate, glycerol, amino acids, and some TCA cycle intermediates.

Q6. Is gastrin a peptide, a steroid, or a tyrosine derivative? Is gastrin likely to bind at a membrane receptor? If so, where would you expect that receptor to be located?

Gastrin is a peptide hormone. Therefore, we would expect it to be soluble in blood without a carrier molecule and to require a membrane receptor because it cannot dissolve through the non-polar interior of the bi-layer membrane.

Q9. The process just described, by which the liver makes new glucose, is called ___________________.

Gluconeogenesis.

Q28. Provide conceptual definitions for each of the following: hematopoeisis, red bone marrow, yellow bone marrow, spongy bone and compact bone.

Hematopoiesis is the name given to the formation and differentiation of blood cells in the bone marrow. The flowchart given previously to illustrate the source of immune system cells demonstrates hematopoiesis. This process occurs in the red bone marrow that fills the pockets of spongy bone. Hematopoiesis does NOT occur in the yellow bone marrow that fills the medullary cavity of long bones. Yellow bone marrow consists mostly of fat. Compact bone is the dense bone that surrounds the outside of all bones, and constitutes the shafts of long bones. The interior of flat and irregular bones, as well as the bulbous ends of the long bones, is filled with spongy bone. Spongy bone contains many open spaces, formed by the interwoven trabeculae. These spaces are filled with red bone marrow. Compact bone is many times more compact. It is organized into orderly units called osteons, and the only spaces it contains are Haversion canals and canaliculi.

Q5. Proteins in the stomach undergo _________, a reaction catalyzed by the enzyme ___________. This enzyme begins as, ________ an example of a zymogen. Zymogens are inactive enzyme precursors.

Hydrolysis; pepsin; pepsinogen

Q29. Provide a conceptual definition for, and describe the primary function of, each of the following: penis, testicles, scrotum, sperm, seminiferous tubules, epididymis, vas deferens, seminal vesicles, prostate gland, bulbourethral gland, and urethra.

In humans, the penis is the male copulatory organ. It can also be thought of as playing a structural role in excretion and ejaculation because the urethra runs through it. The testicles serve the primary functions of making, nurturing and storing sperm. The scrotum is the thin sack of skin in which the testes are located. The external location of the scrotum allows the testicles to exist at a temperature a few degrees lower than the normal human body temperature of 37°C. The optimum temperature for spermatogenesis is 35°C. Sperm cells are the male haploid gametes. They are produced in the seminiferous tubules of the testes and move to the epididymis, where they are nurtured, fully matured, and stored until ejaculation. The vas deferens is a duct that connects each testicle with the urethra. Beginning at the epididymis, it leads up the inside of the scrotum, into the pelvic cavity, past the seminal vesicles, and prostate gland, and dumps into the urethra before the urethra enters the penis.

Cell-Mediated Immunity (a.k.a. "T-cell Immunity"):

Involves T-cells (T-Lymphocytes). When you see T-cells, THINK of cell-mediated immunity. T-cells are made in the bone marrow like B-cells, but mature in the thymus. T-cells have receptor proteins embedded in their cell membrane. Unlike B-cells, they never produce free antibodies. Instead, they are "tested" in the thymus against the host's own membrane proteins (called "self-antigens"). All T-cells matching a self-protein are destroyed, leaving only cells that will recognize invaders. T-cells that pass this test will differentiate into one of the T-cell types previously described in this lesson. To Review: WHEN YOU SEE "CELL-MEDIATED" THINK: self-attack of diseased cells WHEN YOU SEE "HUMORAL" THINK: antibodies and primary/secondary response.

Skeletal Structure:

JointTypes: 1)Fibrous (skull bones) 2)Cartilaginous (ribs to sternum) 3)Synovial (knee, elbow, etc.) Bone Types: Long bones (e.g., femur, humerus), short bones (e.g., tarsals, carpals), flat bones (i.e., skull, sternum), irregular bones (e.g., hip, vertebrae) Anatomy of a Long Bone: Two epiphyses (bulbous ends) cushioned by cartilage; the ends are filled with spongy bone and the shaft in between is made of compact bone; the center is a hollow cavity filled with yellow bone marrow. Hydroxyapatite is an inorganic compound of calcium, phosphate, and hydroxide. It is the mineral matrix responsible for a bone's strength and is the form in which most all of the body's calcium is stored.

Q16. Provide a conceptual definition for each cell type and describe its basic function: macrophages, neutrophils, basophils, eosinophils, mast cells, dendritic cells, natural killer cells, T-cells, B-cells, plasma cells, memory B cells, helper T-cells, suppressor T-cells, killer T-cells, and memory T-cells.

Macrophages are white blood cells, the mature form of a monocyte. They phagocytize pathogens and cellular debris. In terms of their non-specific engulfing of foreign material, they participate in innate immunity. They also present antigens from the pathogens they consume on their cell membrane which are recognized by B and T cells—a role in acquired immunity. Neutrophils are one of three kinds of granulocytes—the other two being basophils and eosinophils. The three cells get their names from how they appear when stained and viewed under a light microscope. Commonly used laboratory preparations stain basophils a dark blue (due to the basic nature of the granules), eosinophils a bright red (due to the acidic nature of the granules), and neutrophils a "neutral" pale pink. Similarly, the term "granulocyte" comes from the fact that all three cells contain large cellular granules easily visible with a microscope. Neutrophils are phagocytes that are recruited to areas of infection and inflammation by chemotaxis. They live for only about 5 days, but are the most abundant of all white blood cells. The pus created at a wound is mostly dead neutrophils. Basophils are the least common white blood cell. Their granules contain mostly histamine, which they release along with other chemicals when activated. These chemicals promote inflammation and are integral in the allergic response, so many think of basophils as roughly associated with allergies. Eosinophils are recruited to areas of parasitic invasion, particularly multicellular parasites where they release their granules containing peroxidases and other enzymes that digest tissue. This would destroy the pathogen but could also destroy host tissue. Note that all granulocytes are short- lived and do not reside permanently in the tissues. All granulocytes circulate in the blood and are recruited to areas of infection/inflammation. Mast cells, contrary to granulocytes, are permanent resident cells within many tissues. They are activated by allergens and other antigens to release histamine and other chemical mediators. They are usually associated with severe allergic reactions, including anaphylactic shock. Dendritic cells are professional antigen- presenting cells. They efficiently phagocytize pathogens and present those antigens on their surface to stimulate other immune cells. Dendritic cells are white blood cells (leukocytes), but are not lymphocytes. They can form from monocytes (which also differentiate into macrophages) or independently in their own cell line from a blood cell precursor. There are three kinds of lymphocytes: T cells, B cells, and natural killer cells. Natural killer cells recognize infected or cancerous cells and release cytotoxic granules that destroy the cell. T cells are lymphocytes that mature in the thymus and participate in cell-mediated immunity. B-cells are lymphocytes that mature in the bone marrow and lymph tissues and participate in humoral immunity. B-cells produce antibodies, T-cells do not. T-cells recognize and bind antigens via a "T- cell Receptor" (TCR) not found on B-cells. Plasma cells are formed when a B-cell binds its matching antigen and is activated (with the help of helper T-cells) to undergo mitosis. This mitosis produces mostly plasma cells—clones of the original B-cell that act as "antibody factories," making and secreting soluble copies of that antibody. A few cells will differentiate into memory B cells that remain in the body, allowing the immune system to mount a more efficient secondary immune response if there is a later infection by the same pathogen. Helper T-cells are T-cells that "help" other immune system cells, such as B-cells and cytotoxic T-cells, to perform their function. The way they "help" other cells is usually to secrete chemicals, such as cytokines, that activate (i.e., "turn-on") functions or activies in the cell that is being "helped" Suppressor T-cells (a.k.a., regulatory T-cells) suppress the body's own immune system—which helps prevent severe allergic reactions or autoimmune disease, and aides in turning off an immune response once an infection has been eliminated. Killer (or cytotoxic) T-cells target infected and cancerous versions of the body's own cells and destroy them. Memory T-cells are just what they sound like—analogues to memory B-cells that have previous experience with a pathogen that allows them to mount a more effective response during a subsequent infection. However, we strongly suggest you forget about memory T-cells for the MCAT! As far as the MCAT is concerned, the concept of immunological "memory" and a "secondary immune response" are so intricately associated with humoral immunity that the wisest choice would be to always associate such concepts with B-cells and humoral immunity only. The algorithm below can help you put together a mental picture of where all of these cells come from and how they relate. You should NOT concern yourself with the progenitor cells. Just focus on the final step of each pathway and perhaps the precursor just before it. For example, it is helpful to observe things such as: a) multiple non-nucleated RBCs form from a single parent cell, b) the three granulocytes are related because they all differentiate from one parent cell, c) platelets (thrombocytes) bud off of megakaryocytes, or d) monocytes differentiate into macrophages. Not shown on this diagram are dendritic cells—they can form from either a monocyte or from another cell that is a precursor to lymphocytes.

Macrophages:

Macrophages engulf microbes and present antigens from those microbes on MHC proteins in their cell membranes to be recognized by B-cells and T-cells.

Q13. As a review from the Biology 1 Lesson, the relationship between humans and the E. Coli in the colon is an example of what type of inter-species relationship? (e.g., commensalism, parasatism, mutualism, etc.)

Mutualism ; Recall that mutualism is a form of symbiosis where both participants benefit. The bacteria benefit by consuming the food in our intestines and we benefit because the bacteria produce vitamins that we absorb.

The Sliding Filament Mechanism:

Myosin filaments contain bead-like arms (often called "heads" or "cross-bridges"). First, understand that the default, low-energy position for these heads is bent. In between contractions, ATP hydrolysis (ATP --> ADP + Pi) is used to provide the energy to straighten-out, or "cock", these myosin heads into the high-energy, straight position. Regardless of which position the myosin heads are in (straight or bent), they are attracted to, and bind readily with, their neighboring actin filaments. The only reason they are not bound continuously is a protein molecule called tropomyosin, which covers the myosin binding site on the actin filament. Tropomyosin is "clamped" into position by another protein, troponin. When calcium is released from the sarcoplasmic reticulum it binds to troponin, releasing the "clamp" and freeing the binding site. Myosin will then immediately bind to actin. Recall that the myosin heads are still in their high-energy, straight position. After binding, the myosin heads release ADP and Pi (ATP is NOT hydrolyzed into ADP and Pi. This already occurred as the heads were cocked into the straight position). The small amount of energy linked to this dissociation is enough to "pull the trigger," allowing the heads to relax back to their bent position. Because they are still bound, they drag the actin filaments with them. This is the power stroke associated with contraction. After the power stroke, ATP binds to the myosin head again facilitating its release from the actin filament. Tropomyosin immediately re-covers the myosin binding site. Finally, the attached ATP is hydrolyzed, providing the necessary energy to push the head back into the high-energy straight, or "cocked," position. If no ATP is present, the myosin heads cannot detach from actin and the muscle will be stuck in a contracted position called "rigor." This is what occurs during "rigor mortis". If no Ca2+ is present, we do NOT get rigor, but the inability to contract (a.k.a., "flaccidity").

Q15. After you have studied each of the above topics to a mastery level, deliver to your tutor a thorough, structure-by-structure account of digestion as it progresses from the mouth to the anus, including all enzymes, where and how each type of macromolecule is digested, the process and location of absorption, and the general anatomy of the entire tract. Include the following terms: mouth, pharynx, epiglottis, esophagus, peristalsis, stomach, pepsin, gastric pits, small intestine (i.e., duodenum, jejunum, and ileum), bile, pancreatic enzymes, villi, microvilli, large intestine, colon (i.e., ascending, transverse, descending, and sigmoid), and rectum.

Note to Tutors: Ensure your students can tell the entire story of digestion—from memory—in at least the level of detail provided here: The primary role of the mouth in digestion is to physically breakup food particles via chewing, and mix the food with saliva and the alpha-amylase enzyme it contains. Amylase begins carbohydrate digestion, and the lubricating properties of saliva aide in ease of movement. There are also specific classes of antibodies found in saliva. The pharynx ensures the bolus is delivered to the esophagus without entering the nasal cavities or the larynx, but no digestion of any kind occurs here and nothing is added to the bolus. The epiglottis is an upward-oriented cartilaginous flap that folds down over the opening to the larynx during swallowing to prevent food from entering the larynx. The esophagus utilizes peristalsis to push the bolus down and into the stomach. Peristalsis is the rhythmic contraction of smooth muscle in the wall of the gastrointestinal track that moves food forward. Once again, no digestion occurs in the esophagus and nothing is added. Food enters the stomach by passing through the cardiac sphincter—which exists at the junction of the esophagus and the stomach. The churning of the stomach continues physical digestion. The stomach is lined with gastric pits. Chief cells are one of four cell types lining these pits. They release pepsinogen, a zymogen that is converted into active pepsin by the acidic environment inside the lumen of the stomach. The stomach acid that creates this low pH environment (pH = 2) is the result of parietal cells that secrete HCl into the same gastric pits. The lining cells themselves are protected from the acid and digestive enzymes by a mucus coating. Pepsin catalyzes the hydrolysis of proteins. The partially digested food mix, now referred to as chyme, passes through the pyloric sphincter and into the upper portion of the small intestine, called the duodenum. The common bile duct and the pancreatic duct both dump into the duodenum. As a result, the duodenum receives bile from the liver/gallbladder plus a bicarbonate rich solution and six digestive enzymes from the pancreas [trypsin (protein), chymotrypsin (protein), pancreatic amylase (carbohydrates), lipase (fats), ribonuclease (RNA) and deoxyribonuclease (DNA)]. The bicarbonate ions are important because they lower the pH of the chyme to around 6—a necessary step because the enzymes just listed could not function at the much lower pH of the mixture arriving from the stomach. The chyme progresses through the small intestine to its middle section, called the jejunum, and then to the final section, called the ileum. There are no distinct boundaries marking these three sections. Most digestion occurs in the duodenum and most absorption (of food molecules, not water) occurs in the jejunum and ileum. The lining of the small intestine features finger-like projections called villi that increase the surface area for absorption. Individual epithelial cells along each villus have microvilli—long fingerlike projections of the cell membrane on their apical surface. Each villus' center is filled with blood vessels and a single lymph vessel called a lacteal. Fats are absorbed into the lacteal, NOT the blood vessels. Carbohydrates and proteins are absorbed into the blood vessels. The ileum empties into the large intestine on the right side of the abdomen (anatomical right) slightly above a blind (dead-end) pouch called the cecum. The appendix is attached to the cecum. The colon rises upward along the right wall of the abdomen (ascending colon), traverses across the abdomen (transverse colon) and then descends along the left wall (descending colon). The final segment of the colon is somewhat twisted and is therefore appropriately named the sigmoid colon. The primary function of the colon is the absorption of water and vitamins. The colon contains resident commensal bacteria that secrete vitamin K, thiamin, riboflavin, and vitamin B12. The rectum is the final segment of the large intestine. It connects to the anus and stores feces.

The Mouth:

Saliva provides lubrication & the first example of a digestive enzyme.

The Sarcomere: Q22. Draw a diagram of a sarcomere in both its relaxed and contracted position.

See the diagram below. Verify visually each of the distances/changes described in the previous question. Some students have difficulty visualizing how the distance between M lines decreases during contraction. The sarcomere in the diagram below would be at the very center of the muscle fiber. Remember that there will be many sarcomeres on either side. With each sarcomere becoming shorter, by definition the distance between the centers of each sarcomere must decrease.

Q18. Draw and describe the structure of an antibody. Include the heavy chains, light chains, disulfide bridges, hypervariable region, and location of antigen binding.

See the labeled diagram. We like this diagram because it shows the N-terminus and C-terminus of each chain—emphasizing that these are protein chains. Antigens bind to the ends of the hypervariable regions. An antigen would contact both the end of the heavy chain and the end of the light chain.

Q19. Why do you shiver? What is occurring physiologically during a shiver?

Shivering is an involuntary response to cold. At sufficiently low temperatures a specific location within the hypothalamus receives signals from the skin and spinal cord. The hypothalamus sends signals to core muscle groups to undergo rapid contractions that generate heat.

Important Features of Skeletal Muscle:

Skeletal muscles store large amounts of glycogen; they also require a lot of oxygen and thus have their own oxygen storage molecule, myoglobin. Myoglobin is basically one subunit of a hemoglobin molecule, capable of holding one O2 molecule. Mature (differentiated) skeletal muscle cells are frozen in Go phase and do not divide (similar to neurons).

2) Exposure to an environmental toxin causes a genetic defect in the normal mechanism that manufactures new antibodies. If a person were exposed to a virus for the first time shortly after such an exposure, and was unable to manufacture new antibodies as a result: A) viral antigens would not be recognized or bound by an antibody due to the host's inability to manufacture new antibodies to match the pathogenic antigens. B) viral antibodies would be bound by host antigens that were created as a match to those antibodies prior to the environmental exposure. C) the antibody producing function of the host would be repaired by insertion of viral DNA into the host genome. D) viral antigens would be bound by host antibodies created before the first exposure to the virus.

Solution: A favorite MCAT principle is the fact that antibodies are never created "in response to" or "to match" antigens. Antibody creation is a random process. The sheer number of antibodies created, coupled with a somewhat flexible binding requirement, ensures that a foreign antigen will be bound by a host antibody. Answer A is false because it employs the false "created to match" premise. Answer B uses this same flawed reasoning and confuses antibodies with antigens. Answer C is illogical because viruses are far too simple to contain a complete set of genes for a complex process such as antibody production. Answer D is therefore the best choice.

4) A common muscle relaxant acts on skeletal muscle by binding ATP and preventing it from interacting with the sarcomere. When this drug is present, which of the following is expected? A) The myosin heads will not be able to bind to the actin filament. B) The muscle fiber will remain in a flaccid state until ATP becomes available. C) The myosin heads will be locked in the bent position. D)The muscle fiber will be hyperstimulated by ADP.

Solution: ATP plays two important roles. First, it is required to release the myosin head from the actin binding site. Second, it must be hydrolyzed to move the myosin heads from their relaxed (bent) position to their high-energy (straight) position. Answer A is incorrect. Myosin can bind actin without ATP. Answer B is incorrect because muscles are locked in "rigor" if no ATP is available. ADP does not stimulate contraction, so Answer D is also false. Answer C is the only true statement.

3) When a relaxed muscle fiber contracts, all of the following will occur, EXCEPT: A) The density of thick filaments per cubic centimeter of muscle fiber increases. B) The distance between Z discs decreases and the distance between M lines remains constant. C) The distance between adjacent M lines and between adjacent Z discs both decrease. D) Myosin heads bind to actin and dissociate from it multiple times during the same contraction.

Solution: Answer A is true because the myosin bundles move closer together as the length of the muscle fiber decreases. Answer C is true because as the entire muscle fiber shortens all repeating segments move closer to one another. Answer D is true because some myosin heads must release while others simultaneously bind to allow the myosin to "crawl" along the actin filament. Answer B is not true, and is thus the correct answer to this EXCEPT question.

1) The lining of the human stomach contains G-cells, cells that are primarily responsible for the secretion of the peptide hormone, gastrin. Some mammals have stomach linings that lack G-cells completely. It is likely that these mammals: A) have developed immunity to the effects of gastrin B) consume a diet high in carbohydrates C) consume a diet high in proteins D) evolved in an environment where the presence of gastrin was a competitive advantage

Solution: Gastrin stimulates the parietal cells to secrete HCl. That HCl serves two purposes: 1) It denatures proteins in the stomach, and 2) it activates pepsinogen to pepsin. Both functions are closely related to the digestion of proteins, so answer B is very plausible. Answer C states just the opposite and is therefore incorrect. Answer A is incorrect because the question asks about why certain mammals do not have G-cells. This answer implies that they do have G-cells but are immune to the gastrin produced by them. Answer D would actually explain the presence of G-cells, not the lack of them. By process of elimination, answer B must be correct.

5) A woman suffering from a heritable hormone abnormality has above-normal levels of estrogen and progesterone in her blood, but little to no luteinizing hormone or follicle stimulating hormone. Based on this information alone, one would expect this woman to: A) develop a thick uterine lining and have heavy menstrual flow. B) develop a thick uterine lining but lack regular menses. C) be at risk for multiple births. D) exhibit an over-developed corpus luteum.

Solution: The MCAT directly associates high estrogen/progesterone levels with the thickening of the uterine lining. These two hormones also relate to secondary female sex characteristics and other functions, but this should be the first thing you think of when you see these two hormones on the MCAT. However, even if a thick uterine lining is in place, it is the luteal surge that causes menstruation. Thus, this woman is likely to have a developed lining, but no menstruation. She will not ovulate without FSH present to mature the follicles, so answers C is not plausible. Because the corpus luteum is the structure left over after ovulation, answer D cannot be correct either. Answer B is therefore the best choice.

The Reproductive System Female:

The Egg: Eggs are the female gametes. Their plural scientific name is ova and the singular form is ovum. An egg begins meiosis as a germ line cell in the ovary of a female fetus and is arrested at Prophase I of Meiosis at birth. Not until puberty and menstruation is Meiosis I completed. Even then, all cells remain in this arrested state except for those that begin maturing in a follicle in preparation for ovulation. Meiosis II is not completed until after the sperm fertilizes the ovum. An immature egg is called an oocyte.

Q25. Provide a conceptual definition for the term "autorhythmic." Do heart muscle cells contract in response to innervation by a nerve?

The SA node acts like a natural pacemaker for the heart. The action potential for each heart beat originates in the SA node, not with a nerve from the nervous system. Nerves do innervate the heart, but they only regulate its rhythm up or down—they do not initiate that rhythm. The vagus nerve (parasympathetic) slows the heart rate, and sympathetic nerves increase heart rate.

Q7. In addition to pepsin, what other molecule assists in protein metabolism in the stomach?

The acid in the stomach denatures proteins. Recall from the Biology 1 lesson that acid is a protein denaturing agent.

Q30. What is the acrosome?

The acrosome is a membrane-bound structure on the tip of the head of each sperm. The acrosome contains hydrolytic enzymes that breakdown the otherwise impenetrable coating around the ovum.

Q4. What is the name of the reaction by which amylase catalyses carbohydrate breakdown?

The breakdown of all nutrient macromolecules into their monomers is accomplished via hydrolysis.

The 3 Germ Layers:

The general rules you often see applied here (ectoderm = outer layers of adult body, mesoderm = middle structures, and endoderm = innermost layers) are somewhat flawed. Because generalizations often lead to errors on these questions, it is best in this case to memorize the following lists: Ectoderm: epidermis, nails, tooth enamel, lens of the eye, pituitary gland, central, peripheral and autonomic nervous systems. Mesoderm: dermis, muscle, bone, connective tissues, kidneys, genitalia and most internal organs EXCEPT the liver and pancreas. Endoderm: the entire digestive tract, thyroid, parathyroid, urinary bladder, the lining ONLY of the lungs, and the liver and pancreas.

Arrival of the Action Potential:

The junction between a skeletal muscle and a motor (i.e., somatic) nerve is called the neuromuscular junction. Acetylcholine (ACh) is the only neurotransmitter used at neuromuscular junctions. When Ach is released from the motor neuron at the neuromusclular junction an action potential is initiated. The action potential will then spread along the muscle cell sarcolemma and down specialized invaginations of the sarcolemma called T-tubules that dive deep into the muscle cell, causing the release of Ca2+.

Q8. What prevents the tissues lining the stomach from being digested by pepsin?

The lining cells of the stomach are protected by a thick layer of mucus secreted by the mucus cells that line the gastric pits (one of four cells found there).

Q2. Why are the liver, gallbladder, and pancreas included in our study of the digestive system?

The liver, gallbladder and pancreas are usually included in a discussion of the digestive system because they play vital roles in this process. The liver manufactures bile. The gall bladder concentrates and stores bile (but does NOT manufacture it). The pancreas secretes bicarbonate into the duodenum to neutralize the acidic chyme coming from the stomach. The pancreas also secretes six pancreatic digestive enzymes.

The Small Intestine:

The majority of all digestion and absorption occurs in the small intestine; digestion occurs primarily in the duodenum with absorption primarily in the jejunum and ileum.

Innate Immunity:

The non-specific attack of pathogens. All forms of innate immunity are present at birth, NOT acquired in any way. o Includes all immune responses that are NOT specific to one particular virus, bacteria, pathogen, etc. Examples include: skin, stomach acid, enzymes in the mucus and saliva, digestive enzymes, blood chemicals, fevers, inflammation, and non-specific phagocytosis.

Q17. Provide a possible explanation for the location of concentrations of lymphoid tissue in the respiratory and gastrointestinal tracks.

The respiratory and gastrointestinal tracks would both be "front line" locations—if you will—in the battle against foreign invaders. Both locations are interfaces between the internal and external environments. For this reason, it would be an advantage for any organism to have lymph tissue, and therefore a concentration of immune cells, at these locations.

IMPORTANT NOTE: Humoral Immunity

The shapes of antibody binding sites are NOT determined in response to the shape of foreign antigens. They are determined by the individuals' genes. Antibodies are made by random splicing of gene products in a way that provides huge numbers of receptors each with a unique shape. Due to the sheer number of antibodies, almost any antigen will happen to match one of the existing antibodies. Although the creation process is random, antibody-antigen pairing is specific: one antibody will bind only one antigen.

Q12. Provide a conceptual definition for each of the following: villi, microvilli, lacteal and brush border.

The villi are finger-like projections of the wall of the small intestine. They are hollow and contain both blood vessels and a single lymphatic vessel called a lacteal. Fats are absorbed into the lymph system via the lacteals and carbohydrates and proteins are absorbed into the blood. The villi dramatically increase the surface area available for absorption. Further, each epithelial cell lining a villus contains fingerlike projections of the cell membrane called microvilli. To be clear, a villus is an undulation of the lining of the small intestine, while a microvillus is an undulation of the cell membrane of a single cell. The brush border is a name given to the microvilli and the collection of mucus and digestive enzymes intermingled within them. The name was chosen because under a light microscope individual microvilli are not easily discernible and they appear instead as a fuzzy line along the apical surface of the epithelial cells.

The Sarcomere: Q21. Provide conceptual definitions for each of the following as they relate to the sarcomere: thick filament, thin filament, actin, myosin, myosin heads, A band, I band, H zone, Z line and M line. For each item, describe how its length, location or size changes during contraction.

The thick filaments are made from myosin fibers. Recall that myosin is a motor protein. The thin filaments consist of mostly actin fibers, also known as microfilaments (thin filaments also feature troponin and tropomyosin). Actin is a protein, of which microfilaments are polymers. A myosin protein has both a head moiety and a tail moiety. A dimer is formed between two myosin proteins with their tails intertwined. Large numbers of these dimers combine to create the long myosin filaments seen in the sarcomere. The globular heads, also called "cross- bridges," stick out from the myosin filament at an angle—which is the relaxed conformation referred to as "bent" in the sliding filament mechanism. Myosin heads have a high affinity for actin and will bind to it unless tropomyosin blocks the binding site. All of the following terms refer to portions of the sarcomere. The A band is the length of the myosin filaments. The length of the myosin filaments does NOT change during contraction. The I band is the distance between the ends of the myosin filaments. It is also the lightest band when viewed under a microscope because only the thin actin filaments are present in this region. The I band will shorten during a contraction. The H zone is the distance between the ends of the actin filaments. The H zone will also shorten during a contraction. The Z lines (a.k.a., Z discs) are zigzag lines that define the edges of each individual sarcomere unit. The actin filaments are anchored here by the protein connectin and stretch out in both directions. During a contraction the distance between Z lines decreases as the sarcomere shortens. The M line is the very center of the myosin filaments. The distance between M lines will decrease during a contraction.

Q27. Describe the hormones that regulate bone cells. Which hormones affect which cells and in what specific ways?

The two hormones that regulate bone maintenance and blood calcium levels are parathyroid hormone (PTH) and calcitonin. Remember that "calcitonin tones your bones." When blood levels are above normal calcitonin inhibits osteoclast activity. Osteoblast activity continues and thereby a net increase in bone structure results. The calcium used by osteoblasts to build new bone matrix comes from the blood and therefore blood calcium levels decrease. Parathyroid hormone has the opposite effect. Parathyroid hormone stimulates osteoclast activity, resulting in the breakdown of bone matrix and release of the associated calcium into the blood. As a result, blood calcium levels rise. The two hormones also have predictable impacts on the absorption of calcium at the gut and the reabsorption of calcium in the kidney.

Q32. Provide a conceptual definition for, and describe the primary function of, each of the following: vagina, cervix, uterus, fallopian tubes and ovaries.

The vagina serves as the female copulatory organ, as the birth canal, and as an exit route for menstrual fluid. The cervix is the conical-shaped bottom portion of the uterus that projects into the rear, upper wall of the vaginal canal. It contains a small opening that allows for exchange of fluids, but must dilate significantly during child birth to allow for delivery. The uterus is an elastic, muscular pouch that receives a fertilized egg via implantation and provides nourishment for the developing fetus. Muscle contractions of the uterine wall, stimulated by oxytocin, facilitate the process of childbirth. The fallopian tubes are ducts that utilize ciliated epithelium to transport the egg from the ovary to the uterus. Fertilization usually occurs in one of the two fallopian tubes. The ovaries are the female gonads, homologous to the testes in males. The ovaries develop and release ova (i.e., eggs) on a regular 28-day cycle (on average). They also function as endocrine glands that secrete both estrogen and progesterone. The diagram that follows illustrates all of the structures discussed relative to other common anatomical structures.

Dendritic Cells:

These cells are antigen presentation experts. They are specialized to do so efficiently and are found in the highest concentration near membranes (such as the skin) that interface with the external environment—where they are most likely to encounter antigens.

Epiglottis:

This is the organ that caused the debunking of the "honey soothes the vocal chords" myth discussed in the previous lesson. This u-shaped flap of cartilage and membrane is oriented nearly vertical in its default position. This allows air to proceed past it and into the trachea. During the act of swallowing this flap folds down over the opening to the portion of the larynx that contains the "voicebox" and trachea, disallowing food down the trachea and allowing food down the esophagus.

Q10. Describe the function of each of the digestive enzymes secreted by the pancreas.

Trypsin and chymotrypsin are both proteases. Each enzyme cuts proteins at its own specific amino acid sequence. Pancreatic amylase catalyzes the hydrolysis of carbohydrates. Lipase catalyzes the hydrolysis of fats. Ribonuclease and deoxyribonuclease catalyze the hydrolysis of RNA and DNA respectively. Some students find this odd, thinking "Who eats DNA and RNA?" While its true we don't think of either as a food group, anything made of living cells—plant, fungi, animal, etc.--will contain both of these polynucleotides.

Q26. Describe the difference between single unit (a.k.a., "unitary" or "visceral"), and multi-unit smooth muscle types.

Unitary, or single-unit, smooth muscle is group of smooth muscle fibers that are innervated by a single nerve and contract simultaneously as a single group. These are the most common smooth muscle unit and are found in most organs, around most blood vessels, the digestive track, etc. A mutli-unit smooth muscle is innervated by multiple nerves and does not act as a single unit. This allows for more precise control (remember, however, that all smooth muscle is involuntary, so it is not conscious control). Multi-unit smooth muscle is quite rare. It is found in some of the larger vessels such as the aorta, and in the retina of the eye.

The Large Intestine:

Whenever you see "water absorption" or "vitamin absorption" THINK of the large intestine. In addition to absorbing vitamins from food, the large intestine also absorbs some vitamins produced by beneficial symbiotic E. Coli bacteria that live in the large intestine (e.g., vitamin K, thiamin, riboflavin, and B12).

Absorption Review:

You must know where and how each class of molecules is digested and absorbed. Carbohydrates: Digestion begins in the mouth (salivary amylase), and is complete by the end of the small intestine. Carbohydrates are broken down entirely to their monomers (e.g., glucose, fructose, etc.) before absorption; they enter the blood stream (NOT the lacteal) and travel to the liver via the hepatic portal vein. Proteins: Digestion begins in the stomach and is complete by the end of the small intestine. Proteins are broken down to small peptides and amino acids before absorption. They enter the bloodstream (NOT the lacteal) and travel to the liver. Lipids: Digestion begins in the small intestine (duodenum) and is complete by the end of the small intestine. Digestion of lipids CANNOT begin prior to their reaching the small intestine where they encounter bile and lipase. Triglycerides are broken down to fatty acids, transported across the membrane, then reformed into triglycerides. Lipids enter the lacteals (NOT the blood stream). In order to travel in blood or lymph, all lipids must either 1) bind to a protein carrier such as albumin, or 2) be formed into a chylomicron or micelle.

Cartilage:

o A connective tissue composed mostly of collagen. o No perfusion or innervation o Found in appendages such as the nose and ears, at the ends of long bones, between vertebrae, and at almost any joint or articulation

Smooth Muscle:

o Characteristics: involuntary, non-striated, one nucleus o Smooth muscle is what controls the gut, viscera, blood vessels, etc. o Important Features of Smooth Muscle: Smooth muscle is NOT arranged in sarcomeres. Smooth muscle does NOT contract via the same sliding filament mechanism described previously for skeletal and cardiac muscle. It is similar—actin and myosin still slide past one another—but several different proteins are involved and the steps are NOT identical. As an example, in skeletal muscle it is calcium binding to troponin that initiates contraction, but in smooth muscle contraction is initiated by the calcium-dependent phosphorylation of the myosin head. Smooth muscle contraction is sometimes referred to as the "calcium-calmodulin cascade." This is not information required for the MCAT, but there's a good chance it could show up in a passage.

Cardiac Muscle:

o Characteristics: involuntary, striated, one nucleus. o Contraction: Cardiac muscle cells contain sarcomeres the same as those found in skeletal muscle and therefor use the same sliding filament mechanism outlined above. o Important Features of Cardiac Muscle: Like skeletal muscle, cardiac muscle utilizes myoglobin. It also contains very large numbers of mitochondria to prevent fatigue. Cardiac muscle cells/fibers are connected by intercalated discs containing gap junctions. It is through these junctions that the ions used to initiate action potential pass effectively making the heart muscle cells into a functional syncitium. Allowing them to act as a single unit. Unlike skeletal muscle cells, cardiac cells continue dividing after differentiation.

Embryology:

o Definitions: Cleavage = mitosis without change in size Morula = 8-cell zygote Gastrulation = at about week 2 cells migrate to form the three germ layers Neurulation = at about week 3, the notochord forms from the mesoderm and induces the overlying ectoderm to form the neural plate, which becomes the neural tube, and eventually the spinal cord.

Bone:

o Functions: Physical support and movement, protection of vital structures, mineral storage and regulation of blood mineral concentration, and blood cell formation. o Bone Cell Types: Osteocytes: Mature bone cells surrounded by a mineral matrix. Osteoclasts: Bone cells that break down and resorb bone matrix, releasing the component minerals (Ca2+ and P) back into the blood. Osteoblasts: Immature bone cells that secrete collagen, organic compounds, and minerals forming a bone matrix around themselves. Once they are completely enclosed by matrix, they differentiate into osteocytes.

Types of Digestion:

o Physical Digestion = chewing (mastication), churning in stomach, and breaking of food into smaller pieces, including emulsification of fats by bile. o Chemical Digestion = all breakdown of food that involves the breaking of bonds through the use of digestive enzymes.

The Reproductive System Male:

o Sperm: Sperm are the male gametes. Their technical, plural name is spermatozoa and the singular form of that same word is spermatozoon. They are produced by the testicles in the seminiferous tubules and stored and nurtured in the epididymis. They are a single, haploid cell consisting of a head (cell body) and tail (flagellum). They contain lots of mitochondria. o Ejaculation Pathway: During ejaculation, sperm leave the epididymis via the vas deferens. The vas deferens arches back up into the pelvis and then back down toward the penis. Along the way, the seminal vesicles, prostate gland and bulbourethral gland (a.k.a. Cowper's gland) all secrete various lubricants and nutrients into the ejaculate. The vas deferens empties into the urethra at the base of the urinary bladder. The urethra then travels down the penis.