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MCAS Biology: Human Systems Review
This booklet is designed to introduce or review the human biology concepts tested on the MCAS exam. Since
this unit comes at the end of the year, it is often a rushed and sometimes we do not get to a particular topic
before you sit the exam.
After reading this booklet, you should be capable of excelling at the MCAS exam. Please note, however, that the
Honors Biology course expects you to know and understand much more than is found in this simple booklet. In
short: mastering this booklet will allow you to master the MCAS, but NOT Honors Biology.
1. The digestive system converts macromolecules into smaller molecules.
The purposes of the digestive system are to mechanically digest food into small
pieces, to chemically digest food into monomers, and then to absorb the
monomers into the blood for use throughout the body.
Mechanical digestion starts in the mouth, with chewing. Saliva moistens and
lubricates the food, and provides the enzyme salivary amylase to begin the
digestion of starch. The chewed food is then pushed down the esophagus by
peristalsis (rhythmic contractions of smooth muscle) into the stomach. Here,
stomach acid sterilizes the food and the enzyme pepsin begins to break down
proteins. From the stomach, food moves into the small intestine, a 20-foot
long tube.
In the first foot of the small intestine, the pancreas adds many different
digestive enzymes to break down all of the biomolecules in your meal. In this
first foot, the liver—via the gall bladder—adds bile, which mechanically breaks down lipids. In the remaining 19
feet of the small intestine, biomolecules are absorbed through little finger-like projections called villi and
microvilli (the use of these small structures increases the surface area of the small intestine). Finally, the
indigestible remains of the meal enter the large intestine, where liters of water are reclaimed.
The nutrients from the food are brought to the liver by the circulatory system. Here, toxins in your blood are
neutralized and the biomolecules are processed. Some of the nutrients may be stored in your liver for future
use. Others may be released into your blood for immediate use by your body.
2. The circulatory system transports nutrients and oxygen, and
removes cell wastes. Kidneys and liver remove waste from
blood.
The main purpose of the circulatory system is to transport oxygen and nutrients
throughout the body. The heart pumps oxygen-rich blood through the arteries. The
vessels narrow into capillaries, where nutrients and oxygen are ken up by the body cells.
The deoxygenated blood then returns to the heart through the veins. The veins return the
oxygen poor blood into the right atrium; it then flows into the right ventricle. The blood
travels to the lungs through the pulmonary artery, where oxygen diffuses into the blood,
and returns to the heart through the pulmonary vein. Finally, it is pumped through the left
atrium into the left ventricle and then back out to the body.
Arteries are thick-walled blood vessels that carry blood away from the heart—remember Artery, Away. Veins
are thin-walled blood vessels that contain valves to keep blood from going backwards. Capillaries are smaller
blood vessels where diffusion of nutrients and oxygen into surrounding cells occurs. Capillaries are one cell
thick.
There are three types of blood cells. Red blood cells use the protein hemoglobin to carry oxygen through the
circulatory system. White blood cells defend the body from foreign invaders. Platelets are small fragments of
cells that clot blood in wounds.
Waste products are carried in the blood plasma. Highly toxic wastes are detoxified in the liver, and then the
safer forms are deposited into the blood. Wastes in the blood plasma—especially nitrogenous wastes from the
breakdown of proteins—are removed by the kidneys, and collected into urine. The kidneys are also responsible
for removing or conserving water to maintain homeostasis in the body.
3. The respiratory system provides exchange of O2 and CO2.
The primary purpose of the respiratory system is to supply the body with
steady amounts of oxygen (O2), and remove carbon dioxide (CO2) from the
body. The respiratory system works closely with the circulatory system.
The air enters through the mouth or nose, travels down the pharynx, larynx,
trachea, bronchi and finally into the lungs. There, the air enters alveoli—small
hollow spheres where gas exchange occurs with the bloodstream. Many small
alveoli increase the surface area of the lungs, allowing more gas exchange. Cilia
line the respiratory surfaces, and help trap and filter out particles that you
inhale.
The contraction of the diaphragm (a muscle in your thorax) causes the chest cavity to expand, and the increase
in volume pulls oxygen-rich air into the lungs. The relaxation of the diaphragm causes a decrease in volume in
the lungs and carbon dioxide-rich air is forced out.
An increase in activity or need for oxygen by the mitochondria causes an increase in breathing rate in order to
maintain homeostasis.
4. The nervous system mediates communication.
The nervous system is a modified endocrine system. Its primary roles are to
provide fast communication about stimuli from outside the body, and allow quick
response (like movement) to that information. It consists of the central nervous
system (the brain and spinal cord) and the peripheral nervous system (nerves in
the body and extremities).
The basic structural unit is the neuron (nerve cell), which is shown in the
adjacent diagram. There are three types of neurons: sensory neurons sense
different conditions (e.g., pain, light, or pressure); interneurons (in the brain and
spinal cord) interpret stimuli and decide how to respond to them; motor neurons
control skeletal muscles and allow precise movement.
One of the simplest responses of the nervous system is a reflex. In a reflex, your sensory neurons detect a
stimulus, for example, you touch something hot. This stimulus travels down the sensory neuron from your hand
to your spine. Interneurons in the spine interpret the stimulus and react to it, which triggers a motor neuron
controlling an arm muscle. This new stimulus travels down the motor neuron and causes your arm muscle to
contract, pulling away from the hot object.
Your brain is one of the most complex collections of interneurons known. Your brain has three general parts:
the cerebrum, where memory, thought, and emotion are processed; the cerebellum, which coordinates
movement and is the home of athletic “muscle memory;” and the medulla and brainstem, which integrate the
brain and the spine and is the site of basic life-support functions like control of heartbeat, breathing, and body
temperature.
5. The muscular/skeletal system supports the body and allows for movement.
Bones produce blood cells.
The skeletal system provides structure and support. Bones of the body protect internal organs such as the lungs
and the heart, and provide a site for muscle attachment, which allows movement. Bone marrow is inside the
bone, and it produces blood cells. Bones also store minerals, especially Ca2+ (calcium ions).
Besides bone, other tissues make up the skeletal system. Tough, flexible cartilage acts as a cushion between
bones (to avoid bone-to-bone contact), and forms your ears and nose. Ligaments are tough cables that attach
bone to bone and tendons are cables that attach bone to muscle.
The point at which two bones meet is known as a joint. There are several types of joints: hinge joints are found
in your elbows and knees; ball and socket joints make up your shoulder and hips; pivot joints allow rotation of
your forearm or to shake your head “no”; and immovable joints, such as the joints between the bones of the
skull, do not move.
Remember the location of these bones: the femur is in your thigh, and your radius and ulna are in your wrist.
6. The sexual reproductive system allows organisms to produce offspring that
receive half of their genetic information from their mother and half from their
father, and that sexually produced offspring resemble, but are not identical to,
either of their parents.
The reproductive system refers to the production of eggs and sperm, as well as the processes that lead to
fertilization. In asexual reproduction, only one mate is needed. The offspring created are identical to the
parent, so the offspring o not vary and may have difficulty surviving in times of hardship for the species. We will
be focusing on sexual reproduction between two members of the same species, with different sexes.
Males produce sperm. Sperm are created through meiosis, and are haploid (n). Spermatogenesis is the meiotic
process that produces the sperm cells. Sperm cells have flagella (a whip-like tail) and a head containing the
chromosomes. They eventually swim to the egg once introduced into the female.
Females produce eggs during their development. They do not develop these eggs further until fertilization, the
introduction of sperm and eggs that yield a zygote. The female undergoes a menstrual cycle. This cycle
provides for moments of a peak fertilization opportunity. The follicular phase is the creation of the egg,
ovulation is the release of the egg, and the luteal phase is the process of estrogen secretion afterwards.
When a sperm cell reaches the egg, it fuses with the egg cell. Once one sperm enters the egg, the egg no longer
allows entrance of other sperm cells. The two cells will eventually yield a diploid (2n) zygote. This will
eventually yield an embryo once development has occurred. The embryo shares DNA from both the father and
mother, and the offspring expresses traits from each parent.
7. Communication among cells is required for coordination of body functions.
The endocrine system secretes hormones from glands. Hormones are chemical
messengers that travel in the bloodstream. They trigger responses in target cells.
One hormone can trigger different responses in different types of target cells.
Since hormones travel through the bloodstream, endocrine communication is
slower than nervous communication. It is also less specific, and can cause a
response in more cells and thus more systems than nervous impulses.
Hormones must be received by the target cells in order to get a response. There
are different classes of hormones, each of which interacts with the target cells in
a different manner. Protein hormones like insulin and glucagon bind to the
outside of the target cell. Steroid hormones like testosterone and estrogen enter
the target cell.
An example is adrenaline, produced in the adrenal glands over the kidneys.
When a person is scared or under stress, adrenaline is released. It travels
through the blood, and has a number of effects at different organs. It increase
your respiration rate, increases your heart rate, releases sugar from your liver into your blood stream, and
redirects blood away from the skin into the core of the body. Note how the endocrine system interacts with
many other systems.
8. Body systems interact to maintain homeostasis using physiological
feedback loops.
Homeostasis is the maintenance of a constant internal environment regardless of changes in the external
environment. For example, whether the temperature outside is 0°C or 45°C, your body temperature is kept at
37°C.
All of your systems work together to maintain homeostasis. If you are competing in the 100m dash, when the
starter pistol fires your adrenal glands (endocrine system) release a flood of adrenaline into your blood stream.
In response to the adrenaline, your liver dumps a huge supply of glucose (stored from a previous meal processed
by your digestive system) into your blood. Your arteries and capillaries carry the adrenaline and the glucose
throughout your body (circulatory system). As your leg muscles (muscular system) pull on your leg bones
(skeletal system), they quickly use up the oxygen in your blood and are starving for more. In response, your
brain speeds up your breathing (respiratory system) to bring more oxygen to your blood. Your heart (also
circulatory) beats faster in order to bring this oxygen to your muscles quicker.