Biology- Chapter 6

The digestion of large molecules is essential because the molecules are too large to pass across cell membranes, so the molecules are digested to a suitable size.

Enzymes allow digestion to happen at a lower temperature and therefore increase the rate of reaction. They do not perform the action, but increase the possibility of the reaction to occur. 

    Function of the stomach:
- Food is brought to the stomach by the esophagus.
- Food is mixed with gastric juice (mucus, HCl, pepsin and water)
- Muscular walls of the stomach create a motion in order to mix the food with the gastric juice.
- Continues to duodenum in the small intestine.

Function of the small intestine:
- In the duodenum, the accessory organs secrete juices (eg. bile and trypsin).
- Later, small molecules (fatty acids/glycerol, amino acids, monosaccharides, vitamins and minerals) are absorbed by the capillary beds in villi.

Function of large intestine:
- What is left of the original food is not absorbed.
- In the large intestine, water is absorbed.
- A lot of bacteria live here. They provide the environment and synthesize vitamin K.
- Any undigested food (mucus, dead cells, bacteria, some mineral ions and water) leave the body as faeces.

Absorption: when soluble products of digestion “enter” the body circulation system (or lymphatic system).

Assimilation: the products enter the body cells for usage or storage.

Structure of a villus

The inner wall of the small intestine in made up of thousand of villi. Each villus contains a capillary bed (a small vessel of your circulatory system) and a lacteal (a small vessel of your lymphatic system). The function of the villi is to increase the surface area for absorption of molecules.

The coronary arteries supply heart muscle with oxygen and nutrients.

Pulmonary circulation:
- Deoxygenized blood flow into the right atrium.  
- The blood begins to move down into the right ventricle through the right atrioventricular valve.
- The right atrium contracts to force remaining blood into the right ventricle.
- The right ventricle contracts, initiating several events, including:
1. Closure of the atrioventricular valve to prevent backflow (the closing produces the audible heart beats)
2. Increase in blood pressure, opening the right semilunar valve.
3. Due to increase in pressure blood leaves the heart through the pulmonary artery.
- Blood flows to one of the two lungs through small arteries (arteriole).
- Any one arteriole leads to a capillary bed and is only one single cell thick à facilitates “exchange” of oxygen.
- The oxygenized blood flows into left atrium.
 Systemic circulation:
- Blood flows into left atrium and events similar to when the blood entered the right atrium occur (both atria contract at the same time and both ventricles contract at the same time).
- Blood accumulates in the left atrium and then enters the left atrioventricular valve.
- Left ventricle is filled with blood and then contracts.
- Blood flows through the aorta to all tissues in the body.
- In the capillaries, oxygen is given off and CO₂ may be taken in by the blood.
- The blood flows back to the original starting point.

Myogenic muscle contraction:
- Cardiac muscle spontaneously contracts and relaxes without nervous system control.
- It needs to be controlled in order to keep the timing of the contractions unified and useful.
The role of the pacemaker:
- The right atrium contains a mass of tissue within its walls known as the sinoatrial node (SA node) à acts as a pacemaker.
- SA node sends out an electrical signal to initiate the contraction of both atria.
- An other mass of tissue, atrioventricular node (AV node), receives the signal from the SA node and waits app. 0,1 seconds and then sends out an other electrical signal.
- This signal goes to the much more muscular ventricles and results in their contraction.
The medulla of the brain:
- While exercising, there is an increase in CO₂ which the medulla senses.
- Medulla sends a signal through a cranial nerve to increase heart rate to an appropriate level (reversed effect when exercise terminates).
Nerve:
- When exercise is terminated, the signal is carried by another cranial nerve.
Adrenaline:
- Heart rate can also be influenced by chemicals such as adrenaline.
- Adrenaline causes the SA node to “fire” more frequently than before à heart rate increases.

Arteries:
- Blood vessels carrying blood from the heart.
- Blood in arteries is at high pressure à relatively thick smooth muscle layer.

Veins:
- Blood vessel carrying blood to the heart.
- Receive blood at low pressure à thin walls and a larger internal diameter.
- Many internal passive valves that keep the blood moving towards the heart.

Capillaries:
- Form capillary beds (network of capillaries) which link arteries and veins.
- When blood enters a capillary bed, pressure is lost.
- Chemical exchanges occur.    

Blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets.

The following are transported by the blood: nutrient, oxygen, CO_2, hormones, antibodies, urea and heat.

Pathogen: an organism or virus that cause a disease (viruses, bacteria, protozoa, fungi and worms of various types).

Antibiotics are effective against prokaryotic cells but not viruses as they make use of eukaryotic cells. They have the ability to damage or kill prokaryotic cells since they have a cell wall dissimilar to our body cells. One type of antibiotic may inhibit the production of a new cell wall by bacteria, thus blocking their ability to grow and divide.

Skin: a barrier to infections which has two layers. The dermis contains sweat glands, capillaries, sensory receptors and it gives structure and strength to the skin. The epidermis consists mainly of dead cells à good barrier as it is not truly alive. As long as skin remains intact, we are protected from pathogens; therefore it is important to cleanse cuts and abrasions.

Mucus: other pathogens enter in the air we breathe (through nasal passageways or mouth). The entrance is lined up with mucous membranes (trachea, nasal passages, urethra, vagina).
Cells of mucous membranes produce sticky mucus, which can trap incoming pathogens, and so prevent them from reaching cells they could infect.
Some mucous membrane tissues lined with cilia (hair like extensions), move pathogens up and out of mucous-lined tissues. The cells that secrete the mucus also secrete and enzyme called lysozyme, which is able to chemically damage many pathogens.

Leucocytes (white blood cells) fight pathogens that enter our bodies and provide us with immunity when encountering pathogens a second time. Macrophages (type of leucocyte) can change their shape to surround an invader, a process called phagocytosis. Macrophages are able to squeeze their way in and out of small blood vessels and therefore encounter invaders outside of the bloodstream.

When encountering a cell it recognizes whether it is a “self” or “not-self”. If it is a “not-self” it engulfs the invader to undergo phagocytosis. Phagocytes contain many lysosome organelles to help digestion of invaders.

Antibody: protein molecules that we produce in response to a specific type of pathogen.  Each type of antibody is different because each type has been produced in response to a different pathogen.

Antigen: a protein on the surface of a pathogen (all the “not-self” proteins are antigens, which trigger an immune response).  
Most pathogens have several different antigens on their surface à may trigger the production of many different types of antibody.

Humans have different types of B lymphocytes, which produce a relatively small number of one type of antibody each. However, our immune response can produce more antibodies:
1. Antigen is identified.
2. B lymphocyte is identifies to produce an antibody which will bind to the antigen.
3. B lymphocytes clone themselves (mitosis) to increase in number.
4. The “army” begins antibody production.
5. Antibodies eventually find their antigen match.
6. Antibodies eliminate the pathogen.
7. Clones B lymphocytes remain and give immunity (memory cells).

All viruses must find a type of cell in the body that matches their own proteins in a complimentary way. This is why only certain body cells are damaged by certain viruses as is typically reflected in the symptoms associated with a particular infection.
I.e. a cold virus located the proteins on mucous membrane cells in your nasal region and damages those cells à symptoms including swelling of the area and excessive mucus production.
This is the same for HIV. Only certain cells in the body have the protein in their membranes that HIV recognizes (one of those is a cell that functions as a communicator cell in the blood stream, known as a helper-T cell). These are the cells that HIV infects. HIV is a type of virus which infects and afterwards remains alive. This is why the AIDS is developed many years after HIV infection.

      Cause of AIDS: the helper-T cells need to communicate with cells producing antibodies. When helper-T cells die, communication is lost à no antibody production à symptoms of AIDS appear

.

Transmission of AIDS: HIV is transmitted from person to person by body fluids (during sex and needle injections). Also, blood transfusions have to be tested for HIV to prevent transmission.

Social implications of AIDS: labeled as a disease affecting homosexuals and drug abusers. People diagnosed as being HIV positive may be discriminated against in terms of employment, insurance, social acceptance etc.

Ventilation: filling our lungs with air, and then breathing that air out.

Gas exchange: during the time when the air is in our lungs diffusion of gases occurs. Oxygen in the lung tissues diffuses into the blood stream, and carbon dioxide from the blood stream diffuses into the lung tissues.

Cell respiration: the process that requires oxygen (and gives off carbon dioxide) is called aerobic cell respiration. Chemical bonds within a glucose molecule are sequentially broken to release energy. Much of this energy is then stored as molecules of ATP. In aerobic organisms, the process requires oxygen molecules, and each of the six carbons of glucose are given off as a carbon dioxide molecule. (See p. 169 “in your book”).

Some organisms need a ventilation system because their bodies are so thick à not allowing gas exchange by diffusion. Another reason is to ensure that the concentration of respiratory gases in the lungs encourages diffusion of gases beneficial to the body.

Mechanism of ventilation:
The tissue that makes up our lungs is passive and not muscular à incapable of purposeful movement. However, there are muscles surrounding the lungs (diaphragm, abdomen, intercostals muscles).
The mechanism of breathing is based on the inverse relationship between pressure and volume. The lungs are located within the thorax, closed to the outside air à closed environment. The lungs have only one opening to the outside air – through the trachea.

Mechanism of inspiration:
1. Diaphragm contracts, abdominal muscles and intercostals muscles raise the rib cage.
2. Pressure inside cavity decreases as activity in thorax has increased its volume à less pressure “pushing on” lung tissue.
3. Lung tissue increases its volume due to less pressure.
4. à Decrease in pressure inside of the lungs (partial vacuum).
5. Air counters the partial vacuum within the lung, filling alveoli.
These steps are reversed for expiration.

The nervous system consists of the CNS and peripheral nerves, and is composed of cells called neurons that can carry rapid electric impulses.

Nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.

Resting potential: the state of being where an area of a neuron is ready to send an AP. This area of a neuron is said to be polarized. Resting potential is carried characterized by the active transport of Na⁺ (transported out) and K⁺ (transported in) in two different directions. In addition, there are negatively charged organic ions located in the cytoplasm of the axon à net positive charge outside the axon.

Action Potential: a self-propagating wave of ion movements in and out of the neuron membrane. The movement consists of ions diffusing from outside to inside and vice versa. Resting potential requires active transport (protein channels and ATP) to set up a concentration gradient between the internal and external environments. Na⁺ diffuses in when channels open, and K⁺ diffuses out. This diffusion is the action potential which is a nearly instantaneous event of an axon called depolarization.

Nerve impulse: the individual neurons within the nerve are each capable of carrying an action potential. Axon is the conductor of a neuron impulse. The axons of neurons in some organisms have a surrounding membranous structure called a myelin sheet. The myelin sheet greatly increases the rate at which an action potential passes down an axon.
An axon which does not have a myelin sheet is known as a non-myelinated neuron.

At the end of axons are areas called terminal buttons, withholding neurotransmitters (eg. acetylcholine, dopamine, and serotonin). When an action potential reaches the terminal buttons, this happens:
1. Ca²⁺ diffuse into the terminal buttons.
2. Vesicles containing neurotransmitter fuse with the plasma membrane à releasing neurotransmitter.
3. Neurotransmitter diffuses across the synaptic gap (pre- to post-)
4. Neurotransmitter binds with a receptor protein à ion channel opens and Na⁺ diffuses in.
5. Action potential begins to move down the postsynaptic neuron due to depolarization.
6. Neurotransmitter is degraded by enzymes.
7. Ion channel closes.
8. Neurotransmitter fragments fuse back across the synaptic gap to be reassembled in the terminal buttons.

Endocrine system consists of gland that release hormones that are transported in the blood.

Homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.   

Homeostasis: the human body typically stays within certain limits for many physiological variables as blood pH, carbon dioxide concentrations, blood glucose concentration, body temperature and water balance within tissues. Each variable has an expected value that is considered to be normal for homeostasis.
Eg. the internal body temperature is 37°C. However, there is an inevitable fluctuation around this exact temperature, for example when exercising or when exposed to cold environments. There are mechanisms that revert the temperature back to the normal state. These are called negative feedback mechanisms (functions to keep a value within the narrow range that is considered normal for homeostasis).

The biological thermostat for temperature control is the hypothalamus. When body temperature rise, due to exercise or warm external environment, the hypothalamus receives information from thermoreceptors, and begins to activate cooling mechanisms. These include increased activity of sweat glands and increased perspiration. In addition, arterioles dilate (enlarges) and fill capillaries with blood à heat leaves by radiation.
When in a cold environment, the hypothalamus activates warming mechanisms. These include constricting of arterioles à blood diverted to deeper organs and tissues à less heat lost by radiation. Hypothalamus also stimulates skeletal muscle to begin shivering.

Cells rely on glucose for the process of cell respiration (thus constantly lowering the concentration of glucose in the blood). Humans eat daily and thereby ingest glucose to raise the blood glucose level. Blood glucose must be maintained close to the body’s set point for blood glucose level, and negative feedback mechanisms ensure this. Glucose concentration in the hepatic vein varies depending on the time since your last meal, and the glucose content of the food you ate. All other blood vessels (not hepatic vein) receive blood after it has been acted on by liver cells called hepatocytes. Hepatocytes are directed to action by two hormones produced in the pancreas; insulin and glucagon.
Blood glucose levels goes above the set point.
Cells known as β cells in the pancreas produce and secrete the hormone insulin, which is absorbed into the blood. Insulin’s effect on body cells is to open channels to allow glucose to diffuse into the cell (facilitated). Also, insulin stimulates the hepatocytes to take in glucose and convert it to glycogen (monosaccharide à polysaccharide) stored as granules. The ultimate goal of insulin is to reduce blood glucose.
Blood glucose level drops below the set point when someone has not eaten or exercised for a long time. Under this circumstance, α cells of the pancreas begin to produce and secrete the hormone glucagon. The glucagon circulates in the bloodstream and stimulates hydrolysis of the granules of glycogen stored in hepatocytes and muscle cells; the hydrolysis produces the monosaccharide glucose, which enters the blood stream. à Increased blood glucose.

     
Type 1: autoimmune disease where the body’s own immune system attacks and destroys the β cells so little to no insulin is produced. Less than 10%
of diabetics have type 1, which most often develops in children or young adults.
Type 2: a result of the body cells no longer responding to insulin, known as insulin resistance. Often associated with genetic history, obesity, lack of exercise, aging and ethnic groups.

The hypothalamus produces a hormone known as GnRH. The target tissue of GnRH is the nearby pituitary gland and it results in the pituitary producing and secreting two hormones (FSH and LH) into the bloodstream. The target tissues for these two hormones are the ovaries. FSH and LH have several effects on the ovaries:
1. Increase the production and secretion of another reproductive hormone by the follicle cells of the ovary (oestrogen which enters the bloodstream) à the endometrium becomes highly vascular.
2. Production of structures within the ovaries (Graafian follicles). In the ovaries are cells known as follicle cells and the true reproductive cells which are at a stage of development called oocytes (accompanied by the inner ring of the follicle cells).
A spike in the level of FSH and LH leads to ovulation.
The follicle cells begin to produce and secrete another hormone (progesterone). The cells of the outer ring begin to divide and fill in the “wound” area left by ovulation à forming a glandular structure known as corpus lutheum (hormonally active for 10-12 days after ovulation). Progesterone maintains the thickened, highly vascular endometrium and as long as progesterone continues to be produced, the endometrium will not break down and an embryo will still be able to implant.
Oestrogen and progesterone are a negative feedback signal to the hypothalamus.
Assuming there is no pregnancy, the corpus lutheum eventually begins to break down à decline in both progesterone and oestrogen levels and the highly vascular endometrium can no longer be maintained.
The capillaries and small blood vessels begin to rupture and menstruation begins.
The drop in progesterone and oestrogen also signals to hypothalamus to begin secreting GnRH and thus another menstrual cycle begins.

Three roles of testosterone in males:
1. Determines the development of male genitalia during embryonic development.
2. Ensures development of secondary sex characteristics during puberty.
3. Maintains the sex drive of males throughout their lifetime.

In-vitro fertilization:
1. Woman injected with FSH during 10 days à ensures development of many Graafian follicles within her ovaries.
2. Oocytes are harvested surgically.
3. The man ejaculates in a container.
4. The harvested eggs are mixed with sperm cells in separate culture dishes.
5. Microscopic observation reveals if the early development appears normal and healthy.
6. Two or three healthy embryos are introduced into the woman’s uterus for implantation to increase chance of success.