Biology and Life Sciences - GED Science

When an organism has only one type of allele for a given gene, it is described as homozygous ("homo-" meaning one). Organisms can be either homozygous dominant, meaning they have two dominant alleles, or homozygous recessive, meaning they have two recessive alleles.

A heterozygote, or hybrid, has two different types of alleles for a given gene. When the dominant allele causes the recessive trait not to be represented in the phenotype, the organism is considered a carrier for the recessive trait.

The circulatory system is responsible for transporting blood, oxygen, nutrients, and wastes throughout the body. The vessels of the circulatory system include arteries, arterioles, capillaries, venules, and veins. The heart is responsible for pumping the fluids of the circulatory system through these vessels, and is a major component of the circulatory system.

The respiratory system is responsible for transporting air to and from the lungs, and facilitates gas exchange with the capillaries of the circulatory system. The lymphatic system helps regulate fluid balance and immune system function; it consists of lymphatic vessels, the spleen, and the thymus. The digestive system breaks down and absorbs nutrients; it consists of the digestive tract from the mouth, through the stomach and small intestine, and to the rectum.

The heart has four chambers: two atria and two ventricles. The atria are responsible for receiving blood returning from the body, while the ventricles pump blood out of the heart. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body.

The flow of blood through the heart is: right atrium, right ventricle, lungs, left atrium, left ventricle, body.

Arteries are the high pressure blood vessels responsible for carrying blood away from the heart. All arteries in the body travel away from the heart toward other tissues.

Most arteries carry oxygenated blood, but the pulmonary arteries carry deoxygenated blood; these vessels are responsible for transporting deoxygenated blood from the right side of the heart to the lungs. Veins are responsible for returning blood back to the heart. Arteries do not allow for diffusion of nutrients; this process primarily occurs in capillaries.

The heart is able to pump independently of the brain thanks to a collection of cells that fire impulses automatically. These cells are located in the sinoatrial node, which leads to the node's nickname as the "pacemaker" of the heart. Even without stimulation by a nerve, the heart can continue beating.

The heart is made of cardiac muscle tissue; smooth muscle tissue lines internal organs and blood vessels. Both sides of the heart pump at the same rate, but the left side pumps with more force. Each side of the heart must pump the same volume, but the left side must pump it a greater distance since it pushes blood to the entire body. A normal resting heart rate is around 60-80 beats per minute. A heart rate of 120 would be more typical during exercise.

The respiratory system allows air to travel to the lungs. Air enters the mouth or nasal passages, travels through the trachea, and then into the bronchi. The bronchi split into smaller and smaller bronchioles before terminating in small sacs called alveoli.

The aorta is a major artery that carries blood away from the heart. The urethra carries urine from the bladder. The small intestine is a portion of the digestive tract that carries nutrients from the stomach to the large intestine.

Blood is composed of multiple cell types that have specific functions in the body. The red blood cells (erythrocytes) help carry oxygen to the tissues in the body. The white blood cells (leukocytes) help defend the body from bacteria and other illnesses. Platelets are cell fragments responsible for helping blood clot, and are created by megakaryocytes.

The plasma membrane of the cell acts as a semi-permeable barrier, regulating what can enter and exit the cell. Only small, nonpolar molecules are able to cross the membrane via diffusion, without the assistance of protein channels. Larger molecules will be blocked, as will molecules that are charged or polar.

This principle is true for both prokaryotic and eukaryotic cells, and is not affected by the presence of a cell wall. Virtually all organic molecules contain carbon and hydrogen; the presence of these atoms will not affect the molecule's ability to cross the membrane.

A hypertonic solution will have a higher solute concentration than the cell. To reach equilibrium, water must flow so that the concentration fo the solution is equal to the concentration of the cell. In order for this to happen, water must enter the solution, diluting it and reducing the concentration. Water will flow out of the cell and into the solution.

Note that the membrane of the cell will prevent ions from crossing.

Homeostasis describes the resistance of the body to change and serves to reinforce equilibrium. Concentration of ions and water in the blood and regulation of body temperature are examples of homeostatic regulation. These processes must be tightly regulated and maintained in order for the body to operate.

Negative feedback reinforces equilibrium and plays a key role in homeostasis. In a negative feedback system, deviations from equilibrium trigger processes that serve to return the body back to equilibrium. In contrast, a positive feedback system will respond to deviations from equilibrium by enhancing the changes, deviating farther and farther from the equilibrium state.

Hormones can play a key role in maintaining homeostasis, but many other molecules also help return the body to equilibrium.

The plasma membrane of the living cell is made up of a phospholipid bilayer, meaning two opposed sheets of phospholipids with their hydrophilic heads facing outwards, and their hydrophobic tails facing one another. This structure allows the cell to remain fluid in shape while also staying "water tight".

Active transport is the movement of individual small molecules across the plasma membrane against their concentration gradient (from lower concentration to higher concentration). This is usually accomplished by proteins embedded in the membrane which use ATP energy to act as "pumps".

In contrast, osmosis, simple diffusion and facilitated diffusion are all forms of passive transport and do not require additional energy from the cell. These transport methods move with the concentration gradient, rather than against.

Osmosis is the movement of water across a selectively permeable membrane, like the plasma membrane. It flows with its concentration gradient (from high concentration of water to low concentration of water).

A hypotonic solution is a solution that has a lower solute concentration than the fluid in the interior of the cell (the cytoplasm). When the cell is placed in this solution water will flow into the cell via osmosis, causing it to expand and potentially burst.

A hypertonic solution is a solution that has a higher solute concentration than the fluid in the interior of the cell (the cytoplasm). When the cell is placed in this solution water will flow out of the cell via osmosis, causing it to contract.

Exocytosis is the transport of waste materials or secretions from the cells. These molecules are packaged in membrane bound vesicles and are released across the membrane to the outside of the cell.

The "water-loving" hydrophilic heads of phospholipids orient themselves outwards towards the watery cytoplasm inside the cell and interstitial fluid outside the cell. Meanwhile, the "water-fearing" hydrophobic tails of phospholipids orient themselves inwards toward one another.

Interstitial fluid is the substance surrounding living tissue cells, its solute concentration is critical to the maintenance of homeostasis for the cells. Cells may have to spend valuable energy to keep unwanted materials from passing through their semi-permeable membranes in some conditions.

Diffusion, facilitated diffusion, and osmosis are all examples of passive transport because no energy is required to move molecules across the cell membrane. The driving force is the concentration gradient of the substance.