Correct answer: (4) equilibrium

Equilibrium is achieved by an ecosystem if the competing factors in the environment are balanced, thus maintaining the number and types of species over the years. Hence, there will be minimal evolution, which usually brings about formation of new organisms.

Incorrect answers:

(1) feedback

Feedback refers to situations that happened due to a similar event that happened in the past.

(2) global instability

When there is global instability, the population of species dramatically change over time.

(3) environmental change

Environmental change, such as climate change, provides a risk to the survival of the population of species, causing their numbers to change over time.

Correct answer: (2) excretory system-remove potentially dangerous materials from the body

The primary organ of the excretory system is the kidney, which filers the blood to remove wastes, thereby maintaining homeostasis. The basic unit of the kidney is a nephron, which serves as a filtering unit. As blood flows through the nephron and then to the capillaries of the glomerulus in high pressure, water, glucose, vitamins, amino acids, protein waste products, salts, and ions from the blood pass out of the capillaries into the Bowman's capsule. Since blood cells and most proteins are too large to pass through the walls of the capillaries, these components are retained in the blood vessels.

From the Bowman's capsule, the filtered liquid passes through a u-shaped tubule where most of the ions and water, and all of the glucose and amino acids are reabsorbed into the bloodstream to maintain homeostasis. Thus, excess water, waste molecules, and excess ions become urine, which flows out of the kidneys through the ureter, then into the urinary bladder, and, after which, exits the body through the urethra.

Incorrect answers:

(1) immune system-intake and distribution of oxygen to cells of the body

The immune system protects the body against invasion of disease-producing agents such as bacteria, fungi, and viruses. The body system in charge of intake and distribution of oxygen to the cells of the body is the respiratory system.

(3) digestive system-transport energy-rich molecules to cells

The main function of the digestive system is to disassemble ingested food into its simpler form so that it can be used as energy for the body. The transport of energy-rich molecules to cells is a function of the circulatory system.

(4) circulatory system-produce building blocks of complex compounds

Correct answer: (3) Each of these cells contains only half the genetic information necessary for the formation of an offspring.

There are two types of cells in the body, the body cells and the sex cells. Body cells divide by mitosis, while sex cells divide by meiosis. A sex cell undergoing different stages of meiosis is illustrated. This division results in gametes with half the chromosome number of the original cells. When this gamete (sperm) unites with another gamete (egg) through fertilization, their genes would combine to form the genetic information of an offspring.

Incorrect answers:

(1) The cells are produced by mitosis and contain all the genetic information of the father.

Mitosis is a process of cell division that conserves the chromosome number of the original cells by equally allocating replicated chromosomes to each of the daughter cells.

(2) If one of these cells fertilizes an egg, the offspring will be identical to the father.

During fertilization, the haploid gametes (egg and sperm) unite and form a diploid offspring. The genes of the gametes are combined, thus the offspring has a new set of genetic information.

(4) An egg fertilized by one of these cells will develop into a female with the same characteristics as the mother.

Correct answers: (2) active transport, recognition of chemical messages, protection

The cell membrane is a semi-permeable membrane that separates and protects the cell from the outside environment. It controls the passage of molecules and ions in and out of the cell through passive or active transport. During passive transport, substances move through diffusion, while in active transport, energy and transport molecules are used to move substance through the cell membrane against concentration or electrochemical gradient.

Incorrect answers:

(1) protein synthesis, respiration, digestion of food molecules

Ribosomes are the sites where cell assembles proteins. On the other hand, respiration occurs in the mitochondria and digestion of molecules is a function of lysosomes.

(3) enzyme production, elimination of chemical messages, duplication of DNA codes

Lysosomes are membrane-enclosed sacs of hydrolytic enzymes that the cell uses to eliminate macromolecules. These enzymes can hydrolyze proteins, complex sugars, fats, and nucleic acids. DNA replication or duplication of DNA codes occurs in the nucleus.

(4) release of ATP molecules, regulation of cell reproduction, food production

Correct answer: (3) weakened viruses associated with the infection

Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen. The term vaccine came from the word "vacca" which means cow in Latin. Edward Jenner, an English physician, formulated the first vaccine against smallpox virus in 1796. He learned from his patients in farm country that milkmaids who had slight cowpox (a milder disease that usually infect cows) were resistant to subsequent smallpox infections. He did his experiment by scratching a boy with a needle that contains a fluid from a sore of a milkmaid who had cowpox. The boy was later exposed to smallpox and he resisted the disease.

Incorrect answers:

(1) live bacteria that ingest viruses

Vaccines may contain weaker versions of pathogenic bacteria, but these will not ingest viruses. The bacteria will turn on the defense system of the body so that when the stronger bacteria invade the system, it has already formed the resistance against them.

(2) white blood cells from an infected individual

White blood cells are naturally occurring components of the blood produced by the hematopoietic stem cell that are involved in the defense against pathogens.

(4) a variety of microbes that will attack the virus

Correct answer: (2) have the same genetic information but perform different specialized functions

Almost all cells in the body contain the same DNA or genetic information. However, not all genes in the DNA are expressed in all body parts. Expression of the genes is dependent of the function of the cells. For example, only the genes that code for the function of the heart are expressed in the cells of the cardiac muscles.

Incorrect answers:

(1) produce a hormone involved in respiration

The heart is an organ of the circulatory system which pumps blood all throughout the body. Thus, the statement is only true for lungs, which is the primary organ for respiration.

(3) use one part of the genetic code to synthesize all enzymes needed by the cell

Genetic codes are represented by nucleic acids that code for the formation of proteins like enzymes. Thus, different genetic codes are needed to form various types of enzymes depending on their structure and function.

(4) contain different numbers of DNA molecules

Correct answer: (3) natural selection

Natural selection refers to the differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Natural selection can bring about adaptive evolution which is a blend of chance and sorting-chance in the origin of new genetic variation and sexual recombination, and sorting in the workings of selection as it favors the propagation of some chance variations over others. From the range of variations available to it, natural selection increases the frequencies of certain genotypes and fits organisms in their environments.

Incorrect answers:

(1) ecosystem stability

Ecosystem stability refers to the unity of organisms on Earth. Unity and diversity are the two main factors that characterize life on Earth.

(2) homeostasis

Homeostasis refers to the steady-state physiological condition of the body.

(4) direct harvesting

Each step represents the following embryonic stages of animals:

1-unicellular zygote

2, 3, and 4-multicellular cleavage

5-blastula formation

6-gastrulation

7-organogenesis

The initial stage of the embryogenesis is the zygote or the unicellular fertilized egg. This then undergoes several divisions forming tissues in a particular fashion until it reaches the 7^th stage, the organogenesis, wherein the cells undergo differentiation to forms structures that will serve as the organs of the organism.

Incorrect answers:

(1) cells and organs

Cell is the basic unit of life. When cells aggregate for a specific function, it becomes a tissue. When tissues work together, these are now called organs. When the unicellular zygote divides, and the cells function together, it is then called a tissue.

(3) tissues and organelles

Tissues are collections of cells that work together for a specific function, while organelles are the membrane-enclosed parts of a cell. Though both of these structures are present in the stages illustrated, this answer is incorrect because cells must also be included in the structures present.

(4) organelles and cells

Correct answer: (3) insert the gene for the protein from species B into a chromosome in species A

Genetic engineering is the processes of altering the genetic makeup of an organism through recombinant DNA technology which involves laboratory tools and enzymes that cut out, insert, and alter species of DNA that contain one or more genes of interest.

Incorrect answers:

(1) mutate fungal DNA and introduce the mutated DNA into species B using a virus

Mutagenesis can be done to improve traits. However, a virus cannot be used to introduce the mutated DNA. Genes can be inserted by particle bombardment using a gene gun, or through the use of Agrobacterium tumefaciens, which is a bacterium, not a virus. The statement is also incorrect because it is not species B that needs transformation because it already produces the protein that provides resistance to the fungal infection.

(2) add DNA from species B into the soil around species A

Adding DNA into the soil is useless because it does not function like nutrients that can sip through the soil then to the plant. The gene from the DNA that code for resistance must be inserted directly to the DNA of species A through particle bombardment or Agrobacterium tumefaciens-mediated transformation.

(4) cross species A and a fungus to stimulate the synthesis of this protein

Correct answer: (4) the origin of new combinations of traits in offspring

The X-shaped region (in the second stage of the illustration) is called the chiasma (plural, chiasmata) which is a physical manifestation of a process called chromosomal cross over. Through this process that is a characteristic of meiosis, genetic rearrangement occurs.

Incorrect answers:

(1) why some offspring are genetically identical to their parents

Based on the illustration, the resulting chromosomes (Bb and Bb) are different from the original ones (BB and bb). Thus, genetic rearrangement occurred through the process of crossing over of chromosome, leading to the formation of a new set of chromosomes, different of the parents.

(2) the process of differentiation in offspring

Differentiation is the structural and functional divergence of cells as they become specialized during a multicellular organism's development. This occurs in the later part of development of body cells. The diagram illustrates what occurs in the gametes or sex cells.

(3) why some offspring physically resemble their parents