Correct Answer: (2) B

The plasma membrane (B) is the boundary of the cell that acts as a selective barrier, thereby regulating the cell's chemical composition.

Incorrect Options:

The nucleus (A) is the organelle that contains the chromosomes.

The vacuole (C) is a membrane-enclosed sac taking up most of the interior of a mature plant cell and containing a variety of substances important in plant reproduction, growth, and development.

The chloroplast (D) is an organelle found in plants and photosynthetic protests that absorbs the sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.

Correct Answer: (4) A=digestion B=amino acid molecules

Protein--like meat, fish, eggs, milk products, some vegetables, nuts, etc,--are long chains of amino acids. These are digested in the system by the aid of pepsin under acidic conditions. Most protein is absorbed in the form of single amino acids (the building blocks of protein), but some are absorbed as two or three amino-acid combinations.

Incorrect Answers:

Protein is not involved in the respiration of gases in the body.

Reproduction is not metabolic activity.

The primary product of excretion is ammonia, which is not a simple sugar molecule.

Correct Answer: (3) organelles→cells→tissues→organs→organ systems

Organ systems are made up of organs with different structures and functions but work together to perform a certain task (e.g. heart and blood work together for cardiovascular functions).

Organs (e.g. stomach) are made up of tissues which have the same structure and function. Cells make up these tissues (e.g. connective tissues). Organelles (e.g. nucleus) are parts of the cell.

Correct Answer: (2) an excess of enzyme X was present, resulting in a decrease in the production of substance B

This is called a negative feedback mechanism. It is a primary mechanism of homeostasis, whereby a change in a physiological variable (in this case, enzyme Y) triggers a response (inability of producing substance C) that counteracts the initial fluctuation.

Correct Answer: (2) ATP

ATP is an adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. The energy is used to drive endergonic reactions in cells.

Incorrect Answers:

DNA is a double-stranded, helical nucleic acid molecule capable of replicating and determining thee inherited structure of a cell's proteins.

A hormone is a chemical signal in multicellular organisms that are formed in specialized cells, travel in body fluids, and coordinate the various parts of the organism by interacting with target cells.

An antibody is an antigen-binding immunoglobulin, produced by B cells, and functions as the effector of immune response.

Correct Answer: (1) antigens

Antigen is a foreign macromolecule that does not belong to the host organism and that elicits an immune response.

Incorrect Answers:

The other choices do not cause immune responses in the human body.

Enzymes refer to the class of protein that serves as catalysts, which are chemical agents that change the rate of reaction without being consumed by the reaction.

Fat or triacylglycerol is a biological compound consisting of three fatty acids linked to one glycerol molecule.

Cytoplasm refers to the entire contents of the cell, exclusive of the nucleus, and bounded by the plasma membrane.

Correct Answer: (3) Requires less energy than thyroid tissue

Mitochondrion is considered the "power house" of the cell because it is where most cellular respiration occurs. The amount of mitochondrion in the cell exemplifies the energy requirement of the cell.

Cell X is a leukocyte or white blood cell. It is characterized by an irregularly-shaped nucleus.

Homeostasis is the steady-state physiological condition of the body. Since leukocytes function for defense and immunity of an individual, whenever there is a recognized pathogen the number of leukocytes temporarily increases to fight the infection. The leukocytes, particularly the monocytes and neutrophils, engulf and digest pathogenic bacteria. The number of leukocytes will go back to normal once the infection is gone.

1. One difference is the movement of substances in the concentration gradient. In diffusion, the substances travel from a more concentrated to a less concentrated area. On the other hand, in active transport, the substances travel against the concentration or electrical gradient.
2. During diffusion, the transport is spontaneous and not aided by energy input, but in active transport, energy and specific transport proteins are needed to cross the biological membrane.

Diagram A

Structure X is a stoma (plural stomata).

Stomata are microscopic pores surrounded by guard cells in the epidermis of leaves      and stems. If these malfunction, the gas exchange between the environment and the    interior of the plant will be impeded, which can further lead to withering of the plant.

Diagram B

Structure X is the pancreas.

Pancreas is soft, flattened gland that secretes digestive enzymes and hormones. One of the most important hormones secreted by the pancreas is insulin, which aids in the       balance of blood glucose levels. Malfunction of the pancreas may lead to pancreatitis or             diabetes mellitus.