NUMS Entry Test Biology Syllabus

Compare DNA and RNA in terms of sugar, bases, and number of strands.

DNA: deoxyribose sugar, bases A/T/G/C, double-stranded. RNA: ribose sugar, bases A/U/G/C, single-stranded.

Who proposed the double-helix model of DNA, and what holds the two strands together?

Watson and Crick proposed the double helix. The two antiparallel strands are held together by hydrogen bonds between complementary bases.

List the three main types of RNA and their functions.

mRNA (messenger – carries the genetic code from DNA to ribosomes), tRNA (transfer – brings amino acids to the ribosome), and rRNA (ribosomal – structural and functional component of ribosomes).

What molecular feature of water makes it a polar molecule?

Oxygen is more electronegative than hydrogen, so the bent water molecule has a partial negative charge on oxygen and partial positive charges on the hydrogens, creating polarity and hydrogen bonding.

Define cohesion and adhesion of water and give a biological significance of each.

Cohesion = attraction between water molecules (enables surface tension and water transport up the xylem). Adhesion = attraction of water to other surfaces (helps capillary action and water movement in plants).

Why is water described as a near-universal solvent, and why is this important biologically?

Because of its polarity, water dissolves many ionic and polar substances. This allows it to act as the medium for biochemical reactions and for transport of substances in organisms.

How do water's high specific heat capacity and high heat of vaporization benefit organisms?

High specific heat resists temperature change, stabilizing internal/external environments; high heat of vaporization makes evaporative cooling (e.g. sweating, transpiration) effective for temperature regulation.

According to the lock-and-key model, how does an enzyme catalyze a reaction?

The substrate has a shape complementary to the enzyme's active site (like a key fitting a lock); they bind to form an enzyme-substrate complex, the reaction occurs, and products are released.

What does the induced-fit model add to the lock-and-key concept of enzyme action?

The active site is not perfectly rigid; binding of the substrate induces a slight change in the active site's shape so it moulds around the substrate, improving catalysis.

How do enzymes speed up reactions in terms of activation energy?

Enzymes lower the activation energy required for a reaction to proceed, increasing the reaction rate without being consumed and without changing the reaction's equilibrium.

Describe the effect of temperature on enzyme activity, including the optimum.

Activity increases with temperature up to an optimum (often ~37 C in humans); beyond the optimum the enzyme denatures and activity falls sharply.

Describe the effect of pH on enzyme activity.

Each enzyme has an optimum pH at which activity is maximal; deviation from optimum (too acidic or too alkaline) reduces activity and extreme pH denatures the enzyme. E.g. pepsin ~pH 2, trypsin ~pH 8.

How do substrate concentration and enzyme concentration affect reaction rate?

Increasing substrate concentration raises the rate until all active sites are saturated (then it plateaus). Increasing enzyme concentration increases the rate proportionally as long as substrate is in excess.

Differentiate between competitive and non-competitive enzyme inhibition.

A competitive inhibitor resembles the substrate and binds the active site, blocking substrate (overcome by more substrate). A non-competitive inhibitor binds elsewhere (allosteric site), changing the active site's shape, and is not overcome by more substrate.

What is a reversible versus an irreversible enzyme inhibitor?

A reversible inhibitor binds non-covalently and can detach, restoring activity; an irreversible inhibitor binds tightly (often covalently), permanently inactivating the enzyme.

What is feedback (end-product) inhibition?

A type of regulation in which the end product of a metabolic pathway inhibits an earlier enzyme in the pathway (often by binding an allosteric site), preventing overproduction.

Define cofactor and distinguish it from a coenzyme.

A cofactor is a non-protein component required for enzyme activity. Inorganic cofactors are metal ions (e.g. Zn2+, Mg2+); organic cofactors that are loosely bound are called coenzymes (e.g. NAD, FAD, often vitamin-derived).

What is a prosthetic group and how does it differ from a coenzyme?

A prosthetic group is a cofactor that is tightly (often permanently) bound to the enzyme, whereas a coenzyme binds loosely/transiently. Example prosthetic group: the heme in catalase.

State the main idea of the fluid mosaic model of the cell membrane.

The membrane is a fluid phospholipid bilayer in which proteins (integral and peripheral) are embedded and can move laterally, giving a 'mosaic' of components; cholesterol and carbohydrates are also present.

Differentiate passive transport from active transport across the cell membrane.

Passive transport moves substances down a concentration gradient without ATP (diffusion, facilitated diffusion, osmosis). Active transport moves substances against the gradient using ATP and carrier proteins.

Define diffusion, osmosis, and facilitated diffusion.

Diffusion: net movement of particles from high to low concentration. Osmosis: diffusion of water across a selectively permeable membrane from high to low water potential. Facilitated diffusion: passive movement of substances through specific membrane proteins.

Differentiate endocytosis from exocytosis.

Endocytosis is the bulk intake of materials into the cell by engulfing them in a membrane vesicle (phagocytosis/pinocytosis); exocytosis is the bulk export of materials out of the cell via vesicles fusing with the membrane. Both require ATP.

State the function of mitochondria and a key structural feature.

Mitochondria are the site of aerobic respiration and ATP production ('powerhouse of the cell'). They have a double membrane, the inner one folded into cristae enclosing the matrix, and contain their own DNA.

Compare rough and smooth endoplasmic reticulum.

Rough ER has ribosomes on its surface and synthesizes/transports proteins; smooth ER lacks ribosomes and synthesizes lipids/steroids and detoxifies substances.

What is the function of the Golgi apparatus and of lysosomes?

The Golgi apparatus modifies, packages, and sorts proteins and lipids into vesicles for secretion. Lysosomes contain digestive (hydrolytic) enzymes for intracellular digestion and breakdown of worn-out organelles.

What is the function of ribosomes and what are they made of?

Ribosomes are the sites of protein synthesis (translation). They are made of rRNA and protein and consist of a large and a small subunit; they may be free in the cytoplasm or attached to rough ER.

State the functions of the nucleus and describe the nuclear envelope.

The nucleus controls cell activities and stores genetic information (DNA). It is bounded by a double-membrane nuclear envelope perforated by nuclear pores, and contains the nucleolus (rRNA/ribosome synthesis) and chromatin.

What is a chromosome, and what is its relationship to chromatin and genes?

A chromosome is a condensed structure of DNA wound around histone proteins; in non-dividing cells it exists as loosely coiled chromatin. Genes are segments of DNA on chromosomes that code for proteins. Humans have 46 chromosomes (23 pairs).

Compare the cell walls of plants, fungi, and bacteria by their main structural material.

Plant cell walls are made of cellulose; fungal cell walls are made of chitin; bacterial cell walls are made of peptidoglycan (murein).

Write the overall balanced equation for photosynthesis.

6CO2 + 6H2O --(light energy, chlorophyll)--> C6H12O6 + 6O2.

Compare the light-dependent reactions and the Calvin cycle (light-independent reactions) of photosynthesis.

Light-dependent reactions occur in the thylakoid membranes, split water (photolysis), release O2, and produce ATP and NADPH. The Calvin cycle occurs in the stroma, uses ATP and NADPH to fix CO2 into glucose.

Write the overall equation for aerobic respiration and state its ATP yield.

C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP). Aerobic respiration of one glucose yields about 36-38 ATP molecules.