Carbohydrates, fats and proteins

Begin by naming what the question is talking about. In Biochemistry, both phenylalanine and tyrosine are well-known small molecules that act as monomers used to build proteins (they are part of the amino acid set).

From prior knowledge of biological molecules, we classify phenylalanine and tyrosine as amino acids.

Because the prompt describes a conversion of one into the other, it points to a reaction where the structure of an amino acid is adjusted so it becomes a different amino acid, rather than being built from scratch or chopped out of a protein.

Consider why the remaining choices don’t match the situation:

• Option B doesn’t fit because the starting substance is already an amino acid, not a simple sugar.
• Option C is about protein digestion, meaning peptide bonds are broken in a chain; it doesn’t describe turning one single amino acid into another.
• Option D refers to nitrogen waste removal (excretion-related pathways), which is different from converting one amino acid into a new amino acid.

Since both molecules are amino acids and one is being converted into the other, the best description is a metabolic modification that changes one amino acid into a different amino acid. That corresponds to Option A.

Start by identifying the substances. In Topic: Biochemistry, phenylalanine and tyrosine are taught as amino acids—the monomers that can be linked to form proteins. The question focuses on converting one into the other.

Option B describes producing an amino acid from simple sugars. Although some amino acids can be formed using carbon skeletons that trace back to sugar metabolism, this particular reaction begins with phenylalanine, which is already a fully formed amino acid. So it is not synthesis from simple sugars.

Option C refers to breaking a protein into its separate building blocks, which is hydrolysis. Hydrolysis can release amino acids such as phenylalanine or tyrosine from a polypeptide, but it does not convert one amino acid into another; it only separates them by cutting peptide bonds.

Option D is about removing excess nitrogen, commonly through deamination and then forming urea in humans. In the conversion of phenylalanine to tyrosine, the amino group ($N{H}_2$) stays on the molecule. The key change is adding a hydroxyl group ($-OH$) to the ring, so nitrogen is not being discarded as waste in this step.

With the other choices removed, the remaining interpretation is that the body is directly altering one amino acid to make another. The enzyme phenylalanine hydroxylase adds an $-OH$ group to phenylalanine, producing tyrosine—an example of modifying one amino acid into a different amino acid.

To see what temperature changes do to reaction speed, we can use the Maxwell–Boltzmann distribution. It’s a graph that shows how kinetic energy is spread out among molecules in a liquid or gas at one temperature.

• The x-axis shows kinetic energy ($E$).
• The y-axis shows how many molecules have that particular energy.

In a real sample, not all molecules move at the same speed. Many have medium energies (near the top of the curve), some have low energy, and a small number have very high energy.

Heating increases the molecules’ average kinetic energy. On a Maxwell–Boltzmann graph, that shows up in two key ways:

1. The highest point of the curve moves to the right (toward higher energy values).
2. The curve becomes lower and wider because the same total number of molecules is now spread across a broader range of (generally higher) energies.

The activation energy ($E_{\text{a}}$) is the minimum energy a collision needs for a reaction to occur. On the graph, we represent $E_{\text{a}}$ as a vertical line at a fixed position on the energy axis.

Only molecules with energies to the right of that line can overcome the barrier and successfully react when they collide with the right orientation and conditions.

When temperature increases and the curve shifts right, the area under the curve beyond the $E_{\text{a}}$ line becomes much larger.

That means the number of molecules with energy $\ge E_{\text{a}}$ rises a lot. Because successful reactions depend on having enough energy, even a small temperature increase can greatly boost reaction rate.

Now we can reject the options that conflict with the Maxwell–Boltzmann explanation:

• Option A: Heating makes particles move faster, so collisions happen more often, not less often.
• Option B: Temperature affects kinetic energy, but it doesn’t turn the transition state into something long-lived; the transition state is still extremely brief.
• Option C: Heating doesn’t reduce the activation-energy barrier; it increases particle energy. Only catalysts (like enzymes) provide an alternative pathway that lowers $E_{\text{a}}$.

From the distribution shift, the best explanation is that warming increases the fraction of molecules that have enough kinetic energy to get over the activation-energy barrier.

Using elimination, start by recalling what temperature means: it reflects the average kinetic energy of particles in a system. Increasing temperature gives molecules more energy, so they move faster.

This idea connects to MYP Biology topics like metabolism and biochemistry, where reaction rates (often enzyme-controlled) depend strongly on energy and molecular motion.

Option A says heating “reduces the number of collisions.”

But faster-moving molecules collide more frequently because they travel and mix more quickly. So the claim in A contradicts how particle motion works.

Option B suggests the transition state becomes stable and lasts for minutes.

The transition state is, by nature, a very high-energy and unstable arrangement of atoms at the top of the energy barrier. It exists for an incredibly short time (around $10^{-13}$ to $10^{-15}$ seconds). Raising temperature doesn’t make it stable; it mainly increases how many molecules can reach that stage.

Option C claims heating lowers the activation energy.

That’s a common misunderstanding. The activation energy ($E_{\text{a}}$) is a set barrier for a particular reaction pathway. Temperature doesn’t change the height of the barrier; it changes how energetic the molecules are. To actually lower $E_{\text{a}}$, you need a catalyst (for example, an enzyme) that provides an alternative pathway.

After removing A, B, and C, Option D remains.

When temperature rises, the spread of kinetic energies shifts upward, so a larger proportion of molecules have kinetic energy ($KE$) that meets or exceeds the activation energy needed for a successful reaction:

$KE\ge E_{\text{a}}$

That increase in the number of sufficiently energetic molecules is why reaction rates can increase sharply with temperature.

Digestion mainly works to change large, complex, often less-soluble food molecules into smaller, simpler, more-soluble molecules. This matters because only small dissolved molecules can cross the cells of the intestinal lining and then enter the bloodstream.

Lactose is a disaccharide, meaning it is made from two smaller sugars (glucose and galactose) linked together. While it is still a “double sugar,” it is generally too large to be carried across the small intestine lining efficiently for absorption into the blood.

Check the other answer choices using basic biology:

• Option B: Glucose and galactose are not limited to plants; animals also contain and use these sugars in many carbohydrates.
• Option C: The body releases heat during many chemical reactions (including cellular respiration), not only when lactose is digested.
• Option D: Lactose is a food molecule, not a poison. Even if it somehow reached the blood, it wouldn’t be “toxic”; it just wouldn’t be useful until enzymes split it into usable monosaccharides.

The enzyme lactase works like a molecular cutter, breaking the bond holding lactose together. This produces glucose and galactose, which are monosaccharides. Single sugars are small enough to pass through the intestinal wall and can then be transported in the blood to cells for energy.

Since absorption requires molecules to be small enough (and soluble enough) to cross the intestinal lining, lactose must be digested first because it is physically too large to be absorbed as a disaccharide.

This question is asking why the body needs digestion—specifically why lactase must split lactose into smaller sugars—before those sugars can be used. In biology, digestion breaks big, less-soluble molecules into small, soluble ones so they can be absorbed into the bloodstream. We can find the best answer by removing the distractors.

This choice says glucose and galactose are only found in plants, which is incorrect. Although plants make glucose during photosynthesis, animals also have glucose in their bodies. Also, galactose is part of lactose, which is found in mammal milk. So it’s not true that the body must make these sugars from lactose because they don’t exist elsewhere.

This option says splitting lactose is the only way to produce heat. Body heat is mainly produced as a side effect of cellular respiration and other metabolic reactions, and many fuels can be used (carbohydrates, fats, and proteins), not just lactose. Therefore, this statement is too absolute to be correct.

This option suggests lactose is toxic, but lactose is simply a nutrient. People with lactose intolerance feel discomfort because lactose isn’t broken down properly in the gut, not because lactose is a toxin. Even if a small amount entered the blood, it wouldn’t act like a poison; it just wouldn’t be readily usable until it was broken into monosaccharides.

After eliminating the incorrect choices, Option A remains. Lactose is a disaccharide (two linked sugar units), so it is larger than the monosaccharides the small intestine is designed to absorb. Because lactose is too large to cross the intestinal lining into the capillaries efficiently, it must be broken down into glucose and galactose first.

This question is asking about the energy change that happens when a chemical bond is broken. That idea shows up often in Biochemistry and energy transfer in IB MYP Biology.

To answer, you need the basic rule linking bond stability with energy changes when bonds are broken or made.

A chemical bond is the attractive force holding atoms together. Separating the atoms means overcoming that attraction, so energy must be supplied from outside the molecule.

Using standard energy terms:

• Breaking bonds is endothermic: energy goes in.
• Making bonds is exothermic: energy comes out.

Apply the bond rule to remove incorrect answers:

• Option D is wrong because energy release is linked to forming bonds, not splitting them.
• Option C is wrong because adding energy to separate atoms leaves them with more potential energy, not less.
• Option B is wrong because bonding increases stability; breaking a bond generally makes the pieces more reactive and less stable.

Because energy is needed to overcome the attractive forces holding atoms together, a bond cannot break without an energy input. That means energy is absorbed, which is exactly what an endothermic change involves.

To figure out what happens during bond breaking, start with what a bond represents. A chemical bond is basically an electrostatic attraction—for example, between positively charged nuclei and negatively charged electrons that are shared or interacting.

Because the bond is an attractive force, atoms together in a bond sit in a lower-energy arrangement than separated atoms. Pulling them apart means working against that attraction.

In physics and chemistry, moving something against an attractive force requires work done on the system, and doing work requires energy input.

If energy were released to the surroundings during bond breaking, that would suggest the system is becoming more stable and dropping to a lower-energy state. But separated atoms/fragments are typically less stable and have higher potential energy than when they are bonded. So energy release cannot describe bond breaking.

When bonds in a molecule are split, the products are often reactive pieces (atoms or fragments). That means the system is not suddenly more stable right after the bond breaks.

Since energy must be supplied to overcome electrostatic attraction, the process requires the system to absorb energy from its surroundings. That is why breaking a chemical bond is an endothermic step.

The question focuses on what happens to energy when chemical bonds are broken. In Biochemistry processes, bonds represent a stable arrangement where electrostatic attractions hold atoms together. Separating them requires work to be done on the system.

Bonds form because the bonded state is typically more stable (lower energy) than the atoms on their own. Breaking a bond moves the system toward a less stable, higher-energy situation. So the statement that it "instantly becomes stable" cannot be correct.

A useful memory rule is "BARF": Break Absorbs, Release Forms. Energy is released when bonds are created, not when they are split. Since bond breaking needs energy input, it cannot be releasing energy to the surroundings.

Separating two attracted atoms increases the system’s potential energy (like stretching a spring or pulling back a rubber band). Because energy is added to overcome the attraction, the atoms/fragments do not end up with less energy overall; the system’s energy increases.

With the other options ruled out, the remaining correct idea is that bond breaking is endothermic. Energy must be absorbed from the surroundings to overcome the forces holding the atoms together.