Diet and nutrition

We need the biological name for the process in which cells get usable energy from glucose molecules. In living things, “usable energy” usually means ATP (adenosine triphosphate).

In the topic of Metabolism, cellular respiration is the set of chemical reactions where cells break down glucose and transfer the released energy into ATP so the cell can do work.

A common word equation for aerobic respiration is: $\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}+\text{Energy (ATP)}$

The other choices describe different ideas:

• Ingestion means bringing food or drink into the body.
• Digestion means physically and chemically breaking food down into smaller molecules (such as producing glucose from starch), but it is not the step where cells capture energy as ATP.
• Homeostasis is keeping the body’s internal conditions steady (for example, stable temperature).

Only respiration refers to the cell-level process that releases energy from glucose for ATP production, so it is the correct answer.

We must choose the process where cells release usable energy from glucose molecules. We can decide by checking each option’s meaning against that description.

Ingestion is the action of eating or drinking (getting food into the body). It helps bring in nutrients, but it is not the chemical energy-release step happening inside cells, so it cannot be correct.

Digestion breaks big food molecules into small, soluble molecules (like converting starch into glucose) so they can be absorbed. However, the energy stored in glucose is released later inside cells, not during digestion, so this option is not right.

Homeostasis is the regulation that keeps internal conditions within safe limits (such as steady body temperature or controlled blood glucose). It uses energy, but it is not the name of the reaction pathway that makes ATP from glucose.

That leaves respiration. In metabolism, respiration is the biochemical process in cells that breaks down glucose and releases energy captured as ATP (adenosine triphosphate), which matches the description.

The question is about how lipids support the structural integrity of cells, not their role as long-term energy stores. When thinking about what gives a cell its shape and keeps its contents contained, the first structure to consider is the cell membrane, which forms the boundary of the cell.

This links to Cells: Cell structure and Processes: Biochemistry in the course content.

Option A connects lipids with making enzymes. But enzymes are catalysts built from proteins (made of amino acids), not from lipids. Lipids can have other roles (like signaling), but they are not the main material used to construct enzymes.

Option B claims lipids are the medium where cell reactions occur. Most reactions happen in the cytoplasm, which is mostly water. Since many lipids are hydrophobic (they don’t mix with water), they are not the general solvent or reaction medium inside cells.

Option C focuses on glucose and cell walls. Cell walls (in plants, fungi, and many bacteria) are mainly made from structural carbohydrates like cellulose or chitin. Even though cells can convert molecules in metabolism, lipids are not the main material used to build or reinforce cell walls.

Option D is correct. A major lipid type called a phospholipid makes up most of the cell membrane. Phospholipids arrange into a phospholipid bilayer, forming a strong, flexible barrier that protects the cell and helps control what substances can enter and leave. This is a key way lipids help maintain the structure of cells.

The question targets the structural integrity of an organism's cells and asks for a lipid function besides energy storage. To choose correctly, we can match each incorrect option to a different macromolecule (or substance) and eliminate them one by one.

Option A talks about building enzymes. Enzymes are made from amino acids, so they are proteins, not lipids.

Because this describes a protein-related role, we can eliminate this option.

Option B suggests lipids are the main environment for chemical reactions. But lipids are usually hydrophobic and don’t blend into the watery interior of cells.

The true reaction medium/solvent in cells is water ($H_{2}O$), not lipids.

Option C mentions turning into glucose and strengthening a cell wall. Glucose is a sugar, and plant cell walls are largely cellulose—both are carbohydrates.

So this option is describing carbohydrate structure and function, not the specific structural role of lipids.

After removing the options that describe proteins, water, and carbohydrates, Option D remains.

In Topic: Biochemistry and Topic: Cell Structure, we learn that phospholipids (a type of lipid) form the phospholipid bilayer. This bilayer is the main structural part of cell membranes, creating a selectively permeable boundary that helps the cell keep its shape and stay intact.

To solve this, start with heterotrophy. Plants and other autotrophs can build organic food molecules using sunlight and $C{O}_2$. Heterotrophs (including humans and most animals) can’t do that, so they must take in organic material. We can’t tap energy straight from inorganic sources the way autotrophs do, and we also can’t assemble every complex substance entirely from basic raw materials.

A major reason organisms eat is to obtain essential nutrients. These include particular amino acids, vitamins, and fatty acids that the body cannot make because the necessary biochemical routes are missing. Since we can’t produce these molecules ourselves, they have to come from our diet.

The statement that the body can only make energy and not structural materials is wrong. Cells regularly construct structures such as muscle proteins and cell membranes, but they need dietary ‘building blocks’ (for example, amino acids) to assemble them.

Options C and D don’t fit. Enzymes regulate metabolic reactions, so the issue isn’t that making molecules internally is “too fast.” Also, although food can contain water, it isn’t the only way organisms get water, and water isn’t the main reason we eat organic molecules (we mainly need them for carbon-containing compounds and energy).

Using elimination and the idea that organisms have limits on what they can biosynthesize, the best explanation is that many organisms must eat because they cannot manufacture every required molecule (including essential amino acids and vitamins).

This question is asking why organisms—especially heterotrophs—need to take in food instead of producing all needed substances inside their bodies. The best approach is to compare each option with basic biology facts and cross out the incorrect claims.

Option B says bodies can make energy but not structural materials. That’s not true: cells constantly synthesize structural proteins (like collagen in skin, actin and myosin in muscles) and many enzymes used in metabolism. Since the body does build structural materials, this option must be eliminated.

Option C argues that internal synthesis is “too fast” to control. In reality, metabolic pathways are managed by enzymes and regulatory systems. There’s no biological reason that being “too fast” forces organisms to eat; if anything, we eat because some substances can’t be made in sufficient amounts—or can’t be made at all.

Option D claims food is the only source of water. But organisms can get water by drinking, and water is also formed during cellular respiration (sometimes called metabolic water). Because food isn’t the only source of $H_{2}O$, this option is incorrect.

With the other choices removed, Option A remains. It matches what we know: humans and other animals are heterotrophs and cannot synthesize all “essential” nutrients (such as certain amino acids, fatty acids, and vitamins), nor can we capture energy directly from sunlight. Therefore, we must eat to obtain these ready-made organic molecules.

The key clue is that the deficiency affects the blood’s ability to transport oxygen effectively. Oxygen is moved around the body mainly by red blood cells, so we need to remember what inside red blood cells actually attaches to oxygen in the first place.

Red blood cells are packed with a protein called hemoglobin. Hemoglobin can only pick up oxygen properly when it contains iron ($F{e}^{2+}$) as a key part of its structure.

If someone doesn’t get enough iron, their body can’t make enough functional hemoglobin. This can cause anemia, meaning less oxygen is carried to tissues, so the person may feel weak or exhausted.

Here’s why the other choices don’t match the specific job of oxygen transport:

• Glucose: This is a fuel molecule used in respiration to release energy; it isn’t a mineral that helps red blood cells carry oxygen.
• Calcium: Important for bones, muscle movement, and clotting, but it is not the substance that binds oxygen.
• Vitamin D: Helps the body take in calcium and supports bone health; it does not attach to oxygen in the blood.

Because iron is required to build hemoglobin, and hemoglobin is what binds oxygen in red blood cells, lacking iron will directly reduce how well the blood can transport oxygen. That means Option A is correct.

We are looking for the substance that is specifically needed for the blood to transport oxygen efficiently. This fits with MYP Biology ideas about transport and circulation. We can decide by checking what each option mainly does and crossing out those that don’t match.

Vitamin D is best known for helping the intestines absorb calcium. It supports bones and also has roles in immunity, but it does not directly help red blood cells bind or move oxygen through the bloodstream.

Calcium is essential for strong bones and teeth, and it is also used in muscle contraction and nerve communication. Even though calcium is present in blood, it is not responsible for transporting gases like oxygen.

Glucose ($C_6{H}_{12}{O}_6$) is a sugar that cells use as a main energy source in aerobic respiration. The blood delivers glucose to cells, but glucose itself is not what carries oxygen—oxygen is actually required to break down glucose to release energy.

After eliminating the others, Iron remains. Iron is a crucial part of hemoglobin in red blood cells. It allows hemoglobin to pick up oxygen in the lungs and let go of oxygen in body tissues where it’s needed.

When iron is lacking, anemia can develop, and the blood cannot carry enough oxygen, often causing tiredness and weakness.