Plant responses

Phototropism is when a plant shoot shows directional growth by curving toward a light source. The hormone auxin (IAA) controls this response. In shoots, a greater amount of auxin causes cells to elongate more, leading to bending when one side grows faster than the other.

To curve toward the light, the shaded side of the shoot has to lengthen more than the lit side. That only happens if auxin is moved so it becomes more concentrated on the shaded side. So the idea that auxin ends up higher on the illuminated side is inconsistent with the growth pattern needed for phototropism.

If the illuminated side elongated first, the shoot would curve the opposite way (away from the light), which is not the normal shoot response. Also, IB plant physiology explains that light doesn’t simply break auxin down in the tip; instead, the plant changes where auxin is sent using specific transport proteins.

In the IB description of phototropism, the sequence is:

1. Blue light is sensed by phototropins.
2. This signal causes PIN proteins (auxin efflux pumps) to change position.
3. The PIN pumps relocate to the membrane side facing away from the light.
4. Auxin is then transported laterally toward the shaded side. Therefore, the redistribution depends on the sideways relocation of auxin efflux pumps.

Phototropism describes a shoot growing toward a light source. Auxin controls the unequal growth that produces bending. Using elimination, we compare each statement with what the IB syllabus says about auxin movement during this response (Topic 9.3: Growth in Plants).

Option A says the bright side grows first. If that happened, the shoot would curve away from the light because the lit side would become longer. In real shoot phototropism, the shaded side elongates more, causing the shoot to bend toward the light.

Option C claims auxin becomes higher on the lit side. Since auxin promotes elongation in shoots, the side with more auxin should grow faster. Because the shoot bends toward the light, the shaded side must be the faster-growing side, meaning auxin must be higher on the shaded side, not the lit side.

Option D suggests light destroys auxin. Although this was suggested historically, current understanding is that the amount of auxin in the shoot tip doesn’t mainly decrease during phototropism. Instead, auxin is moved from one side to the other, creating an uneven distribution without needing it to be broken down.

With the other options removed, the remaining statement matches the known mechanism: phototropins detect blue light and cause PIN proteins (auxin efflux pumps) to shift within the cell membrane. By relocating to the shaded-facing side, these pumps drive sideways auxin transport across the shoot, producing the gradient that makes the shoot curve toward the light.

Start by unpacking the vocabulary. A tropism is a plant growth pattern triggered by a stimulus from the environment. Gravitropism (also called geotropism) is growth in response to gravity.

Roots show positive gravitropism, meaning they grow in the same direction as gravity pulls—down into the soil.

Using Direct Biological Reasoning, ask: What survival benefit comes from growing downward?

A major job of roots is anchorage. Growing down into the ground allows roots to grip the soil and hold the plant steady against forces like wind, rain, or flowing water. If roots didn’t reliably grow downward, the plant would be easier to tip over or pull out.

Roots also function as the plant’s main system for taking in water and mineral ions. These materials are located in the soil below the surface.

When roots grow downward, they increase their reach into moist areas and nutrient-containing layers, improving the plant’s ability to gather the inputs needed for growth and for making sugars in photosynthesis.

Now check why the other statements don’t describe the real advantage:

• Option A: Growing into soil doesn’t automatically reduce disease risk; in fact, deeper soil can contain many pathogens, so this isn’t the key adaptive reason.
• Option B: Gravitropism sets the direction a root grows, not a built-in maximum length.
• Option D: Photosynthesis needs light, and underground conditions lack light; roots also usually don’t have enough chloroplasts for photosynthesis.

Putting the two main root roles together—keeping the plant secure and collecting water/minerals—shows why positive gravitropism is so valuable. Downward root growth supports both stability and resource uptake, which are essential for the plant to survive.

First define the key idea. Tropisms are directional growth responses to stimuli. Positive gravitropism means a plant part (such as a root) grows in the same direction as gravity acts—down into the ground.

This links to Topic: Processes (Tropisms) in MYP Biology.

Option D doesn’t fit. Photosynthesis needs light energy to produce chemical energy, but soil blocks light. Also, most root cells don’t contain the chloroplasts needed to carry out photosynthesis effectively.

Option A is not a good biological explanation. If the goal were to avoid soil-borne disease, growing further into the soil—where many microbes and pathogens exist—would not be a reliable avoidance strategy.

Option B is incorrect because tropisms guide which way growth happens. How long a root becomes depends on things like genetics, rates of cell division, and availability of water and nutrients—not on gravitropism itself.

After eliminating the others, Option C remains. This is the best adaptive outcome: downward growth helps roots reach water and dissolved minerals in the soil and also anchors the plant so it can stay upright and stable.

Let’s break down how plants adjust their growth to improve survival. In Biology, under Processes: Tropisms, tropisms are directional growth responses where a plant grows either toward or away from an outside stimulus.

Typical examples are:

• Phototropism: growth in the direction of light.
• Hydrotropism: root growth toward water.
• Geotropism: growth guided by gravity.

Plants don’t grow this way by accident. When shoots grow toward light (phototropism), the leaves can absorb more light energy for photosynthesis. When roots grow toward moisture (hydrotropism), the plant is more likely to take up enough water for its life processes.

So, these growth directions help the plant place its organs where they can collect the most useful materials from the environment.

In science and technology, improving something so it works as well as it can is called optimization.

Tropisms function like an optimization strategy in living organisms. Rather than growing in random directions, the plant adjusts its growth to reduce waste and increase success—reaching better light and water while using energy wisely.

Now compare the other options to see why they don’t describe the overall role of tropisms:

• Reproduction: means making new plants (seeds/spores). Tropisms can support health, but they aren’t the process of producing offspring.
• Respiration: is the cellular chemical reaction that releases energy from glucose. Tropisms need energy, but they are not the mechanism of respiration.
• Transpiration: is water evaporating from leaves. Even though water-related tropisms help find water, the growth response itself isn’t transpiration.

Because tropisms help plants adjust growth to gather resources in the most effective way, the correct term is optimization.

In IB MYP Biology, tropisms are taught as growth responses where plants grow toward or away from a stimulus such as light, gravity, or water. The question is asking for the broad reason this helps survival.

Reproduction is about creating offspring to continue the species. Even if growth toward light helps a plant become strong enough to reproduce later, tropisms are not the actual process of making seeds or pollen, so this option doesn’t fit.

Respiration happens inside cells to release energy from glucose. Tropisms are growth-direction changes at the whole-plant level, not the chemical pathway for energy release or gas exchange, so respiration is not the best answer.

Transpiration is the evaporation-driven loss of water vapor from leaves. Although roots may grow toward water due to hydrotropism, transpiration is a different process and doesn’t describe the overall strategy of directing growth to obtain resources.

After removing the other options, the remaining choice is optimization. Tropisms help a plant fine-tune its growth—for example, bending toward light to capture more energy—so it can obtain resources as efficiently as possible.

The prompt is asking for the name of the root cells that help a plant detect gravity using packed starch grains. This links to the plant growth response called gravitropism (also called geotropism), which is part of the Processes: Tropisms content area.

Consider the first two choices:

• Epidermal cells make up the outside covering of plant organs and act as protection.
• Guard cells occur in leaf epidermis and open/close stomata to control gas exchange and water loss.

These are not the gravity-detecting cells in the root cap, so they don’t match the description.

Phloem cells belong to vascular tissue and move sugars/organic nutrients around the plant (translocation). Even though phloem can be present in roots, its job is distribution of food, not detecting gravity.

The correct name is statocytes. These are specialized cells in the root cap. Inside them are dense, starch-filled organelles called amyloplasts (also called statoliths). Because the starch is heavy, it settles to the lower side of the cell when the root’s orientation changes, which helps signal a change in auxin distribution and guides growth.

After excluding the other choices, the only option that fits the gravity-detecting, starch-grain-containing root cells is statocytes.

We need a specialized cell type in the root that uses dense starch grains to detect gravity. That describes the mechanism behind gravitropism (geotropism). We can reach the answer by crossing off options that don’t fit.

Epidermal cells are the outer layer of the root, important for protection and for taking in water and minerals (especially with root hairs). They are not the specialized gravity-sensing cells with dense starch grains.

Guard cells are paired cells mostly found in leaves and young stems, forming stomata to regulate gas exchange and transpiration. They aren’t root-cap sensors for gravity, so this option doesn’t match.

Phloem is vascular tissue responsible for translocation—moving sugars like sucrose from sources (often leaves) to other parts of the plant. Since this is a transport role rather than a gravity-detection role, it can be eliminated.

With the other options removed, Statocytes remain. These root-cap cells contain starch-filled amyloplasts (statoliths) that sink to the lower side due to gravity, helping the root respond by growing downward.

Before choosing an option, remember what gravitropism (also known as geotropism) refers to. It is a growth response where a plant organ grows in a particular direction in relation to gravity.

Biologists describe gravitropism using two directions:

• Positive gravitropism: growth with gravity (down into the ground).
• Negative gravitropism: growth against gravity (upward, away from the ground).

Root parts are adapted to hold the plant in place and absorb water and minerals, so they generally grow downward in the direction gravity pulls. That means root structures show positive gravitropism, not negative. So the root-related choices can be ruled out.

Shoots need to lift leaves and flowers into the air to access light and help reproduction. Because stems grow upward, away from gravity’s pull, they show negative gravitropism.

Start by unpacking the key idea. Gravitropism (geotropism) is how plant growth responds to gravity.

• Positive gravitropism = growing toward gravity (downward).
• Negative gravitropism = growing away from gravity (upward).

The question is asking for the structure that typically grows upward.

Now consider the options linked to roots. Roots are built for anchoring and for taking in water and minerals from the soil, so their overall growth direction follows gravity.

• A primary root tends to grow straight down.
• Lateral roots spread out from the main root but still remain within the soil and often extend downward or sideways.
• The root cap protects the growing tip as the root pushes through soil.

Because these are associated with downward growth (positive gravitropism), they can be eliminated.

After excluding root structures, the remaining option is stems.

Stems are part of the shoot system and typically grow upward to reach light. Since that growth is away from gravity, stems show negative gravitropism.