Tissues, organs and systems

To solve this, we need to remember what secondary growth is in woody plants. Secondary growth increases the diameter (girth) of stems and roots. It happens because lateral meristems keep dividing and add new tissues that later become the main parts of wood and bark.

Now consider the choices linked to the plant’s outside covering. The cork cambium makes cork cells that build the periderm (outer bark). It contributes to the protective outer layers, not to the internal material we call wood.

The vascular cambium is a narrow ring of dividing cells. It produces new xylem and phloem, so it helps create wood, but it is only a very thin layer and does not make up most of the trunk’s mass.

The secondary phloem is made by the vascular cambium on the outside side. Its job is moving sugars around the plant. In woody stems it becomes part of the inner bark and often gets compressed or shed as the plant thickens, so it is not what mainly forms wood.

From plant anatomy, wood is secondary xylem.

The vascular cambium adds secondary xylem toward the inside of the stem. These cells develop thick walls strengthened with lignin, and they pile up year after year, creating the strong bulk of the trunk.

We need the tissue that makes up most of wood. In biology, wood is the built-up inner material formed during secondary growth. We can narrow it down by ruling out tissues that don’t match that description.

Eliminate Vascular cambium (Option C) and Cork cambium (Option D).

Both are meristems, meaning they are thin layers of cells that divide to make other tissues. They are important for growth, but they are not the thick, accumulated material that forms the wooden part of the stem.

Next remove Secondary phloem (Option B).

During secondary growth, phloem is produced on the outside of the vascular cambium and becomes part of the bark. As the stem expands, this tissue is commonly crushed or peeled away, so it doesn’t build up into the hard inner mass we identify as wood.

That leaves Secondary xylem.

In woody plants, new secondary xylem is laid down on the inside of the vascular cambium. Its cells have thick walls containing lignin, giving strength. Over many seasons, these xylem layers accumulate to form wood, which supports the plant and also helps move water.

To answer this, start by remembering what biologists mean by a tissue. In levels of organization, a tissue is a collection of similar cells that cooperate to do one particular job.

This idea is a core part of the Organisms content in MYP Biology. If you understand how cells group into tissues, it becomes much easier to understand how organs and organ systems function together in the body.

Consider the first choice: it says a tissue forms from different types of cells joining together. But a tissue is made from similar cells. When several different tissues work together, that higher level is an organ (for example, the heart), not a single tissue.

The next statement suggests cells behave independently. If that were true, the tissue couldn’t carry out coordinated tasks such as a muscle shortening together or a gland releasing a substance at the right time. Tissues depend on working together, not acting separately.

This choice says cells give up their specialized roles when they form a tissue. In reality, it’s the reverse: cells become specialized through differentiation, which helps them perform their role in that tissue even better.

Using the definition and eliminating the incorrect ideas, the best description is Option D. It states that a tissue contains many cells of the same type and highlights that they coordinate what they do to create an outcome one cell alone could not achieve (an emergent effect).

To work this out, recall the definition of a tissue from the Organisms topic. A tissue is a group of similar cells cooperating to perform a specific function.

An important linked idea is emergent properties: when many parts interact, the group can do something that each part alone cannot.

Option A suggests that different kinds of cells combine to make one tissue.

However, in biology, multiple different tissues working together form an organ (such as a heart or a leaf). A single tissue is made of the same, or very similar, cell types—so this option is not correct.

Option B says the cells act independently.

If tissue cells truly worked independently, they wouldn’t be able to produce coordinated functions like contracting as one muscle or passing signals along a nerve pathway. The key feature of a tissue is cooperation, not isolated action, and independence would not explain the high performance of multicellular organisms.

Option C asks us to accept that cells lose specialization when they form a tissue.

Actually, cells become more specialized through differentiation so they can do their job effectively within the tissue. For instance, muscle cells are specialized for contraction. They don’t discard specialization; they rely on it to contribute to the tissue’s function.

Once the incorrect statements are removed, the remaining choice is the most accurate.

A tissue is made of many cells of the same type (or extremely similar types) that coordinate their actions. This teamwork allows a complex function that a single cell could not accomplish by itself, which is exactly how tissues support multicellular life.

In biology, emergent properties describe how a system can do more than its individual parts can do alone.

In other words, complex abilities appear because simpler parts interact. A single cell can do basic life processes, but it cannot carry out the full job of a tissue (such as pumping blood or transmitting a coordinated signal) by itself. This is a key idea in Topic: Organisms.

Next, recall the IB definition of a tissue. A tissue is a group of similar cells (the same cell type) that are specialized in a similar way to complete a particular function.

With that in mind, we can judge each option.

Now eliminate options that conflict with the definitions:

• Option A is wrong because tissues are made of similar cells; combining different tissues creates an organ, not one tissue.
• Option B is wrong because independent action wouldn’t create a shared emergent function—interaction and coordination are required.
• Option C is wrong because cells typically gain specialization through differentiation; they do not lose it when forming tissues.

The correct description is that many cells of the same type work together. By coordinating their actions, they produce a complex outcome that one cell alone cannot.

For example, a single cardiac muscle cell can contract, but only a huge number contracting in a coordinated pattern can pump blood effectively—an excellent example of an emergent property.

We need a plant tissue that is specially designed to do two jobs at once:

1. Transporting water
2. Providing structural support

In MYP Biology, these paired functions point us toward tissues in the plant’s vascular system that are built for moving water and adding strength at the same time.

Mesophyll tissue is found inside leaves (palisade and spongy layers). Its main function is photosynthesis and helping gases move around in the leaf. Even though water is present in leaf cells, mesophyll is not designed for long-distance water flow or for acting as the plant’s main support tissue.

Phloem is part of the vascular system, but it mainly carries dissolved food (such as sucrose and amino acids) around the plant. The cells involved are living, and phloem is not the key tissue that gives the plant its strongest internal framework.

The epidermis forms the plant’s outer covering. Its job is mostly protection and reducing water loss (often using a waxy cuticle). It doesn’t function as the main internal pipeline for water, and it isn’t the primary tissue that supports the plant from the inside.

Remembering the role of xylem, it is the tissue that fits both requirements:

• Water Transport: It is made of long, hollow tubes (vessels and tracheids) formed from dead cells, allowing water to travel efficiently from roots up to leaves.
• Structural Support: Xylem walls are reinforced with lignin, a hard, woody material that strengthens the tubes and helps the plant stay upright.

To answer this, we’re looking for a plant tissue that can do two things:

1. Transporting water
2. Providing structural support

We can compare each option to these requirements and cross off the ones that don’t fit.

Mesophyll is mainly a leaf tissue full of chloroplasts, so it is adapted for photosynthesis. It is not built for moving large amounts of water through the whole plant and it doesn’t act as the main strengthening tissue.

Phloem does transport materials, but it transports organic solutes during translocation (for example sugars like sucrose and amino acids). Because its transport cells must stay alive and are not heavily lignified, it does not provide major mechanical support compared with true support/strength tissues.

The epidermis is the plant’s outer layer, acting as a barrier against damage and infection, and often limiting water loss with a cuticle. It is not responsible for carrying water internally from roots to leaves.

The only option left is Xylem, and it matches both required functions:

• It contains hollow, dead tubes (vessels and tracheids) that efficiently transport water and minerals upward.
• Its walls are strengthened with lignin, giving strong structural support so plants can stand tall.

So xylem is the best answer.

To solve this, start with the meaning of analogous structures in biology. Analogous features in different organisms:

1. Do the same (or very similar) job.
2. Did not come from the same ancestral structure (they arose separately).

This idea links to Topic 5: Evolution (unity and diversity). It often happens due to convergent evolution, where unrelated groups face similar challenges and end up with similar solutions.

Because analogous structures are defined as having similar function without a shared origin for that trait, any option claiming the tissues come from the same ancestral source does not match.

Plants and animals are in different kingdoms and split extremely long ago, so their surface-covering tissues did not evolve from one shared “covering tissue” in a common ancestor.

Therefore, remove options that claim a shared starting point or developmental origin.

Now check the cell features being described. Plant epidermal cells are plant cells, so they typically have a cellulose cell wall and a large central vacuole. Animal epithelial cells, however, are animal cells and do not have cell walls.

So an option saying both tissues have cellulose walls cannot be correct, even before thinking about analogy.

Analogous structures are grouped together mainly because they do a similar function.

Plant epidermal tissue forms the outside layer of many plant organs (like leaves and stems). Animal epithelial tissue forms coverings and linings (like skin and the digestive tract lining). In both cases, the key job is to form a barrier: covering surfaces and protecting the organism.

Since these tissues carry out the same protective, surface-covering role but come from very different evolutionary histories and structures, they fit the definition of being analogous.

Use elimination by first remembering what each term means:

• Analogous structures: Similar function, but different evolutionary origin (for example, insect wings vs bird wings).
• Homologous structures: Similar origin/embryonic development, even if the function is different (for example, the limb bones in humans and whales).

Because the question already says the tissues are analogous, the correct reason should be about what they do, not where they came from.

Any answer claiming the tissues share an evolutionary origin or come from the same embryonic germ layer would describe homology, not analogy.

Also, plants and animals evolved multicellularity and tissues separately. Plants don’t use animal-style embryonic germ layers (like ectoderm/endoderm), so that claim doesn’t fit.

Check the statement about cell structures. Cellulose cell walls and large vacuoles are plant-cell features, but animal epithelial cells lack a cell wall. Since the statement is wrong for animals, it cannot explain the classification.

The remaining choice describes the shared function: both tissues form a covering/lining that protects and separates the organism from its environment. That functional match is exactly why they are considered analogous.

We need to match a tissue type to three functions: giving the body support, joining/holding structures together, and storing substances like fat. These roles come from the Organisms: Tissues content in the course expectations.

Muscular tissue is designed to contract so the body (or organs) can move. Even though it is vital, it isn’t mainly responsible for fastening structures together or storing fat.

Epithelial tissue makes coverings and linings, such as the outer layer of skin or the lining of the digestive tract. Its major functions include protection, secretion, and absorption—not the support/binding or fat-storage roles described.

Nervous tissue focuses on communication: sensing changes and sending electrical impulses to coordinate responses. It doesn’t act as the body’s structural support or “glue,” and it doesn’t store fat.

Connective tissue, by definition, includes tissues like bone and cartilage (support), tendons and ligaments (binding/connecting), and adipose tissue (fat storage). That combination matches every function listed in the question.

The prompt is pointing to a tissue type that provides support, connects body parts together, and may store fat. That set of functions is part of the Organisms unit under Tissues in the IB Biology syllabus.

Muscular tissue’s main feature is contraction, which creates movement (like moving your limbs or pushing food through the gut, or pumping blood in the heart). Because it is not mainly a fat-storage tissue or a structural connector, it can be removed.

Epithelial tissue forms surfaces—covering the body and lining organs. It is best known for protection, absorption, and secretion, not for acting as a supportive framework or an energy store.

Nervous tissue specializes in receiving information and transmitting nerve impulses to control body activities. It doesn’t connect tissues together mechanically and it isn’t where fat is stored.

After eliminating the other choices, Connective tissue remains, and it fits the description well because it includes:

• Support & Binding: bone, cartilage, tendons (and ligaments).
• Storage: adipose (fat) tissue, which stores energy and helps with insulation.

So this option is the correct one.

To solve this, start by recalling the role of mesophyll tissue. In MYP Biology, when studying photosynthesis in plant metabolism, we learn that mesophyll cells (including palisade and spongy mesophyll) are the main cells where photosynthesis happens in a leaf.

Photosynthesis needs light energy to be absorbed. Mesophyll cells can do this well because they contain many chloroplasts, which hold chlorophyll (the pigment that traps light). So, mesophyll tissue must have lots of chloroplasts to carry out its key function.

Body structures are usually positioned where they work most effectively.

• Leaves are shaped and positioned to receive sunlight.
• Roots are usually underground, where light is extremely limited.

Because mesophyll depends on light to photosynthesize, it makes sense that it is in leaves rather than roots.

Now remove the options that don’t correctly explain mesophyll location:

• Option A: Moving water is mainly done by xylem, not mesophyll. So water transport doesn’t explain why mesophyll is in leaves.
• Option B: Plants are covered by epidermal tissue, not animal epithelial tissue. That makes this statement incorrect.
• Option C: Mesophyll does help with gas exchange (especially spongy mesophyll), but it is not its only role. Also, gas exchange alone wouldn’t restrict it to leaves, since roots also exchange gases with the soil.

The best explanation is that mesophyll is concentrated in leaves because it is specialized for capturing light to power photosynthesis. That matches the final option.

Begin by remembering what mesophyll tissue refers to. In the MYP Biology topics on plant tissues and photosynthesis, mesophyll is the inner ground tissue of a leaf and its main purpose is photosynthesis, which depends on sunlight. Leaves are exposed to light, while roots are commonly in darkness underground, so location is part of the adaptation.

Option A implies roots don’t need water transport, which is wrong because roots absorb water for the whole plant. Also, the tissue specialized for water transport is xylem, not mesophyll. So this option fails both in plant function and in tissue identification.

Option B describes roots as having “animal-like epithelial tissue.” Plants and animals have different tissue types: plants have an epidermis/epidermal tissue, while epithelial tissue is an animal tissue category. That mismatch makes the option incorrect.

Option C says mesophyll is for gas exchange “only.” Spongy mesophyll does create air spaces that help gas exchange, but the most important role of mesophyll is photosynthesis. If gas exchange were the sole reason, mesophyll wouldn’t need to be mainly in sunlit leaves.

After eliminating A, B, and C, Option D remains. This fits plant anatomy: mesophyll—especially palisade mesophyll—has many chloroplasts. Since chloroplasts absorb light for photosynthesis, mesophyll is concentrated in leaves where light is available, not in roots where light is absent.

Biologists describe living things using levels that go from simpler to more complex. To answer, we need the level that comes after a cell but before an organ. That middle level is discussed in the Organisms topic under Tissues.

In biology, a tissue is made of many cells that are alike and come from the same starting source, and they cooperate to do a particular function.

For cells to count as a tissue, two key ideas must be true:

1. The cells are similar in their structure and origin.
2. The cells are specialized so they can carry out a shared task.

We can remove answers that don’t match the tissue level in the organization hierarchy:

• Option A is describing an organ system (for example, the circulatory or digestive system), which is more complex than a tissue.
• Option B refers to just one cell, like in a unicellular organism, not a group of cells.
• Option C doesn’t fit because tissues are arranged in an organized way; they are not an unplanned mix of cells.

Option D matches the definition exactly: it describes cells that are similar, have special roles, and work together to achieve one particular function.

To pick the correct meaning of tissue, we can eliminate choices by checking them against the standard order of biological organization:

$\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Systems}\to \text{Organisms}$

This fits within the Organisms topic, where you learn how these levels connect.

Option A talks about multiple organs working together to carry out a major body function. That is the definition of an organ system, which is well above the tissue level.

Option B describes a single cell doing all life processes. Even though some single-celled organisms can do this, a tissue requires many cells working together, so this cannot be correct.

Option C suggests cells are just mixed together randomly. Real tissues are arranged and coordinated; their cells usually have similar structures and share an embryonic origin so they can function effectively together. A random assortment of cells is not a tissue.

After eliminating A, B, and C, Option D is left. It matches the definition: tissues are made of similar cells (such as nerve cells or muscle cells) that are specialized and cooperate to perform one shared function.

We need to name the biological level based on what it is made from. The clue tells us it contains several tissue types that join forces to carry out one function, so we’re looking for the level formed by multiple tissues working as a unit.

Multicellular organisms are arranged in levels from simple to complex (this is part of the Organisms content). The usual order is:

1. Cells: the smallest living units.
2. Tissues: groups of similar cells doing a shared job.
3. Organs: structures made from different tissues working together.
4. Organ Systems: groups of organs that cooperate.
5. Organism: the entire living individual.

Now compare each option with the description of “different tissues working together”:

• A single cell that has a special role: that’s only one cell, not multiple tissues.
• A cluster of separate organisms living together: this describes a group living near each other (often microbes), not a functional part inside one organism.
• A set of different organs that work as a team: that’s the next level up, where several organs coordinate (like the respiratory system).

From the hierarchy, an organ is defined as a structure made of multiple tissue types (such as muscle, epithelial, connective, and nervous tissues) that combine to perform a particular function—so the correct choice is the one describing a single structure built from several tissues.

We can answer by using the standard biological hierarchy in multicellular organisms:

$\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Organ Systems}\to \text{Organisms}$

The prompt is asking for the level where different types of tissues combine to do one specific job.

A specialized cell is just one unit at the lowest level here. Since tissues are made from many cells, a single cell cannot be the level made from several tissues.

A colony usually means many similar cells or separate organisms living together (such as bacteria growing on agar). It isn’t one of the standard internal organization levels within a complex multicellular organism.

An organ system is above the level we need: it is made of several organs working together (for example, the digestive system includes multiple organs). The question is asking for the level made from tissues, not from organs.

Following the hierarchy and eliminating the other choices, the correct level is an organ. An organ sits one step above tissues and contains multiple tissue types (like connective, epithelial, and muscle tissues) that coordinate to carry out a particular function.

To explain tissue organization, start with how each organism survives. Most animals are motile (able to move), so they need tissues that allow movement and fast communication, such as muscle tissue and nervous tissue.

Plants are sessile (fixed in place). Since they can’t walk away from threats or travel to find resources, their tissues are adapted for staying upright, growing toward light, and moving water and sugars through the body over long distances (for example, xylem and phloem). This difference in lifestyle strongly shapes which tissue types are most important.

Plants and animals are both multicellular organisms with division of labor. That means both rely on cells that are adapted for particular functions. Plants include specialized cells like guard cells that control gas exchange, while animals include specialized cells like red blood cells that carry oxygen. So it’s not true that only animals have specialized cells.

This statement flips the real pattern. Animals commonly have well-developed muscle and nerve tissues that support movement and quick responses. Although plants can show limited movements (like leaves turning toward light or a Venus flytrap snapping shut), their tissues are mainly organized for photosynthesis, support, and transport rather than moving the whole organism from place to place.

Living things are organized in levels: $\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Systems}$

Plants follow this pattern too. For instance, plant tissues (like epidermal tissue) make up organs (like leaves), and organs belong to larger systems (like the shoot system). So plants do not “skip” hierarchical organization.

The best explanation is that plants are stationary and must make food through photosynthesis while remaining supported without an internal skeleton. This leads to distinctive tissues for strong support and for efficient transport of water, minerals, and sugars—different priorities than animals, which emphasize tissues for movement and rapid coordination.

We’re comparing how plant and animal cells are grouped into tissues. Using the Process of Elimination, we can check each statement against what we know about tissues, organs, and systems from the Organisms unit.

Option A says only animals have specialized cells. But plants clearly have many specialized cell types, such as root hair cells for absorbing water and minerals and guard cells for regulating gas exchange. Since plants do have specialized cells, this option is incorrect.

Option D conflicts with the biological hierarchy: $\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Systems}\to \text{Organism}$ Plants fit this model: they have organs like roots, stems, and leaves and systems such as the vascular and shoot/root systems. So plants absolutely do have hierarchical structure.

Option C suggests plants are designed for organism-wide movement while animals are not. In reality, animals have muscle and nervous tissues that enable locomotion. Plants may bend or grow toward stimuli (tropisms), but they don’t travel from place to place. So this option is backwards.

With the incorrect choices removed, Option B remains. It matches the key idea: because plants are fixed in place, their tissues emphasize support and transport (moving water and sugars throughout the plant), while animals prioritize tissues that support movement and fast responses.

To answer this, focus on the way the statement is written. It is comparing two situations:

1. One muscle cell working by itself.
2. A very large number of muscle cells working together.

The word "but" shows that the second situation should contrast with the first one.

The word "but" tells us the second part should not match the first part—it should point to an opposite or much bigger outcome.

The first part says: $\text{One cell}\to \text{contracts weakly}$

The second part mentions "millions" of cells, so the combined effect should be much greater than weak.

Now rule out answers that don’t make sense with the contrast:

• Option A doesn’t work because it repeats the idea of weakness, which clashes with the contrast created by "but."
• Option B doesn’t fit because muscle cells respond to nerve messages; they aren’t mainly described as producing signals to cause movement.
• Option C doesn’t fit because glucose is used up to supply energy during contraction, not produced as the outcome.

If one muscle cell makes only a small contraction, then many cells contracting together add their forces. That teamwork creates a much stronger effect, allowing noticeable movement of a limb.

So the blank should be filled with "strong movement."

The statement contrasts a single muscle cell’s weak contraction with the combined action of "millions of cells." The missing phrase should describe what their combined work produces to move a body part.

This links to Topic: Organisms (Tissues and Systems) in MYP Biology, where we learn that many cells working together create a larger function.

If one cell is described as "weak," it wouldn’t make sense for millions of cells acting together to still give a weak outcome. The word "but" signals we should expect something stronger than weak.

Muscles need sugar (glucose) as fuel and they receive signals from the nervous system, but neither of these is the movement/force produced by contracting muscle tissue.

• Sugar is an energy source, not the movement itself.
• Signals are instructions, not the force that moves bones.

After eliminating the other choices, the only option left describes what happens when many tiny contractions add together: the overall force increases enough to create large, noticeable movement.

To decide, we think about how living things are organized. Biology often describes a sequence of organization:

$\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Systems}\to \text{Organism}$

This question is focused on what is true at the organ level, which is discussed in your syllabus under Organisms.

An organ is defined as a body part made from several tissue types that cooperate to carry out a complicated job. An organ needs at least two tissue types, but in many animal organs you typically see all four major tissue types working together:

1. Epithelial tissue: forms coverings and linings (for example, the stomach’s inner lining).
2. Connective tissue: supports, holds parts together, and forms structures (such as supportive layers and blood vessels).
3. Muscle tissue: produces movement (like squeezing, pumping, or mixing).
4. Nervous tissue: carries messages that help control and coordinate what the organ does.

The statement says the tissues appear in different proportions, which fits real organs. The tissue that takes up the most space depends on the organ’s main function.

For instance, the heart contains mostly muscle tissue because it must contract strongly to move blood, but it also includes epithelial tissue (smooth lining), connective tissue (support and valves), and nervous tissue (signals that time the beats).

Because complex organs are built from multiple tissues that work together, the option that contradicts this general pattern can be removed.

This is a key idea in animal biology: most well-developed organs rely on the combined roles of all four tissue groups to work properly, even though one type may dominate. Therefore, the statement is correct.

First, recall the four major tissue types from Topic: Tissues:

1. Epithelial tissue: covers body surfaces and lines internal spaces.
2. Muscle tissue: contracts to create movement.
3. Connective tissue: supports, links, and protects (including tissues like bone and blood).
4. Nervous tissue: sends electrical signals for control and communication.

Using Specific Example Analysis, we can test the claim by looking at a complex organ such as the heart and asking whether it contains all four tissue types:

• Muscle tissue: most of the heart is cardiac muscle (myocardium) responsible for pumping blood.
• Epithelial tissue: the heart has smooth linings—endocardium on the inside and protective coverings like the pericardium on the outside.

Continuing with the heart:

• Connective tissue: tough connective tissue provides support and forms structures such as valves; blood, which is a fluid connective tissue, is also found moving through the heart.
• Nervous tissue: specialized conducting tissue (including the SA node) generates and spreads electrical impulses that coordinate contractions.

So, although the proportions are not equal (muscle makes up far more than nervous tissue), all four tissue types appear.

This heart example shows that one organ can include epithelial, connective, muscle, and nervous tissues working together. Many other complex organs (like lungs or the stomach) show the same idea: multiple tissues cooperate, but in different ratios depending on the job.

Because the example supports the claim that organs usually contain all four tissue types, the option saying the statement is not true should be eliminated.

The statement correctly describes how most animal organs are built and function, so the correct choice is True.

Multicellular organisms are arranged in an ordered set of levels. This idea appears in the Organisms part of the course. The sequence goes from the smallest living units to the complete individual:

$\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Organ Systems}\to \text{Organism}$

When you move upward in the hierarchy, you are combining many parts from the level below into a larger structure.

Because these parts work together, the higher level shows emergent properties—abilities that the smaller pieces alone cannot achieve. For instance, one heart muscle cell can shorten, but only the heart as an organ can push blood around the body. This shows that each step up the hierarchy involves more coordination and therefore greater complexity.

Option B says the levels operate independently. That does not match biology: organs and organ systems depend on the cells and tissues that form them. If the lower levels stop working properly, the higher levels cannot carry out their functions.

Option C suggests organization becomes simpler from cells to organ systems. Actually, going from one cell to a whole system (such as the digestive system) requires many specialized structures and careful coordination among them, so complexity increases rather than decreases.

Option D treats specialized cells as the most complex level. Specialized cells are important, but they are the lowest level in this hierarchy. Higher levels (organs and organ systems) include huge numbers of cells working together, making them more complex overall.

Since each level is built by combining and coordinating the level below, the amount of organization and complexity rises at every step. Therefore, Option A is the correct description.

In biology (in the Organisms topic), multicellular organisms are described using a structural ladder. It begins with the simplest living units and builds up to the whole individual:

$\text{Cells}\to \text{Tissues}\to \text{Organs}\to \text{Organ Systems}\to \text{Organism}$

In a hierarchy, each level depends on the one beneath it. A tissue needs functioning cells, an organ needs working tissues, and an organ system needs properly functioning organs. So the levels are interdependent, not independent.

As you move upward from cell to organ system, groups of parts interact to create emergent properties—new abilities the smaller parts cannot do alone. More parts plus more coordination means complexity increases toward the organ system level.

Specialized cells are the basic functional unit in this hierarchy. An organ system includes many organs and enormous numbers of cells working together, so it is far more complex than a single specialized cell.

After removing the incorrect choices, the remaining statement correctly describes biological organization: each higher level (from cell up to organism) shows greater complexity and organization than the level below it.