Q.Structural organisation in animals attains different levels as cell - organ - organ system. What is missing in this chain? Mention the significance of such an organisation.
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From a Single Cell to a Complete Animal
Imagine you are building a house. You start with bricks. A single brick is just a brick. Stack bricks together in a pattern, and you get a wall. Combine walls, a roof, doors, and windows, and you have a room. Join several rooms with plumbing and wiring, and you have a functioning house. The house is far more than a pile of bricks — it has a structure that lets it do something.
Your body is built the same way. The "bricks" are cells. The "walls" are tissues. The "rooms" are organs. And the "house with all its systems" is the whole animal. This step-by-step increase in complexity is called the levels of organisation.
The Four Levels, One by One
1. Cellular Level — The Brick
The simplest living unit is a single cell. In some animals, the entire body is just one cell. An amoeba, for example, eats, moves, and reproduces using just its single cell. There is no division of labour inside the body — that one cell does everything.
At this level, the cell is the organism. There are no tissues, no organs — just a self-sufficient cell.
2. Tissue Level — The Wall
When many similar cells group together to perform a common function, they form a tissue. For instance, muscle cells join to make muscle tissue, which can contract. Nerve cells join to make nervous tissue, which carries signals.
A jellyfish or a hydra reaches this level. Its body has distinct tissues (like a nerve net and muscle fibres), but these tissues are not yet organised into specific organs. The animal can do more than a single cell — it can sense touch and move — but it still lacks specialised structures like a heart or a stomach.
3. Organ Level — The Room
Different tissues combine to form an organ. The stomach, for example, contains muscle tissue (to churn food), epithelial tissue (to line the inner surface), and nervous tissue (to sense when food is present). All these tissues work together so the stomach can digest food.
A flatworm (planaria) is a good example. It has a simple eye spot (an organ for light detection) and a branched gut (an organ for digestion). But it does not have a full set of organ systems — its organs work somewhat independently.
4. Organ-System Level — The House
Multiple organs join forces to form an organ system. The stomach, intestines, liver, and pancreas together make the digestive system. The heart, blood vessels, and blood make the circulatory system. Each system has a specific job, and all systems work together to keep the animal alive.
Most familiar animals — frogs, birds, dogs, humans — are at this level. This is the highest grade of organisation. It allows for extreme specialisation: the heart only pumps blood, the lungs only exchange gases, and the brain only processes information. No single cell or tissue could do all that alone.
The levels are hierarchical — each level contains the one below it. A tissue is made of cells. An organ is made of tissues. An organ system is made of organs. You cannot skip a level.
The Big Picture: Why This Matters …
The chain "cell - organ - organ system" is missing the tissue level, which comes between cells and organs. The complete sequence should read: cell - tissue - organ - organ system.
- Cells first group together and organise into tissues, and it is these tissues, combining together, that then form organs; organs in turn combine to form organ systems.
- The significance of this layered organisation is division of labour: each level takes on a specific part of the overall workload, so that the many cells of a complex animal's body function more efficiently and in a better-coordinated way than if each cell acted independently. …
The tissue level is missing between cell and organ; the full hierarchy is cell to tissue to organ to organ system, and its significance is that it allows division of labour for efficient, coordinated body function.
A multicellular animal's body is not simply a mass of cells acting alone — it is built up through a series of increasingly complex levels of organisation. The chain given, cell straight to organ to organ system, skips a crucial intermediate step: cells first organise into tissues, groups of similar cells (together with any surrounding intercellular material) that work as a coordinated unit to perform one or more functions. It is only after this tissue level is reached that tissues combine, in specific proportion and pattern, to build organs, and organs in turn cooperate to form organ systems. …
Method: Reconstruct the Hierarchy of Structural Organisation
When a chain of organisational levels is given and something is described as "missing," first write out the complete, correct hierarchy from memory, then compare it term-by-term against what's given in the question. For animal body organisation this hierarchy is: cell, tissue, organ, organ system. Laying both sequences side by side makes the gap immediately visible — here, jumping from "cell" straight to "organ" skips the intermediate grouping step. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.Assertion (A) : Molecular interactions results in emergent properties at a higher level of organization. Reason (R) : All living phenomena are due to underlying interactions. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation for (A) (B) (A) and (R) are true. But (R) is not correct explanation for (A) (C) (A) is true, but (R) is false. (D) (A) is false, but (R) is true.
›Reveal solutionSolution
Both the Assertion and Reason are true, and the Reason correctly explains the Assertion — emergent properties at higher levels of organisation arise precisely because of interactions among molecules.
Concept and Intuition
A single enzyme in a test tube can catalyse a reaction, but it is not 'alive'. Put many such molecules together inside a cell in an organised, interacting system, and properties like metabolism, growth and reproduction 'emerge' — properties that no individual molecule possesses on its own. This is the idea of an emergent property: a property of the whole that cannot be predicted from, or seen in, any single component in isolation. It appears only because of the interactions between the components.
Step-by-Step Solution
- Assertion (A): interactions among molecules give rise to emergent properties at a higher level of organisation — this is a textbook description of biological emergence (e.g., a cell shows properties an isolated molecule does not).
- Reason (R): all living phenomena are ultimately manifestations of underlying (molecular) interactions — this is the mechanistic basis of (A). …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Assertion (A) : In cellular level of organisation different types of cells are functionally isolated. Reason (R) : This is due to the absence of sensory cells and nerve cells. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation of (A) (B) (A) and (R) are true. (R) is not the correct explanation of (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
At the cellular grade of organisation (e.g. sponges), cells act relatively independently because there is no nervous system to coordinate them.
Concept and Intuition
Organisms with cellular level of organisation (such as sponges) show division of labour among cell types, but lack true tissues, and there is no nervous system (no sensory or nerve cells) to integrate the activity of different cells. Because of this absence of a coordinating nerve net, the cells function in a relatively independent, functionally isolated manner rather than as a tightly coordinated tissue.
Step-by-Step Solution
- Assertion: at the cellular level of organisation, cells act in a functionally isolated manner — true, since there is no tissue-level integration. …
- AP EAPCET 2023Set ap-2023-05-23-FN1 markMCQQ.In eucoelomates the mesoderm comes into contact with endoderm of alimentary canal to form gizzard and stomach. This is referred to as (A) Cephalization (B) Primary induction (C) Tube-in-a-tube plan (D) Solid body plan
›Reveal solutionSolution
The key idea is that the interaction between mesoderm and endoderm to form the gizzard and stomach is a classic example of primary induction — the process where one embryonic tissue influences the development of another. The correct answer is (B).
In embryology, the formation of complex organs often depends on inductive interactions between different germ layers. Here, the mesoderm signals to the underlying endoderm (the lining of the alimentary canal), causing it to differentiate into specialized structures like the gizzard and stomach. This is not merely a structural arrangement (like a tube-in-a-tube) but a dynamic, instructive event.
Let’s break down why the other options don’t fit and why (B) is correct.
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Understanding the process described
The question states that mesoderm “comes into contact with” endoderm to “form” gizzard and stomach. This contact is not just physical proximity; it triggers a change in the endoderm’s fate. In developmental biology, when one tissue (the inducer) influences the development of another tissue (the responder) through direct contact or signaling, this is called induction. Because this happens early in development, involving the primary germ layers (mesoderm and endoderm), it is specifically primary induction.
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Why not (A) Cephalization?
Cephalization refers to the evolutionary trend of concentrating sensory organs and a brain at the anterior (head) end of an organism. It has nothing to do with tissue interactions during organ formation. The gizzard and stomach are not head structures, so this is irrelevant.
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Why not (C) Tube-in-a-tube plan?
This describes the basic body architecture of many animals, where the digestive tract (inner tube) is surrounded by the body wall (outer tube), with mesoderm in between. While eucoelomates do have a tube-in-a-tube plan, the formation of specific organs like the gizzard and stomach is not explained by this static anatomical description. The question asks about the process of organ formation, not the overall body layout.
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Why not (D) Solid body plan? …
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- AP EAPCET 2021Set ap-2021-09-06-FN1 markMCQQ.Match the following and choose correct combination? Column I | Column IIi) Cellular grade level | a) Pheretimaii) Tissue grade level | b) Schistosomaiii) Organ level | c) Obeliaiv) Organ system level | d) Sycon (A) (i – b), (ii – c), (iii – d), (iv – a) (B) (i – d), (ii – c), (iii – a), (iv – b) (C) (i – d), (ii – b), (iii – c), (iv – a) (D) (i – d), (ii – c), (iii – b), (iv – a)
›Reveal solutionSolution
This tests the grades of body organisation across invertebrate phyla, illustrated by a representative genus each; the correct match is (D).
Concept and Intuition
Animal body plans show a progression of increasing structural complexity: cells acting semi-independently (cellular grade), cells organised into tissues (tissue grade), tissues combined into organs (organ grade), and organs cooperating as full organ systems (organ-system grade). Each grade is illustrated by a phylum that best represents it.
Step-by-Step Solution
- Cellular grade → Porifera, e.g. Sycon → (i – d).
- Tissue grade → Cnidaria, e.g. Obelia → (ii – c).
- Organ grade → Platyhelminthes, e.g. Schistosoma → (iii – b). …
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