Unit 6 Levels of organisation in multicellular organisms
Key unit competence: Establish the relationship between specialisation of cells and the levels of organisation in multicellular organisms.
Key ideas
- In a multicellular organism, cells are specialised: each type has a structure suited to one job. Specialisation lets cells work efficiently, like a team in which everyone does what they do best.
- Levels of organisation, from simplest to most complex: organelle → cell → tissue → organ → organ system → organism. A tissue is a group of similar cells with the same function; an organ is made of several tissues working together; an organ system is several organs working together for a major function.
- Ciliated cells line the air passages; their cilia sweep mucus (made by goblet cells) with trapped dust and germs away from the lungs. They also line the oviducts.
- Nerve cells (neurones) are long and thin with branched endings, carrying nerve impulses quickly over long distances.
- Red blood cells are biconcave discs (large surface area for oxygen), have no nucleus (more room for haemoglobin, which carries oxygen) and are flexible (squeeze through capillaries).
- Sperm cells have a tail (flagellum) for swimming, many mitochondria in the middle piece for energy, an acrosome with enzymes to digest the egg membrane, and a nucleus with half the genetic material. Egg cells (ova) are large with stored food in the cytoplasm and a membrane that lets only one sperm enter.
- Root hair cells have a long, thin extension that gives a large surface area for absorbing water and mineral salts. Xylem vessels are dead, hollow, end-to-end tubes with thick walls (lignin) carrying water upwards from roots. Phloem is living tissue that carries food (sugars) made in leaves up and down the plant.
- Palisade mesophyll cells are long, packed with chloroplasts and placed near the upper leaf surface to trap light. Spongy mesophyll cells are loosely packed with air spaces for gas exchange.
- Examples: animal tissues: muscle, nerve, blood, epithelium; plant tissues: xylem, phloem, mesophyll. Organs: heart, stomach, lungs, brain, skin; leaf, stem, root, flower. Systems: digestive, circulatory, respiratory, nervous; the plant shoot and root transport system.
Common misconceptions
- "An organ is made of one kind of cell." No: an organ contains several tissues; the stomach has muscle, glandular and nerve tissues.
- "Red blood cells have a large nucleus to control oxygen carrying." No: mature human red blood cells have no nucleus, leaving more space for haemoglobin.
- "Xylem carries food." No: xylem carries water and mineral salts; phloem carries food.
- "Organ systems are the highest level of organisation." No: the whole organism is the highest level; it is made of several organ systems.
Worked examples
Example 1. Arrange in order from smallest to largest: organ, cell, organism, tissue, organelle, organ system. Give a human example of each.
Solution
Organelle (mitochondrion) → cell (muscle cell) → tissue (muscle tissue) → organ (heart) → organ system (circulatory system) → organism (human).
Example 2. Explain three ways in which a sperm cell is adapted to its function.
Solution
A flagellum lets it swim to the egg; many mitochondria release energy for swimming; the acrosome holds enzymes that digest a way into the egg; its nucleus carries half the genetic information.
Practice questions
Define: (a) specialised cell; (b) tissue; (c) organ; (d) organ system.
Show answer
(a) A cell whose structure is adapted to a particular function. (b) A group of similar cells performing the same function. (c) A structure made of several tissues working together. (d) Several organs working together to carry out a major function.
Common mistake: Defining a tissue as "a group of organs".
Describe three ways a red blood cell is adapted to carry oxygen.
Show answer
Biconcave disc: large surface area. No nucleus: more space for haemoglobin. Full of haemoglobin, which combines with oxygen. Flexible: can squeeze through narrow capillaries.
Common mistake: Saying it has many mitochondria to carry oxygen.
Explain how a root hair cell is adapted for absorption.
Show answer
It has a long, thin extension between soil particles, giving a very large surface area; its thin, permeable wall lets water in easily; its large vacuole holds water.
Common mistake: Saying root hairs have chloroplasts.
Compare xylem and phloem in three ways.
Show answer
Xylem carries water and minerals, phloem carries food; xylem flow is upward only, phloem is both ways; xylem cells are dead and hollow, phloem cells are living.
Common mistake: Mixing up which tissue carries food.
Arrange these in order from smallest to largest and give a plant example of each: tissue, organism, cell, organ.
Show answer
Cell (palisade cell) → tissue (mesophyll/xylem) → organ (leaf/stem) → organism (bean plant).
Common mistake: Putting tissue before cell.
Explain the role of cilia and goblet cells in protecting the lungs.
Show answer
Goblet cells make mucus that traps dust and microorganisms; cilia beat to sweep the mucus up to the throat, where it is swallowed or coughed out.
Common mistake: Saying cilia kill germs directly.
Give one example of each, in a human: a cell, a tissue, an organ and an organ system.
Show answer
Cell: nerve cell; tissue: nerve tissue; organ: brain; system: nervous system. (Other correct sets accepted.)
Common mistake: Mixing examples from different systems without a link.
Explain one advantage of cell specialisation to a multicellular organism.
Show answer
Each type of cell does one job very well (for example red blood cells carry oxygen efficiently), so the whole organism works more efficiently than if every cell had to do every job.
Common mistake: Saying specialised cells can do all jobs.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Nerve cells are long and thin so that they can: a. carry impulses over long distances 2. Which cells carry oxygen in the blood? b. Red blood cells 3. Several organs working together form: c. Acrosome 4. Which part of the sperm contains enzymes to penetrate the egg? d. an organ system Show answer
1 → a (carry impulses over long distances) · 2 → b (Red blood cells) · 3 → d (an organ system) · 4 → c (Acrosome)
- Long axons link distant parts of the body.
- They contain haemoglobin.
- For example, the digestive system.
- The acrosome is at the tip of the head.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
organ · has several tissues working together · tissue · mucus- The stomach is called an organ because it .
- Goblet cells produce .
- A group of similar cells performing the same function is a .
Show answer
1: has several tissues working together · 2: mucus · 3: tissue
- Muscle, gland and nerve tissues all work together.
- Mucus traps dust and microorganisms.
- For example, muscle tissue is made of muscle cells.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Red blood cells have a biconcave shape to: a. dead, hollow tubes joined end to end 2. Xylem cells are described as: b. food made in the leaves to other parts 3. Phloem carries: c. each cell type is adapted to a particular job 4. Specialisation of cells means that: d. give a large surface area for oxygen uptake Show answer
1 → d (give a large surface area for oxygen uptake) · 2 → a (dead, hollow tubes joined end to end) · 3 → b (food made in the leaves to other parts) · 4 → c (each cell type is adapted to a particular job)
- More surface means faster uptake of oxygen.
- Their contents and end walls have disappeared.
- Phloem transports sugars in both directions.
- Structure is linked to function.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
absorbing water and mineral salts · chlorophyll · haemoglobin · cell, tissue, organ, organ system, organism- Root hair cells are adapted for .
- The correct order from simplest to most complex is .
- The substance in red blood cells that carries oxygen is .
Show answer
1: absorbing water and mineral salts · 2: cell, tissue, organ, organ system, organism · 3: haemoglobin
- Their long extension gives a large surface area.
- Each level is made of the one before it.
- Haemoglobin combines with oxygen in the lungs.