Lesson focus: Multicellular organismA living thing, such as a plant, animal or bacterium. organise specialised parts into interacting levels, so failure in one level can propagate through the whole system.
Explain the hierarchy organelle → cell → tissueA group of similar cells working together to do a job, such as muscle tissue. → organA body part made of different tissues working together to do a job, such as the heart or a leaf. → organ systemA group of organs working together, such as the digestive system. → organism using linked plant and animal examples.
Build on earlier learning
Specialised cellsThe previous lesson explained specialised cells. This lesson shows how specialised cells cooperate at larger levels.
You will be successful when you can…
- Order levels of organisation.
- Distinguish tissue, organ and system.
- Trace a function across levels.
- Explain interdependence between systems.
Words that carry the science
- tissue
- A group of specialised cells working together to perform a function.
- organ
- A structure made from different tissues working together.
- organ system
- A group of organs cooperating in a major body function.
- organism
- An individual living thing.
- interdependence
- Mutual reliance of parts or organisms on one another.
- exchange surface
- A surface adapted for movement of substances between regions.
Organisation forms a nested hierarchy
01Organisation forms a nested hierarchy
Organelles operate inside cells; specialised cells cooperate in tissueA group of similar cells working together to do a job, such as muscle tissue.; different tissueA group of similar cells working together to do a job, such as muscle tissue. combine into organA body part made of different tissues working together to do a job, such as the heart or a leaf.; organA body part made of different tissues working together to do a job, such as the heart or a leaf. cooperate in organ systemA group of organs working together, such as the digestive system.; interacting systems form an organismA living thing, such as a plant, animal or bacterium..
The levels are nested, not separate: heart muscle cells contain mitochondria, form cardiac muscle tissue, contribute to the heart, and support the circulatory system.
02An organ contains several tissues
The stomach contains muscle tissue for mixing, glandular tissue for secretion, epithelial tissue for protection and nervous tissue for coordination. Calling it “a muscle” ignores the tissues needed for its complete function.
A leaf is also an organ: palisade tissue absorbs light, spongy mesophyll supports gas exchange, epidermal tissues protect and regulate exchange, and vascular tissues transport substances.
03Systems exchange matter and depend on one another
The digestive system supplies soluble nutrients, the gas-exchange system supplies oxygen, and the circulatory system transports both to respiring cells. Excretory organs remove metabolic wastes and regulatory systems coordinate conditions.
Large multicellular organisms need transport systems because many cells are far from the external environment and their overall surface-area-to-volume ratio is relatively small.
Organ systems
Multicellular organisms are organised in levels. Similar cells form a tissue, several tissues form an organ, organs work together in an organ system, and organ systems make up the organism. Systems depend on each other, so a failure in one can affect the whole body.
The moving dot follows blood around the body.
Controls
Trace glucose from food to a muscle cell
Build a hierarchy-and-systems explanation for glucose reaching a contracting muscle.
- Digestive organs break large molecules into soluble glucose and absorb it in the small intestine.
- Blood tissue transports glucose through vessels driven by the heart.
- Capillaries bring blood close to muscle tissue and glucose moves into cells.
- Mitochondria use glucose and oxygen in aerobic respiration pathways that support contraction.
How to make the reasoning visible: The outcome depends on cooperation between digestive, circulatory and respiratory systems and on specialised structures at each level.
Trace failures through the hierarchy
Predict how a change at one level affects higher levels.
| Change | Immediate level | Next consequence | Whole-organism effect |
|---|---|---|---|
| damaged cilia in airway | cell/tissue | mucus clearance falls | infection risk rises |
| blocked coronary artery | organ supply | heart muscle gets less oxygen | pumping may fail |
| stomata remain closed | leaf tissue | carbon dioxide entry falls | photosynthesis and growth fall |
| villus surface damaged | organ tissue | nutrient absorption falls | less material reaches cells |
Why is blood classified as a tissue?
Why is a leaf an organ rather than a tissue?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a tissue?
What is an organ?
Why do large organisms need transport systems?
Explain why damage to alveoli can reduce muscle performance.
Why does increasing body size usually decrease surface-area-to-volume ratio?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The small intestine is an organ in the digestive system.
Explain why the small intestine is described as an organ.
(2)Explain how it works with other organ systems to supply respiring cells.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., move through the hierarchy and then trace the causal link between systems.