MYP Integrated Sciences Tissues, Organs and Systems Notes

How Are Cells Organized in Multicellular Organisms?

1. Multicellular organisms don't function as loose collections of cells.
2. Instead, cells are arranged in a hierarchical structure, where each level builds on the one below:
   1. Cells form tissues,
   2. Tissues form organs,
   3. And organs form organ systems.

How Do Specialized Cells Form Tissues?

1. Tissues form when many cells of the same type organise themselves to cooperate.
2. This cooperation allows tissues to perform tasks that no single cell can manage alone.
3. For example, one muscle cell contracts weakly, but millions together create strong movement.The structure of the tissue always reflects its function.

What Are the Main Tissue Types in Animals?

1. All animal tissues fall into four major categories, each serving a distinct biological purpose.
2. The table below summarizes them clearly.

Note

All four tissue types appear in most organs, but in different proportions depending on the organ’s purpose.

What Are the Main Tissue Types in Plants?

1. Plant tissues are organized differently from animal tissues because plants do not move and have different survival needs.
2. Plant tissue types include:

Tip

• Mesophyll tissue contains many chloroplasts.
• This is why most photosynthesis happens in the leaves, not roots.

How Do Tissues Combine to Form Organs?

1. An organ isn't defined by size or shape, but by coordination between multiple tissues.
2. Each tissue contributes a distinct role that supports the organ’s overall job.
3. This structure–function link is important because if one tissue fails, the entire organ is affected.

Example

The heart pumps blood only because several tissues interact in a precise pattern.

What Tissues Make Up an Organ?

1. Most organs use a core set of tissue types arranged in predictable ways.

1. The table below shows how different tissues contribute to organ function.

How Does Tissue Arrangement Create Organ Function?

1. Tissues are arranged deliberately, not randomly.
2. For example, in the stomach:
   1. Muscle tissue churns food
   2. Epithelial tissue protects the lining
   3. Nervous tissue coordinates contractions
   4. Connective tissue holds everything together
3. This specific layout allows the stomach to digest food efficiently.
4. If the tissue arrangement changed, the organ would lose function.

Why Do Organs Have Specialised Shapes?

1. By now it should be clear that form follows function is a recurring theme.
2. Organs are no different, for example:
   1. The heart’s chambers optimise blood flow direction.
   2. The lungs’ branching structure maximises gas exchange surface area.
   3. The small intestine’s folds and villi increase absorption efficiency.

1. These shapes develop during growth and are supported by connective tissues that give strength and flexibility.

How Do Organs Maintain Their Structure Over Time?

1. Organs face constant forces: pressure, movement, chemical exposure, stretching.
2. They maintain structure using:
   1. Connective tissue (collagen for strength, elastin for flexibility)
   2. Continuous repair from stem cells
   3. Regulation by hormones and the nervous system
3. Some organs, like the skin and intestine, repair quickly.
4. Others, like the brain, have limited repair because of highly specialised cells.

Why Can Some Organs Be Transplanted and Others Cannot?

1. Some organs tolerate transplantation because they can survive brief periods without blood supply and resume function when reconnected.
2. The kidney, liver, and heart are all examples of this.
3. Other organs, such as the brain, depend on extremely fragile neural connections that cannot be repaired once cut.
4. Immune rejection also limits transplantability.

How Do Organs Work Together to Form Organ Systems?

1. Systems form when several organs link their functions to achieve a task that no single organ could complete.
2. Just as tissues combine to create an organ, organs combine to create a functional pathway, where each step depends on the previous one.
3. The digestive and respiratory systems are clear examples of how tissue-level structures scale up into system-level processes.

Why Must Organ Systems Be Structured in a Specific Order?

1. As we just covered, systems rely on ordered structures, not random organ placement.
2. For example, in the digestive system:
   1. The mouth breaks food down
   2. The stomach churns and dissolves food further.
   3. The small intestine absorbs nutrients.
   4. The large intestine reabsorbs water.

Note

• Each organ's internal tissue structure supports its role:
  • The stomach has thick muscle tissue to churn food.
  • The small intestine has villi to maximise absorption.

How Do Systems Coordinate Without Conscious Control?

1. The respiratory system depends on automatic coordination between organs and tissues.

1. Air passes from trachea → bronchi → lungs → alveoli, a structure built from tissues designed for gas exchange:
   1. Epithelial tissue forms thin alveolar walls.
   2. Capillary networks allow oxygen to move into blood.
   3. Muscle tissue in the diaphragm controls breathing rate.

1. None of this requires thinking.
2. The nervous system monitors carbon dioxide levels and adjusts breathing automatically.

How Do Different Systems Work Together?

1. No system works in isolation.
2. They're all interdependent, relying on input or support from others:
   1. The respiratory and circulatory systems depend on each other to supply oxygen.
   2. The digestive system depends on the circulatory system to transport nutrients.
   3. The muscular system depends on the nervous system to activate movement.
3. To do this, organs communicate using electrical signals (nerves) and chemical signals (hormones).