Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms (Louisiana Student Standards for Science, High School Biology, HS-LS1-2).

What this topic is asking

Louisiana's LS1 standards (HS-LS1-2) ask you to model how a multicellular organism is organized as a hierarchy of interacting systems. For LEAP 2025 Biology you should know the levels of organization (cell to tissue to organ to organ system to organism), the major organ systems and what each does, and, importantly, how systems interact to keep the organism alive. Because this is a modeling standard, the test often asks you to order the levels or explain how two systems cooperate.

The hierarchy of organization

Working up the hierarchy:

• A cell is the basic unit of life.
• A tissue is a group of similar cells working together (such as muscle tissue).
• An organ is made of different tissues working together for a function (such as the heart or stomach).
• An organ system is a group of organs working together for a major function (such as the circulatory system).
• The organism is all the organ systems working together.

This nested structure is the model the standard asks you to use, and the test commonly asks you to put the levels in order.

The major organ systems

Each organ system carries out a specific function:

• Circulatory system. Transports blood, carrying oxygen, nutrients, hormones, and wastes (heart and blood vessels).
• Respiratory system. Brings in oxygen and removes carbon dioxide (lungs).
• Digestive system. Breaks down food and absorbs nutrients (stomach, intestines).
• Nervous system. Senses and responds rapidly, coordinating the body (brain, nerves).
• Endocrine system. Coordinates more slowly using hormones (glands).
• Skeletal and muscular systems. Support the body and produce movement.
• Excretory system. Removes waste and helps balance water (kidneys).
• Immune system. Defends against pathogens.

How systems interact

This interaction is the heart of HS-LS1-2: the organism is a system of interacting systems, and its overall function emerges from how the parts work together.

Try this

Q1. List the levels of organization from smallest to largest. [2]

• Cue. Cell, tissue, organ, organ system, organism.

Q2. Explain how the digestive and circulatory systems work together to nourish body cells. [2]

• Cue. The digestive system breaks down food and absorbs nutrients into the blood; the circulatory system then carries those nutrients to the body cells.