In which of the following organisms does oxygen diffuse directly across a respiratory surface to cells, without being carried by the blood?
a. A grasshopper
b. A whale
c. An earthworm
d. A mouse

Verified step by step guidance

1. Understand the concept of respiratory surfaces: A respiratory surface is the site where gas exchange occurs between the organism and its environment. Oxygen diffuses across this surface to reach cells, and the method of transport varies among organisms.
2. Review the respiratory systems of the organisms listed: Grasshoppers use a tracheal system, whales and mice rely on lungs and blood to transport oxygen, and earthworms use their skin as a respiratory surface.
3. Focus on the grasshopper: Grasshoppers have a tracheal system where oxygen directly diffuses through spiracles (openings) into tracheae, which transport oxygen directly to cells without involving blood.
4. Compare with the other organisms: Whales and mice use lungs to oxygenate blood, which then carries oxygen to cells. Earthworms use their skin as a respiratory surface, but oxygen is carried by blood to cells.
5. Conclude that the organism where oxygen diffuses directly across a respiratory surface to cells, without being carried by the blood, is the grasshopper.

Key Concepts

Diffusion

Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. In the context of respiration, oxygen diffuses across respiratory surfaces, such as skin or gills, directly into cells when the concentration of oxygen is higher in the environment than in the cells. This passive transport mechanism is crucial for organisms that lack a circulatory system to transport oxygen.

Respiratory Surfaces

Respiratory surfaces are specialized structures in organisms that facilitate gas exchange. These surfaces must be thin, moist, and have a large surface area to allow efficient diffusion of gases like oxygen and carbon dioxide. Examples include the skin of earthworms, gills in fish, and lungs in mammals, each adapted to their specific environments and modes of respiration.

Circulatory Systems

Circulatory systems are networks that transport blood and nutrients throughout an organism's body. In animals with complex circulatory systems, such as mammals and whales, oxygen is carried by blood cells (hemoglobin) from the respiratory organs to tissues. In contrast, some simpler organisms, like earthworms, rely on direct diffusion for gas exchange, bypassing the need for a circulatory system to transport oxygen.