Final answer:
Diffusion efficiency and gravitational forces limit the size of land animals. As size increases, the surface area to volume ratio becomes too low for efficient diffusion, necessitating larger and stronger skeletal and muscular systems, which are in turn limited by gravity.
Step-by-step explanation:
The key limiting factor in how large land animals can become is the efficiency of diffusion as a means of transport in their bodies. In larger animals, as the volume increases, the surface area to volume ratio decreases, leading to a reduction in the efficiency of diffusion for exchanging nutrients and wastes. Additionally, land animals are constrained by the need for a robust skeletal system to support their body mass and muscles for movement. However, as size increases, the gravitational effects also become a significant limiting factor, as the skeleton and muscles must be proportionally larger to support and move the increased mass. Therefore, both diffusion limitations and gravitational forces prevent land animals from growing beyond a certain size.
For single-celled organisms, like an amoeba, diffusion alone can satisfy its needs. In larger multicellular organisms, diffusion distances increase and specialized respiratory systems and circulatory systems have evolved to meet the oxygen and nutrient demands of the body.