Note in Figure 3 the many mitochondria just inside the plasma membrane. Protists are single cell eukaryotes found in aquatic environments. Note that in this image a clear nucleoli in the center of the nucleus is seen. The nucleolus is the site of ribonucleic acid synthesis. RBC transport gases to and from tissues.
Cells come in different shapes.
Shapes reflex the function of the cells. This is the optimal shape for diffusion of gases. A primary function of RBC is to transport oxygen to the tissues where oxygen diffuses out of the RBC and into the tissues. It also picks up carbon dioxide also by diffusion and transports this waste gas back to the lungs. The function of platelets is in blood coagulation.