Membrane proteins are essential for the cell to communicate with its environment. They function as a gateway across the lipid bilayer, allowing stimuli transmission and controlling molecular transport into or out of the cell. The structure of membrane proteins plays a pivot role in their function and mechanism. However, determination of membrane protein structure remains a great challenge due to difficulties associated with expression and purification. This dissertation focuses on utilizing X-ray crystallography to study the structure and the function of various membrane proteins from different biological systems. Chapter 2 explores the role of MmpL family transporters in the development of antibiotic resistance in Mycobacterium tuberculosis. The cell wall of M. tuberculosis is crucial to its virulence and antimicrobial resistance. The MmpL transporters are known to participate in cell wall formation by exporting fatty acid derivatives. We present the structural insights into the TetR transcriptional regulator Rv0302, which controls the expression of several MmpL proteins. Also, by combining functional studies and structural analysis, we demonstrate how the investigation of Rv0302 improves our understanding of substrate transport by the MmpL family proteins. Chapter 3 details our investigations of the the carbon concentrating mechanism (CCM) of the green algae Chlamydomonas reinhardtii. Due to their aquatic habitat, the photosynthetic efficiency of phytoplankton is hindered by the difficulty of maintaining sufficient supply of inorganic carbon (Ci). To overcome this hurdle, these microorganisms have developed CCM to enhance Ci uptake from its Ci limited environment. We reveal the structure of a CCM related membrane protein transporter LCI1, whose structure is the first membrane transporter solved in the CCM pathway of C. reinhardtii and is unique in the protein database. Finally, in chapter 4 we apply our knowledge of protein crystallography to study plant receptor-like kinase FERONIA from Arabidopsis thaliana. FERONIA is known to play an important role in many plant signaling pathways such as growth development, root growth, and drought response. We demonstrated different expression, purification and crystallization attempts of acquiring structural information of FERONIA and the plant hormones RALF1 and RALF23. While these efforts did not result in a solved structure, based on our acquired experience with this system, we suggest several directions for future structural study of membrane protein in higher evolved organisms.