Protoplanetary disks are the natal place of planets and ALMA observations are now revealing the detailed physical and chemical structure of planet forming regions in the disks. Understanding chemical components of gas, dust and ice in the disks is essential to investigate the origins of materials in our Solar system and other planetary systems. As a first step of planet formation, dust grains are thought to grow through collisional sticking, and then drift radially and sometimes azimuthally to be piled up, depending on the gas distribution, which has been observed in some disks by ALMA. Also, spatially resolved observations of dust size distribution by ALMA give us information of detailed physical structure, such as possible planet formation in the disks. Meanwhile, protoplanetary disks are known as the object of chemical diversity because the physical structure dramatically changes, depending on the distance from the central star which is the main source of heating and UV radiation. ALMA is now revealing the detailed chemical structure in the disks. Also, ALMA enables us to detect complex organic molecules in the disks, CH3CN and CH3OH, which could lead to more complex organic compounds found in comets and meteorites in our Solar system. In this talk I will review some of our recent work of ALMA observations and modelling of gas and dust in protoplanetary disks. In addition, I will introduce our recent model calculations for studying the effect of possible carbon grain destruction on chemical structure in the disks. Elemental abundance ratio of carbon to oxygen (C/O) in gas is a fundamental parameter to control the chemical composition in the disks and then the materials in planetary systems. The abundance distributions of carbon-bearing and oxygen-bearing species dramatically change, depending on the C/O ratio, near the midplane of the disks where CO is not photodissociated. I will discuss the effect on the composition of gas and ice in the disks, and then possible observational probes to diagnose the C/O ratio, such as HCN and its isotopologues.

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