What are the websites for human body structure exercises?

The basic unit of human body structure is cells, and there are non-cellular substances between cells, which are called intercellular stroma. Cell structure can be divided into three parts: cell membrane, cytoplasm and nucleus. Epithelial tissues are divided into three types: covering epithelium, glandular epithelium and sensory epithelium.

It can be divided into cell membrane, cytoplasm and nucleus. Cell membrane is mainly composed of protein, lipids and sugars, which can protect cells, maintain the stability inside cells and control the material exchange inside and outside cells. Cytoplasm is the center of cell metabolism, which is mainly composed of water, protein, RNA, enzymes and electrolytes. There are also various organelles suspended in the cytoplasm. The main organelles are mitochondria, endoplasmic reticulum, lysosomes and centrosomes. The nucleus is surrounded by a nuclear membrane, which contains nucleoli and chromatin. Chromatin contains nucleic acids and protein. Nucleic acid is a substance that controls biological inheritance.