The Extracellular Matrix
Animal cells synthesize an extracellular matrix (ECM) on the outside of the cell membrane. The ECM is composed of proteins and carbohydrates:
- The protein collagen forms long collagen fibers;
- These collagen fibers are modified with carbohydrates and interwoven with proteoglycan;
- Proteoglycan is a protein heavily modified with carbohydrates and form branched proteoglycan complexes.
- ECM components attach to membrane proteins called integrins and also connects to surrounding cells.
- The ECM provides strength and structure to tissues.
Cell Junctions and Adhesions
Cells need to stick together! There are different types of junctions and adhesions that hold cells in our tissues together:
- Gap junctions
- Tight junctions
- Desmosomes
- Plasmodesmata
Gap Junctions
- From channels between cells.
- Allows the passage of fluid and molecules between their borders through the channel.
- The channels are called connexons and each one is composed of six connexin proteins.
Tight Junctions
- Form a tight seal between cells, preventing the passage of fluids and molecules between their border.
- Cells are held together by tight junction proteins.
- Main proteins are claudins and occludins.
Desmosomes
- Form links between cells (not water tight) and give strength to tissues.
- Cadherins are proteins located on the cell membrane.
- They connect with intermediate filaments on the inside of the cells and with other cadherins on the neighbor cell.
Plasmodesmata
- Neighboring plant cells are separated by two thick cell walls and the middle lamellae.
- Plasmodesma is a channel between the cell walls of two plant cells adjacent to each other.
- Allows for the sharing of cytoplasm between plant cells.
