From Discover Magazine:

Question:

How do cells communicate with each other to form organisms? For example, how does the side of a plant stem facing sunlight tell the one on the shady side to grow more so the whole plant can bend toward the light?

Answer:

Cells communicate through their own language of chemical signals. Different compounds, such as hormones and neutrotransmitters, act like words and phrases, telling a cell about the environment around it or communicating messages. When the pancreas detects a person has just eaten, for example, it releases the hormone insulin to tell other cells in the body to remove glucose from the blood.

Just as a person needs ears as much as a mouth to have a conversation, cells use receptor proteins either on the outer cell wall or inside the cell itself to “hear” different signals. Once the signal chemical binds to a receptor, that protein turns on a signaling cascade in the cell that ultimately leads to the cell’s response. Every cell has receptors that can detect a lot of different signals, so they are constantly bombarded with biological conversation. Imagine being in a room and having everyone talking at you at once!

So how does a plant use cellular communication to grow toward sunlight? The growing tips of plants produce auxin — a hormone that tells cells to grow and divide — which is then sent to the rest of the plant. The shady parts of a plant receive more auxin, which causes those cells to elongate while the sunny-side cells don’t. When one side lengthens while the other side stays the same, the plant will bend.