tion plants have because, with their rigid cell wall, they have no need of anchoring junctions. Plasmodesmata connect every cell in a plant with its neighboring cells, allowing the passage of cytoplasm from cell to cell. In a sense, then, plant cells connected in this way form one mega-cell containing many nuclei.

The outer membranes of bacteria, mitochondria, and chloroplasts are permeated by pore proteins called porins that function similarly to gap junctions, although they are structurally very different. They allow the passage of small molecules through the membrane by passive transfer.

But a hole in a cell is a dangerous thing! In fact, one ancient part of our immune system, the complement system, functions mainly to poke holes in invading bacterial cells. The cell leaks its guts out, so to speak. In general, cells don't want drafty open doors, yet they must communicate. This explains the diverse and highly elaborated means animal and plant shave for getting things into and out of themselves and of communicating with other cells without becoming too vulnerable.

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