Cell-Cell Interactions 8.1 The Cell Surface The Structure and Function of an Extracellular Layer The Plant Cell Wall The Extracellular Matrix of Animals 8.2 How Do Adjacent Cells Connect and Communicate? Cell-Cell Attachments Tight Junctions; Desmosones Selective Adhesion; The Discovery of Cadherins Cell-Cell Gaps 8.3 How Do Distant Cells Communicate? Signal Reception Signal Processing G Proteins Receptor Tyrosine Kinases Signal Response Signal Deactivation

Plant cell, with chloroplasts and large vacuole

Most cells secrete extracellular material that generally consists of long crosslinked filaments surrounded by a stiff ground substance. The rods or filaments protect against stretching forces, and the ground substance protects against compression. This is especially obvious in plants.

The Plant Cell Wall The extracellular material secreted by plant cells first builds a primary cell wall of long strands of cellulose bundled into microfibril filaments that form a crisscrossed network. This network becomes filled with a gelatinous polysaccharide such as pectin, which is hydrophilic and keeps the cell wall moist. In young cells, enzymes called expansins allow microfibrils to slide past each other, and turgor pressure then forces the cell wall to elongate, allowing cell growth.

Animal cells secrete an extracellular matrix (ECM) that consists of a gelatinous polysaccharide as ground substance and protein fibers like collagen instead of polysaccharide filaments.

The ECM provides structural support for the cell and also helps cells stick together. Cytoskeletal actin filaments connect to transmembrane integrins that bind to ECM fibronectins, which bind to collagen. These protein-protein attachments link the cytoskeletons within cells directly to the ECM

In multicellular organisms, cells can form tissues, which may combine to form an organ specialized for one biological function.

A middle-lamella-like layer exists between cells in many animal tissues. Some tissues, such as epithelia, have additional proteins that connect neighboring cells more strongly. In tight junctions, this seal is water-tight. Example: epithelial cells

Each major cell type has its own cell adhesion proteins Each major cell type has its own cell adhesion proteins. The cadherins are a group of cell adhesion proteins found in plasma membranes that bind only to other cadherins of the same type, causing cells of the same tissue type to bind together.

In most animal tissues, gap junctions connect adjacent cells by forming channels that allow the flow of small molecules between cells to coordinate their activities

Chemical signals (hormones, etc) travel throughout animals and plants to target cells and convey information from one tissue or organ to another. This intercellular signaling involves four steps: signal reception, signal processing, signal response, and signal deactivation. The function and chemical structure of plant and animal hormones vary widely (Table 8.1). However, all hormones play a role in coordinating cell activity in response to information from outside or inside the body.

Signal receptors are proteins that change their conformation or activity when a hormone binds to them. There are many types of receptors, each of which is found only in certain cell types. In all cases, a change in receptor structure indicates that a signal has been received. Hormones that cannot diffuse across the plasma membrane rely on a signal-transduction pathway to convert the extracellular hormone signal to a new intracellular signal. Many of these involve transmembrane so-called G proteins or else receptor protein kinases.

Common second messengers are normal cell constituents such as Ca2+ ions, cyclic adenosine monophosphate (cAMP), cyclic GMP (cGMP), or one of two molecules made from membrane lipids: diacylglycerol (DAG) or inositol triphosphate (IP3) Cells can produce large quantities of these second messenger molecules in a short time after each hormone-receptor binding event, so this signal transduction amplifies the original signal. Signal transduction converts an easily transmitted extracellular message into a greatly amplified intracellular message that carries information throughout the cell and induces a pre-programmed cell response.

Receptor Tyrosine Kinases Receptor tyrosine kinases are transmembrane proteins that bind a hormone signal and phosphorylate one another. The phosphorylated form activates other enzymes by phosphorylating them. These enzymes phosphorylate yet more enzymes and so on, creating a phosphorylation cascade that activates many enzymes. Example: insulin receptor

Signal Response and Deactivation Signal responses include changes in gene expression and changes in the activity of specific proteins. Turning off cell signals is just as important as turning them on. Cells have automatic and rapid mechanisms for signal deactivation. These mechanisms allow the cell to remain sensitive to future hormone signals.

The end result of cell sensitivity to hormonal signaling is an integrated whole-organism response to changing conditions both inside and outside the multicellular organism.