Biological Hierarchy:

Cell to Cell Communication

• Involves (and you should review):• Structure of plasma membranes

• Diffusion vs. active transport

• Hydrophobic vs. hydrophilic compounds

• Proteins

• Surface vs. intermembrane proteins

• Enzyme activity

Figure 8.7 The structure of a transmembrane protein

Figure 8.9 Some functions of membrane proteins

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Receptor protein

Receptor protein

Cells can communicate by direct contact between cells

Cells can communicate by sending chemical signals to other cells:

Mating in yeast cells depends on

cell to cell communication

Chemical signals can be sent to cells at various distances from each other

Once a chemical signal reaches its target cell:

• Signal molecule binds to a receptor protein on the target cell (it is a ligand)

• The receptor molecule changes shape

• The shape change causes a series of reactions (transduction) to take place in the cytoplasm of the cell

• This series of reactions leads to a response of the cell to the chemical signal

• This process is called a signal-transduction pathway

Figure 11.5 Overview of cell signaling (Layer 1)

Figure 11.5 Overview of cell signaling (Layer 2)

Figure 11.5 Overview of cell signaling (Layer 3)

Common Types of Receptors

• G-protein-linked receptors

• Tyrosine-kinase receptors

• Ligand-gated ion-channel receptor

• Intracellular receptors

G-protein-linked receptors

When G-protein-linked receptors receive a signal, they will activate a “G protein” inside the cell

The G-protein starts the transduction pathway leading to the cell response

Tyrosine-kinase receptors act as an enzyme, using ATP to start several transduction pathways simultaneously

Ligand-gated ion-channel receptors open or close in response to a chemical signal, allowing ions to DIFFUSE in or out of the cell based on relative concentrations

Lipid-soluble chemical signals, such as steroid hormones, can pass through the plasma membrane, and bind to an intracellular receptor

Inside the cell, the activated receptor may turn on or off certain genes (a transcription factor)

Once the signal has been received by the receptor, this triggers the transduction pathway, that eventually leads to the cell’s response

Fig. 11.11: A phosphorylation cascade

Some signal-transduction pathways involve the use of small, non-protein molecules or ions. These are called second messengers

Second messengers often bind to and activate other enzymes in the pathway

Cyclic AMP (cAMP) is a common second messenger

Calcium ions are also used as second messengers

The concentration of calcium is kept low in the cytoplasm of animal cells by actively pumping them into the mitochondria and the ER and out of the cell

Calcium is used by both G-protein and tyrosine-kinase pathways as a second messenger

Inositol triphosphate (IP3) opens a channel that releases Ca2+ into the cytoplasm

Calcium activates calmodulin which turns on or off other proteins

An example of an entire pathway

In times of physical stress, the adrenal glands release the hormone epinephrine

Fuel reserves are mobilized by the breakdown of glycogen to individual glucose molecules

The response of a cell to signal can be varied:

1. Turning on certain enzymes

2. Turning on/off genes and protein production

3. Muscle contraction

4. Cell division

etc…

The response of a cell to a signal depends on what type of cell it is and what other types of proteins are present in the cell