Chapter 31 Phototropism

You Must Know The three components of a signal transduction pathway and how changes could alter cellular responses. (This is review from Chapter 5) The role of auxins in plants. (This is the only plant hormone you will be expected to remember.) Phototropism

The Three Stages of Cell Signaling: A Review Cells receiving signals undergo three processes – Reception – Transduction – Response © 2014 Pearson Education, Inc.

Figure EXTRACELLULAR FLUID Plasma membrane Reception Receptor Signaling molecule CYTOPLASM Cell Signals are detected by receptors that undergo changes in shape in response to a specific stimulus.

Figure EXTRACELLULAR FLUID CYTOPLASM Plasma membrane ReceptionTransduction Relay molecules Receptor Signaling molecule Transduction is a multistep pathway that amplifies the signal. This allows a small number of signal molecules to produce a large cellular response

Figure EXTRACELLULAR FLUID CYTOPLASM Plasma membrane Response ReceptionTransduction Relay molecules Activation Receptor Signaling molecule Cellular response is primarily accomplished by two mechanisms: (1) turning genes on or off and thereby increasing or decreasing mRNA production, or (2) activating existing enzyme molecules.

Concept 31.1: Plant hormones help coordinate growth, development, and responses to stimuli Plant hormones: – chemical signals that modify or control one or more specific physiological processes within a plant. – produced in very low concentration. – Most aspects of plant growth and development are under hormonal control.

Video clip Phototropism

Control Light Illuminated side Shaded side Phototropism

Tip removed Light What part of a grass coleoptile senses light? Opaque cap Trans- parent cap Opaque shield over curvature Darwin and Darwin: Phototropism occurs only when the tip is illuminated.

Auxin Results Growth-promoting chemical diffuses into agar cube Excised tip on agar cube Control (agar cube lacking chemical) Offset cubes Plants grown in the dark.