1.3 Membrane transport and cellular communication

Membrane transport and cellular communication are crucial for plant survival. These processes allow plants to move nutrients, regulate water balance, and respond to environmental cues. From passive diffusion to energy-driven active transport, plants use various mechanisms to maintain cellular homeostasis.

Plants rely on complex signaling pathways to communicate within and between cells. These pathways involve receptors, signaling molecules, and transduction cascades that enable plants to sense and respond to their environment. Understanding these processes is key to grasping plant physiology and cell biology.

Passive Membrane Transport

Movement of molecules across membranes

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• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient
• Osmosis is the movement of water across a semipermeable membrane from an area of high water potential to an area of low water potential
  • Water moves from a hypotonic solution to a hypertonic solution across a selectively permeable membrane
  • Osmotic pressure is the minimum pressure required to prevent the inward flow of water across a semipermeable membrane
• Facilitated diffusion is the movement of molecules across a membrane with the assistance of transport proteins
  • Allows for the movement of larger or charged molecules that cannot pass through the membrane on their own (glucose, amino acids)

Specialized channels for transport

• Ion channels are protein pores in the membrane that allow specific ions to pass through
  • Gated ion channels open or close in response to a stimulus such as a change in voltage or ligand binding
  • Examples include potassium channels, sodium channels, and calcium channels
• Aquaporins are water channel proteins that facilitate the rapid movement of water across cell membranes
  • Play a crucial role in maintaining water balance in cells and tissues (root hair cells, guard cells)
  • Aquaporins can be regulated by phosphorylation or pH changes to control water flow

Active Membrane Transport

Energy-dependent transport processes

• Active transport is the movement of molecules against their concentration gradient using energy from ATP
  • Primary active transport directly uses ATP to power the transport of molecules (sodium-potassium pump, proton pumps in plant vacuoles)
  • Secondary active transport uses the electrochemical gradient created by primary active transport to move molecules against their concentration gradient (sodium-glucose cotransporter)
• The symplast is the continuous system of cytoplasm connected by plasmodesmata that allows for the transport of molecules between cells
  • Plasmodesmata are channels that connect the cytoplasm of adjacent cells, allowing for intercellular communication and transport

Apoplastic transport

• The apoplast is the continuous system of cell walls and intercellular spaces that allows for the transport of water and solutes external to the cells
  • Water and dissolved minerals move through the apoplast of root cells before entering the symplast
  • Apoplastic transport is driven by transpiration, which creates a water potential gradient that pulls water and solutes up through the xylem

Cellular Communication

Signal transduction components

• Signal transduction is the process by which cells convert external signals into cellular responses
  • Involves the binding of a signaling molecule to a receptor, which initiates a series of biochemical reactions that ultimately lead to a cellular response
• Receptors are proteins that bind to specific signaling molecules and initiate signal transduction pathways
  • Cell surface receptors are located on the plasma membrane and bind to extracellular signaling molecules (receptor kinases, G protein-coupled receptors)
  • Intracellular receptors are located within the cell and bind to signaling molecules that can pass through the membrane (steroid hormone receptors)

Signaling molecules and pathways

• Signaling molecules are chemical compounds that convey information between cells or within a cell
  • Examples include hormones (auxins, cytokinins), neurotransmitters, and growth factors
  • Signaling molecules can act locally (paracrine signaling) or over long distances (endocrine signaling)
• Transduction pathways are series of biochemical reactions that relay the signal from the receptor to the target molecule, often involving second messengers and protein kinases
  • Second messengers are small molecules that amplify and propagate the signal within the cell (cyclic AMP, calcium ions)
  • Protein kinases are enzymes that phosphorylate target proteins, modulating their activity and leading to cellular responses (MAP kinases, CDPKs in plants)