3.5 Transport

4 min read•august 22, 2024

Cells are like bustling cities, constantly moving materials in and out. Transport across cell membranes is crucial for survival, allowing nutrients in and waste out. Some molecules can slip through easily, while others need a helping hand or energy boost.

Active and passive transport mechanisms work together to maintain cellular balance. From simple diffusion to energy-hungry pumps, cells have evolved diverse ways to control what enters and exits. Understanding these processes is key to grasping how cells function and communicate.

Passive vs Active Transport

Mechanisms and Energy Requirements

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  • Passive transport moves molecules across a membrane without the use of energy, while active transport requires the expenditure of cellular energy, typically in the form of ATP
  • Passive transport moves molecules down their concentration gradient from regions of high concentration to regions of low concentration
  • Active transport moves molecules against their concentration gradient, requiring energy input to overcome the concentration difference

Examples of Passive and Active Transport

  • Examples of passive transport include simple diffusion (movement of small, nonpolar molecules), facilitated diffusion (movement of larger or polar molecules with assistance from carrier or channel proteins), and osmosis (diffusion of water across a selectively permeable membrane)
  • Examples of active transport include the sodium-potassium pump (maintains resting potential in neurons by pumping sodium ions out and potassium ions in) and the proton pump (creates proton gradient in mitochondrial inner membrane for ATP synthesis during cellular respiration)

Membrane Transport Mechanisms

Simple Diffusion, Facilitated Diffusion, and Osmosis

  • Simple diffusion is the movement of small, nonpolar molecules across a membrane down their concentration gradient without the assistance of membrane proteins
  • Facilitated diffusion is the movement of larger or polar molecules across a membrane down their concentration gradient with the assistance of carrier proteins or channel proteins
  • Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration)
  • The rate of diffusion is influenced by factors such as the concentration gradient, temperature, and the size and polarity of the molecules

Factors Affecting Rate of Diffusion

  • Concentration gradient: A steeper concentration gradient leads to a faster rate of diffusion
  • Temperature: Higher temperatures increase the kinetic energy of molecules, leading to a faster rate of diffusion
  • Size and polarity of molecules: Smaller and nonpolar molecules diffuse more quickly than larger and polar molecules
  • Membrane permeability: The presence of channel proteins or carrier proteins can increase the rate of diffusion for specific molecules

Membrane Proteins in Active Transport

Pumps and Transporters

  • Pumps, such as the sodium-potassium pump and the proton pump, use ATP to actively transport specific ions across a membrane against their concentration gradient
  • Transporters, such as the sodium-glucose cotransporter and the calcium ATPase, use energy from ATP or the concentration gradient of another molecule to transport specific molecules across a membrane against their concentration gradient

Examples and Functions of Pumps and Transporters

  • The sodium-potassium pump maintains the resting potential of neurons by pumping sodium ions out of the cell and potassium ions into the cell, creating a concentration gradient that drives the propagation of action potentials
  • The proton pump in the mitochondrial inner membrane creates a proton gradient that drives the synthesis of ATP during cellular respiration, providing energy for cellular processes
  • The sodium-glucose cotransporter (SGLT) in the small intestine uses the sodium gradient to transport glucose into the cell, facilitating the absorption of glucose from the digestive tract
  • The calcium ATPase (PMCA) in the plasma membrane uses ATP to pump calcium ions out of the cell, maintaining low intracellular calcium levels and regulating calcium signaling

Endocytosis vs Exocytosis

Definitions and Mechanisms

  • Endocytosis is the process by which a cell takes in material from the extracellular environment by invaginating its plasma membrane to form a vesicle
  • Exocytosis is the process by which a cell releases material to the extracellular environment by fusing a vesicle with the plasma membrane
  • Endocytosis can be divided into phagocytosis (uptake of large particles) and pinocytosis (uptake of fluid and small molecules), while exocytosis is a uniform process

Functions and Examples

  • Endocytosis is used by cells to take in nutrients (amino acids, glucose), signaling molecules (hormones, growth factors), and other essential materials (low-density lipoproteins)
  • Receptor-mediated endocytosis is a specific type of endocytosis that involves the selective uptake of specific molecules, such as low-density lipoproteins (LDLs), by receptors on the cell surface
  • Exocytosis is used to release waste products (cellular debris), signaling molecules (neurotransmitters, hormones), and secretory proteins (digestive enzymes, antibodies)
  • Examples of exocytosis include the release of neurotransmitters at synapses, the secretion of insulin by pancreatic beta cells, and the release of digestive enzymes by cells in the pancreas and small intestine
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