Movement Across the Membrane

Selective Permeability of the Plasma Membrane

• The cell membrane is a selectively permeable barrier, allowing certain substances to pass freely while restricting others.

• Factors influencing selective permeability include:

  • Lipid Solubility: Lipid-soluble substances (e.g., oxygen, carbon dioxide) pass easily; others struggle; not lipid-soluble

  • Size of Molecules: Smaller molecules (e.g., water, urea) pass through the membrane more readily than larger ones, like starch molecules.

  • Charge of Molecules: Charged molecules (ions) face difficulty crossing without specialized transport proteins.

  • Transport Proteins: The cell can pump specific molecules in or out against concentration gradients using embedded proteins.

Membrane Transport

• Passive Transport

  • Movement of substances across cell membranes without the cell having to use any of its energy: cellular energy.

  • Occurs along a high-to-low concentration gradient.

  • Examples: Diffusion, Osmosis, Facilitated diffusion, Filtration.

• Active Transport

  • Requires cellular energy (usually ATP) for movement.

  • Moves substances against a concentration gradient (low to high concentration).

  • Examples: Solute pumping, Endocytosis, Exocytosis.

  Concentration Gradients and Homeostasis.

  • A concentration gradient is the difference in concentration between two regions.

  • It creates a physical force on the components of a solution.

  • This results in the movement of a solute, solvent, or both components of the solution.

Simple Diffusion

• Diffusion occurs due to the random movement of molecules or ions. This continues until concentrations equalize on both sides of a membrane.

• Examples include gases like oxygen and carbon dioxide.

Osmosis

• Osmosis is the diffusion of water through a selectively permeable membrane. It pertains to the movement of the solvent, rather than the solute.

• Water moves toward areas of higher solute concentration until equilibrium is achieved.

Tonicity: Hypertonic, Isotonic, Hypotonic

• Tonicity measures solute concentration on either side of a semipermeable membrane.

• It influences the movement of water across the membrane via osmosis.

Comparing and Contrasting Tonicity

• A hypertonic environment has a higher concentration of solute. This leads to the net movement of water into the hypertonic region by osmosis.

• For example, if the extracellular space is hypertonic, water will flow out of the cytoplasm.

• Isotonic solutions have equal solute concentrations on both sides of a membrane. This equality results in no net water movement across the membrane.

• Example: In isotonic conditions, water flows equally in both directions.

• A hypotonic solution has a lower concentration of solute. This leads to water moving away from this area through osmosis.

• For example, if the extracellular space is hypotonic, water will enter the cytoplasm.

Critical Concept

• Osmosis is crucial for maintaining homeostasis in all living organisms, as water is essential for life.

• Fluctuations in water levels can disrupt these metabolic processes.

• Cytosol shapes cells, which is vital for their function; alterations in cell shape can hinder specific cellular abilities.

Facilitated Diffusion

• Facilitated diffusion uses transport proteins in the cell membrane. These proteins assist specific molecules in crossing the membrane along their concentration gradient.

• They function as "channels" or "carriers" for substances such as glucose and amino acids.

Filtration

• Filtration involves the movement of substances across a membrane driven by hydrostatic pressure differences. It does not depend on concentration gradients.

• In the kidneys, blood pressure forces small molecules and water through filtration membranes, leading to urine formation.

Solute Pumps

• Solute pumping involves specialized proteins in the cell membrane known as "ion pumps" or "carrier proteins." These proteins use energy, primarily in the form of ATP, to transport specific molecules or ions.

• The transport occurs against the concentration gradient.

Active Transport - Symport and Antiport

• The net direction of solute transport influences the type of active transport.

• Types of active transport include:

  • Symport: Two different solutes are transported in the same direction across the membrane.

  • Antiport: Two different solutes are transported in opposite directions across the membrane.

  Sodium-Potassium Pump

  • The sodium-potassium pump is an example of an antiport mechanism found in neuron and muscle cell membranes. It transports 3 Na+ ions out of the cell and 2 K+ ions into the cell. Both ions are moved against their concentration gradients.

  • This process maintains a concentration gradient and generates an electrochemical potential across the membrane.

  • The electrochemical potential is essential for the rapid activation of signal transduction pathways.

  Glucose Transporters

  • Glucose transport in muscles occurs via symport mechanisms that move glucose against its concentration gradient.

  • This process relies on the sodium-potassium pump to establish a [Na+] gradient.

  • The glucose symporter first binds extracellular [Na+], enabling subsequent binding of glucose.

  Vesicular Transport/Bulk Transport

  • Bulk transport/vesicular transport involves the movement of large quantities of materials into and out of cells.

  • There are two primary forms: endocytosis and exocytosis.

  • This process requires energy to move materials against concentration gradients or to modify the cell membrane to engulf or expel substances.

  Phagocytosis - Cell “Eating”

  • Phagocytosis is the process by which cells engulf large particles, such as pathogens or cellular debris.

• It is performed by immune cells, such as macrophages, to eliminate bacteria and other foreign invaders.

Pinocytosis - Cell “Drinking”

- A type of endocytosis. Cells engulf extracellular fluid and its dissolved solutes during this process. The cell membrane indents to form a pocket that encloses the fluid.

- Inside, the vesicle may fuse with lysosomes for digestion or release its contents into the cytoplasm.

Receptor-Mediated Endocytosis

- A selective process for internalizing specific ligands via membrane-bound receptors. Enables cells to absorb substances available in low concentrations in the extracellular environment.

- Commonly involves hormones and nutrients.

Exocytosis: Pt. 2

• Exocytosis is the process by which cells transport and release substances from their interior to the exterior environment.