Cellular Function and Ion Channels Quiz G 4 - 1.3

Transport of Substances Through Cell Membranes

• The cell membrane is a lipid bilayer with embedded proteins, acting as a barrier between intracellular and extracellular fluids.
• The extracellular fluid has high sodium and chloride concentrations, while intracellular fluid has high potassium and phosphate concentrations. These differences are vital for cell function.
• Lipid-soluble substances can diffuse directly through the lipid bilayer.
• Water-soluble substances or ions enter via transport proteins in the membrane.
• Channel proteins form hydrophilic pathways, enabling water and some dissolved substances to pass through.
• Carrier proteins facilitate transport by binding to molecules to be transported and undergoing conformational changes.
• Diffusion, a passive process, entails random movement of molecules due to kinetic energy.
• Simple diffusion involves a substance moving across a membrane without a carrier protein.
• Facilitated diffusion, a passive process, involves carrier proteins in moving substances across a membrane down a concentration gradient.
• Active transport is an energy-consuming process that moves substances against their concentration gradient.

Diffusion Through the Membrane

• Diffusion is the random movement of molecules from areas of high concentration to areas of low concentration.
• Lipid-soluble substances diffuse readily through the lipid bilayer, while water-soluble substances utilize channel or carrier proteins.
• The rate of diffusion is influenced by the concentration difference, molecular size, and the presence of membrane openings.
• Facilitated diffusion involves the interaction of a carrier protein, and the rate plateaus as concentration increases in facilitated diffusion, unlike simple diffusion which continues to increase.

Diffusion of Lipid-Soluble Substances

• Lipid solubility plays a crucial role in how quickly a substance diffuses through the lipid bilayer.
• High lipid solubility, as seen in oxygen, nitrogen, carbon dioxide, and alcohols, leads to rapid diffusion.

Diffusion of Water

• Water readily diffuses through protein channels in the membrane, primarily aquaporins, which are highly selective.
• Water transport is significantly faster than that of other small water-soluble molecules.

Diffusion Through Protein Pores/Channels

• Pores are permanent open pathways allowing selective passage.
• Channels, which may be gated, regulate permeability based on factors such as voltage or chemical signals.
• Channels exhibit selectivity due to size, charge, and/or binding interactions with specific molecules, e.g., potassium channels allow potassium passage more than sodium.

Factors Affecting Net Diffusion Rate

• Concentration difference across the membrane drives net diffusion.
• A higher concentration difference results in a faster diffusion rate.
• Electrical potential difference can also influence the movement of charged molecules.
• Pressure difference can also play a role in molecular or ionic movement.

Osmosis

• Osmosis is the net diffusion of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.
• Osmotic pressure is the hydrostatic pressure that would need to be applied to oppose the osmotic flow of water.
• The concentration of solutes influences the osmotic pressure of a solution.
• Solutions with equal concentrations on both sides of a membrane do not demonstrate osmosis.

Active Transport

• Active transport involves the use of energy to move substances against their concentration gradients.
• The sodium-potassium pump is a critical example, actively moving sodium ions out and potassium ions in.
• This is crucial for maintaining cellular osmotic balance and creating electrochemical gradients.
• Counter-transport and co-transport, are secondary active transport processes that utilize the energy stored in a concentration gradient to move other molecules along with the primary ion.

Primary Active Transport

• This involves directly using energy from molecules like ATP to move substances across a membrane.
• The sodium-potassium pump is a major primary active transport system, maintaining sodium and potassium gradients.
• Other pumps transport calcium and hydrogen ions and other substances as needed.

Secondary Active Transport

• Secondary active transport utilizes the energy stored by the primary active transport of ions, like sodium, to move other substances.
• Examples include the sodium-glucose co-transport, commonly seen in the intestines and kidneys.
• Calcium and hydrogen ions are often moved by secondary active counter-transport.