IAS22 Cell Membrane Transport 2025-26

Microvilli increase surface area to facilitate absorption of extracellular materials.

The Golgi apparatus is responsible for the storage, elaboration, and packaging of secretory products and lysosomal enzymes.

Centrosome and centrioles are essential for the movement of chromosomes during cell division and the organization of microtubules in the cytoskeleton.

Mitochondria produce 95% of the ATP required by the cell.

Lysosomes are responsible for the intracellular removal of damaged organelles or pathogens through the action of digestive enzymes.

Ribosomes are involved in protein synthesis, with fixed ribosomes bound to rough endoplasmic reticulum and free ribosomes scattered in the cytoplasm.

The rough endoplasmic reticulum is involved in the synthesis of proteins, while the smooth endoplasmic reticulum synthesizes lipids and carbohydrates.

Peroxisomes are involved in the catabolism of fats and other organic compounds, as well as the neutralization of toxic compounds generated in the process.

The plasma membrane provides isolation, protection, sensitivity, support, and controls the entry and exit of materials.

The plasma membrane consists of a phospholipid bilayer with:

• Hydrophilic heads facing outward
• Hydrophobic tails facing inward

Key components include:

• Integral proteins with channels for substance transport
• Glycolipids on the extracellular side, part of the glycocalyx
• Integral glycoproteins that span the membrane and are attached to carbohydrate chains
• Cholesterol molecules interspersed within the bilayer
• Peripheral proteins associated with the inner surface
• Cytoskeleton (microfilaments) connected to the membrane via proteins.

Permeability determines what moves in and out of a cell. It can be categorized as:

• Impermeable: lets nothing in or out
• Freely permeable: lets anything pass
• Selectively permeable: restricts movement, allowing some materials to move freely while restricting others.

The two main types of transport through a plasma membrane are Active transport (which requires expenditure of ATP) and Passive transport (which occurs without expenditure of ATP).

The three different mechanisms of membrane transport are:

1. Diffusion (passive)
2. Carrier-mediated transport (can be passive or active)
3. Vesicular transport (active)

The plasma membrane of cells is a semipermeable structure.

The plasma membrane contains lipids, proteins, and carbohydrates.

Membrane proteins allow the passage of substances across the membrane.

Diffusion is the process where molecules in a solution move randomly, leading to mixing. It occurs due to a concentration gradient, where there is a difference in concentration of molecules in different parts of the solvent.

The rate of diffusion is influenced by several factors:

1. Distance - The greater the distance, the slower the diffusion.
2. Molecule size - Smaller molecules diffuse faster than larger ones.
3. Temperature - Higher temperatures increase molecular movement, enhancing diffusion.
4. Concentration gradient - A steeper gradient results in faster diffusion.
5. Electrical forces - Charged particles may be influenced by electric fields, affecting their movement.

• Lipid-soluble compounds: alcohols, fatty acids, and steroids
• Dissolved gases: oxygen and carbon dioxide

• Size of the ion
• Charge of the ion
• Interaction with the channel, such as leak channels

Osmosis is the diffusion of water across the cell membrane. It occurs when there are more solute molecules, resulting in a lower concentration of water molecules. The membrane must be freely permeable to water and selectively permeable to solutes. Water molecules will diffuse toward the solution with more solutes, leading to an increase in volume on that side.

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of lower solute concentration (solution A) to a region of higher solute concentration (solution B). This process continues until the solute concentrations on both sides of the membrane are equal, resulting in an increase in the volume of solution B and a decrease in the volume of solution A.

At equilibrium, the solute concentrations on both sides of the membrane are equal. The volume of solution B increases while the volume of solution A decreases, indicating that water has moved from A to B until the concentrations are balanced.

Osmosis can be prevented by applying an external hydrostatic pressure to the side with the higher solute concentration (solution B). This pressure counteracts the osmotic pressure, stopping the flow of water and maintaining equal volumes of both solutions.

The relationship is defined by the equation: Osmotic pressure = Hydrostatic pressure. When these two pressures are equal, there is no osmotic flow of water across the membrane, indicating that the system is at equilibrium.

Osmolarity refers to the solute concentration of a solution, indicating how many solute particles are present in a given volume of solvent.

Tonicity describes how a solution affects a cell, particularly in terms of osmotic flow and cell volume changes.

In an isotonic solution, no osmotic flow occurs, meaning the cell maintains its normal shape and volume as the concentration of solutes is equal inside and outside the cell.

A hypotonic solution causes water to flow into the cell, resulting in cell swelling as the osmotic pressure increases inside the cell.

In a hypertonic solution, water moves out of the cell, leading to cell shrinkage or crenation as the concentration of solutes is higher outside the cell than inside.

There are two types of channel proteins:

1. Leak channels:

   • Always open, allowing continuous flow of ions.

2. Gated channels:

   • Open only upon stimulation, regulating the flow of ions based on specific signals.

Channel-mediated diffusion primarily facilitates the transport of:

• Small inorganic ions
• Small water-soluble molecules

These molecules pass through the membrane channels created by channel proteins.

Facilitated diffusion is a type of passive transport that allows molecules, such as glucose and amino acids, to cross the cell membrane with the help of carrier proteins. Unlike simple diffusion, which allows small molecules to pass directly through the lipid bilayer, facilitated diffusion requires specific proteins to transport larger molecules that cannot pass through the membrane directly. This process does not require energy and relies on the concentration gradient.

Carrier proteins are essential in facilitated diffusion as they transport molecules that are too large to fit through channel proteins, such as glucose and amino acids. These proteins have specific receptor sites that bind to the target molecules, allowing them to cross the membrane. Once the carrier proteins are saturated, increasing the concentration gradient will not change the rate of transport, indicating a maximum transport capacity.

When carrier proteins become saturated, they reach their maximum transport capacity. At this point, increasing the concentration gradient of the transported molecule (e.g., glucose) will not result in an increase in the rate of transport. This saturation indicates that all available carrier proteins are occupied and cannot transport additional molecules until some are released into the cytoplasm.

Primary active transport moves substrates against their concentration gradient, requiring an energy supply from ATP.

Ion pumps move ions such as Na+, K+, Ca2+, and Mg2+.

An exchange pump counter-transports two ions simultaneously; for example, sodium (Na+) moves from the cytoplasm to extracellular fluid while potassium (K+) moves from extracellular fluid to the cytoplasm.

Sodium ions (Na+) play a crucial role in secondary active transport by providing the energy needed for the transport of other molecules, such as glucose, into the cell. This occurs through a sodium-glucose linked transporter, which utilizes the sodium gradient established by the Na+-K+ pump to facilitate glucose absorption.

The Na+-K+ pump helps maintain the sodium gradient across the cell membrane by exporting 3 Na+ ions out of the cell and importing 2 K+ ions into the cell. This gradient is essential for secondary active transport mechanisms, as it allows sodium to flow back into the cell, driving the transport of other substances like glucose against their concentration gradient.

Secondary active transport does not directly require energy in the form of ATP for the transport process itself. However, the cell must use ATP later to maintain the sodium gradient, which is critical for the function of secondary active transporters like the sodium-glucose linked transporter.

Vesicular transport is the process by which materials move into or out of a cell in vesicles, utilizing active transport that requires ATP. It includes two main mechanisms: endocytosis and exocytosis.

The types of endocytosis include:

1. Receptor-mediated endocytosis: Involves the uptake of specific molecules based on their binding to receptors on the cell surface.
2. Pinocytosis: Often referred to as 'cell drinking', it involves the ingestion of liquid and small particles.
3. Phagocytosis: Known as 'cell eating', it is the process by which cells engulf large particles or even other cells.

During exocytosis, granules or droplets are released from the cell. This process is essential for the secretion of substances such as hormones, neurotransmitters, and enzymes.

Target molecules (ligands) bind to receptors in the plasma membrane.

The pockets pinch off, forming endosomes known as coated vesicles.

Coated vesicles fuse with primary lysosomes to form secondary lysosomes.

Ligands are removed and absorbed into the cytoplasm during the process.

The endosome fuses with the plasma membrane, and the receptors are again available for ligand binding.

Pinocytosis, also known as 'cell drinking', is primarily responsible for the uptake of extracellular fluid and dissolved solutes into the cell. It allows cells to sample their environment and absorb nutrients.

Phagocytosis, or 'cell eating', involves the engulfing of large particles, such as bacteria, by extending pseudopodia to form a phagosome. In contrast, pinocytosis involves the uptake of small particles and fluids without the formation of pseudopodia.

Lysosomes fuse with phagosomes to form secondary lysosomes, where enzymes break down the engulfed material, allowing the cell to digest and recycle nutrients while expelling waste through exocytosis.

Exocytosis is significant in phagocytosis as it allows the cell to expel waste products after digestion of engulfed materials, maintaining cellular homeostasis and preventing accumulation of debris.

Molecular movement of solutes where the direction is determined by relative concentrations. Factors affecting the rate include the size of the concentration gradient, size of molecules, electrical charge, lipid solubility, and temperature.

Osmosis is the movement of water molecules toward a solution with a higher solute concentration, requiring a selectively permeable membrane. Factors affecting its rate include the concentration gradient, opposing osmotic or hydrostatic pressure, and the number of aquaporins (water channels).

Facilitated diffusion involves carrier proteins that passively transport solutes across a membrane down a concentration gradient. Factors affecting its rate include the size of the gradient, temperature, and availability of carrier proteins. Common substances involved are glucose and amino acids.

Active transport uses carrier proteins to actively transport solutes across a membrane, often against a concentration gradient. Factors affecting its rate include the availability of carrier proteins, substrates, and ATP. Common substances involved are Na+, K+, Ca2+, and Mg2+.

Secondary active transport involves carrier proteins that passively transport two solutes, with one (usually Na+) moving down its concentration gradient. The cell must later expend ATP to eject the Na+. It differs from primary active transport, which directly uses ATP to move solutes against their gradient.

Endocytosis is the creation of membranous vesicles containing fluid or solid material. The stimulus and mechanics are incompletely understood, but it requires ATP. It involves fluids and nutrients in all cells, and debris and pathogens in specialized cells.

Exocytosis is the fusion of vesicles containing fluids or solids with the plasma membrane, allowing the contents to be released outside the cell. The stimulus and mechanics are incompletely understood, but it requires ATP. It involves fluids and debris in all cells.

The potency of anesthetics such as chloroform, ether, halothane, and nitrous oxide is directly correlated with their lipid solubility. Higher lipid solubility accelerates the drug's entry into cells and enhances its ability to block ion channels or alter plasma membrane properties, reducing the sensitivity of neurons and muscle cells.

Local anesthetics such as procaine and lidocaine work by blocking sodium channels in the plasma membranes of nerve cells. This blockage reduces or eliminates the responsiveness of these cells to painful stimuli.

Although both procaine and lidocaine are effective local anesthetics, procaine has very low lipid solubility, which affects its potency and mechanism of action compared to lidocaine, which has higher lipid solubility and is more effective in blocking sodium channels.

Carrier-mediated transport involves the binding and transporting of specific molecules by integral proteins in the plasma membrane.

Active transport is a process that consumes ATP and is not dependent on concentration gradients for the movement of substances across the membrane.

Vesicular transport is a mechanism that moves materials such as ligands, fluids, or solids into or out of the cell using membrane vesicles.

Diffusion is a passive transport mechanism that occurs until the concentration gradient is eliminated. It allows substances to move from areas of higher concentration to areas of lower concentration without the need for energy.

Lipid-soluble molecules and dissolved gases can pass through the plasma membrane by simple diffusion due to their ability to move freely across the lipid bilayer.

Channel and carrier proteins are essential for the passive transport of:

1. Small water-soluble molecules/ions - transported through channel proteins.
2. Large molecules - transported through carrier proteins.
   Both types of proteins facilitate movement down their concentration gradients without using energy.