Chapter 3 BOOK NOTES PART 2

Types of Cellular Transport

Simple Diffusion

Substances diffuse directly through the lipid bilayer.
Small nonpolar molecules like gases (oxygen, carbon dioxide), steroid hormones, and fatty acids can pass through.
Criteria for passage include lipid solubility and size.
Not all substances diffuse equally; some diffuse readily while others do not.
Example: Oxygen and carbon dioxide moving through the plasma membrane.

Facilitated Diffusion

Certain molecules like glucose, sugars, amino acids, and ions are transported passively.
These substances are unable to pass through the lipid bilayer and require assistance.
Two types: carrier-mediated and channel-mediated facilitated diffusion.
Example: Glucose moving down its concentration gradient through carrier proteins.
Transport is limited by the availability of protein carriers.

Osmosis

Movement of water through a selectively permeable membrane.
Essential for determining water distribution in the body's compartments.
Water can pass through the membrane by osmosis due to its small size.
Aquaporins are transmembrane proteins that facilitate water movement.
Example: Water moving through aquaporins in red blood cells.

Mechanisms of Facilitated Diffusion

Carrier-Mediated Facilitated Diffusion

Involves carrier proteins specific for transporting polar molecules or large substances.
Carrier proteins change shape to transport substances across the membrane.
Example: Glucose transport into cells through carrier proteins.
Transport occurs down the concentration gradient, similar to simple diffusion.
Transport is limited by the saturation of carrier proteins.

Channel-Mediated Facilitated Diffusion

Channels are transmembrane proteins that transport substances, usually ions, through aqueous channels.
Selective channels based on pore size and amino acid charges.
Gated channels can be controlled by signals, while leakage channels are always open.
Example: Ions moving through channels according to concentration gradients.
Channels can be inhibited by certain molecules and show saturation.

Osmosis and Its Significance

Osmosis Process

Movement of water through a selectively permeable membrane.
Crucial for maintaining fluid balance in the body's compartments.
Water can pass through aquaporins due to its small size and polarity.
Example: Water movement in kidney tubule cells.
Osmosis occurs when solute concentration differs across a membrane.

Factors Influencing Osmosis

Osmosis depends on the number of solute particles, not their type.
Net movement of water occurs towards higher solute concentration.
Equilibrium is reached when solute concentration is equal on both sides of the membrane.
Example: Movement of water towards higher solute concentration to dilute it.
Osmolarity refers to the total concentration of solute particles in a solution.

Passive Membrane Transport and Osmolarity

Osmosis and Selective Permeability

Osmosis is the movement of water across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.
In a U-shaped glass tube with two different solutions separated by a membrane, water moves down its concentration gradient to equalize osmolarity.
If the membrane is freely permeable to both solute molecules and water, at equilibrium, both sides have the same osmolarity and volume.
In contrast, if the membrane is selectively permeable to water but impermeable to solutes, water moves by osmosis from low to high solute concentration, resulting in equal osmolarity but different volumes.
Real cells mimic this process, with differences based on cell type (plant or animal) and the presence of a cell wall affecting volume changes.

Osmosis in Living Cells

In plant cells, water diffuses until hydrostatic pressure equals osmotic pressure, balancing water movement.
Higher nonpenetrating solutes increase osmotic pressure, requiring greater hydrostatic pressure to resist further water entry.
Animal cells lack rigid cell walls, leading to swelling or shrinking in response to osmotic imbalances.
Tonicity refers to a solution's ability to change cell shape by altering internal water volume.
Isotonic solutions maintain cell shape, hypertonic solutions cause cell shrinkage, and hypotonic solutions lead to cell swelling.
Tonicity is based on solute concentration and permeability, affecting cell volume differently than osmolarity.

Osmolarity and Tonicity Relationship

Osmolarity is solely based on total solute concentration, while tonicity considers the effect on cell volume.
Isotonic solutions have the same nonpenetrating solute concentration as cells, maintaining normal shape.
Hypertonic solutions have higher solute concentration, causing cell shrinkage, while hypotonic solutions lead to cell swelling.
Osmolarity is expressed in osmoles per liter, while tonicity depends on solute concentration and membrane permeability.
Osmolarity and tonicity differ in their considerations of solute permeability and effects on cell volume.

Passive Membrane Transport and Transport Processes

Characteristics of Transport Processes

Transport processes exhibit specificity and saturability.
Processes dependent on transport proteins are saturable due to a limited number of proteins in the membrane.
Transport proteins, like enzymes, show high specificity in binding to specific molecules.
Simple diffusion and osmosis through the plasma membrane are nonspecific and unsaturable processes.
These processes do not rely on proteins and are not saturable, allowing molecules to pass through the lipid barrier.

Passive Membrane Transport Processes

Overview of Passive Membrane Transport

Passive membrane transport processes are not specific to the shape of the molecule and do not involve proteins, making them unsaturable.
Transport rate is solely dependent on the size of the concentration gradient; a larger gradient results in greater movement.
Processes of passive membrane transport include simple diffusion, facilitated diffusion, and osmosis.
Examples of molecules transported through passive processes include lipids, oxygen, carbon dioxide, glucose, sodium ions, potassium ions, and water.
Table 3.2 summarizes the characteristics of passive membrane transport processes.

Passive Membrane Transport Processes

					Process Energy Source Description Membrane Transport Protein Required Specific & Saturable Examples
Simple Diffusion	Kinetic energy	Net movement down the concentration gradient from higher to lower concentration.	No	No	Lipids, oxygen, carbon dioxide
Facilitated Diffusion	Kinetic energy	Similar to simple diffusion but involves a membrane carrier protein or channel protein.	Yes	Yes	Glucose, sodium ions, potassium ions
Osmosis	Kinetic energy	Movement of water through a selectively permeable membrane.	No, except for aquaporins	No, except for aquaporins	Water

Passive Membrane Transport Examples

Table 3.2
Passive Membrane Transport Processes

| Process               | Energy Source  | Description                                                                             | Membrane Transport Protein Required | Specific & Saturable      | Examples                             |
|:--------------------- |:-------------- |:--------------------------------------------------------------------------------------- |:----------------------------------- |:------------------------- |:------------------------------------ |
| Simple Diffusion      | Kinetic energy | Net movement down the concentration gradient from higher to lower concentration.        | No                                  | No                        | Lipids, oxygen, carbon dioxide       |
| Facilitated Diffusion | Kinetic energy | Similar to simple diffusion but involves a membrane carrier protein or channel protein. | Yes                                 | Yes                       | Glucose, sodium ions, potassium ions |
| Osmosis               | Kinetic energy | Movement of water through a selectively permeable membrane.                             | No, except for aquaporins           | No, except for aquaporins | Water                                |

Active Membrane Transport

Active Membrane Transport Overview

Active membrane transport processes require energy to move solutes across the membrane.
Substances moved actively are usually unable to pass through passive transport due to size, polarity, or concentration gradient.
Major means of active membrane transport include active transport and vesicular transport.

Active Transport vs. Vesicular Transport

Active Transport: Requires transport proteins that bind specifically with substances; moves against concentration gradients.
Vesicular Transport: Involves the formation of vesicles to transport large molecules across membranes.

Types of Active Membrane Transport

Primary vs. Secondary Active Transport:

Primary: Directly uses ATP to transport molecules.
- Secondary: Utilizes the energy stored in ionic gradients to transport molecules.

Endocytosis vs. Exocytosis:

Endocytosis: Internalizes substances into the cell via vesicle formation.
- Exocytosis: Expels substances out of the cell via vesicle fusion with the membrane.

Pinocytosis, Phagocytosis, Receptor-mediated Endocytosis:

Pinocytosis: Cell drinking, non-specific uptake of fluids and solutes.
- Phagocytosis: Cell eating, ingestion of large particles or cells.
- Receptor-mediated Endocytosis: Specific uptake of ligands bound to receptors.

Active Transport

Mechanisms of Active Transport

Active transport moves solutes, especially ions, against their concentration gradients, requiring cells to expend energy.
It involves transport proteins that combine specifically and reversibly with substances being transported.
Primary active transport derives energy directly from ATP hydrolysis by pumps, while secondary active transport uses energy stored in ion concentration gradients.
Active transport allows cells to be highly specific in transporting substances that cannot diffuse through the membrane.

Primary Active Transport

In primary active transport, ATP hydrolysis phosphorylates the pump protein, enabling it to move solutes across the membrane.
Examples include calcium and hydrogen pumps, with the sodium-potassium pump being the most crucial.
The sodium-potassium pump, driven by ATP, moves three sodium ions out and two potassium ions in for each ATP molecule used.
This process maintains essential ionic concentration differences crucial for cell function, especially in excitable cells like muscles and neurons.
The pump operates continuously to counteract the leakage of sodium and potassium ions through the membrane.

Secondary Active Transport (Cotransport)

Secondary active transport couples the downhill movement of one solute with the uphill movement of another using a cotransport protein.
The energy for secondary active transport is provided by the concentration gradient created by primary active transport, such as the sodium-potassium pump.
As sodium moves back into the cell through the pump, other substances are cotransported along, driven by their concentration gradients.
This process is crucial for transporting sugars, amino acids, and ions into cells, like those lining the small intestine.
The concentration gradient of ions, particularly sodium, is essential for secondary active transport and cell function.

Sodium-Potassium Pump

Sodium ions (Na+) are more concentrated outside the cell, while potassium ions (K+) are more concentrated inside the cell.
Primary active transport is driven by ATP, where the sodium-potassium pump creates a concentration gradient for Na+ entry into the cell.
The pump expels Na+ out of the cell and pumps K+ back into the cell.
Secondary active transport involves Na+ diffusing back across the membrane through a cotransporter protein, driving glucose against its concentration gradient into the cell.
Two types of secondary active transport are symport (substances move in the same direction) and antiport (substances move in opposite directions).

Sodium-Potassium Pump

Function and Process

The sodium-potassium pump moves solutes across cell membranes against their electrochemical gradients using energy from ATP.
It involves six steps: binding of sodium ions, ATP hydrolysis, phosphorylation of the pump, shape change pushing sodium out, binding of potassium ions, and release of phosphate to resume the original shape.
This cycle repeats to push sodium out and pump potassium back into the cell.
The pump maintains essential ionic concentration differences for cell function, especially in excitable cells.
Ions diffuse according to electrochemical gradients, considering both electrical and concentration forces.

Importance and Examples

The sodium-potassium pump is crucial for maintaining normal cell function, especially in excitable cells like muscles and neurons.
It underlies most secondary active transport processes by creating and maintaining ion concentration gradients.
The pump is an example of primary active transport that directly uses ATP to transport ions against their gradients.
The pump continuously operates to counteract ion leakage through the membrane.
The pump's action is essential for normal fluid volume maintenance in body cells.

Secondary Active Transport

Cotransport Process

Secondary active transport uses a cotransport protein to move one solute downhill and another uphill, driven by the energy stored in ion concentration gradients.
The sodium-potassium pump creates the concentration gradient that provides energy for secondary active transport.
As sodium moves back into the cell through the pump, other substances are cotransported along, utilizing their concentration gradients.
This process is vital for transporting various substances, including sugars, amino acids, and ions, into cells.
The concentration gradient of ions, particularly sodium, plays a crucial role in secondary active transport and cell function.

Examples and Significance

Secondary active transport is essential for transporting nutrients and ions into cells, such as those in the small intestine.
It allows substances to be transported against their concentration gradients by utilizing the energy stored in ion gradients.
The process of cotransport ensures efficient uptake of essential molecules for cellular functions.
The sodium-potassium pump's role in creating and maintaining ion gradients is fundamental for secondary active transport.
Understanding secondary active transport mechanisms is crucial for comprehending nutrient absorption and cell function.

Vesicular Transport

Overview of Vesicular Transport

Vesicular transport uses cellular energy to move large substances or large amounts of a substance across cellular membranes in vesicles.
It includes endocytosis (moving substances into the cell) and exocytosis (moving substances out of the cell).
Vesicular transport processes are energized by ATP or GTP.
Transcytosis moves substances into, across, and out of the cell, common in endothelial cells.
Vesicular trafficking moves substances between different areas or organelles within the cell.

Endocytosis

Endocytosis uses vesicles to bring bulk solids, macromolecules, and fluids into the cell.
It begins with a coated pit on the cell's plasma membrane, which forms a vesicle to transport substances.
Three possible fates for vesicles include fusion with lysosomes for digestion, delivery to the opposite side of the cell (transcytosis), or recycling back to the plasma membrane.
Types of endocytosis include phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.
Phagocytosis involves engulfing large particles like bacteria, while pinocytosis surrounds small volumes of extracellular fluid.
Receptor-mediated endocytosis is a selective mechanism for specific uptake of macromolecules by cells.

Exocytosis

Exocytosis ejects substances from the cell into the extracellular fluid.
It is triggered by cell-surface signals like hormone binding or changes in membrane voltage.
Exocytosis is responsible for hormone secretion, neurotransmitter release, mucus secretion, and waste removal.
Substances to be removed are enclosed in secretory vesicles that fuse with the plasma membrane and release their contents.
The process involves vesicle migration, fusion with the plasma membrane, and release of vesicle contents.