Lab 3 – Cell Transport

Cell Transport

Cells rely on the acquisition of water, nutrients, and various chemicals from the surrounding fluids to sustain their vital functions.  Conversely, they also release water and particles into their environment.  Essential to this dynamic exchange are processes such as osmosis (primarily to water), diffusion (for particles other than water), and active transport (for specific particles).  These mechanisms facilitate the movement of substances across the cellular membrane.

Furthermore, diffusion serves as fundamental mechanism for particle movement within and between various bodily fluid compartments.  For instance, oxygen and glucose (particularly in fenestrated or continuous capillaries) utilize diffusion to traverse between interstitial fluid and blood plasma.

Osmosis, specifically, plays a pivotal role in regulating water movement across membranes in vital systems like the gastrointestinal tract, urinary system, and throughout the extensive network of capillaries in the body.  Figure 1 provides a classic laboratory example illustrating osmosis.

Throughout this laboratory session, you will explore the factors influencing diffusion and osmosis while gaining insight into other models of cellular transportation.

Vocabulary

ICF: Intracellular Fluid – refers to the fluid inside the cell.

ECF: Extracellular Fluid – denotes the fluid outside the cell.

Osmosis: The process of water diffusing through a semipermeable membrane.

Simple Diffusion: The movement of particles from areas of high to low concentration across a cell membrane.

Facilitated Diffusion: The transport of particles across the cell membrane from areas of high to low concentration facilitated by carrier or channel proteins.

Selectively Permeable: Characterized by the ability to selectively allow particles to cross using active biochemical transport, diffusion, and osmosis.  An example is the cell membrane.

Semi-permeable: Permeability determined by the size of particles, such as dialysis tubing.

Permeant Solutes: Solutes capable of crossing the plasma membrane, leading to equilibrium between ICF and ECF. They do not affect the eventual volume of the cell and are irrelevant for measuring tonicity.

Impermeant Solutes: Solutes unable to cross the plasma membrane, thus enable to reach equilibrium between ICF and ECF.  They affect the eventual volume of the cell.

Crenate: The process of cell shrinkage due to fluid exiting the cell through osmosis.

Lysis: The bursting open of a cell due to excessive fluid entering the cell through osmosis. (The term “hemolysis” is used specifically for red blood cells.)

Tonicity: The capacity of solutions to draw water across the membrane, determined solely by the concentration of impermeant solutes.

Isotonic Solution: A solution with the same concentration of impermeant solutes as the ICF, resulting in no net movement of water between ECF and ICF.

Hypotonic Solution: A solution with a lower concentration of impermeant solutes than the ICF, leading to the movement of water from ECF to ICF.

Hypertonic Solution: A solution with a high concentration of impermeant solutes than the ICF, causing water to move from the ICF to the ECF.

Transport in Human Physiology

The cell membrane plays a crucial role in regulating the movement of substances into and out of the cell.  Some chemicals can freely pass through membranes, while others are unable to do so.  We’ll delve into the specifics of which substances can or cannot traverse the membrane later in this discussion.  Importantly, the cell lacks direct control over the passage of substances through its membrane.

For substances capable of passing through the membrane unaided, they utilized a process knows as simple diffusion.  This entails the free movement of substances across the cell membrane, following their concentration gradient.  These substances lead what might call a “simple life”, effortlessly entering or exiting the cell as needed, without assistance.

However, substances that cannot traverse the membrane through simple diffusion require assistance.  These helpers come in the form of carrier or channel proteins.  Among these substances, some move along their concentration gradient and others against it.  Those moving with their concentration gradient require minimal assistance to cross the membrane and undergo facilitated diffusion.  The term “facilitated” signifies the easing of a process, while “diffusion” refers to movement along the concentration gradient.

Conversely, substances moving against their concentration gradient, often termed “pumping,” necessitate more assistance than those undergoing facilitated diffusion.  Such substances rely on active transport, which involves carrier proteins.  Below, we’ll discuss the two types of active transport in detail.

To grasp the distinction and similarities between osmosis, simple diffusion, and facilitated diffusion, it’s crucial to focus on specific categories:

Substances Unable to Traverse the Cell Membrane Independently:

These substances are typically hydrophilic or too large to navigate the phospholipid bilayer autonomously.  This category encompasses proteins, peptides, carbohydrates, nucleic acids, ions like Na+, K+, Ca2+, Cl-, H+, and triglycerides.  While it’s noted that under certain conditions, like elevated termpertures which accelerate diffusion, very small molecules such as simple sugars like glucose and fructorse (and also minute amino acids) can pass through cell membranes in small quantitites, they usually don’t significantly contribute to the movement of these particular substances due to their overall rarity.

Substances Capable of Independent Movement Across the Cell Membrane:

These substances tend to be hydrophobic or small enough to maneuver between the phospholipids.  This group includes gases like O2 and CO2, steroids, carotenoids, such as Vitamin A, eiocasonoids like thromboxanes, alcohols such as ethanol and propanol (with larger alcohol molecules being more lipid soluble and thus having an easier time slipping through the cell membrane), and H2O. Though water is notably hydrophilic, its small size allows it to penetrate cell membranes.  While some water molecules cross through simple diffusion, the majority traverse cell membranes via aquaporins (water channels) and ion channels.

The Two Types of Active Transport

1. Primary Active Transport (also known as direct active transport): This mechanism relies on the cell’s primary energy source, ATP, to drive the movement of a substance, typically ions, against their concentration gradient. It earns the term “direct” as ATP is directly utilized at the pump itself.
2. Secondary Active Transport (also known as indirect active transport): This mode of transport utilize a secondary energy source, namely a concentration gradient, to transport substances (usually sugars, amino acids, and vitamins) against their concentration gradient. The term “indirect” arises from its dependence on ATP indirectly; without primary active transporters establishing the concentration gradient, secondary active transport would have no gradient to leverage.

A notable example of primary active transport is the Sodium-Potassium Pump, prominently found in human cell membranes.

So, what purpose does primary/direct active transport serve? It establishes an ion gradient! This seemingly redundant process is crucial as the cell utilizes the ionic gradient generated by primary active transporters to power secondary active transporters.

How Primary and Secondary Active Transports Work Together

Voltage represents the separation of charges between two distinct locations, while current signifies the movement of charged particles.  In practical terms, current powers devices like lights and heaters.

Within cells, charges arise from ions residing inside and outside the cell.  Due to their positive or negative charge, ions, no matter how small, require transport proteins to traverse the cell membrane.  Voltage measures the disparities in types and amounts of charged particles between these locations.  The cell membrane effectively segregates charges into two distinct compartments: the intracellular (ICF) and extracellular (ECF) compartments, each filled with their respective fluids.

Primary active transporters utilize ATP – the primary energy source – to move ions against their gradients.  This process generates charge separation across the cell membrane, known as voltage.  Consider the Na+/K+ pump, which expels 3 Na+ ions while importing 2 K+ ions (requiring 1 ATP).  Over time, this creates a positive exterior compared to the negative interior of the cell, owing to components like DNA and RNA with negatively charged phosphate groups.  Cells employ primary active transporters to establish the membrane potential (Vm), a voltage potential capable of performing work.

Secondary active transporters leverage this voltage difference between the ECF and ICF to generate a current, defined as moving charges.  An example is the Na+/glucose symporter, which utilizes the current of Na+ moving down its gradient to propel glucose against its gradient.  Here, the current from Na+ movement powers the transportation of glucose.  Thus, secondary active transporters utilize a secondary form of energy – voltage, in the form of the concentration gradient of ions across the membrane (membrane potential, Vm).  However, they indirectly rely on ATP, as without primary active transporters, there would be no energy (voltage) for secondary active transporters to exploit.

Note: Understanding the electrochemical gradient is crucial, as it governs not only transport but also the function of excitable cell types like neurons and muscles.  We’ll delve deeper into this concept in Lab 4: Nerve Cells and Electrical Signaling.

Distinguishing Membrane Proteins

Channels, carriers, and pumps – essential membrane proteins – aid the transit of molecules across the membrane.  Let’s explore their unique characteristics:

Channels: These are narrow pathways designed for molecules unable to pass through the lipid bilayer efficiently due to hydrophilicity or charge.  Channels merely create an opening for molecules to pass through without direct interaction.  Consequently, molecules can only move down their concentration gradient through channels, without expending energy (a process known as diffusion through a channel). Polar, hydrophilic molecules like Na+, K+, and Ca2+ commonly utilize channels.  Channels may remain open continually (leak channels) or be activated by specific conditions (ligand-gated channels, voltage-gated channels, etc.).

Carriers: Unlike channels, carriers interact with the molecules they transport.  Molecules may travel down their concentration gradient (facilitated diffusion) or against it (active transport) via carriers.  When molecules traverse the membrane through carriers, they bind to the carrier, prompting it to change shape and facilitate passage.  As molecules must bind to carriers, this mode of transport limits the rate at which molecules cross the membrane.  Once carriers reach saturation, molecules must await available carriers, making this process slower than simple or facilitated diffusion through channels.  Molecules resort to carriers for facilitated diffusion when channels or simple diffusion are impractical.  For instance, large molecules like amino acids and glucose often utilize carriers due to their size.

Pumps: When a carrier proteins transport molecules against their concentration gradients, they are termed pumps.  Just as pumping water or lifting weights requires energy, pumps necessitate energy expenditure.  Pumps are crucial for maintaining concentration gradients, such as the Na+/K+ pump mentioned earlier.

These distinctions highlight the diverse roles and mechanisms employed by membrane proteins in cellular transport.