Cell Transport Cells need to obtain water, nutrients, and other chemicals from the fluids that surround them. Water and other particles also move out of cells. Osmosis (for water only), diffusion (for other particles), and active transport (for other particles) are important processes that allow for the movement of substances across the plasma membrane of cells. In addition, diffusion is an important mechanism of the movement of particles between and within many-body fluid compartments. For example, diffusion is the mechanism by which particles like oxygen and glucose (if in fenestrated or continuous capillaries) travel between interstitial fluid and blood plasma. Osmosis is important in the movement of water into and out of the GI tract, the urinary system, and capillaries throughout the body. Vocabulary ICF: intracellular fluid (inside the cell) ECF: extracellular fluid (outside the cell) Osmosis: diffusion of water through a semipermeable membrane Simple diffusion: movement of particles from high to low concentration through a cell membrane Facilitated diffusion: movement of particles from high to low concentration through the cell membrane using a carrier or channel protein Selectively permeable: uses active biochemical transport, diffusion, and osmosis to choose which particles will cross (ex: cell membrane) Semi-permeable: permeability is based on the size of particles (ex: dialysis tubing) Permeant solutes: solutes are able to cross plasma membrane (can reach equilibrium between ICF and ECF, so they have NO effect on the eventual volume of the cell and no relevance for measuring tonicity) Impermeant solute: solutes are NOT able to cross plasma membrane (cannot reach equilibrium between ICF and ECF, so they WILL have an effect on the eventual volume of the cell) Crenate: shrinking of cell in response to fluid exiting the cell through osmosis Lysis: bursting open of cell in response to too much fluid entering the cell through osmosis (term hemolysis is used for RBCs) Tonicity: the ability of solutions to pull water across the membrane; determined by the concentration of impermeant solutes only Isotonic solution: solution outside the cell has the same concentration of impermeant solutes as the ICF; causes no net movement of water between ECF and ICF Hypotonic solution: solution outside the cell has a lower concentration of impermeant solutes than the ICF; causes water to move from ECF to ICF Hypertonic solution: solution outside the cell has a higher concentration of impermeant solutes than the ICF; causes water to move from the ICF to the ECF. Transport in Human Physiology One of the major roles of the cell membrane is to regulate the movement of substances into and out of the cell itself. There are certain chemicals that can move through membranes and other chemicals that cannot move through membranes. Details on which substances can/cannot move through the membrane will be discussed later in this reading. The cell has no direct control over things that can move through its membrane. The substances that can go through the membrane use simple diffusion- moving freely through the cell membrane from high to low concentration (or down its concentration gradient). You can think of these substances as living the "simple life" being able to get in/out of the cell whenever they want, without help. So, what about the substances that cannot move through a membrane by simple diffusion? They need "help" getting in/out of the cell. These "helpers" come in the form of carriers or channel proteins. Of the substances that need help, we have those that are moving with their concentration gradient and those moving against their concentration gradient. If something is moving with its concentration gradient, it only needs a little help getting across the membrane. This type of transport is called facilitated diffusion. ("facilitated" meaning making a process easy/easier and "diffusion" because it is moving with its concentration gradient). If the substance is moving against its concentration gradient (this act is often called "pumping"), it needs a little more help than those using facilitated diffusion. These substances will use active transport which relies on the use of carrier proteins. There are 2 types of active transport discussed below. Now you should remember/understand the differences and similarities between osmosis and simple and facilitated diffusion. In order to really understand active transport and how that differs from diffusion and osmosis, you will need to pay special attention to the following categories: Things that CanNOT move on their own through the cell membrane: These substances are generally hydrophilic and/or too large to slip between the phospholipid bilayer. This list includes proteins/peptides (and amino acids), carbohydrates (and simple sugars), nucleic acids (and nucleotides), all ions (such as Na+, K+, Ca2+, Cl-, H+, etc.), and triglycerides (even though these are lipids they are way too big to cross the membrane). Note: In certain circumstances, such as high temperature (which increases the rate of diffusion), very small substances such as simple sugars like glucose and fructose (and also very small amino acids) can actually slip through the cell membranes in small numbers (between gaps between the phospholipids as they move). However, the majority of these substances do not move through human cell membranes on their own, so this small amount of simple diffusion does not usually have any significant contribution to the movement of these particular substances. Things that CAN move on their own through the cell membrane: These substances are generally hydrophobic and/or small enough to slip between the phospholipids. This list includes gasses (such as O2 and CO2), steroids, carotenoids (such as vitamin A), eicosanoids (such as thromboxanes), alcohols (such as ethanol and propanol). Generally speaking, the larger an alcohol molecule is, the more lipid-soluble it is and can "slip" through the cell membrane easier), and H2O (this is the weird one since it is clearly NOT hydrophobic, but it is very small. It makes up 2/3 of every cell so there is a LOT of it). Some water crosses the cell membrane by simple diffusion, but the majority of water diffuses across cell membranes through aquaporins (water channels) and ion channels. The 2 Types of Active Transport
How Primary & Secondary Active Transports Work Together Voltage is the separation of charges between two different locations. Current is what happens when charged particles move. We use current to power lights, heaters, etc. In cells, charges are created by the ions that reside inside/outside of the cell. Ions, no matter how small they are cannot move through a cell's membrane without the help of a transport protein due to their +/- charge. Voltage is a measurement of the differences in the amounts/types of charged particles between two locations. Your cell's membrane effectively separates charges into two very distinct locations: the intracellular (intra- means "within") compartment (full of intracellular fluid - ICF) and the extracellular (extra - means "outside of or not part of") compartment (full of extracellular fluid - ECF).
Primary active transporters use the primary energy source, ATP, to move ions against their gradients. Why? Because they are moving charges in a way to create a charge separation across the membrane = voltage. Think of the Na+/K+ pump. It forces 3 Na+ out and 2 K+ in (using 1 ATP to do it), and over time it becomes more positive (+) outside compared to inside the cell (which is, in comparison negative because it is full of things like DNA and RNA, which have negatively charged phosphate groups). Cells use these primary active transporters to make the voltage across the membrane. The voltage across cell membranes is called the membrane potential (Vm). The Vm has the potential to do work, hence the term "membrane potential".
Secondary active transporters USE this voltage difference between the ECF and ICF to create a current (which is defined as moving charges). An example of this type of active transport is the Na+/glucose symporter. This specific transporter uses the current of Na+ moving down its gradient to power glucose being forced against its gradient. The current from Na+ moving is used as the energy source to pump glucose against its concentration gradient! Therefore, secondary active transporters use a secondary form of energy: voltage, the concentration gradient of ions across the membrane - membrane potential (Vm). However, they indirectly use ATP because without the primary active transporters (which use ATP) there would be NO energy (voltage: the separation of charges across the membrane) for secondary active transporters to use! ~Note: Ions are atoms/molecules that have gained or lost electrons to stabilize their valence shells, so they have chemical concentration gradients. But ions also have charges, and physical laws dictate that like charges (+ and + or - and -) repel each other, and opposite charges (+ and -) attract each other. Therefore, ions also have another force, the electrical gradient force, that acts upon them. The result of both the chemical gradient and electrical gradient acting upon an ion is the resulting force called the electrochemical gradient. You will really need to understand the electrochemical gradient and its effect upon ions in order to understand not only transport but also to understand excitable cells types: neurons and muscles. We will learn more about this in Lab 4: Nerve Cells & Electrical Signaling. Differences in Membrane Proteins Channels, carriers, and pumps are all membrane proteins that assist molecules across the membrane. Let's look at the differences between them. Channels are small passageways for molecules that cannot (or slowly) pass through the lipid bilayer due to being hydrophilic or charged. Channels do not interact with the molecules that pass through them; they only create an opening conducive for them to pass. This means molecules can only travel down/with their concentration gradient through channels and do not use energy (diffusion through a channel). Molecules that use channels are polar, hydrophilic molecules (ex: Na+, K+, Ca2+). Channels can be open all the time (leak channels) or closed until opened by a certain condition (ligand-gated channels, voltage-gated channels, etc.) Carriers do interact with the molecules that travel through them. Molecules that pass through the membrane via carriers can travel down their concentration gradient (facilitated diffusion) or against their concentration gradients (active transport). When molecules use carriers to cross the membrane, they bind to the carrier and the carrier will then change its shape to allow the molecule across. Since molecules must bind to carriers, this type of transport will limit the rate of molecules passing through the membrane. Once carriers are saturated, molecules have to "wait" for a free carrier on which it can bind which makes this process slower than simple diffusion or facilitated diffusion using channels. Molecules will only use carriers for facilitated diffusion when simple diffusion and channels will not work for them. For example, things like amino acids and glucose commonly use carriers because these molecules are too big to use channels. When a carrier protein is used to "pump" molecules against their concentration gradients, we call it a pump. When you "pump" something (water pump, pumping iron) you have to use energy. We use pumps to maintain concentration gradients (like the Na+/K+ pump mentioned earlier). Membrane Protein Direction of Molecules Type of Transport Channel only down/with concentration gradient diffusion through a channel Carrier down or against concentration gradient facilitated diffusion or active transport Pump against concentration gradient active transport