BIOMG 1350 Lecture 9 - Transport Across Cell Membranes

Membrane Transport

Large polar molecules and ions cross through the cell membrane via transport proteins as they are unable to pass through normally. 

Membrane transport proteins fall into two distinct classes: channels and transporters (transporters include active and passive transport proteins).

Transport across cell membranes can result in an osmotic imbalance that will cause cells to swell. Different cells have different mechanisms for dealing with osmolarity.

Both the plasma membrane and the membranes of organelles have their own sets of channels and transporters.

Two Distinct Classes of Membrane Transport

Membrane transport proteins fall into two distinct classes: channels and transporters (transporters include active and passive transport proteins).

• Channels discriminate based on size and charge and allow many ions to pass quickly when they are open or if they are open at a given time. They discriminate based on various characteristics.

• Transporters discriminate based on direct binding specificity and transports only one or a few molecules at a time. So they are more selective but slower at transporting ions/small molecules compared to channel proteins.

Channel Protein (Type of Transport Protein)

Channel proteins are a type of transport protein that provide a hydrophilic passageway (pore) for molecules to move across the lipid bilayer.

• Channel proteins allow specific molecules to diffuse across the cell membrane through passive transport only.

• They are usually constructed of alpha helixes that extend through the cell membrane (transmembrane). The alpha helixes have nonpolar or hydrophobic side chains sticking out that interact well with the bilayer's fatty acid tails.

• Many channels open and close depending on various inputs that are able to change channel conformation. The inner helix can compress so that the channel pore is no longer present and passive transport doesn’t occur.

• Gated channels respond to different types of stimuli.

• Channels are much faster at transporting molecules across the bilayer than transporters/carrier proteins.

Passive Transport

Passive transport does not require additional energy as molecules move down or with the concentration gradient. This means that they move from high to low concentration which can happen naturally without energy input.

Within the concentration gradient molecules pass from one side to another.

This is only true for uncharged molecules. Uncharged molecules only rely on the concentration (chemical) gradient rather than the electrochemical gradient like ions.

The electrochemical gradient for ions or charged molecules will determine which direction the molecule gets transported in.

Passive transporters are completely at the whim of the already established electrochemical or concentration gradient present in the areas outside of the cell membrane.

Against Concentration Gradient

Moving against the concentration gradient means that a molecule is transported from an area of lower concentration to an area of higher concentration, which is the opposite of natural diffusion. Requires a transport protein through either active transport. Always requires ATP. Channel proteins cannot make a molecule go against the concentration gradient as they do not use energy.

With the Concentration Gradient

Moving with the concentration gradient means that a molecule travels from an area of higher concentration to an area of lower concentration. This movement is energetically favorable and occurs naturally. Does not require energy. Can occur: Directly through the lipid bilayer (small nonpolar molecules like O₂, CO₂). Via transport proteins or diffusion through the bilayer, does not require ATP.

Active Transport

Active transport requires additional energy to actively transport molecules up or against their concentration gradient. This means they can move from low to high concentration given enough energy is inputted into the transport protein.

Low to high concentration is true only for uncharged molecules. Uncharged molecules only rely on the concentration (chemical) gradient rather than the electrochemical gradient like ions. Ions rely on both the chemical and electrical gradient and thus are pushed up their electrochemical gradient in active transport.

Some active transport proteins use the same "energy coupling" principle that we discussed for enzyme catalyzed coupled reactions.

Gradient driven pump (symport or antiport), ATP driven pump, and light driven pump are the 3 main types of transport proteins/carrier proteins involved in active transport.

Types of Active Transporter Proteins Move Molecules Against the Concentration Gradient

There are many different types of ways active transporter proteins or pumps can move molecules or ions against their electrochemical or concentration gradient.

• One is the gradient driven pump which couples an unfavorable transport of one molecule/ion with the favorable transport of another molecule/ion.

• Another is a ATP driven pump which hydrolyzes ATP into Pi and ADP such that it can drive the unfavorable transport of a molecule against the concentration gradient.

• Finally, a light driven pump uses light to power the unfavorable transport of a molecule/ion against its concentration gradient. Light provides the energy for this to occur rather than anything else.

Amphipathic

A molecule that has both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region.

Membrane Sidedness

The plasma membrane has distinct inside (cytoplasmic) and outside (extracellular) faces, with different lipid and protein compositions on each side.

Lipid composition can differ between the two layers of the bilayer. Proteins have directional orientation; their cytoplasmic and extracellular regions are distinct. This asymmetry is established during synthesis in the endoplasmic reticulum (ER) and Golgi apparatus. Important for functions like signal reception, cell recognition, and transport.

Selective Permeability

Selective permeability is the property of biological membranes that allows some substances to cross more easily than others. This results from the structure of the lipid bilayer and the presence of specific transport proteins.

It enables cells to regulate what enters and leaves, maintain internal conditions, and support essential functions like nutrient uptake, waste removal, ion balance, and cell signaling. Without selective permeability, cells couldn’t control their internal environment or sustain life.

Things the cell needs: sugars, amino acids, O2, Na, K, Ca, and Cl to name a few.

Things the cell expells: Waste products and CO2 to name a few.

Aquaporins

Aquaporins are specialized channel proteins in the cell membrane that facilitate rapid, selective transport of water molecules across the lipid bilayer. Water is small and polar, so it can slowly diffuse through the bilayer on its own, but diffusion is too slow for most physiological needs. Aquaporins allow water to move quickly along its concentration gradient (passive transport).

Sodium Potassium Pump

An example of an essential and famous pump that uses active transport is the sodium potassium pump. It is an antiport.

• The Na+ and K+ ATPase is an ATP driven pump.

• This pump utilizes about 30% of the total ATP hydrolysis in the animal which demonstrates the essential nature of the pump for all animals in maintaining a proper cell membrane difference on either side.

• In the sodium potassium pump, there are 3 binding sites for Na+ and 2 binding sites for K+.

• The sodium potassium pump transports Na+ up the concentration gradient and K+ also up its concentration gradient.

• A complete cycle takes about 10ms.

• The sodium potassium pump is a kinase and phosphatase which phosphorylates itself so that it can perform its function and dephosphorylates itself to change its conformation back to normal.

• The sodium potassium pump is neutrally open to the cytosol and then when changed by the phosphorylation of the protein, it can be briefly open to the outside of the cell.

• Hydrolysis of one ATP transfers 3 Na+ to the outside and 2K+ to the inside of the cell per cycle. This also creates a generally negative change interior of the cell membrane.

How the Sodium Potassium Pump Functions

Sodium Potassium pump function in the following steps.

• 3 Na+ ions insert themselves into the pump which is default open to the cytosol of the cell.

• The pump then phosphorylates itself and hydrolyzes ATP into Pi and ADP. The ADP is released while the Pi remains attached to the protein.

• Phosphorylation triggers conformational change and the Na+ is ejected from the pump.

• The pump dephosphorylates itself releasing the Pi after 2 K+ ions bind to the pump which was open to the extracellular side after the conformational change.

• Finally the pump returns to its original conformation and K+ is ejected with the dephosphorylation.

Results of the Sodium Potassium Pump

3 Na+ is transfered to the extracellular space for every 2 K+ ions that are transfered into the cell. This creates a generally negative interior of the cell membrane due to a higher number of positively charged cations being present outside of the cell. Contributes to the membrane potential of the cell and is crucial in neurons.

Electrochemical Gradient and Ion Transport

For charged molecules the electrochemical gradient determines the direction of passive transport.

The electrochemical gradient is a combination of the voltage (electrical) gradient and the concentration (chemical) gradient.

The chemical and electrical gradient don't always point in the same direction. They can point in different directions depending on the charge on either side of the membrane and the concentration of ions on either side of the membrane as well.

If the chemical gradient is pointing in an opposite direction as the electrochemical gradient, the molecule passively travels to whichever side has the stronger force. Active transport ignores this entirely.

For an uncharged molecule, the membrane potential and its strength is irrelevant: The electrochemical gradient is equal to the concentration gradient all the time. For a charged particle or ion, the electrochemical gradient (formed from 2 seperate gradients) matters rather than just the concentration gradient.

K+ Leak Channels Function

K+ leak channels help generate the membrane potential. The Na+-K+ pump generates high Na+ outside the cell and high K+ inside the cell. The K+ channel allows K+ to move down its electrochemical gradient out of the cell. This movement of charge generates a voltage across the membrane (the ‘membrane potential’) and so K+ will move until its electrochemical gradient is zero (ie the concentration gradient balances the voltage gradient).

Equilibrium (Biological Context)

Equilibrium is the state where there is no net movement of a substance across a cell membrane because the forces driving movement in opposite directions are balanced. Molecules and ions can still move across the membrane, but they move at equal rates in both directions, so there is no overall change in concentration.

Types of Gated Channels

• Voltage gated channels respond to voltage changes on the cell membrane to open or close accordingly.

• Ligand gated channels (either affected by extracellular ligands or intracellular ligands) open when a ligand binds to its structure.

• Stress gated channels open in response to physical or environmental stress applied to them. An example of this are the proteins in the cell membranes in your ear which respond to sound as the "stress."

• Note: Not all channels are gated. Aquaporins, for example, are usually ungated.

Transport or Carrier Protein

Passive transport by transporters moves molecules down or with their electrochemical or concentration gradient.

Some transporters mediate active while others mediate passive transport.

Passive transporters permit the flow of a solute, but play no role in determining the direction of the flow as they only engage in passive transport.

Transporters only transport molecules that fit into a binding pocket. They transport single or small numbers of molecules based on a conformational change, so they are more limited in the number of

Gradient driven transport is driven by the flow of a molecule down its electrochemical gradient.molecules they can transport. They are like a ‘turnstile’.

Can be active or passive transport. Symport, antiport, and uniports are examples of transport proteins. Symport or antiport are active if at least one of the molecules it transports is moving up the electrochemical gradient and it is passive only if both of the molecules are moving down their electrochemical gradient.

Gradient driven transport is driven by the flow of a molecule down its electrochemical gradient.

Symport

A symport co transports an ion with another molecule that is favorably transported both in the same direction. A symport is usually an example of active transport but can be passive.

Antiport

A antiport co transports an ion with another molecule that is unfavorably transported in opposite directions. A symport is usually an example of active transport but can be passive.

Uniport

Passively transports one molecule in a single direction

Glucose Transport Protein (Passive Transport)

The glucose transporter is one example of a passive transporter protein.

• Transport via the glucose transporter requires the conformational change of the transporter.

• It contains a glucose binding site that is present within the glucose transporter.

• The glucose transporter transports glucose in whatever direction of the concentration gradient whereas the transporter itself controls the specificity of what molecule it transports.

• Passive transport and it is a type of uniport. Not to be confused with the glucose Na+ symport.

Why did cells evolve transporters, rather than just channels, for molecules like glucose?

Because transporters can be highly specific as regular channel proteins also allow other small molecules to be transported through itself. A cell needs specific molecules at specific times and thus transporters only allow a certain molecule in, helpful for the cell's function. 

Simple and Facilitated Diffusion

The passive movement of molecules from an area of higher concentration to lower concentration due to their natural kinetic energy. Diffusion continues until particles reach dynamic equilibrium (evenly distributed but still moving).

Key Features: No ATP required. Driven by the concentration gradient. Occurs in gases, liquids, and across membranes.Rate increases with higher temperature, steeper gradient, shorter distance, and greater surface area.

Simple diffusion does not require transport proteins like channel or carrier proteins, but facilitated diffusion does. Only difference between diffusion and facilitated diffusion is that one uses the help of a transmembrane protein to speed up the diffusion process.

Cotransport

Cotransport is a type of membrane transport in which two substances are moved across a membrane simultaneously by a single transport protein, with at least one of them moving down its electrochemical gradient. The energy from the gradient drives the transport of the other substance against its gradient.

Key Features:

Also called secondary active transport because it relies on an existing ion gradient (usually created by a primary active transport pump like the Na⁺/K⁺ pump).

Types of cotransport is Symport: both substances move in the same direction across the membrane. Example: Na⁺/glucose symporter in intestinal cells.

Antiport (exchanger): substances move in opposite directions. Example: Na⁺/Ca²⁺ exchanger in heart cells.

No direct ATP usage: Energy comes from the ion gradient, not directly from ATP.

Glucose Coupled Active Transport (via Symport)

Transport of glucose up its concentration gradient is driven by Na+ moving down its concentration gradient. Essentially a favorable transport is moved by a unfavorable transport.

However, both molecules in the glucose coupled active transport are moved into the cell which is an example of a symport.

The glucose, Na+ symport works because there is extra Na+ outside of the cell from the sodium potassium pump. This particular transporter transports both glucose and Na+ from the outside to the inside of the cell only.

The glucose sodium symport pump changes conformation randomly but glucose and Na+ only bind strongly when both are bound at the same time (cooperative binding).

Process of Glucose Uptake from the Gut Lumen and into the Bloodstream or Extracellular Space

Three different transporters are involved in glucose uptake from the gut lumen where food is digested and glucose isolated

• Glucose is drawn in from the gut lumen

• The first is a symport that draws both Na+ and glucose into the epithelial cells. The glucose is transported against its electrochemical gradient via coupling to the transport of Na+ with the electrochemical gradient. This is a form of active transport.

• The glucose uniport or type of passive transport transporter/carrier protein which transports the glucose that is inside the epithelial cells lining the intestine out to the extracellular fluid or the bloodstream.

• Finally, the sodium potassium pump utilizes active transport to keep the concentration of potassium inside the epithelial cells high while the concentration of sodium on the outside of the epithelial cells are high. This way, the symport can effectively transport glucose by coupling it to sodium that is moving down its concentration gradient as there is less sodium inside of the intestinal cell than outside.

Membrane Proteins Role in the Production of ATP

• The mitochondria is the site of ATP production and contains and inner and outer membrane along with an intermembrane space. It also has the matrix formed by the folded inner membrane cristae.

• Three H+ pumps generate an electrochemical gradient of H+ across the inner membrane of the mitochondria.

• Specifically, H+ pumps are transmembrane proteins embedded within the inner mitochondrial membrane and pump H+ ions outside of the inside of the inner membrane into the intermembrane space. At which point the H+ ions travel back through the inner membrane into the inside of the inner membrane via ATP synthase proteins which combine ADP and Pi into ATP through a rotation movement.

• Three H+ pumps use energy from the electron transport chain that travels through these proteins to pump H+ from the matrix to the intermembrane space of the mitochondria.

• Using respiration (oxidative phosphorylation), 1 molecule of glucose can be converted or used to power the production of 30 to 32 molecules of ATP.

Electron Transport Chain in the Production of ATP

• Electron transport releases energy in a controlled manner to power H+ pumps. NADH being hydrolyzes into NAD which releases 2 electrons that can be fed through the electron transport chain which hits the 3 H+ proton pumps in the inner mitochondrial membrane. The remaining lower energy electrons are then combined with 1/2O2 to form H2O.

• NADH is formed from the citric acid cycle or Krebs cycle when glucose is metabolized. CO2 is a major output of these reactions.