Module 5.2 CELL MEMBRANES-STRUCTURE AND TRANSPORT

Size of molecule:

Smaller molecules tend to travel further before colliding with other molecules so diffusion rates are faster for smaller molecules

GLUT4

is found in skeletal muscle and fat tissue.

Facilitated diffusion can occur in two different ways:

The first is via Channel Proteins. The second type of facilitated diffusion utilizes Carrier Proteins in the membrane.

Concentration gradient:

The greater the difference between the concentrations on the two sides of the membrane the faster the rate of diffusion.

Passive Diffusion

The movement of solutes from an area of higher concentration to an area of lower concentration.

Once the molecules are evenly distributed we say that we have reached a state of

DIFFUSION EQUILIBRIUM, and even though the molecules are still moving, there is no longer any net change in concentration.

Na+,K+-ATPase pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell with the aid of ATP.

ATP energy is required to generate the sodium concentration gradient but is not directly involved in moving the desired solute across the membrane, hence the designation as secondary active transport.

Distance

Diffusion is quite rapid over short distances but gets slower the further it goes. The time it takes for something to diffuse is proportional to the square of the distance. Therefore, if it takes 1 second to diffuse 1 cm, it would take 100 seconds to diffuse 10 cm, and 10,000 seconds to diffuse 100 cm. So to go 100 times further takes 10,000 times longer. In the body diffusion is quite sufficient to cross the thin cell membrane, but to travel long distances by diffusion would be very slow. This is why we have other mechanisms like the blood circulation for moving substances long distances.

carrier proteins

Unlike the channel proteins these carriers do not open to both sides of the membrane simulataneosly. Instead, they bind to a specific solute on one side of the membrane. This binding causes the carrier to change shape, which moves the solute to the other side of the membrane (think of a revolving door). Like the channel proteins, these carriers can be very specific for the solute they transport since the solute must bind to a receptor site that is designed to fit the specific solute.

Solutes that cross the membrane by simple diffusion tend to be

hydrophobic. Examples of substance that cross the membrane by simple diffusion are the gasses CO2 and O2.

Simple Diffusion: Diffusion

is a process that results from the fact that molecules are constantly in a state of random movement.

Endocytosis

is the bulk transport of material into the cell.

In receptor mediated endocytosis

ligands bind to specific receptors which then migrate to a clathrin‐coated pit. The contents are then brought into the cell by a process similar to pinocytosis.

channel proteins.

proteins resemble fluid filled tubes through which the solutes can move down their concentration gradients across the membrane. These channels are often responsible for assisting ions such as Na+, K+, Ca2+ and Cl-cross the membranes. Even though they are open tubes they are often very specific for the ions that pass through them. For instance a K+ channel may allow K+ to pass through but not Na+ or Cl-. -These channels, therefore, are often gated (they have doors or gates that can be opened or closed). -Depending on the channel, these gates may respond to voltage difference across the membrane (voltage-gated channels), specific signal molecules (ligand-gated channels), or even stretching or compressing the membrane (mechanically-gated channels).????

In the body, if the material in question can pass through the cell membrane without the aid of a membrane protein we refer to the process as

simple diffusion

(voltage-gated channels),

specific signal molecules

If no energy input is required for the transport then we say particles move via a

transport process.

phagocytosis

which means "cell eating". a type endocytosis. - Only a limited number of cells are capable of phagocytosis, specifically cells of the immune system. -In this process the cell sends extensions of its plasma membrane called pseudopodia out and around the particle to be phagocytized. -As these pseudopodia surround the particle they eventually fuse creating a vesicle containing the particle. -This phagosome can then unite with a lysosome inside the cell and the engulfed material can be digested for use within the cell.

To this point the transport processes we have discussed have all been passive processes in which the solute or the water movement has been down a concentration gradient with no input of energy required.

. However, there are times when it is important for the cell to be able to move solutes against their concentration gradient. Just like moving water from the first to the top floor of a high-rise building these processes requires an energy source.

Factors That Affect the Rate of Diffusion: The rate at which the solute diffuses is affected by several factors.

.Concentration gradient, Temperature,Size of molecule, Viscosity of the medium, Membrane permeability, Surface area, and Distance.

symport)

A mechanism of transport across a membrane in which two different molecules move in the same direction.

Another interesting characteristic of these carriers proteins is that they have a maximum rate of transport and can thus become SATURATED if the solute concentration is high enough.

Carriers proteins have a maximum rate of transport and can thus become SATURATED

2 main types of Transport Proteins?

Channel Proteins and Carriers Proteins. -Channel proteins are tube-like and provide opening through which small, dissolve particle especially IONS can diffuse. -Each type of carrier protein has a shape that fits a specific molecule or ion. when the proper molecule binds with the protein it changes shape and moves the molecule across the membrane

Video about Active Transport Mechanisms:: -3 types of facilitated transport: 1.Passive facilitated diffusion 2.Active transport or primary active transport 3.Secondary active transport -All 3, Passive facilitated transpoart, Active transport, and secondary active transport all demonstration specificity, competition, and saturation. -ACTIVE TRANSPORT DEFINITION- a type of mediated transport that requires the use of ATP as an energy source. -can move things against the concentration gradient - it starts with 3 Na+ and an ATP binding to the SODIUM/POTASSIUM PUMP (Na+/K+). -Then the ATP breaks down to form ADP and a Phosphate. this prove the energy to move Na+ across the membrane. Then K+ (2 K+ moves in)from the outside binds to the transport protein, releasing the phosphate group. -SECONDARY ACTIVE TRANSPORT-uses an ion gradient to provide energy for the transport. (Electrochemical Gradient). Uses active transport to move ions out of the cell to establish a concentration gradient. IMP-In secondary active transport there must be at least 2 transport proteins. One for ACTIVE TRANSPORT and the other for PASSIVE FACILITATED DIFFUSION. The use of both transport proteins together makes SECONDARY ACTIVE TRANSPORT. for this process Na+ and Glucose are transported across inside the cell. This is also called Secondary active symport. -concentration gradient from Na+ provides the energy to continue facilitate the GLUCOSE across the lipid bilayer. VIDEO REVIEW NOTES: -ACTIVE TRANSPORT-requires ATP as an energy source. -Moves with or against the gradient -A type of facilitated transport, which means it the rules of SPECIFICITY, COMPETITION< SATURATION. -SECONDARY ACTIVE TRANSPORT- requires two transport proteins -uses active transport to established a concentration gradient. -substances are facilitated across the membrane as a result of that gradient. -It is also a type of facilitated transport which means that it follows the rules of SPECIFICITY, COMPETITION, SATURATION

Facilitated transport -TRANSPORT PROTEINS assist the movement of water-soluble molecules or ions across the plasma membrane. -PRoteins that helps moves substances across the plasma membrane have 3 important characteristics called: ---1.specificity-means that is it only specific to only one molecule or ion ---2.competition-means that other molecules or ions can sometimes compete for the same transport proteins ---3. saturation-refers to the rate that molecules can move across the membrane. when there are too much molecules, the rate of which molecule can cross remain constant.

A second type of endocytosis is PINOCYTOSIS or "cell drinking"

In this process rather than send out pseudopodia the cell membrane simply invaginates, forms a pocket, and engulfs anything in the fluid that is taken into to cell. -Unlike phagocytosis, pinocytosis occurs in most cells of the body. -It is prominent in cells involved in moving large amounts of material across the membrane, like cells of the intestines and the kidneys.

One of the most important active transport systems is the

Na+,K+-ATPase. -This system moves sodium out of the cell and moves potassium into the cell. Each cycle of the pump moves 3 sodium ions out and 2 potassium ions into the cell. -Potassium is the primary intracellular cation in the body while sodium is the primary extracellular cation and Na+,K+-ATPase is responsible for maintaining this distribution.

A much more efficient mechanism for bringing specific solutes into the cell is

RECEPTOR MEDIATED ENDOCYTOSIS. -As the name implies, this mechanism employs specific receptors that bind to the material (ligand) to be brought into the cell. -Once the material binds, the receptor‐ligand complex migrates to a specific area of the membrane, a clathrin‐coated pit, which is then brought into the cell by a process similar to pinocytosis. IMP-The advantage of receptor‐mediated endocytosis is that it can engulf large amounts of a specific solute.

Membrane permeability

Since we are talking about movement of solutes into and out of the cell, the permeability of the membrane to the solute will affect how fast solutes diffuse across the cell membrane. For example, ions and other charged molecules that are hydrophilic do not readily cross the membrane due to the hydrophobic core of the bilayer. Conversely, oxygen and carbon dioxide, both non-polar molecules, can readily diffuse through the membrane.

Surface area:

The greater the surface area of the membrane the faster the rate of diffusion across the membrane. In our bodies, areas where diffusion is especially important typically employ structural modifications that increase the available surface area. For example, in the lungs the hundreds of millions of small alveoli have a total surface area of nearly 70 square meters for gas exchange! Approximately the same size as a typical 2 bedroom apartment in Rexburg.

Temperature:

The higher the temperature the faster molecules move so as temperature increases the rate of diffusion increases.

Viscosity of the medium

The viscosity is a measure of the "thickness" of the solvent. An increase in viscosity decreases the rate of diffusion.

GLUT2

Their distribution and specificity varies. For example, GLUT2 is found in the liver and pancreatic islets.

Recall that embedded in the cell membrane are several types of proteins that pass completely through the membrane, the integral membrane proteins. There are a number of specialized integral proteins that assist in the diffusion of solutes across the membrane.

This type of diffusion is referred to as facilitated diffusion

On the other hand, if the process requires cellular energy, usually in the form of ATP, then it is an

active transport process

Processes that require energy are called active transport processes.

active transport processes.

(antiport),

against its concentration gradient.

Primary active transport

can move solutes such as ions against their concentration gradient. -However, in this case the carrier has a site for the binding of ATP, which pro-vides the energy to move the solute against its gradient. - These transport systems can move one or multiple ions across the membrane.

This process of gradual movement from where they are more concentrated to where they are less concentrated is called

diffusion. We refer to the concentration difference as the concentration gradient.

As mentioned above, one of the characteristics of carrier proteins is that they can become

saturated. One of the symptoms of uncontrolled diabetes is the presence of glucose in the urine. This is due to the fact that so much glucose is entering the kidney tubules that the transporters that normally move the glucose back into the blood become saturated and the excess glucose ends up in the urine.