5.2 Passive Transport

By the end of this section, you will be able to explain why and how passive transport occurs.

Some substances can pass through, but not others.
The cell would no longer be able to sustain itself if they lost this selectivity.
Some cells need more substances.
They need a way of getting the materials from the fluids.
As certain materials move back and forth, or as the cell has special mechanisms that facilitate transport, this may happen passive.
Some materials are so important to a cell that it spends some of its energy to get them.
Red blood cells use some of their energy.

Most cells spend the majority of their energy to maintain an ion balance between the inside and outside of the cell.

Passive forms of transport are the most direct.

Substances move from an area of higher concentration to an area of lower concentration in passive transport.

The interior of a plasm membranes is not the same as the exterior.

There is a significant difference between the array of phospholipids and the array of proteins between the two leaflets.
Some proteins anchor the membrane to the cytoskeleton's fibers.
The outside of the cell's exterior is home to peripheral proteins that bind the matrix elements.
Carbohydrates are on the exterior surface of the cell.
The cell can bind required substances in the fluid.
The addition of this adds to the nature of the membrane.

The exterior surface of the plasma is not the same as the interior surface.

They have both hydrophilic and hydrophobic regions.

The characteristic helps move some materials through the system.
A material with a low weight can easily slip through the core.
Substances such as the fat-soluble vitamins A, D, E, and K can be found in the body.
Fat-soluble drugs and hormones are easy to get into the body's tissues and organs.

Some polar molecules can connect with the cell's outside, but they can't pass through the core of the cell.

Small ion can easily slip through the spaces in the mosaic, but their charge prevents them from doing so.
Ions such as sodium, potassium, calcium, and chloride must have special ways of penetrating.
Simple sugars and amino acids need the help of various transmembrane proteins to move across the plasma membranes.

When the concentration is equal across a space, a single substance moves from a high concentration to a low concentration area.

You are familiar with air movement.
Imagine a person opening a bottle of ammonia in a room filled with people.
The bottle has the highest concentration of ammonia.
The room's edges have the lowest concentration.
Gradually, more people will smell the ammonia as it diffuses from the bottle.
The materials move through the cell's cytosol by diffusion.
Diffusion doesn't have any energy.
Concentration gradients are a form of potential energy, which can be dissipated as the gradient is eliminated.

A substance from a high concentration area is moved down its concentration gradient into the cytoplasm.

The concentration of different substances in the same medium has their own concentration gradients.

Each substance will diffuse according to the gradient.
There will be different rates of dispersal of different substances within a system.

Molecules move at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature.

Molecules are dispersed through whatever medium they are in.
A substance moves through space until it evenly distributes itself.
There will be no net movement of the number of Molecules from one area to another after a substance has diffused through a space.
The substance has no net movement dynamic equilibrium because of the lack of a concentration gradient.
In the presence of a substance's concentration, several factors affect the rate of diffusion.

The slower the diffusion rate is, the closer the distribution of the material gets to equilibrium.

The heavier the molecule, the slower it diffuses.

It's true for lighter molecules.

The temperature increases the energy and the movement of the molecule.

Lower temperatures decrease the energy of the molecule.

diffusion increases if the medium is less dense.

The increase in the density of the cytoplasm will affect the movement of the materials.
A person is experiencing dehydration.
The cells' functions get worse when the body's cells lose water.
Neurons are very sensitive to this effect.
Dehydration can lead to unconsciousness and possibly coma because of the decrease in the cell's diffusion rate.

A faster diffusion rate can be achieved by using nonpolar or lipid-soluble materials.

Increased surface area increases the rate of diffusion.

The slower the diffusion rate is, the greater the distance that a substance must travel.

This limits the size of the cell.
A large spherical cell will die because it can't leave its center.
In the case of prokaryotes, cells must either be small in size or flattened.

The process of filtration is a variation of diffusion.

Material moves according to its concentration through a separator.
Pressure can cause the substances to filter more quickly.
Pressure is almost completely dependent on the diffusion rate.
One of the effects of high blood pressure is the appearance of a substance in the urine.

There is a concentration that allows these materials to diffuse into the cell.

The materials repel the cell's parts.
The materials can diffuse into the cell with the help of the transport proteins.

The transported material can attach to the exterior surface of the cell.

The material that the cell needs can be removed.
The substances are able to pass through specific integral proteins.
Some of the integral proteins are collections of sheets that form a channel through thelipid bilayer.
Others are carriers that help the substance spread through the membranes.

Both of them are transmembrane proteins.

There are specific channels for the substance.

They have a channel through their core that provides a hydrated opening.

The passage through the channel allows polar compounds to avoid the nonpolar central layer of the cell.

Facilitated transport moves substances.

They can use the aid of channel proteins.

The channel's opening is either open at all times orgated.

The opening of the channel may be affected by a particular ion attaching to it.
In some tissues, a gate is not required to allow passage, whereas in other tissues a gate is required.
In the kidneys, there are two different forms of channel in different parts.
Nerve and muscle cells that transmit electrical impulses have gated channels in their membranes.
In the case of nerve cells, opening and closing these channels can change the concentrations on opposing sides of the ion in a way that facilitates electrical transmission.

The bound molecule can be moved from the cell's outside to its interior depending on the gradient.

Carrier proteins can be specific to a single substance.
The overall selectivity is increased by this.
The mechanism for the change of shape is not understood by scientists.
When hydrogen bonds are affected, the shape of the proteins can change.
There are a finite number of the same carrier proteins in different parts of the body.

Problems in transporting enough material for the cell can be caused by this.

The rate of transport is at its maximum when all of the proteins are bound to their ligands.
The increase in concentration at this point will not result in an increase in transport rate.

Some substances are able to move down their concentration with the help of carrier proteins.

The shape of the carrier proteins changes as they move.

An example of this process taking place in the body.

In one part of the body, the kidneys filters various substances.
This filtrate contains a lot of sugar and reabsorbs in another part of the body.
The excess is not transported and the body excretes it through urine because there are only a finite number of carriers.
There is a group of carriers that are involved in transporting sugars through the body.

Material is transported at different rates.

Channel proteins move faster than carrier proteins.
At a rate of tens of millions of molecules per second, channel and carrier proteins work at the same rate.

Osmosis limits the solutes' diffusion in the water, while diffusion only transports material across the membranes.

The aquaporins that facilitate water movement are found in the red blood cells and the kidneys.

Osmosis is a special case.

Imagine a beaker with a semipermeable separator.
If the solution's volume on both sides is the same, but the solute's concentrations are different, then there are different amounts of water.

Water always moves from one area of higher water concentration to another.

In the diagram, the solute can't pass through the protective barrier, but the water can.

Imagine two full water glasses.

One has a small amount of sugar in it, while the other has a large amount.
The first cup has more water than the second cup because of the large amount of sugar in it.

The beaker example has a solute mixture on either side.

The principle of diffusion is that the Molecules will spread evenly throughout the Medium if they can move around.
Only the material that can diffuse through it will do so.
The water can diffuse the solute in this example.
This system has a concentration of water.
Water will diffuse to the side where it is less concentrated.
Until the water's concentration goes to zero or until the water's osmotic pressure is balanced, this will continue.
In living systems, Osmosis proceeds constantly.

A solution's tonicity can correlate with the solution's osmolarity.

A solution with low osmolarity has more water than solute particles.
A solution with high osmolarity has less water in it.
Water will move from the side with lower osmolarity to the side with higher osmolarity in a situation in which the solute doesn't separate the two osmolarities.
The only component in the system that can move is the water, which moves along its own concentration gradient.
Osmolarity is a measure of the number of particles in a solution.
If the second solution contains more dissolved molecule than there are cells, it may have a lower osmolarity than a solution that is clear.

Hypotonic, isotonic, and hypertonic are three terms used by scientists to describe the cell's osmolarity.

The water concentration in the solution is higher for the extracellular fluid than it is for the cell.
Water will enter the cell in this situation.

The cell has a higher concentration of water.

There will be no net movement of water into or out of the cell if the cell's osmolarity matches that of the extracellular fluid.

The shape of red blood cells is changed by osmotic pressure.

A doctor injects a patient with a solution he thinks is isotonic.

An autopsy shows that many red blood cells have been destroyed.

You can watch a video about the process of dispersion in solutions.

The relative solute and solvent concentrations are the same on both sides.

There is no change in the cell's size because there is no net water movement.
In a hypertonic solution, water leaves a cell.
The cell may be destroyed if the hypo- or hyper- condition goes to excess.

When the red blood cell swells, it will burst, or lyse.

There are spaces between the molecule that make up the membrane.
The cell will break apart if the spaces between the lipids and proteins become too large.

When excessive water amounts leave a red blood cell, it shrinks.

The effect of concentrating the solutes left in the cell is to make the cytosol denser.
It is possible that the cell's ability to function will be compromised and that it will die.

There are ways in which living things can control the effects of osmoregulation.

Some organisms, such as plants, fungi,bacteria, and protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution.
The cell won't lyse because the cell can only expand to the wall's limit.
Water will always enter a cell if water is available, and the cytoplasm in plants is slightly hypertonic to the cellular environment.
The water inflow stiffens the plant's cell walls.
Turgor pressure supports the plant in nonwoody plants.
The water will leave the cell if you don't water the plant.

The cell does not shrink because the wall is not flexible.

The cell membrane detaches from the wall.
Plants lose turgor pressure in this situation.

Turgor pressure within a plant cell depends on the solution's tonicity.

The plant on the left has lost turgor pressure because of the lack of water.

The turgor pressure can be restored by watering the plant.

Tonicity is a concern for all living things.

Paramecia and amoebas are protists that lack cell walls.
The cell is kept from lysing as it takes water from its environment by collecting excess water from the cell and pumping it out.

A paramecium's contractile vacuole, visualized using bright field light microscopy at 480x magnification, continuously pumps water out of the organism's body to keep it from bursting in a hypotonic medium.

Many marine invertebrates have internal salt levels that match their environments, making them isotonic with the water in which they live.

Five percent of the fish's metabolism is needed to maintain osmotic homeostasis.
The environment that freshwater fish live in is hypotonic.
The fish take in salt through their gills and excrete it as urine.
The reverse environment where saltwater fish live is hypertonic to their cells and they excrete highly concentrated urine.

The amount of water in the body is regulated by the kidneys.

The brain contains specialized cells that monitor concentration in the blood.
If the solute levels increase beyond a certain range, a hormone releases that slows water loss through the kidneys and reduces blood pressure.
Animals have high albumin concentrations in their blood.