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3.5 Passive Transport
By the end of this section, you will be able to explain why and how passive transport occurs.
Some substances are allowed but not others.
The cell would no longer be able to sustain itself if they lost this selectivity.
Some cells need more specific substances than others, and they need a way to get them from the extracellular fluids.
As certain materials move back and forth, or as the cell has special mechanisms that ensure transport, this may happen passive.
Most cells use most of their energy to create and maintain an even distribution of ion on the opposite side of their membranes.
There are some problems with the structure of the plasma membrane.
Passive forms of transport are the most direct.
In passive transport, substances move from an area of higher concentration to an area of lower concentration.
The asymmetric nature of the membranes means that the interior is not the same as the exterior.
There are channels or pumps that work in one direction.
Carbohydrates are found on the exterior surface of the cell.
The cell uses these complexes to bind things in the fluid.
It adds to the nature of the membranes.
There are two regions in the plasma membranes.
The characteristic helps the movement of certain materials and hinders the movement of others.
The material 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 are readily transported into the body's tissues and organs.
Oxygen and carbon dioxide have no charge.
Some polar molecules can connect with the outside of a cell, 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 means of penetrating.
Simple sugars need help with transport.
When the concentration is equal across the space, a single substance tends to move from an area of high concentration to an area of low concentration.
You are familiar with air movement.
Imagine a person opening a bottle of perfume in a room filled with people.
The perfume is at its highest concentration in the bottle and lowest at the edges of the room.
As the perfume diffuses from the bottle, more and more people will smell the perfume as it spreads.
The materials move within the cell's cytosol by diffusion.
The different concentrations of materials in different areas are a form of potential energy that can be dissipated as materials move down their concentration gradients.
A substance can be moved from an area of high concentration to one of low concentration with the help of a permeable membrane.
The concentration of different substances in the same medium has their own concentration.
The substance will diffuse according to the gradient.
There are a number of factors that affect the rate of diffusion.
The slower the rate of diffusion becomes, the closer the distribution of the material gets to equilibrium.
The mass of the molecule diffuses more slowly if it is more difficult for them to move between the substance they are moving through.
The movement of the molecule increases when the temperature is higher.
The slower the molecule is, the harder it is to get through the denser medium.
The substances that are transported through the air would not diffuse easily or quickly.
The solution to moving polar substances and other substances is dependent on the proteins that span its surface.
The material being transported is first attached to the surface of the cell.
The material that is needed by the cell can be removed.
The substances are passed through channels or pores that allow them to pass through the membranes.
The transport proteins are either channels for the material or the carriers, and they are collectively referred to as transport proteins.
Osmosis is a special case.
Imagine a beaker with a semipermeable separator.
If the volume of the water is the same, but the concentrations of solute are different, there are also different concentrations of water on either side of the membranes.
Water always moves from an area of higher concentration to one of lower concentration.
In this system, the solute cannot pass through a barrier.
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.
The concentration of water in the air will continue until it goes to zero.
In living systems, Osmosis proceeds constantly.
There is a video that shows the difference between hot and cold solutions.
Hypotonic, isotonic, and hypertonic are three terms used to describe the osmolarity of a cell.
The cell has a lower concentration of water than the extracellular fluid.
Water will enter the cell in this situation.
This can cause an animal cell to burst.
The fluid has a higher concentration of solutes than the cell's cytoplasm, so it contains less water.
The water will leave the cell.
The water is being drawn out of the cell by the solute.
An animal cell may be damaged.
There will be no net movement of water into or out of the cell if the concentration of solutes of the cell matches that of the extracellular fluid.
The features of blood cells in hypotonic, isotonic, and hypertonic solutions are shown in Figure 3.22.
Red blood cells in hypertonic, isotonic, and hypotonic solutions have their shape changed by osmotic pressure.
A doctor injects a patient with a solution that isotonic.
An autopsy shows that many red blood cells have been destroyed.
Some organisms, such as plants, fungi,bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis.
The cell won't lyse because the cell can only expand to the limit of the wall.
Water will always enter a cell if water is available, and the cytoplasm in plants is slightly hypertonic compared to the cellular environment.
This influx of water stiffens the cell walls of the plant.
Turgor pressure supports the plant in nonwoody plants.
Water will leave the cell if it becomes hypertonic or if a plant is not watered adequately.
Plants lose turgor pressure in this situation.
Turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in.
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