Cellular Transport

 

The concepts of active and passive transport.

 

Cells need to have a way to move molecules areound, including bringing molecules from outside the cell to the inside, moveing molecules around once they are in the cell, and from the inside to  the outside.  
SOme of this motion happens without the cell spending any energy at all, and some requires the cell to work and use energy.  We'll be looking at both of these types of transport in this section.

 

Definitions:

 

Passive Transport:

We will look at three ways that cells move molecules without spending any energy.  We call this type of transport passive transport, since no energy is input from the cell.  Let's take a look at types of passive transport:

 

1. Diffusion: 

The net movement of a substance (e.g., an atom, ion or molecule) from a region of high concentration to a region of low concentration.  We experience diffusion all the time in our daily lives. Color dropped in water will eventually spread out until all of the water is an even color.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initial Condition                                                              Mid-Point                                                                     Equilibrium

Equilibrium:

Molecules will continue to distribute throughout the liquid until it is evenly distributed.  Equilibrium is the condition where all influences are balanced.  Equilibrium refers to a stable, unchanging system.  So in the images below, we can see the progression of diffusion, from initial condition through equilibrium.

 

 

 

 

 

 

 

 

So lets take a closer look at diffusion and equilibrium:

You should take notes and outline the information presented in this video.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.  Osmosis:

A specific type of diffussion, where water molecules diffuse across a semi-permeable membrane.  That's quite a sentence, so lets deconstruct it.

 

Osmosis is the diffusion (movement from areas of higher concentration to areas of lower concentration) of water molecules across a semi-permeable (solids that allow only some substances to pass through) membrane (A membrane is a selective barrier - in life science it is also the outer covering of cell or cell organelle that allows the passage of certain constituents and retains other constituents).  

 

This definition seems even more complex than the original definition, but it actually clarified quite a bit.

 

Osmosis is something special that happens only with water and a membrane.  Osmosis is what we call it when water can pass through a membrane that many other substances can't, and when water moves from areas of HIGH concentration (of water) to areas of lower concentration (of water). 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

We can see in the diagram above that the water molecules move through the membrane from the side where there are more water molecules across the membrane into areas with fewer water molecules.

 

Challenge:

Watch the  video and answer the question presented.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3. Facilitated Diffusion:

Facilitated diffusion is the process of  passive transport of molecules or ions across a membrane via specific proteins embded in the membrane.  Being passive, facilitated transport does not require chemical energy from the cell in the transport; rather, molecules and ions move down their concentration gradient (from areas of higher concentration to areas of lower concentration).

 

 

 

 

 

 

 

 


 

 

 

 

 


Polar molecules and large ions dissolved in water cannot diffuse freely across the cell membrane due to the hydrophobic nature of that membrane. Only small, non-polar molecules, such as oxygen and carbon dioxide, can diffuse easily across the membrane (via diffusion).  All polar molecules are transported by proteins in the form of channels that cross the membrane. These channels are gated, meaning that they open and close, and thus regulate the flow of ions or small polar molecules across membranes. Larger molecules are transported by transmembrane carrier proteins, such as permeases, that change their shape as the molecules are carried across.

Glucose, sodium ions, and chloride ions are just a few examples of molecules and ions that must efficiently cross the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass mechanism

 

Active Transport:

Active transport is the movement of molecules across a cell membrane against their concentration gradient (moving from an area of lower concentration to an area of higher concentration). Active transport is usually associated with accumulating high concentrations of molecules that the cell needs.  Unlike passive transport, active transport uses cellular energy. Active transport is a good example of a process for which cells require energy. Let's take a look at some forms of active transport:

 

1. Sodium-Potassium Pump:

Cells must keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). Outside cells (extracellular), there are high concentrations of sodium and low concentrations of potassium, so diffusion occurs through ion channels in the plasma membrane. This causes sodium to enter the cell and potassium to leave the cell (the opposite of what is needed).  In order to keep the appropriate concentrations, the sodium-potassium pump pumps sodium out and potassium in through active transport.  This requires the cell use energy to move the sodium and potassium ions from areas of lower concentration to areas of higher concentration.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Endocytosis:

Endocytosis is an energy-using process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane. The opposite process is exocytosis. [1]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

So, can we see Endocytosis with real cells?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3. Exocytosis:

Exocytosis is an energy-using process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane. The opposite process is exocytosis. [1]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

So, can we see Exocytosis with real cells?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

So, let's compare and contrast Endocytosis & Exocytosis: