Definition of Active/Passive Cellular Transport (through the membrane)

Cells are the basic units of the structure of life. They are like small cities full of activity and, just like in a city, transportation and the exchange of materials between the exterior and the interior is essential for everything to work and must be rigorously checked. The cell membrane is the barrier that separates the cell from the outside world, and all the substances that enter and leave the cells must pass through it, taking care of regulating the passage.

The cell membrane: a selective frontier

The membrane is like a filter that allows the passage of certain substances, and blocks others. It is composed of a double layer of a type of lipid known as phospholipids with proteins embedded in it. These proteins are the carrier proteins and, as their name indicates, they facilitate the passage of substances while controlling the flows that enter and leave the cell.

Some carrier proteins form channels, comparable to gates, which open or close to allow the passage of materials. Are

channel proteins they open and close based on the needs of the cells and respond to a multitude of signals. This type of protein participates in a type of cellular transport known as passive facilitated transport or facilitated diffusion.

There are other types of transporter proteins, known as bombs and they act in a similar way to a catapult, which captures a molecule on one side, and throws it towards the other side of the membrane. These types of proteins act during the Active transport.

Concentration Gradients: The Driving Force of Cellular Transport

On both sides of the membrane, there is an aqueous solution (this means that the solvent is water) of organic and mineral molecules. For each of the substances present, the solution has a different concentration; that is, there is a certain amount of solute dissolved.

For example, if we prepare two glasses of lemonade in a 250ml glass (the amount of liquid that enters a glass) but one of the glasses has we put 2 tablespoons of sugar and the other 4, the one with 4 tablespoons will surely be too sweet and the sugar concentration will be high. The other glass will have a lower concentration and will taste less sweet. If we mix the contents of both glasses, the flavor of the mixture will be homogenized at a midpoint between both solutions, and possibly now we have half a liter of lemonade with the right point of sugar. This is an example of how solutes move down the concentration gradient. By mixing the beakers, the sugar molecules moved from the more concentrated solution to the less concentrated one, until the entire solution reached the same concentration and the movement stopped.

passive transport

Passive transport is like turning on the faucet and simply letting the water flow uncontrollably. without wasting energy. In this stage, substances move down their concentration gradient, that is, from where there is more concentration to where there is less, until reaching equilibrium, as in the example of the glasses of lemonade. There are two types of passive transport: simple diffusion and facilitated diffusion.

Plain Diffusion

In this type of transport, small molecules, such as oxygen and carbon dioxide, cross the cell membrane down their concentration gradient.

This process is similar to the example of glasses of lemonade or when the smell of a perfume spreads through a room: molecules move from where there is more perfume to where there is less until the scent disperses evenly.

Facilitated Diffusion

Larger or electrically charged molecules cannot cross the membrane, and need help to cross it. This is where the channel transporter proteins.

The Molecules traverse the channels down the gradient., but those channels can be closed or opened in response to cellular conditions. If the channel is closed, even though there is a concentration gradient on both sides of the membrane, there will be no movement.

Osmosis

It is the simple diffusion of water across the cell membrane.. Water has an incredible ability to pass through membrane fats, which means cells must carefully control their water content.

If a cell is in a saltier environment than its interior, water will leak out of the cell to dilute the external salt, which can lead to cell shrinkage. On the other hand, if the external environment is less salty, water will enter the cell, causing it to swell and possibly burst. To avoid this, plant cells have a rigid cell wall that contains the cell and prevents it from increasing in volume beyond a limit.

animal cells without wall, must be in an environment with a strictly controlled salinity, otherwise they could suffer a osmotic shock and die. For this reason, the saline balance of the blood, which is in charge of the excretory system, is very important.

Active Transport and examples

Unlike passive transport, active transport requires energy expenditure. The cells use energy to move substances against their concentration gradient, that is, from where there is less concentration to where there is more. The cells use energy to activate pump proteins, the catapults we were talking about when we mentioned the structure of the cell wall.

During active transport, transporter proteins use energy directly to be able to pump substances against their gradient. Ions and mineral salts are substances that sometimes move against their gradient by processes of this type. An example is the sodium-potassium pump, essential for the functioning of muscles and neurons.

Other times, carrier proteins function coupled to passive transport. In this case, the step in favor of the gradient gives a "push" or drags the substance that crosses against its gradient. It is as if an inertia were used to move forward. An example is glucose transport in intestinal cells, where sodium is pumped out of the cell by a sodium-potassium pump, generating a gradient that allows glucose to enter the cell taking advantage of that "push".

endocytosis

Another active transport mechanism is endocytosis, which also transports substances against its gradient, and is used for larger particles, about the size of a bacterium or cell. In this case, the cell “swallows” the particle. This mechanism is the main form of food for unicellular organisms and some of the cells of the immune system, known as macrophages, eat the invading agents of the body.

There are other transport mechanisms, but the exposed mechanisms are the main ones and the most common ones in cells.