Transportation is one of the life-sustaining processes that take place in higher organisms, most notably human beings, animals, and plants. This physiological and naturally occurring process involves moving essential materials, such as gases, water, nutrition minerals, and hormones, to parts of an organism where they are required to facilitate effective functioning. Active transport is one of the major transportation models applied in living beings. It is defined as the movement of essential materials against a gradient, from an area of lower concentration to an area of higher concentration, by means of external energy. Below is a detailed breakdown of this concept and some real life examples of its applications in living organisms.
What is Active Transport?
In most cases, essential substances involved in cellular transportation move along the concentration gradient. This implies that they move across a biological membrane from a region of high concentration to a region of low concentration. It is an automatic process whereby the cell’s input is not required as the molecules simply diffuse from one area to the next. This process is called passive transportation.
Notably, passive transportation is not always applicable. Case in point, a cell’s sugar glucose concentration might be higher than that of its surroundings. However, the cell will still require more sugar to fully realize its metabolic requisites. The most likely course of action will be for the sugar molecules to diffuse out of the cell as dictated by the concentration gradient. However, the cell needs to maintain its sugar concentration and still bring in more from the surrounding low concentration. Herein, the cell will undertake active transportation to move sugar molecules from the lower concentration exterior to the cell’s higher concentration. To achieve this, the cell will dispense energy in the form of ATP (adenosine triphosphate) to help push the sugar molecules against the concentration gradient and into the cell. By and large, passive transport occurs naturally along a concentration gradient while active transport requires energy to propel the molecules against the concentration gradient.
Examples of Active Transport in Everyday Life
1. Sodium-Potassium Pump
Also referred to as the Na+/K+ pump or the Na+/K+-ATPase, the sodium-potassium pump is an electrogenic transmembrane enzyme located in all animal cell membranes. Among its main functions is to maintain a high concentration of intracellular potassium ions and a low concentration of intracellular sodium ions. It achieves this by tapping energy from ATP such that, for every ATP molecule it utilizes, this pump is able to import three potassium ions and export three sodium ions.
2. Absorption of Amino Acids
Once ingested, proteins are broken down into oligopeptides and amino acids by means of pepsin action. Sodium cotransporters in the small intestines then allow the body to absorb the amino acids through active transportation mechanisms. These sodium-amino transporters are located in the basolateral membrane whereby they push amino acids from an area of high concentration in the intracellular space to an area of low concentration in the extracellular space.
3. Movement of Glucose In and Out of a Cell
In the renal and intestine proximal tubules, glucose transportation occurs against the concentration gradient. This movement is facilitated by glucose symporters that also cotransport sodium ions, including SGLT proteins and SLC5 genes. This movement qualifies as secondary active transportation since the SGLT proteins use energy leftover after sodium ion transportation to carry glucose across renal and intestinal cell membranes, against a concentration gradient.
4. Bacterial Ingestion by a Macrophage
Macrophages are a type of WBCs (white blood cells) that are always among the first to reach an infection site within the body. They are gigantic cells with a size of approximately 21 micrometers. The term ‘macrophage’ is actually a Greek terminology meaning ‘big eater’. This name is used in reference to the macrophages’ task of getting rid of unwanted bacteria or viruses by eating them up. Normally, macrophages first squeeze themselves into blood vessels within the site of infection. Once inside, they begin eating up the bacteria through active transportation, specifically phagocytosis. They start by engulfing the unwanted particle before sucking it in and breaking it down into smaller particles that are later forced out of the cell.
5. Neurotransmission
Chemical messages in the body are transmitted by neurotransmitters. These are molecules that transmit signals from one neuron to the next or to the muscles, hence facilitating operations of the nervous system. Neurotransmitters are stored in synaptic vesicles. For signal transmission to take place, cell membranes fuse with these vesicles, allowing neurotransmitters to move across the membrane and into the cell, thereby transmitting the intended signal. This process is a form of active transport, specifically endocytosis.
6. Mineral Ions Uptake by Plants
Another notable example of active transport is the absorption of mineral ions by plants, via root hairs. Soil contains diluted solutions of ions while the plant has a higher concentration of these minerals. However, plants require a steady supply of ions to meet their metabolic needs, such as respiration and photosynthesis. Therefore, active transport takes place at the root hairs as mineral ions move from the diluted soil concentration into the root hairs, and up the plant.
7. Calcium Uptake in Plants
Calcium moves up a plant in unidirectional fashion as it climbs from the roots and gradually enters young tissues and meristematic zones. This upward stream occurs through the xylem wherein ATP is used to facilitate active transportation. Studies show that Ca(2+)-ATPase facilitates the absorption of calcium from the xylem parenchyma into the stellar apoplast, thus allowing the gradual uptake of calcium ions throughout the plant.
8. Root Pressure
As aforementioned, the uptake of ions into root hairs occurs through active transportation. This process is partly facilitated by root pressure wherein the root absorbs water via osmosis, bloating up xylem cells and placing more pressure on the plant’s outer cells. This pressure forces water up the plant against gravity and minerals up the plant against concentration gradients.
Types of Active Transport
I. Primary Active Transport
This is the most common form of active transport. It involves breaking down ATP to facilitate the movement of molecules from low concentration to high concentration through a biological membrane. Cell membrane pumps are integral in this process as they help dispense sufficient amounts of energy for this transportation.
II. Secondary Active Transport
This model relies heavily on leftover energy packs from primary transportation processes. Energy is mostly dispensed from electrochemical gradients as carrier proteins combine with left over energy to facilitate transportation.
III. Endocytosis
This is an active transportation model involving the movement of molecules into a cell. It can happen through pinocytosis (absorption of liquid materials by the cell) or phagocytosis (absorption of solid materials by the cell).
IV. Exocytosis
This active transportation involves the movement of molecules out of the cell. Herein, the cell produces energy to dispel molecules across the cell membranes.