It's important that cell membranes establish and maintain their internal environments through selective permeability.
It's important that cell membranes establish and maintain their internal environments through selective permeability.(1:40) There are three main types of active transport - membrane pumps,(2:45) endocytosis (5:37) and exocytosis.(6:30)
The Question of the Day asks (8:10) How is a cotransporter classified that moves molecules in opposite directions?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Active transport.
Zoom out:
Unit 2 - Cell Structure and Function
Topics 2.6 and 2.9
Big idea - Energetics
There’s no room in the Uber, but your friend begs the driver - “please, can’t we just fit one more person? Look, we will all just scooch over” - as he hops into the back seat, stepping on toes and struggling to close the door. It's uncomfortable - for everyone. Just like you, molecules need their personal space (well, at least as it pertains to molecular interactions) and will not crowd themselves without some effort. And since in biology, these molecules are typically contained in cells - that effort will often come from good ‘ol adenosine triphosphate.
Let’s Zoom in:
There are two types of cellular transport – passive, and active. These categories are divided by the use of energy and direction of molecular movement. As we’ve discussed previously, passive transport is the net movement of molecules from an area where they are more highly concentrated to an area where they are less concentrated without the input of metabolic energy, or ATP. For a full recap, check out episode 53. It's important that cell membranes establish and maintain their internal environments through selective permeability. After all, homeostasis is the maintenance of constant internal conditions, despite external changes. And environments are always changing! One of ways that cells accomplish this is through the strategic movement of molecules by active transport.
How does the cell move molecules against the concentration gradient, especially those that are large or charged? The movement of molecules from regions of low concentration to high concentration is known as active transport and requires the spending of cellular energy, or ATP. This movement is often referred to as “up” or “against” the concentration gradient. But be careful with the word up - there is no up, down, left or right in the 3D world of cells. Instead, we will want to use phrases like “into the cell” or “out of the cell” to reference the movement of solutes.
There are three main types of active transport - membrane pumps, endocytosis and exocytosis. Let’s start with membrane pumps - which are transmembrane carrier proteins, just like with those used for facilitated diffusion. But since this is active transport, the molecules move against their concentration gradient and use of ATP. Transport proteins are solute specific due to their unique chemical folding and R-group interactions. When ATP is hydrolyzed, a phosphate group is removed and often attached to the pump itself. Phosphorylation causes a conformational change in the proteins shape, adjusting the microenvironment within the protein channel and shuttling the solutes across the membrane.
One of the most common examples of active membrane transport is the sodium potassium pump, which performs a crucial job in the nervous system to maintain membrane potential, or charge. The sodium potassium pump moves three sodium ions out of the cell for every two potassium ions moved into the cell, in somewhat of a seesaw motion, where the protein is open to one side of the membrane at a time. How can there be a difference in membrane charge if these are both positive ions? It comes down to ratios - 3 out for every 2 in, so the extracellular environment has a net positive charge. Another example is the proton pump (spoiler alert….its moves protons). Proton pumps are used during the electron transport chain in chloroplasts and mitochondria to establish an electrochemical gradient of hydrogen ions. Both the sodium potassium pump and the proton pump work to accumulate ions on one side of a membrane, which the cell may later use during cotransport or with ATP synthase. Molecules rarely stay concentrated for long.
The two other types of active transport involve vesicle formation for endo and exocytosis. Since all membranes are primarily composed of phospholipids, the formation and fusion of vesicles is a fairly seamless process. Endocytosis is the process of bringing substances into the cell in larger quantities and has three types. Phagocytosis, commonly referred to as cell eating, pinocytosis, or cell drinking, and receptor mediated endocytosis, where specific ligands bind to cell surface receptors, causing vesicle formation. Receptor mediated endocytosis is the pickiest of the three. Remember the endosymbiosis theory? Well the foundation of that theory is phagocytosis! The engulfing of a small prokaryote, resulting in the double membrane energy transducers like chloroplasts and mitochondria. Exocytosis involves the fusion of a vesicle with the plasma membrane for molecules to exit the cell. These might be intentional products that were shipped out by the Golgi or waste products that the cell is removing.
Time for unit connections. Active transport is involved in Unit 2 with the endosymbiosis theory, Unit 3 with photosynthesis and cellular respiration and Unit 4 with cell communication and signal transduction. Alright - what about the exam? You may be asked to predict molecule movement based upon experimental design, draw conclusions about concentration gradients, assign experimental questions to membrane protein models, describe the pathway of vesicle movement, or to describe the movement of molecules to a new scenario (like absorption in the human intestines!)
To recap……
Active transport moves molecules against their concentration gradient from low to high with the use of ATP. The three types of active transport are membrane pumps, endocytosis, and exocytosis.
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Coming up next on the Apsolute RecAP Biology Edition: Glucose Combusting
Today’s question of the day is about transport
Question of the day: How is a cotransporter classified that moves molecules in opposite directions?