Cell membrane transport
50 flashcards covering Cell membrane transport for the MCAT Biology & Biochemistry section.
Cell membrane transport refers to the ways cells move substances like nutrients, waste, and ions in and out through their outer layer, the membrane. This process is essential for maintaining a cell's internal environment, allowing it to survive and function properly. There are two main types: passive transport, which doesn't require energy and includes diffusion and osmosis, relying on concentration gradients; and active transport, which uses energy to move substances against those gradients. Understanding these mechanisms helps explain how cells interact with their surroundings, from absorbing food to expelling toxins.
On the MCAT, cell membrane transport appears in biology and biochemistry questions that test your grasp of concepts like diffusion rates, osmosis in hypotonic or hypertonic solutions, and the role of carrier proteins. Common traps include confusing passive processes with active ones or overlooking factors like membrane permeability, so pay attention to details in passage-based questions. Focus on applying these ideas to scenarios, such as how drugs cross membranes, as exams often integrate this with physiology.
Practice sketching transport diagrams to visualize processes quickly.
Terms (50)
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Cell membrane
The semi-permeable barrier surrounding the cell, composed primarily of a phospholipid bilayer that regulates the passage of substances in and out.
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Phospholipid bilayer
The basic structure of the cell membrane, consisting of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward, which controls molecular movement.
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Selective permeability
The property of the cell membrane that allows it to permit certain substances to pass while blocking others, based on size, charge, and solubility.
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Diffusion
The passive movement of molecules from an area of higher concentration to lower concentration until equilibrium is reached, driven by random molecular motion.
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Simple diffusion
A type of passive transport where small, nonpolar molecules like oxygen pass directly through the phospholipid bilayer without assistance.
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Facilitated diffusion
Passive transport of molecules, such as glucose, through specific membrane proteins like channels or carriers, without requiring energy when moving down a concentration gradient.
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Channel proteins
Membrane proteins that form pores allowing ions or small molecules to pass through based on size and charge, often rapidly and without changing shape.
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Carrier proteins
Membrane proteins that bind to specific molecules and undergo a conformational change to transport them across the membrane, used in facilitated diffusion.
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Osmosis
The diffusion of water across a semi-permeable membrane from an area of lower solute concentration to higher solute concentration, essential for maintaining cell volume.
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Tonicity
A measure of the solute concentration relative to a cell, with terms like hypotonic (lower outside, causing water influx), hypertonic (higher outside, causing water efflux), and isotonic (equal, no net change).
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Active transport
The movement of substances across the membrane against their concentration gradient, requiring energy typically from ATP to move molecules like ions.
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Primary active transport
Active transport that directly uses ATP to power the transport process, such as in pumps that move ions against their gradients.
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Secondary active transport
Active transport that uses the energy from an ion's electrochemical gradient, established by primary active transport, to move another substance, like in symporters.
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Sodium-potassium pump
A primary active transport protein that uses ATP to pump three sodium ions out of the cell and two potassium ions in, maintaining the cell's electrochemical gradient.
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Proton pump
A primary active transport mechanism that uses ATP to move protons across membranes, creating gradients used in processes like ATP synthesis in mitochondria.
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Electrochemical gradient
The combined effect of a concentration gradient and an electrical gradient across the membrane, driving the movement of charged ions in transport processes.
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Membrane potential
The electrical voltage difference across the cell membrane, typically negative inside, which influences ion transport and cellular activities.
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Ion channels
Proteins that allow specific ions to pass through the membrane, often gated by voltage, ligands, or mechanical signals, regulating processes like nerve impulses.
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Voltage-gated channels
Ion channels that open or close in response to changes in membrane potential, crucial for action potentials in neurons.
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Ligand-gated channels
Ion channels that open when a specific molecule binds to them, such as neurotransmitters binding to receptors on postsynaptic membranes.
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Aquaporins
Specialized channel proteins that facilitate the rapid diffusion of water across cell membranes, important in kidney function and osmotic balance.
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Uniport
A type of transport where a single substance moves across the membrane, either alone in facilitated diffusion or against a gradient in active transport.
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Symport
A co-transport mechanism where two substances move in the same direction across the membrane, often using the gradient of one to drive the other.
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Antiport
A co-transport system where one substance moves into the cell while another moves out, such as in the sodium-calcium exchanger.
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Endocytosis
The process of engulfing substances into the cell by forming vesicles from the membrane, used for large particles or fluids.
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Phagocytosis
A type of endocytosis where the cell engulfs solid particles, like bacteria, by extending its membrane to form a phagosome.
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Pinocytosis
A form of endocytosis that involves the uptake of extracellular fluid and its dissolved solutes into small vesicles.
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Receptor-mediated endocytosis
Endocytosis that uses specific receptors on the cell surface to bind and internalize particular molecules, like LDL cholesterol.
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Exocytosis
The process of exporting substances from the cell by fusing vesicles with the plasma membrane, releasing contents like neurotransmitters.
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Vesicular transport
The movement of materials into or out of the cell via membrane-bound vesicles, encompassing both endocytosis and exocytosis.
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Osmotic pressure
The pressure required to prevent the inward flow of water across a semi-permeable membrane, calculated based on solute concentration differences.
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Van't Hoff equation
The formula π = iCRT, where π is osmotic pressure, i is the van't Hoff factor, C is molar concentration, R is the gas constant, and T is temperature in Kelvin, used to quantify osmosis.
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Factors affecting diffusion rate
Variables such as concentration gradient, temperature, molecule size, and membrane solubility that influence how quickly diffusion occurs.
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Permeability coefficient
A measure of how easily a substance can pass through the membrane, depending on its lipid solubility and the presence of transport proteins.
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Common trap: Diffusion vs. osmosis
Students often confuse diffusion, which involves any molecule moving down its gradient, with osmosis, which specifically refers to water movement across a membrane.
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Strategy for distinguishing active and passive transport
Determine if the substance moves against its gradient; if it does, it's active transport requiring energy, whereas down the gradient indicates passive transport.
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Worked example: Sodium-potassium pump
In this process, for every ATP molecule hydrolyzed, three Na+ ions are pumped out and two K+ ions are pumped in, establishing a gradient essential for nerve function.
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Role of ATP in transport
ATP provides the energy for active transport processes, such as powering pumps that move ions against their gradients to maintain cellular homeostasis.
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Inhibitors of transport
Substances like ouabain, which block the sodium-potassium pump, can disrupt ion balances and lead to cellular dysfunction.
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Bulk transport
The movement of large particles or volumes of material via vesicles, including endocytosis and exocytosis, as opposed to transport of individual molecules.
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Fluid mosaic model
A description of the cell membrane as a dynamic structure with proteins embedded in a phospholipid bilayer, allowing for transport and signaling.
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Glycoproteins in transport
Proteins with carbohydrate attachments on the membrane that can act as receptors or markers, influencing processes like receptor-mediated endocytosis.
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Cholesterol and membrane fluidity
Cholesterol in the bilayer modulates fluidity, affecting the rate of transport by maintaining membrane stability across temperature changes.
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Temperature effects on transport
Higher temperatures increase molecular kinetic energy, speeding up diffusion and transport processes up to a point where proteins may denature.
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pH effects on transport
Changes in pH can alter protein conformation, potentially opening or closing channels and affecting the transport of ions or molecules.
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Size and polarity of molecules
Smaller, nonpolar molecules diffuse more easily through the membrane, while larger or polar ones require proteins, influencing transport efficiency.
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Equilibrium in diffusion
The point where the concentration of a substance is equal on both sides of the membrane, resulting in no net movement during passive transport.
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Steady state vs. equilibrium
In steady state, transport continues but rates balance out, as in active transport maintaining gradients, unlike true equilibrium with no net flux.
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Transport in prokaryotes
Prokaryotic cells use similar transport mechanisms but lack membrane-bound organelles, relying on the plasma membrane for processes like nutrient uptake.
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Transport in eukaryotes
Eukaryotic cells have compartmentalized transport via organelles, with the plasma membrane handling external exchanges and internal membranes managing intracellular traffic.