Definition of Transport Process Across Cell Membrane
- During Transport Process Across Cell Membrane, The permeability of substances across cell membranes depends on their solubility in lipids. Not on their molecular size.
- Water-soluble compounds are typically impermeable and require carrier-mediated transport.
- There are different transport mechanisms, including passive and active transport.
- Passive transport contains simple and facilitated diffusion.
- Active transport involves the use of pumps to drive molecules against the gradient using energy.
- Ion channels enable the passage of molecules in accordance with the concentration gradient.
- The fluid mosaic model of the membrane is a model that describes the structure and function of cell membranes.
Types of Transport Process Across Cell Membrane with examples.
Simple Diffusion
- Solutes and gases can passively enter cells, driven by the concentration gradient.
- The rate of entry is proportional to the solubility of the solute in the hydrophobic core of the membrane.
- Simple diffusion occurs from higher to lower concentration and does not require any energy during Transport Process Across Cell Membrane
- However, simple diffusion is a slow process.
- Passive transport is a type of transport mechanism that does not require energy.
- There are different types of passive transport mechanisms, including simple diffusion and facilitated diffusion.
- Simple diffusion takes place when a substance moves from high concentration to low concentration.
Example of Simple Diffusion
An example of simple diffusion is the movement of oxygen and carbon dioxide between the alveoli in the lungs and the blood vessels surrounding them. Oxygen molecules move from an area of high concentration (in the alveoli) to an area of low concentration (in the blood vessels), while carbon dioxide molecules move from an area of high concentration (in the blood vessels) to an area of low concentration (in the alveoli). This occurs because the cell membrane of the alveoli and blood vessels is permeable to these gases, and the concentration gradient drives the movement of the molecules across the membrane. This process is passive, meaning it does not require any energy from the cell.
Facilitated Diffusion
- Facilitated diffusion is a type of carrier-mediated process in Transport Process Across Cell Membrane
- Structurally similar solutes can competitively inhibit the entry of the solutes during facilitated diffusion.
- Facilitated diffusion does not require energy, but the rate of transport is more rapid than the diffusion process.
- Facilitated diffusion is dependent over concentration gradient.
- Hormones can regulate the number of carrier molecules involved in facilitated diffusion.
- An example of facilitated transport is glucose transport across the membrane by glucose transporters.
Example of Facilitated Diffusion
An example of facilitated diffusion is the transport of glucose into cells. Glucose is a polar molecule and cannot passively diffuse across the nonpolar lipid bilayer of the cell membrane. Instead, it requires the help of glucose transport proteins, also known as glucose carriers or GLUTs, which are embedded in the cell membrane.
The GLUTs bind to glucose on one side of the membrane and undergo a conformational change, allowing the glucose to be transported across the membrane and released on the other side of the membrane. This process is passive and does not require energy from the cell.
Facilitated diffusion is used to transport many other polar or charged molecules across the cell membrane, such as amino acids and ions, and it relies on specific membrane transporters or channels to assist in the transport process.
Ion Channels
- Ion channels in membranes facilitate the quick transport of electrolytes like Ca++, K+, Na+, and Cl-.
- Ion channels are selective ion conductive pores made of specialized protein molecules that span the membrane.
- Cation conductive channels usually remain closed but open in response to stimuli, allowing rapid flux of ions down the gradient.
- This type of regulation is called “gated,”
- Based on the nature of stimuli that trigger the opening of the gate, ion channels are classified into “voltage gated” or “ligand-gated” channels.
- Voltage-gated channels open in response to membrane depolarization, while ligand-gated channels open in response to the binding of effectors.
Example of Ion Channels
An example of ion channels is the voltage-gated sodium (Na+) channels in neurons. These channels are important for the generation and propagation of action potentials, which are the electrical signals that neurons use to communicate with each other and with other cells in the body.
When a neuron is stimulated, a rapid change in the membrane potential occurs, causing the voltage-gated Na+ channels to open. This allows Na+ ions to move into the neuron, which depolarizes the cell membrane and triggers the generation of an action potential.
The voltage-gated Na+ channels are selective for Na+ ions and can only open in response to a certain voltage threshold. This ensures that the channels are only activated when the neuron needs to generate an action potential.
Ion channels are also involved in many other processes, such as the regulation of muscle contraction, the maintenance of electrolyte balance, and the control of insulin secretion in the pancreas.
Active Transport
- Active transport requires energy, and about 40% of the total energy expenditure in a cell is used for the active transport system.
- Active transport requires specialized integral proteins called transporters, which are susceptible to inhibition by specific organic or inorganic compounds in Transport Process Across Cell Membrane.
- The sodium-potassium activated ATPase, or sodium pump, maintains a low intracellular sodium concentration and a high intracellular potassium concentration. Hydrolysis of one molecule of ATP results in the expulsion of 3 Na+ ions and influx of 2 K+ ions. The ion transport and ATP hydrolysis is very tightly coupled and linked.
- Digoxin, a cardiotonic drug, inhibits the sodium-potassium pump, leading to an increase in Na+ levels inside the cell and extrusion of Ca2+ from the myocardial cell, enhancing the contractility of the cardiac muscle and improving heart function.
- The ATP-dependent calcium pump functions to regulate muscle contraction. A specialized membrane system called sarcoplasmic reticulum is found in skeletal muscles, which regulates the Ca++ concentration around muscle fibers.
- The calcium pump removes cytosolic calcium and maintains a low cytosolic concentration, so that the muscle can receive the next signal. For each ATP hydrolyzed, 2Ca++ ions are transported.
Example of Active Transport
An example of active transport is the sodium-potassium (Na+/K+) pump, which is present in most cells. The Na+/K+ pump actively transports three Na+ ions out of the cell and two K+ ions into the cell for every ATP molecule hydrolyzed.
This pump is important for maintaining the electrochemical gradient across the cell membrane and regulating the volume of cells. By pumping Na+ out of the cell, the pump helps to maintain a low concentration of Na+ ions inside the cell, which is essential for many cellular processes. By pumping K+ into the cell, the pump helps to maintain a high concentration of K+ ions inside the cell, which is important for many physiological processes such as muscle contraction and nerve transmission.
