5.1 Passive transport: diffusion and osmosis
3 min read•july 22, 2024
Passive transport mechanisms are the unsung heroes of cellular life. They move molecules across cell membranes without energy, relying on concentration gradients. and are key players, allowing cells to exchange gases, nutrients, and waste.
Cells must maintain balance in the face of changing environments. Osmotic regulation mechanisms, like and ion pumps, help cells control their volume and internal composition. These processes are crucial for cell survival and function in diverse conditions.
Passive Transport Mechanisms
Process of simple diffusion
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- Movement of molecules from high concentration region to low concentration region
- Driven by concentration gradient
- Requires no energy input
- Molecules move randomly but net movement occurs down concentration gradient
- Crucial role in cellular transport
- Allows small nonpolar molecules to cross cell membrane (O2, CO2)
- Facilitates exchange of gases nutrients and waste products between cell and environment (, amino acids)
Concept of osmosis
- Movement of across from high water potential (low solute concentration) to low water potential (high solute concentration)
- Driven by difference in solute concentrations across membrane
- Water moves to equalize solute concentrations on both sides
- Affects cell volume
- In (lower solute concentration than cell) water enters cell causing swelling
- In (higher solute concentration than cell) water leaves cell causing shrinkage
- Affects solute concentration
- As water moves into or out of cell solute concentration inside cell changes
- In hypotonic solution solute concentration decreases as water enters cell (dilution)
- In hypertonic solution solute concentration increases as water leaves cell (concentration)
- As water moves into or out of cell solute concentration inside cell changes
Types of osmotic solutions
- Hypotonic solution
- Lower solute concentration than cell
- Causes water to move into cell by osmosis
- Results in cell swelling and potential lysis (bursting) if cell cannot regulate volume (red blood cells in water)
- Hypertonic solution
- Higher solute concentration than cell
- Causes water to move out of cell by osmosis
- Results in cell shrinkage and potential crenation (shriveling) if cell cannot regulate volume (plant cells in salt water)
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- Same solute concentration as cell
- No net movement of water across cell membrane
- Cell maintains normal volume and shape (blood cells in saline solution)
Factors affecting diffusion rate
- Concentration gradient
- Greater difference in concentration between two sides of membrane faster rate of diffusion (steep gradient)
- Membrane permeability
- More permeable membrane is to particular molecule faster rate of diffusion (lipid-soluble molecules)
- Depends on size charge and polarity of molecule as well as presence of specific transport proteins (ion channels)
- Temperature
- Higher temperatures increase kinetic energy of molecules leading to faster diffusion rates ( effect)
- Molecular size and shape
- Smaller molecules diffuse faster than larger molecules (H2O vs glucose)
- Molecules with more compact shape diffuse faster than those with more extended shape (spherical vs linear)
- Pressure
- Higher pressure on one side of membrane can increase rate of diffusion towards lower pressure side (gas exchange in lungs)
Cellular Osmotic Regulation
Osmotic regulation mechanisms
- Aquaporins are water that facilitate rapid movement of water across cell membranes
- Selectively permeable to water molecules (size exclusion)
- Allow cells to quickly respond to changes in extracellular osmolarity (renal collecting duct cells)
- Ion pumps and transporters help maintain cell's osmotic balance
- Na+/K+ ATPase pumps sodium ions out of cell and potassium ions into cell
- Maintains cell's resting membrane potential ( mV)
- Helps regulate cell volume by controlling ion concentrations (nerve cells)
- Other ion transporters also contribute to osmotic regulation (Cl- channels, H+/K+ antiporter)
- Na+/K+ ATPase pumps sodium ions out of cell and potassium ions into cell
Key Terms to Review (19)
Aquaporins: Aquaporins are specialized channel proteins embedded in the cell membrane that facilitate the rapid transport of water molecules in and out of cells. These proteins play a crucial role in maintaining cellular hydration, regulating osmotic balance, and enabling efficient water transport across biological membranes. Their selective permeability ensures that only water molecules pass through, while preventing the movement of ions and other solutes.
Brownian motion: Brownian motion is the random movement of microscopic particles suspended in a fluid (liquid or gas) due to collisions with the fast-moving molecules of the surrounding medium. This phenomenon is crucial for understanding how substances diffuse through membranes and move within cells, connecting it to passive transport processes like diffusion and osmosis.
Carrier proteins: Carrier proteins are integral membrane proteins that facilitate the transport of specific substances across a cell membrane by binding to the substance and undergoing a conformational change. They play a crucial role in both passive and active transport, ensuring that essential molecules like glucose and ions can enter or exit the cell efficiently.
Channel proteins: Channel proteins are specialized membrane proteins that form pores or channels in the cellular membrane, allowing specific ions and molecules to pass through the membrane down their concentration gradient. These proteins facilitate the movement of substances across the membrane without the need for energy, making them essential for passive transport processes such as diffusion and osmosis. They play a crucial role in maintaining cellular homeostasis and enabling communication between cells.
Cytolysis: Cytolysis is the process in which a cell bursts due to an influx of water, causing it to swell beyond its capacity and eventually rupture. This event occurs when a cell is placed in a hypotonic solution, leading to an imbalance in osmotic pressure. Understanding cytolysis is crucial as it highlights the consequences of osmotic imbalances and the importance of maintaining homeostasis within cells.
Down a concentration gradient: Moving down a concentration gradient refers to the process where molecules travel from an area of higher concentration to an area of lower concentration. This movement occurs naturally without the input of energy, making it a fundamental concept in understanding how substances diffuse through membranes and the role of osmosis in cell biology. It highlights how cells can maintain homeostasis by allowing certain molecules to flow passively, balancing concentrations inside and outside the cell.
Equilibrium: Equilibrium refers to a state in which the concentrations of substances are balanced and remain constant over time. In biological systems, this often relates to the movement of molecules across membranes, where processes like diffusion and osmosis play a key role in maintaining homeostasis. Achieving equilibrium is essential for cell function, as it ensures that the internal environment of the cell remains stable despite changes in external conditions.
Glucose: Glucose is a simple sugar and an essential carbohydrate that serves as a primary energy source for cells in the body. As a key player in cellular respiration, glucose is crucial for both passive and facilitated transport processes, where it moves across cell membranes to be utilized for energy production and metabolic functions.
Hypertonic solution: A hypertonic solution is a type of solution that has a higher concentration of solutes compared to another solution, often leading to water moving out of a cell when the two are separated by a semipermeable membrane. This movement of water occurs through osmosis, which is a specific form of passive transport, and results in the cell losing water and potentially shrinking. Understanding hypertonic solutions is essential for grasping how cells interact with their environment and maintain homeostasis.
Hypotonic solution: A hypotonic solution is a type of solution that has a lower concentration of solutes compared to another solution, usually referring to the fluid inside a cell. When cells are placed in a hypotonic solution, water moves into the cell through osmosis, causing the cell to swell and potentially burst if the pressure becomes too great. This process is an important aspect of passive transport, where movement occurs without energy input, and plays a key role in maintaining cellular homeostasis.
Isotonic Solution: An isotonic solution is a type of solution that has the same solute concentration as another solution, typically referring to the cytoplasm of a cell. This balance in concentration means that there is no net movement of water into or out of the cell, allowing it to maintain its shape and function properly. Understanding isotonic solutions is crucial for grasping how cells manage water and nutrients through processes like diffusion and osmosis.
Osmosis: Osmosis is the movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process is essential for maintaining cellular homeostasis, as it helps regulate the balance of fluids and solutes within cells and their environments, affecting overall cell function and integrity.
Osmotic potential: Osmotic potential is the measure of the tendency of water to move into a solution due to solute concentration. It reflects how much pressure is needed to prevent water from entering a solution, which is crucial in understanding how cells maintain their internal environment during passive transport processes like diffusion and osmosis. The concept is closely tied to the movement of water across semipermeable membranes, where it moves from areas of lower solute concentration to areas of higher solute concentration, helping to balance concentrations on either side of the membrane.
Oxygen: Oxygen is a colorless, odorless gas that is essential for life, making up about 21% of the Earth's atmosphere. It plays a critical role in cellular respiration, where it acts as the final electron acceptor in the electron transport chain, allowing cells to produce energy efficiently. Beyond its biological importance, oxygen is also involved in various chemical reactions, such as combustion and oxidation processes.
Plasmolysis: Plasmolysis is the process in which plant cells lose water in a hypertonic solution, causing the cell membrane to pull away from the cell wall. This occurs when water moves out of the cell due to osmosis, leading to a decrease in turgor pressure, which is essential for maintaining the structural integrity of plant cells. The phenomenon highlights the importance of osmotic balance and the effects of different solute concentrations on cellular behavior.
Selectively permeable membrane: A selectively permeable membrane is a biological barrier that allows certain substances to pass through while restricting others. This property is essential for maintaining homeostasis within cells, as it controls the movement of ions, molecules, and nutrients in and out of the cell, facilitating processes like diffusion and osmosis.
Simple Diffusion: Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration without the involvement of energy or transport proteins. This process occurs until equilibrium is reached, allowing substances like gases, lipids, and small uncharged molecules to move freely across a selectively permeable membrane. Simple diffusion is a fundamental mechanism that underlies passive transport, playing a crucial role in maintaining cellular homeostasis and regulating the internal environment of cells.
Turgor Pressure: Turgor pressure is the pressure exerted by the fluid (usually water) inside the central vacuole of plant cells against the cell wall. This pressure is crucial for maintaining the structural integrity and rigidity of plant cells, allowing them to stay upright and support the overall structure of the plant. Turgor pressure plays a vital role in osmosis, where water moves into cells, increasing their volume and thereby exerting pressure on the cell wall.
Water: Water is a vital polar molecule made up of two hydrogen atoms and one oxygen atom (H2O), which plays a crucial role in biological processes. Its unique properties, such as cohesion, adhesion, and its ability to dissolve many substances, make it essential for maintaining homeostasis within cells. Additionally, water is the medium through which passive transport processes like diffusion and osmosis occur, facilitating the movement of molecules across cell membranes.