5.2 Passive Transport
3 min read•june 14, 2024
is the movement of molecules across cell membranes without using energy. It's driven by concentration gradients, pressure, and electrical charge differences. This process continues until is reached, balancing concentrations on both sides.
and are key mechanisms. moves molecules from high to low concentration areas, while osmosis is the diffusion of water across membranes. Various factors affect these processes, including temperature, molecular size, and .
Passive Transport
Concept of passive transport
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- Movement of molecules across a membrane down their without using cellular energy
- Driven by differences in concentration (high to low), pressure, and electrical charge between two sides of a membrane
- Continues until is reached, where concentrations on both sides are equal (sodium and )
Diffusion and osmosis processes
- Diffusion: Net movement of molecules from high to low concentration areas until equilibrium ( and exchange in lungs)
- Occurs through random molecular motion and collisions
- involves direct movement through the
- Osmosis: Diffusion of water across a membrane from high to low ( absorbing water from soil)
- depends on solute concentration; high solute means low water potential
Types of Passive Transport
- Simple diffusion: Direct movement of molecules through the phospholipid bilayer
- : Movement of molecules across the membrane with the help of transport proteins
- : Change shape to move specific molecules across the membrane
- : Form pores in the membrane for specific molecules to pass through
Tonicity and cell behavior
- compares solute concentrations inside and outside a cell, affecting water movement and cell size
- : Equal solute levels, no net water movement (normal blood cells in plasma)
- : Lower solute outside, water enters cell causing swelling or (freshwater amoeba)
- : Higher solute outside, water leaves cell causing shrinkage (marine fish eggs)
Concentration gradients in diffusion
- Steeper gradients drive faster diffusion until equilibrium is reached
- Initial rapid net movement slows as gradient decreases (dye spreading in water)
- No net movement once concentrations equalize on both sides
Molecular mass vs diffusion rates
- Smaller, lighter molecules diffuse faster due to higher and less resistance ( vs )
- Larger, heavier molecules move slower with more inertia and medium resistance
Temperature effects on transport
- Higher temperatures increase molecular kinetic energy and diffusion rates (hot tea brewing faster than cold)
- Lower temperatures reduce kinetic energy and slow passive transport
Solvent density and diffusion
- Denser solvents provide more resistance, slowing diffusion (syrup vs water)
- Dehydration increases cytoplasmic density, impairing diffusion and cell function (severe diarrhea)
Solubility and membrane crossing
- Lipid-soluble nonpolar molecules easily cross phospholipid bilayer ()
- Water-soluble polar/charged molecules cannot pass through hydrophobic membrane core without transport proteins (glucose and )
Surface area in diffusion
- Greater surface area increases contact points and diffusion rates (lung )
- Thinner membranes reduce distance for faster diffusion (gills)
- High surface area to volume ratio enhances efficient exchange ( in small intestine)
Distance impact on diffusion
- Longer distances require more time for molecules to traverse, slowing diffusion (gas exchange in insects vs mammals)
- Cell size is limited by diffusion constraints as volume increases faster than surface area
- Large cells develop adaptations like membrane folds or multiple nuclei to compensate (skeletal muscle fibers)
Membrane permeability
- Affects the rate of passive transport across the membrane
- Depends on the composition of the phospholipid bilayer and presence of transport proteins
Key Terms to Review (44)
Alveoli: Alveoli are tiny air sacs located in the lungs that are crucial for gas exchange in mammals. They provide a large surface area for oxygen to diffuse into the blood and for carbon dioxide to diffuse out, making them essential components of the respiratory system.
Amino acids: Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains an amino group, a carboxyl group, and a unique side chain (R-group).
Amino Acids: Amino acids are organic compounds that serve as the building blocks of proteins, consisting of an amino group, a carboxyl group, and a unique side chain. They play critical roles in various biological processes, including protein synthesis, metabolism, and the regulation of cellular functions.
Carbon dioxide: Carbon dioxide is a colorless, odorless gas that is produced by the respiration of living organisms and the combustion of organic matter. It plays a crucial role in various biological and environmental processes, including photosynthesis, respiration, and climate regulation.
Carrier protein: A carrier protein is a type of membrane protein involved in the facilitated diffusion of substances across the cell membrane. It binds to specific molecules and undergoes a conformational change to transport them across the membrane without using energy.
Carrier proteins: Carrier proteins are integral membrane proteins that facilitate the transport of specific molecules across a cell membrane by changing their shape. They are crucial for the movement of substances that cannot freely diffuse through the lipid bilayer, playing vital roles in both passive and active transport mechanisms, as well as maintaining cellular homeostasis.
Channel proteins: Channel proteins are integral membrane proteins that form pores in the plasma membrane, allowing specific molecules or ions to pass through by diffusion. They facilitate passive transport and do not require energy input from the cell.
Channel proteins: Channel proteins are integral membrane proteins that facilitate the movement of specific ions or molecules across a cell membrane, creating a passageway for substances to enter or exit the cell. These proteins are essential for maintaining the balance of nutrients and ions inside the cell while allowing waste products to be expelled, highlighting their role in cell structure and function as well as their contribution to passive transport mechanisms.
Concentration gradient: A concentration gradient is the difference in the concentration of a substance across a distance, often across a membrane. It plays a crucial role in the movement of substances in and out of cells, influencing how molecules diffuse or are transported. This gradient affects various biological processes, including the transport of ions and gases, as well as how efficiently organisms exchange vital substances with their environment.
Diffusion: Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration. This process continues until equilibrium is reached, and it does not require cellular energy (ATP).
Diffusion: Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration, driven by the random motion of particles. This fundamental concept is crucial for understanding how substances like gases and solutes are exchanged and transported in biological systems, influencing processes such as nutrient uptake, waste elimination, and gas exchange in organisms.
Equilibrium: Equilibrium is a state in which opposing forces or influences are balanced. In biological systems, it often refers to the balance of chemical concentrations within cells and ecosystems.
Equilibrium: Equilibrium refers to a state of balance where opposing forces or influences are equal and stable, resulting in no net change over time. This concept is crucial in understanding how substances move across membranes and how sensory systems maintain homeostasis, ensuring that organisms respond appropriately to their environments.
Facilitated diffusion: Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the help of special proteins, known as transport proteins, without the need for energy. This process is crucial for moving polar and charged molecules, which cannot easily pass through the lipid bilayer of cell membranes. By utilizing specific channels or carriers, facilitated diffusion ensures that essential nutrients and ions can enter or exit cells, maintaining cellular homeostasis.
Facilitated transport: Facilitated transport is a type of passive transport that involves the movement of molecules across a cell membrane via specific transmembrane proteins. It does not require energy and relies on concentration gradients to drive the process.
Glucose: Glucose is a simple sugar and a vital carbohydrate that serves as a primary energy source for living organisms. This monosaccharide is crucial for various biological processes, including cellular respiration, energy production, and as a building block for larger carbohydrates.
Glucose-sparing effect: Glucose-sparing effect is a metabolic process where the body prioritizes the use of fats and proteins for energy to conserve glucose for the brain. This mechanism is crucial during fasting or intense exercise when glucose levels are low.
Hydrogen Gas: Hydrogen gas is a diatomic molecule (H₂) consisting of two hydrogen atoms bonded together, and it is the simplest and most abundant element in the universe. It plays a crucial role in various biological and chemical processes, including cellular respiration and photosynthesis, where it acts as an electron donor or acceptor. In the context of passive transport, hydrogen gas can influence the movement of other molecules across membranes due to its small size and nonpolar nature.
Hydrophobic core: The hydrophobic core refers to the region within a biological membrane or protein structure that is predominantly composed of nonpolar, hydrophobic molecules. This core plays a crucial role in maintaining the stability and integrity of membranes and proteins by driving the self-assembly of lipid bilayers and influencing protein folding.
Hypertonic: A hypertonic solution is one that has a higher concentration of solutes compared to another solution, usually across a semipermeable membrane. When a cell is placed in a hypertonic environment, water will move out of the cell in an attempt to equalize solute concentrations, leading to cell shrinkage or crenation. Understanding hypertonic solutions is essential for grasping concepts like passive transport and the ways organisms regulate their internal environments.
Hypotonic: Hypotonic refers to a solution with a lower concentration of solutes compared to another solution, typically causing water to move into cells by osmosis. This process can lead to cell swelling and even bursting if the imbalance is significant. Understanding hypotonic solutions is crucial for grasping passive transport mechanisms, maintaining osmotic balance in organisms, and the functioning of excretion systems.
Isotonic: Isotonic refers to a situation where two solutions have the same solute concentration, resulting in no net movement of water across a semipermeable membrane. It is crucial in maintaining cell stability and function by preventing excessive water influx or efflux.
Isotonic: Isotonic refers to a solution that has the same concentration of solutes as another solution, typically across a semipermeable membrane. In biological systems, isotonic environments are crucial for maintaining cell stability, allowing for balanced movement of water and solutes without causing cell swelling or shrinkage. This balance is essential for proper cellular function and overall organismal health.
Kinetic energy: Kinetic energy is the energy an object possesses due to its motion, which depends on both its mass and velocity. It plays a crucial role in various biological processes, as it relates to how substances move across membranes, the energy transformations within cells, and the principles governing thermodynamic systems.
Lysis: Lysis refers to the process of breaking down or disintegrating cells, often leading to their destruction. This phenomenon is especially relevant in understanding how cells interact with their environment during passive transport, as the movement of water and solutes can lead to the swelling and bursting of cells in certain conditions, known as osmotic lysis.
Membrane permeability: Membrane permeability refers to the ability of a biological membrane to allow certain substances to pass through while restricting others. This selective passage is crucial for maintaining homeostasis within cells, as it regulates the movement of ions, molecules, and water in and out of the cell. The degree of permeability is influenced by various factors including the composition of the membrane, the size and charge of the substances, and the presence of specific transport proteins.
Microvilli: Microvilli are tiny, finger-like projections that extend from the surface of epithelial cells, primarily in the intestines and kidneys. They serve to increase the surface area of these cells, enhancing their ability to absorb nutrients and other substances. This structural adaptation is crucial for efficient nutrient uptake and plays a role in various biological processes.
Osmolarity: Osmolarity is the concentration of solute particles in a solution, expressed as osmoles per liter. It plays a crucial role in determining the movement of water across plasma membranes through passive transport mechanisms like osmosis.
Osmosis: Osmosis is the movement of water molecules through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration, aiming to balance solute concentrations on both sides of the membrane. This process is vital for maintaining cellular homeostasis and plays a crucial role in various biological systems, including plant hydration and nutrient transport.
Oxygen: Oxygen is a diatomic molecule (O₂) essential for cellular respiration and energy production in living organisms. As a highly reactive non-metal, it plays a critical role in various biochemical processes, including the formation of water, facilitating metabolic reactions, and participating in gas exchange mechanisms in both plants and animals.
Passive transport: Passive transport is the movement of molecules across a cell membrane without the use of cellular energy. It relies on the concentration gradient to drive the movement of substances from areas of higher concentration to areas of lower concentration.
Passive Transport: Passive transport is the movement of substances across a cell membrane without the need for energy input from the cell. This process relies on the natural tendency of molecules to move from areas of higher concentration to areas of lower concentration, allowing essential nutrients and waste products to passively diffuse in and out of cells. It plays a critical role in maintaining cellular homeostasis and is fundamental to many biological processes.
Phospholipid bilayer: The phospholipid bilayer is a fundamental structure of cell membranes, consisting of two layers of phospholipids arranged tail-to-tail. This arrangement creates a semi-permeable barrier that separates the interior of the cell from the external environment, allowing for the selective passage of substances. The unique properties of phospholipids, with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, are crucial for maintaining cellular integrity and facilitating various cellular functions.
Plasmolysis: Plasmolysis is the process where cells lose water in a hypertonic solution, causing the plasma membrane to pull away from the cell wall. It typically occurs in plant cells when they are exposed to high salt or sugar concentrations.
Potassium ions: Potassium ions (K+) are positively charged particles that play a vital role in various physiological processes, including maintaining cellular homeostasis and generating electrical signals in nerve and muscle cells. These ions are essential for processes like muscle contraction and the transmission of nerve impulses, contributing to overall cellular function and communication.
Root hair cells: Root hair cells are specialized cells located on the surface of plant roots that are crucial for water and nutrient absorption. These cells increase the surface area of the root, allowing for more efficient uptake of essential minerals and water from the soil, utilizing passive transport mechanisms to facilitate movement across their membranes.
Selectively permeable: Selectively permeable refers to the property of a cell membrane that allows certain molecules or ions to pass through it by means of active or passive transport. This selectivity is essential for cellular homeostasis and function.
Simple diffusion: Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration without the use of energy. This process relies on the inherent kinetic energy of molecules and occurs until equilibrium is reached, allowing substances like oxygen and carbon dioxide to cross cell membranes freely. It plays a crucial role in maintaining cellular homeostasis and is essential for various physiological processes.
Sodium ions: Sodium ions (Na\(^+\)) are positively charged particles that play a critical role in various biological processes, including the maintenance of cellular homeostasis and the generation of electrical signals in nerve and muscle cells. These ions are essential for maintaining osmotic balance and are involved in the transport of nutrients and waste across cell membranes, which is a key aspect of passive transport mechanisms.
Solutes: Solutes are substances that are dissolved in a solvent, forming a solution. In biological systems, solutes can include ions, small molecules, and larger macromolecules.
Steroid hormones: Steroid hormones are a class of hormones derived from cholesterol, characterized by their lipid-soluble nature, allowing them to easily pass through cell membranes and bind to intracellular receptors. They play critical roles in regulating various physiological processes, including metabolism, immune response, and reproductive functions, by influencing gene expression and protein synthesis within target cells.
Tonicity: Tonicity refers to the ability of a solution to affect the volume and pressure of a cell by influencing the movement of water across its membrane. It is crucial in understanding how cells maintain their shape and functionality in different environments, particularly in relation to osmotic pressure. Tonicity helps determine whether a cell will swell, shrink, or remain stable when placed in various solutions, impacting processes such as passive transport and osmoregulation.
Water potential: Water potential is a measure of the potential energy in water and drives the movement of water through plants. It is influenced by factors like solute concentration and pressure.
Water potential: Water potential is a measure of the potential energy of water in a system, influencing the direction and movement of water across membranes. It combines both osmotic potential and pressure potential, determining how water moves through plants and organisms. Understanding water potential is crucial for grasping how water is transported in plant cells, affects passive transport, and plays a role in maintaining osmotic balance within cells.