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Questions and Answers
What is the primary role of the plasma membrane in biological cells?
What is the primary role of the plasma membrane in biological cells?
Which of the following is NOT a characteristic of the plasma membrane?
Which of the following is NOT a characteristic of the plasma membrane?
How do biological cells utilize the plasma membrane for substance exchange?
How do biological cells utilize the plasma membrane for substance exchange?
What surrounds biological cells, providing them with selective permeability?
What surrounds biological cells, providing them with selective permeability?
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What is the interstitial fluid's relationship to the plasma membrane?
What is the interstitial fluid's relationship to the plasma membrane?
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What distinguishes selective ion channels from pores?
What distinguishes selective ion channels from pores?
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Which of the following correctly describes the role of the narrow region in selective ion channels?
Which of the following correctly describes the role of the narrow region in selective ion channels?
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Which type of channel is characterized by its response to changes in membrane voltage?
Which type of channel is characterized by its response to changes in membrane voltage?
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Which of the following is NOT a characteristic of pores?
Which of the following is NOT a characteristic of pores?
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What is the main evolutionary significance of channels?
What is the main evolutionary significance of channels?
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What is the primary factor that affects the rate of diffusion through a membrane?
What is the primary factor that affects the rate of diffusion through a membrane?
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Which of the following methods of transport requires energy expenditure?
Which of the following methods of transport requires energy expenditure?
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According to Fick's Law, the rate of transport is positively influenced by which of the following?
According to Fick's Law, the rate of transport is positively influenced by which of the following?
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What property of substances significantly reduces their permeability through a biological membrane?
What property of substances significantly reduces their permeability through a biological membrane?
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If the diffusion coefficient increases, what happens to the rate of transport?
If the diffusion coefficient increases, what happens to the rate of transport?
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What is the effect of increasing the molecular weight of a substance on its diffusion through a membrane?
What is the effect of increasing the molecular weight of a substance on its diffusion through a membrane?
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What is the primary characteristic of substances that can achieve simple diffusion across a phospholipid bilayer?
What is the primary characteristic of substances that can achieve simple diffusion across a phospholipid bilayer?
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Which equation describes the relationship between permeability and the diffusion coefficient?
Which equation describes the relationship between permeability and the diffusion coefficient?
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What is the effect of increasing the viscosity of a medium on the diffusion coefficient?
What is the effect of increasing the viscosity of a medium on the diffusion coefficient?
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How does the distance of diffusion affect the time required for diffusion?
How does the distance of diffusion affect the time required for diffusion?
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What type of substances must utilize selective ion channels to cross a biological membrane?
What type of substances must utilize selective ion channels to cross a biological membrane?
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In what scenario would diffusion be most effective?
In what scenario would diffusion be most effective?
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What ratio illustrates the permeability of propanol versus glycerol across the membrane?
What ratio illustrates the permeability of propanol versus glycerol across the membrane?
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What is the expected outcome if the absolute temperature of the medium is increased?
What is the expected outcome if the absolute temperature of the medium is increased?
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Study Notes
Transport in Biological Systems
- All biological systems consist of cells surrounded by a selective plasma membrane.
- This membrane controls the exchange of substances between the cell and its surroundings.
- Various substances like oxygen, carbon dioxide, ions, glucose, amino acids, fatty acids, and more, all have different sizes, shapes, and molecular weights.
Types of Transport
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Passive Transport
- Simple Diffusion: Movement of molecules from high to low concentration, driven by random thermal motion.
- Facilitated Diffusion: Movement of molecules across a membrane with the help of transport proteins, still following the concentration gradient.
- Osmosis: The specific diffusion of water across a selectively permeable membrane.
- Filtration: Movement of substances across a membrane due to pressure differences.
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Active Transport: Movement of molecules against their concentration gradient, requiring energy.
- Primary Active Transport: Directly uses ATP (energy) to move ions against their gradient.
- Secondary Active Transport: The movement of one substance down its concentration gradient drives the movement of another substance against its gradient.
- Endocytosis: The process of capturing a substance or particle from outside the cell by engulfing it within a membrane-bound vesicle.
Diffusion
- Diffusion is a random process of movement of any molecules (solid, liquid, or gas) from one location to another through the random thermal motion.
- The amount of substance crossing a biological membrane per unit time is called flux.
- Net flux always occurs from a region of higher concentration to a region of lower concentration.
- Diffusion rate depends on the substance's molecular weight: heavier molecules diffuse slower.
- Fick's Law of Diffusion describes the relationship between different factors that influence diffusion.
- RT (rate of transport) is proportional to the concentration gradient (C1-C2) across the membrane, the surface area (S) of diffusion, and the diffusion coefficient (D) and inversely proportional to the distance of diffusion (∆x).
Diffusion Coefficient
- The Stokes-Einstein equation describes how the diffusion coefficient (D) is related to variables such as Boltzmann Constant (KB), absolute temperature (T), the molecular radius (r) and the viscosity of the medium (η).
- Molecules progressively spread in space over time, following a square root relationship with time (d=√6Dt).
Diffusion in Biological Systems
- Diffusion limits of space constraints biological activity
- Distance and time scale of diffusion is significant for cellular process
- Typical distance = micrometers, time in seconds to hours.
- Water molecule collisions occur very frequently.
Permeability
- Permeability (P) quantifies how easily a substance passes through a membrane.
- Permeability depends on the substance's diffusion coefficient (D) and its partition coefficient (K). This relationship has a direct correlation between the velocity of a substance (cm/s).
- Substances that are highly soluble in lipids have high permeabilities.
- Water, oxygen, carbon dioxide, and anesthetic gases exhibit high permeability across cell membranes.
Permeability and Molecular Size
- Smaller molecules have higher permeability.
- Larger molecules have lower permeability.
- Biological membranes are more permeable to smaller, nonpolar molecules than to large polar molecules.
Permeability and Polarity
- Nonpolar molecules are more readily permeable than polar molecules across biological membranes.
- Lipid bilayers are more permeable to nonpolar molecules than to polar molecules.
Permeability and Ions
- Cell membranes are generally impermeable to ions due to their charge and hydration shells.
- Specialized ion channels exist to facilitate ion transport.
Simple Diffusion and Channels
- Channels are integral membrane proteins which act as selective pathways allowing controlled transport. Types of channels include selective ion channels, porins, and aquaporins.
- Three main types of selective ion channels:
- Voltage-gated channels
- Ligand-sensitive channels
- Mechanosensitive channels
Aquaporins
- Aquaporins are channel proteins specialized in water transport.
- They are highly specific for water and other small polar solutes.
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Description
Explore the mechanisms of transport in biological systems, focusing on the functions of the plasma membrane and the various types of transport. Learn about passive and active transport processes, including diffusion, osmosis, and filtration. This quiz covers essential concepts of substance exchange in cells.
