120 Questions
What is the primary reason for the establishment of a concentration gradient across the cell membrane?
To maintain the selective permeability of the membrane
Which of the following best describes the movement of ions across the cell membrane?
They are influenced by both their concentration gradient and the electrical gradient
What is the primary function of the selective permeability of the cell membrane?
To maintain the concentration gradient across the membrane
What is the net movement of water through a selectively permeable membrane?
From an area of high water concentration to an area of low water concentration
What percentage of resting energy is used to maintain concentration and electrical gradients?
30%
What is the term for the difference in charged ions between the inside and outside of the membrane?
Membrane potential
What is the term for the stored energy represented by the concentration and electrical gradients?
Potential energy
What is the analogy used to describe the cell membrane in terms of storing and separating charge?
Capacitor
What is the primary function of membrane proteins in the cell membrane?
To mediate the transport of substances across the membrane
Which of the following molecules can permeate the hydrophobic core of the lipid bilayer?
K+
What is the primary driving force behind diffusion?
Kinetic energy of particles
What is the effect of increasing the surface area of a cell on diffusion?
It increases the rate of diffusion
What is the maximum size of a cell that can rely solely on diffusion for transport?
20 µm
What is the effect of membrane thickness on the rate of diffusion?
It slows down the rate of diffusion
Which of the following molecules is impermeable to the lipid bilayer?
Glucose
What is the effect of increasing the temperature on the rate of diffusion?
It increases the rate of diffusion
What is the condition for osmosis to occur across a biological membrane?
The membrane is permeable to water but not to certain solutes.
What is the primary function of aquaporins in cell membranes?
To facilitate the transport of water across the membrane.
What is the effect of mercury on the permeability of water through the cell membrane?
It specifically inhibits the permeability of water through water channels.
What is the relationship between the permeability of water through the lipid bilayer (Pd) and the permeability of water through water channels (Pf)?
Pd is less than Pf.
What determines the difference in osmolarity between two solutions?
The concentration of solutes in both solutions.
What is the effect of temperature on the permeability of water through the lipid bilayer?
It increases the fluidity of the lipid bilayer, increasing the permeability of water.
What is the definition of osmotic pressure?
The pressure required to prevent the inward flow of water across a semi-permeable membrane.
What is the result of a difference in osmolarity between two solutions separated by a semi-permeable membrane?
Water will move from the solution with lower osmolarity to the solution with higher osmolarity.
What is the primary function of proteins in the fluid mosaic model of membrane structure?
To act as gatekeepers and regulate traffic across the membrane
What type of lipid molecules comprise 75% of the lipids in the cell membrane?
Phospholipid molecules
What is the thickness of the cell membrane?
8 nm (8 x 10^-9 metre)
What is the primary reason for the selective permeability of the cell membrane?
The lipid bilayer is impermeable to polar substances
What is the primary mechanism of transport across cell membranes?
All of the above
What is the primary function of the lipid bilayer in the cell membrane?
To act as a barrier to entry or exit of polar substances
What is the term that describes the movement of substances from an area of higher concentration to an area of lower concentration?
Diffusion
What is the structure of the phospholipid bilayer in the cell membrane?
Two parallel layers of phospholipid molecules
What is the primary reason for the asymmetry of lipid composition in membrane leaflets?
Lipids rarely flip-flop between membrane leaflets
What determines the fluidity of a membrane?
Lipid tail length and number of double bonds
Which type of protein is attached to the surface of the cell membrane and is easily removed?
Peripheral protein
What is the function of hydrophobic regions in integral membrane proteins?
To span the hydrophobic core of the lipid bilayer
Which type of molecule is permeable through the lipid bilayer?
Nonpolar, uncharged molecules
What is the primary function of receptor proteins in the cell membrane?
To act as cell identity markers
What is the result of the molecular organization of the cell membrane?
Selective permeability and fluidity
What is the characteristic of hydrophilic regions in integral membrane proteins?
They are polar and interact with the aqueous solution
What is the primary reason for the limitation of cell size to approximately 20 μm in diameter?
The rate of diffusion sets a limit on the size of cells.
What is the effect of increasing the surface area of a cell on diffusion?
The rate of diffusion increases.
What is the effect of temperature on the rate of diffusion?
The rate of diffusion increases.
What is the result of a difference in concentration between two sides of a membrane?
The solvent will move from the area of lower concentration to the area of higher concentration.
What is the effect of membrane thickness on the rate of diffusion?
The thicker the membrane, the slower the rate of diffusion.
Which of the following molecules can diffuse through the hydrophobic core of the lipid bilayer?
CO2
What is the primary benefit of increasing the membrane area available for exchange?
It increases the rate of diffusion.
What is the result of an increase in the surface area of a cell?
The rate of facilitated diffusion increases.
Which of the following best describes the energy requirement for the maintenance of concentration and electrical gradients across the cell membrane?
It requires 30% of resting energy.
What is the primary driving force behind the movement of ions across the cell membrane?
Electrochemical gradient
What is the term for the difference in charged ions between the inside and outside of the cell membrane?
Membrane potential
What is the primary consequence of osmosis across a biological membrane?
Net movement of water from low to high concentration
What is the primary function of the selectively permeable cell membrane in terms of osmosis?
To regulate the movement of certain molecules
What is the primary reason for the energy storage in concentration and electrical gradients across the cell membrane?
To store energy for cellular processes
What is the term for the movement of substances from an area of higher concentration to an area of lower concentration?
Diffusion
What is the primary analogy used to describe the cell membrane in terms of storing and separating charge?
Capacitor
What is the primary reason for water movement across a biological membrane?
To eliminate an osmotic gradient
Which of the following is a characteristic of Pf, the permeability of water through water channels?
It is mercury-insensitive
What is the result of a difference in osmolarity between two solutions separated by a semi-permeable membrane?
Water will move from the higher to the lower osmolarity solution
What is the term for the pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane?
Osmotic pressure
Which of the following determines the difference in osmolarity between two solutions?
The concentration of solutes
What is the effect of increasing the temperature on the permeability of water through the lipid bilayer?
It increases
What is the relationship between Pf and Pd?
Pf is larger than Pd
Which of the following is a characteristic of the movement of water across a biological membrane?
It is a passive process
What is the primary mechanism by which cholesterol and glycolipids are scattered among the phospholipid molecules in the cell membrane?
Hydrophobic interactions
What is the primary consequence of the amphipathic nature of phospholipid molecules in the cell membrane?
Selective permeability of the membrane
What is the primary role of membrane proteins in the fluid mosaic model of membrane structure?
To regulate traffic across the membrane
What is the primary driving force behind the movement of substances from an area of higher concentration to an area of lower concentration?
Concentration gradient
What is the primary consequence of the difference in osmolarity between two solutions separated by a semi-permeable membrane?
Osmosis
What is the primary function of the lipid bilayer in the cell membrane?
To act as a barrier to the entry or exit of polar substances
What is the primary characteristic of facilitated diffusion?
Requires the presence of membrane proteins
What is the primary consequence of increasing the surface area of a cell?
Increased rate of diffusion
What is the primary mechanism by which nonpolar, uncharged molecules cross the cell membrane?
Passive transport
Which type of molecules are permeable through the lipid bilayer due to their lipid solubility?
Steroids, fatty acids, and some vitamins
What is the primary function of hydrophilic regions in integral membrane proteins?
To interact with the aqueous solution
What determines the fluidity of a membrane?
Lipid tail length, number of double bonds, and amount of cholesterol
What is the result of the molecular organization of the cell membrane?
The membrane allows some substances to cross but excludes others
Which type of protein is attached to the surface of the cell membrane and is easily removed?
Peripheral membrane protein
What is the primary function of transporter proteins in the cell membrane?
To facilitate the transport of specific molecules
What is the primary reason for the asymmetry of lipid composition in membrane leaflets?
Lipids rarely flip flop between membrane leaflets
What is the primary reason for the difference in permeability of water through the lipid bilayer (Pd) and through water channels (Pf)?
Aquaporin isoforms
What is the result of a difference in osmolarity between two solutions separated by a semi-permeable membrane?
Osmosis
What determines the permeability of water through the lipid bilayer (Pd)?
Lipid fluidity
What is the primary function of aquaporin isoforms in cell membranes?
To mediate water permeability
What is the primary driving force behind the movement of water across a semi-permeable membrane?
Concentration gradient
What determines the rate of diffusion across a cell membrane?
Concentration gradient
What is the result of an increase in the surface area of a cell?
Increased diffusion rate
What is the term that describes the movement of substances from an area of higher concentration to an area of lower concentration?
Diffusion
What is the primary factor that determines the rate of diffusion?
Concentration gradient
Which of the following factors increases the rate of diffusion?
Increase in surface area
What is the result of a large concentration gradient across a membrane?
Faster rate of diffusion
What is the effect of increasing the size of the diffusing substance?
Decrease in rate of diffusion
What is the primary reason for the limitation of cell size?
Diffusion rate limitation
What is the effect of increasing the temperature on the rate of diffusion?
Increase in rate of diffusion
What is the effect of increasing the membrane thickness on the rate of diffusion?
Decrease in rate of diffusion
What is the primary benefit of increasing the membrane area available for exchange?
Increase in diffusion rate
What is the primary factor that determines the rate of diffusion across a cell membrane?
Concentration gradient
What is the effect of increasing the surface area of a cell on the rate of diffusion?
Increases the rate of diffusion
What is the primary mechanism of transport across cell membranes?
Passive transport
What is the primary driving force behind the movement of ions across the cell membrane?
Electrochemical gradient
What is the primary benefit of increasing the membrane area available for exchange?
Increases the rate of diffusion
What is the result of a difference in concentration between two sides of a membrane?
Osmosis
What is the primary reason for the limitation of cell size to approximately 20 μm in diameter?
Diffusion rates
What is the effect of temperature on the rate of diffusion?
Increases the rate of diffusion
What determines the direction of diffusion of non-charged molecules across a cell membrane?
The concentration gradient of the molecule
What is the primary reason for the establishment of an electrical gradient across a cell membrane?
The difference in concentration of charged ions between the inside and outside of the membrane
Which of the following is a characteristic of the movement of ions across a cell membrane?
Ions are influenced by the electrical gradient
What is the primary function of the selective permeability of the cell membrane?
To establish a concentration gradient across the membrane
What determines the rate of diffusion of non-charged molecules across a cell membrane?
The concentration gradient of the molecule
What is the result of the difference in concentration of charged ions between the inside and outside of a cell membrane?
An electrical gradient is established
What is the primary driving force behind the movement of non-charged molecules across a cell membrane?
The concentration gradient of the molecule
What is the result of the movement of ions down their concentration gradient across a cell membrane?
An electrical gradient is established
What is the primary factor that determines the rate of diffusion across a cell membrane?
The concentration gradient of the substance
What is the effect of increasing the concentration of a substance on one side of a cell membrane?
It increases the rate of diffusion
What is the relationship between the concentration gradient and the rate of diffusion across a cell membrane?
Directly proportional
What is the effect of decreasing the temperature on the rate of diffusion across a cell membrane?
It decreases the rate of diffusion
What is the primary mechanism by which cells maintain concentration gradients across their cell membranes?
Active transport
What is the effect of increasing the surface area of a cell membrane on the rate of diffusion?
It increases the rate of diffusion
What is the relationship between the concentration gradient and the direction of diffusion across a cell membrane?
The concentration gradient determines the direction of diffusion
What is the primary benefit of maintaining a concentration gradient across a cell membrane?
It allows for the regulation of intracellular ion concentrations
Study Notes
Selective Permeability and Diffusion
- The cell membrane is selectively permeable, allowing certain substances to cross while excluding others
- The lipid bilayer is permeable to nonpolar, uncharged molecules (e.g. O2, N2, benzene) and lipid-soluble molecules (e.g. steroids, fatty acids, some vitamins)
- The lipid bilayer is impermeable to large uncharged polar molecules (e.g. glucose, amino acids) and ions (e.g. Na+, K+, Cl-, Ca2+)
Diffusion
- Diffusion is the random mixing of particles in a solution due to kinetic energy
- Diffusion occurs from an area of high concentration to an area of low concentration
- Factors affecting the rate of diffusion:
- Concentration gradient: the greater the difference in concentration, the faster the rate of diffusion
- Temperature: higher temperatures increase the rate of diffusion
- Particle size: larger particles diffuse slower
- Surface area: increasing surface area increases the rate of diffusion
- Distance: increasing diffusion distance slows down the rate of diffusion
Osmosis
- Osmosis is the diffusion of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration
- Osmosis occurs to eliminate osmotic gradients
- Cell membranes are permeable to water but not to certain solutes, allowing osmosis to occur
Membrane Permeability to Water
- The cell membrane has a permeability coefficient (Pw) that is the sum of two components:
- Pd: permeability through the lipid bilayer, which is small, mercury-insensitive, and temperature-dependent
- Pf: permeability through water channels, which is large, mercury-sensitive, and temperature-independent
- Aquaporins are proteins that mediate Pf, and cells express different isoforms to regulate water permeability
Gradients across the Cell Membrane
- Concentration gradients: non-charged molecules diffuse down their concentration gradients
- Electrical gradients: ions are influenced by the membrane potential in addition to their concentration gradient
- The selective permeability of the membrane enables the establishment of concentration and electrical gradients across the membrane
- Cells maintain concentration and electrical gradients at a cost of energy (~30% of resting energy)
Selective Permeability and Diffusion
- The cell membrane is selectively permeable, allowing certain substances to cross while excluding others
- The lipid bilayer is permeable to nonpolar, uncharged molecules (e.g. O2, N2, benzene) and lipid-soluble molecules (e.g. steroids, fatty acids, some vitamins)
- The lipid bilayer is impermeable to large uncharged polar molecules (e.g. glucose, amino acids) and ions (e.g. Na+, K+, Cl-, Ca2+)
Diffusion
- Diffusion is the random mixing of particles in a solution due to kinetic energy
- Diffusion occurs from an area of high concentration to an area of low concentration
- Factors affecting the rate of diffusion:
- Concentration gradient: the greater the difference in concentration, the faster the rate of diffusion
- Temperature: higher temperatures increase the rate of diffusion
- Particle size: larger particles diffuse slower
- Surface area: increasing surface area increases the rate of diffusion
- Distance: increasing diffusion distance slows down the rate of diffusion
Osmosis
- Osmosis is the diffusion of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration
- Osmosis occurs to eliminate osmotic gradients
- Cell membranes are permeable to water but not to certain solutes, allowing osmosis to occur
Membrane Permeability to Water
- The cell membrane has a permeability coefficient (Pw) that is the sum of two components:
- Pd: permeability through the lipid bilayer, which is small, mercury-insensitive, and temperature-dependent
- Pf: permeability through water channels, which is large, mercury-sensitive, and temperature-independent
- Aquaporins are proteins that mediate Pf, and cells express different isoforms to regulate water permeability
Gradients across the Cell Membrane
- Concentration gradients: non-charged molecules diffuse down their concentration gradients
- Electrical gradients: ions are influenced by the membrane potential in addition to their concentration gradient
- The selective permeability of the membrane enables the establishment of concentration and electrical gradients across the membrane
- Cells maintain concentration and electrical gradients at a cost of energy (~30% of resting energy)
Gradients across the Cell Membrane
- A concentration gradient is the difference in concentration of non-charged molecules between the inside and outside of the cell membrane, causing molecules to diffuse down their concentration gradients.
- An electrical gradient, or membrane potential, is the difference in electrical charge between the inside and outside of the cell membrane, influencing the movement of ions.
- The selective permeability of the cell membrane allows for the establishment of concentration and electrical gradients across the membrane.
- The membrane's selectivity enables cells to maintain differences in charged ions between the inside and outside of the membrane, establishing an electrical gradient.
Ion Gradients across the Membrane
- The concentration of ions (Na+, K+, Cl-) is higher in the extracellular fluid than in the cytoplasm.
- The cell membrane acts as a capacitor, separating and storing charge.
Osmosis
- Osmosis is the diffusion of water through a selectively permeable membrane from an area of high water concentration to an area of low water concentration.
- Osmosis occurs when the membrane is permeable to water but not to certain solutes.
- Water moves to eliminate osmotic gradients.
Membrane Permeability to Water
- The permeability of the cell membrane to water is determined by the presence of aquaporins (9 isoforms) and the lipid bilayer.
- Cells have different permeability to water due to the expression of different aquaporin isoforms.
Selective Membrane Permeability
- The cell membrane is selectively permeable to small uncharged polar molecules (water, urea, glycerol, CO2) and impermeable to large uncharged polar molecules (glucose, amino acids) and ions (Na+, K+, Cl-, Ca2+, H+).
- Membrane proteins mediate the transport of substances across the membrane that cannot permeate the hydrophobic core of the lipid bilayer.
Diffusion
- Diffusion is the random mixing of particles in a solution as a result of kinetic energy.
- The rate of diffusion is influenced by the concentration gradient, temperature, and surface area.
- Diffusion is limited by the size of the cell (approximately 20 µm).
Membrane Fluidity
- Membranes are fluid structures, and lipids can move around within the plane of the membrane.
- Fluidity is determined by lipid tail length, number of double bonds, and the amount of cholesterol.
Membrane Proteins
- Integral proteins extend into or across the cell membrane and are amphipathic, with hydrophobic regions spanning the hydrophobic core of the lipid bilayer.
- Peripheral proteins are attached to the inner or outer surface of the cell membrane and are easily removed.
- Membrane proteins can act as receptors, cell identity markers, linkers, enzymes, ion channels, and transporter proteins.
Selective Permeability of Membrane
- The molecular organization of the membrane results in selective permeability, allowing some substances to cross while excluding others.
- The lipid bilayer is permeable to nonpolar, uncharged molecules and lipid-soluble molecules.
Understand the selective permeability of cell membranes, including the passage of ions, uncharged polar molecules, and large molecules. Learn about the lipid soluble molecules and their role in membrane permeability.
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