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Questions and Answers
What is the primary function of cell membranes?
What is the primary function of cell membranes?
- To regulate the movement of substances into and out of the cell (correct)
- To produce energy through cellular respiration
- To store genetic information
- To synthesize proteins
In eukaryotic cells, what is a key role of compartmentalization achieved by cell membranes?
In eukaryotic cells, what is a key role of compartmentalization achieved by cell membranes?
- Reducing the number of chemical reactions
- Increasing the overall cell size
- Segregating processes and compartments (correct)
- Enhancing the rate of diffusion
Which of the following is NOT a main component of cell membranes?
Which of the following is NOT a main component of cell membranes?
- Lipids
- Proteins
- Carbohydrates
- Nucleic acids (correct)
What property of the plasma membrane allows it to reseal if broken?
What property of the plasma membrane allows it to reseal if broken?
What is the approximate thickness of the lipid bilayer in cell membranes?
What is the approximate thickness of the lipid bilayer in cell membranes?
Which of the following is true regarding the lipid composition of cell membranes?
Which of the following is true regarding the lipid composition of cell membranes?
Which type of microscopy is best suited for viewing the 3D structure of a cell membrane's surface?
Which type of microscopy is best suited for viewing the 3D structure of a cell membrane's surface?
What is a key difference between eukaryotic and prokaryotic cell membranes regarding sterols?
What is a key difference between eukaryotic and prokaryotic cell membranes regarding sterols?
Which of the following is a primary function of transmembrane proteins?
Which of the following is a primary function of transmembrane proteins?
What is the main type of interaction holding lipid and protein molecules together in a cell membrane?
What is the main type of interaction holding lipid and protein molecules together in a cell membrane?
What does 'amphipathic' mean in the context of membrane lipids?
What does 'amphipathic' mean in the context of membrane lipids?
What is the predominant type of lipid in cell membranes?
What is the predominant type of lipid in cell membranes?
Which structural feature is common to all cell membranes when viewed in cross-section?
Which structural feature is common to all cell membranes when viewed in cross-section?
Which process involves the splitting of a membrane into two parts?
Which process involves the splitting of a membrane into two parts?
What is the role of a 'freeze-fracture' technique in studying cell membranes?
What is the role of a 'freeze-fracture' technique in studying cell membranes?
What is guaranteed by the differences in length and saturation of fatty acid tails in phospholipids?
What is guaranteed by the differences in length and saturation of fatty acid tails in phospholipids?
How does the presence of a cis-double bond affect a hydrocarbon tail in a phospholipid?
How does the presence of a cis-double bond affect a hydrocarbon tail in a phospholipid?
Which microscope is used to observe the interior of a cell by transmitting a beam of electrons through a thin specimen?
Which microscope is used to observe the interior of a cell by transmitting a beam of electrons through a thin specimen?
What is the main function of vesicles and liposomes?
What is the main function of vesicles and liposomes?
If a cell membrane is described as a 'selective barrier', what does this mean?
If a cell membrane is described as a 'selective barrier', what does this mean?
Which cellular process is NOT facilitated by membrane fusion and fission?
Which cellular process is NOT facilitated by membrane fusion and fission?
For a red blood cell membrane, what is the appearance of the membrane when viewed in cross-section under an electron microscope?
For a red blood cell membrane, what is the appearance of the membrane when viewed in cross-section under an electron microscope?
What role do membrane proteins play in the attachment of the cell membrane to the cytoskeleton?
What role do membrane proteins play in the attachment of the cell membrane to the cytoskeleton?
What happens when a phospholipid bilayer spontaneously closes to form a sealed compartment?
What happens when a phospholipid bilayer spontaneously closes to form a sealed compartment?
Which of the following statements is NOT a function of the cell membrane?
Which of the following statements is NOT a function of the cell membrane?
Which of the following is required for a phospholipid to form a bilayer instead of a micelle?
Which of the following is required for a phospholipid to form a bilayer instead of a micelle?
Which of the following is considered a property that is summarized as being part of the cell membrane?
Which of the following is considered a property that is summarized as being part of the cell membrane?
What is the role of noncovalent interactions between lipids and proteins in a cell membrane?
What is the role of noncovalent interactions between lipids and proteins in a cell membrane?
A researcher wants to examine protein distribution within the cell membrane. Which of the following methods could provide the most direct visualization?
A researcher wants to examine protein distribution within the cell membrane. Which of the following methods could provide the most direct visualization?
Under what circumstances is the production of heat by peroxisomes most relevant?
Under what circumstances is the production of heat by peroxisomes most relevant?
How does the selective permeability of the plasma membrane contribute to cellular homeostasis?
How does the selective permeability of the plasma membrane contribute to cellular homeostasis?
Which characteristic is unique to membrane phospholipids compared to other lipids?
Which characteristic is unique to membrane phospholipids compared to other lipids?
What is the main reason why lipids spontaneously form bilayers in an aqueous environment?
What is the main reason why lipids spontaneously form bilayers in an aqueous environment?
If a researcher introduces short, saturated fatty acids into a cell membrane, what effect would this likely have on membrane fluidity?
If a researcher introduces short, saturated fatty acids into a cell membrane, what effect would this likely have on membrane fluidity?
Which major phospholipid carries a net negative charge at physiological pH?
Which major phospholipid carries a net negative charge at physiological pH?
A researcher is studying the effect of a new drug on a particular membrane receptor. What percentage of human genome's proteins must be taken into account for consideration, as they are membrane proteins?
A researcher is studying the effect of a new drug on a particular membrane receptor. What percentage of human genome's proteins must be taken into account for consideration, as they are membrane proteins?
Why is membrane fluidity essential for cell function?
Why is membrane fluidity essential for cell function?
Which technique is most suitable for directly visualizing the lipid bilayer structure of a cell membrane?
Which technique is most suitable for directly visualizing the lipid bilayer structure of a cell membrane?
Flashcards
Cell Membrane Function
Cell Membrane Function
Regulates movement into/out of cell, facilitates electrical signaling, defines organelle boundaries.
Cellular Compartmentalization
Cellular Compartmentalization
Compartmentalization segregates processes within eukaryotic cells.
Antiparallel Process Separation
Antiparallel Process Separation
Separates anabolic/catabolic processes, preventing substrate competition/interference.
Main cell membrane components
Main cell membrane components
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Membrane Proteins Function
Membrane Proteins Function
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Amphipathic Lipids
Amphipathic Lipids
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Bilayer formation
Bilayer formation
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Fluid mosaic model
Fluid mosaic model
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Membrane Thickness
Membrane Thickness
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Trilaminar appereance
Trilaminar appereance
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Selective barriers
Selective barriers
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Membrane Fusion and Fission
Membrane Fusion and Fission
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Membrane Fusion
Membrane Fusion
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Membrane Fission
Membrane Fission
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Lipid structures
Lipid structures
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Energetically unfavorable
Energetically unfavorable
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Energetically Favorable
Energetically Favorable
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Vesicles and liposomes
Vesicles and liposomes
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Liposome uses
Liposome uses
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SEM
SEM
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Freeze-fracture
Freeze-fracture
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Sterols in Prokaryotes vs. Eukaryotes
Sterols in Prokaryotes vs. Eukaryotes
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Cell Membrane: Phospholipids
Cell Membrane: Phospholipids
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Fatty Acid Tails
Fatty Acid Tails
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Major phospholipids
Major phospholipids
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Asymmetric
Asymmetric
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How is the lipid monolayer setup?
How is the lipid monolayer setup?
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Study Notes
- Without biological membranes, there is no life.
- A cell membrane regulates the movement of material into and out of a cell.
- Cell membranes facilitate electrical signaling between cells.
- Organelle boundaries are defined because of cell membranes.
- Cell membranes also separate complex chemical reactions.
- Membranes are flexible; this allows for growth and movement.
- Membranes are selectively permeable to polar solutes and regulate molecular traffic across boundaries.
- Membranes are central to energy conservation and cell-to-cell communication.
- Fusion or fission is possible because of self-sealing.
- The cytoskeleton and extracellular structures attach to membranes.
- In eukaryotic cells, compartmentalization segregates processes and compartments.
- Cellular compartmentalization allows:
- Functional separation within the cell.
- Separation of antiparallel processes such as anabolic and catabolic processes which can be held in separate compartments. This avoids competition for the substrate or interference in reactions.
- Separation of similar reactions which serve different purposes, needing to be held in independent compartments.
- Coordination of different reactions involved in the same pathway for energy efficiency.
- Cell membranes are made of lipids, proteins, and carbohydrates.
- Membrane lipids include phospholipids, sterols, and glycolipids.
- Lipids comprise approximately 50% of the mass of most animal cell membranes.
- Transmembrane and peripheral proteins are components of the cell membrane.
- Glycan groups of carbohydrates are components of the cell membrane.
- Membrane proteins are part of or interact with biological interfaces.
- Most membrane proteins are transmembrane and mediate many functions like transport and catalysis.
- Some transmembrane proteins are structural links that connect the cytoskeleton through the lipid bilayer to the extracellular matrix or an adjacent cell.
- Membrane proteins act as receptors to detect and transduce signals.
- About 30% of the human genome's proteins are membrane proteins.
- Membrane proteins are targets of over 50% of all modern medicinal drugs.
- Three main kinds of lipids occur in membranes:
- Phospholipids (glycerophospholipids and sphingophospholipids).
- Sterols (cholesterol in eukaryotic cells).
- Glycolipids (glycosphingolipids).
- All lipid molecules in cell membranes are amphipathic, meaning they have a hydrophilic polar end and a hydrophobic nonpolar end.
- Phospholipids are the most abundant membrane lipids.
- A universal basis for membrane structure is the lipid bilayer.
- The special properties of lipid molecules result in the bilayer structure.
- Lipids cause spontaneous assemblage into bilayers.
- The common general structure is a thin film (average 5 nm thick) of lipid and protein molecules, held together mainly by noncovalent interactions.
- Electron microscopy is used to see cell membranes.
- Specialized techniques are necessary (x-ray diffraction, freeze-fracture).
- Membranes are fluid and have dynamic structures; most molecules can move freely within the membrane's plane (Fluid Mosaic Model).
- All cell membranes share a characteristic trilaminar appearance.
- Plasma membrane has a three-layer structure, 5-8 nm thick.
- Trilaminar image consists of two electron-dense layers separated by a less dense central region.
- Membrane thickness affects the rate of solute diffusion.
- The following table shows the composition, by percentage, of lipids, proteins, and carbohydrates in various membranes:
- Nuclear Membrane (Rat): 35% lipids, 59% proteins, 2-4% carbohydrates.
- Liver Cell (Rat): 42% lipids, 53% proteins, 5-10% carbohydrates.
- Liver Cell (Mouse): 54% lipids, 46% proteins, 3% carbohydrates.
- Erythrocytes (Human): 43% lipids, 49% proteins, 8% carbohydrates.
- Outer Mitochondrial Membrane: 48% lipids, 52% proteins, 2-4% carbohydrates.
- Inner Mitochondrial Membrane: 24% lipids, 75% proteins, 1-2% carbohydrates.
- Gram-Positive Bacteria: 24% lipids, 75% proteins, 1% carbohydrates.
- Prokaryotic cells have a simpler composition:
- One main type of phospholipid.
- No sterols (cholesterol), with some rare exceptions.
- Mechanical stability is due to an overlying cell wall.
- Eukaryotic cells have a varied composition:
- Mixtures of different phospholipids.
- Large amounts of sterols (cholesterol).
- Sterols offer stability and fluidity in membranes.
- Membranes act as selective barriers and allow selective transport of molecules into and out of the cell.
- Membranes contain enzymes, transporters, and receptors, allowing exchanges between the environment in a controlled manner.
- Membranes are extremely dynamic and can gain information from the cell's metabolic status.
- Membrane fusion and fission is central to many cellular processes in organelles and the plasma membrane.
- Fusion allows for a combination of compartmentalization and transport of material.
- Both processes involve membrane reorganization without loss of continuity.
- Membrane fusion: two separate lipid bilayers merge to become one.
- Transport vesicles from the ER fuse with Golgi membranes, or a virus membrane fuses with a cell membrane.
- Membrane fission: splitting of a membrane into two parts.
- Formation of vesicles by the ER/Golgi apparatus transports lipids and proteins to other organelles or to the cell membrane.
- Hydrophilic molecules dissolve readily in water because of charged groups or uncharged polar groups.
- These molecules form favorable electrostatic interactions or hydrogen bonds with water molecules.
- Hydrophobic molecules are insoluble in water because most or all of their atoms are uncharged and nonpolar.
- They cannot form energetically favorable interactions with water molecules.
- Spontaneous formation of bilayers in aqueous environments relates to the shape and amphipathic nature of membrane lipids.
- This can form spherical micelles, bilayers (sheets), and vesicles/liposomes.
- Lipids spontaneously form micelles or bilayers in an aqueous environment.
- Cone-shaped amphipathic molecules (fatty acids) form micelles. Cylinder-shaped molecules (phospholipids) form bilayers or liposomes.
- Planar phospholipid bilayers with edges exposed to water are energetically unfavorable.
- The spontaneous closure of a phospholipid bilayer to form a sealed compartment is energetically favorable.
- Closed structures avoid exposure of hydrophobic hydrocarbon tails to water, which is energetically unfavorable.
- This provides the bilayer with self-healing properties: all free edges are avoided by closing in on themselves.
- Vesicles are intra or extracellular structures consisting of a liquid/cytoplasm enclosed by a lipid bilayer.
- Function: Exocytosis, endocytosis, lysosomes, and transport (ER to Golgi).
- The environment inside the vesicle can differ from the cytosolic environment due to metabolism, transport, temporary storage of food and enzymes, and chemical reaction chambers.
- Liposomes are vesicles made artificially in a laboratory.
- Three techniques are used for visualizing cells are electron microscopy (EM):
- Scanning electron microscope (SEM): Directly produces an image of the 3D structure of a specimen's surface.
- Transmission electron microscope (TEM): A beam of electrons transmits through a specimen to form an image, capturing fine detail.
- Freeze-fracture and freeze-etch electron microscopy: Provides views of surfaces inside the cell.
- The freeze-fracture technique involves breaking apart a frozen biological sample and coating the fracture plane with a metal layer for visualization via electron microscopy.
- It gives views from inside the membranes.
- Allows for understanding of the cell membranes.
- Phospholipids have a polar head group and two hydrophobic hydrocarbon tails (fatty acids).
- Tails generally differ in length (normally: 14-24 C).
- One fatty acid usually contains one or more cis-double bonds (unsaturated).
- The other fatty acid does not contain the double bonds and is saturated.
- A double bond creates a small kink in the tail.
- Length and saturation of fatty acid tails affect the ability of phospholipid molecules to pack together, and so guarantee the membrane fluidity.
- The four main phospholipids predominate in the plasma membrane of mammalian cells:
- Phosphatidylcholine.
- Phosphatidylethanolamine.
- Phosphatidylserine.
- Sphingomyelin.
- These phospholipids account for more than half the lipid mass in most membranes.
- The bilayer is asymmetric with unequal distribution of phospholipids.
- Phosphatidylinositol is present only in small quantities, but is very important functionally (e.g., cell signaling).
- At physiological pH, phosphatidylserine carries a net negative charge, and other phospholipids are electrically neutral.
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