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Questions and Answers
All living things need membranes to survive.
All living things need membranes to survive.
True
Biological membranes are passive barriers that merely prevent substances from entering or leaving cells.
Biological membranes are passive barriers that merely prevent substances from entering or leaving cells.
False
The first membranes were observed with electron microscopes in the 1950s.
The first membranes were observed with electron microscopes in the 1950s.
True
What are the two primary components of biological membranes?
What are the two primary components of biological membranes?
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Membrane lipids are arranged in a single layer.
Membrane lipids are arranged in a single layer.
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Which type of lipid primarily forms the membrane bilayer?
Which type of lipid primarily forms the membrane bilayer?
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Phospholipids contain glycerol and two fatty acid chains.
Phospholipids contain glycerol and two fatty acid chains.
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What is the function of double bonds in the fatty acid chains of phospholipids?
What is the function of double bonds in the fatty acid chains of phospholipids?
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Cholesterol is a common component of both animal and plant membranes.
Cholesterol is a common component of both animal and plant membranes.
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Cholesterol can either increase or decrease membrane fluidity, depending on the saturation level of the fatty acid chains.
Cholesterol can either increase or decrease membrane fluidity, depending on the saturation level of the fatty acid chains.
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Membrane proteins are only found on the surface of the bilayer.
Membrane proteins are only found on the surface of the bilayer.
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Which of the following is NOT a function of membrane proteins?
Which of the following is NOT a function of membrane proteins?
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What type of bond typically holds peripheral proteins to the membrane surface?
What type of bond typically holds peripheral proteins to the membrane surface?
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Integral proteins can be removed from the membrane using mild treatments like changes in pH or ionic strength.
Integral proteins can be removed from the membrane using mild treatments like changes in pH or ionic strength.
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Transmembrane proteins extend across the entire membrane, connecting the intracellular and extracellular environments.
Transmembrane proteins extend across the entire membrane, connecting the intracellular and extracellular environments.
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What type of membrane protein is responsible for the transport of specific molecules across the membrane?
What type of membrane protein is responsible for the transport of specific molecules across the membrane?
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What is the key difference between channel proteins and carrier proteins?
What is the key difference between channel proteins and carrier proteins?
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Passive transport requires an input of energy.
Passive transport requires an input of energy.
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Active transport moves molecules against their concentration gradient.
Active transport moves molecules against their concentration gradient.
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The sodium/potassium pump is an example of passive transport.
The sodium/potassium pump is an example of passive transport.
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Exocytosis is the process of releasing materials from the cell.
Exocytosis is the process of releasing materials from the cell.
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Endocytosis is the process of taking materials into the cell.
Endocytosis is the process of taking materials into the cell.
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Which type of endocytosis involves the binding of specific molecules to receptors on the cell surface?
Which type of endocytosis involves the binding of specific molecules to receptors on the cell surface?
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Fluid mosaic model describes the membrane as a dynamic structure, with lipids and proteins moving freely within the bilayer.
Fluid mosaic model describes the membrane as a dynamic structure, with lipids and proteins moving freely within the bilayer.
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The two sides of a membrane are identical in their composition.
The two sides of a membrane are identical in their composition.
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What is the primary factor that determines membrane fluidity?
What is the primary factor that determines membrane fluidity?
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Membranes are highly permeable, allowing any molecule to pass through.
Membranes are highly permeable, allowing any molecule to pass through.
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Simple diffusion is a type of passive transport that requires energy input.
Simple diffusion is a type of passive transport that requires energy input.
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Facilitated diffusion requires the help of membrane proteins.
Facilitated diffusion requires the help of membrane proteins.
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Active transport can be driven by energy stored in ion gradients.
Active transport can be driven by energy stored in ion gradients.
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Bulk transport involves the movement of large particles, such as macromolecules and even cells, across the membrane.
Bulk transport involves the movement of large particles, such as macromolecules and even cells, across the membrane.
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What is the main difference between exocytosis and endocytosis?
What is the main difference between exocytosis and endocytosis?
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Receptor-mediated endocytosis requires specific receptors on the cell surface to bind to target molecules.
Receptor-mediated endocytosis requires specific receptors on the cell surface to bind to target molecules.
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Study Notes
Membrane Structure
- Biological membranes are crucial for all living things to survive.
- These membranes control the movement of substances into and out of cells.
- Membranes also regulate the composition of the fluid within cells.
- Membranes control the flow of information between cells.
- Membranes are involved in energy capture and release processes like photosynthesis and oxidation.
Brief History of Membrane Models
- Models of membranes were developed long before they could be seen with electron microscopes.
- In 1915, membranes of red blood cells were analyzed and found to be composed of lipids and proteins.
- In 1925, scientists reasoned that cell membranes are a continuous phospholipid bilayer using extraction of RBCs and various other methods.
- They concluded that membranes are approximately 5-8nm thick.
Sandwich Model
- In 1935, scientists proposed the sandwich model, describing a phospholipid bilayer positioned between two layers of protein globules.
- Support for this model stemmed from early electron microscope images.
- This model was widely accepted until the 1960s, representing the structure of plasma and internal membranes.
Problems with the Sandwich Model
- Measurements indicated that membrane proteins are not highly water-soluble.
- Proteins are amphipathic, possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
- If these proteins were positioned at the membrane's surface, their hydrophobic regions would come into contact with water, thus challenging the sandwich model.
Fluid Mosaic Model
- The fluid mosaic model proposed by Singer and Nicolson (1972) revised the previous model.
- This model suggests that membrane proteins are dispersed within the phospholipid bilayer rather than positioned in layers.
- Hydrophilic regions of proteins and phospholipids face the watery environment, whereas hydrophobic regions are positioned in the non-aqueous environment within the membrane. This arrangement positions hydrophilic regions of proteins and phospholipids in maximum contact with water.
Structural Features of Membranes
- Membranes are fluid structures.
- They are sheet-like structures primarily composed of proteins and lipids, with carbohydrates attached to lipids and proteins.
- Membranes are amphipathic, having both hydrophilic and hydrophobic properties.
- Proteins, embedded in the lipid bilayer, are important as pumps, channels, receptors, and enzymes.
- Membranes possess two distinct sides—the extracellular and cytosolic sides—that differ in composition and function.
Fluid Mosaic Model Details
- Membranes are 2-dimensional fluids of lipids and proteins contributing to the transverse movement of molecules.
- Lipids are arranged in a bilayer structure.
- Proteins can be embedded within or span the entire bilayer.
- Carbohydrate chains are found outside the membrane.
Membrane Components
- Membranes have a general structure consisting of sheets of lipid molecules with proteins embedded within them.
- Lipids and proteins can move freely in the membrane.
- Carbohydrates are almost always on the outside of the membrane.
Types of Lipids
- Phospholipids are most common, with glycerol as their backbone. Sphingophospholipids are also significant.
- Major Glycerophospholipids: Phosphatidyl choline, Phosphatidyl serine, and Phosphatidyl ethanolamine.
- Glycolipids contain either a glycerol or sphingosine backbone.
- Cholesterol: a structurally diverse lipid found mainly in animal membranes. It is not found in most bacterial or plant membranes.
Membrane Proteins
- Membrane proteins perform various functions crucial for cell processes.
- Protein amounts vary between cells and across different membrane types.
- Proteins serve important functions like energy transduction and help overcome permeability barriers within the lipid bilayer.
Types of Membrane Proteins
- Peripheral or extrinsic proteins are located on cell membrane surfaces (extracellular or cytosolic) and are bound by non-covalent interactions, like electrostatic and hydrogen bonds.
- Integral or intrinsic proteins are embedded within the membrane, some being completely embedded and others spanning the entire membrane (transmembrane). They require more drastic treatments to remove (e.g. detergents or organic solvents).
Examples of Membrane Proteins
- Peripheral: G protein, glyceraldehyde-3-PO4 dehydrogenase, fibronectin, spectrin of RBC, cytochrome c
- Integral: G-protein & GPCR, enzyme cholinesterase (in synapses), glycophorin of RBC, adenylate cyclase, anion channels in RBC
Membrane Carbohydrates
- Carbohydrates in eukaryotes comprise 2-10% of the membrane's weight.
- They are predominantly located on the outer surface of the membrane as glycoproteins or glycolipids.
- They participate in cell-cell recognition, playing roles in the appearance and individuality of cells.
- Different biological, chemical, and structural features vary between individuals within the same species.
- Glycolipids are mainly found on the outer surface of the plasma membrane.
Asymmetric Composition of Membranes
- Membranes are not symmetric.
- Carbohydrates are located on the outer surface.
- Lipids have different compositions in the inner and outer membrane layers.
- Certain proteins are positioned predominantly on the inner or outer surface.
Membrane Fluidity
- Membranes are fluid structures.
- Lipids and proteins are constantly moving within the membrane.
- Three types of movement are: Lateral diffusion (along the plane of the membrane), rotational movement (around their axis), and transverse diffusion or "flip-flop" (movement across the layer).
Factors Affecting Membrane Fluidity
- Temperature: higher temperatures increase fluidity, lower temperatures decrease it.
- Lipid Composition: Short tails and the presence of cis double bonds in hydrocarbon tails increase fluidity, whereas long tails decrease fluidity
- Cholesterol Content: In saturated lipids, cholesterol stabilizes the membrane. In unsaturated lipids, cholesterol increases fluidity, by filling in the gaps caused by kinks in the tails.
Transport Across Membranes
- Materials continuously cross cell membranes.
- Membranes are selectively permeable because of the hydrophobic lipid bilayer.
- This barrier controls molecule passage.
- Selective permeability helps cells maintain the needed solute concentration differences between their cytosol and the extracellular fluids (ECF)
3 Types of Membrane Transport
- Transport of Small Molecules: Passive transport (downhill) and active transport (uphill)
- Transport of Large Particles (Bulk Transport): Exocytosis (export), endocytosis (import)
- Exocytosis: Constitutive (occurs continuously) or regulated (occurs in response to signals).
- Endocytosis: Pinocytosis, phagocytosis, and receptor-mediated endocytosis.
Downhill & Up-hill Transport
- Downhill transport occurs from high to low concentration, often passive.
- Up-hill transport requires energy, typically ATP, to move molecules against their concentration gradient.
- Electrochemical gradients affect ions, considering both concentration and electrical differences.
Transport Driven by ATP
- Active transport, like the Na+/K+ pump, uses ATP to maintain specific ion concentrations.
Transport Driven by Ion Gradients
- Secondary active transport uses ion gradients to power the transport of different molecules. This energy source derives from the primary active transport system, as energy is still necessary to maintain the concentration difference in the first place.
- Examples include co-transport (symport, antiport) and other similar mechanisms where the use of one gradient powers the movement of another substance against its own gradient.
Getting Things Across Cell Membranes
- Channels: water-filled pores that allow specific inorganic ions to pass through.
- Carriers (transporters): bind to specific molecules, inducing a conformational change and transferring them across the membrane. Uniporters move one molecule while co-transporters move two or more substances.
Mechanisms of Sodium Pump
- Binding of Na+ to the cytosolic side triggers ATP binding and phosphorylation.
- This change in shape releases Na+.
- Binding of K+ triggers dephosphorylation.
- The pump's original shape returns, releasing K+ inside the cell.
Other Examples of Active Transport
- Examples include proton pumps in the stomach and calcium pumps in muscle cells.
Transport Driven by Light
- Light-driven transport mechanisms, like those involved in vision, utilize specific proteins for transporting molecules in response to light absorption by them.
Membrane Channels
- Gated channels; Ligand-gated or chemically gated channels.
- Voltage-gated or electrically gated channels.
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