Biomembrane Structure and Functions
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Questions and Answers

What type of proteins are responsible for moving phospholipids from one leaflet to the other in membranes?

  • Passive diffusion proteins
  • ATP-powered transport proteins (correct)
  • Receptor proteins
  • Co-transport proteins

Which phospholipid is primarily found in the outer leaflet of the membrane?

  • Phosphatidylcholine (PC) (correct)
  • Phosphatidylinositol (PI)
  • Phosphatidylethanolamine (PE)
  • Phosphatidylserine (PS)

What are lipid rafts enriched with?

  • Glycolipids and proteins only
  • Cholesterol and sphingolipids (correct)
  • Phospholipids and cholesterol only
  • Integral proteins and peripheral proteins

Which type of membrane protein does not span the membrane?

<p>Lipid-anchored proteins (A)</p> Signup and view all the answers

How are transmembrane proteins characterized in terms of their structure?

<p>Two hydrophilic exterior regions and one hydrophobic interior region (A)</p> Signup and view all the answers

Which type of proteins are exclusively found in the exoplasmic face of the plasma membrane?

<p>Glycoproteins and glycolipids (C)</p> Signup and view all the answers

Which characteristic is most commonly associated with the secondary structure of hydrophobic regions in transmembrane proteins?

<p>Hydrophobic α-helices (C)</p> Signup and view all the answers

What role do arginine and lysine residues play in glycoprotein structure?

<p>They help anchor the protein in the membrane (A)</p> Signup and view all the answers

What is the primary composition of prokaryotic cell membranes?

<p>Single plasma membrane containing proteins essential for cell function (D)</p> Signup and view all the answers

Which function is NOT typically associated with proteins in eukaryotic cell membranes?

<p>Catalyzing ATP synthesis (A)</p> Signup and view all the answers

Which term describes the ability of a cell membrane to allow certain substances to pass while blocking others?

<p>Selective permeability (A)</p> Signup and view all the answers

What is meant by 'tonicity' in relation to cellular membranes?

<p>The relative concentration of solutes outside the cell compared to inside (A)</p> Signup and view all the answers

What structural feature do all biomembranes share?

<p>Phospholipid bilayers forming leaflets (D)</p> Signup and view all the answers

What characteristic distinguishes eukaryotic from prokaryotic cell membranes?

<p>Complex internal membrane system (B)</p> Signup and view all the answers

How do phospholipids behave in an aqueous environment?

<p>They spontaneously form bilayers (D)</p> Signup and view all the answers

What term describes the lateral movement of lipids and proteins within the cell membrane?

<p>Fluidity (C)</p> Signup and view all the answers

What is the primary role of desaturase enzymes in fatty acid metabolism?

<p>They add double bonds to fatty acids. (C)</p> Signup and view all the answers

What do fatty acid binding proteins (FABPs) primarily interact with?

<p>Hydrophobic pockets of long fatty acid chains. (D)</p> Signup and view all the answers

Which enzyme is crucial for converting HMG-CoA to mevalonate in cholesterol synthesis?

<p>HMG-CoA reductase. (B)</p> Signup and view all the answers

What is one of the proposed mechanisms for lipid transport between organelles?

<p>Direct contact between membranes mediated by integral proteins. (B)</p> Signup and view all the answers

Which statement accurately describes the effect of statins in relation to cholesterol metabolism?

<p>They inhibit the activity of HMG-CoA reductase. (B)</p> Signup and view all the answers

What do phosphatidic acid and diacylglycerol have in common in phospholipid synthesis?

<p>Both contain a glycerol backbone. (B)</p> Signup and view all the answers

What characterizes atherosclerosis in the arterial walls?

<p>Formation of plaque due to foam cell accumulation. (B)</p> Signup and view all the answers

What type of proteins play a role in the movement of fatty acids within cells?

<p>Cytosolic fatty acid binding proteins. (D)</p> Signup and view all the answers

What is the primary structural component of a micelle?

<p>Fatty acyl chains (C)</p> Signup and view all the answers

What class of membrane lipids is primarily involved in cell membrane fluidity?

<p>Phosphoglycerides (A)</p> Signup and view all the answers

Which fatty acid length is NOT commonly associated with phospholipids?

<p>12 carbons (A)</p> Signup and view all the answers

What role does cholesterol play in cell membranes?

<p>Maintains membrane fluidity at temperature extremes (C)</p> Signup and view all the answers

Which of the following is a characteristic of sphingolipids?

<p>Have a long hydrocarbon chain (D)</p> Signup and view all the answers

What type of fatty acid attachment do plasmalogens have?

<p>One ether and one ester linkage (A)</p> Signup and view all the answers

Which lipid is the most abundant in mammalian plasma membranes?

<p>Cholesterol (D)</p> Signup and view all the answers

How does the presence of sphingolipids affect membrane fluidity?

<p>Decreases membrane fluidity (D)</p> Signup and view all the answers

What is the significance of uneven lipid distribution in cellular membranes?

<p>Influences signaling pathways in the cell (C)</p> Signup and view all the answers

Which of the following statements about membrane proteins is true?

<p>All transmembrane proteins are also integral membrane proteins. (B), All integral membrane proteins are amphipathic. (C)</p> Signup and view all the answers

Which components of biological membrane are considered amphipathic?

<p>Glycolipids and membrane steroids (A), Phospholipids and glycolipids (C)</p> Signup and view all the answers

Which of the following best describes phosphoglycerides?

<p>Have diverse head group attachments (B)</p> Signup and view all the answers

Which of the following about membrane lipids is accurate?

<p>Flippases are responsible for lipid transfer across the membrane. (B)</p> Signup and view all the answers

In which leaflet of the plasma membrane are glycolipids primarily found?

<p>The outer leaflet of the plasma membrane (D)</p> Signup and view all the answers

What can be said about lipid movement in biological membranes?

<p>Both integral and peripheral proteins affect lipid movement. (D)</p> Signup and view all the answers

What determines the blood type in individuals?

<p>The presence or absence of glycosyltransferases (D)</p> Signup and view all the answers

Which fatty acids are primarily synthesized in the cytosol from Acetyl-CoA?

<p>14 and 16 Carbon fatty acids (B)</p> Signup and view all the answers

Which statement best describes the role of detergents in membrane biology?

<p>Detergents can both denature and solubilize proteins (D)</p> Signup and view all the answers

From where are many lipids synthesized in the cell?

<p>Smooth ER (A)</p> Signup and view all the answers

What are the characteristics of amphipathic molecules like detergents?

<p>They have both hydrophilic and hydrophobic parts (A)</p> Signup and view all the answers

What role do oligosaccharide chains play in determining blood types?

<p>They identify blood types by varying sugar compositions (A)</p> Signup and view all the answers

What is the final location for the last steps of membrane lipid synthesis?

<p>Destination membranes (D)</p> Signup and view all the answers

Which of the following is a function of glycosyltransferases?

<p>To add sugars to O Antigen (C)</p> Signup and view all the answers

Which of the following blood type percentages is the highest among Caucasians in the United States?

<p>O+ (C)</p> Signup and view all the answers

How are fatty acids generated from stored triacylglycerol in adipocytes?

<p>By enzymatic hydrolysis (B)</p> Signup and view all the answers

Flashcards

Biomembrane structure

All biomembranes share common features: a phospholipid bilayer with embedded proteins.

Phospholipid bilayer

Two layers of phospholipids forming a membrane; hydrophobic tails face inward, hydrophilic heads face outward.

Membrane protein function

Proteins in the membrane perform various roles including transport, catalysis, and signaling.

Cellular membrane types

Prokaryotic cells have a single plasma membrane, while eukaryotic cells have a plasma membrane and internal membranes surrounding organelles.

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Tonicity

The ability of a solution to cause a cell to gain or lose water.

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Membrane fluidity

Lipids and proteins can move laterally within the membrane.

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Isotonic solution

A solution with the same solute concentration as the cell's cytoplasm; no net water movement.

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Membrane proteins

Proteins embedded within the membrane that carry out many specific functions, like transport or signaling.

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Membrane lipid composition

Membrane lipids are mainly phosphoglycerides, sphingolipids, and sterols (like cholesterol).

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Phosphoglycerides

A major type of membrane lipid with a hydrophilic head and hydrophobic tails.

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Sphingolipids

Membrane lipids derived from sphingosine, containing a long hydrocarbon chain.

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Sterols

Amphipathic lipid molecules that include cholesterol, important for membrane fluidity.

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Amphipathic lipids

Lipids with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

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Fatty acids

Long hydrocarbon chains with a carboxyl group at one end, part of phospholipids.

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Lipid asymmetry

Uneven distribution of lipids between the inner and outer leaflets of a cell membrane.

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Phospholipid

The most common type of lipid that helps to form the membrane bilayer, having both a hydrophilic phosphate head and a hydrophobic tail of fatty acids.

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Cholesterol

Important for membrane fluidity and structure; a sterol lipid present in abundance in the membranes of animal cells.

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Flippase

A type of ATP-powered transporter protein that moves phospholipids from the inner leaflet of a membrane to the outer leaflet.

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ABC Superfamily

A class of ATP-powered transport proteins that includes flippases and other pumps involved in moving molecules across membranes.

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Phospholipid Asymmetry

The uneven distribution of phospholipids in a membrane's leaflets; for example, phosphatidylserine (PS) is primarily found in the inner leaflet.

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Lipid Rafts

Specialized regions within the cell membrane enriched with specific lipids (cholesterol, sphingolipids) and proteins. They play a role in signal transduction.

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Peripheral Membrane Protein

A protein associated with the membrane's surface, but not embedded within its structure. They can be easily removed.

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Integral Membrane Protein

A protein firmly embedded within the membrane structure, spanning the entire lipid bilayer.

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Lipid-Anchored Protein

A protein attached to the membrane through a lipid molecule, instead of directly interacting with the bilayer.

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Transmembrane Protein Structure

A protein that spans the membrane with three distinct regions: hydrophilic ends interacting with water, and a hydrophobic interior interacting with the lipid tails.

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Transmembrane proteins

A type of integral membrane protein that spans the entire lipid bilayer, with its amino acid chain extending through the hydrophobic core and into the aqueous environment on either side of the membrane.

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Amphipathic molecules

Molecules that have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

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Blood Type Antigens

Sugars found on glycolipids or glycoproteins of red blood cells, determining blood type (A, B, AB, or O) by the presence or absence of specific sugars.

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Glycosyltransferases

Enzymes that add specific sugars (like galactose or GalNAc) to the O antigen on red blood cells, determining blood type.

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What determines blood type?

The presence or absence of glycosyltransferases responsible for adding specific sugars to the O antigen on red blood cells.

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Detergents

Amphipathic molecules that disrupt cell membranes by inserting into the phospholipid bilayer and solubilizing lipids and proteins.

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How do detergents work?

Detergents insert into the phospholipid bilayer due to their amphipathic nature, ultimately disrupting the membrane and solubilizing lipids and proteins.

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Membrane Synthesis

New biomembranes are created by expanding pre-existing membranes, with synthesis often occurring on the smooth ER.

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Fatty Acid Synthesis: Sources

Fatty acids can be synthesized from: triacylglycerol breakdown (stored fat) or de novo from Acetyl CoA.

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Fatty Acid Synthesis: Process

14 and 16 carbon fatty acids are synthesized in the cytosol using enzymes like Acetyl-CoA carboxylase and fatty acid synthase.

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How are new membranes formed?

New biomembranes are synthesized by expanding pre-existing membranes, using the smooth ER as a primary site for lipid synthesis.

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Fatty Acid Elongation

The process of adding carbon units to existing fatty acids, increasing their chain length. This occurs mainly in the smooth endoplasmic reticulum (ER) membrane, resulting in saturated fatty acids.

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Fatty Acid Desaturation

The process of introducing double bonds into saturated fatty acids, creating unsaturated fatty acids. This occurs in the ER membrane with the help of desaturase enzymes.

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Fatty Acid Binding Proteins (FABPs)

Cytosolic proteins that bind to long-chain fatty acids, facilitating their transport within cells. They show up- and down-regulation based on cellular needs for fatty acid uptake and release.

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Phosphatidic Acid

A key intermediate in phospholipid synthesis. It's formed by combining fatty acyl CoA and glycerol phosphate using acyl transferases on the smooth ER membrane.

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Diacylglycerol

A molecule formed from phosphatidic acid by removing a phosphate group. It's a crucial intermediate used in the synthesis of phospholipids, including choline phospholipids.

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HMG-CoA Reductase

An enzyme embedded in the smooth ER membrane responsible for converting HMG-CoA into mevalonate, a crucial step in cholesterol biosynthesis.

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Atherosclerosis

The hardening of arteries caused by buildup of plaque within the arterial walls, leading to narrowing of the arteries. It's a major contributor to heart disease.

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Statins

Anti-atherosclerosis medications that work by inhibiting HMG-CoA reductase, thereby reducing cholesterol biosynthesis and slowing down plaque formation.

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Study Notes

Biomembrane Structure

  • Biomembranes are integral components of all cells
  • Prokaryotic cells have a single plasma membrane, usually without internal membrane compartments
  • Prokaryotic cell membranes contain hundreds of proteins crucial for cell function (e.g., catalyzing ATP synthesis, initiating DNA replication, transporting ions, molecules)
  • Proteins can also be involved in cell signaling
  • Eukaryotic plasma membranes are studded with proteins with various functions
  • Membrane transport proteins facilitate ion, sugar, amino acid, and vitamin passage

Membrane Functions

  • Membranes contribute to the shape of cells and organelles
  • Membranes possess stability due to hydrophobic and van der Waals interactions between lipid chains
  • Membranes have selective permeability (tonicity) including isotonic, hypertonic and hypotonic conditions

Lipid Composition

  • Biomembranes share common phospholipid features
  • Phospholipids spontaneously form bilayers
  • The bilayer is composed of two leaflets
  • Lipids are amphipathic

Membrane Protein Distribution

  • In membranes, proteins can be peripheral, integral or lipid-anchored
  • Transmembrane proteins span the membrane, typically with hydrophobic α-helices interacting with fatty acid tails
  • Integral membrane proteins have multiple portions embedded within the membrane
  • Peripheral proteins are loosely associated with the membrane surface
  • Cell types and subcellular locations affect membrane protein amounts and types

Membrane Proteins

  • Porins: a class of trimeric transmembrane proteins that form barrel-like structures with central pores, allowing molecules to traverse the membrane
  • Aquaporins: transmembrane proteins that transport water and other hydrophilic molecules

Lipid Distribution

  • Membrane lipids are asymmetrically distributed between exoplasmic and cytoplasmic leaflets
  • Cholesterol is generally evenly distributed
  • Mechanisms like flippases facilitate asymmetric lipid distribution and lipid transport between leaflets to maintain asymmetries
  • Lipid composition influences membrane physical properties (e.g., fluidity, membrane thickness, curvature)

Lipid Rafts

  • Regions of cell membranes with enriched specific lipids (cholesterol, sphingolipids) and proteins
  • They may promote specific functions
  • Decreased cholesterol content in membranes can disrupt lipid rafts

Membrane Biogenesis

  • New bio-membranes are formed by expanding existing ones
  • Lipids are synthesized mainly on smooth endoplasmic reticulum membranes
  • Steps of lipid synthesis complete on destination membranes
  • Lipid distribution needs to occur after synthesis to appropriate leaflets and organellar membranes

Fatty Acid Synthesis

  • Fatty acids can be synthesized from other organic molecules or Acetyl CoA
  • They are primarily synthesized in the cytoplasm
  • Elongation to longer chains may occur in the ER membrane
  • Some fatty acids have double bonds, created by desaturase enzymes

Intracellular Fatty Acid Movement

  • Fatty acids are transported within cells using cytosolic protein carriers known as fatty acid binding proteins (FABPs)
  • FABP upregulation/downregulation is responsive to cellular demands

Phospholipid Synthesis

  • Fatty acyl CoA and glycerol phosphate are synthesized into phosphatidic acid
  • Then the molecule is produced into diacylglycerol
  • Lastly phosphocholine is transferred to diacylglycerol to form the phospholipid
  • Flipases transfer lipids between cytosolic and exoplasmic leaflets

Cholesterol Synthesis

  • Acetyl CoA and acetoacetyl CoA form HMG-CoA, which converts to mevalonate, a 6-carbon molecule
  • HMG-CoA reductase converts HMG-CoA to mevalonate
  • Mevalonate, IPP and farnesyl pyrophosphate are intermediates in cholesterol biosynthesis
  • Squalene is converted to cholesterol on the SER

Atherosclerosis

  • Atherosclerosis is the hardening of arteries due to lumen blockage
  • It arises from damage to the endothelial cells
  • Monocytes enter and are converted into macrophages which then take up LDL cholesterol
  • Excess LDL forms foam cells, eventually accumulating to form plaques that obstruct arteries

Transport of Lipids between Organelles

  • Cholesterol and phospholipids are often transported via Golgi-dependent and Golgi-independent processes
  • There is also transport via vesicles; direct interaction between organellar membranes mediated by integral proteins
  • Lipid transfer proteins can mediate transport

Membrane Proteins

  • Membrane proteins are diverse in kind and amount dependent on the cell and subcellular location
  • Proteins interact with membranes in 3 ways: peripheral, integral and lipid-anchored proteins

Typical Single-pass Transmembrane Protein

  • Glycophorin A (an integral protein in RBC membranes) has a hydrophobic alpha-helix portion and carbohydrate chains
  • The protein is usually a dimer

Multipass Membrane Protein

  • Bacteriorhodopsin (a transmembrane protein) crosses the membrane multiple times with hydrophobic alpha-helices
  • Other multipass membrane proteins are also integral to the membrane

Porins

  • Porins are a type of transmembrane protein that forms channels
  • The channels are made up of Beta pleated sheets
  • Examples include mitochondrial porin and E.coli porin (OmpX)

Aquaporins

  • Aquaporins are tetrameric transmembrane proteins
  • They transport water and hydrophilic molecules

Anchored Membrane Proteins

  • Anchored membrane proteins are covalently linked to lipids by hydrocarbon chains
  • These chains provide direct connections to the membrane

Human ABO Blood Group Antigens

  • Antigens are oligosaccharide chains covalently attached to glycoproteins or glycolipids
  • Differences in these chains determine blood type (A, B, O, AB)

Q1. Membrane Protein Characteristics

  • A: Incorrect
  • B: Correct
  • C: Correct
  • D: Incorrect

Q2. Amphipathic Membrane Components

  • a: Partially correct. Integral membrane proteins have hydrophobic and hydrophilic regions, making them amphipathic.
  • b: Correct
  • c: Correct
  • d: Correct

Q3. Properties of Membrane Lipids

  • a: Incorrect. Membrane lipids have hydrophobic and hydrophilic regions, making them amphipathic.
  • b: Correct. Flippases catalyze the transfer of lipids across the membrane.
  • c: Incorrect. While lipids can move, it's not spontaneous in all cases. Flippases and other mechanisms are required, in most cases, for movement to occur.
  • d: Correct. Different lipid compositions are found in the different membrane leaflets.

Q4. Glycolipid Location in Plasma Membrane

  • c: Correct. Glycolipids are predominantly found in the outer leaflet of the plasma membrane

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