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Questions and Answers
What characterizes hereditary spherocytosis?
Which component of the cell membrane forms the glyocalyx?
What is the primary role of spectrin in red blood cells?
Which lipid is the most common in biological membranes?
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What is the arrangement of lipids in the plasma membrane called?
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What defines the amphipathic nature of plasma membrane lipids?
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What is the primary symptom of anemia associated with hereditary spherocytosis?
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Which of the following is NOT a component of the plasma membrane structure?
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What defines the structure of the lipid bilayer in the plasma membrane?
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What happens to the phospholipids synthesized in the smooth endoplasmic reticulum?
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Which component affects the fluidity of the plasma membrane by stiffening it?
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Which aspect of phospholipid structure contributes to increased membrane fluidity?
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Identify the correct statement about the flexibility of the plasma membrane.
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How is the plasma membrane described due to the distribution of phospholipids?
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What is the role of flippase enzymes in the plasma membrane?
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What function do glycolipids serve in the lipid bilayer?
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What is the primary function of tight junctions in epithelial cells?
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What is the glycocalyx primarily composed of?
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Which of the following is NOT a function of the cell membrane?
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How do transmembrane proteins span the lipid bilayer?
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What role do integral proteins play in the cell membrane?
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How does the removal of lipid bilayer components affect membrane properties?
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What is a key characteristic of the lipid bilayer?
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What is a primary role of the carbohydrate layer or glycocalyx?
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What is the role of detergents like SDS and Triton in relation to integral membrane proteins?
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Which type of membrane protein typically crosses the lipid bilayer as an alpha helix?
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What characterizes the distribution of membrane proteins?
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How can the movement of membrane proteins be restricted within a cell?
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What is a primary function of plasma membrane proteins?
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Which type of membrane protein is not embedded within the lipid bilayer?
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What is a characteristic of multipass transmembrane proteins?
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What role do receptors in membrane proteins play?
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Study Notes
Hereditary Spherocytosis
- A genetic disease and a type of hemolytic anemia
- Causes red blood cells to lose their biconcave, flexible shape becoming spherical
- This affects their ability to pass through narrow capillaries
- Anemia symptoms include fatigue, dizziness, hair loss, and eye yellowing
- Caused by dysfunctional proteins attaching the cell membrane to the cytoskeleton, such as spectrin and ankyrin
Red Blood Cell Membrane Structure
- Plasma membrane is supported by a meshwork of proteins called the cell cortex (actin, myosin, and actin-binding proteins like spectrin and ankyrin in red blood cells)
- Plasma membrane is a thin fatty film studded with proteins and coated with carbohydrates
- The carbohydrates attached to the plasma membrane proteins and lipids on the outside form a sugar coating called glycocalyx
All Plasma Membranes consist of Lipids and Proteins
- All plasma membranes (surrounding the cell or organelles) consist of lipids and proteins
- Membrane lipids are arranged in two sheets called a lipid bilayer
Plasma Membrane Lipids
- Main lipid components are phospholipids, cholesterol, and glycolipids
- All plasma membrane lipids are amphipathic (hydrophilic head and hydrophobic tail)
- Phosphatidylcholine is the most common phospholipid in biological membranes
- The hydrophilic head of phosphatidylcholine is composed of a choline molecule and a phosphate group
- Glycerol links the hydrophilic head to the hydrophobic tails
- The hydrophobic tails are composed of two fatty acids (14 and 24 carbon atoms)
- The hydrophobic tails can be saturated (no double bonds between the carbon atoms) or unsaturated (with double bond(s) between the carbon atoms)
Plasma Membrane Phospholipids form a Lipid Bilayer
- The hydrophilic heads face water on both surfaces of the bilayer
- The hydrophobic tails are shielded from water and lie next to each other in the interior of the bilayer
Phospholipid Bilayers form Sealed Compartments
- Phospholipid bilayers spontaneously close in on themselves to form sealed compartments
- The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water
- The layer of the lipid bilayer facing the cytosol is called the cytosolic monolayer (or face) and the layer facing the exterior of the cell or the lumen of an organelle is called the non-cytosolic monolayer (or face)
The Lipid bilayer is a Flexible Two-Dimensional Fluid
- The plasma membrane is flexible and its components (lipids and proteins) can move freely within the layer
- Phospholipids can rotate, move laterally within the same layer and flip to the other layer (rarely, with the aid of flippase enzymes)
Several Factors affect the Fluidity of the Plasma Membrane
- Temperature: fluidity increases as the temperature increases
- Lipid composition: cholesterol tends to stiffen plasma membranes
- Phospholipid tail saturation degree: lipid bilayers with unsaturated hydrocarbon tails are more fluid
- Phospholipid tail length: lipid bilayers with shorter fatty acid chains are more fluid
New Membrane is synthesized on the SER
- Phospholipids are made and incorporated into the cytosolic face of the smooth endoplasmic reticulum plasma membrane
- Phospholipids are randomly distributed to the non-cytosolic face of the smooth endoplasmic reticulum’s membrane
New Membrane is then matured in the Golgi Membrane
- The new plasma membrane is then delivered to the Golgi plasma membrane
- In the Golgi membrane, specific phospholipids are transferred back to the cytosolic monolayer (ex: phosphatidylserine)
Therefore, the Plasma Membrane is Asymmetrical
- The plasma membrane is described as asymmetrical because phospholipids are asymmetrically distributed to the cytosolic and non-cytosolic monolayer
Phospholipids and Glycolipids are distributed Asymmetrically
- Glycolipids are in the non-cytosolic monolayer of the lipid bilayer and face the exterior of the cell
Plasma Membrane Asymmetry is preserved during Membrane Transfer
- Plasma membrane is transported by a process of vesicle budding (from the Golgi apparatus) and fusion (with the cell membrane or the plasma membrane of other organelles)
- Membranes retain their orientation during transfer between cell compartments (lipids in the cytosolic monolayer always face the cytosol)
Most Plasma Membrane functions are carried out by Membrane Proteins
- Plasma membrane proteins have a variety of functions:
- Transporters
- Ion channels
- Anchors
- Receptors
- Enzymes
Membrane Proteins associate with the Lipid Bilayer in Different Ways
- There are two general types of membrane proteins:
- Integral transmembrane proteins, monolayer-associated, lipid-linked (anchored)
- Peripheral proteins-attached or lipid-attached
- The distribution of membrane proteins is also asymmetrical
Transmembrane Proteins usually cross the Lipid Bilayer as an α Helix
- The backbone of a polypeptide chain is hydrophilic
- Single pass transmembrane proteins have a hydrophobic alpha helix
- Multi pass transmembrane proteins have amphipathic alpha helices
Detergents are required to solubilize Integral Proteins
- Detergents (SDS and Triton) are required to solubilize integral membrane proteins
- Detergents are amphipathic molecules
Plasma Membrane Proteins can move laterally in the Lipid Bilayer
- Staining membrane proteins allows us to visualize membrane fluidity
The movement of membrane proteins can be restricted
- Cells can confine particular proteins to localized areas:
- By binding the cell cortex
- By binding extracellular matrix molecules
- By binding proteins on the surface of another cell
- Be restricted by diffusion barriers
- Example: epithelial cells in the gut
- Tight junctions prevent proteins from the apical plasma membrane to mix with proteins in the lateral and basal plasma membrane
The Cell Surface is coated with Carbohydrates
- All carbohydrates added to proteins and lipids face outside of the cell
- This sugar coating is called the carbohydrate layer or glycocalyx
- The carbohydrates of glycoproteins and proteoglycans often function in cell recognition and adhesion
Glycocalyx allows cell-cell adhesion
The Functions of the Cell Membrane
- All these properties allow the cell membrane to get involved in various essential functions such as:
- Cell signaling
- Transport
- Cell growth and motility
- Cell-cell recognition
- Intercellular adhesion
Learning Outcomes
- Describe the structure, location and function of the cell cortex.
- Explain how the lipid bilayer is formed and why is it considered fluid and asymmetrical.
- List the main membrane components, explain how they move within the lipid bilayer and how their movement (membrane fluidity) is regulated and restricted.
- Illustrate in a diagram, a cell membrane with its phospholipids and different membrane proteins. Include examples of lipid bilayer asymmetry.
- Predict how removal (or inhibition) of any components of the cell membrane affects its properties and function.
- Predict the results of cell fusion experiments.
- Contrast different types of integral proteins (single pass, multipass, β-barrel transmembrane protein, anchored protein, etc.) and integral and peripheral proteins in terms of structure and function.
- Explain how the polypeptide chain of a transmembrane protein, with its hydrophilic backbone, can span the hydrophobic interior of the lipid bilayer.
- Explain how detergent molecules can extract proteins from a cell membrane.
- Explain where and how new membrane is synthesized and explain how phospholipids are asymmetrically distributed to produce even lipid bilayers.
- Describe how membranes retain their orientation during transfer between cell compartments.
- Describe the structure and function of the glycocalyx and with an example explain its role in tissue formation.
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Description
This quiz explores hereditary spherocytosis, a genetic disease characterized by the transformation of red blood cells into a spherical shape. Learn about its causes, symptoms, and the structural aspects of red blood cell membranes, including the role of proteins like spectrin and ankyrin in cell integrity.
