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Questions and Answers
What are the characteristics of amphiphilic molecules?
What are the characteristics of amphiphilic molecules?
How does cholesterol affect membrane fluidity at high temperatures?
How does cholesterol affect membrane fluidity at high temperatures?
What is the driving force behind the spontaneous formation of lipid bilayers?
What is the driving force behind the spontaneous formation of lipid bilayers?
Which statement best describes the composition of a phospholipid?
Which statement best describes the composition of a phospholipid?
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What distinguishes transverse diffusion from lateral diffusion in membranes?
What distinguishes transverse diffusion from lateral diffusion in membranes?
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What effect does temperature have on membrane fluidity according to the phase transition concept?
What effect does temperature have on membrane fluidity according to the phase transition concept?
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What is a major characteristic of liposomes?
What is a major characteristic of liposomes?
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Which statement is true regarding flippases?
Which statement is true regarding flippases?
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What is a key function of biological membranes?
What is a key function of biological membranes?
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What happens to amphiphilic molecules when they are in an aqueous solution?
What happens to amphiphilic molecules when they are in an aqueous solution?
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Which statement correctly describes the differences between prokaryotic and eukaryotic membranes?
Which statement correctly describes the differences between prokaryotic and eukaryotic membranes?
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Why do amphiphatic molecules orientate themselves at the interface between organic solvents and water?
Why do amphiphatic molecules orientate themselves at the interface between organic solvents and water?
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What characteristic makes water a strong solvent for charged or polar molecules?
What characteristic makes water a strong solvent for charged or polar molecules?
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How does the inner membrane of prokaryotic cells function as a permeability barrier?
How does the inner membrane of prokaryotic cells function as a permeability barrier?
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What role do surfactants or detergents play in relation to biological membranes?
What role do surfactants or detergents play in relation to biological membranes?
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What is the outcome of phase separation observed when water interacts with hexane?
What is the outcome of phase separation observed when water interacts with hexane?
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Study Notes
Cell Membrane Overview
- Cell membranes act as barriers enclosing reaction compartments
- They maintain concentration gradients
- They determine what enters & exits the compartments
- They provide an interaction surface with the environment
Learning Objectives
- Properties and locations of membranes
- Structure of membrane lipids
- How membrane lipids order to form bilayers
- How lipid bilayers form membranes
- How membranes are selective
- How biological membranes are formed
Membranes as Barriers
- Enclose reaction compartment
- Maintain concentration gradients
- Control what enters/exits the compartment
- Provide an interaction surface with the environment
A Barrier in Aqueous Environments
- Membranes form due to phase separation from water's insolubility in non-polar solvents (like hexane) and vice versa.
- Water is a strong dipole while compounds like hexane are apolar.
Aqueous Solubility
- Charged/polar molecules dissolve readily in water due to interactions with the water dipole.
- Non-polar molecules are insoluble in water because there is no interaction with the water dipole.
Compartment of Amphipathic Molecules
- Phase separation between water and organic solvents (like hexane) is due to the insolubility of each
- Surfactants/detergents are amphiphilic (both polar and non-polar)
- Amphipathic molecules organize at the interphase (boundary layer) between water and non-polar solvents.
Behaviour of Amphipathic Molecules in Aqueous Solution
- Micelle formation reduces exposure of lipophilic (non-polar) residues to the aqueous environment
- Hydrophillic heads face the water, hydrophobic tails cluster inside.
Behaviour of Amphipathic Molecules (continued)
- Micelles form in polar solvents
- Inverse micelles form in non-polar solvents
How Detergents Work
- Soaps/detergents dissolve in water
- Surfactants orientate themselves in grease and water
- Agitation separates grease from the surface
- Process continues until cleaning is complete.
Locations of Membranes: Prokaryotes
- Some prokaryotes have a single membrane
- Outer membrane provides protection
- Outer membrane is relatively permeable to small molecules
- Inner membrane acts as a permeability barrier.
- Region between the membranes is called the periplasm.
Locations of Membranes: Eukaryotes
- Eukaryotic cells have a single lipid bilayer cell membrane
- Internal compartments are surrounded by specialized membranes (e.g., mitochondria, nucleus, endoplasmic reticulum)
Terminology
- Hydrophilic: Water-loving, interactions with water are thermodynamically favorable.
- Hydrophobic: Water-fearing, non-polar molecules can’t interact with water.
- Lipophilic: Lipid-loving, tending to combine or dissolve in lipids/fats
- Amphiphilic: Molecules with both hydrophilic and hydrophobic parts
- Amphipathic: Molecules with both hydrophilic and hydrophobic parts.
Phospholipids
- Naturally occurring amphipathic molecules
- Have a glycerol backbone
- Two fatty acyl groups
- A phosphate group
- A head group
Example Phospholipids
- PE (phosphatidylethanolamine)
- PC (phosphatidylcholine)
- PS (phosphatidylserine)
- PI (phosphatidylinositol)
Cholesterol
- Intercalates with acyl chains, reducing mobility
- Decreases fluidity at high temperatures
- Increases fluidity at low temperatures
- Maintains fairly constant fluidity across temperature ranges
- Found in eukaryotic but not prokaryotic intracellular membranes.
Phase Transition
- Membrane fluidity depends on acyl chain composition and temperature.
- Examples: compositions of butter and olive oil.
Spontaneous Formation of Membranes
- Lipid bilayers form spontaneously
- Growth of bimolecular sheets is rapid and spontaneous in water.
- Driven by hydrophobic interactions
- Leads to creation of cell membranes.
Lipid Structures in Aqueous Environments
- Planar bilayers are energetically unfavorable
- Micelles (polar head group outside, non-polar tails inside)
- Vesicles/liposomes are formed from planar bilayers and are energetically favorable (sealed compartments formed by phospholipid bilayers)
Movement of Lipids in Membranes: Lateral Diffusion
- Lipids move laterally within the leaflet.
- 2D lateral diffusion is fast (e.g., bacterial cell length in 1 second at 37°C)
Movement of Lipids in Membranes: Transverse Diffusion (Flip-Flop)
- Movement between leaflets
- Polar heads have large solvation shells that must be shed
- Massive energy requirements, much slower than lateral diffusion(~20 hours).
- Catalyzed by flippases.
Lipid Structures (continued)
- Bilayer membranes are two lipid sheets (monolayers or leaflets)
- Polar heads are on outside, hydrophobic tails are inside
- Typically 4-6 nm thick
- Liposomes/vesicles: aqueous environments enclosed by lipid bilayers
- Useful experimental tools, vary in size (50 nm to >10 µm).
Membranes are Asymmetric
- Compare head groups to understand how they affect membrane properties.
- Phosphatidylcholine & Phosphatidylserine
Membranes are Asymmetric (continued)
- Phosphatidylcholine and Sphingomyelin are in the outer monolayer
- Phosphatidylserine is in the inner leaflet (is - charged, a difference in charge).
- Membrane asymmetry is functionally important
Membrane Composition Inside the Cell
- Diagram illustrating pathways of endocytosis (internalization of materials by the cell).
- Membrane-bound components are labeled and described
Composition of Biological Membranes
- Lipids (25-50% by mass) – phospholipids, glycosphingolipids, cholesterol
- Carbohydrates (glycolipids and glycoproteins)
- Proteins (50-75% by mass)
Glycocalyx
- Carbohydrate layer on the cell surface.
- Consists of glycoproteins, glycolipids etc
Integral vs Peripheral Membrane Proteins
- Integral proteins are embedded within the membrane
- Peripheral proteins are associated with the membrane surface.
- Various classifications of integral proteins are mentioned, including α-helices, helical bundles, and β-barrels with descriptions.
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Description
Explore the essential properties and structures of cell membranes with this insightful quiz. Understand how membranes act as barriers, maintain concentration gradients, and regulate the passage of substances. Dive into the formation and function of lipid bilayers and their role in biological systems.
