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Chapter 6- Lipids, Membranes, and the First Cells

6.1 Lipid Structure and Function

  • Molecules that contain only carbon and hydrogen are known as hydrocarbons.

  • Fatty acid is a simple lipid consisting of a hydrocarbon chain bonded to a polar carboxyl functional group

  • Hydrocarbon chains that consist of only single bonds between the carbons are called saturated.

    • If one or more double bonds exist in the hydrocarbon chains, then they are unsaturated.

    • Saturated lipids that have extremely long hydrocarbon tails, like waxes, form particularly stiff solids at room temperature

    • Highly unsaturated lipids are liquid at room temperature and called oils

  • Steroids a family of lipids distinguished by the bulky, four-ring structure

  • Fats are nonpolar molecules composed of three fatty acids that are linked to a three-carbon molecule called glycerol.

  • In organisms, energy storage is the primary role of fats.

  • The glycerol and fatty acid molecules become joined by what is called an ester linkage.

    • An ester linkage occurs when two atoms ( one of them carrying a double-bonded oxygen, often a carbonyl group) are linked together by an oxygen.

  • Phospholipids consist of a glycerol that is linked to a phosphate group and two hydrocarbon chains of either isoprenoids or fatty acids.

  • Substances that contain both hydrophilic and hydrophobic regions are amphipathic.

6.2 Phospholipid Bilayers

  • A lipid bilayer is created when lipid molecules align in paired sheets.

  • Micelles tend to form from free fatty acids or other simple amphipathic lipids with single hydrocarbon chains.

    • Micelles and phospholipid bilayers form spontaneously in water-no input of energy is required.

  • Vesicles are small bubble-like structures consisting of lipid bilayers surrounding a small amount of aqueous solution.

  • Artificially generated membrane-bound vesicles like these are called liposomes.

  • Liposomes provide a three-dimensional model that mimics a membrane-bound cell.

  • The permeability of a structure is its tendency to allow a given substance to pass through it.

  • Selective permeability means that some substances cross a membrane more easily than other substances do.

    • A membrane’s permeability is closely related to its level of fluidity, which is a measure of molecular mobility. As temperature drops, molecules in a bilayer move more slowly and become less fluid.

6.3 How Substances Move across Lipid Bilayers: Diffusion and Osmosis

  • Spontaneous movement of molecules and ions is known as diffusion.

  • A difference in solute concentrations creates what is called a concentration gradient.

  • Solutes move randomly in all directions, but when a concentration gradient exists, there is a net movement from regions of high concentration to regions of low concentration.

  • When substances diffuse across a membrane in the absence of an outside energy source, it is known as passive transport.

  • The movement of water is a special case of diffusion that is given its own name: osmosis.

  • Osmosis occurs only when solutions are separated by a membrane that permits water to cross, but holds back some or all of the solutes-that is, a selectively permeable membrane

  • If the solution outside the vesicle has a higher concentration of solutes than the interior has, the solution outside is said to be hypertonic relative to the inside of the vesicle.

  • If the solution outside the vesicle has a lower concentration of solutes than the interior has, the outside solution is said to be hypotonic relative to the inside of the vesicle.

  • If solute concentrations are equal on both sides of the membrane, the outside is said to be isotonic.

  • Simple vesicle-like structures that harbor nucleic acids are referred to as protocells.

6.4 Proteins Alter Membrane Structure and Function

  • Their hypothesis was called the fluid-mosaic model. Singer and Nicolson suggested that membranes are a dynamic and fluid mosaic of phospholipids and different types of proteins.

  • The method is called freeze-fracture electron microscopy because the steps involve freezing and fracturing the membrane before examining it with a scanning electron microscope (SEM), which produces images of an object’s surface

  • Some proteins span the membrane and have segments facing both the interior and the exterior of the cell which are called integral membrane proteins or transmembrane proteins.

  • Proteins that bind to membrane lipids or integral membrane proteins without passing through it are called peripheral membrane proteins.

  • In cells, ions routinely cross membranes by way of specialized transmembrane proteins called ion channels.

  • When considered together, concentration and electrical gradients are called an electrochemical gradient.

  • Cells have many different types of pore-like channel proteins in their membranes.

  • Gated channels open or close in response to a signal, such as the binding of a particular substance or a change in the electrical voltage across the membrane

  • When transmembrane proteins assist the passive transport of substances that otherwise would not cross a membrane readily, the process is called facilitated diffusion.

  • The movement of water and K+ are examples of facilitated diffusion through channel proteins, but facilitated diffusion can also occur through specialized membrane proteins called carrier proteins.

  • Transport against a gradient is called active transport.

    • A classic example of how structural changes can lead to active transport is provided in the sodium-potassium pump,

    • Gradients are crucial to the function of the cell, in part because they make it possible for cells to engage in secondary active transport-also known as cotransport.

Chapter 6- Lipids, Membranes, and the First Cells

6.1 Lipid Structure and Function

  • Molecules that contain only carbon and hydrogen are known as hydrocarbons.

  • Fatty acid is a simple lipid consisting of a hydrocarbon chain bonded to a polar carboxyl functional group

  • Hydrocarbon chains that consist of only single bonds between the carbons are called saturated.

    • If one or more double bonds exist in the hydrocarbon chains, then they are unsaturated.

    • Saturated lipids that have extremely long hydrocarbon tails, like waxes, form particularly stiff solids at room temperature

    • Highly unsaturated lipids are liquid at room temperature and called oils

  • Steroids a family of lipids distinguished by the bulky, four-ring structure

  • Fats are nonpolar molecules composed of three fatty acids that are linked to a three-carbon molecule called glycerol.

  • In organisms, energy storage is the primary role of fats.

  • The glycerol and fatty acid molecules become joined by what is called an ester linkage.

    • An ester linkage occurs when two atoms ( one of them carrying a double-bonded oxygen, often a carbonyl group) are linked together by an oxygen.

  • Phospholipids consist of a glycerol that is linked to a phosphate group and two hydrocarbon chains of either isoprenoids or fatty acids.

  • Substances that contain both hydrophilic and hydrophobic regions are amphipathic.

6.2 Phospholipid Bilayers

  • A lipid bilayer is created when lipid molecules align in paired sheets.

  • Micelles tend to form from free fatty acids or other simple amphipathic lipids with single hydrocarbon chains.

    • Micelles and phospholipid bilayers form spontaneously in water-no input of energy is required.

  • Vesicles are small bubble-like structures consisting of lipid bilayers surrounding a small amount of aqueous solution.

  • Artificially generated membrane-bound vesicles like these are called liposomes.

  • Liposomes provide a three-dimensional model that mimics a membrane-bound cell.

  • The permeability of a structure is its tendency to allow a given substance to pass through it.

  • Selective permeability means that some substances cross a membrane more easily than other substances do.

    • A membrane’s permeability is closely related to its level of fluidity, which is a measure of molecular mobility. As temperature drops, molecules in a bilayer move more slowly and become less fluid.

6.3 How Substances Move across Lipid Bilayers: Diffusion and Osmosis

  • Spontaneous movement of molecules and ions is known as diffusion.

  • A difference in solute concentrations creates what is called a concentration gradient.

  • Solutes move randomly in all directions, but when a concentration gradient exists, there is a net movement from regions of high concentration to regions of low concentration.

  • When substances diffuse across a membrane in the absence of an outside energy source, it is known as passive transport.

  • The movement of water is a special case of diffusion that is given its own name: osmosis.

  • Osmosis occurs only when solutions are separated by a membrane that permits water to cross, but holds back some or all of the solutes-that is, a selectively permeable membrane

  • If the solution outside the vesicle has a higher concentration of solutes than the interior has, the solution outside is said to be hypertonic relative to the inside of the vesicle.

  • If the solution outside the vesicle has a lower concentration of solutes than the interior has, the outside solution is said to be hypotonic relative to the inside of the vesicle.

  • If solute concentrations are equal on both sides of the membrane, the outside is said to be isotonic.

  • Simple vesicle-like structures that harbor nucleic acids are referred to as protocells.

6.4 Proteins Alter Membrane Structure and Function

  • Their hypothesis was called the fluid-mosaic model. Singer and Nicolson suggested that membranes are a dynamic and fluid mosaic of phospholipids and different types of proteins.

  • The method is called freeze-fracture electron microscopy because the steps involve freezing and fracturing the membrane before examining it with a scanning electron microscope (SEM), which produces images of an object’s surface

  • Some proteins span the membrane and have segments facing both the interior and the exterior of the cell which are called integral membrane proteins or transmembrane proteins.

  • Proteins that bind to membrane lipids or integral membrane proteins without passing through it are called peripheral membrane proteins.

  • In cells, ions routinely cross membranes by way of specialized transmembrane proteins called ion channels.

  • When considered together, concentration and electrical gradients are called an electrochemical gradient.

  • Cells have many different types of pore-like channel proteins in their membranes.

  • Gated channels open or close in response to a signal, such as the binding of a particular substance or a change in the electrical voltage across the membrane

  • When transmembrane proteins assist the passive transport of substances that otherwise would not cross a membrane readily, the process is called facilitated diffusion.

  • The movement of water and K+ are examples of facilitated diffusion through channel proteins, but facilitated diffusion can also occur through specialized membrane proteins called carrier proteins.

  • Transport against a gradient is called active transport.

    • A classic example of how structural changes can lead to active transport is provided in the sodium-potassium pump,

    • Gradients are crucial to the function of the cell, in part because they make it possible for cells to engage in secondary active transport-also known as cotransport.

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