bio 10 lecture 5
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BIO 10 Lecture 5. A SEPERATE SELF: THE PLASMA MEMBRANE. Diameter of plasma membrane: ~ 5 nm Diameter of DNA: 2 nm Diameter of cell: 5,000-50,000 nm. The Plasma Membrane has Two Main Functions:. Forms a physical, largely impermeable barrier between the cell and its surroundings - PowerPoint PPT PresentationTRANSCRIPT
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BIO 10 Lecture 5
A SEPERATE SELF:
THE PLASMA MEMBRANE
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Diameter of plasma membrane: ~ 5 nm
Diameter of DNA: 2 nm
Diameter of cell: 5,000-50,000 nm
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The Plasma Membrane has Two Main Functions:
• Forms a physical, largely impermeable barrier between the cell and its surroundings– Keeps “the goods” concentrated while keeping harmful
materials out• requires transport in two directions
– Provides a “bag” in which DNA can evolve separately from the DNA of other organisms
– All cells have plasma membranes; some also have cell walls
• Facilitates communication between the cell and other cells and the environment
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• Major components:– Phospholipid Bilayer– Cholesterol (animals only)– Proteins – Glycolayx (carbohydrate groups)
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Phospholipid Bilayer
• Phospholipids: two fatty-acid chains and a polar phosphate group attached to glycerol.
• Arrangement of phospholipids in water (two layers, heads pointed out, tails pointed in).
• Permeability of bilayer: lipid center is a barrier to passage of most hydrophilic molecules, but it allows nonpolar, hydrophobic molecules to pass. – Water, although hydrophilic and polar, can move
across the membrane because it is so small
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Cholesterol
• Helps prevent passage of some small molecules
• Adds fluidity to the membrane– If not for cholesterol, the membrane would
harden at low temperatures• Much like peanut oil left in the refrigerator
– This would be incompatible with life in many creatures
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Proteins in the Plasma Membrane:
• Two types: – Integral (span entire membrane) – Peripheral (lie on either side)
• Functions:– Structural support—attach to cytoskeleton – Recognition—helps immune system determine
self from foreign – Communication—receptors and binding sites – Transport—allow molecules to pass
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Glycocalyx
• Sugars or carbohydrates protruding from lipids and proteins in the membrane
• Functions:– Binding sites for proteins in communication and
recognition• Forms basis of ABO blood typing in humans
– Lubricate cells– Attach cells to extracellular matrix
• Enables tissues to form
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• Diffusion:– Is the spontaneous movement of molecules from
regions of higher to lower concentrations
– Concentration gradient = difference between the highest and lowest concentration of a solute
• Like bike coasting downhill, the tendency is for molecules to travel from high to low concentration.
Moving Materials In and Out: Diffusion and Gradients
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• Permeability verses semi-permeability– A permeable membrane is one that lets
everything in or out– A semi-permeable membrane is one that lets
some things in but not others– An impermeable membrane is one that does
not let anything in or out (like a plastic bag)• Cells have semi-permeable membranes
– They are highly selective• Osmosis
– Is the net movement of water across a semi-permeable membrane from an area of lower solute concentration to higher solute concentration.
• Membrane lets water through but not solutes
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Osmosis in Cells:
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1. Passive Transport– Simple diffusion
• Water, gases, fat soluble molecules• Membrane is permeable to these molecules, so they
travel down their concentration gradients and enter without energy output by the cell.
– Facilitated diffusion • Larger polar molecules (e.g. ions, sugars)• Membrane is impermeable, so even if they want to
travel down the concentration gradient, they can’t without help from a membrane channel (transport protein)
• However, energy is not expended in this process because the molecule is still following its concentration gradient
Two Main Types of Transport across Cell Membranes
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2. Active Transport– Molecules have to
pass across the membrane up their concentration gradient
– They cannot use the energy of diffusion, but must expend energy in the form of ATP
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Example: Sodium-Potassium Pump
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Movement of Big Molecules or Whole Organisms
• Exocytosis– movement of materials out of the cell by fusion of vesicles
with the plasma membrane• Endocytosis:
– infolding of the plasma membrane to bring large molecules into the cell.
• Pinocytosis: – water and solvents are enclosed in invaginating vesicle
• Receptor-mediated endocytosis: – more specific, with receptor capturing ligand and concentrating
it into an invaginating pit.• Phagocytosis:
– how the human immune system ingests whole bacteria and one-celled creatures eat using pseudopodia.
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Short Review of Lecture 5
• What are the two main functions of the plasma membrane?
• What are its 4 main components?• What does it mean to say that a membrane is semi-
permeable? Why is it important that the plasma membrane has this quality?
• What is the difference between passive and active transport of small molecules across the plasma membrane?
• How can large molecules move in and out of a cell?• Why do you think steroids affect all cells in the body,
not just those in the skeletal muscles?