BIOL 1406 Lecture 7: Biology notes ch. 7

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8 Feb 2017

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Chapter 7: Membranes
Overview: Life at the Edge
All cells have a plasma membrane
the boundary that separates the living cell from its surroundings
The plasma membrane exhibits selective permeability
This semi-permeability is maintained in several different ways
Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
The plasma membrane is composed of phospholipids and proteins
Phospholipids contain hydrophobic and hydrophilic regions that maintain a bilayer
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various
proteins embedded in it
An ocean of phospholipids with islands of protein
A mosaic of proteins is dispersed within the bilayer
These serve numerous functions
Membrane Proteins and Their Functions
Membrane-proteins determine most of the membrane’s specific functions
Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core
Integral proteins that span the membrane are called trans-membrane proteins
Six major functions of membrane proteins:
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to cytoskeleton
Why do cells need a selectively permeable membrane?
Diffusion is the tendency for molecules to spread out evenly into the available space
Molecules naturally try to diffuse in and out of cells
Cells are constantly fighting diffusion to maintain internal homeostasis
Substances diffuse down their concentration gradient
From high concentration to low concentration
The diffusion of a substance across a biological membrane is passive transport
It requires no energy to make it happen
Water is always diffusing into and out of cells
Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of
higher solute concentration
Isotonic: Solute concentration is the same; no net water movement
Hypertonic solution: Solute concentration is greater; cell loses water
Hypotonic solution: Solute concentration is less; cell gains water
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