Download Membrane Structure and Function

Membrane Structure and Function
 Cells must contain a cell membrane,
cytoplasm and genetic material.
 The cell membrane is the EDGE, “boundary
of life”, while the cytoplasm is the site of all
the reactions of life and the genetic material
is the information required for life.
 SELECTIVELY PERMEABLE means that the
CM allows some substances across more
easily than others… some it helps and some
it inhibits or rejects all together.
Membrane Structure
 Development of models as a scientific
process (linked to technology)
 Models include
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Overton – proposed lipid base
Langmuir (phospholipids) – made membrane
Gorter and Grendel (two layers)
Davson and Danielli (sandwich image, protein outside)
Singer and Nicolson = Fluid Mosaic Model
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Required electron microscopy
1972
Membrane Structure
 Phospholipids – phosphate and fatty acid tails
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Hydrophilic phosphates
Hydrophobic fatty acid tails
 Two layers
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With tails together
 Protein
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Embedded throughout - integral
Some just surface – peripheral
Motility
Idea of Fluidity
 Cholesterol molecules prevent membrane
from becoming ‘crispy’
 Proteins ‘bob’ along surface
 Proteins are anchored by fibers in the
cytoskeleton
 Phospholipids can even flip flop
 Merging of membranes ( pg. 128)
Proteins - many purposes within CM
 Transport – substance across membrane,
specificity to substance, active pumps
 Enzymatic activity – sequence reactions
 Transmit signals – conformational change
when with substrate = message.
 Junctions – glue cells into tissues
 Recognition – glycoproteins act as targets or
ID
 Attach to cytoskeleton – change shape of cell
Other organelles
 The cell membrane is continuous with other
membranes in the cell
 Endoplasmic reticulum starts/ends at CM
 ER makes CM and it is turned inside out as
the new CM flows out of the ER….
Traffic Across the Membrane
 Small particles and ions move in both
directions; sugars, amino acids, CO2, NH4
and O2. Also Na+, K+, Ca+2 and Cl Hydrophobic molecules ( like CO2) can
dissolve in lipid layer
 Hydrophilic core of CM impedes movement of
ions and polar molecules that are hydophilic.
 Proteins built into membrane facilitate the
process
Transport Proteins
 Span the CM
 Function as a channel
 Some also physically move passengers
 Specificity of fit ( form and function ) allows
for membrane selectivity. (emergent property
and homeostasis )
Passive Transport
 Molecules have intrinsic kinetic energy
 They are in constant motion
 Cytoplasm, extracellular fluid and CM vary in
concentrations and pressure gradients.
 Concentration refers to the overall
POPULATION of molecules as well as the
RATIO in that location compared to another.
 Net Movement
 DYNAMIC EQUILIBRIUM
Passive transport includes
 Diffusion
 Tendency of molecules of any substance to spread out
into the available space. Substances will diffuse
DOWN their concentration gradient. (Entropy)
 Osmosis
 The diffusion of water molecules across a selectively
permeable membrane.
 Hypertonic = solution with higher [conc.] of solutes
 Hypotonic = solution with lower [conc.] of solutes
 Isotonic = solutions are equal in solute concentration
 Facilitative Diffusion
 Transport proteins are helping molecules to cross
membrane, but still diffusion (lowering overall free
energy) thus doesn’t require energy from cell.
Active Transport
 Movement of solutes, UP their concentration
gradient, with the help of transport proteins in
a CM.
 Requires cell to expend energy / do work.
 Solute becomes more concentrated or
“stock- piled” in a particular area.
Example is Sodium-Potassium Pump. (Na-K)
Electrogenic Pumps
 In addition to moving ions, some active
transport pump also generate electric current
and/ ATP.
 An enzyme called ATP synthase is built into
the membrane and ADP is phosphorylated as
a result of ions (often H+) passing through the
membrane.
 Examples are found in the Electron Transport
Chains of respiration (inner mitochondrial
membrane) and photosynthesis (chloroplasts
thylakoid membrane)
THE “Answers” for AP Biology 
If you don’t know an answer choose either
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Hydrogen bond
(water, organic compounds, etc)
OR
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Hydrogen ion
(membrane pumps, nerve impulses, etc)