ACTIVITY-Membranes Organize Cellular Complexity
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Membranes Organize Cellular Complexity
Membranes Define Compartments
Membranes organize proteins and other molecules enabling the cell to run much more efficiently than if
everything were floating freely. Mitochondrial membranes, for example, keep protein assembly lines
together for efficient energy production. And the lysosome safely holds enzymes that would destroy
essential proteins if released into the cytoplasm. Membrane-enclosed vesicles form packages for cargo
so that they may quickly and efficiently reach their destinations. In this way, membranes divide the cell
into specialized compartments, each carrying out a specific function inside the cell.
Real life complexity inside an insulin-producing pancreas cell. Cells like this contain thousands of membrane-enclosed
compartments. Each color represents a structure with a unique function and composition. mage courtesy of Dr. Brad Marsh,
Institute for Molecular Bioscience, The University of Queensland, Australia. © 2001 National Academy of Sciences,
U.S.A. More about this image
Membranes Form Spontaneously
Phospholipids provide the framework for all membranes in the cell. Phospholipids are made up of a
phosphate head region and a lipid tail region. The two ends of a phospholipid have very different
chemical properties. The head end is attracted to water, while the tail end moves away from water.
When phospholipids are placed into water, they organize themselves into a structure called a bilayer. The
water-fearing tail regions clump together on the inside of the bilayer, as far from water as possible. The
head regions move toward the outside of the bilayer where they contact the surrounding water molecules.
The shape and chemical nature of phospholipids drives them to organize themselves one level further. A
flat phospholipid bilayer leaves the edges of the tails exposed to water. By forming into a sphere, all of the
tail regions are protected inside the bilayer as far from water as possible. The result is a membraneenclosed compartment.
Membrane Proteins Control Traffic
Phospholipid membranes form a barrier that most molecules cannot cross. But living things need to be
able to interact with the outside world. At the very least, waste must be able to go out and raw
materials need to come in. That's where membrane proteins come in. Membrane proteins that contact
the spaces on both sides of the membrane are the gate keepers of cellular compartments. Each type of
compartment has a specific population of membrane proteins that largely define its function. On
average, proteins make up about half the mass of membranes.
The nuclear pore complex (see scanning electron micrographs at right) is a unique protein structure that
controls traffic flow in and out of the nucleus. Each nuclear pore complex is made up of hundreds of
individual proteins.
Nuclear pore complexes on the nuclear membranes of frog oocytes as seen from the cytoplasm (left) and from inside the
nucleus (right). Images courtesy Dr. Martin W. Goldberg, School of Biological and Biomedical Sciences, Durham University,
UK.
Membrane proteins represent a large number of proteins with diverse functions.
Membranes Are Fluid
Phospholipids with their embedded proteins form a dynamic, fluid environment. Individual proteins and
phospholipids flow freely. Complexes of proteins and specific subtypes of phospholipids form "rafts"
that move through the membrane. Organelles stretch and bend and even flow through the cell. Fluid
membranes allow cells to be dynamic and responsive to their environment.
Active membranes in fish skin cells migrate across a culture dish. The shift in focus about halfway through the
sequence reveals nuclei, lysosomes and elongated mitochondria inside the cells. Video courtesy Dr. Mark
Cooper, Associate Professor, Department of Zoology,
Chapter 5 - Membrane Structure and Function
The Plasma Membrane
fluid mosaic model, semi-permeable (selectively permeable), double layer of
phospholipids with embedded proteins
Jobs of the cell membrane
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Isolate the cytoplasm from the external environment
Regulate the exchange of substances
Communicate with other cells
Identification
Phospholipids (fats) contain a hydrophilic head and a nonpolar hydrophobic
tail, which creates a barrier.
Cholesterol - stiffens the membrane by connecting phospholipids
Glycolipids - signal molecules
Glycoproteins - have an attached chain of sugar (antibodies)
Proteins embedded in membrane serve different functions
1. Channel Proteins - form small openings for molecules to difuse through
2. Carrier Proteins- binding site on protein surface "grabs" certain molecules
and pulls them into the cell, (gated channels)
3. Receptor Proteins - molecular triggers that set off cell responses (such as
release of hormones or opening of channel proteins)
4. Cell Recognition Proteins - ID tags, to idenitfy cells to the body's immune
system
5. Enzymatic Proteins - carry out metabolic reactions
Transport Across Membrane
-The membrane is selectively permeable (also called semipermeable )
- Small particles, or particles with no charge can pass through the bilayer
(carbon dioxide and oxygen)
- Water has a charge, does not easily cross the membrane - a channel
protein, Aquaporin helps water across
Passive Transport
Simple Diffusion - water, oxygen and other molecules move from areas of
high concentration to areas of low concentration, down a concentration
gradient
Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier
proteins)
Osmosis - diffusion of water
Solutions: Hypertonic | Isotonic | Hypotonic
Contractile vacuoles can help pump out excess water in freshwater organisms
The central vacuole of plants can store excess water, creating a turgor
pressure; plants are less likely to burst due to cell walls
Active Transport
- involves moving molecules "uphill" against the concentration gradient, which
requires energy (ATP)
Endocytosis - taking substances into the cell (pinocytosis for water,
phagocytosis for solids)
Exocytosis - pushing substances out of the cell, such as the removal of waste
Sodium-Potassium Pump - pumps out 3 sodiums for ever 2 potassium's taken
in against gradient