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Chapter 7: Membrane structure and function - Coggle Diagram
Chapter 7: Membrane structure and function
7.1: Cellular membranes are fluid mosaics of lipids and proteins
Some are covalently bonded to lipids, forming molecules called glycolipids. (Recall that glyco refers to carbohydrate.) However, most are covalently bonded to proteins, which are thereby glycoproteins
A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic ("water-loving") region and a hydrophobic ("water-fearing") region
Phosphatidylcholine – one of the most common phospholipids in cell membranes.
Phosphatidylethanolamine – helps maintain membrane structure and flexibility.
Phosphatidylserine – important for cell signaling.
Phosphatidylinositol – involved in communication inside cells.
A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water
In this fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
EVOLUTION Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions.
Example: Archaea that live in extremely hot environments (like hot springs) have different membrane lipids than most organisms
Now we come to the mosaic aspect of the fluid mosaic model.
Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer
Integral proteins penetrate the hydrophobic interior of the lipid bilayer.
The majority are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior.
Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, often to exposed parts of
ral proteins
7.2: Membrane structure results in selective permeability
The biological membrane has emergent properties beyond those of the many individual molecules that make it up.
A membrane exhibits selective permeability; that is, it allows some substances to cross more easily than others.
hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane.
For example, the passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins
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7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Molecules have a type of energy called thermal energy, due to their constant motion (see
diffusion, the movement of particles of any substance so that they spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional.
concentration gradient, the region along which the density of a chemical substance increases or decreases
The diffusion of a substance across a biological membrane is called passive transport because it requires no energy.
The diffusion of free water across a selectively permeable membrane, whether artificial or cellular, is called osmosis.
tonicity, the ability of a surrounding solution to cause a cell to gain or lose water.
If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means "same"),
there will be no net movement of water across the plasma membrane.
Let's transfer the cell to a solution that is hypertonic to the cell (hyper means "more," in this case referring to nonpenetrating solutes).
If we place the cell in a solution that is hypotonic to the cell hypo means "less"), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.
osmoregulation, the control of solute concentrations and water balance.
For example,
the unicellular protist Paramecium caudatum lives in pond water, which is hypotonic to the cell.
the cell is turgid (very firm), which is the healthy state for most plant cells. Plants that are not woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution.
If a plant's cells and surroundings are isotonic, there is no net tendency for water to enter and the cells become flaccid (limp);
the plant wilts.
plasmolysis, causes the plant to wilt and can lead to plant death. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.
many polar molecules and ions blocked by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion.
Channel proteins that transport ions are called ion channels.
Many ion channels function as gated channels, which open or close in response to a stimulus
The Scientific Skills Exercise gives you an opportunity to work with data from an experiment related to glucose transport.
7.4: Active transport uses energy to move solutes against their gradients
Despite the help of transport proteins, facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient, a process that requires no energy.
To pump a solute across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport.
One transport system that works this way is the sodium-potassium pump, which exchanges Nat for Kt across the plasma membrane of animal cells
The voltage across a membrane, called a membrane potential, ranges from about -50 to -200 millivolts (mV). (The minus sign indicates that the inside of the cell is negative relative to the outside.)
Thus, two forces drive the diffusion of ions across a membrane: a chemical force
(the ion's concentration gradient, which has been our sole consideration thus far in the chapter) and an electrical force (the effect of the membrane potential on the ion's movement). This combination of forces acting on an ion is called the electrochemical
gradient.
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A transport protein that generates voltage across a membrane is called an electrogenic pump. :
The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports protons (hydrogen ions, Ht) out of the cell.
In a mechanism called cotransport, a transport protein (a cotransporter) can couple the "downhill" diffusion of the solute to the "uphill" transport of a second substance against its own concentration gradient.
7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Large molecules, such as proteins and polysaccharides, generally don't cross the membrane by diffusion or transport proteins.
Instead, they usually enter and leave the cell in bulk, packaged in vesicles.
The cell secretes certain molecules by the fusion of vesicles with the plasma membrane; this process is called exocytosis
In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane. :
