B2.1 Membranes and membrane transport Flashcards
B2.1.2—Lipid bilayers as barriers
Students should understand that the hydrophobic hydrocarbon chains that form the core of a membrane
have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so
membranes function as effective barriers between aqueous solutions.
Large molecules and hydrophilic particles do not
pass easily between the hydrophobic hydrocarbon
chains that form the core of a membrane. This gives
membranes low permeability to these substances
and allows membranes to function as effective
barriers between the aqueous solutions.
Low membrane permeability makes it possible to
maintain differences in concentration (concentration
gradients) across a membrane. The plasma
membrane has a particularly important role
as it holds useful substances inside the cell and
prevents many potentially harmful substances
from entering.
Examples of low permeability
Large molecules: proteins, starch, glycogen, cellulose
Polar molecules: glucose, amino acids
lons: chloride, sodium, potassium, phosphate
B2.1.1—Lipid bilayers as the basis of cell membranes
Phospholipids and other amphipathic lipids naturally form continuous sheet-like bilayers in water.
Phospholipids naturally form continuous sheet-like
bilayers in water. The reasons for this are described
in Section B1.1.12. Other amphipathic lipids such as
cholesterol join with phospholipids to form the bilayers
that are the basis of cell membranes-both the plasma
membrane that forms the outer boundary of the cell
and the membranes of organelles in eukaryote cells.
hydrophobic core
of membrane
c o m p o s e d of
n o n - p o l a r parts
of phospholipids
and other
amphipathiclipids 10000000000003000
hydrophilic exterior of cholesterol
membrane composed phospholipids
of polar/charged parts
of lipids
B2.1.3—Simple diffusion across membranes
Use movement of oxygen and carbon dioxide molecules between phospholipids as an example of simple
diffusion across membranes.
Diffusion is net movement of particles from a region with a higher concentration to a region of lower concentration. Diffusion is passive because it is a natural consequence of the continual random motion of particles in a liquid or gas. Diffusion does not happen in solids.
* Small non-polar molecules can diffuse across membranes because they can pass between phospholipid molecules.
* The rate at which such molecules move from one side of a membrane to the other depends on their concentration. The higher the concentration on one side of a membrane, the more molecules move from that side to the other side per unit time.
* If the concentration of small non-polar molecules is the same on the two sides of a membrane, the molecules will move in both directions across the membrane, but because rates of movement are equal, they cancel out and there is no net movement.
* If there is a higher concentration on one side of a membrane than the other side, the molecules move across in both directions, but more move from the higher to the lower concentration than vice versa.
There is therefore a net movement from the higher to the lower concentration. This net movement is simple diffusion, because no special structures are required in the membrane-the molecules move through the phospholipid bilayer.
phospholipid bilayer
high concentration
low concentration
solute molecules dissolved in water (water not shown)
net movement
Oxygen and carbon dioxide are both small, non-polar molecules so they can enter or leave cells by simple diffusion. In a respiring animal cell for example, oxygen enters by simple diffusion and carbon dioxide leaves.
Oxygen is non-polar because the two oxygen atoms share electrons evenly in the double covalent bond between them.
Carbon dioxide has two polar carbon-oxygen bonds, but as the molecule is linear, the polarity is cancelled out, so CO, is non-polar.
B2.1.4—Integral and peripheral proteins in membranes
Emphasize that membrane proteins have diverse structures, locations and functions. Integral proteins are
embedded in one or both of the lipid layers of a membrane. Peripheral proteins are attached to one or
other surface of the bilayer.
In addition to the basic phospholipid bilayer, the membranes of cells contain proteins. The protein molecules are mostly globular, with a wide range of structures, functions and positions in the membrane.
Integral proteins are embedded in the phospholipid bilayer. Peripheral proteins are attached to the surface of the membrane, on one side or the other.
the two sides of most
integral proteins have
membranes have different
a mostly hydrophobic
functions so the proteins
surface so they are
are different
embedded in the bilayer
peripheral proteins are attached to proteins or lipids in the bilayer on one side of the membrane or the other
00
transmembrane proteins stretch across from one side of the membrane to the other
B2.1.5—Movement of water molecules across membranes by osmosis and the role of aquaporins
Include an explanation in terms of random movement of particles, impermeability of membranes to
solutes and differences in solute concentration
Water is the solvent, both in the cytoplasm of cells and in the fluids that surround cells. Particles dissolve in water by forming hydrogen bonds or other intermolecular interactions with water molecules.
In aqueous solutions, both the water molecules and solutes are in continual random motion. Membranes are typically very permeable to water but have low permeability to many solutes. For this reason, water moves across membranes far more readily than most solutes. It can move in either direction across a membrane, but because of water’s attraction to solutes, there is a net movement from the side with lower solute concentration to the side with higher solute concentration. The direction of net water movement across membranes is thus governed by differences in solute concentration, not water concentration, so it is not typical diffusion and instead is defined as osmosis.
osolute
4 water
net movement of water by osmosis
region of lower
solute concentration (in this case pure water)|
- partially permeable membrane
region of higher solute concentration
It is the overall concentration of solutes that causes movement of water by osmosis, not concentrations of particular dissolved substances.
Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane.
Plasma membranes are all permeable to water to some extent, but the permeability can vary. Permeability is increased by placing aquaporins in a membrane.
These are transmembrane integral proteins with a pore through which water molecules can pass in either direction. The properties of the pore prevent other particles, such as protons or chloride ions, from passing through. conical entrances to the pore -
narrow part of pore that acts as a selectivity filter
phospholipid bilayer
aquaporin
water molecules pass through in single file
Aquaporins do not use energy to make water move.
Water movement across membranes is always passive-water molecules are never pumped.
B2.1.6—Channel proteins for facilitated diffusion
Students should understand how the structure of channel proteins makes membranes selectively
permeable by allowing specific ions to diffuse through when channels are open but not when they are
closed.
lons such as chloride and sodium and polar molecules such as glucose only pass between the phospholipids in a membrane at very slow rates. To allow these substances to diffuse through membranes at the rates required by cells, channel proteins are needed. Inside each channel protein is a pore that allows particles to pass across the membrane in either direction. The diameter of the narrowest part of the pore and charges (+ or -) on the amino acids lining the pore make channel proteins specific only one substance or group of related substances can pass through. For example, potassium channels only allow potassium ions (K*) through. No energy from ATP is used
solutes (water not shown)|
arrows show movement through pore
channel| protein with pore
higher solute concentration
ower solute concentration
Facilitated diffusion is passive movement of particles across a membrane from a higher concentration to a lower concentration via channel proteins.
Some channel proteins have mechanisms for opening and closing their pores.
B2.1.7—Pump proteins for active transport
Students should appreciate that pumps use energy from adenosine triphosphate (ATP) to transfer specific
particles across membranes and therefore that they can move particles against a concentration gradient.
Active transport is the movement of substances across membranes using energy from ATP. Active transport moves substances against the concentration gradient (from a lower to a higher concentration). Protein pumps in the membrane carry out active transport. Pumps work in a specific direction-the substance can only enter the pump on one side and exit on the other side. This is due to the pump alternating between two conformations. ATP causes a change from the more stable to a less stable conformation. The reverse change happens without input of energy.
2. energy from ATP causes the pump to change conformation
higher solute concentration
T. particle enters from one side of the membrane, and binds to a site inside the pump protein
lower solute concentration
3. particle exits the membrane on the other side
4. the pump reverts spontaneously to its original conformation
B2.1.8—Selectivity in membrane permeability
Facilitated diffusion and active transport allow selective permeability in membranes. Permeability by
simple diffusion is not selective and depends only on the size and hydrophilic or hydrophobic properties
of particles.
Membranes have been described as semi-permeable and partially permeable, because they allow some substances through, but not others. This fits simple diffusion across membranes: particles that are large or hydrophilic cannot pass, but any small and non-polar (hydrophobic) particle can move across, with a net movement down the concentration gradient (from high to low concentration).
Facilitated diffusion and active transport allow cells to exert more control over membrane permeability than this.
* Channel proteins used for facilitated diffusion are specific to one type of particle or group of related particles. Cells can select which particles enter or exit, by controlling the types of channel protein placed in their plasma membrane. Also, the pores of some channel proteins can be closed temporarily, preventing movement of particles. Direction of movement cannot be controlled-net movement is always down the concentration gradient.
* Pump proteins used in active transport are specific to particular particles and their asymmetrical structure ensures that the particles are only moved in one direction across the membrane. Particles can be moved from the lower to the higher concentration, so active transport can generate concentration gradients. Because of these properties of facilitated diffusion and active transport, the membranes of cells can be described as selectively permeable.
B.2.1.9—Structure and function of glycoproteins and glycolipids
Limit to carbohydrate structures linked to proteins or lipids in membranes, location of carbohydrates on
the extracellular side of membranes, and roles in cell adhesion and cell recognition
Glycoproteins are polypeptides with carbohydrate attached (ranging from a single sugar up to small
polypeptides). They are therefore partly protein and partly carbohydrate. In a similar way, glycolipids are lipids with carbohydrates attached (usually a short chain of up to four glucose subunits). The lipid part usually consists of one or two hydrocarbon chains. Both glycoproteins and glycolipids are components of plasma membranes, with the protein or lipid embedded in the membrane and the hydrophilic carbohydrate part projecting outwards into the extracellular environment.
They have two main roles.
Cell adhesion: glycoproteins and glycolipids together form a carbohydrate-rich layer on the outer face of
the plasma membrane of animal cells. This layer is the glycocalyx. The glycocalyxes of adjacent cells can fuse, binding the cells together and preventing the tissue from falling apart.
Cell recognition: differences in the types of glycoproteins and glycolipids within plasma membranes allow cells to recognize other cells. This helps in the development of the tissues and organs of multicellular organisms. Cell recognition allows the immune system to distinguish between self and non-self cells, so pathogens and foreign tissue can be recognized and destroyed.
B2.1.10—Fluid mosaic model of membrane structure
Students should be able to draw a two-dimensional representation of the model and include peripheral
and integral proteins, glycoproteins, phospholipids and cholesterol. They should also be able to indicate
hydrophobic and hydrophilic regions.
A mosaic is a two-dimensional array of many small and diverse subunits. In the fluid mosaic model of membrane structure, which is widely accepted, the subunits are proteins that float in a lipid bilayer. Hydrophobic parts of the proteins (light grey in the diagram) are embedded in the core of the bilayer and hydrophilic parts (dark grey) are on the surface, or project from the bilayer. The lipids and proteins are mostly free to rotate or move aterally in the membrane because the lipid bilayer is liquid, but they cannot flip over, so differences between the two sides of the membrane can persist.
carbohydrate part of a glycolipid.
hydrocarbon chains may be saturated (straight) or unsaturated (with a kink)
carbohydrate part of a glycoprotein
wole
through channel
cholesterol (an amphipathic lipid occurring with phospholipids in the bilayer)
hydrophilic phosphate head of
phospholipid
000000
hydrophobic tails
(hydrocarbon chains) of
phospholipids
phospholipid bilayer
(about 8 nm
across)
integral proteins
embedded in the bilayer with the left-hand protein transmembrane
to the membrane surface
B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity
Unsaturated fatty acids in lipid bilayers have lower melting points, so membranes are fluid and therefore
flexible at temperatures experienced by a cell. Saturated fatty acids have higher melting points and make
membranes stronger at higher temperatures. Students should be familiar with an example of adaptations
in membrane composition in relation to habitat.
1 1 . Fluidity of lipid bilayers
Saturated fatty acids have straight chains, so allow
phospholipids to pack together tightly in bilayers.
This reduces the fluidity of a membrane and therefore
its flexibility and permeability by simple diffusion. In
contrast, unsaturated fatty acids have one or more
kinks in their hydrocarbon chain, so phospholipids
pack together more loosely, making a membrane
more fluid, flexible and permeable. The diagrams
show bilayers with (right) and without (left) unsaturated
hydrocarbon chains.
Relative amounts of saturated and unsaturated fatty
acids are regulated so that the membranes have the
required properties. They must remain fluid but be
strong enough to avoid becoming perforated. They
must be permeable but not too porous. The ideal ratio
of saturated to unsaturated fatty acids d e p e n d s on the
temperatures experienced by a cell. For example, fish
from Antarctic waters have more unsaturated fatty acids
n their membranes than fish from warmer waters.
B2.1.12—Cholesterol and membrane fluidity in animal cells
Students should understand the position of cholesterol molecules in membranes and also that cholesterol
acts as a modulator (adjustor) of membrane fluidity, stabilizing membranes at higher temperatures and preventing stiffening at lower temperatures
Cholesterol makes up between 20% and 40% of the lipids
in the plasma membranes of eukaryotes. The diagram
in Section B2.1.10 shows the position of cholesterol in
m e m b r a n e s .
Cell membranes do not correspond exactly to any of
the three states of matter-they are in what is called a
liquid-ordered phase. The lipid molecules are packed
densely but are still free to move laterally. The fluidity of
membranes needs to be carefully controlled.
At high temperatures cholesterol helps to maintain the
orderly arrangement of phospholipids. This prevents
the membrane from becoming too fluid, so it does
not become too porous and remains impermeable to
hydrophilic particles such as sodium and hydrogen ions.
At low temperatures cholesterol ensures that
saturated fatty acid tails do not solidify, preventing
membranes from becoming viscous and inflexible
(stiff), which would restrict cell movement and make
the cell more likely to burst.
at one end of the molecule is a
*– hydrophilic -OH group which is
attracted to hydrophilic heads of
phospholipids on the periphery
phospholipids in the membrane
most of the molecule is hydrophobic so
it is attracted to the hydrocarbon tails of
B2.1.13—Membrane fluidity and the fusion and formation of vesicles
Include the terms “endocytosis” and “exocytosis”, and examples of each process.
A vesicle is a small spherical sac of membrane with a
droplet of fluid inside. Vesicles are a dynamic feature
of most eukaryotic cells, with a continuous cycle of
formation, movement and fusion. These changes are
possible because of the fluidity of membranes.
Vesicles move materials around inside cells.
Examples of intracellular movement using vesicles:
* Proteins synthesized by ribosomes on the rough ER
are carried to the Golgi apparatus.
* Proteins processed by the Golgi apparatus are
carried to the plasma membrane.
* Phospholipids and cholesterol synthesized by the
smooth ER are transported in the vesicle membrane
to the plasma membrane of a growing cell to
increase its area.
Endocytosis is formation of a vesicle in the cytoplasm by
pinching off a piece of plasma membrane. Vesicles made
by endocytosis contain water and solutes from outside
the cell. They may contain larger molecules needed by
the cell that cannot pass through the plasma membrane.
Examples of endocytosis:
* Foetal cells in the placenta absorb proteins from the
mother’s blood, including antibodies.
* Unicellular organisms including Amoeba and
Paramecium absorb large undigested food particles.
* Phagocytic white blood cells absorb pathogens
including bacteria and viruses.
Exocytosis is fusion of a vesicle with the plasma membrane,
expelling the contents of the vesicle from a cell.
Examples of exocytosis:
expelled by exocytosis.
into the contractile the vacuole by
vacuole, increasing osmosis so
its solute
it swells
concentration
* Gland cells secrete proteins by exocytosis, for
example digestive enzymes and protein hormones.
* Neurons secrete neurotransmitter by exocytosis.
* Unicellular organisms such as Amoeba and
Paramecium load excess water into vesicles
(sometimes called contractile vacuoles) so it can b e
1. lons are pumped 2. Water enters
3. Contractile
vacuole moves
to the plasma
membrane and
expels its
contents,
In a growing cell, the area of the plasma membrane
needs to increase. Phospholipids are synthesized and
carried to the plasma membrane in vesicles.
B2.1.14—Gated ion channels in neurons
Include nicotinic acetylcholine receptors as an example of a neurotransmitter-gated ion channel and sodium and potassium channels as examples of voltage-gated channels.
Gated ion channels open for a fraction of a second
to allow a pulse of ions to diffuse through, then close
again. They are used in nerve impulses and also in
synaptic transmission.
Voltage-gated sodium and potassium channels are
used in nerve impulses. They are membrane proteins
that can change conformation (shape) in response
to changes in the voltage across the membrane. The
conformational changes open and close a pore through
the channel. Voltages across membranes are due to an
imbalance of positive and negative charges. A negative
voltage indicates that the overall balance of charges
inside the neuron is less positive than outside. The
diagrams show how a K* channel opens and closes.
3. Rapidly reclosed at +40 mV
more -ve
inside
2. Open at +40 mV
a globular protein “bal” attached by
a flexible polypeptide “chain” soon
blocks the pore at +40 mV then i s
ejected at -70 mV
more +ve
inside
A nerve impulse is a brief movement of sodium
ons (Na) and then potassium ions (K) across the
membrane of a neuron. If the voltage is below -50 mV,
Nat and K* channels remain closed. If it rises above
-50 mV, Na* channels open, allowing sodium ions to
diffuse into the neuron. This causes the voltage to rise
more. When it reaches +40 mV, K* channels open,
allowing potassium ions to diffuse out, returning the
voltage to its original level of -70 mV.
Nicotinic acetylcholine receptors are used at
synapses where acetylcholine is the neurotransmitter.
They have proteins in the postsynaptic membrane that
are both receptors and channels. They have a site to
which both acetylcholine and nicotine can bind.
Binding causes a conformational change, which opens
a pore in the protein, through which sodium and other
positively charged ions can
1. Binding sites are vacant
pass. Sodium diffuses into the so the pore is closed
postsynaptic neuron, raising its
voltage above -50 mV, which
initiates a nerve impulse by
causing voltage gated sodium
channels to open. Binding
of acetylcholine is reversible.
2. Binding of acetylcholine
causes the pore to open
When it dissociates from the
receptor, the conformational
change caused by binding is
reversed and the pore in the
receptor is closed.
B2.1.15—Sodium–potassium pumps as an example of exchange transporters
Include the importance of these pumps in generating membrane potentials.
Exchange transporters are multi-taskers because they
transport different substances in opposite directions
across a membrane. Sodium-potassium pumps are an
example. Each time a cycle of conformational changes
happens, three sodium ions are transported out of
the neuron and two potassium ions in. Energy from
ATP is needed because both Na* and K* are pumped
from lower to higher concentration, but the amount of
energy is reduced by exchanging cations.
The sodium-potassium pump generates concentration
gradients of both Nat and K* across the membrane,
which allows a nerve impulse, by facilitated diffusion of
these ions through voltage-gated ion channels
ATP ADP 3Na* 2K+
3 N a 2K
Nat concentration already:
higher on this side
K+ concentration
higher on this side
