Biology Chapter 5
Terms in this set (38)
Phospholipids arranged in a bilayer
• Globular proteins inserted in the lipid
• Fluid mosiac model - mosaic of proteins
float in or on the fluid lipid bilayer like
boats on a pond
Four Components of Cellular Membranes
1. Phospholipid bilayer -provides a flexible matrix and, at the same time, imposes a barrier to permeability.
2. Transmembrane proteins- These proteins have a variety of functions, including transport and communication across the membrane.
• Integral membrane proteins
3. Interior protein network- Membranes are structurally supported by intracellular proteins that reinforce the membrane's shape.
• Peripheral membrane proteins
4. Cell surface markers
• Glycoproteins and glycolipids
Similar in structure to a fat, but having only two fatty acids attached to the glycerol backbone, with the third space linked to a phosphorylated molecule; contains a polar hydrophilic "head" end (phosphate group) and a nonpolar hydrophobic "tail" end (fatty acids).
• Structure consists of
- Glycerol - a 3-carbon polyalcohol
- 2 fatty acids attached to the glycerol
• Nonpolar and hydrophobic ("water-fearing")
- Phosphate group attached to the glycerol
• Polar and hydrophilic ("water-loving")
• Spontaneously forms a bilayer
- Fatty acids are on the inside
- Phosphate groups are on both surfaces
• Bilayers are fluid • Hydrogen bonding
of water holds the 2 layers together
• Individual phospholipids and unanchored proteins can move through the membrane.
Environmental Influences on Plasma Membranes
- Saturated fatty acids make the membrane
less fluid because they pack together well
.Unsaturated fatty acid more fluid
• "Kinks" introduced by the double bonds keep them
from packing tightly
• Most membranes also contain steroids such as
cholesterol, which can either increase or
decrease membrane fluidity, depending on the
- Warm temperatures make the membrane
more fluid than cold temperatures
• Cold tolerance in bacteria due to fatty acid desaturases. Fatty acid desaturases that can introduce double bonds into fatty acids in membranes. The double bonds introduced by fatty acid desaturase make the membrane more fluid, counteracting the environmental effect of reduced temperature.
Functions of Membrane Proteins
1. Transporters: Membranes are very selective, allowing only certain solutes to enter or leave the cell, either through channels or carriers composed of proteins.
2. Enzymes: Cells carry out many chemical reactions on the interior surface of the plasma membrane, using enzymes attached to the membrane.
3. Cell-surface receptors:Membranes are exquisitely sensitive to chemical messages, which are detected by receptor proteins on their surfaces.
4. Cell-surface identity markers:Membranes carry cell-surface markers that identify them to other cells.
5. Cell-to-cell adhesion proteins:Cells use specific proteins to glue themselves to one another.
6. Attachments to the cytoskeleton: Surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins.
Proteins associated with the surface of the membrane. Include anchoring proteins.
The anchoring molecules are modified lipids that have
(1) nonpolar regions that insert into the internal portion of the lipid bilayer and
(2) chemical bonding domains that link directly to proteins.
Proteins embedded in the membrane.
- Span the lipid bilayer (transmembrane
• Nonpolar regions of the protein are embedded in
the interior of the bilayer
• Polar regions of the protein protrude from both
sides of the bilayer
- Transmembrane domain
• Spans the lipid bilayer
• Hydrophobic amino acids arranged in α helices
• Proteins need only a single transmembrane
domain to be anchored in the membrane, but
they often have more than one such domain
- Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane
- Cylinder of sheets in the protein secondary
structure called a -barrel
• Interior is polar and allows water and small polar
molecules to pass through the membrane
Protein molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.
Lipid molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.
Diffusion is movement of molecules from high concentration to low concentration.
- Will continue until the concentration is the same in all
• Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not
- Nonpolar molecules will move until the
concentration is equal on both sides
- Limited permeability to small polar molecules
- Very limited permeability to larger polar
molecules and ions
The movement of substances across a cell's membrane without the expenditure of energy.
• Passive transport is movement of molecules through the membrane in which
- No energy is required
- Molecules move with the concentration gradient.
• Two main methods
- Simple diffusion - essentially limited to CO2 and O2.
- Facilitated diffusion - "helped" with membrane proteins.
Movement of specific molecules across cell membranes through protein channels.
Condition in which a membrane is permeable to some substances but not to others.
A transmembrane protein with a hydrophilic interior that provides an aqueous channel allowing diffusion of species that cannot cross the membrane. Usually allows passage of specific ion.
• Hydrophilic channel when open.
• Ion channels
- Allow the passage of ions
- Gated channels - open or close in response to stimulus (chemical or electrical)
- 3 conditions determine direction
• Relative concentration on either side of membrane.
• Voltage differences across membrane
• Gated channels - channel open or closed
A membrane protein that binds to a specific molecule that cannot cross the membrane and allows passage through the membrane.
• Bind specifically to molecules they assist.
• Can help transport both ions and other solutes, such as some sugars and aminoacids
• Requires a concentration difference across the membrane
• Must bind to the molecule they transport
- Saturation - rate of transport limited by
number of transporters.
• Cytoplasm of the cell is an aqueous
- Water is solvent
- Dissolved substances are solutes
• Osmosis - net movement of water across
a membrane toward a higher solute concentration.
• Osmosis means "only water moves"
A membrane channel that allows water to cross the membrane more easily than by diffusion through the membrane.
A solution that has a higher concentration of solutes than another.
Describes a solution whose solute concentration is equal to the solute concentration inside a cell
Having a lower concentration of solute than another solution
• Force needed to stop osmotic flow
• Cell in a hypotonic solution gains water
causing cell to swell - creates pressure.
• If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out.
- Cell wall of prokaryotes, fungi, plants, protists
• If membrane is not strong, may burst
- Animal cells must be in isotonic environments
Most plant cells are hypertonic to their immediate environment, containing a high concentration of solutes in their central vacuoles. The resulting internal hydrostatic pressure, known as turgor pressure, presses the plasma membrane firmly against the interior of the cell wall, making the cell rigid. Most green plants depend on turgor pressure to maintain their shape, and thus they wilt when they lack sufficient water.
Requires energy - ATP is used directly or indirectly to fuel active transport.
• Moves substances from low to high concentration (against concentration gradient)
• Requires the use of highly selective carrier proteins.
A carrier protein in a cell's membrane that transports only a single type of molecule or ion.
A carrier protein in a cell's membrane that transports two molecules or ions in the same direction across the membrane.
A carrier protein in a cell's membrane that transports two molecules in opposite directions across the membrane.
Sodium-potassium (Na+-K+) pump
Direct use of ATP for active transport
• Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
- Against their concentration gradient
• ATP energy is used to change the
conformation of the carrier protein
• Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane.
Sodium-potassium (Na+-K+) Pump Steps
1. Carrier in membrane binds intracellular sodium. (Na+).
2. ATP phosphorylates protein with bound sodium,
3. Phosphoyrylation causes conformational changes in protein, reducing it affinity for Na+, which diffuses out to the outside environment.
4. Protein now has higher affinity for K+, which binds to protein.
5.Binding of potassium causes dephosphorlation of protein.
6. This causes the protein to revert back to its original affinity for Na+ and release the K+ inside the cell.
• Uses ATP indirectly
• Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different
• Symporter is used
• Glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient.
Some molecules are moved against their concentration gradient by using the energy stored in a gradient of a different molecule. In this process, called coupled transport, the energy released as one molecule moves down its concentration gradient is captured and used to move a different molecule against its gradient.
The transfer of large, polar molecules across the lipd bilayer.
- Movement of substances into the cell
- Phagocytosis - cell takes in particulate matter
- Pinocytosis - cell takes in only fluid
- Receptor-mediated endocytosis - specific
molecules are taken in after they bind to a receptor
Process by which specific macromolecules are transported into eukaryotic cells at clathrin-coated pits, after binding to specific cell-surface receptors.
-Movement of substances out of cell
Used in plants to export cell wall material
- Used in animals to secrete hormones, neurotransmitters, digestive enzymes.
Endocytosis of a solid particle; the plasma membrane folds inward around the particle (which may be another cell) and engulfs it to form a vacuole.
The process of fluid uptake by endocytosis in a cell.
Collapse of a walled cell's cytoplasm due to a lack of water