Membrane Structure and Transport

Exam #2
Which of the following types of molecules are the major structural components of the cell membrane?
phospholipids and proteins
The presence of cholesterol in the plasma membranes of some animals
enables the membrane to stay fluid more easily when cell temperature drops.
Which of the following is one of the ways that the membranes of winter wheat are able to remain fluid when it is extremely cold?
by increasing the percentage of unsaturated phospholipids in the membrane
In order for a protein to be an integral membrane protein it would have to be which of the following?
When a membrane is freeze-fractured, the bilayer splits down the middle between the two layers of phospholipids. In an electron micrograph of a freeze-fractured membrane, the bumps seen on the fractured surface of the membrane are
integral proteins
Which of the following is a reasonable explanation for why unsaturated fatty acids help keep any membrane more fluid at lower temperatures?
The double bonds form kinks in the fatty acid tails, forcing adjacent lipids to be further apart.
Of the following functions, which is most important for the glycoproteins and glycolipids of animal cell membranes?
a cell's ability to distinguish one type of neighboring cell from another
An animal cell lacking oligosaccharides on the external surface of its plasma membrane would likely be impaired in which function?
cell-cell recognition
Which of these are not embedded in the lipid bilayer at all?
peripheral proteins
Which of these often serve as receptors or cell recognition molecules on cell surfaces?
What kinds of molecules pass through a cell membrane most easily?
small and hydrophobic
Which of the following is a characteristic feature of a carrier protein in a plasma membrane?
It exhibits a specificity for a particular type of molecule.
Which of the following would likely move through the lipid bilayer of a plasma membrane most rapidly?
Which of the following statements is correct about diffusion?
It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.
Water passes quickly through cell membranes because
it moves through aquaporins in the membrane.
Cystic fibrosis is a genetic disease in humans in which chloride ion channels in cell membranes are missing or nonfunctional.

Chloride ion channels are membrane structures that include which of the following?
hydrophilic proteins
Which of the following factors does not affect membrane permeability?
The polarity of membrane phospholipids
How can a lipid be distinguished from a sugar?
Lipids are mostly nonpolar.
True or false? Osmosis is a type of diffusion.
What property of dishwashing liquid (detergent) makes it useful to wash grease from pans?
Amphipathic nature
Which of the following particles could diffuse easily through a cell membrane?
Oxygen (O2)
True or false? The water-soluble portion of a phospholipid is the polar head, which generally consists of a glycerol molecule linked to a phosphate group.
If a red blood cell is placed in a salt solution and bursts, what is the tonicity of the solution relative to the interior of the cell?
What name is given to the process by which water crosses a selectively permeable membrane?
Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left in a salt solution become limp and soft. From this we can deduce that the cells of the celery stalks are
hypertonic to fresh water but hypotonic to the salt solution.
Which of the following statements correctly describes the normal tonicity conditions for typical plant and animal cells?
The animal cell is in an isotonic solution, and the plant cell is in a hypotonic solution.
Which of the following membrane activities require energy from ATP hydrolysis?
Na+ ions moving out of the cell
What are the membrane structures that function in active transport?
integral proteins
Glucose diffuses slowly through artificial phospholipid bilayers. The cells lining the small intestine, however, rapidly move large quantities of glucose from the glucose-rich food into their glucose-poor cytoplasm. Using this information, which transport mechanism is most probably functioning in the intestinal cells?
facilitated diffusion
What is the voltage across a membrane called?
membrane potential
The sodium-potassium pump is called an electrogenic pump because it
contributes to the membrane potential.
The movement of potassium into an animal cell requires
an energy source such as ATP or a proton gradient.
Endocytosis moves materials _____ a cell via _____.
into ... membranous vesicles
A white blood cell engulfing a bacterium is an example of _____.
White blood cells engulf bacteria through what process?
In receptor-mediated endocytosis, receptor molecules initially project to the outside of the cell. Where do they end up after endocytosis?
on the inside surface of the vesicle
Which of the following factors would tend to increase membrane fluidity?
a greater proportion of unsaturated phospholipids
The presence of cholesterol in the plasma membranes of some animals
enables the membrane to stay fluid more easily when cell temperature drops.
The Plasma Membrane
- Are fluid mosaics of lipids and proteins
(phospholipids most abundant lipid in the plasma
- Boundary that separates the living cell from its surroundings
- Exhibits selective permeability
Fluid mosaic model
- A membrane is a fluid structure with a "mosaic" of various proteins embedded in it
- Hugh Davson and James Danielli proposed a sandwich model: the phospholipid bilayer lies between two layers of globular proteins = problem because the placement of membrane proteins, which have hydrophilic and hydrophobic regions
- J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer and only the hydrophilic regions exposed to water
Selective Permeability
- allowing some substances to pass through the plasma membrane more easily than others
Selective Permeability of the lipid bilayer: Hydrophobic (nonpolar) molecules
- Hydrocarbons
- Can dissolve in the lipid bilayer and pass through the membrane rapidly
Selective Permeability of the lipid bilayer: Polar molecules
- Such as sugars
- Do not cross the membrane easily
Amphipathic molecules
containing hydrophobic and hydrophilic regions
Specialized preparation technique which splits a membrane along the middle of the phospholipid bilayer
Synthesis and Sidedness of Membranes
- Asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane determined when the membrane is built by the ER and Golgi apparatus
The Fluidity of Membranes
- Phospholipids in the plasma membrane can move within the bilayer
- Most of the lipids, and some proteins, drift laterally
- Molecule rarely flip-flops transversely across the membrane
- At least some membrane proteins move sideways within the plane of the plasma membrane

- As temperatures cool, membranes switch from a fluid state to a solid state (temperature at which a membrane
solidifies depends on the types of lipids, membranes rich in unsaturated fatty acids = more fluid that those rich in
saturated fatty acids)

- Membranes must be fluid to work properly
- Cholesterol within animal cells have different effects on membrane fluidity at different temperatures
- Hinders solidification at low temperatures, preventing tight packing
Membrane Proteins
- Embedded in the fluid matrix of the lipid bilayer
- Proteins determine most of membrane's specific functions
Peripheral proteins
Bound to the surface of the membrane
Integral proteins
- Penetrate the hydrophobic core
- The hydrophobic regions of an integral protein consists of one or more stretches of nonpolar amino acids (often coiled into alpha helixes)
Transmembrane proteins
Integral proteins that span the membrane
Six Major Functions Of Membrane Proteins
1. Transport
2. Enzymatic activity
3. Signal transduction
4. Cell-cell recognition
5. Intercellular joining
6. Attachment to the cytoskeleton and extracellular matrix (ECM)
Role of Membrane Carbohydrates in Cell-Cell Recognition
- Cells recognize each other by binding to surface molecules on the plasma membrane (often carbohydrates)
- Membrane carbohydrates may be covalently bonded to
lipids or proteins = forming glycolipids or glycoproteins
Transport Proteins
- Allow passage of hydrophilic substances across the membrane
- specific for the substance it moves

Channel proteins
- type of transport proteins that have a hydrophilic channel
(certain molecules or ions can use as a tunnel)
- Aquaporins: Facilitate the passage of water

Carrier proteins
- type of transport proteins that bind to molecules to change shape to shuttle them across the membrane
Passive Transport
- diffusion of a substance across a membrane with no energy investment
- substances diffuse down their concentration gradient: the difference in concentration of a substance from one area to another
- no work must be done to move substances down the concentration gradient
- the diffusion of a substance across a biological membrane is passive transport (it requires no energy from the cell to make it happen)
- The tendency for molecules to spread out evenly into the available space
- Diffusion of a population of molecules may exhibit a net movement in one direction, although each molecule moves randomly
- Dynamic equilibrium, as many molecules cross one way as cross in the other direction
Effects of Osmosis on Water Balance
- Osmosis: The diffusion of water across a selectively permeable membrane
- Water selectively diffuses across a membrane, from the region of lower solute concentration to the region of higher solute concentration
the ability of a solution to cause a cell to gain or lose water
Isotonic solution
solute concentration is the same as that inside the cell (no net water movement across the plasma membrane)
Hypertonic solution
- solute concentration is greater than that inside the cell (cell loses water)
- create osmotic problems for organisms
Hypotonic solution
- solute concentration is less than that inside the cell (cell gains water)
- create osmotic problems for organisms
- The control of water balance
- Necessary adaptation for life in such environments
Water Balance of Cells with Walls
- Cell walls: help maintain water balance
- If a plant cell and its surroundings are isotonic: there is no net movement of water into the cell , the cell becomes flaccid, the plant may wilt
- A plant cell in a hypotonic solution: swells until the wall opposes uptake, the cell is firm
- In a hypertonic environment: the plant cells lose water, eventually, the membrane pulls away from the wall (usually a lethal effect called plasmolysis)
Facilitated diffusion
- Transport proteins speed the passive movement of molecules across the plasma membrane
- Passive Transport Aided by Proteins
- Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
- Channel proteins include aquaporins (for facilitated diffusion of water), ion channels that open or close in response to a stimulus (gated channels)
- Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
Active Transport Uses Energy To Move Solutes Against Their Gradients
- Still passive because the solute moves down its concentration gradient
- Some transport proteins can move solutes against
their concentration gradients
The Need for Energy in Active Transport
- Active transport moves substances against their concentration gradient
- requires energy (usually in the form of ATP): Performed by specific proteins embedded in the membranes
- Active transport allows cells to maintain concentration gradients that differ from their surroundings
How Ion Pumps Maintain Membrane Potential
- Membrane potential: voltage difference across a membrane
- Voltage: Created by differences in the distribution of positive and negative ions
- Function: allows a cell to function as a battery, use this potential to transport other ions, neurons transmit signals between different parts of a cell (opening/closing of ion channels)
Electrogenic Pump
- Electrochemical gradient: two combined forces drive the diffusion of ions across a membrane
• A chemical force: the ion's concentration gradient
• An electrical force: the effect of the membrane potential on the ion's movement
• An electrogenic pump: a transport protein that generates
voltage across a membrane
The sodium-potassium pump
The major electrogenic pump of animal cells
Proton pump
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
Cotransport: Coupled Transport
By a Membrane Protein
- Cotransport: occurs when active transport of a solute indirectly drives transport of another solute
- Plants commonly use the gradient of hydrogen ions generated by drive active transport of nutrients into cell
- transport vesicles migrate to the membrane, fuse with it, and release their contents
- Many secretory cells use exocytosis to
export their products
- the cell takes in macromolecules by forming vesicles from the plasma membrane
- reversal of exocytosis, involving different proteins
- Three types of endocytosis
1. Pinocytosis (cellular drinking)
2. Receptor-mediated endocytosis
3. Phagocytosis (cellular eating)
- cell engulfs a particle in a vacuole
- the vacuole fuses with a lysosome (digests the particle)
- Molecules are taken up when extracellular fluid is "gulped" into tiny vesicles
Receptor-Mediated Endocytosis
- Binding of ligands to receptors triggers vesicle formation
- Any molecule that binds specifically to a receptor site of another molecule