Exam 2 Chapter 5 Flashcards
Biological membranes consist of ___ and ___ in a thin film
lipids, proteins
What are the two major types of lipids in cell membranes?
Phospholipids and sterols (in animal cell membranes, cholesterol)
Does the proportion of lipids and proteins vary or stay the same across the cell and across different cells?
Varies across the cell and across different cells
What are four of the main polar groups attached to the phosphate group in phospholipids?
Either an alcohol or an amino acid
- Phosphatidyl choline
- Phosphatidyl serine
- Phosphatidyl ethanolamine
- Phosphatidyl inositol
Describe the structure of phospholipids
Phosphate group linked to a polar amino acid or alcohol and a glycerol on the other side; the glycerol is attached to two fatty acid nonpolar tails that are hydrophobic
How are the hydrophobic tails of phospholipids attracted to other nonpolar molecules?
Via Van Der Waals forces
Phospholipids are _____ molecules
Amphiphilic
What structure is spontaneously formed by the phospholipid molecules?
A bilayer
Describe the characteristics of a fluid and frozen bilayer
Fluid:
- Individual molecules are free to flex, rotate, and exchange places
Frozen:
- Molecules remain fixed in place
Which end faces the aqueous solution in a bilayer?
The polar phosphate heads
What happens when a phospholipid bilayer sheet is shaken in water?
Vesicles are spontaneously formed
Phospholipids are ____ molecules
amphiphilic (both hydrophobic and hydrophilic properties)
True or false: membrane sterols have dual solubility properties
True; sterols have an outward facing polar OH group that extends into the aqueous surface, and the four carbon rings and nonpolar chain that face inwards are nonpolar
What sterols are found in plants?
Phytosterols
True or false: the similarity of membrane structure in eukaryotes and prokaryotes tells us that basic membrane structure evolved during the earliest stages of life on earth
True
What role does cholesterol play in animal cell membranes?
Helps in membrane fluidity
True or false: plants produce cholesterol
False
True or false: membrane proteins have hydrophobic and hydrophilic regions, so also display amphiphilic properties
True
Describe the fluid mosaic model
The membrane consists of a fluid phospholipid bilayer in which proteins are embedded and float freely. The molecules are fluid in that they freely exchange places with one another within a sheet. “Mosaic” refers to the fact that membrane proteins float freely in the membrane/lipid bilayer.
How are the hydrophobic sections of membrane proteins formed?
Segments of polypeptide chains are enriched in amino acids with nonpolar R groups. These segments are often coiled into alpha helices with loops of hydrophilic amino acids that face the aqueous exteriors.
How do secondary structures contribute to membrane proteins?
Alpha helices may form a hydrophobic channel with a hydrophilic loop through the center. Beta sheets may ‘weave’ together.
Where are integral proteins found?
Embedded in phospholipid bilayer
Where are peripheral proteins found?
Held to membrane surfaces by noncovalent bonds
Describe structure and function of bacteriorhodopsin
Bacteriorhodopsin absorbs light for energy in photosynthetic archeans.
Where are glycolipids found?
In the part of the membrane facing outside the cell.
Where are glycoproteins found?
Carbohydrate groups are attached to the part of proteins facing the exterior of the cell, forming glycoproteins
What role do glycoproteins and glycolipids play in animals?
Glycolipids and glycoproteins form a glycocalyx which protects against chemical and mechanical damage.
How are peripheral proteins attached?
Attached to integral membrane proteins or membrane lipids mainly on the cytoplasmic side
What is the purpose of membrane proteins?
Transport: form channels that allow polar molecules and ions to pass through the membrane
Recognition: In plasma membrane, allows for recognition of foreign bodies
Receptors: Recognize and bind molecules from other cells that act as chemical signals, such as hormones
Cell adhesion: bind cells together by recognizing and binding receptors or chemical groups on other cells
What role does the unsaturated fatty acid chain play in membranes?
Keeps membrane fluid at low temperatures
What role does cholesterol play in membranes at low and high temperatures?
Low temps: intercalates between fatty acid chains to prevent stiffening
High temps: stabilizes membrane and reduces fluidity
How do eukaryotes adapt to colder temperatures?
They change their membrane lipids to prevent freezing
- Unsaturated fatty acid and cholesterol proportion both increase in colder temperatures
Describe Frye and Edidin’s experiment
The two scientists grew human and mouse cells in tissue culture. They added antibodies that were labeled red for humans and green for mice (fluorescently labeled). The researchers then fused the cell, and within 40 minutes, membrane proteins were completely intermixed, proving the fluid nature of the membrane.
True or false: biological membranes are selectively permeable
True
Which molecules are most easily able to move through the membrane?
Hydrophobic (nonpolar) molecules
Which molecules have more difficulty moving through the membrane?
Hydrophilic (polar) molecules (charged atoms and molecules are completely blocked by the hydrophobic core)
Differentiate between the two types of transport
Passive transport uses energy from the concentration gradient; active transport either directly or indirectly uses energy from ATP
What are the types of passive transport?
Simple diffusion, facilitated diffusion, and osmosis
What are the types of active transport?
Primary (uses energy from ATP) and secondary (indirectly uses energy from ATP)
Describe simple diffusion
Simple diffusion involves diffusion through the lipid part of a biological membrane; it only depends on the size of the molecule and the solubility of the lipids. In simple diffusion, only the concentration gradient drives the movement of molecules.
What types of molecules are transported via simple diffusion?
Nonpolar inorganic cases (O2, N2, CO2) and organic molecules
What is the difference between simple and facilitated diffusion?
Simple diffusion does not use transport proteins, but facilitated diffusion does; also, facilitated diffusion involves transportation of polar and charged molecules
Describe facilitated diffusion
Transport of charged and polar molecules down a concentration gradient with the help of transport proteins in the bilayer
What types of proteins can be found in facilitated diffusion?
Channel proteins, aquaporins, ion channels, and gated channels
What are channel proteins?
Channel proteins are formed by integral membrane proteins that form hydrophilic (polar) channels, which allow ions and water to pass
What are aquaporins?
Aquaporins are a water channel. It transports water, and it does so via multiple H-bonding sides on the channel in this protein.
What are ion channels?
Ion channels facilitate the transport of ions such as Na+, K+, Ca2+, or Cl-. Most ion channels are gated, meaning they switch between open, closed, or intermediate in response to voltage channels.
Describe gated channels
Gated channels are a type of transport protein involved in facilitated diffusion. They help transport ions.
What is the K+ voltage-gated channel?
With normal voltage across the membrane, the activation gate of K+ is closed and K cannot move across the membrane. If a voltage change occurs, K+ opens and K+ moves down the concentration gradient from cytoplasm to outside the cell.
What are carrier proteins?
Involved in facilitated diffusion; transport ions and other solutes across the plasma membrane. They do this by physically binding a molecule on one side, undergoing a conformational change, and releasing the molecule on the other side.
T or F: Carrier proteins are specific to its own molecule, and it is passive
T
What happens if there are too many molecules and too few carrier proteins?
The carrier protein becomes saturated
Describe the process of carrier proteins
- Carrier protein binding site is exposed to region of high concentration.
- Solute molecule binds to carrier protein.
- Protein undergoes conformational change, after which it no longer favors the original molecule.
- After releasing the molecule, carrier protein returns to original conformation.
How can you increase the rate of transport of carrier proteins?
Adding more proteins and molecules
Give a specific example of a carrier protein
The glucose transporter
What is osmosis? Is it active or passive?
Osmosis is the diffusion of water across a selectively permeable membrane in response to concentration gradients. It is a form of passive transport. Osmosis causes cells to swell and burst or shrivel and shrink. Animal cells expend energy to counteract the inward and outward movement of water by osmosis
Describe the movement of water in osmosis
Water moves from area with less solutes to area with more solutes. This is because the concentration of free water molecules is lower on the solute side.
Describe the glucose osmosis experiment
A glucose solution with a cellophane membrane is placed in a beaker. The distilled water diffuses into the glucose solution, causing it to rise.
What is osmotic pressure?
The force needed to stop osmotic flow.
What happens to a cell in a hypotonic solution
Gains water, causing swelling, which creates pressure. If the membrane is strong enough, the cell reaches a counterbalance of osmotic pressure driving water in and hydrostatic pressure driving water out. The cell wall of a plant is strong enough to prevent bursting.
Why must an animal cell be in an isotonic environment?
The membrane is not strong enough.
What does it mean if a solution is hypotonic to a cell?
If a solution is hypotonic to a cell, that means there are less solutes and more water outside the cell. This causes the cell to expand.
What does it mean if a solution is hypertonic to a cell?
More solute and less water outside cell, which causes water to leave the cell and the cell shrinks.
What is an isotonic solution?
Concentration of solute inside and outside is balanced. In animal cells, energy is used to transport Na+ from inside to outside to facilitate isotonicity.
Describe what happens to a human RBC in different tonicity solutions
Hypertonic solution- there is more solute outside the cell, so water flows from the cell out into the solution. This causes the cell to shrink.
Isotonic solution: Net movement in and out of cell is balanced
Hypotonic solution: There is less solute and more water outside the cell in comparison to inside the cell, so water flows into the cell, causing it to expand and burst.
Describe what happens to a plant cell in different tonic solutions
Hypotonic solution- more solute inside cell than outside, so water flows inwards, leading to a normal turgid cell.
Isotonic solution- flow of water in and out is balanced, but does not provide enough turgor pressure, causing the cell to become flaccid.
Hypertonic solution- more solute outside the cell, meaning water flows out of the plant cell and causes plasmolysis as the cell body shrinks from the wall. This means plant cells prefer a hypotonic solution.
What happens if the membrane is strong enough?
The cell reaches a counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out; this can be seen in prokaryotes, fungi, plants, and protists
What is isomotic regulation?
Keeps cells isotonic with their environment. Some cells use extrusion in which water is ejected through contractile vacuoles.
What do plant cells use turgor pressure for?
To push cell membrane against cell wall and keep the cell rigid.
When do we use active transport?
When we transport substances across a membrane against a concentration gradient (low concentration to high concentration)
Why do we use active transport?
It allows cells to uptake essential nutrients, even when there are more of the nutrient inside the cell.
It allows for the cell to secrete or expel waste materials, even when outside concentration is higher.
It allows for maintenance of intracellular concentrations of H+, Na+, K+, and Ca2+
What is membrane potential?
The electrical charge difference that is caused by the active transport of ions across plasma membrane. The inside of the membrane may be more negative or positive relative to the outside. This is the basis for transmission of a nerve impulse since neurons and muscle cells use changes in membrane potential in response to a stimulus.
Distinguish between primary and secondary active transport.
Primary active transport: ATP is directly used and hydrolyzed in order to power the transport protein. It requires energy and the use of a carrier protein. It always involves moving substances from low to high concentrations.
Secondary active transport: indirectly driven by ATP hydrolysis, so transporters use a favorable concentration gradient of ions facilitated by primary transport as the energy source for active transport of a different ion or molecule.
Describe the sodium potassium pump
The sodium potassium pump is active transport and moves 3 Na out and 2 K into the cell. The carrier protein uses ATP to change its conformation, and the affinity of the carrier protein for Na or K+ changes based on the conformation so the ions can be carried out. Transport of these ions causes a positive charge to accumulate outside the cell, creating a membrane potential of -50 to -200 mV. The differences in concentration of ions and charge causes an electrochemical gradient, which is a form of potential energy.
Describe the steps of the sodium-potassium pump
- The carrier protein first has 3 spots for Na+ and 2 spots for K+. When facing the inside of the cell, the 3 spots become high affinity. There is low Na+ inside the cell and high Na+ outside the cell.
- When the 3 Na+ ions bind to the pump, it splits ATP into ADP and phosphate. The phosphate binds to the pump. ADP is released
- Phosphorylation causes the pump to change conformation so that the protein opens into the extracellular space. This reduces the binding strengths holding Na+ in place and causes the 3 Na+ to be released.
- This change in shape also allows for K+ to bind since affinity for K+ becomes higher.
- When the K+ binds, the phosphate detaches and causes the pump to return to its original conformation, where the K+ are released due to a lower affinity for K+.
What is the Ca2+ pump?
It moves calcium ions from the cytoplasm to the exterior, and from cytosol into the ER vesicles.
Why is the Ca2+ pump important?
It regulates cellular activities like secretion, microtubule assembly, and muscle contraction. In muscle contraction, pumps release the Ca2+ into muscle fibers, which eventually leads to contraction. Also helps in pollen growth and fertilization
What is secondary active transport and what does it use as its energy source?
Secondary active transport pumps use an ion concentration gradient that was established by a primary pump. This is why it is called “secondary”.
What is an ion always present in secondary active transport?
The driving ion
What are the protein domains of the carrier protein in the Na+ K+ pump?
- Nucleotide binding protein- binds atp
- Phosphorylation domain- binds the phosphate
- Mediator domain- facilitates the conformational change
- Transmembrane domain- spans the membrane and aids with affinity
What is symport and antiport?
In symport, solute moves through membrane channel in same direction as driving ion
In antiport, solute and driving ion move through in opposite directions
What is coupled transport?
Coupled transport uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule (a symporter is used), Coupled transport can either be symport or antiport
How does the glucose- Na+ symporter work?
It moves glucose against a concentration gradient through symport. The high concentration of Na+ outside is facilitated by the sodium potassium pump.
The driving ion always goes from ___ concentration to ___ concentration
High to low
The transported solute always goes from ___ to ____ concentration
Low to high
What conformational changes does a transport protein experience during symport?
Diffusing and pumped molecules both bind, then conformation changes to release cargo molecules
What conformational change occurs for a transport protein in antiport?
Driving ion binds, conformation changes, and it is released inside the cell; pumped molecule binds and conformation changes, after which it is released.
What are some other terms for non-specific endocytosis?
Bulk endocytosis or pinocytosis, where the cell only takes in fluid
When does endocytosis occur?
When proteins and other substances are trapped in a pit like inward depression from the plasma membrane
What is specific endocytosis?
Specific endocytosis, or receptor mediated endocytosis, occurs when specific molecules are taken in after binding to a receptor.
Describe the process of receptor-mediated endocytosis
Substances attach to receptors, membrane pockets inwards (called the coated pit), and pocket pinches off as an endocytic vesicle. The vesicle is coated by clathrin. Clathrin reinforces the cytoplasmic side. Afterwards, the vesicle may fuse with lysosomes, and molecular products cross the vesicle membrane into the cytoplasm via transport proteins. Membrane proteins are then recycled to the plasma membrane.
What is phagocytosis?
Phagocytes in blood stream and protists like amoeba take in particles or cells through phagocytosis.
Process of phagocytosis
Receptor binds to material, cytoplasmic lobes extend and surround the material to form a pit that pinches off as a large endocytic vesicle, and enzymes digest the materials, and cell stores residues in vesicles or expels them
What happens when the human aquaporin-1 gene is mutated?
Inability to make concentrated urine
- Membrane component responsible for transport in different types of transport
- Binding of transported substance
- Energy source
- Direction of transport
- Specificity?
- Can it become saturated?
Simple, Facilitated, Primary, Secondary
- Lipids, proteins, proteins, proteins
- No, yes, yes, yes
- gradient, gradient, atp, atp and gradient (atp is indirect)
- With gradient, with gradient, against, against
- Nonspecific, specific, specific, specific
- No, yes, yes, yes
What is the predominant sterol of animal cell membranes?
Cholesterol
What is the composition of the hydrophobic regions of membrane proteins?
They are enriched in amino acids with nonpolar side chains
Describe the structure of bacteriorhodipsin
Alpha-helical segments that span the membrane, connected by flexible and hydrophilic loops at the surfaces
Where can glycolipids be found in the cell?
In the exterior facing membrane, with carbohydrate groups attached
How are glycoproteins in the cell formed?
Carbohydrates attach to proteins in the exterior facing lipid layer
Where can glycolipids and glycoproteins be found in animals?
Intestinal epithelial cells, where they form the glycocalyx
Why do membrane lipids rarely switch between layers?
Polar heads are attracted by H=bonds to the water, making such reactions energetically unfavorable
How do organisms adapt to temperatures?
They synthesize lipids with a different mix of fatty-acid tails
What changes do mammals in hibernation and amphibians, fish, and reptiles exhibit in cold temperatures?
They introduce double bonds and cholesterol into cell membranes
How did frye and edidin know that it was the proteins moving around and not new synthesis of proteins that led to the findings of their expeirment?
Introducing protein synthesis inhibitors had no effect on protein mixing because the proteins were still mixed, but temperature decreases did reduce the amount of mixing
T or F: Integral membrane proteins are usually also transmembrane proteins
T
T or F: Membranes in cells vary in composition of proteins and lipids, even within one cell
T
What are cell adhesion molecules?
Glycoproteins that link cells to cells, cells to the ECM, and the ECM to the cytoskeleton; also help organize cells into tissues
T or F: Integrins are not a class of CAM
F
Why are cadherins important?
A cell of one type recognizes other cells with the same cadherin
T or F: Enzymes are usually not found in cell membranes
F
Why is transport considered specific and directional?
Specific in that only certain molecules move across membranes, and directional in that some molecules move in, others move out
Is passive transport a form of diffusion?
Yes, because there is a net movement from a region of high concentration to low concentration
What is dynamic equilibrium?
When concentration does not change anywhere but there is still movement of particles in and out
What type of molecule moves most easily through membranes and why?
Small, hydrophobic molecules because they are not attracted to the aqueous solution on either side of the cellular membrane.
Why can’t hydrophilic molecules enter the cell?
There are stronger attractions between hydrophilic molecules and the aqueous solution
What proteins carry out facilitated diffusion?
Integral membrane proteins that extend entirely throughout the membrane. They are selective for one molecule
T or F: Saturation occurs in both facilitated and simple diffusion of there is a high enough concentration of solute
F; saturation only occurs in facilitated diffusion and active transport.
For osmosis to occur, the selectively permeable membrane (can/cannot) ________ allow water molecules, and (can/cannot) allow all solutes
can, cannot
Osmosis occurs as long as some solutes are _________
nonpenetrating
What is tonicity?
The effect a solution has on a cell when the solution surrounds it
Why is turgor pressure important in plants?
It supports the softer tissues against gravity and keeps them upright
What is plasmolysis?
In hypertonic solutions, when cells shrink from walls
What is a membrane poteintal?
Electrical charge difference across a membrane that results from active transport of ions
What type of protein is used in active transport?
Carrier proteins
What is the H+ pump?
A proton pump that moves ions across membranes, binding a phosphate group and helping to generate membrane potential and keep pH low
What is the kiss-and-run mechanism
Occurs when a vesicle fuses with membrane, releases contents, then reforms and moves back inside cell
How do cells use exocytosis
Animals: peptide hormones, milk proteins, mucus, digestive enzymes
Plants, funci, bacteria: secrete molecules associated with cell wall and also digestive enzymes
All organisms use exocytosis to place integral membrane proteins in the plasma membrane
Why is receptor mediated endocytosis important?
Mammals take in substances like insulin, growth factors, neurotransmitters, enzymes, antibodies, blood proteins, iron, and vitamin B12 through endocytosis. HIV also binds to receptors and LDL proteins use this type of endocytosis to enter the cell and release cholesterol to cytoplasm
