Chapter 7 Flashcards
How does the plasma membrane regulate traffic for the cell?
Passive transport, transport proteins, active transport, or bulk transport (exocytosis or endocytosis)
How do large molecules move in and out of the plasma membrane?
They use bulk transport; exocytosis or endocytosis
What are the main components of the membranes?
Lipids, proteins, and carbohydrates are also important. Include a phospholipid bilayer.
What are phospholipids?
Amphipathic molecules, that contain two hydrophobic regions and one hydrophilic region
What are the structures of phospholipids?
Hydrophobic tails on the inside of the membrane, and hydrophilic heads that are exposed to the water on each side of the tails.
What is the fluid mosaic model?
The membrane structure that depicts the membrane as a mosaic of protein molecules in a fluid bilayer of phospholipids.
How are membranes mainly held together?
Weak hydrophobic interactions.
Do membrane proteins/lipids move?
A lipid may very rarely flip flop across the membrane.
What happens to a membrane in different temperatures?
In cooler temperatures, they switch from a fluid state to a solid state; the temperature this happens at depends on the type of lipids it’s made out of.
Which type of membrane is more fluid?
Membranes rich in unsaturated fatty acids are more fluid than membranes rich in saturated fatty acids.
What state does a membrane have to be in to work properly?
Fluid.
What does cholesterol do in membranes?
A component of animal cells that has variable effects on membrane fluidity at different temperatures.
What does cholesterol do in higher temperatures?
Restrains movements of phospholipids, making the membrane less fluid.
What does cholesterol do in lower temperatures?
Maintains fluidity by preventing tight packing of the fatty acids.
What do plant cells use instead of cholesterol?
Different but related steroid lipids that also buffer membrane fluidity.
What does fluidity affect in membranes?
Both permeability and movement of transport proteins. Membranes that are too fluid cannot support protein function.
What are the adaptations of cell membranes in fish that live in extreme cold?
They have a high proportion of unsaturated hydrocarbon tails.
In winter wheats, the % of unsaturated phospholipids increases in autumn to prevent membrane solidification during winter.
What determines the membrane’s functions?
The proteins imbedded in the phospholipid bilayer.
What are the two major types of membrane proteins?
Peripheral proteins that are bound to the surface of the membrane.
Integral proteins that penetrate the hydrophobic core.
What are transmembrane proteins?
Integral proteins that span the length of the membrane. Hydrophobic regions of an integral protein consist of nonpolar amino acids that are coiled into alpha helices.
How are some membrane proteins held in place?
By attachments to the cytoskeleton inside the cell.
Others attach to materials outside the cell (integrins attach to fibers of the extracellular matrix).
What functions do cell-surface membrane proteins carry out?
Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton and extracellular matrix (ECM).
Why are cell-surface proteins important in medicine?
HIV enters immune cells by binding to cell-surface protein CD-4 and a “co-receptor” CCR-5. Individuals lacking CCR-5 are immune to HIV infection.
How do cells recognize each other?
By binding to molecules on the surface of the membrane.
What are the surface molecules bonded to?
Glycolipids, which are carbohydrates bonded to lipids.
Glycoproteins, which are carbohydrates bonded to proteins.
Are membranes the same shapes?
Membranes have distinct inside/outside faces, and the composition and distribution of proteins, lipids, and associated carbohydrates is asymmetrical across the membrane.
What is selective permeability?
Some substances cross more easily than others.
Which molecules pass through the membrane rapidly?
Hydrocarbons, which are hydrophobic (nonpolar), pass through the membrane easily.
Which molecules pass through slowly?
Hydrophilic molecules (polar) pass through the membrane slowly, and sometimes not at all. The hydrophobic interior of the membrane impedes their passage. (Sugars, water, ions)
How do hydrophilic substances cross membranes quickly?
Transport proteins. Two types of proteins are used called channel proteins and carrier proteins.
What are channel proteins?
They have a hydrophilic channel that certain molecules or ions can use as a tunnel.
What are carrier proteins?
They bind to molecules and change shapes to shuttle them across the membrane.
What are aquaporins?
They increase the rate of passage of water molecules. They’re composed of four polypeptide subunits that form a channel for the passage of water.
What do transport proteins do?
They move only specific substances; glucose carrier proteins only transport glucose, and will not transport fructose which is a structural isomer of glucose.
What is the selective permeability dependent on?
The lipid bilayer and the specific transport proteins it contains.
What is diffusion?
The movement of particles of any substance so that they spread out evenly into the available space. Each molecule moves randomly, diffusion of population of molecules may be directional.
What is a concentration gradient?
The region along which the density of a chemical substance increases or decreases.
What is passive transport?
The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell.
What does the rate of diffusion depend on?
Membrane permeability to the specific substance.
What is osmosis?
The diffusion of free water (water molecules not clustered around another substance) across a selectively permeable membrane.
What do water molecules gravitate towards?
Free water molecules diffuse across a membrane from a region of lower solute concentration to the region of higher solute concentration.
Until when does water move from concentrations?
Until the solute concentration is equal on both sides.
What is tonicity?
The ability of a surrounding solution to cause a cell to gain or lose water.
What does tonicity depend on?
The concentration of solutes in the solution that cannot pass the membrane, relative to that inside the cell.
What happens if a solution has higher concentration of these solutes than the inside of the cell?
Water will tend to leave the cell, and vice versa.
What is an isotonic solution?
If its solute concentration is the same as that inside the cell.
When will there be no net movement of water?
When water diffuses across the membrane at the same rate in both directions.
When is the volume of a cell without a cell wall stable?
Only in an isotonic solution.
What is a hypertonic solution?
If the solute concentration is greater than that inside the cell.
What will happen to cells in hypertonic solutions?
Cells without cell walls will lose water, shrivel, and likely die.
What is a hypotonic solution?
Solute concentration is less than that inside the cell.
What will happen to a cell in a hypotonic solution?
Cells without cell walls will gain water, swell, and lyse (burst) in a hypotonic solution.
Why do hypotonic or hypertonic environments cause problems?
Cells without cell walls cannot tolerate excess water loss or uptake.
What is osmoregulation?
The control of solute concentration and water balance. e.g. Paramecium that live in hypotonic environments have a contractile vacuole to push excess water out of the cell.
Which cells have cell walls?
Plants, prokaryotes, fungi, and some protists.
What happens to a plant cell in a hypotonic solution?
Takes up water and swells until the inelastic wall exerts back a pressure on the cell, called turgor pressure.
What is a healthy state for most plant cells?
Turgid (very firm) is the healthy state for most plant cells.
When do plant cells become flaccid?
In an isotonic solution and the plant starts to wilt.
When do plant cells lose water?
In a hypertonic environment. The cell shrivels and the membrane pulls away from the cell wall in multiple locations, which is called plasmolysis.
What is facilitated diffusion?
Transport proteins speed the passive movement of molecules across the plasma membrane.
What do transport proteins include?
Channel proteins and carrier proteins.
What are the different types of channel proteins?
Aquaporins facilitate the diffusion of water.
Ion channels facilitate the transport of ions.
What do channel proteins provide/do?
Provide corridors that allow a specific molecule or ion to cross the membrane.
What are gated channels?
Ion channels that open or close in response to a stimulus.
What are examples of gated channels?
Nerve cells have potassium ion channels that open in response to electrical stimulus.
Others open in response to chemical stimulus, binding of a specific substance to the proteins.
What triggers carrier proteins changing shape?
The binding and release of the transported molecule.
How do carrier proteins move substances?
Ones involved in facilitated diffusion move substances down their concentration gradients, no energy input is required.
What type of transport is facilitated diffusion?
Passive, the solute moves down its concentration gradient and the transport requires no energy.
What is an active transport?
Transport proteins that use energy to move solutes against their concentration gradients.
What does active transport require?
Energy in the form of ATP; all proteins involved in active transport are carrier proteins.
Examples of active transport?
Transfer of a phosphate group from ATP to the sodium potassium pump energizes the transport of ions into the cell and out of the cell.
What is membrane potential?
The voltage across a membrane.
What is the voltage?
It is created by differences in the distribution of positive and negative ions across a membrane.
The inside of the cell is negative in charge relative to the outside, and favors passive transport of cations into the cell and anions out of the cell.
What is the electrochemical gradient?
Two combined forces that drive the diffusion of ions across a membrane.
What are the two forces of electrochemical gradients?
A chemical force; the ion’s concentration gradient.
An electrical force; the effect of the membrane potential on the ion’s movement.
What is an electrogenic pump?
A transport protein that generates voltage across a membrane, and stores energy that can be used for cellular work.
What are the main electrogenic pumps in plant and animal cells?
In animals, it’s the sodium-potassium pump.
In plants, fungi, and bacteria, it’s the proton pump that actively transports hydrogen ions out of the cell.
What is cotransport?
It occurs when active transport of a solute indirectly drives transport of other substances.
What is the “downhill” diffusion?
Diffusion of solute is coupled to the “uphill” transport of a second substance against its own concentration gradient.
How do plants load sucrose into their veins for transport around the body?
Proton pumps to generate a gradient across the cell membrane.
A cotransporter couples the movement back down its concentration gradient to the active transport of sucrose into the cell.
What is an animal cell’s equivalent for loading glucose in the body?
Sodium potassium pumps actively out of the cell to maintain the electrochemical gradient.
What happens when someone has diarrhea?
Waste is expelled too fast for reabsorption, causing sodium levels to drop.
How do small molecules and water enter or leave the cell?
Through the lipid bilayer or via transport proteins
How do large molecules (like polysaccharides and proteins) cross membranes?
Bulk, inside vesicles
What is exocytosis?
Transport vesicles migrate to the membrane, fuse with it, and release their contents out of the cell.
What type of cell uses primarily exocytosis?
Secretory cells. Ex. pancreas cells secrete insulin by exocytosis.
What is endocytosis?
Macromolecules are taken into the cell in vesicles. The membrane forms a pocket that deepens and pinches off forming a vesicle around the material for transport.
What are the three phases of endocytosis?
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor mediated endocytosis
What happens in phagocytosis?
A cell engulfs a particle by extending its pseudopodia around it and packing it in a membranous sac called a food vacuole. The vacuole then fuses with a lysosome to digest the particles.
What happens in Pinocytosis?
Molecules are taken up when extracellular fluid is “gulped” into tiny vesicles. Parts of the plasma membrane that form vesicles are on the inner side with coat proteins, forming coated vesicles.
Is pinocytosis specific?
No, it is nonspecific for the substances it transports, any and all solutes are taken into the cell.
What is receptor mediated endocytosis?
Vesicle formation is triggered by solute binding to receptors. Receptor proteins bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles.
How are emptied receptors dealt with?
Recycled to the plasma membrane by the same vesicle.
How do human cells use receptor mediated endocytosis?
To take in cholesterol, which is carried in particles called low density lipoproteins (LDLs).
What do families with hypercholesterolemia have?
Missing or defective LDL receptor proteins.
When can cholesterol be harmful?
Cholesterol accumulates in the blood, building up lipids and narrowing the space in the blood vessels, resulting in potential heart damage or stroke.
