CHAPTER 6 Flashcards
what is the fluid mosaic model and what does it describe?
A model for biological membranes with a fluid bilayer of phospholipids where proteins can move freely, like floating in a lake
describe the structure of a phospholipid.
They have polar hydrophilic “heads” facing outward and non-polar hydrophobic fatty acid “tails” facing inward.
how do phospholipids vary?
-fatty acid chain length
-degree of unsaturation
-phosphate groups
why is cholesterol important?
for membrane integrity and regulates fluidity
information about the membrane interior.
-The membrane interior is fluid, enabling lateral molecule movement
-Molecules rarely flip across the membrane
-The inner and outer sides of the bilayer can differ significantly.
membrane fluidity depends on?
-Depends on lipid composition and temperature
-Cholesterol and saturated fatty acids tightly pack, reducing membrane fluidity
-Unsaturated long-chain fatty acids create “kinks,” increasing membrane fluidity
-Membrane fluidity decreases as temperature decreases
-Some organisms adjust membrane lipid content based on external temperature.
which ways do organisms change the lipid content of the cell membrane in relation to external temperature?
-Cold: Replace saturated with unsaturated fatty acids, shorten tails.
-Hot: Replace unsaturated with saturated fatty acids, lengthen tails.
what are the proteins contained in the membrane (plus other information)
-integral
-peripheral
-anchored
-Protein quantity varies by membrane function
-Some proteins move freely within the bilayer
integral membrane proteins
-Proteins partly embedded in the bilayer.
-Hydrophilic domain extends inward or outward.
-Hydrophobic domain interacts with fatty acids inside.
-Some extend across the lipid bilayer, some partially embed.
peripheral membrane proteins
-Lack hydrophobic regions, don’t penetrate bilayer
-Located on one side of the membrane.
anchored membrane proteins
-are covalently attached to fatty acids or other lipids
-are anchored to specific regions
transmembrane proteins
-Extend through bilayer with transmembrane domains
-Inner and outer domains can have distinct functions
what happens when cells are fused experimentally
Proteins distribute uniformly around the membrane.
what are membranes constantly doing? what is an example of this
-Membranes are dynamic, forming, transforming, fusing, and breaking down.
-Example: Endomembrane system.
information about the chemistry of the membrane
-Sub cellular membranes differ chemically.
-Membranes change chemically when forming parts of specific organelles.
what do membranes also have on the outer surface? what are the names?
-Carbohydrates on outer surface serve as recognition sites for cells and molecules.
-glycolipids
-glycoproteins
glycolipids
carbohydrate + lipid
glycoproteins
carbohydrate (oligosaccharide) + proteins
-proteoglycans have higher percentage of carbohydrates
how do cells arrange themselves in tissues?
Cell recognition and adhesion rely on surface proteins and carbohydrates.
cell adhesion
-Cell adhesion can occur through interactions between carbohydrates, proteins, or both.
-usually homotypic
homotypic
the same molecule sticks out from both cells and bind to each other
heterotypic
the cells have different proteins that bind together
define cell junctions. what are the types of junctions?
-cell junctions are specialized structures that hold cells together
-tight junctions
-desmosomes
-gap junctions
tight junctions
-help ensure directional movement of materials
desmosomes
-like “spot welds”
-An adhering junction between animal cells
gap junctions
-allow communication
what do cell membranes also adhere to?
-They adhere to the extracellular matrix.
-Transmembrane protein integrin binds to matrix outside epithelial cells and actin filaments inside.
-Binding is noncovalent and reversible.
what is integrin?
In animals, this transmembrane protein is integrin, facilitating epithelial cell attachment to the extracellular matrix.
how do cells move within a tissue?why is this important?
-Cells move within tissues via integrin binding and reattaching to the extracellular matrix.
-This process is vital for cell movement in developing embryos and cancer cell spread.
what does it mean for a membrane to be selectively permeable
-this means some substances can pass through but others can not
passive transport
no energy input required (diffusion)
active transport
-energy required
what does the energy for passive transport come from?
-came from concentration gradient.
-Signifies a difference in concentration of ions or substances between locations, typically across a membrane.
particles in a solution.
-they move randomly and at equilibrium they are evenly distributed
diffusion
-the process of random movement toward equilibrium
net movement?
-Net movement is directional until equilibrium (from high to low concentration).
-Diffusion is the net movement from regions of higher to lower concentration.
what does diffusion rate depend on?
-size and mass of the molecules or ions
-temperature of the solution
-density of the solution
-concentration gradient
-area and distance
what does surface area or short distances do for diffusion?
-larger surface area permits more rapid diffusion
-diffusion works very well over short distances (ex: within a cell)
membranes relating to molecular movement
-Molecules move across a permeable membrane until concentrations are equal on both sides.
-Diffusion persists across the membrane, yet there’s no net change in concentrations.
simple diffusion
-small molecules pass through the lipid bilayer
-lipid-soluble molecules can diffuse across the membrane
-electrically charged and polar molecules cannot pass through easily
what is osmosis? what are the types?
-diffusion of water (depends on the relative concentrations of water molecules)
-isotonic
-hypertonic
-hypotonic
isotonic
equal solute concentrations
hypertonic
higher solute concentration
hypotonic
lower solute concentration
if two solutions are separated by membrane that allows water, but not solutes, to pass through:
water will diffuse from the region of higher water concentration (hypotonic) to the region of lower water concentration (hypertonic)
what happens to animal cells in hypotonic solutions and hypertonic solutions
-in hypotonic solutions the cell may burst
-in hypertonic solution the cell will shrink
what is turgor pressure?
-plant cells with rigid cell walls build up internal pressure that keep more water from entering
facilitated diffusion
polar and charged molecules diffuse with concentration gradients, but facilitated by protein channels or carriers
channel proteins
integral membrane proteins that form a tunnel
carrier proteins
-Membrane proteins bind substances, accelerating their diffusion through the bilayer.
-They transport polar molecules like glucose across membranes bidirectionally.
how does glucose go through carrier proteins?
-glucose binds to the glucose transporter, causing it to change shape and release the glucose on the other side
ion channels
-channel proteins with hydrophilic pores
-most are gated: can be closed or open to ion passage
when do ion channel gates open?
-chemical signal (ligand)
-electrical charge difference (voltage-gated)
what is aquaporins?
-water can cross membranes through special channels called aquaporins (ions are excluded)
-first identified by injection aquaporin proteins into a frog oocyte
what does rate of diffusion depend on?
-Depends on concentration gradient and number of carrier proteins in the cell membrane.
-If carriers are fully loaded, diffusion is slow due to saturation.
-Cells requiring abundant energy, such as muscle cells, possess numerous glucose transporters for faster diffusion.
define active transport
Moves substances against gradients, requiring energy, typically ATP.
what are the proteins involved in active transport?
-uniporter
-symporter
-antiporter
define uniporter. give examples.
-moves one substance in one direction
define symporter. give examples.
-moves two substances in one direction
define antiporter. give examples
-moves two substances in opposite directions
-EX: the sodium-potassium (Na+ - K+) pump
what are the type of active transports?
-primary active transport
-secondary active transport
define primary active transport. give example
-requires direct hydrolysis of ATP
-the sodium-potassium (Na+ - K+) pump
define secondary active transport. give example
-Energy is sourced from an ion concentration gradient set by primary active transport.
-The Na⁺–K⁺ pump creates a Na⁺ concentration gradient. Some Na⁺ passively diffuses back into the cell, powering glucose transport against its gradient.
what is the sodium-potassium pump?
- it is an integral membrane glycoprotein; where it brings two K+ ions into the cell and exports three Na+ ions
describe in detail what happens during the sodium-potassium pump.
-3 Na⁺ ions and 1 ATP bind to the pump protein.
-ATP hydrolysis yields ADP and phosphorylates a pump protein amino acid.
-Shape change moves 3 Na⁺ ions outside and 2 K⁺ ions inside.
-2 K⁺ ions bind to the pump.
-Dephosphorylation releases the phosphate, leading to 2 K⁺ ions moving inside and returning the pump to its original form.
how are larger molecules like macromolecules transported to a cell?
-they can be taken in or secreted by means of membrane vesicles
define endocytosis
-it brings molecules and cells into a eukaryotic cell
-the cell membrane folds inwards, or invaginates, around the material forming a vesicle
define phagocytosis
-molecules or entire cells are engulfed
-a food vacuole or phagosome forms, which fuses with a lysosome where the contents are digested
when is phagocytosis used?
-some protists feed in this way
-some white blood cells engulf foreign substances in this way
define pinocytosis
-a vesicle forms to bring small dissolved substances or fluids into a cell
-vesicles are much smaller than in phagocytosis
-is constant in endothelial (capillary) cells
Describe the process of receptor-mediated endocytosis.
Receptor-mediated endocytosis is like a targeted delivery system. Special receptor proteins on the cell membrane grab specific molecules. These receptors are coated inside with proteins such as clathrin.
define receptor mediated endocytosis
Endocytosis initiated by macromolecular binding to a specific membrane receptor.
define receptor porteins
-integral membrane proteins located at specific sites on the cell membrane
-these sites are also coated with proteins such as clathrin on the inside
example of receptor proteins used to transport cholesterol
-mammalian cells take in cholesterol by receptor-mediated endocytosis
-in the liver, cholesterol is packaged into low-density lipoprotein, or LDL, and secreted to the bloodstream
-cells that need cholesterol have receptors for the LDLs in clathrin-coated pits
define exocytosis
-materials packaged in vesicles (like digestive enzymes and neurotransmitters) are secreted from a cell when the vesicle membrane fuses with the cell membrane
-or, the vesicle touches the cell membrane and a pore forms, releasing the vesicle’s contents; no membrane fusion
