Chapter 5 ~ Membrane Transport And Cell Signaling Flashcards
Amphipathic
A molecule - phospholipid - has both a hydrophilic and hydrophobic region
Describe the fluidity of the components in the cell membrane
Held together by hydrophobic interactions, most lipids and some proteins can shift laterally rapidly. Many membrane proteins are held mobile by attachments to the cytoskeleton
How membrane mobility is influenced by temperature
- A membrane remains fluid as temperature decreases until phospholipids settle into closely packed arrangements until solidified
- Membrane remains fluid at a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails (since the double bonds give them a bent geometry)
How membrane fluidity is influenced by membrane composition
- Cold temps: high proportion of unsaturated hydrocarbon tails
- Hot temps: unusual lipids
How cholesterol resists changes in membrane fluidity with temperature change
Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement; at low temperatures it hinders solidification by disrupting the regular packing of phospholipids
Integral membrane protein
- Penetrate the hydrophobic interior of the lipid bilayer
- Extend only partway into the interior
- Hydrophobic regions consist of one or more stretches of non polar amino acids could into alpha helices
Peripheral membrane protein
- not embedded in the lipid bilayer at all
- loosely bound to the surface of the membrane, often to exposed parts of the integral proteins
Major functions of membrane proteins
- transport (hydrophilic channel across the membrane that is selective for a particular solute)
- enzymatic activity (can be a protein built into a membrane with its active site)
- signal transduction (binding site with a specific shape that fits the shape of a chemical messenger)
- cell-cell recognition
- intercellular joining
- attachment to the cytoskeleton and ECM
What is the role of membrane carbohydrates in cell to cell recognition?
Cells recognize other cells by binding to the molecules, often containing carbs, on the extracellular surface of the plasma membrane
Membrane carbs: short, branched chains of fewer than 15 sugar units
How do hydrophobic molecules cross the cell membrane?
through simple diffusion
Channel Proteins
- transmembrane proteins with a hydrophilic channel
- form water-filled channels in the cell membrane, allowing passive movement of ions through it
- do not bind to transported molecule, instead provide a direct passageway fro them
- highly specific to the type of molecule being transported
Carrier Proteins
- transmembrane proteins, but have temporary binding sites
- facilitate the movement of larger or polar molecules through conformational changes
- bind to the specific molecule, undergo change in shape, then release it on the other side of the membrane
- highly specific to the molecules they transport
Diffusion
The movement of particles of any substance from an area of high concentration to an area of lower concentration (driven by natural tendency of particles to spread out and achieve equilibrium)
Why does a concentration gradient of a substance across a membrane represent potential energy?
It stores the potential for that substance to move and perform work, which is closely related to energy (the substance has potential energy stored when it seeks to create an equilibrium)
Isotonic
there is no net movement of water across the plasma membrane (its the same rate in both directions)
Hypertonic
The cell will lose water, shrivel, and probably die (increase in salinity in a lake can kill animals living there)
Hypotonic
Water will enter the cell faster than it leaves, the cell will swell and lyse like a water balloon
Osmosis
the movement of water across cell membranes and the balance of water between cell and its environment. Water diffuses from an area of less concentration to more concentration
How living cells regulate water balance
Animal cells: Through osmoregulation (the control of solute concentrations and water balance)
Cell wall cells: the cell wall regulates the water balance
How do transport proteins facilitate diffusion?
Channel proteins: provide hydrophilic passageways that allow specific molecules or ions across the membrane
Carrier proteins: go under a subtle shape change that translocates the solute-binding site across the membrane
Facilitated diffusion
Passive mechanism of molecular transport that relies on transport proteins to move substance across a biological membrane
Active transport
A biological princess that moves molecules or ions across a cell membrane against their concentration gradient (an area of lower to higher) in which the cell must expend energy
How does an elecrogenic pump create voltage across a membrane?
The net transfer of of one positive charge from the cytoplasm to the extracellular fluid stores energy as voltage (three Na+ for every K+)
Cotransport process
A transport protein (cotransporter) can couple the downhill diffusion of the solute to the uphill transport of a second substance against its own concentration gradient
How are large molecules transported across a cell membrane?
Endocytosis: the process by which cells engulf large molecules or particles by wrapping the membrane around them, forming a vesicle (phagocytosis & pinocytosis: require energy and involve receptor proteins)
Exocytosis: the process by which cells expel large molecules or particles by fusing membrane-bound vesicle with the cell membrane, releasing contents (ex. Hormones, neurotransmitters, waster products) into extracellular space
Phagocytosis
A cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole. It will be digested by a lysosome containing enzymes
Pinocytosis
A cell gulps droplets of extracellular fluid into tiny vesicles formed by infoldings of the plasma membrane
Receptor-mediated endocytosis
Specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances even if they are not concentrated in the fluid
Paracrine signaling
A form of cell communication in which cells release signaling molecules into the extracellular fluid to influence neighboring cells, signaling them to grow and divide. Numerous cells can simultaneously receive and respond to the molecules growth factor
Synaptic signaling
Occurs in the animal nervous system, its an electrical signal moving along a nerve cell that triggers the secretion of neurotransmitter molecules carrying a chemical signal (it diffuses across the synapse)
Endocrine signaling
In both animals and plants, specialized cells release hormone molecules, which travel via circulatory system, where they reach target cells that can recognize and respond to the hormones
