Chapter 5 Membrane transport and cell signaling Flashcards
Plasma Membrane
Seperates living cells from its surroundings.
controls traffic in and outside of the cell
Selective permeability
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others.
amphipathic
Phospholipids are the most abundant lipid in most membranes
Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
A phospholipid bilayer can exist as a stable boundary between two aqueous compartments
Fluid mosaic model
Most membrane proteins are also amphipathic and reside in the bilayer with their hydrophilic portions protruding
The fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
Groups of certain proteins or certain lipids may associate in long-lasting, specialized patches
Integral proteins
Integral proteins penetrate the hydrophobic interior of the lipid bilayer
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into a helices
Peripheral proteins
Peripheral proteins are loosely bound to the surface of the membrane
What are the six major functions of membrane proteins?
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition
- Intercellular joining
- Attachment to the cytoskeleton and extracellular matrix (ECM)
The permability of the lipid bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer of the membrane and cross it easily
Polar molecules, such as sugars, do not cross the membrane easily
Transport proteins
allow passage of hydrophilic substances across the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Aquaporins
Channel proteins called aquaporins facilitate the passage of water
Diffusion
is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly, diffusion of a population of molecules may be directional
At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
Concentration gradient
Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated
No work must be done to move substances down the concentration gradient
Passive transport
The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
Osmosis
is the diffusion of free water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
Tonicity
is the ability of a surrounding solution to cause a cell to gain or lose water
Isotonic solution
Solute concentration is the same as inside the cell; no net water movement across the plasma membrane
Hypertonic solution
Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
Hypotonic
Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
Osmoregulation
Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
example,
The protist Paramecium caudatum, which is hypertonic to its pondwater environment, has a contractile vacuole that can pump excess water out of the cell
turgid
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (very firm)
Flaccid
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
plasmolysis
In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
Facilitated diffusion
- In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
- Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
Channel proteins include
Channel proteins include
- *Aquaporins**, for facilitated diffusion of water
- *Ion channels** that open or close in response to a stimulus (gated channels)
Ion Channels
Ion channels that open or close in response to a stimulus (gated channels)
Active transport
Active transport moves substances against their concentration gradients
Active transport requires energy, usually in the form of ATP
Active transport allows cells to maintain concentration gradients that differ from their surroundings
Sodium potassium pump
is one type of active transport system
Exchanges Na+ for K+ across the plasma membrane of animal cells
Membrane potential
Membrane potential is the voltage across a membrane
Voltage is created by differences in the distribution of positive and negative ions across a membrane
Electrochemical gradient
Two combined forces, collectively called the electrochemical gradient, 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)
Electrogenic pump
An electrogenic pump is a transport protein that generates voltage across a membrane.
Example:
The sodium-potassium pump is the major electrogenic pump of animal cells
Proton pump
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
Electrogenic pumps help store energy that can be used for cellular work
Most protein pumps are powered by ATP
Cotransport
Cotransport occurs when active transport of a solute indirectly drives transport of other solutes
Plant cells use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export products
Endocytosis
Name the three types
In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
There are three types of endocytosis
- Phagocytosis (“cellular eating”)
- Pinocytosis (“cellular drinking”)
- Receptor-mediated endocytosis
local signaling
- Eukaryotic cells may communicate by direct contact
- Animal and plant cells have junctions that directly connect the cytoplasm of adjacent cells
- These are called gap junctions (animal cells) and plasmodesmata (plant cells)
- The free passage of substances in the cytosol from one cell to another is a type of local signaling
paracrine signaling
In many other cases of local signaling, messenger molecules are secreted by a signaling cell
- These messenger molecules, called local regulators, travel only short distances
- One class of these, growth factors, stimulates nearby cells to grow and divide
- This type of local signaling in animal cells is called paracrine signaling
Synpatic signaling
Another more specialized type of local signaling occurs in the animal nervous system
This synaptic signaling consists of an electrical signal moving along a nerve cell that triggers secretion of neurotransmitter molecules
These diffuse across the space between the nerve cell and its target, triggering a response in the target cell
long-distance signaling
In long-distance signaling, plants and animals use chemicals called hormones
In hormonal signaling in animals (called endocrine signaling), specialized cells release hormone molecules that travel via the circulatory system
Hormones vary widely in size and shape
What are the three stages of cell signaling?
- Reception: target cells detection of a signaling molecule coming from outside of the cell. A chemical signal is detected when the signaling molecule binds to a receptor protein located at the cells surface
- Transduction: a step or series of steps that converts the signal to a form that can bring about a specific cellular response.
- Response: transduced signal triggers cellular response. The response may be an imaginable cellular activity. such as catylist by enzyme, rearrangemnt of cytoskeleton.
Submerging a red blood cell in distilled water will result in
Cell will become lysis (it will burst) hypotonic
