Topic 7 Flashcards
Plasma Membranes
The boundary that separates the living cell from its surroundings
* Plasma membrane is a dynamic structure
* Exhibits selective permeability
selective permeability
Some substances cross more easily than others
Transport proteins
can control passage across cellular membranes
most abundant lipid in the PM
Phospholipids
Phospholipids are
amphipathic (Contains both hydrophobic and
hydrophilic components)
Forming bilayer means
it can create a boundary between two aqueous compartments
The PM is a containing
dynamic structure
* Phospholipid heads
* Phospholipid tails
* Water molecules
* Transmembrane channel
Cellular membranes are
fluid mosaics of lipids
Fluid mosaic model:
- Membrane made up of many protein molecules bobbing in a fluid bilayer of phospholipids
Membrane Fluidity
Membranes are held together mainly by
weak hydrophobic interactions
* Most lipids and some proteins can move sideways within a membrane
* Rarely, a lipid may flip-flop from one phospholipid layer to the other
What affects membrane fluidity?
- as temp drops, fluidity of emmbrane decreases
- reduction in fluidity affects function of membranes
reduction in fluidity affects
function of membranes
temperature when membrane solidifies depends on
composition of lipids
Saturated and unsaturated fatty acids in membranes
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Unsaturated fats
the kinks in the tails prevent packing
* Stays fluid at lower temp
Saturated fats
straight tails allow tight packing
* Becomes solid at lower temp
Role of cholesterol as a buffer for membrane fluidity
The steroid cholesterol has different effects on the membrane fluidity of animal cells at different temperatures
- at warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
- at cool temps it maintains fluidity by preventing tight packing
Evolution of differences in membrane lipid composition
Some species have variations in lipid composition of cell membranes
* Appear to be adaptations to specific environmental conditions
* Ability to change the lipid compositions in response to temperature changes
* Evolved in organisms that live where temperatures vary
Fluid mosaic model: What makes it mosaic?
- Collection of multiple different proteins, embedded in the lipid bilayer
- Phospholipids form the bulk of the membrane
- Proteins determine most of the membrane’s
functions - Structural and function mosaic
2 major types of membrane proteins:
1) Integral proteins
2) Peripheral proteins
Integral proteins
Enters into hydrophobic region of lipid bilayer
* Includes transmembrane proteins
* Hydrophobic core made of nonpolar amino acids, coiled into α-helices
Peripheral proteins
- BOUND TO THE SURFACE OF THE MEMbrane (not embeded
- do not pass through the hydrophobic core of the bilayer
- membrane associated proteins
- on cytoplasmic side : can be attached to cytoskeleton components
- on extra celluar side: cell recognition
Cell-surface membrane proteins can carry out several functions
1 transport
2 enzymatic activity
3 signal transduction
4 cell-cell recognition
5 intercellular joining
6 attachment to the cytoskeleton and extracellualr matrix
Cell-cell recognition by membrane carbohydrates
- Cells can recognize each other by surface molecules
- On extracellular surface of PM
Glycocalyx
Carbohydrates outside the cell, can be covalently bound to:
* Lipids > forming glycolipids
* Proteins > forming glycoproteins
Lipids > forming
glycolipids
Proteins > forming
glycoproteins
Membranes have a sidedness to them
- Membranes have distinct inside and outside faces
- Distribution of proteins, lipids, and associated carbohydrates in the PM is asymmetrical
- Determined when the membrane is built by the ER and Golgi apparatus
Membrane structure results in selective permeability
- Emergent Properties of membrane:
- Properties of membrane is vastly different and broader than properties of individual components
- Plasma membranes are selectively permeable
- Regulates the cell’s molecular traffic and exchange of materials with surroundings
The Selective Permeability of the Lipid Bilayer
- Hydrophobic molecules can dissolve in the lipid bilayer and pass through the membrane rapidly
- Hydrophilic molecules including ions and polar molecules do not cross the membrane easily
- Proteins built into the membrane play key roles in regulating transport
Transport proteins includes
- Allow passage of hydrophilic
substances across the membrane - Channel proteins
- Carrier proteins
Channel proteins:
- Hydrophilic channel that certain molecules or ions can use as a tunnel
- Ie) Aquaporin - facilitates the passage of water molecules
Carrier proteins:
- Bind to molecules and change shape
to shuttle them across the membrane
A transport protein is specific for
the substance it moves
* Ie) Glucose transporter, Na+/K+ pump
Diffusion
is molecules spreading out evenly into the available space
* Ie)perfumeinair,dyeinwater
* Each molecule moves randomly, but overall population of molecules moves down concentration gradient
Dynamic equilibrium:
equal number of molecules cross the membrane in each
Substances diffuse down their
concentration gradient
-No work is done to move substances down the concentration gradient
Passive transport: Diffusion
The diffusion of a substance across a biological membrane is passive transport as no energy is expended by the cell
* Each substance moves down its own concentration gradient
Osmosis:
The diffusion of water across a sleectively permable membrane
permeable membrane
* Water diffuses across a membrane moving from a lower to a higher solute concentration
* Water is trying to dilute a concentrated solution
* Until the solute concentration is equal on both sides
Water diffuses across a membrane moving from
a lower to a higher solute concentration
- High free water (low solute)»_space; low free water (high solute)
The behaviour of a cell in a solution depends on
solute concentration and membrane permability
Tonicity:
the ability of a surrounding solution to cause a cell to gain or lose water
tonicity of a solution depends on
its concentration of solutes that cannot cross the mmebrane, relative to that inside the cell
Osmoregulation
the control of solute concentrations and water balance
Isotonic solution:
Solute concentration is the same as that
inside the cell
* No net water movement across the plasma membrane
Hypertonic solution:
- Solute concentration in solution is greater than
inside the cell; cell loses water - Water wants to dilute the higher solute concentration outside of cell
Hypotonic solution:
- Solute concentration in solution is less than inside the
cell; cell gains water - Water wants to dilute the higher solute concentration inside of cell
Cells without cell walls will shrivel in hypertonic solution and lyse (burst) in a hypotonic solution
Water balance of cells with cell walls
- Cell walls help maintain water balance
Isotonic solution:
- There is no net movement of water into the cell
- The cell becomes flaccid (limp)
Hypertonic solution:
- In a hypertonic environment, plant
cells lose water - The membrane pulls away from the cell wall, causing the plant to wilt, a potentially lethal effect called plasmolysis
Hypotonic solution
- A plant cell in a hypotonic solution swells
until the wall opposes uptake - The cell is now turgid (firm)
- Turgor pressure provides non-woody plants mechanical support
facilitated diffusion
transport proteins speed the passive movement of molecules across the plasma membrane
- no energy needed, passive
- molecules still moving down concentration gradient
- channel and carrier proteins involved
Channel Proteins
- Provide corridors to allow a specific molecule or ion to cross the membrane
-ex aquaporins (facillitate diffusion of water)
Ion channels
facilitate the transport of ions
gated channels
open or close in response to a stimulus
- ex in nerve cells ion channels open in response to electrical stimulus
Carrier Proteins
Undergo a change in shape that translocates the solute-binding site across the membrane
- ex glucose transporter
The change in shape can be triggered by the binding and release of the transported molecule
Active transport uses
energy to move solutes against their gradients
Active transport
requires energy
- usually in the form of atp hydrolysis to move substrances against their concentration gradient
- terminal phosphate group from atp is transferred to the transport protein
* Active transport allows cells to maintain concentration gradients that differ from their surroundings
* All proteins involved in active transport are carrier proteins
All proteins involved in active transport are
carrier proteins
sodium-potassium pump
a transport protein that is energized by transfer of a phosphate group from the hydrolysis of ATP
Electrochemical gradient
the combined forces that drives diffusion of ions across a membrane
- chemical and electrical
Membrane potential
voltage across membrane, due to differences in distribution of ions across a membrane
- cytoplasm side, negative
- extracellular side, positive
Cotransport
occurs when active transport of a solute indirectly drives transport of other substances
- secondary active transport
Diffusion of first substance down concentration gradient is coupled to transport of second substance against own concentration gradient
Bulk transport across the plasma membrane
- Small molecules and water pass through the lipid bilayer or via transport proteins
- Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
- Bulk transport occurs via endocytosis or exocytosis and requires energy
- Can also recycle and remodel plasma membrane
Exocytosis
- In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
- Many secretory cells use exocytosis to export their products
- Sidedness of membranes
- Exo - expelling
Endocytosis
- The cell takes in macromolecules by forming vesicles from the plasma membrane
- Endocytosis is a reversal of exocytosis, involving different proteins
- There are three types of endocytosis
There are three types of endocytosis
- phagocytosis (cellular eating)
- pinocytosis (cellular drinking)
- receptor-mediated endocytosis
phagocytosis
a cell engulfs a particle in a vacuole
* Extends pseudopodia to engulf particle
* The vacuole fuses with a lysosome to digest the particle
pinocytosis
molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
* Cellular drinking
Endocytosis: Receptor mediated
- In receptor-mediated endocytosis, binding of specific solutes to receptors triggers vesicle formation
- Receptor proteins, receptors, and other molecules from the extracellular fluid are transported in the vesicles
- Emptied receptors are recycled to the plasma membrane
