Transport Mechanisms Flashcards
How do we maintain homeostasis?
through the exchange of nutrients, salts, gases and waste in and out of the body
The plasma membrane separates what? main exchanges?
The PLASMA MEMBRANE separates ICF and the ISF
Main exchanges: gases, waste, energy sources
The capillary wall separates what? main exchanges?
The CAPILLARY WALL separates the ISF and the Plasma
Main exchanges: nutrients and gases
The plasma also exchanges with the lungs (O2/CO2), the skin (cooling/evaporation), the kidneys (excretion of urea, retention of salts) and the GI tract (intake of nutrients, expulsion of water)
Role of cholesterol
Has a different role depending on temperature:
At cooler temperatures, cholesterol reduces the packing of fatty acid tails and tends to increases/preserve membrane fluidity
At warmer temperatures, when the membrane has higher fluidity, cholesterol keep it more stable
Plays an additional role in lipid rafts and the formation of vesicles
Types of proteins in the cell membrane
Integral Proteins: closely associated with phospholipids and embedded within the cell membrane
- Transmembrane proteins: a type of amphipathic integral protein that spans the entire cell membrane. Plays an important role in transport.
Peripheral Proteins: more loosely associated with phospholipids, involved in signaling within the cell and the cytoskeleton
- Usually found on the cytoplasmic side of the membrane
- Usually non amphipathic
Role of glycocalyx?
Provides protection from infection and enables cells to identify each other and interact
The cell membrane is highly permeable to…
H2O
Lipid-soluble substances
Dissolved gasses (O2 & CO2)
Small uncharged molecules
The cell membrane is less permeable to…
Larger molecules
Charged particles
The cell membrane is impermeable to…
Very large molecules
Functions of Plasma Membrane Proteins (6)
- Transport & Diffusion
- Cell Surface Identity Markers
- Enzymes that catalyze membrane-associated reactions
- Cell-to-cell adhesion
- Receptors of chemical signals
- Attachment to cytoskeleton
Transmembrane transport can occur via… (2 possible ways to cross the cell membrane)
Via the Phospholipid Bilayer – only available for molecules that can permeate the bilayer
Via interactions with protein clusters – pathway for molecules that cannot permeate the bilayer
- Channels (for ions that diffuse down their concentration gradient)
- Carriers/Transporters/Pumps (can involve the use of ATP)
2 Major Categories of Transport Mechanisms Across the Cell Membrane
Active & Passive Transport
Methods of Passive Transport
Diffusion
Carrier-Mediated Facilitated Diffusion
Osmosis
Methods of Active Transport
Carrier-Mediated Active Transport (Primary and Secondary)
Pino/Phagocytosis
What is flux?
number of particles crossing a surface per unit of time
Where is Net Flux Directed?
Net Flux is always from regions of higher concentration to regions of lower concentration
Describe the flux at equilibrium
At equilibrium: diffusion fluxes are equal & net flux = 0 because molecules are distributed everywhere in equal concentrations. There is no gradient, thus the fluxes are equal and in all directions.
Factors that affect diffusion across a cell membrane
- Mass of molecule
- Concentration gradient
- lipid solubility
- electrical charge (charged molecules require channels or carriers)
- Availability of ion channels or membrane carriers
Limitation of Diffusion?
diffusion time increases in proportion to the square of the distance travelled by the solute molecules
How do particles penetrate the cell in diffusion? (2)
- Dissolving in the lipid component of the cell membrane
2. Diffusing through channels
What determines the diffusion of ions across an ion channel?
Electrochemical Gradient: simultaneous existence of an electrical gradient and a concentration gradient for a particular ion
The electrochemical balance is what determines the flow of ions
Gating of Ion Channels
Ligand-Gated Channel – gated by a molecule that binds to the channel and causes conformational changes (ex. a neurotransmitters binds to a channel to open it)
Mechanically-Gated – gated by stretch or pressure (ex. touch-sensitive or pain-related channels)
Voltage-Gates – gated by voltage differences across the cell membrane the channel responds to
The total number of ions that flow through voltage-gated ion channels depends on…(3)
Channel Conductance (How big is channel? How easily do ions flow though it?) How often the channel opens How long the channel stays open
What are mediated transport systems?
The movement of ions/molecules by integral membrane proteins called transporters or carriers.
Properties of Mediated Transport Systems (3)
Specificity, Saturation, Competition
Factors that Determine magnitude of mediated transport systems (4)
- Solute concentration
- Affinity of transporter
- number of transporters
- rate of transporter conformational change
How is facilitated diffusion different from diffusion?
facilitated diffusion is distinct from diffusion in that it involves a transporter or carrier molecule which enables the solute to penetrate the membrane more readily than it could by simple diffusion
Steps of facilitated diffusion:
Solute binds transporter
Transporter configuration changes
Solute is delivered to the other side of the membrane
Transporter resumes original configuration
(down the concentration gradient)
Primary Active Transport
Involves the hydrolysis of ATP by a transporter
The phosphorylation of the transporter changes its conformation and solute-binding affinity
The Na+/K+ Pump
- ATP is bound to the pump, increasing the affinity of the three binding sites for Na+
- 3 Na+ molecules bind to the pump. This causes the hydrolysis of the ATP and the phosphorylation of the ATPase.
ADP is released - Another conformation change occurs that reduced the affinity of the sodium binding sites. Na is released outside the cell while the two K+ binding sites increase their affinity
- 2x K+ ions bind to the pump and the phosphate is released. This causes another conformation changes to the ATPase resulting in the K+ binding site facing the inside of the cell where the K+ molecules are released
Secondary Active Transport
In secondary active transport, the movement of Na+ down its concentration gradient is coupled with the transport of another solute molecule (ion, glucose, amino acid, etc.) uphill against its concentration gradient
Cotransport/Symport
transport of solute X is in the same direction as the Na+’s movement (i.e. both into the cell)
Countertransport/Antiport
transport of solute X is in the opposite direction as the Na+’s movement (i.e. out of the cell)
2 forms of Exocytosis
Constitutive & Regulated
Constitutive Exocytosis
Constitutive Exocytosis: function is to replace the plasma membrane, deliver membrane proteins to the membrane and get rid of substances from the cell. It is part of the natural turnover of the cell membrane and can be released by metabolic pathways
Regulated Exocytosis
Regulated Exocytosis: function is to secrete hormones, digestive enzymes and neurotransmitters. Tends to involve a vesicle waiting to be triggered by an extracellular signal and the increase of cytosolic Ca2+
3 forms of Endocytosis
Pinocytosis, Phagocytosis & Receptor Mediated Endocytosis
Pinocytosis
an endocytotic vesicle engulfs the ECF and whatever solutes are present. The vesicle travels into the cytoplasm and fuses with other vesicles such as endosomes or lysosomes. (nonspecific)
Phagocytosis
process by which cells bind and internalize particulate matter (>75 microns, ex. small dust particles, cell debris, microorganisms). Involves pseudopodia, extensions of the cell membrane, folding around the particle and engulfing it. The pseudopodia fuse to form large vesicles called phagosomes that pinch off the cell membrane. Phagosomes migrate to and fuse with lysosomes where the contents of the phagosome are degraded. (specific)
2 types of Receptor Mediated Endocytosis
Clathrin-Dependant & Potocytosis
Clathrin Dependant Endocytosis
Clathrin is recruited to the plasma membrane by ligand binding and linked to the ligand-receptor complex by adaptor proteins. The entire complex forms a cage-like structure that leads to the aggregation of ligand-bound receptors into a localized region of the membrane, forming a Clathrin-coated pit which then invaginates and pinches off to form a Clathrin-coated vesicle. Clathrin coat is removed and Clathrin proteins are recycled back to the membrane
Potpcytosis
similar to Clathrin-Dependent RME, but the ligand is restricted to low molecular weight molecules (most of the time, ex. vitamins). Also, it is tiny vesicles called caveolae pinch off form the plasma membrane and they can deliver their contents directly to the cell cytosol as well as to the ER, other organelles or performing transcytosis.
What protein channels allow for the facilitated diffusion of water across the cell membrane?
Aquaporins
Osmosis (Definition)
the net diffusion of water across a semi-permeable membrane
Osmotic Pressure (definition and formula)
pressure required to resist the movement of water across a semi—permeable membrane. It is equal to the difference in hydrostatic pressure of the two solutions
P = nRT/V
Non-Penetrating vs Penetrating Particles (Osmotic Pressure)
Particles that can penetrate the cell membrane do not exert a sustained osmotic pressure and distribute until equilibrium is reaches
Non-penetrating particles exert osmotic pressure
Osmolarity reflects what?
the net number of particles in the solution
If a solution is hypotonic…
a solution has a concentration of non-penetrating solutes that is <300 mOsm. Water enters the cell and it swells.
If a solution is hyper
a solution has a concentration of non-penetrating solutes that is >300 mOsm. Water leaves the cell and it shrinks
If you have excessive intake of water, what happens to the ECF volume/osmolarity and ICF volume/osmolarity?
ECF osmolarity decreases, volume increases.
ICf osmolarity decreases, volume increases
Four main processes by which molecules get transported across the capillary wall:
- diffusion across cell membrane
- diffusion through water-filled channels (between cells)
- Transcytosis
- Bulk Flow (but capillary wall acts as a filter)
Fundamental principle of physiology?
At all levels of organization, functional activities are directed at maintaining homeostasis, the relative constancy of the ‘Milieu Intérieur’ (internal environment)
Water is the medium in which
Solutes are dissolved (water = solvent)
Metabolic reactions take place
Individuals with less subcutaneous fat have a…
a higher % water content (i.e. the proportion of water in their body is greater)
Trends in variations in Body Water
Age
- Newborns have little subcutaneous fat, and therefore a higher % water composition
- As people age, they gain more subcutaneous fat and lose muscle tissue, and therefore have a lower % water composition
Gender
- Women deposit adipose tissues in the breast/buttock at puberty, and therefore have a lower % water composition
For the same dosage of water soluble-medication, having a lower % water composition…
leads to higher concentrations of medication
For the same dosage of water soluble-medication, having a higher % water composition…
leads to lower concentrations of medication
Water balance occurs when…
output = input
Major Homeostatic organ for water balance
Kidneys
Intake of water:
oral fluid intake, intake as food, metabolic processes
Output of water
Obligatory loss, facultative loss, sweat
Obligatory Water losses
Required each day in order to flush out water from the body
Insensible perspiration from lungs/skin
Sensible perspiration from kidneys/stoll
Facultative loss
additional water loss via the kidneys in order to maintain water balance
Water exchange over 24 hours
3-4% of the total body weight for Adults
10% of the total body weight for newborns
Constant body water help maintain (2) and ensures what?
Normal solute concentration
Normal blood volume and pressure
This ensures an adequate supply of oxygen to tissues
Negative Water Balance causes
reduced intake, excessive loss from gut, sweating, urine or in expired air
Causes of water intoxication
Excessive intake or renal system failure
What are transcellular fluids?
Aggregates of small fluid volumes in specific body cavities secreted by epithelial cells lining the body cavities
Fluids have a specialized function depending on their location
Do not contribute significantly to overall water exchange (i.e. local changes do not affecter the body fluid balance), but play an important functional role locally
1-2% of ECF
Composition of ICF
ICF is high in potassium and low in NaCl ions, contains some proteins
Composition of ECF
ECF is low in potassium and high in NaCl ions, plasma contains more proteins than ISF, but they are otherwise similar in ionic concentration
How to measure ICf using Indicator Dilution Method?
Cannot be done directly: use one indicator to measure total body water and one to measure ECF water component. The difference is the volume of the ICF
How to measure ISF using Indicator Dilution Method?
Cannot be done directly: use one indication to measure total ECF volume and one to measure plasma volume. The difference is the volume of the ISF.
Indicators to measure total body water:
Indicator must be able to distribute through the plasma, cross capillary walls to travel into the ISF, and then across the cell membrane Antipyrine (fat-soluble) Radioactive Water (D2O or T2O)
Indicators to measure ECF
Indicator must enter the capillaries, but cannot cross the cell membrane
Sugars & starches (sucrose, mannitol, inulin) – usually used with a radioactive tag
Indicators to measure plasma
Indicator cannot cross capillary walls
Evan’s Blue (dye that sticks to plasma proteins) or Radioactive Albumin
