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Cell membrane structures and functions:
- Mass balance and homeostasis
- diffusion
- protein-mediated transport
- vesicular transport
- transepithelial transport
- osmosis and tonicity
- resting membrane potential
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Law of mass balance:
if the amount of a substance in the body is to remain constant, any input must be offset by an equal loss
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Output options:
- Execretion- Ex. waste materials
- Metabolism- Ex. production of metabolites
- Clearance: the rate at which a material is removed from the blood by either execretion or metabolism.
- The liver, kidneys, lungs, and skin clear substances from the blood.
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If a person eats 12 mg of salt in a day and excretes 11 mg of it in the urine, what happened to the remaining 1 mg?
the remaining salt stays in the body
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Glucose metabolism effect on mass balance in the body
Glucose metabolism adds CO2 and water to the body, disturbing the mass balance of these two substances. To maintain mass balance both substances must be either excreted or further metabolized.
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Body compartments:
- ICF
- ECF: interstitial fluid and plasma
*the whole body is electrically neutral
*chemical and electrical disequilibrium is due to selective membrane permeability
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Bulk flow:
a pressure gradient moves a large quantity of fluid along with its dissolved and suspended materials
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Distribution of solutes in body compartments:
- Na+= high in ECF; low in ICF
- K+= low in ECF; high in ICF
- Cl-= high in ECF; low in ICF
- HCO3-= more in ECF, than ICF but not high in either
- Large anions/ proteins= some in the plasma of ECF; high in ICF
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Active Transport:
Requires direct or indirect use of ATP
Phagocytosis, exocytosis, and endocytosis
ex: Two compartments are separated by a membrane that is permeable to glucose. Each compartment is filled with 1 M glucose. After 6 hours, compartment A contains 1.5 M glucose and compartment B contains 0.5 M glucose
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Indirect/ Direct Active Transport:
- DIRECT: which derives energy directly from ATP; Na+K+ATPase
- INDIRECT: which couples the kinetic energy of one molecule moving down its concentration gradient to the movement of another molecule against its concentration gradient.
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Passive Transport:
uses energy stored in concentration gradient
simple and facilitated diffusion, osmosis
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Properties of Diffusion:
- passive
- open system/ across partitions
- movement down concentration gradient; from high to low
- until state of equilibrium is reached
- lipid solubility: pass through the lipid core of the membrane
- faster with increased tempertaure (more kinetic E)
- faster with smaller molecules
- slower with increase in distance
- slower across thicker membranes
- faster where there is more surface area
- faster where there is increase concentration gradient
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If the distance over which a molecule must diffuse triples from 1 to 3, diffusion takes how many times as long?
takes 9 times as long
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Where does the energy for diffusion come from?
molecular motion
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Three physical methods by which materials enter cells:
- simple diffusion
- protein mediated transport
- vesicular transport
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Simple Diffusion:
movement of lipophilic molecules directly through phospholipid bilayer
- faster with increased surface area
- faster with increased concentration gradient
- faster with increased membrane permeability
- slow with increased membrane thickness
*small, non-polar molecules
Ex: Emphysema
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Which is more likely to cross a cell membrane by simple diffusion: a fatty acid molecule or a glucose molecule?
because its lipophilic the fatty acid is more likely to cross by simple diffusion
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Functions of membrane proteins:
- Structural proteins - link cell to matrix
- transporters - water channels
- receptors - hormone receptors
- enzymes - intestinal digestive enzymes
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Protein Mediated Trasport:
For lipophobic molecules
Passive (facilitated diffusion) or active transport
Channel or Carrier proteins
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Channel Proteins:
form water filled channels that link the intracellular and extracellular compartments. Gated channels regulate movement of substances through them by opening and closing; regulated by ligands, the electrical state of the cell, or by physical changes ie. pressure
for small molecules: H2O, Na+, K+, Ca2+, Cl-
*Aquaporins: family of channels for H2O; all cells have!
*selectively based on diameter and charge
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Open Channels:
- =pores!
- have gates but they are open most of the time
- "leak channels"
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Gated Channels:
- Chemically gated: controlled by messenger molecule or ligand; molecule binds = gate opens
- Voltage gated: controlled by electrical state of the cell; axons; channel that opens when resting membrane potential changes
- Mechanically gated: controlled by physical state of the cell; temp, stretching of cell membrane, etc. *Inner ear- organ of corti- hairs bending
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Why doesn’t glucose cross the cell membrane through open channels?
because its too large
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Which kinds of particles pass through open channels?
ions and water molecules
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If a channel is lined with amino acids that have a net positive charge, which of the following ions is/are likely to move freely through the channel? Na+, Cl-, K+, Ca2+
a channel lined with positive charges attracts anions, which means Cl- is more likely to move freely through the channel.
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Carrier Proteins:
- never form a continuous connection between the intracelluar and extracellular fluid. They bind to substrates then change conformations.
- used for small organic molecules; glucose
- ions may use channels or carriers
- slow
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Name two ways channels differ from carriers.
Channel proteins form continuous connections between the two sides of a membrane and transport molecules more quickly.
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Cotransporter:
- a protein that moves more than one molecule at a time
- Symport or Antiport
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Molecules are moved in the same direction, the cotransporters are called:
- Symport Carriers
- Ex: glucose, Na+
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Molecules are transported in opposite directions, the cotransporters are called:
- Antiport Carriers
- Ex: Na+/K+ pump
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A transport protein that moves only one substrate is called:
Uniport Carrier
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Sodium Postassium ATPase
most important primary active transporter; pumps Na+ out of the cell and K+ into the cell
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Facilitated Diffusion:
protein mediated diffusion; has same properties as simple diffusion.
Specific, competitive, and saturation
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Define the following terms and explain how they differ from one another: specificity, competition, saturation. Apply these terms in a short explanation of facilitated diffusion of glucose.
- specificity- GLUT is specific for hexose sugars
- competition- if two hexoses are present (glucose, fructose) they compete for GLUT binding sites
- saturation- when enough sugar is present, transport saturates
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If you give a patient saline soltuion (NaCl):
body volumes are going to INCREASE; osmolarity is going to DECREASE.
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Primary Active Transport:
ATP DIRECTLY FUELS!
EX: Na+/K+ pump = Na+/K+ATPase (antiport)
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Mechanism of the Na+-K+ATPase
3 Na+ from ICF bind to protein -> ATPase is phosphorylated with Pi from ATP; protein changes conformation -> 3 Na+ released into ECF -> 2 K+ bind from ECF; protein changes conformation -> 2 K+ released in ICF
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Secondary Active Transport:
Used stored E from concentration gradient
coupling of E of one molecule with movement of another molecule
Ex: SGLT
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Mechanism of SGLT transport:
Na+ binds to carrier (in ECF Na+ is high and glucose is low; in ICF Na+ is low and glucose is high) -> Na+ binding creates a site for glucose -> glucose binding changes carrier conformation -> Na+ released into the cytosol and glucose follows
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Name two ways active transport by the Na+-K+-ATPase differs from secondary transport by the SGLT.
The ATPase is an antiporter, but the SGLT is a symporter. The ATPase requires energy from ATP to change conformation, whereas SGLT uses energy stored in the Na+ concentration gradient
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What does Na+ movement from the cytoplasm to the extracellular fluid require energy?
Sodium movement out of the cell requires energy because the direction of ion flow is against the concentration gradient.
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Vesicular Transport:
Movement of large molecules across the cell membrane
- 1. Phagocytosis
- 2. Endocytosis
- 3. Exocytosis
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Phagocytosis:
- Requires E
- cell engulfs particle into vesicle via pseudopodia formation
- vesicles formed are much larger than those formed by endocytosis
- phagolysosome = phagosome fused with a lysosome
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Endocytosis:
- Requires E
- membrane surfaces indent
- smaller vesicles
- nonselective: Pinocytosis for fluids and dissolved substances
- when vesicles that come into the cytoplasm are returned to the cell membrane = membrane recycling
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Name the two membrane protein families associated with endocytosis.
- clathrin - receptor mediated endocytosis Ex. LDL cholesterol and Familial Hypercholesterolemia; ligands bind to membrane receptors that concentrate in coated pits (site of endocytosis)
- caveolin - potocytosis; receptors are located in caveolae that have a caveolin protein coating
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Exocytosis
- Intracellular vesicle fuses with membrane; releases its contents into extracellular space
- Requires E and Ca2+
- Active transport
- docking and fusion of vesicles
- Ex: goblet cells, fibroblasts; receptor insertion, waste removal
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How do cells move large proteins into the cell? Out of the cell?
proteins move into cells by endocytosis and out of the cell by exocytosis
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How does phagocytosis differ from endocytosis?
In phagocytosis, the cytoskeleton pushes the membrane out to engulf a particle in a large vesicle. In endocytosis, the membrane surface indents and the vesicle is much smaller.
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Transepithelial Transport:
- combination of secondary active and passive transport
- molecules must cross two phospholipid bilayers
- Polarity of epithelial cells: apical and basolateral; allows one way movement of molecules across the epithelium
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Apical membrane:
- Na+-glucose transporter
- Na+ leak channels but no water pore
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Basolateral membrane:
- Na+/K+ATPase
- Na+-K+ATPase and K+ leak channels. May also have water channels
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Transepithelial transport of glucose:
Na+ glucose symporter brings glucose into the cell against the concentration gradient using E stored in the Na+ concentration gradient -> GLUT transporter transfers glucose to ECF by facilitated diffusion -> Na+/K+ATPase pumps Na+ out of the cell, keeping ICF Na+ concentration low
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Transcytosis:
- endocytosis -> vesicular transport -> exocytosis
- moves large proteins intact
- Ex: absorbtion of maternal antibodies from breast milk, movement of proteins across capillary endothelium
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Osmosis:
- the movement of water across a membrane in response to a concentration gradient
- causes volume change
- can be opposed by other force (pressure) so that water does not move
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Osmolarity
concentration; the number of particles per liter of solution; mOsM
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Molarity -> Osmolarity
- 1 M glucose = 1 OsM glucose
- 1 M NaCl= 2 OsM NaCl
- 1 M MgCl2= 3 OsM MgCl2
osmolarity of the human body = 300 mOsM
isosomotic, hyperosmotic, hyposmotic
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A tissue placed in a 0.1 OsM NaCl soultion gained H20; therefore...
the soultion was hyposmotic to the tissue
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What does it mean if we say that a solution is hypotonic to a cell? Hypertonic to the same cell? What determines the tonicity of a solution relative to a cell?
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