Do the majority of biochemicals pass through the lipid bilayer?
NO
What substances can cross the lipid bilayer?
Lipid-soluble sources AND
Water (too small)
Extra cellular fluid contains:
Na+ and Cl-
Intracellular fluid contains:
K+, Phosphate, and proteins
What is imprortant for membrane transport?
Membrane proteins
What two things can membrane proteins act as?
- Channels
2. Carrier proteins
Are channels and carrier proteins selective or non selective?
Usually selective
Transport through the cell membrane occurs via what two processes?
- Passive Transport or Diffusion
2. Active Transport
What is passive transport (diffusion)
When molecules move down their concentration gradient
Does not require the cell to expend energy
What type of energy does passive transport/diffusion use if not cell provided energy?
Kinetic energy
What is active transport?
Molecules move down their concentration gradient
What energy does active transport use to move molecules?
The cell’s energy
What are the main two types of passive transport?
- Simple diffusion
2. Facilitated diffusion
What is simple diffusion?
Molecules diffuses through the lipid bilayer by:
- Selective protein channels
- Gated Channels
- Osmosis
What is facilitative diffusion?
Diffusion through
- Carriers or
- Transporters
Regarding simple diffusion, what is the rate of diffuion directly proportional to?
It is directly proportional to lipid solubility of the molecule
WHat molecules can dissolve by simple diffusion directly?
Oxygen Nitrogen CO2 and alcohol
What TYPE of molecules readily dissolve in lipid bilayers?
Small hydrophobic molecules
O2, CO2
What TYPE of molecules dissolve more slowly across lipid bilayers?
Small uncharged hydrophilic molecules
H20, Urea, glycerol
What TYPE of molecules are impermeable to the lipid bilayer?
Ions, they will not cross the lipid bilayer no matter how small they are
Protein channels are usually highly selective: true or false?
TRUE
What usually determines selectivity in protein channels?
- Diameter
- Shape
- The nature of the electrical charge or bonds along the inside surface
What is an example of a selective protein channel?
Potassium channel
How does the potassium channel permit potassium ions to pass?
A narrow selective filter has transient binding sites for potassium which allows passage of the ions
For gated channels, what controls ion permeability?
The gate
What kind of stimuli can open ion gated channels?
- Changes in voltage across a membrane
- Ligand (intracellular or extracellular)
- Mechanical stress
Voltage gated channels are present in the plasma membrane of what type of cells?
All excitable cells
What are some types of excitable cells?
- Nerve cells
- Muscle cells
- Endocrine cells
- Egg cells
What is the job of voltage gated channels in nerve cells?
Voltage gated channels:
- Responsible for the neurons ability to transmit information along its length
- Release neurotransmitters
….in neurons
What causes voltage gated channels to open?
A range of membrane potentials
In regards to voltage gated channels,
What is a “channel’s threshold”?
A channel’s threshold = the minimum membrane potential needed to open the channel
Where are ligand gated channels found?
Ligand gated channels are found:
- In the membrane of skeletal muscle cells
- Some neurons of the autonomic Nervous System and the brain
How do ligand gated channels open?
Via ligand binding
What is another name for facilitated diffusion?
Carrier mediated diffusion
Properties of Facilitated Diffusion:
- Specific proteins facilitate diffusion across a membrane
- No cellular energy required
- The carrier only acts on specific substrates
- The rate of transport will reach a maximum based on the number of proteins available for transport
In facilitated diffusion, can the rate of diffusion rise higher than the Vmax of the carrier protein?
NO
What is the difference between the rate of simple diffusion and facilitated diffusion?
In simple diffusion the rate of diffusion is proportional to the concentration of the diffusing substance whereas in facilitated the rate of diffusion is never greater than the Vmax of the carrier protein
What is osmosis?
The net movement of water caused by a difference in water concentration across a membrane
What are the two ways water diffusion typically occurs?
- Diffusion through the lipid bilayer (unaided) (simple diffusion)
- Through selective protein channels (simple diffusion) called “Aquaporins”
Are aquaporins highly specialized? And are there different types?
Yes they are highly specialized, and yes there are at least 13 types in various mammalian cells
What other factor can also influence the diffusion of water?
Pressure
What is osmotic pressure?
The amount of pressure required to stop OSMOSIS is osmotic pressure.
Remember: osmosis is the diffusion of water across the membrane or through aquaporins
What are the two main types of active transport?
- Primary active transport
2. Secondary active transport
What is primary active transport?
- The energy is derived directly from ATP or something similar
- Transport is against the concentration gradient or electrochemical gradient
What is secondary active transport?
- The energy is provided by the concentration gradient of the driving ion
- Transporter proteins couple the movement of an ion going down its concentration gradient (usually Na+ or H) with an ion or molecule moving against its concentration gradient
- uses “CoTransport” or “Counter transport”
What is co-transport (secondary active transport)?
What is the membrane protein called in this situation?
The simultaneous transport of two substances across a membrane in the SAME direction
-one is moving with its concentration gradient, the other is moving against
The membrane protein used is called a “Symporter”
What is another name for Co-transport (secondary active transport)?
Symport
What is another name for counter transport (secondary active transport)?
Anti port
What is counter-transport (secondary active transport)?
What is the membrane protein called in this situation?
The simultaneous transport of two substances across a membrane in OPPOSITE directions.
-one is moving with its concentration gradient the other is moving against
The protein is called an antiporter.
What is transcellular transport or epithelial transport?
- Substance is carried across the cell
- from one ECF to another
What is the mechanisms used to cross the cell membrane to get to the other ECF in transcellular transport?
One side uses active transport
The other side of a membrane uses simple diffusion or facilitated
What is the other method of transporting molecules across the membrane by means other than membrane proteins, where the membrane itself is a carrier?
Exocytosis
Endocytosis
What are the 3 types of Endocytosis and what is their main function?
- Phagocytosis
- Ingestion of large molecules
- Pinocytosis
- Ingestion of small molecules
- Receptor-mediated endocytosis
- ex: cholesterol
What are the two types of secretion associated with exocytosis?
- Constitutive secretion
2. Regulated secretion
Properties of constitutive secreted exocytosis:
- All cells
- No signal sequence
- Proteins are incorporated into plasma membrane, extracellular matrix, or signaling proteins
Properties of regulated secretion of exocytosis:
- Specialized cells
2. Need signal to stimulate fusion with cell membrane and release of secreted proteins