Membrane 3 (WS 5)

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Function of membrane proteins

1. Structural proteins
2. Enzymes
3. Membrane receptors


Transport processes

1. Transport across a selectively permeable membrane
2. Channels
3. Transport across membranes-carrier mediated


Transport across a selectively permeable membrane

a transport process



a transport process

EX: water channels, ion channels, open channels, gated channels

Chemically gated channel
Mechanically gated channels


Transport across membranes- carrier mediated

1. Facilitated Diffusion- starts with momentum
EX: Like a boat being let in through a gate, and let out through another gate
Glucose transporter: Glucose goes in and out through a Glucose Transporter
Moves sugar from HI to LO concentration
2. Primary Active Transport- moves ions against their electro-chemical gradient
EX: Na+-K+-ATPase (aka Sodium Potassium Pump): use of ATP to move Na+ and K+ through channel, opposite directions---Intracellular fluid consists of low concentration of Na+, and high concentration of K+
3. Secondary Active Transport- Needs primary active transport in the process for this to occur
Needs a working pump
Moves against concentration gradient
EX: Na+- Glucose (SGLT- Sodium glucose transporter)


Carrier Proteins

Uniport (one), Contransport (multi-porters>1)

Primary active transport: direct use of ATP to move molecules up electrical/chemical gradient

Secondary Active transport: Co-transport process using established ionic concentration gradient (used energy to generate) to move substance uphill against electrical/chemical gradient


Carrier mediated transport characteristics

1. Specificity-selective
2. Competition- Two or more ions compete for the same carrier
3. Saturation- Limited transport amount
Not the case if you have simple diffusion across a membrane
Glucose and Galactose move through same transport system


Vesicles in membrane transport

Endocytosis- small membrane vesicles pinch material into cell
Moves material into cell
Vesicles pinch off material (drawing in notebook)

1. Pinocytosis- budding of vesicles from cell membrane
2. Receptor mediated- Similar to a filter feeder
EX: Hormones, neurotransmitters
Exocytosis- Moves material out
Vesicle present fuses with plasma membrane and releases content (exact opposite of what endocytosis does)


Trans-membrane movement of ions

1. Trans-membrane movement of ions impacts electrical charge balance across membrane