BIOL 2420 Lecture Notes - Lecture 6: Moving Walkway, Proximal Tubule, Reabsorption
26 Apr 2018
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Unit 7 – Lecture 6
Renal Transport Can Reach Saturation
- Most transport in the nephron uses membrane proteins and exhibits the three characteristics of
mediated transport
1. Saturation
2. Specificity
3. Competition
- Saturation: the maximum rate of transport that occurs when all available carriers are occupied
by substrate
o Substrate concentrations below the saturation point = transport rate is directly related
to substrate concentration
o Substrate concentration equal to or above saturation point = transport occurs at a
maximum rate
o Transport maximum (Tm): transport rate at saturation
- Glucose reabsorption in the nephron is an excellent example of the consequences of saturation
o Normal plasma glucose concentrations
▪ All glucose that enters the nephron is reabsorbed before it reaches the end of
the proximal tubule
▪ Tubule epithelium is well supplied with carriers to capture glucose as the filtrate
flows past
o Glucose concentrations excessive (diabetes mellitus)
▪ Glucose is filtered faster than the carriers can reabsorb it
▪ The carriers become saturated and are unable to reabsorb all the glucose that
flows through the tubule
▪ Resulting in some glucose escaping reabsorption and being excreted in the urine
▪ Analogy:
• Assume the carriers are like seats on a train at Disney world
• Instead of boarding the stationary train from a stationary platform,
passengers step onto a moving sidewalk that rolls them past the train
• As the passengers see an open seat, they grab it
• However, if more people are allowed onto the moving sidewalk than
there are seats in the train, some people will not find seats
• Because the sidewalk is moving people past the train toward the exit,
they cannot wait for the next train
• Instead they end up being transported out the exit
- Glucose olecules eterig Bowa’s capsule in the filtrate are like passengers stepping onto
the moving sidewalk
o To be reabsorbed, each glucose molecule must bind to a transporter as the filtrate flows
through the proximal tubule
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Most transport in the nephron uses membrane proteins and exhibits the three characteristics of mediated transport: saturation, specificity, competition. Instead they end up being transported out the exit. Glucose (cid:373)olecules e(cid:374)teri(cid:374)g bow(cid:373)a(cid:374)"s capsule in the filtrate are like passengers stepping onto the moving sidewalk: to be reabsorbed, each glucose molecule must bind to a transporter as the filtrate flows through the proximal tubule. If only a few glucose molecules enter the tubule at a time, each one can find a free transporter and be reabsorbed, just as a small number of people on the moving sidewalk all find seats on the train. If glucose molecules filter into the tubule faster than the glucose carriers can transport them, some glucose remains in the lumen and is excreted in the urine. Indicates an elevated blood glucose concentration: this rarely occurs despite normal blood glucose concentrations, occurs due to genetic disorder nephron does not make enough carriers for glucose.
