Active Transport Flashcards
What characterises active transport?
Requires input of metabolic energy (can be ATP-driven or ion-driven)
What is Na+, K+, Ca2+ and H+ transport directly coupled to during ATP-driven active transport?
ATP hydrolysis
What is Na+/K+ gradient of mammalian cells?
Have high [K+] and low [Na+] inside cell and opposite outside cell
What is purpose of Na+/K+ gradient?
- Controls cell volume
2. Electrochemical gradient –> makes nerve and muscle cells electrically excitable
How is Na+/K+ gradient maintained?
By Na+/K+ ATPase (an integral membrane protein)
Pumps 3 Na+ ions out and 2 K+ ions into cell –> polarises cell membrane (net +ve charge on outside)
Why is an input of energy required with Na+/K+?
Working against concentration gradient
How is Na+/K+ and ATP a coupled system?
ATP not hydrolysed unless Na+ and K+ are transported an vice versa
What is ion-driven active transport?
Movement of molecule is coupled to movement of ion (Na+ or H+) down its concentration gradients –> cotransporters
What are the 2 examples of cotransporters?
- Symport
2. Antiport
What is Na+/glucose transporter an example of?
Symport
What is Na+/Ca2+ exchanger an example of?
Antiport
What is a symport?
Explain in relation to Na+/glucose
Both molecules travelling in same direction
- Glucose comes across membrane in company with Na+
- Concentration gradient of Na+ is used to drive active transport of glucose
What is an antiport?
Explain in relation to Na+/Ca2+
Molecules travelling in different direction
- Na+ movement into cell can be coupled to movement of Ca2+ out of cell
- Na+/Ca2+ exchanger helps maintain a low concentration of Ca2+ inside cell (3 Na+ in and 1 Ca2+ out)
- Energy comes from electrochemical gradient
What is heart contraction caused by?
An increase of [Ca2+] within cell
What is needed to maintain [Ca2+] gradient?
Na+/K+ ATPase
How does inhibition of Na+/K+ ATPase affect heart?
- Inhibition leads to increased [Na+] inside cell and decreased Na+ gradient across membrane (due to initial low [Na+] inside cell)
- Leads to increased [Ca2+] inside cell (as Na+ gradient is required for Na+/Ca2+ exchanger to move Ca2+ out of cell)
- Increased [Ca2+] inside cell of cardiomyocytes
- Leads to enhanced strength of contractions
How was digitalise (foxglove) used to treat heart failure?
Inhibits Na+/K+ ATPase –> strengthens heartbeat
Where are intestinal epithelial cells found?
Line the lumen of small intestine
Similar cells found in kidney tubules that are involved in reabsorption of molecules from urine
What is function of intestinal epithelial cells and how are they adapted for this?
Absorb nutrients from digested food into blood
There are villi on intestinal folds which increase SA for absorption
These intestinal epithelial cells are polarised. What does this mean?
They have 2 distinct sides:
- Apical
- Basolateral
What is the apical side of intestinal epithelial cells?
Apical (brush border) membrane that faces the lumen of gut.
Has finger-like projections of the membrane –> increase SA
What is the basolateral surface of intestinal epithelial cells?
Faces blood stream
How do these intestinal epithelial cells take up glucose?
- Glucose transporter (GLUT2)
2. Na+/glucose transporter
Where is GLUT2 present in intestinal epithelial cells?
Only in basolateral membrane
What is purpose of GLUT2 in intestinal epithelial cells?
- Enables glucose that has built up in intestinal epithelial cells to move down concentration gradient into blood
- Moves via facilitated diffusion
Where is Na+/glucose transporter found in intestinal epithelial cells?
Only in apical membrane
What is purpose of Na+/glucose transporter in intestinal epithelial cells?
- Moving glucose across concentration gradient across apical membrane (from low conc in lumen of intestine to high conc in cytosol of epithelial cells)
- Requires energy –> ion-driven active transport
How is low conc of Na+ maintained inside epithelial cells?
By Na+/K+ ATPase on the basolateral membrane (example of ATP driven active transport)
Why is sodium required for uptake of glucose?
Provides electrochemical gradient in order to allow uptake of glucose
What are the 2 methods by which glucose can move across the cell membrane?
Active transport and facilitated diffusion
What are oral rehydration therapies made of?
Solution of glucose and Na+
How do oral rehydration solutions work?
Movement of Na+ and glucose from lumen of intestine across the epithelial cell to blood sets up difference in osmotic pressure (increases osmotic pressure in epithelial cells)
Water flows through cell by simple diffusion
