Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

6. The Membrane

Simple and Facilitated Diffusion

General Biology

6. The Membrane

Simple and Facilitated Diffusion

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3:16 minutes

Problem 13

Textbook Question

Cells lining kidney tubules function in the reabsorption of water from urine. In response to chemical signals, they reversibly insert additional aquaporins into their plasma membranes. In which of these situations would your tubule cells have the most aquaporins: after a long run on a hot day, right after a large meal, or after drinking a large bottle of water? Explain.

Verified step by step guidance

Understand the role of aquaporins: Aquaporins are specialized membrane proteins that facilitate the movement of water across cell membranes. Their presence in kidney tubule cells increases water reabsorption from urine back into the bloodstream.

Identify the physiological conditions: After a long run on a hot day, the body loses water through sweat, leading to dehydration. After a large meal, digestion is the primary focus, and water balance may not be as critical. After drinking a large bottle of water, the body has excess water and needs to excrete it, reducing the need for aquaporins.

Relate aquaporin insertion to dehydration: When the body is dehydrated, such as after a long run on a hot day, chemical signals like antidiuretic hormone (ADH) are released. ADH triggers the insertion of aquaporins into the plasma membranes of kidney tubule cells to maximize water reabsorption.

Consider the other scenarios: After drinking a large bottle of water, the body has sufficient water and does not need to reabsorb as much, so fewer aquaporins would be inserted. After a large meal, water balance is not as urgent, and aquaporin levels would likely remain at baseline.

Conclude the situation with the most aquaporins: The tubule cells would have the most aquaporins after a long run on a hot day because the body is dehydrated and needs to conserve water by reabsorbing as much as possible from the urine.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Aquaporins

Aquaporins are specialized membrane proteins that facilitate the transport of water across cell membranes. They play a crucial role in regulating water balance in various tissues, including the kidneys. When the body needs to conserve water, such as during dehydration, more aquaporins are inserted into the membranes of kidney tubule cells to enhance water reabsorption from urine.

Kidney Function and Water Reabsorption

The kidneys filter blood to produce urine, and a significant part of this process involves reabsorbing water back into the bloodstream. This reabsorption occurs primarily in the nephron's tubules, where the presence of aquaporins can significantly increase water reabsorption efficiency. The body's hydration status influences how many aquaporins are present, adjusting to maintain homeostasis.

Physiological Responses to Hydration Status

The body's physiological responses to hydration status dictate how kidney tubule cells manage water reabsorption. After intense exercise in hot conditions, the body is likely dehydrated, prompting the release of hormones like vasopressin, which increases aquaporin insertion. Conversely, after consuming a large amount of water, the body will reduce aquaporin levels to excrete the excess fluid, demonstrating a dynamic response to hydration needs.

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Related Practice

Textbook Question

Based on Figure 7.18, which of these experimental treatments would increase the rate of sucrose transport into a plant cell?

a. Decreasing extracellular sucrose concentration

b. Decreasing extracellular pH

c. Decreasing cytoplasmic pH

d. Adding a substance that makes the membrane more permeable to hydrogen ions

2001

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Textbook Question

Draw and label the plasma membrane of a cell that is placed in a solution with concentrations of calcium ions and lactose that are greater than those on the inside of the cell. Use arrows to show the relevant gradients and the activity of the following membrane proteins:

(1) A pump that exports protons

(2) A calcium channel

(3) A lactose carrier

1979

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Textbook Question

In terms of structure, how do channel proteins differ from carrier proteins?

1948

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Textbook Question

Facilitated diffusion across a membrane requires ______________ and moves a solute ______________ its concentration gradient.

a. transport proteins . . . up (against)

b. transport proteins . . . down

c. energy and transport proteins . . . up

d. energy and transport proteins . . . down

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Textbook Question

Mercury is known to inhibit the permeability of water channels. To help establish that the protein isolated by Agre's group was a water channel, the researchers incubated groups of RNA-injected oocytes (which thus made aquaporin proteins) in four different solutions: plain buffer, low concentration and high concentration of a mercury chloride (HgCl₂) solution, and low concentration of a mercury solution followed by an agent (ME) known to reverse the effects of mercury. The water permeability of the cells was determined by the rate of their osmotic swelling. Interpret the results of this experiment, which are presented in the graph below. Control oocytes not injected with aquaporin RNA were also incubated with buffer and the two concentrations of mercury. Predict what the results of these treatments would be.

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Multiple Choice

How does facilitated diffusion differ from simple diffusion?

273

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Multiple Choice

By which mechanism would a steroid molecule diffuse into the cell?

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Multiple Choice