Facilitated diffusion across the

cell membrane

While

water molecules can pass through the cell membrane by simple diffusion, the

rate of transport is very low. However, this slow diffusion cannot account for

the rapid water movement observed in cells. To facilitate efficient water

transport, cells rely on a specialized channel protein called aquaporin.

Refer

to the figure below and choose the correct answers. 

(Note: The figure displays

amino acid labels along the aquaporin channel. For example, 'H' represents the

amino acid Histidine, and 'N' represents Asparagine. The numbers 180 and 192

indicate the positions of these amino acid residues within the aquaporin

protein sequence.)

As Prerna walks through the hallway, she continues

thinking about how energy-intensive it is to generate clean water using reverse

osmosis. This leads her to wonder how water moves across cell membranes in the

body. Water transport is essential for maintaining homeostasis—whether for

thermoregulation or osmolarity balance. The Reverse Osmosis (RO) membrane

reminds her of the cell membrane—both are semi-permeable, allowing selective

movement of water. Could nature have optimized water transport in a way that

engineers could learn from? Her thoughts turn to biomolecules... aquaporins.

Practice Question: What does Prerna wonder about

while comparing reverse osmosis to water transport in the body?

Osmosis

Prerna reaches her hostel and looks at

the water filtration units installed along the floor. The key component of the

water filtration units include a Reverse Osmosis (RO) membrane, which similar

to cell membrane is semi-permeable. The RO membrane requires electricity to

supply the external pressure to push water against the osmotic gradient. Based

on the following figure,  which of the

following statements correctly distinguish reverse osmosis (RO) from osmosis?

Role of membrane proteins

Prerna

is drinking a fruit juice which has vitamins, electrolytes, and sugar. The

sugar is broken down in our gastro-intestinal (GI) tract into glucose, and this

glucose must be transported inside our cells to make energy.  Also, the

whole action which enables Prerna to perceive the thirst and drink the juice is

governed by electrical potentials across neurons, which are dependent on the

transport of sodium and potassium ions. Considering the selective permeability

of our cell membrane and by looking at the figures, which of the following

options are true:

(Hint:

For answering regarding the sodium and potassium ions, look at the bottom

figure and the concentration plots)

Osmolarity

The

feedback loop of sweat loss is to feel thirsty and reach out for a drink.

Prerna did the same.  She had a bottle of

water as well as a fruit juice.

The following figure shows the nutritional information of the fruit juice. Assume that the

primary contributors for the osmolarity are sodium, potassium and total sugars; 

Osmolarity

is a colligative property, meaning it depends on the total number of solute

particles in a solvent and not on their chemical identity. The plot on the

right shows the major solutes present in our body fluids. Given this

information, choose all the correct options.

Homeostasis and thermoregulation

Prerna

weighs approximately 50 kg. During her practice session, her smartwatch tracked

the calories burned and estimated sweat loss. Sweat is produced by sweat glands

and is primarily composed of water, electrolytes (such as sodium and chloride),

and some waste products. As sweat is secreted, plasma volume decreases,

prompting the body to shift water from the interstitial fluid into the plasma

to maintain circulation.

Physical

exercise also causes an increase in body temperature, making thermoregulation

essential. The body's internal temperature is controlled by the hypothalamus,

which detects changes and triggers responses to restore balance.

The

graph below illustrates how the hypothalamus detects temperature changes and

triggers an effector response. The accompanying diagram shows how an artery

constricts (vasoconstriction) or dilates (vasodilation) in response to

temperature regulation. Pick the best options relating to the interstitial

fluid, and by the effector response points (A and B) in the graph with their

effects on blood vessels.

Role of Diffusion

Intense

physical activity requires a rapid exchange of O₂ and CO₂ between the

atmosphere and the body's cells. O₂ is essential for cellular respiration,

which produces the energy needed for muscle function. A higher heart rate means

faster oxygen delivery to tissues and more efficient carbon dioxide removal.

Red

blood cells (RBCs) are responsible for oxygen transport because they contain hemoglobin,

a protein that binds O₂. However, before oxygen can bind to hemoglobin, it must

first enter the RBC.

Looking

at the images, which of the following statements are correct?

From Hydration to Filtration:

Nature’s Blueprint

Prerna, a student at Plaksha University, was finishing

her tennis practice.

A key mode of

transport in the body is diffusion, where molecules move from a region of higher

concentration to a region of lower concentration. As the heart pumps to

maintain  the oxygen level, the oxygen

saturation (SpO

Your blood contains hemoglobin

(Hb), a protein in red blood cells that carries oxygen. When hemoglobin is fully

loaded with oxygen, we call it oxygenated hemoglobin (HbO₂). If it is not

carrying oxygen, it is called deoxygenated hemoglobin (Hb).

A pulse oximeter

shines two types of light—red and infrared—through your finger (or another thin

body part).

• Oxygenated hemoglobin (HbO₂) absorbs

  more infrared light and lets more red light pass through.

• Deoxygenated hemoglobin (Hb) absorbs

  more red light and lets more infrared light pass through.

A sensor measures

the amount of light absorbed and calculates the percentage of hemoglobin that

is carrying oxygen—this is your oxygen saturation (SpO₂).

Practice question:

How does a pulse oximeter determine the

oxygen saturation (SpO₂) of blood?

Some tips to help you make the most of the graded quiz:

• Discuss

  with your peers.

  Engaging in discussions can help you reflect and

  deepen your understanding.

• Pay

  attention to details.

  Questions (including images) contain valuable

  information. For e.g., examine conformational changes and reactions shown next

  to transport proteins to determine the type of transport process. However,

  feel free to use additional resources if you need to explore further.

• Check

  the units carefully.

  Notations like milli (m) and micro (µ

  )

  can significantly impact the correct answer.

• Connect

  new themes with what you’ve learned.

  If you encounter a new topic,

  think about how it relates to concepts you already know. Often, the theme

  provides an opportunity to apply prior knowledge.

• Follow

  the narrative.

  Don’t view questions in isolation—consider how they

  connect. This approach will help you recognize the interconnections

  between multiple topics.

Practice question:

Which of the following is NOT a

suggested tip for making the most of the graded quiz?

How does the addition of 10mM urea affect solution A? (Hint: Urea can freely pass through the membrane, unlike glucose and the other ions)