Transport Across Cell Membrane Quiz

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Questions and Answers

What is the primary function of the Na+ pump in the epithelial cells?

To allow passive flow of water through the basal membrane

To increase intracellular Na+ concentration

To extrude Na+ and create a concentration gradient across the apical membrane (correct)

To facilitate glucose uptake through tight junctions

Which transporter is responsible for sodium-glucose co-transport in the intestinal lumen?

Na+ channel

Glucose uniporter

Na+/Glucose symporter (correct)

Na+/K+ pump

How does the Na+ pump influence osmosis in the interstitial space?

By lowering intracellular Na+ concentration and facilitating water movement (correct)

By allowing passive diffusion of Na+ into the interstitial space

By preventing H2O from moving into the epithelial cells

By increasing Na+ concentration in the lumen

What role do tight junctions play in the absorption process of Na+ and glucose?

They prevent leakage of substances between cells Signup and view all the answers

Which statement is true regarding the transport of Na+ across the epithelial cells?

Na+ must be transported actively into the cells against its gradient Signup and view all the answers

What role does AQP-1 play in the kidney collecting duct?

It is always in the basolateral membrane of the collecting duct cells. Signup and view all the answers

How is urine affected by the presence of ADH in the collecting duct?

Urine is concentrated and water is retained. Signup and view all the answers

What is the impact of dehydration on the activity of ADH and AQP-2?

ADH stimulates AQP-2 insertion to enhance water reabsorption. Signup and view all the answers

What happens to water reabsorption during diarrhea?

Secretion increases while reabsorption decreases. Signup and view all the answers

Why is simply providing Na+ and water ineffective for rehydrating a severely dehydrated individual?

Interstitital fluid concentration of [Na+] is too low to facilitate osmosis. Signup and view all the answers

Which factor is essential for driving water reabsorption in the kidney collecting duct?

Presence of both ADH and an osmotic gradient. Signup and view all the answers

How does the Na+/Glucose cotransporter contribute to water uptake?

It facilitates glucose absorption, which indirectly promotes water reabsorption. Signup and view all the answers

What is the main mechanism by which osmotic gradients drive water reabsorption in the kidney?

Water follows solutes from an area of low osmolarity to high osmolarity. Signup and view all the answers

What must glucose do after absorption from the intestine?

Cross four lipid membranes and enter muscle cells Signup and view all the answers

What characterizes epithelial cells in terms of their structure?

They are polarized and joined by tight junctions. Signup and view all the answers

Why can't glucose enter epithelial cells by diffusion against its concentration gradient?

It requires active transport to move against the gradient. Signup and view all the answers

What is the role of tight junctions in epithelial cells?

They form an impermeable barrier to solutes and water. Signup and view all the answers

How does glucose move through the interstitial space?

By spontaneous diffusion down its concentration gradient Signup and view all the answers

What happens when the membrane permeability (P) for glucose is zero?

A carrier protein is required for glucose movement. Signup and view all the answers

Which of the following statements is true about sodium ions (Na+) in relation to glucose absorption?

Na+ can enter cells from the lumen down its concentration gradient. Signup and view all the answers

During the glucose absorption process, what characterizes the intestinal lumen in relation to glucose concentration?

It has a lower concentration of glucose than epithelial cells. Signup and view all the answers

What type of transport occurs when glucose leaves the blood to enter muscle cells?

Diffusion down its concentration gradient Signup and view all the answers

Which component primarily forms the structure of tight junctions in epithelial cells?

Proteins Signup and view all the answers

What is the primary mechanism for glucose absorption in the small intestine?

Active transport via Na+/glucose cotransporter Signup and view all the answers

What is the role of tight junctions in the absorption of water in the gut?

They allow for some leakage to facilitate water absorption. Signup and view all the answers

Under what condition is the bulk uptake of water a constitutive process?

In healthy individuals without excess solutes. Signup and view all the answers

How does water generally flow in relation to its concentration gradient?

Water flows down its concentration gradient. Signup and view all the answers

What best describes the absorption process of large volumes of water?

It happens through a paracellular pathway influenced by osmotic gradients. Signup and view all the answers

Which of the following best explains the concept of an osmotic gradient?

It is defined as the difference in solute concentration across a membrane. Signup and view all the answers

What physiological mechanism is involved in the bulk uptake of water in the kidneys?

Filtration followed by selective reabsorption. Signup and view all the answers

Which statement is true regarding the regulated uptake of water?

It is influenced by hormonal control and osmotic conditions. Signup and view all the answers

What is considered the biggest medical breakthrough of the 20th century in relation to fluid absorption?

Discovery of hydration therapies for rehydration. Signup and view all the answers

What is the primary function of the Na+/glucose co-transporter in the gut?

It uses energy from the sodium gradient to facilitate glucose uptake. Signup and view all the answers

How does water primarily move from the lumen into the interstitial fluid in the gut?

By bulk absorption through 'leaky' tight junctions. Signup and view all the answers

In the kidney collecting duct, what role do aquaporins play when ADH levels are high?

They facilitate passive water reabsorption. Signup and view all the answers

What occurs in the kidney collecting duct when ADH levels are low?

AQP-2 channels are retained in the cytoplasm. Signup and view all the answers

Which pathway allows for regulated absorption of water in the kidney?

Transcellular pathway via aquaporins. Signup and view all the answers

What is the osmolarity difference between the lumen and interstitial fluid that drives water movement?

High osmolarity in the interstitial space promotes water movement from the lumen. Signup and view all the answers

What is the primary difference between bulk absorption of water and regulated absorption?

Regulated absorption is dependent on hormone action. Signup and view all the answers

What physiological effect results from the action of Na+ and glucose in the osmotic gradient?

It causes water to flow from high to low solute concentration. Signup and view all the answers

Which type of transport is primarily responsible for glucose entering the interstitial space from epithelial cells?

Facilitated diffusion via uniporters. Signup and view all the answers

What is the significance of 'leaky' tight junctions in gut and kidney tubule function?

They allow passive and bulk water absorption to occur. Signup and view all the answers

Study Notes

Transport Across the Cell Membrane

Water will move across the cell membrane via a concentration gradient
The concentration gradient of water is dictated by the concentration of solutes present
Water will move from a region of high water concentration (low solute concentration) to an area of lower water concentration (high solute concentration)
This is driven by osmosis.
The difference in concentration across a membrane is referred to as the osmotic pressure.

Glucose Absorption

Glucose is absorbed from the gut into the epithelial cells and then across the basolateral membrane into the interstitium
It cannot move across the cell membrane against its concentration gradient
This means glucose can enter the epithelial cell from the gut lumen via an active transport mechanism
The concentration gradient of glucose is low in the gut lumen and high in the epithelial cell
However, glucose can leave the epithelial cell across the basolateral membrane by moving down its concentration gradient

Sodium and Glucose

Sodium can enter the epithelial cell from the gut lumen down its concentration gradient by moving through sodium channels
This creates a concentration gradient of sodium across the cell membrane
Movement of sodium across the cell membrane is an important physiological process

Sodium-Glucose Symporter

The sodium-glucose symporter (SGLT) is a protein that is located in the apical membrane of epithelial cells in the gut
It transports glucose into the epithelial cell against its concentration gradient by using the energy from the movement of sodium down its concentration gradient.

Water Absorption

Water moves from the lumen of the gut to the interstitium via paracellular transport.
Paracellular movement is the passage of substances between cells, as opposed to through cells.
This process is known as bulk water absorption
Bulk absorption of water from the gut occurs in the small intestine.
This bulk movement of water is dependent on the osmotic gradient between the lumen of the gut and the interstitium
In a healthy organism, water absorption from the gut is largely constitutive, meaning it occurs continuously

Water Reabsorption in Renal Tubules

Water is also absorbed from the kidney tubules, as the body tries to retain water.
The proximal tubule is where the majority of water re-absorption occurs.
Water moves from the lumen of the renal tubule to the interstitium via paracellular transport, as with the small intestine
The movement of water is driven by the osmotic gradient that exists between the lumen of the tubule and the interstitium

Aquaporins

Aquaporins (AQPs) are specialized proteins that transport water across the cell membrane, using the transmembrane proteins to form a pore through which water moves.
Aquaporins are found in a variety of cells in the body, which plays a crucial role in the regulation of water balance.
The movement of water across the basolateral membrane of epithelial cells in the collecting duct is regulated by the presence of aquaporins.
In the kidneys, aquaporins are responsible for the reabsorption of water from the urine, in a process known as ‘regulated water uptake’.
The water movement in this process is regulated by the hormone Anti-diuretic Hormone (ADH).

Anti-diuretic Hormone (ADH)

ADH is known as vasopressin, and it’s released by the posterior pituitary gland
When the body is dehydrated, ADH levels increase.
This causes AQP-2 to be inserted into the apical membrane of epithelial cells lining the collecting duct, allowing water to be reabsorbed into the body.
ADH levels decrease when the body is hydrated, causing AQP-2 to be removed from the apical membrane, and water cannot be reabsorbed into the body.
When AQP-2 is absent or present in small concentrations, urine is dilute and a large volume of water is lost

Oral Rehydration Therapy (ORT)

Oral rehydration therapy is the use of oral fluids to treat dehydration, especially in cases of diarrhea
It's the most efficient way to rehydrate a person who is suffering from dehydration
The fluid given to the patient is usually a solution of electrolytes and glucose
The glucose in the solution is essential for the absorption of sodium from the gut, which is crucial for the absorption of water
This combination works because the sodium and glucose are absorbed together via the sodium-glucose cotransporter (SGLT), which is located in the apical membrane of epithelial cells in the gut.

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Description

Test your understanding of how water and glucose are transported across the cell membrane. This quiz covers concepts such as osmosis, concentration gradients, and active transport mechanisms. Perfect for students studying cell biology and physiology.