Active transport

1. Overview

Active transport is the process used by cells to move substances "uphill" against their natural direction of flow. Unlike diffusion or osmosis, this process requires the cell to spend energy to capture specific molecules or ions that are in low supply in the environment but are needed in high concentrations inside the cell.

Key Definitions

• Active Transport: The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e., against a concentration gradient), using energy from respiration.
• Concentration Gradient: The difference in the concentration of a substance between two regions. Moving "against" the gradient means moving toward the area where there is already more of that substance.
• Carrier Protein: Specialized proteins embedded in the cell membrane that "pump" specific molecules across the membrane using energy.

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Core Content

Active transport is essential for maintaining the correct concentrations of molecules within a cell.

Key Characteristics:

• Direction: Particles move from Low → High concentration.
• Energy: It is an "active" process because it requires energy. This energy is provided by respiration in the form of ATP.
• Location: It occurs across a cell membrane.

Step-by-Step Process:

1. The specific molecule or ion (e.g., a nitrate ion) approaches the cell membrane.
2. The molecule binds to a specific site on a protein that spans the membrane.
3. The cell uses energy from respiration to change the shape of the protein.
4. The change in shape "pushes" or carries the molecule through the membrane and releases it on the other side.

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Extended Content (Extended curriculum only)

In the extended curriculum, you must understand the specific mechanism and real-world biological applications of active transport.

The Role of Protein Carriers

Active transport does not happen through the lipid bilayer itself. It is carried out by protein carriers (sometimes called "pumps"). These proteins are highly specific; a protein that transports glucose will not transport sodium ions.

Real-World Examples

1. Ion Uptake by Root Hair Cells:

   • Context: Plants need mineral ions (like nitrates and magnesium) for growth. Often, the concentration of these ions in the soil is much lower than the concentration inside the root hair cell.
   • Function: Root hair cells use active transport to "pump" these ions from the soil into the cytoplasm against the concentration gradient.
   • Adaptation: Root hair cells have a large surface area and many mitochondria to provide the energy (via respiration) needed for this transport.

2. Glucose Uptake in the Small Intestine:

   • Context: After a meal, glucose is absorbed into the blood via diffusion. However, once the concentration in the blood is higher than in the intestine, diffusion stops.
   • Function: To ensure no food is wasted, the cells of the small intestine (villi) use active transport to absorb the remaining glucose molecules.

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Key Equations

While there is no single "Active Transport Equation," exams often require you to calculate the Percentage Change in concentration or mass to determine if active transport has occurred.

Percentage Change Formula: $$\text{Percentage Change} = \frac{\text{Final Value} - \text{Initial Value}}{\text{Initial Value}} \times 100$$

• Final Value: Concentration/Mass at the end of the experiment.
• Initial Value: Concentration/Mass at the start.
• Units: Expressed as a percentage (%).

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Common Mistakes to Avoid

• ❌ Wrong: Saying active transport goes from "high to low" concentration.
  • ✓ Right: It always moves from low to high (against the gradient).
• ❌ Wrong: Stating that energy comes from "the sun" or "the gut."
  • ✓ Right: Energy specifically comes from respiration within the cell.
• ❌ Wrong: Confusing active transport with osmosis.
  • ✓ Right: Osmosis is specifically the movement of water; active transport involves particles like ions and glucose.

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Exam Tips

• Command Word - "Describe": If asked to describe active transport, focus on the definition: Low to high concentration, against a gradient, using energy.
• Command Word - "Explain": If asked to explain the importance in root hairs, mention that mineral concentration is lower in the soil and the plant needs energy to pump them in using protein carriers.
• Calculation Questions: You may be given values like a soil concentration of 1.0 mmol/dm³ and a root concentration of 30.0 mmol/dm³. If the plant is still taking in minerals, you must identify this as active transport because it is moving against the gradient.
• Look for Mitochondria: In "Label the Diagram" questions, if a cell (like a root hair or intestinal cell) has an unusually high number of mitochondria, it is likely involved in active transport.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Root hair cells in plants are responsible for absorbing water and mineral ions from the soil.

(a) Define active transport. [2]

(b) Explain why root hair cells use active transport to absorb some mineral ions from the soil. [3]

Worked Solution:

(a)

1. Active transport is the movement of molecules or ions. This defines the basic process.
2. ...from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy from respiration. This specifies the direction and energy source.

How to earn full marks:

• Mention the movement of molecules or ions.
• State that it moves substances against a concentration gradient.
• State the energy source is respiration.

(b)

1. The concentration of some mineral ions in the soil is lower than in the root hair cell. This establishes the concentration gradient.
2. Therefore, diffusion would result in the mineral ions moving out of the root hair cell, not in. This explains the problem with diffusion.
3. Active transport enables the root hair cell to take up mineral ions against this concentration gradient by using energy from respiration. This explicitly links active transport to overcoming the gradient and using energy.

How to earn full marks:

• State that the concentration of mineral ions is lower in the soil than in the root hair cell.
• Explain that diffusion cannot be used to absorb mineral ions.
• Mention that active transport uses energy to move ions against the concentration gradient.

Common Pitfall: Many students forget to mention that active transport requires energy from respiration. Also, be sure to clearly state the direction of movement against the concentration gradient, from low to high concentration.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

The diagram shows a simplified representation of a cell membrane.

(a) Identify the process being represented in the diagram. [1]

(b) State the role of the protein carrier in this process. [2]

(c) Explain why this process requires energy. [3]

Worked Solution:

(a)

1. Active transport Identifies the process.

How to earn full marks:

• Correctly identify the process as active transport.

(b)

1. The protein carrier binds to the molecule or ion. This describes the initial interaction.
2. The protein carrier changes shape to move the molecule or ion across the membrane. This explains how the molecule is transported.

How to earn full marks:

• State that the protein carrier binds to the molecule or ion.
• Explain that the protein carrier changes shape to move the molecule across.

(c)

1. The molecules or ions are being moved from a region of lower concentration to a region of higher concentration. This reiterates the movement against the gradient.
2. This is against the concentration gradient. This explicitly states it's against the gradient.
3. Energy is needed to enable the protein carrier to change shape and move the molecules or ions against the concentration gradient. This links energy to the carrier protein and movement against the gradient.

How to earn full marks:

• State that the molecules are moving from low to high concentration.
• State it is against the concentration gradient.
• Explain that energy is needed for the protein carrier to change shape.

Common Pitfall: A common mistake is forgetting that active transport moves substances against the concentration gradient. Also, remember that the protein carrier needs energy, usually from ATP, to change its shape and facilitate the transport.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

The sodium-potassium pump is a protein found in the cell membranes of many animal cells. It actively transports sodium ions ($Na^+$) out of the cell and potassium ions ($K^+$) into the cell. For each ATP molecule used, the pump transports three $Na^+$ ions out and two $K^+$ ions in.

(a) State the source of energy for the sodium-potassium pump. [1]

(b) Explain the importance of the sodium-potassium pump in maintaining the resting potential of a nerve cell. [5]

(c) A nerve cell uses 1.5 x $10^6$ ATP molecules per second. Calculate the number of sodium ions transported out of the cell in one minute. [2]

Worked Solution:

(a)

1. ATP (adenosine triphosphate) Identifies the energy source.

How to earn full marks:

• Correctly state ATP.

(b)

1. The sodium-potassium pump maintains a concentration gradient of $Na^+$ and $K^+$ ions across the cell membrane. This introduces the concept of maintaining a concentration gradient.
2. The pump transports more $Na^+$ ions out than $K^+$ ions in. This identifies the imbalance in ion movement.
3. This results in a higher concentration of $Na^+$ ions outside the cell and a higher concentration of $K^+$ ions inside the cell. This describes the resulting concentration difference.
4. This difference in ion concentration creates an electrochemical gradient, with a negative charge inside the cell relative to the outside. This links the ion difference to the charge difference.
5. This negative charge inside the cell is the resting potential, which is essential for nerve impulse transmission. This connects the resting potential to nerve impulse transmission.

How to earn full marks:

• Explain that the pump maintains a concentration gradient of $Na^+$ and $K^+$ ions.
• State that more $Na^+$ ions are transported out than $K^+$ ions in.
• Explain that this creates a higher concentration of $Na^+$ outside and $K^+$ inside.
• Explain that this generates an electrochemical gradient.
• Link the negative charge to the resting potential and nerve impulse transmission.

(c)

1. Number of ATP molecules used in one minute = 1.5 x $10^6$ x 60 = 9.0 x $10^7$ Calculates the total ATP used in one minute.
2. Number of sodium ions transported = 9.0 x $10^7$ x 3 = $\boxed{2.7 \times 10^8}$ Calculates the total number of sodium ions transported.

How to earn full marks:

• Calculate the total ATP molecules used in one minute as 9.0 x $10^7$.
• Multiply the number of ATP molecules by 3 to find the number of sodium ions transported.
• State the final answer as $\boxed{2.7 \times 10^8}$ sodium ions.

Common Pitfall: Students often forget to convert the time to the correct units (seconds to minutes). Also, make sure you understand the ratio of ATP molecules used to the number of sodium ions transported.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Cholera is a disease caused by the bacterium Vibrio cholerae. The bacteria release a toxin that affects the cells lining the small intestine. This toxin causes chloride ions ($Cl^−$) to be actively transported from the cells into the lumen (the space inside the intestine). Water then follows the chloride ions by osmosis.

(a) Define osmosis. [2]

(b) Explain how the active transport of chloride ions into the lumen leads to diarrhea, a common symptom of cholera. [4]

(c) Suggest how oral rehydration therapy (ORT), a treatment for cholera, works to restore fluid balance in the body. Your answer should include the role of active transport. [3]

Worked Solution:

(a)

1. Osmosis is the net movement of water molecules. This defines the basic process.
2. ...from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration), through a partially permeable membrane. This specifies the direction and membrane type.

How to earn full marks:

• Mention the net movement of water molecules.
• State that it moves from high to low water potential (or equivalent).
• State that the movement is through a partially permeable membrane.

(b)

1. Active transport of $Cl^−$ ions into the lumen increases the solute concentration in the lumen. This explains the effect of active transport on solute concentration.
2. This decreases the water potential in the lumen. This links solute concentration to water potential.
3. Water moves by osmosis from the cells lining the intestine into the lumen, down the water potential gradient. This explains the movement of water due to osmosis.
4. This increased water in the lumen causes diarrhea. This links the water movement to the symptom.

How to earn full marks:

• State that active transport increases solute concentration in the lumen.
• Explain that this decreases water potential in the lumen.
• Explain that water moves by osmosis into the lumen.
• State that this causes diarrhea.

(c)

1. ORT contains glucose and salts (e.g., sodium chloride) dissolved in water. This describes the contents of ORT.
2. The glucose and sodium ions are actively transported from the lumen of the small intestine into the cells lining the intestine. This explains the role of active transport in ORT.
3. The water then follows the glucose and sodium ions by osmosis, from the lumen into the cells, rehydrating the body. This describes how water is absorbed by osmosis.

How to earn full marks:

• State that ORT contains glucose and salts.
• Explain that glucose and sodium ions are actively transported into the cells.
• Explain that water follows by osmosis, rehydrating the body.

Common Pitfall: Many students confuse the direction of ion movement in cholera and ORT. Remember that in cholera, chloride ions are actively transported out of the cells, while in ORT, glucose and sodium ions are actively transported into the cells.