3.3 Movement of Substances across a Plasma Membrane in Living Organisms
In living organisms, the continuous movement of substances across the plasma membrane is crucial to maintaining internal balance and sustaining vital life processes. Both passive and active transport mechanisms take place in animal and plant organs, such as the exchange of respiratory gases in the alveoli, nutrient absorption in the villi, and water and mineral uptake by plant roots. In addition, the surrounding fluid tonicity—whether hypotonic, hypertonic, or isotonic—directly affects cell shape, internal pressure, and overall cell survival.
Learning Objectives
- State examples of passive transport and active transport in living organisms.
- Define isotonic, hypotonic, and hypertonic solutions based on solute concentration and water potential.
- Explain the effects of hypotonic, hypertonic, and isotonic solutions on animal cells (red blood cells).
- Explain the effects of hypotonic, hypertonic, and isotonic solutions on plant cells.
- Describe the processes of haemolysis, crenation, turgidity, plasmolysis, and deplasmolysis.
Comparison of Cellular Responses in Different Solutions (Animal vs. Plant Cells)
A side-by-side illustrated summary showing the distinct physical changes undergone by animal cells and plant cells when placed in hypotonic, isotonic, and hypertonic environments.
Hypotonic
Water enters cells. Animal cells burst; plant cells become turgid.
Isotonic
No net water movement. Animal cells stay normal; plant cells become flaccid.
Hypertonic
Water leaves cells. Animal cells shrink; plant cells undergo plasmolysis.
Short Explanation
Passive and Active Transport in Living Organisms
Passive transport requires no energy and moves substances down a concentration or potential gradient. Active transport moves substances against a concentration gradient using energy.
Passive Transport Examples
Gaseous exchange between an alveolus and a blood capillary occurs through simple diffusion. Oxygen diffuses from the alveolus into the blood capillaries down its concentration gradient; carbon dioxide diffuses from the blood capillaries into the alveoli down its concentration gradient. Reabsorption of water occurs by osmosis through the renal tubule in the kidney. Absorption of water by plant roots occurs by osmosis from soil water into root hair cells. Absorption of fructose in the villus occurs by facilitated diffusion.
Active Transport Examples
Absorption of glucose and amino acids in the villus, reabsorption of glucose through the renal tubule in the kidney, transport of sucrose from a leaf to phloem tissue, and absorption of mineral ions by plant roots from soil into root hair cells against their concentration gradient.
Isotonic, Hypotonic, and Hypertonic Solutions
An isotonic solution has the same solute concentration as another solution, so there is no net movement of water. A hypotonic solution has a lower solute concentration and higher water potential, so water diffuses into the cell. A hypertonic solution has a higher solute concentration and lower water potential, so water diffuses out of the cell.
Effects on Animal and Plant Cells
In a hypotonic solution, red blood cells swell and burst (haemolysis), while plant cells become turgid. In a hypertonic solution, red blood cells shrink (crenation), while plant cells undergo plasmolysis. In an isotonic solution, red blood cells remain normal and plant cells become flaccid.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why do red blood cells burst when placed in distilled water, but plant cells do not?
What term describes the condition when a plasmolysed plant cell is immediately placed into distilled water and regains its turgidity?
Why do market hawkers spray water on their fresh vegetables and fruits?
Common Mistakes
- Using the term haemolysis for non-red blood cells. The term haemolysis is only used for the bursting of red blood cells.
- Confusing the terms flaccid and plasmolysed. Plant cells in an isotonic solution are flaccid (no net water movement). Plant cells in a hypertonic solution undergo plasmolysis (vacuole shrinks, plasma membrane pulls away from the cell wall).
- Confusing crenation with plasmolysis. Animal cells shrink and undergo crenation; plant cells do not crenate because the cell wall maintains outer cell boundaries while internal contents pull inward during plasmolysis.
- Reversing solute concentration and water potential. A hypertonic solution has a high solute concentration and low water potential.
Concept Misunderstandings
No water moves across the membrane in an isotonic solution.
Water molecules continue to diffuse across the membrane in both directions at equal rates. There is simply no net movement of water.
Active transport is used to absorb all nutrients in the small intestine.
While glucose and amino acids are absorbed by active transport, fructose is absorbed via facilitated diffusion (a passive transport process).
Plant cells burst if left in a hypotonic solution for too long.
Plant cells never burst in hypotonic solutions because their cell wall is rigid and exerts wall pressure to prevent over-expansion.
Summary
- Passive transport requires no energy and moves substances down a concentration/potential gradient (e.g., \(O_2\) and \(CO_2\) exchange in alveoli by simple diffusion, water uptake in roots and renal tubules by osmosis, fructose uptake in villi by facilitated diffusion).
- Active transport moves substances against a concentration gradient using energy (e.g., glucose and amino acid absorption in villi, glucose reabsorption in kidneys, sucrose loading into phloem, mineral ion uptake by plant roots).
- In a hypotonic solution, water moves in: red blood cells burst (haemolysis); plant cells become turgid due to turgor pressure.
- In a hypertonic solution, water moves out: red blood cells shrink (crenation); plant cells undergo plasmolysis (which can be reversed via deplasmolysis in a hypotonic solution).
- In an isotonic solution, water flows in and out equally: red blood cells remain normal; plant cells become flaccid.
Short Activity
Objective: identify transport processes, solution effects, and cellular conditions.
A. Objective Quiz
Which of the following transport processes in living organisms requires cellular energy via active transport?
What happens to a plant cell when it is placed in a solution that has a higher solute concentration and lower water potential than its cell sap?
B. Fill in the Blanks
The bursting of red blood cells when placed in a hypotonic solution is known as .
In plant cells, a plasmolysed cell can regain its turgidity when returned immediately to a hypotonic solution through a process called .
C. Matching / Drag and Drop
Match each physiological condition to its correct description or outcome in living cells. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Biological Condition to Its Description
Write the correct biological condition next to each description.
| Description / Effect | Biological Condition |
|---|---|
| 1. Shrinking of red blood cells when placed in a hypertonic solution. | |
| 2. State of plant cells when vacuoles expand and push against the rigid cell wall in a hypotonic solution. | |
| 3. State of plant cells when immersed in an isotonic solution where water potential is equal inside and outside. | |
| 4. Rupturing of red blood cells caused by water entering under hypotonic conditions. |
Answer choices: Crenation • Turgid • Flaccid • Haemolysis
