Chapter 7 — Front Matter
Purpose and Overview
Every living cell exists in a world full of molecules it needs and molecules it must exclude. Nutrients, water, oxygen, hormones, and waste products are constantly moving through the environment around a cell — but the cell cannot simply open itself to everything. It must select what enters, what leaves, and when.
The cell membrane makes this possible. It is a thin, flexible boundary that separates the inside of the cell from the outside world. But calling it just a boundary understates what it does. The cell membrane is selective, dynamic, and responsive. It controls traffic in both directions, responds to signals, and helps the cell maintain the internal conditions it needs to survive.
This chapter explains how the cell membrane is built, how it controls the movement of molecules, and why membrane transport is essential to homeostasis, health, and life itself.
Opening Question
Imagine a city without walls, gates, or borders. Goods, people, and vehicles would flow without any direction or control — the city could not concentrate resources, remove waste, or protect what matters. A cell without a functioning membrane faces the same problem. By the end of this chapter, you will understand exactly how the membrane works as a dynamic gatekeeper, and why disrupting it even slightly causes serious disease.
10Sections
12Objectives
30Key Terms
5+Worked Examples
Learning Objectives
- 1Describe the structure of the phospholipid bilayer and explain why phospholipids spontaneously form this arrangement in water.
- 2Identify the major components of the cell membrane and explain the role of each (phospholipids, proteins, cholesterol, carbohydrates).
- 3Explain the fluid mosaic model of membrane structure.
- 4Predict which types of molecules can cross the membrane freely and which cannot, based on size, polarity, and charge.
- 5Describe diffusion and explain how concentration gradients, temperature, and molecule size affect diffusion rate.
- 6Explain osmosis and predict how cells will respond to isotonic, hypotonic, and hypertonic solutions.
- 7Compare facilitated diffusion and simple diffusion, including the role of channel proteins and carrier proteins.
- 8Explain active transport and identify why it requires energy, using the sodium-potassium pump as the primary example.
- 9Distinguish between the three types of endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and explain exocytosis.
- 10Explain how all membrane transport mechanisms work together to maintain cellular homeostasis.
- 11Connect transport failure to specific diseases, including cystic fibrosis and cholera.
- 12Apply knowledge of membrane transport to real-world contexts including medical fluids, drug action, and oral rehydration therapy.
Sections 7.1 – 7.3
Membrane Structure and Selective Permeability
Why Membranes Matter · Phospholipid Bilayer · Fluid Mosaic Model · What Can Cross
Section 7.1
Why Cell Membranes Matter
All the molecules the cell has carefully assembled — its enzymes, its DNA, its ATP, its structural proteins — would simply diffuse away into the surrounding environment if the cell had no boundary. The cell would lose its identity, its organization, and its ability to function.
The cell membrane solves this problem while doing far more than simply holding things together. It controls which molecules can enter and which can leave, maintains the chemical conditions the cell needs for metabolism, allows the cell to communicate with its environment, and helps the cell respond to signals from hormones, nutrients, and neighboring cells.
Five Essential Functions of the Cell Membrane
1. Containment — holds the cell's contents together and maintains cell shape
2. Selective transport — controls the movement of molecules into and out of the cell
3. Communication — receives and transmits signals from hormones and other cells
4. Homeostasis — maintains stable internal chemical conditions
5. Recognition — identifies the cell to other cells through surface carbohydrates
Think About Scale
The cell membrane is only about 7–10 nanometers thick — roughly 10,000 times thinner than a human hair. Yet this ultra-thin layer controls everything that enters and leaves the cell. Its power comes entirely from the chemistry of its molecules, not its thickness.
- Name the five essential functions of the cell membrane. For each one, give a brief biological example of why that function matters.
- What would happen to a cell if its membrane suddenly became completely permeable to all molecules? Explain at least two specific consequences.
- The cell membrane is sometimes described as a "dynamic gatekeeper." What makes it dynamic? What makes it a gatekeeper?
Section 7.2
The Phospholipid Bilayer
The cell membrane is built from a class of molecules called phospholipids. Each phospholipid has two distinct regions: a hydrophilic head (water-loving) that is attracted to water, and two hydrophobic tails (water-fearing) made of fatty acid chains that repel water.
When phospholipids are placed in water, they spontaneously arrange themselves into a bilayer — two layers of phospholipids with their hydrophilic heads facing outward toward the watery environment and their hydrophobic tails facing inward, away from water. This arrangement is not forced by any external process. It is thermodynamically favorable — the molecules settle into the most stable configuration available.
Additional Membrane Components
| Component | Location | Function |
|---|
| Phospholipids | Entire bilayer | Form the structural foundation; create a selectively permeable barrier |
| Transmembrane proteins | Span the entire membrane | Act as channels, carriers, or pumps for specific molecules |
| Peripheral proteins | Inner or outer surface | Serve as receptors, enzymes, or structural anchors |
| Cholesterol | Between phospholipid tails (animal cells) | Regulates membrane fluidity; buffers against temperature changes |
| Carbohydrates | Outer surface only | Form the glycocalyx; enable cell recognition and immune function |
The Fluid Mosaic Model
The modern understanding of membrane structure is called the fluid mosaic model. "Fluid" means that phospholipids are not locked in place — they drift and exchange positions with neighboring phospholipids, giving the membrane flexibility. Proteins can also move laterally within the membrane, though some are anchored to the cytoskeleton. "Mosaic" means the membrane is a patchwork of different molecules — phospholipids, proteins, cholesterol, and carbohydrates — distributed in a complex, nonuniform pattern.
Worked Example 7.2.1
Why phospholipids form a bilayer — reasoning from molecular structure
- QuestionA phospholipid has one head and two tails. Why does it form a double layer (bilayer) instead of a single layer in water?
- Key factThe hydrophilic heads are attracted to water. The hydrophobic tails are repelled by water. Both sides of the cell are watery (extracellular fluid outside; cytoplasm inside).
- ReasoningIn a single layer, one set of tails would face water — an unstable arrangement. By forming two layers, tails from both layers face each other inward, shielded from water on both sides. Heads face outward into water on each side.
- ConclusionThe bilayer is the only arrangement that satisfies the chemical preferences of all parts of the molecule simultaneously. It forms spontaneously because it is the lowest-energy, most stable configuration.
Answer: The bilayer places hydrophilic heads toward water on both sides and shields hydrophobic tails from water in the interior — the only stable arrangement in an aqueous environment.
Common Misconception
Students sometimes think the cell membrane is solid or rigid, like a wall. In fact, the membrane is fluid at normal body temperatures. Phospholipids constantly move sideways, and so do many membrane proteins. This fluidity is essential — without it, the membrane could not flex, cells could not divide, and vesicles could not fuse with the membrane surface.
- Draw a simple diagram of a phospholipid and label: hydrophilic head, hydrophobic tails, fatty acid chains. Indicate which region faces water and which faces the membrane interior.
- Explain what the word "fluid" means in the fluid mosaic model. Give one biological consequence of membrane fluidity.
- What is the glycocalyx? Where is it located? Name two functions it serves.
- Why do animal cells have cholesterol in their membranes? What would happen to a cell membrane without cholesterol in a cold environment?
- A transmembrane protein spans the entire bilayer. What type of function would you expect it to serve? Why does its position make it suited for this role?
Section 7.3
Selective Permeability
The cell membrane does not let everything through equally. It is selectively permeable, meaning it allows some substances to pass freely, restricts others, and blocks some entirely. Whether a molecule can cross the membrane depends mainly on three properties: its size, its polarity, and its charge.
Small nonpolar molecules (like oxygen and carbon dioxide) dissolve in the fatty interior of the bilayer and cross easily. Polar molecules and ions are repelled by the hydrophobic core and generally cannot cross without transport proteins.
| Substance | Can cross freely? | Reason |
|---|
| O₂, CO₂ (small nonpolar gases) | Yes | Small and nonpolar; dissolve in the lipid interior |
| Water (H₂O) | Mostly yes | Very small; also uses aquaporin protein channels for faster transport |
| Small polar molecules (ethanol) | Somewhat | Small enough to pass slowly despite polarity |
| Glucose (large polar molecule) | No | Too large and polar; requires carrier proteins |
| Ions (Na⁺, K⁺, Cl⁻, Ca²⁺) | No | Charged; strongly repelled by the hydrophobic interior |
| Large molecules (proteins, DNA) | No | Far too large for any passive crossing; require vesicle-based mechanisms |
A Useful Rule of Thumb
Ask two questions about any molecule: Is it small? Is it nonpolar? If the answer to both is yes, it can probably cross the membrane by simple diffusion. If the answer to either question is no, the cell likely needs a transport protein or vesicle to move it.
- Steroid hormones are small and nonpolar. Can they cross the membrane freely?
- Sodium ions (Na⁺) are small but charged. Can they cross freely?
- Urea is a small, polar waste molecule. How would it cross?
- A large protein needs to leave a cell. What mechanism is likely used?
- Why can CO₂ exit cells easily despite being a product of metabolism?
- Why is selective permeability important for a cell's ability to do work?
Sections 7.4 – 7.7
Passive and Active Transport
Diffusion · Osmosis · Facilitated Diffusion · Active Transport · Na⁺/K⁺ Pump
Section 7.4
Diffusion
Molecules in solution are always in motion. They bump into each other, bounce off surfaces, and move in every direction at random. This random motion produces a predictable result: over time, molecules spread out from areas where they are concentrated to areas where they are less concentrated. This spontaneous movement of molecules from an area of high concentration to an area of low concentration is called diffusion.
Diffusion requires no energy input — it is driven entirely by the random kinetic energy of molecules. The difference in concentration across a membrane is called the concentration gradient. Diffusion always moves molecules down (with) the concentration gradient.
Four Factors That Affect Diffusion Rate
Concentration gradient — a steeper gradient (larger concentration difference) produces faster diffusion
Temperature — higher temperature means more kinetic energy and faster movement
Molecule size — smaller molecules diffuse faster than larger ones
Distance — diffusion is only effective over very short distances; this is one reason cells are microscopic
Worked Example 7.4.1
Predicting the direction of diffusion
- ScenarioA muscle cell is actively respiring. Inside the cell, CO₂ concentration is 0.08%. In the surrounding capillary blood, CO₂ concentration is 0.04%. In which direction will CO₂ diffuse, and why?
- IdentifyCO₂ is small and nonpolar — it can cross the membrane by simple diffusion. Concentration inside cell (0.08%) is higher than outside (0.04%).
- Apply ruleDiffusion moves molecules from high concentration to low concentration — down the gradient. CO₂ will diffuse from the cell into the blood.
- EquilibriumDiffusion will continue until concentrations equalize on both sides. Because the blood is flowing, fresh blood continuously carries CO₂ away, maintaining the gradient and keeping diffusion going.
Answer: CO₂ diffuses out of the muscle cell into the blood, moving from higher to lower concentration. Continuous blood flow maintains the gradient.
Common Misconception
Students sometimes say molecules "want" to move toward where they are needed. Diffusion is not purposeful — molecules do not have goals or intentions. Diffusion happens because of random molecular motion. The net movement toward lower concentration is a statistical outcome of many random collisions, not a directed process. Cells can take advantage of diffusion, but they do not cause it.
- Define diffusion. Explain why it requires no energy input from the cell.
- Oxygen concentration in the lungs is higher than in the blood arriving from the body. In which direction will oxygen diffuse? What type of transport is this?
- A cell is at equilibrium with its surroundings for a particular molecule. What happens to net movement? What happens to individual molecule movement?
- Why are cells small? Connect your answer to the limitations of diffusion over distance.
- A student heats a beaker of water containing dye. The dye spreads faster than in cold water. Explain this using the concept of kinetic energy and diffusion rate.
Section 7.5
Osmosis
Water is the most abundant molecule in every living cell, and its movement across the membrane is critically important. The diffusion of water across a selectively permeable membrane is called osmosis. Water moves from areas of lower solute concentration (more water, fewer dissolved particles) to areas of higher solute concentration (less water, more dissolved particles).
A useful way to think about osmosis: water moves toward where there is more stuff dissolved. The dissolved particles attract and hold water molecules, effectively pulling water in that direction.
Tonicity
The term tonicity describes the relative solute concentration of a solution compared to the inside of a cell. It predicts how a cell will behave in a given environment.
| Solution Type | Solute Concentration Outside | Water Movement | Effect on Animal Cell | Effect on Plant Cell |
|---|
| Isotonic | Equal to cell interior | Equal in and out — no net movement | Normal; cell maintains volume | Normal; slightly less firm than turgid |
| Hypotonic | Lower than cell interior | Water moves into cell | Cell swells; may lyse (burst) | Cell swells; becomes turgid (firm, healthy) |
| Hypertonic | Higher than cell interior | Water moves out of cell | Cell shrinks (crenation) | Cell loses water; plasmolysis (wilting) |
Worked Example 7.5.1
Predicting osmotic behavior — potato experiment
- DataPotato cubes placed in saltwater solutions of varying concentrations for 30 minutes show the following mass changes: 0% → +8%; 1.0% → 0%; 2.0% → −9%
- Analyze 0%Pure water has a lower solute concentration than potato cells. Water enters cells by osmosis. Mass increases. The 0% solution is hypotonic to potato cells.
- Analyze 1.0%No mass change means no net water movement. Solute concentration inside potato cells equals 1.0%. This is the isotonic concentration for potato cells.
- Analyze 2.0%Solution has higher solute concentration than potato cells. Water exits cells by osmosis. Mass decreases. The 2.0% solution is hypertonic to potato cells.
Answer: The isotonic concentration for these potato cells is approximately 1.0% salt. Below this, cells gain water; above this, cells lose water.
Real-World Osmosis
Osmosis explains why intravenous (IV) fluids must be isotonic with blood (0.9% NaCl — called "normal saline"), why wilting plants recover when watered, why salted food resists bacterial growth, and why freshwater fish produce large volumes of dilute urine. Whenever water moves predictably across a membrane, osmosis is at work.
- Define osmosis. How is osmosis similar to diffusion? How is it different?
- A red blood cell is placed in pure water. Predict what happens. Explain using the concept of tonicity.
- A plant cell placed in a hypertonic solution loses water and becomes limp. What is this condition called? Why don't plant cells burst in hypotonic solutions?
- Ocean water has a very high salt concentration. What challenge does this create for marine fish? How do you predict their kidneys respond?
- A hospital patient is accidentally given a hypertonic IV solution. What would happen to their red blood cells? What symptoms might result?
Section 7.6
Facilitated Diffusion
Many molecules that cells need — including glucose, amino acids, and many ions — cannot cross the phospholipid bilayer by simple diffusion. They are too large, too polar, or too charged. Yet they must enter and leave cells constantly. The solution is facilitated diffusion: diffusion assisted by transport proteins embedded in the membrane.
Like simple diffusion, facilitated diffusion moves molecules down the concentration gradient and requires no ATP. Unlike simple diffusion, it requires a specific protein.
| Protein Type | How It Works | Example |
|---|
| Channel protein | Forms an open pore; molecules that fit the channel pass through freely | Ion channels (K⁺, Na⁺, Cl⁻ channels) |
| Aquaporin | Specialized channel protein for rapid water transport | Kidney tubule cells; red blood cells |
| Carrier protein | Binds a specific molecule, changes shape, releases molecule on the other side | Glucose transporters (GLUT proteins) |
Two Key Properties of Facilitated Diffusion
Specificity — each transport protein works only with one type of molecule or ion; glucose carriers transport glucose, not fructose; sodium channels transport Na⁺, not K⁺
Saturation — if all transport proteins are occupied, the transport rate cannot increase even if the concentration gradient steepens; this is unlike simple diffusion, which has no such ceiling
Worked Example 7.6.1
Distinguishing simple diffusion from facilitated diffusion using a graph
- ScenarioA graph shows transport rate (y-axis) vs. concentration gradient (x-axis) for two molecules. Curve A rises steeply and then levels off at a maximum rate. Curve B rises steadily without leveling off.
- Identify ACurve A levels off because all transport proteins become occupied (saturated). This is the hallmark of facilitated diffusion — a maximum rate limited by protein availability.
- Identify BCurve B has no maximum — rate keeps rising with gradient. No protein is involved; the molecule crosses directly. This is simple diffusion.
- PredictionAdding more transport proteins to the membrane would raise the plateau of Curve A (higher maximum rate), but would not affect Curve B.
Answer: Curve A = facilitated diffusion (saturation plateau); Curve B = simple diffusion (no plateau). Saturation distinguishes protein-dependent from protein-independent transport.
- Why can't glucose cross the cell membrane by simple diffusion? What type of protein does glucose use?
- Explain what "saturation" means in the context of facilitated diffusion. What determines the maximum transport rate?
- How are aquaporins different from ordinary channel proteins? In which types of cells would you expect to find many aquaporins?
- Does facilitated diffusion require ATP? Explain why or why not, connecting your answer to the direction of movement relative to the concentration gradient.
- A GLUT1 protein transports glucose but not galactose, even though both are similar sugars. What property of transport proteins explains this?
Section 7.7
Active Transport
Diffusion and facilitated diffusion only move molecules from high to low concentration. But cells often need to maintain concentrations very different from those in their surroundings — sometimes far higher than outside, sometimes far lower. To do this, they must move molecules against (up) the concentration gradient. This process is called active transport, and it requires cells to spend energy in the form of ATP.
Active transport uses carrier proteins that act as pumps. The protein binds the molecule on the low-concentration side, uses ATP to change its shape, and releases the molecule on the high-concentration side — pumping it uphill.
The Sodium-Potassium Pump
The most important active transport pump in animal cells is the sodium-potassium pump (Na⁺/K⁺ pump). Each cycle pumps 3 Na⁺ out of the cell and 2 K⁺ in, consuming one ATP molecule. The result: sodium is kept high outside, potassium is kept high inside.
Worked Example 7.7.1
Tracing the sodium-potassium pump cycle
- Step 1Three Na⁺ ions inside the cell bind to the pump protein on the cytoplasmic side.
- Step 2One ATP molecule is hydrolyzed (broken down). The energy released causes the pump protein to change shape.
- Step 3The shape change exposes the Na⁺ binding sites to the outside. The Na⁺ ions are released into the extracellular fluid. The pump now has low affinity for Na⁺ and high affinity for K⁺.
- Step 4Two K⁺ ions from outside the cell bind to the pump protein.
- Step 5The pump returns to its original shape, releasing K⁺ into the cytoplasm. One cycle is complete.
Net result per cycle: 3 Na⁺ pumped out, 2 K⁺ pumped in, 1 ATP consumed. Both ions move against their concentration gradients.
| Comparison | Passive Transport (diffusion/facilitated) | Active Transport |
|---|
| Direction of movement | Down concentration gradient (high → low) | Against concentration gradient (low → high) |
| Energy requirement | None — driven by concentration gradient | ATP required |
| Protein required? | Not always (simple diffusion needs none) | Always — pump proteins required |
| Reaches equilibrium? | Yes — stops at equilibrium | No — maintains concentration differences indefinitely |
| Examples | O₂, CO₂, glucose (via GLUT), water | Na⁺/K⁺ pump, Ca²⁺ pump, proton pumps |
Important Distinction
Active transport is not simply "faster" than diffusion. They are fundamentally different processes. Diffusion can only move molecules from high to low concentration. Active transport moves them the other way. Speed is not the distinction — direction against the gradient, requiring ATP, is what defines active transport.
- Define active transport. What two features distinguish it from facilitated diffusion?
- The sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in per cycle. Which ion has a higher concentration inside the cell? Which has a higher concentration outside?
- A cell is treated with a drug that blocks ATP production. Predict what happens to the sodium-potassium pump. What downstream effects might this have on the cell?
- Calcium pumps in muscle cells actively remove Ca²⁺ from the cytoplasm after muscle contraction. Why must this be active transport (not diffusion)? What is the consequence for muscle function?
- Some bacteria have evolved pumps that push antibiotics out of the cell. How does this illustrate active transport? Why is this a serious medical problem?
Sections 7.8 – 7.10
Bulk Transport, Homeostasis, and Real-World Applications
Endocytosis · Exocytosis · Cellular Homeostasis · Disease · Medicine
Section 7.8
Endocytosis and Exocytosis
Simple diffusion, facilitated diffusion, and active transport all move individual molecules through transport proteins. But some substances are too large for this — whole bacteria, large protein complexes, and clumps of material cannot pass through protein channels one molecule at a time. Cells handle large-scale transport using a completely different strategy: the membrane itself engulfs or releases material by folding and fusing.
Endocytosis: Bringing Material In
Endocytosis is the process by which the cell membrane folds inward around material outside the cell, forming a membrane-enclosed sac called a vesicle that is brought inside the cell. There are three types.
| Type | What Is Taken In | Biological Example |
|---|
| Phagocytosis ("cell eating") | Solid particles — bacteria, cellular debris, large objects | White blood cells (macrophages) destroying pathogens |
| Pinocytosis ("cell drinking") | Small droplets of liquid and dissolved materials | Intestinal cells sampling extracellular fluid; general sampling |
| Receptor-mediated endocytosis | Specific molecules that bind to receptor proteins on the cell surface | Cells taking in LDL cholesterol; cells absorbing hormones; iron uptake |
Exocytosis: Releasing Material Out
Exocytosis is the reverse process: a vesicle inside the cell moves to the membrane, fuses with it, and releases its contents to the outside. Exocytosis is how cells secrete digestive enzymes, release hormones into the bloodstream, export antibodies from immune cells, release neurotransmitters from nerve cells, and export waste materials that cannot cross the membrane individually.
Worked Example 7.8.1
Classifying transport — phagocytosis vs. receptor-mediated endocytosis
- Scenario AA macrophage encounters a bacterium. The macrophage membrane wraps around the entire bacterium and engulfs it in a large vesicle. Lysosomal enzymes then digest the bacterium.
- Classify AThis is phagocytosis. The target is a large solid particle (the bacterium). The uptake is nonspecific in terms of binding — it targets the physical presence of the bacterium, not a specific receptor-molecule pair.
- Scenario BA liver cell has receptor proteins for LDL particles on its surface. When LDL binds to these receptors, the membrane folds inward specifically at those sites, taking in the LDL particles.
- Classify BThis is receptor-mediated endocytosis. Uptake is triggered by a specific molecular binding event — LDL binds to its receptor, then and only then does the membrane respond.
Answer: Scenario A = phagocytosis (large solid particle, general engulfment); Scenario B = receptor-mediated endocytosis (specific binding triggers targeted uptake).
- Name the three types of endocytosis. For each, describe what is taken in and give one biological example.
- Explain exocytosis. Give two examples of substances that cells secrete using exocytosis.
- How does receptor-mediated endocytosis differ from pinocytosis? What advantage does receptor-mediated endocytosis provide?
- When a nerve cell releases neurotransmitters at a synapse, which process is involved? Describe what happens to the vesicle and the membrane.
- Exocytosis adds membrane material to the cell surface. Why is this a necessary consequence of endocytosis?
Section 7.9
Transport and Cellular Homeostasis
All the transport processes in this chapter work together to maintain homeostasis within the cell — the stable internal conditions that allow biochemical reactions to proceed correctly. No single transport process does this alone. The cell membrane coordinates all these mechanisms simultaneously, adjusting their rates as internal and external conditions change.
| What the Cell Must Regulate | Transport Mechanisms Involved |
|---|
| Water balance | Osmosis; aquaporins regulate rate of water movement |
| Ion concentrations | Active transport pumps (Na⁺/K⁺ pump, Ca²⁺ pump) maintain ion gradients |
| Nutrient supply (glucose, amino acids) | Facilitated diffusion; receptor-mediated endocytosis |
| Waste removal (CO₂, urea, H⁺) | Simple diffusion; active transport; exocytosis |
| pH (H⁺ concentration) | Proton pumps and ion channels regulate H⁺ movement |
| Signal reception (hormones, nutrients) | Receptor-mediated endocytosis; membrane receptor proteins |
Transport Failure and Disease
Cystic Fibrosis
Defective CFTR chloride channel protein
When the CFTR channel fails, chloride ions cannot exit lung and digestive cells normally. Water follows chloride by osmosis — without chloride exiting, water stays inside cells. The result is thick, sticky mucus that clogs airways, traps bacteria, and causes progressive lung damage. A single defective transport protein disrupts the water balance of entire tissues.
Cholera
Bacterial toxin locks chloride channels permanently open
Cholera toxin forces chloride channels in intestinal cells to remain open continuously. Chloride floods out; water follows by osmosis — now in the wrong direction. The result is massive, life-threatening diarrhea. Cholera kills through dehydration caused by uncontrolled osmosis triggered by a single malfunctioning transport protein.
- Explain how the sodium-potassium pump contributes to cellular homeostasis. What would happen to a nerve cell if the pump stopped working?
- In cystic fibrosis, the defective protein is a chloride channel, not a water channel. Yet the main problem is thick mucus (abnormal water content). Trace the chain of events from the protein defect to the mucus symptom using your knowledge of osmosis.
- Cholera and cystic fibrosis both involve defective chloride transport, but in opposite directions. Describe the difference. What does this tell you about the importance of precisely regulated transport?
- A cell needs to keep potassium concentration high inside and sodium concentration high outside. Which transport process maintains this? What would happen if the cell ran out of ATP?
Section 7.10
Membrane Transport in Health and Disease
Understanding membrane transport is not just abstract science — it is the foundation for understanding how medications work, how the body responds to illness, and how living cells manage their complex chemistry.
Medications and the Membrane
| Drug or Therapy | Transport Mechanism Targeted | How It Works |
|---|
| Diuretics (blood pressure) | Ion transporters in kidney cells | Block Na⁺ reabsorption; water follows by osmosis into urine; blood pressure drops |
| Lidocaine (local anesthetic) | Sodium channels in nerve cells | Blocks Na⁺ channels; prevents nerve signals from firing; eliminates pain sensation |
| Proton pump inhibitors (acid reflux) | H⁺ pumps in stomach cells | Block proton pumps in stomach lining; reduce acid production |
| Statins (high cholesterol) | Receptor-mediated endocytosis of LDL | Reduce LDL uptake and increase LDL receptor production; lower blood cholesterol |
Biology in Everyday Life — Oral Rehydration Therapy
Diarrheal diseases kill hundreds of thousands of children per year, mostly in low-income countries. The treatment for most of these cases is remarkably simple: oral rehydration solution — water mixed with specific concentrations of sodium, potassium, and glucose. The glucose activates coupled transporter proteins in intestinal cells that bring both glucose and sodium into the cell together. Water follows by osmosis. The solution works precisely because its composition is matched to the transport mechanisms of intestinal epithelial cells. This life-saving treatment is entirely based on the biology of facilitated diffusion and osmosis.
- Lidocaine blocks sodium channels in nerve cells. Explain how this stops pain signals. Which type of transport (from this chapter) does it affect?
- Dialysis removes waste products from the blood of patients with kidney failure using a membrane permeable to small molecules but not blood cells or proteins. Which transport mechanism drives waste removal across the dialysis membrane?
- Oral rehydration solution contains glucose and sodium. Why is glucose necessary for water absorption? What would happen if you used a glucose-free solution?
- A vaccine must be taken up by immune cells to trigger a response. Which type of endocytosis is likely involved? How does this connect to vaccine design?
- Some cancer cells use active transport pumps to push chemotherapy drugs out of the cell before they can act. What is this phenomenon called? How does it illustrate the concepts of active transport and drug resistance?
Chapter 7 — Back Matter
Chapter Summary
- Sec 7.1The cell membrane holds the cell together, controls molecular traffic, enables communication, maintains homeostasis, and allows cell recognition. Without a functioning membrane, no cell can survive.
- Sec 7.2The cell membrane is a phospholipid bilayer — two layers with hydrophilic heads facing outward toward water and hydrophobic tails facing inward. Embedded proteins, cholesterol, and surface carbohydrates complete the structure, described by the fluid mosaic model.
- Sec 7.3The membrane is selectively permeable. Small nonpolar molecules cross freely. Polar molecules and ions require transport proteins. Large molecules use vesicle-based processes. Size, polarity, and charge determine permeability.
- Sec 7.4Diffusion is the movement of molecules from high to low concentration driven by random kinetic energy. It requires no ATP. Rate increases with steeper gradients, higher temperature, and smaller molecule size. Diffusion stops at equilibrium.
- Sec 7.5Osmosis is the diffusion of water across a selectively permeable membrane from lower to higher solute concentration. Tonicity (isotonic, hypotonic, hypertonic) predicts whether cells will maintain volume, swell, or shrink.
- Sec 7.6Facilitated diffusion uses channel or carrier proteins to move polar molecules and ions down their concentration gradients without ATP. It shows specificity (each protein is selective) and saturation (a maximum rate determined by protein number).
- Sec 7.7Active transport uses ATP to move molecules against their concentration gradients via pump proteins. The sodium-potassium pump (3 Na⁺ out, 2 K⁺ in per ATP) is essential for nerve signals, muscle contraction, and nutrient uptake.
- Sec 7.8Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) brings large material into the cell in vesicles. Exocytosis releases vesicle contents outside the cell. These bulk processes handle substances too large for protein channels.
- Sec 7.9All transport mechanisms together maintain cellular homeostasis — stable internal conditions. Transport failures cause specific diseases: cystic fibrosis results from a defective chloride channel; cholera results from a toxin that locks chloride channels open.
- Sec 7.10Many medications work by targeting membrane transport proteins: diuretics, anesthetics, antacids, and statins all alter specific channels, pumps, or receptors. Dialysis and oral rehydration therapy are also built on transport biology.
Key Terms Review
active transport
Movement of molecules against their concentration gradient using ATP and pump proteins.
aquaporin
A protein channel specialized for rapid water transport across the membrane.
carrier protein
A membrane protein that binds a specific molecule and changes shape to transport it across the membrane.
channel protein
A protein that forms a pore through the membrane through which specific molecules or ions can pass.
cholesterol
A lipid found in animal cell membranes that buffers membrane fluidity against temperature changes.
concentration gradient
The difference in concentration of a substance between two regions; the driving force for diffusion.
diffusion
The movement of molecules from high to low concentration driven by random molecular motion; requires no energy.
endocytosis
A process in which the cell membrane folds inward to engulf material in a vesicle and bring it into the cell.
exocytosis
A process in which vesicles fuse with the cell membrane and release their contents outside the cell.
facilitated diffusion
Diffusion aided by transport proteins; moves molecules down their gradient without ATP.
fluid mosaic model
The model of membrane structure: a fluid phospholipid bilayer with a mosaic of embedded proteins, cholesterol, and carbohydrates.
glycocalyx
The carbohydrate coat on the outer membrane surface, involved in cell recognition and immune function.
homeostasis
The maintenance of stable internal conditions in a cell or organism despite external changes.
hydrophilic
Water-loving; describes polar or charged molecules that interact favorably with water.
hydrophobic
Water-fearing; describes nonpolar molecules that do not interact with water and are repelled by it.
hypertonic
A solution with higher solute concentration than the cell interior; causes water to move out of the cell.
hypotonic
A solution with lower solute concentration than the cell interior; causes water to move into the cell.
isotonic
A solution with the same solute concentration as the cell interior; no net water movement occurs.
osmosis
The diffusion of water across a selectively permeable membrane from lower to higher solute concentration.
phagocytosis
A type of endocytosis in which the cell engulfs large solid particles such as bacteria or debris.
phospholipid
A molecule with a hydrophilic head and two hydrophobic tails that forms the structural basis of cell membranes.
phospholipid bilayer
The double layer of phospholipids forming the core of the membrane, with heads facing outward and tails facing inward.
pinocytosis
A type of endocytosis in which the cell takes in small droplets of liquid and dissolved materials.
receptor-mediated endocytosis
Endocytosis triggered by specific molecules binding to receptor proteins on the cell surface.
selective permeability
The property of the membrane that allows some substances to cross while restricting others.
sodium-potassium pump
An active transport protein that pumps 3 Na⁺ out and 2 K⁺ in per ATP cycle; essential for nerve and muscle function.
tonicity
The relative solute concentration of a solution compared to the interior of a cell; determines direction of osmosis.
transmembrane protein
A protein that spans the entire width of the cell membrane; typically serves as a channel, carrier, or receptor.
vesicle
A small membrane-enclosed sac used to transport materials into or out of the cell during endocytosis or exocytosis.
concentration gradient
The difference in solute concentration across a space or membrane; the driving force for passive transport.
Membrane Diagram Practice
Draw a cross-section of the phospholipid bilayer. Label: hydrophilic head, hydrophobic tail, extracellular fluid, cytoplasm, channel protein. Indicate which regions are water-based and which are lipid-based. Add a water molecule and a glucose molecule to your diagram and indicate which can cross the bilayer directly and which cannot.
Examine a diagram of the cell membrane showing various proteins. For each protein, classify it as: channel protein, carrier protein, receptor protein, or pump. Explain your reasoning based on the protein's structure and position.
Compare the membrane composition of an animal cell and a plant cell. Which components are shared? Which differ? What does cholesterol do in animal cell membranes, and how do plant cells compensate with other mechanisms?
Diffusion and Osmosis Practice
- A glucose solution is separated from pure water by a membrane permeable to water but not glucose. In which direction will water move? Explain using solute concentration and osmosis.
- A red blood cell is placed in each of three solutions — 0.9% NaCl, 0.3% NaCl, and 3.0% NaCl. Predict what will happen to the cell in each solution. Classify each as isotonic, hypotonic, or hypertonic.
- Oxygen is produced inside a plant cell during photosynthesis. The oxygen concentration inside the cell rises above the concentration in surrounding air spaces. Describe how oxygen exits the cell. What type of transport is this? What determines the direction and rate?
- A student measures the mass of potato cubes placed in different concentrations of saltwater for 30 minutes. Results: 0% → +8%; 0.5% → +3%; 1.0% → 0%; 1.5% → −4%; 2.0% → −9%. (a) At which concentration is the solution isotonic? (b) What was happening in pure water? (c) Explain the mass loss at 2.0%. (d) Sketch a graph of the results with concentration on the x-axis and mass change on the y-axis.
Transport Type Classification
Classify each scenario as simple diffusion, facilitated diffusion, active transport, endocytosis, or exocytosis. Justify your classification.
- Oxygen moves from the lungs into red blood cells.
- Glucose enters muscle cells through carrier proteins when blood glucose is higher than muscle glucose.
- A macrophage engulfs and destroys a bacterium.
- A nerve cell pumps Na⁺ out to restore resting conditions after firing.
- A pancreatic cell releases digestive enzymes into the small intestine.
- Neurotransmitters are released from a nerve cell into a synapse.
- Water moves into a plant root cell from surrounding soil.
- CO₂ from cellular respiration exits the cell.
- A cell takes up LDL cholesterol through specific receptor proteins.
- Calcium ions are pumped into the endoplasmic reticulum using ATP.
Graph Interpretation
- A graph shows a linear relationship between concentration gradient and diffusion rate. (a) What pattern does the graph show? (b) What happens to diffusion rate as the gradient increases? (c) Does diffusion require energy? How does this graph relate to that fact? (d) If the concentration gradient reaches zero, what happens to diffusion rate? What does this represent biologically?
- Two curves show transport rate vs. concentration gradient. Curve A rises steeply and then levels off. Curve B rises gradually without leveling. (a) Which curve represents facilitated diffusion? Which represents simple diffusion? (b) What causes Curve A to level off? (c) What is this maximum rate called? (d) What would happen to Curve A if more transport proteins were added to the membrane?
- A graph shows cell volume over time in hypotonic (volume increases), isotonic (no change), and hypertonic (volume decreases) solutions. (a) Which condition poses the greatest threat to an animal cell? Why? (b) Why don't plant cells burst in hypotonic solutions? (c) How do freshwater organisms cope with constant osmotic water gain?
Concept Check
- What are the two main components of the cell membrane, and what role does each play?
- Why can oxygen cross the membrane freely but glucose cannot?
- How does osmosis differ from diffusion?
- What is the difference between facilitated diffusion and active transport? How does energy use distinguish them?
- Give one example each of endocytosis and exocytosis in the human body. What does each accomplish?
- Why is the sodium-potassium pump important for nerve function?
- What does "selectively permeable" mean, and why is this property essential for living cells?
- A cell is placed in a hypertonic solution. Describe what happens. Explain using osmosis.
Error Analysis
Each statement below contains an error in biological reasoning. Identify the error and write a corrected version.
Statement 1
"Because diffusion moves molecules toward where they're needed, cells use diffusion to build up concentration differences."
What is the error? How should this statement be corrected?
Statement 2
"Active transport is faster than diffusion because it uses energy."
What is wrong with this comparison? What actually distinguishes active transport from diffusion?
Statement 3
"Osmosis only occurs in plant cells because plant cells have rigid cell walls."
Identify the error. Where else does osmosis occur, and what role do cell walls actually play?
Statement 4
"A carrier protein creates energy to push molecules through the membrane."
What is wrong? Where does the energy for active transport actually come from, and what does the carrier protein do?
Statement 5
"The sodium-potassium pump moves sodium and potassium down their concentration gradients."
This is the opposite of the truth. Correct it and explain what actually happens.
Review Questions
- Describe the structure of a phospholipid. How does its structure explain why phospholipids spontaneously form a bilayer in water?
- What is the fluid mosaic model? What does "fluid" mean in this context? What does "mosaic" mean?
- Explain why a cell placed in pure water would swell, while the same cell in a concentrated salt solution would shrink.
- A student claims that active transport and facilitated diffusion are similar because both use membrane proteins. Evaluate this claim: in what ways are they similar, and in what important ways do they differ?
- Describe receptor-mediated endocytosis. What makes it different from phagocytosis and pinocytosis? Give a biological example of when a cell would use each.
- A hospital patient is receiving intravenous (IV) fluids. A nurse explains that the fluid must be the right concentration — neither too concentrated nor too dilute. Using your understanding of osmosis and tonicity, explain what would happen to red blood cells if the IV fluid were too concentrated, and what would happen if it were too dilute. What concentration is used, and why?
- Cystic fibrosis results from a defective chloride channel protein in cell membranes. In the lungs, chloride cannot exit cells normally. Using your understanding of osmosis, explain why this malfunction leads to thick, sticky mucus. Trace the chain from the protein defect to the patient symptom.
- Compare and contrast glucose entering a cell through a carrier protein (facilitated diffusion) versus sodium ions being pumped out by the sodium-potassium pump (active transport). Address: energy use, direction of movement relative to the concentration gradient, the role of proteins, and what would happen if the process stopped.
Chapter 7 Checkpoint
Chapter 7 Checkpoint
Complete without assistance — assesses all key skills and concepts from Chapter 7
Section 1 — Vocabulary and Concepts
- Define selective permeability. Why is it essential for cellular function?
- What is the fluid mosaic model? Name four types of molecules found in the cell membrane.
- Distinguish between a channel protein and a carrier protein. Which type does the sodium-potassium pump use?
Section 2 — Transport Mechanisms
- Classify each as simple diffusion, facilitated diffusion, or active transport: (a) CO₂ leaving a cell; (b) glucose entering a muscle cell through GLUT proteins; (c) Na⁺ being pumped out of a nerve cell.
- Explain why active transport requires ATP but facilitated diffusion does not. Use the concept of concentration gradient in your answer.
- Name the three types of endocytosis and give one example of each.
Section 3 — Osmosis and Tonicity
- A plant cell is placed in a hypertonic solution. Predict what happens to the cell. Use the terms osmosis, water movement, and plasmolysis.
- Red blood cells are normally found in isotonic plasma. What would happen if plasma became hypotonic? Hypertonic? Explain each outcome.
- At which salt concentration is a solution isotonic to a cell if the cell loses mass at 2% and gains mass at 0.5%? Explain your reasoning.
Section 4 — Disease and Application
- Cystic fibrosis is caused by a defective chloride channel. Explain how this defect leads to thick mucus in the lungs. Identify which transport mechanism fails and how osmosis is involved.
- Local anesthetics block sodium channels. Explain how this prevents pain. Which transport process is blocked?
- Oral rehydration solution contains glucose, sodium, and water. Explain why glucose is essential for the solution to work. Which transport mechanism does it activate?
Section 5 — Error Analysis
- A student says: "Diffusion moves molecules to where they are needed." Identify the error in this statement and write a scientifically accurate version.
- A student says: "The sodium-potassium pump is a type of facilitated diffusion because it uses proteins." Identify two errors in this statement and correct them.
Looking Ahead
Chapter 8: Enzymes and Metabolism
Chapter 7 explained how cells control what enters and what leaves. But what happens once molecules like glucose arrive inside the cell? Chapter 8 introduces metabolism — the complete set of chemical reactions that sustain life — and the enzymes that make those reactions fast enough to matter. You will learn how enzymes lower activation energy, how enzyme shape determines what reactions they catalyze, how temperature and pH affect enzyme activity, and how cells regulate entire metabolic pathways. Everything you have learned about transport connects directly: the glucose a GLUT carrier brings across the membrane becomes the starting material for the metabolic reactions Chapter 8 will explain.
MGU Library Connection
- Cell Membrane Transport Chart — Biology Appendix D of this textbook
- Cell Organelle Reference — Biology Appendix D: Cell Structure Reference
- Chapter 6: Cell Structure and Function — for background on organelles and the cell as a living unit
- Chapter 5: Biomolecules and Water — for background on phospholipid chemistry and protein structure
- Chapter 8: Enzymes and Metabolism — for what happens to molecules after they cross the membrane
- MGU Dictionary entries: osmosis, diffusion, active transport, phospholipid, tonicity, endocytosis, fluid mosaic model