Cell Membrane and Transport
Explore the structure of the phospholipid bilayer, understand how substances move across cell membranes via diffusion, osmosis, and active transport.
The Phospholipid Bilayer
The cell membrane (plasma membrane) is primarily made of a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty acid tails. This structure is described by the fluid mosaic model.
Fluid Mosaic Model
Extracellular fluid (outside)
Hydrophobic tails (fatty acids) -- interior of membrane
Cytoplasm (inside)
Blue = phospholipid heads, P = protein channel, C = cholesterol
Selectively permeable: The membrane controls what enters and leaves the cell. Small non-polar molecules (O2, CO2) pass freely. Charged ions and large molecules need transport proteins.
Passive Transport
Passive transport requires no energy (ATP). Substances move down their concentration gradient -- from an area of high concentration to an area of low concentration.
Simple Diffusion
Small, non-polar molecules move directly through the phospholipid bilayer. Example: O2, CO2 crossing the membrane.
Facilitated Diffusion
Larger or charged particles pass through channel proteins or carrier proteins. Example: glucose, Na+ ions.
Osmosis
The diffusion of water through a selectively permeable membrane, from high water concentration (low solute) to low water concentration (high solute).
Osmotic effects on cells: In a hypotonic solution, water enters the cell (may lyse). In a hypertonic solution, water leaves the cell (crenation/plasmolysis). In an isotonic solution, water moves equally in both directions.
Active Transport
Active transport moves substances against the concentration gradient (from low to high concentration). This requires cellular energy in the form of ATP.
Types of Active Transport
Protein Pumps
Carrier proteins use ATP to move ions against gradient. E.g., Na+/K+ pump.
Endocytosis
Cell engulfs large molecules by folding the membrane inward to form a vesicle.
Exocytosis
Vesicles fuse with the membrane to release contents outside the cell.
Comparing Transport Methods
Passive: No energy required, moves with gradient. Includes diffusion, facilitated diffusion, osmosis.
Active: ATP required, moves against gradient. Includes protein pumps, endocytosis, exocytosis.
Key factor: The concentration gradient direction determines whether transport is passive or active.
Key Vocabulary
Phospholipid Bilayer
The double layer of phospholipid molecules forming the cell membrane, with hydrophilic heads facing outward and hydrophobic tails inward.
Osmosis
The net movement of water molecules from a region of higher water potential to lower water potential through a selectively permeable membrane.
Concentration Gradient
The difference in concentration of a substance between two regions. Substances naturally move down the gradient (high to low).
Active Transport
The movement of substances against the concentration gradient, requiring cellular energy (ATP) and transport proteins.
Worked Examples
Explain what happens to a red blood cell placed in distilled water.
Step 1: Distilled water is hypotonic (lower solute concentration) relative to the cell.
Step 2: Water moves into the cell by osmosis (from high water concentration to low).
Answer: The cell swells and may burst (haemolysis) because animal cells lack a rigid cell wall to prevent expansion.
Why can oxygen cross the cell membrane without a transport protein?
Step 1: O2 is a small, non-polar molecule.
Step 2: The interior of the phospholipid bilayer is hydrophobic (non-polar).
Answer: Small non-polar molecules can dissolve in and pass through the hydrophobic interior by simple diffusion, without needing a protein channel.
Explain how the sodium-potassium pump works and why it is active transport.
Step 1: The pump moves 3 Na+ ions out of the cell and 2 K+ ions into the cell per cycle.
Step 2: Both ions are moved against their concentration gradients.
Answer: It is active transport because it moves ions from low to high concentration, which requires energy from the hydrolysis of ATP.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
The cell membrane is described as "selectively permeable". What does this mean?
Question 2
In which direction does water move during osmosis?
Question 3
Which type of transport requires ATP?
Question 4
What is the function of cholesterol in the cell membrane?
Question 5
A plant cell placed in a hypertonic solution will:
Key Concepts Summary
- ●The cell membrane is a phospholipid bilayer described by the fluid mosaic model.
- ●Diffusion moves particles from high to low concentration (no energy required).
- ●Osmosis is the diffusion of water through a selectively permeable membrane.
- ●Active transport uses ATP to move substances against the concentration gradient.
- ●Cells in hypotonic solutions swell; in hypertonic solutions they shrink.