Biology: Cell Membrane Structure and Selective Permeability
How membrane structure controls what enters and leaves cells
Biology: Cell Membrane Structure and Selective Permeability
How membrane structure controls what enters and leaves cells
Biology - Grade 9-12
- 1
Describe the basic structure of the phospholipid bilayer. Include the terms hydrophilic and hydrophobic in your answer.
Hydrophilic means water-loving, and hydrophobic means water-fearing.
The phospholipid bilayer is made of two layers of phospholipids. The hydrophilic phosphate heads face the watery outside and inside of the cell, while the hydrophobic fatty acid tails face inward away from water. - 2
Explain why the cell membrane is called selectively permeable.
The cell membrane is called selectively permeable because it allows some substances to pass through more easily than others. This helps the cell control its internal environment. - 3
A small nonpolar oxygen molecule and a large polar glucose molecule are both outside a cell. Which one is more likely to diffuse directly through the phospholipid bilayer, and why?
The hydrophobic interior of the membrane allows nonpolar molecules through more easily than polar molecules.
The oxygen molecule is more likely to diffuse directly through the phospholipid bilayer because it is small and nonpolar. Glucose is large and polar, so it usually needs a transport protein. - 4
Identify two functions of membrane proteins.
Membrane proteins can transport substances across the membrane and help cells communicate by receiving signals. Some membrane proteins also help cells attach to other cells or to their surroundings. - 5
In the fluid mosaic model, what does the word fluid mean, and what does the word mosaic mean?
Think of the membrane as flexible rather than rigid.
Fluid means that phospholipids and some proteins can move sideways within the membrane. Mosaic means that the membrane contains many different components, such as proteins, cholesterol, phospholipids, and carbohydrates, arranged like a patchwork. - 6
A cell is placed in a hypotonic solution. Predict what will happen to the movement of water and explain the effect on the cell.
Water moves by osmosis from lower solute concentration to higher solute concentration.
Water will move into the cell because the solution outside has a lower solute concentration than the inside of the cell. An animal cell may swell or burst, while a plant cell becomes turgid. - 7
Compare diffusion and facilitated diffusion. State one similarity and one difference.
Both diffusion and facilitated diffusion move substances down their concentration gradients without using cellular energy. The difference is that facilitated diffusion requires a transport protein, while simple diffusion does not. - 8
A diagram shows more sodium ions outside a cell than inside. Sodium ions move into the cell through a channel protein without ATP. What type of transport is occurring?
Ions cannot easily pass through the hydrophobic center of the membrane on their own.
Facilitated diffusion is occurring because sodium ions are moving down their concentration gradient through a channel protein without using ATP. - 9
Explain why ions such as Na+ and Cl- usually need protein channels to cross the cell membrane.
Ions usually need protein channels because they are charged particles. The hydrophobic interior of the phospholipid bilayer repels charged substances, so ions cannot pass through easily on their own. - 10
A plant cell is placed in a hypertonic salt solution. Predict what happens to the cell membrane and central vacuole.
Hypertonic solutions have a higher solute concentration outside the cell.
Water will leave the plant cell by osmosis because the outside solution has a higher solute concentration. The central vacuole will shrink, and the cell membrane may pull away from the cell wall in a process called plasmolysis. - 11
What role does cholesterol play in animal cell membranes?
Cholesterol helps regulate membrane fluidity. It prevents the membrane from becoming too rigid at low temperatures and too fluid at high temperatures. - 12
A cell uses ATP to move calcium ions from an area of low concentration to an area of high concentration. What type of transport is this, and why?
Moving against a concentration gradient requires energy.
This is active transport because the calcium ions are being moved against their concentration gradient. ATP is required to provide energy for this movement. - 13
Label the following membrane components by function: A forms the main barrier, B recognizes chemical signals, C helps identify the cell to other cells, and D moves specific molecules across the membrane.
A is the phospholipid bilayer because it forms the main barrier. B is a receptor protein because it recognizes chemical signals. C is a carbohydrate chain or glycoprotein because it helps identify the cell. D is a transport protein because it moves specific molecules across the membrane. - 14
Explain how surface area to volume ratio affects how efficiently a cell exchanges materials with its environment.
The cell membrane is the exchange surface for nutrients, wastes, gases, and water.
A cell with a larger surface area compared with its volume can exchange materials more efficiently because more membrane is available for substances to enter and leave. As a cell gets larger, its volume increases faster than its surface area, making exchange less efficient. - 15
A student says, "All molecules can cross the cell membrane if they are small enough." Explain why this statement is incomplete or incorrect.
The statement is incomplete because size is not the only factor that affects membrane permeability. Polarity and charge also matter. Small nonpolar molecules can cross easily, but charged ions and many polar molecules need transport proteins even if they are small.