Cell Membranes and Transport: Question 3

Worked solution

Part (a): Facilitated diffusion versus simple diffusion

Facilitated diffusion is a passive process: like simple diffusion, it moves the glucose molecules down their concentration gradient and does not require ATP. The key difference is the pathway. Small, non-polar or lipid-soluble molecules (such as oxygen or carbon dioxide) can diffuse directly through the hydrophobic core of the phospholipid bilayer during simple diffusion. Glucose, however, is a relatively large, polar molecule, so it cannot cross the bilayer directly. Instead, it moves through a specific carrier protein, which provides a pathway across the membrane for facilitated diffusion. Both processes still rely only on the kinetic energy of the diffusing particles, not on ATP.

Part (b): Active transport versus facilitated diffusion

Active transport differs from facilitated diffusion in two connected ways:

• Energy requirement. Active transport requires energy from ATP, hydrolysed by the carrier protein as it moves the calcium ions. Facilitated diffusion requires no metabolic energy at all.
• Direction of movement. Active transport moves calcium ions against their concentration gradient, from a region of lower concentration to a region of higher concentration. Facilitated diffusion always moves substances down their concentration gradient, from higher to lower concentration.

Both processes are protein-mediated (both use specific carrier proteins), which is why energy requirement and direction, not the presence of a protein, are the features that distinguish them.

Part (c): Why active transport requires many mitochondria

Moving calcium ions against their concentration gradient is energetically unfavourable, so it can only happen if the carrier protein is continuously supplied with ATP. ATP is generated in large quantities by aerobic respiration, which occurs in the mitochondria. A cell that performs a large amount of active transport therefore has a correspondingly high demand for ATP, and containing a large number of mitochondria allows the cell to produce ATP fast enough to sustain this demand.

Part (d): Endocytosis and its relationship to exocytosis

To remove the damaged patch of membrane, a region of the cell-surface membrane folds inward, surrounding the patch, and pinches off inside the cytoplasm to form a vesicle. This process, endocytosis, directly involves the cell-surface membrane because the vesicle’s membrane is formed from a piece of the original cell-surface membrane, which is removed from the cell surface as the vesicle forms.

This is the reverse of exocytosis, in which a vesicle already inside the cytoplasm moves to the cell-surface membrane and fuses with it, releasing the vesicle’s contents to the outside of the cell and adding the vesicle’s membrane to the cell-surface membrane. So endocytosis takes material and membrane into the cell, while exocytosis releases material and adds membrane out of the cell.

Final answers

• (a) Facilitated diffusion is passive like simple diffusion, but needs a channel/carrier protein and moves larger, polar/charged molecules that cannot cross the bilayer directly.
• (b) Active transport needs ATP and moves substances against their concentration gradient; facilitated diffusion needs no ATP and moves substances down their concentration gradient.
• (c) Active transport needs a large, continuous ATP supply, produced by aerobic respiration in mitochondria, so many mitochondria are needed to meet this demand.
• (d) Endocytosis: cell-surface membrane folds inward and pinches off to form a vesicle inside the cell. Exocytosis: an internal vesicle fuses with the cell-surface membrane to release its contents outside, the reverse process.