2.5 Membrane Transport
- Syllabus
- 2025
- Topic
- 2.5
- Level
- —
Learning objectives
2.5A—Describe the mechanisms that organisms use to maintain solute and water balanceDescribe the mechanisms that organisms use to maintain solute and water balance.• The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane.• Passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy.• Active transport requires the direct input of energy to move molecules. In some cases, active transport is utilized to move molecules from regions of low concentration to regions of high concentration.2.5B—Describe the mechanisms that organisms use to transport large molecules across the plasma membraneDescribe the mechanisms that organisms use to transport large molecules across the plasma membrane.• The processes of endocytosis and exocytosis require energy to move large substances or large amounts of substances into and out of cells.- i. In endocytosis, the cell takes in large molecules and particulate matter by folding the plasma membrane in on itself and forming new (small) vesicles that engulf material from the external environment.- ii. In exocytosis, internal vesicles release material from cells by fusing with the plasma membrane and secreting large molecules from the cell.
Passive and Active Transport Control Gradients
A selectively permeable membrane allows different solute concentrations to exist on opposite sides, forming a concentration gradient. Cells regulate movement across these gradients to maintain solute balance; because solute distribution influences water movement, this regulation also supports water balance.
| Mechanism | Direct metabolic-energy input | Net movement relative to concentration | Effect on a gradient |
|---|---|---|---|
| Passive transport | Not required | High concentration → low concentration | Reduces the concentration difference |
| Active transport | Required | Can move low concentration → high concentration | Can build or maintain the concentration difference |
If a cell must keep an ion more concentrated on one side of its membrane, passive movement alone cannot maintain that unequal distribution: active transport can restore it using energy. If the ion is allowed to move down its gradient, passive transport produces net movement without direct metabolic-energy input.
Active transport is defined by its direct energy requirement, not only by direction. Moving from low to high concentration is an important active-transport case, but the CED states that active transport does this in some cases rather than making it the only possible description.
Vesicles Move Cargo Across the Plasma Membrane
Large molecules, particles, or large amounts of material cannot move through the lipid bilayer as individual small solutes do. Cells use energy to reshape the plasma membrane and move this cargo in membrane-bound vesicles.
| Process | Direction | Membrane action | Result |
|---|---|---|---|
| Endocytosis | Into the cell | Plasma membrane folds inward around external material and pinches off | A new small vesicle encloses the cargo inside the cell |
| Exocytosis | Out of the cell | An internal vesicle fuses with the plasma membrane | The vesicle releases large molecules outside the cell |
A phagocytic cell can engulf a pathogen by endocytosis, enclosing the particle in a vesicle. A synaptic vesicle can fuse with the plasma membrane by exocytosis and release its contents from the cell. In both cases, energy supports movement of cargo through membrane remodeling.
Endocytosis and exocytosis do not carry cargo through a protein channel. They move cargo by forming or fusing vesicles with the plasma membrane.