AP Biology 2.5 Membrane Transport Study Notes - New Syllabus Effective 2025
AP Biology 2.5 Membrane Transport Study Notes- New syllabus
AP Biology 2.5 Membrane Transport Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
2.5.A: Describe the mechanisms that organisms use to maintain solute and water balance.
2.5.B: Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.
Key Concepts:
- Membrane Transport
2.5.A.1 – Formation of concentration gradients
Selective permeability is a property of cellular membranes that only allows certain molecules to enter or exit the cell. This is important for the cell to maintain its internal order irrespective of the changes to the environment. For example, water, ions, glucose and carbon dioxide may need to be imported or exported from the cell depending on its metabolic activity. Similarly, signaling molecules may need to enter the cell and proteins may need to be released into the extracellular matrix. The presence of a selectively permeable membrane allows the cell to exercise control over the quantum, timing and rate of movement of these molecules.
Movement across a selectively permeable membrane can occur actively or passively. For example, water molecules can move passively through small pores on the membrane. Similarly, carbon dioxide released as a byproduct of respiration quickly diffuses out of the cell. Some molecules are actively transported. For example, cells in the kidney expend energy to reabsorb all the glucose, amino acids and vitamins from the glomerular filtrate even against the concentration gradient. Failure of this process leads to the presence of glucose or the byproducts of protein metabolism in urine; a tell tale sign of diabetes.
2.5.A.2 – Passive transport
Passive transport is of two types – free diffusion or facilitated diffusion – and movement is always along a concentration gradient. Free diffusion is seen most often in the movement of uncharged molecules such as carbon dioxide or ethanol across the cell membrane, without the involvement of any other molecules.
Facilitated diffusion requires the presence of another molecule, usually a protein, that acts as a carrier and helps the substrate cross the cell membrane. Carrier proteins bind to the substrate on one side of the membrane and change conformation to release the substrate on the other side. Classic examples of facilitated diffusion are the movement of oxygen through binding to haemoglobin or the transport of water through minute pores formed by aquaporins.
2.5.A.3 – Active transport
Active transport is the process of transferring substances into, out of, and between cells, using energy. In some cases, the movement of substances can be accomplished by passive transport, which uses no energy. However, the cell often needs to transport materials against their concentration gradient. In these cases, active transport is required.
Examples include:
- Movement of Ca2+ ions out of the cardiac muscle cells.
- Transportation of glucose molecules across membranes.
- Movement of amino acids in the human gut across the intestinal lining.
There are two types of active transport namely – Primary active transport and secondary active transport.
Primary active transport – In this process of transportation, the energy is utilized by the breakdown of the ATP – Adenosine triphosphate to transport molecules across the membrane against a concentration gradient. Therefore, all the groups of ATP powered pumps contain one or more binding sites for the ATP molecules, which are present on the cytosolic face of the membrane. Basically, the primary active transport uses external chemical energy such as the ATP.
Sodium-potassium pump, the most important pump in the animal cell is considered as an example of primary active transport. In this process of transportation, the sodium ions are moved to the outside of the cell and potassium ions are moved to the inside of the cell.
Secondary active transport – Secondary active transport is a kind of active transport that uses electrochemical energy. It takes place across a biological membrane where a transporter protein couples the movement of an electrochemical ion (typically Na+ or H+) down its electrochemical gradient to the upward movement of another molecule or an ion against a concentration or electrochemical gradient.
2.5.B.1 – Exocytosis and Endocytosis
Exocytosis
Exocytosis is the process by which a cell releases molecules (such as enzymes, hormones, and neurotransmitters) into the extracellular space. This process is mediated by the fusion of the plasma membrane with vesicles that contain the molecules to be released.
Endocytosis
Endocytosis is the process where cells take in materials from the outside environment through the cell membrane. This can be done either by pinocytosis, which is the uptake of small amounts of fluid, or phagocytosis, which is the uptake of larger particles. Endocytosis is important for cells to take in nutrients, eliminate waste, and process signals from the extracellular environment.
Features of Exocytosis
-It is a process by which a cell releases molecules
-The fusion of the plasma membrane with vesicles mediates the release
-It decreases the cell’s membrane size
-It absorbs the nutrients for the efficient functioning of cells and eliminates the damaged cells
Features of Endocytosis
– Endocytosis is a process that helps cells intake material from the external environment
– This process can be used to take in nutrients, but also to remove toxins and other materials that could be harmful to the cell
– Endocytosis can be either receptor-mediated or non-receptor-mediated
– Receptor-mediated endocytosis is a process that involves special proteins on the cell surface called receptors
– These receptors bind to specific molecules in the environment and then bring them into the cell
– Non-receptor-mediated endocytosis does not use receptors but instead uses the cell’s membrane to engulf materials
– This process is often used to bring large particles into the cell
– Endocytosis can be either active or passive
– Active endocytosis requires the cell to use energy to bring in the material
– Passive endocytosis does not require any energy and is the result of forces such as diffusion or osmosis