AP Biology 2.3 Plasma Membrane Study Notes - New Syllabus Effective 2025
AP Biology 2.3 Plasma Membrane Study Notes- New syllabus
AP Biology 2.3 Plasma Membrane Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
2.3.A : Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.
2.3.B : Describe the fluid mosaic model of cell membranes.
Key Concepts:
- Plasma Membrane Structure
- Membrane Permeability
2.3.A.1 – Phospholipids
Phospholipids are major components of the plasma membrane, the outermost layer of animal cells. Like fats, they are composed of fatty acid chains attached to a glycerol backbone. Unlike triglycerides, which have three fatty acids, phospholipids have two fatty acids that help form a diacylglycerol. The third carbon of the glycerol backbone is also occupied by a modified phosphate group. However, just a phosphate group attached to a diacylglycerol does not qualify as a phospholipid. This would be considered a phosphatidate (diacylglycerol 3-phosphate), the precursor to phospholipids. To qualify as a phospholipid, the phosphate group should be modified by an alcohol. Phosphatidylcholine and phosphatidylserine are examples of two important phospholipids that are found in plasma membranes.
Membrane’s Fluidity
A cell’s plasma membrane contain proteins and other lipids (such as cholesterol) within the phospholipid bilayer. Biological membranes remain fluid because of the unsaturated hydrophobic tails, which prevent phospholipid molecules from packing together and forming a solid.
If a drop of phospholipids is placed in water, the phospholipids spontaneously form a structure known as a micelle, with their hydrophilic heads oriented toward the water. Micelles are lipid molecules that arrange themselves in a spherical form in aqueous solution. The formation of a micelle is a response to the amphipathic nature of fatty acids, meaning that they contain both hydrophilic and hydrophobic regions.
2.3.A.2 – Polarity of proteins
The cell membrane consists of two adjacent layers of phospholipids, which form a bilayer. The fatty acid tails of phospholipids face inside, away from water, whereas the phosphate heads face the outward aqueous side. Since the heads face outward, one layer is exposed to the interior of the cell and one layer is exposed to the exterior. As the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid.
Because of the phospholipds’ chemical and physical characteristics, the lipid bilayer acts as a semipermeable membrane; only lipophilic solutes can easily pass the phospholipd bilayer. As a result, there are two distinct aqueous compartments on each side of the membrane. This separation is essential for many biological functions, including cell communication and metabolism.
2.3.B.1 – Plasma membrane
Phospholipids spontaneously self-organize into a bilayer. These interactions with water enables formation of plasma membrane.
Proteins are packed between the lipids which constitute the membrane. Such transmembrane proteins enables their passing into cells through channels, gates or pores which otherwise could not enter. Hence, cells regulate the molecule flow and also perform other roles such as cell recognition and signaling.
Carbohydrates usually seen in the plasma membrane form a part of glycoproteins which take form when carbohydrates associate with proteins. The glycoproteins are significantly involved in the interaction taking place between cells which includes cell adhesion.
Membrane Protein
While lipids are the fundamental structural elements of membranes, proteins are responsible for carrying out specific membrane functions. Most plasma membranes consist of approximately 50% lipid and 50% protein by weight, with the carbohydrate portions of glycolipids and glycoproteins constituting 5 to 10% of the membrane mass. Since proteins are much larger than lipids, this percentage corresponds to about one protein molecule per every 50 to 100 molecules of lipid. In 1972, Jonathan Singer and Garth Nicolson proposed the fluid mosaic model of membrane structure, which is now generally accepted as the basic paradigm for the organization of all biological membranes. In this model, membranes are viewed as two-dimensional fluids in which proteins are inserted into lipid bilayers.
The second major component is formed by the proteins of the plasma membrane. Integrins or integral proteins integrate fully into the structure of the membrane, along with their hydrophobic membrane, ranging from regions interacting with hydrophobic regions of phospholipid bilayer. Typically, single-pass integral membrane proteins possess a hydrophobic transmembrane segment consisting of 20-25 amino acids. Few of these traverses only a portion of the membrane linking with one layer whereas others span from one to another side of the membrane, thereby exposing to the flip side.
Few complex proteins consist of 12 segments of one protein, highly convoluted to be implanted in the membrane. Such a type of protein has a hydrophilic region/s along with one or more mildly hydrophobic areas. This organisation of areas of the proteins has the tendency to align the protein along with phospholipids where the hydrophobic area of the protein next to the tails of the phospholipids and hydrophilic areas of protein protrudes through the membrane is in touch with the extracellular fluid or cytosol.