AP Biology: The Traffic Across Membranes

AP Biology: Fluids and molecules are able to pass through the plasma membrane. The ability of molecules to move across the cell membrane depends on two things: semipermeability of the plasma membrane and the size and charge of the particles that want to pass through. Get more information on the AP Biology Exam here.

Cells must be selective in what they allow to cross their membranes. This ensures that the internal environment is maintained. Since the plasma membrane is composed primarily of phospholipids, lipid-soluble substances cross the membrane without any resistance. This is because “like dissolves like”. The lipid membrane has an ‘open-door policy’ for substances that are made up of lipids. These substances can cross the plasma membrane without any problem. However, if a substance is not lipid-soluble, the lipid bilayer won’t let it in.

The only exception is water. Although water molecules are polar (not lipid-soluble) they can rapidly cross a lipid bilayer (a rate of about 3 billion water molecules a second) through aquaporins, which are integral membrane proteins that regulate the flow of water.


Lipid-soluble substances can pass through the plasma membrane without much difficulty. However, the direction of the flow is determined by various factors.

Some substances move across a membrane by simple diffusion when there is a concentration gradient. If there is a high concentration of a substance outside the cell and a low concentration inside the cell, the substance will move into the cell down the concentration gradient.

It is like riding a bike downhill. The bike flows naturally. Another name for this is passive transport. Therefore, simple diffusion does not require energy.

Diffusion that involves the movement of water is called osmosis.

Facilitated Transport

What about lipid-insoluble substances? These substances (solutes) are dissolved in the fluid on either side of the cell membrane. They rely on proteins embedded in the plasma membrane. Special proteins, called channel proteins, can help lipid-insoluble substance get in or out.

The proteins pick up substances from one side of the membrane and carry it across to the other. This is known as facilitated transport or facilitated diffusion. It is similar to simple diffusion as the flow of the substance occurs down the concentration gradient. Therefore, it doesn’t require any energy.

Active Transport

How does a substance travel in the opposite direction, from a region of lower concentration to a region of higher concentration? This process requires a transport protein, which helps transfer the substance across the plasma membrane. Also, the process requires energy as the process is like riding a bike uphill. Movement against the natural flow is called active transport.

Some proteins in the membrane are powered by ATP. An example of this is the protein called sodium-potassium pump. The pump transports sodium ions (Na+) out of the cell, and brings in potassium ions (K+). These pumps require ATP to transfer ions across the membrane against the concentration gradient.


When particles are too large to enter a cell, the cell uses a portion of the cell membrane to engulf a substance. The cell membrane forms a pocket, pinches in, and eventually forms a vacuole or a vesicle. This is called endocytosis.

There are three types of endocytosis: pinocytosis, and receptor-mediated endocyctosis.

-          Pinocytosis – The cell ingests liquids

-          Phagocytosis – The cell takes in solids

-          Receptor-mediated endocytosis – Ligands bind to cell surface receptors that covered in clathrin-pits, which causes the cell membrane to fold inwards. A vesicle them forms around the incoming ligand and carries it into the cell’s interior.

Bulk Flow

Bulk flow is the one-way movement of fluids brought about by pressure. For example, the movement of blood through a blood vessel or movement of fluids in xylem and phloem of plants occur as a result of bulk flow.


Dialysis is the diffusion of solutes across a selectively permeable membrane. For example, a cellophane bag is often used as an artificial membrane to separate small molecule from large molecules.


In exocytosis, a cell ejects waste products or specific secretion products (such as hormones) by the fusion of a vesicle with the plasma membrane.

AP Biology Study tips:

The best way to learn the different processes is to draw a table or a spider diagram. Also, remember to include examples for each process.