AP Board Class 9 Biology Chapter 4 Movement of Materials Across The Cell Membrane

AP Board Class 9 students have already learned about cells, its various properties, its different parts and their specific duties in chapter 1. While most of the organisms in the world are made up of cells, students will further increase their understanding of things that go on in cells. They will basically learn how substances pass through cells in chapter 4. The chapter focuses on topics like;

  • Diffusion
  • Osmosis
  • Facilitated Diffusion
  • Active Transport
  • Endocytosis and exocytosis


Diffusion involves the movement of substances from a region of higher concentration to a region of lower concentration. Diffusion is an important process for living things as it helps in moving substances in and out of cells.


Like diffusion, osmosis is the process which involves water moving from an area of higher concentration to a region of lower concentration. Osmosis takes place via semipermeable membrane and only certain molecules are allowed to pass through it.

Facilitated Diffusion

There are instances where water-soluble molecules like amino acids, ions, and monosaccharides, cannot move through the plasma membrane. In order for them to pass these molecules have to diffuse with transport proteins namely channel and carrier proteins. This whole process is termed as facilitated diffusion.

Active and Passive Transport

Whenever substances move across the membrane without any usage of cellular energy, it is known as passive transport. However, the movement of substances across the membrane using ATP energy is known as active transport.

Endocytosis and exocytosis

Endocytosis is basically the process of importing material into the cell by formation of a membrane-bound vesicle. In exocytosis, a cytoplasmic vesicle exports or moves substances from the cytoplasm to the plasma membrane.

Students can read through some important questions below to learn about some of the other chapter topics.

Question 1: Define permeability.

Answer: Permeability is the ability or a property of a cell membrane to allow substances to diffuse across the membrane.

Question 2: Does size and concentration matter in diffusion?

Answer: Yes it matters. Normally, larger molecules are physically blocked from diffusing across membranes whereas molecules cannot diffuse against their concentration gradient by simple diffusion.

Question 3: What are vesicles?

Solution: Vesicles are membrane-bound structures or bubble-like sacs that contain some fluid formed within or outside of the cell.

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Practise This Question

The addition of harmful or toxic chemicals to the soil is called _________.