Anatomy and Physiology Resource Site

GLOSSARY

1 - Anatomy and Physiology Basics

2 - Cells and Tissues

3 - Cell Physiology

4 - Skin and Membranes

5 - Skeletal System

6 - Muscular System

7 - Nervous System (Part I)

8 - Nervous System (Part II)

9 - Endocrine System

10 - Reproductive Systems

11 - Circulatory System

12 - Respiratory System

13 - Digestive System

14 - Urinary System

15 - Microbiology

16 - Lymphatic System and Immunity

Video Tutorials

3.2 - Cell Transport

In cell transport, substances move across the cell membrane. Solvent molecules (water) and solute molecules both cross the cell membrane, either towards the intracellular space or the extracellular space.

What drives the movement of substances across a cell membrane?

A concentration gradient is the difference in concentration of solutes between two adjacent regions of a solution. In the cell, a concentration gradient is the difference in concentration of solutes between the intracellular an extracellular spaces.

In passive transport, the movement of a substance across the cell membrane is driven by a concentration gradient, and requires no energy input from the cell. In passive transport, a substance moves from an area of higher concentration to an area of lower concentration; in other words moving down their concentration gradient.

In active transport, the movement of a substance across the cell membrane is driven by energy input from ATP. In active transport, a substance moves from an area of lower concentration to an area of higher concentration gradient; in other words moving up their concentration gradient.

Filtration is a special type of cell transport that uses fluid pressure instead of concentration as the driving force for the movement of substances.

Click here for the cell transport tutorial about the different types of passive and active transport.

Passive transport

In passive transport, substances move from an area of higher concentration to an area of lower concentration.

Simple diffusion

Diffusion is the passive movement of a solute from an area of higher concentration to an area of lower concentration. Diffusion will continue until equilibrium is reached, such that the concentration of solutes is equal in both areas. In cells, it is mostly small, lipid-soluble solutes that are capable of simple diffusion across the cell membrane.

Osmosis

Osmosis is a special type of diffusion in which water (solvent molecules) move from an area of higher concentration to an area of lower concentration through membrane proteins called aquaporins (“water pores”).
Osmosis occurs when the concentration of solutes is unequal across the membrane, but the solutes themselves are not able to cross the cell membrane. Osmosis will continue until equilibrium is reached, such that the concentration of solutes is equal on both sides of the membrane.

Facilitated diffusion

Facilitated diffusion is the passive movement of solutes from an area of higher concentration to an area of lower concentration through a membrane channel protein or with the aid of a transport protein. Facilitated diffusion does not require the use of energy by the cell, and is the method of transport for larger molecules that cannot move across the cell membrane by simple diffusion.

Filtration

Filtration involves a fluid called the filtrate being forced across the cell membrane due to fluid pressure. The filtrate contains both solute and solvent, and only large molecules and cells are not contained in the filtrate. Filtration does not depend on the concentration of substances.

Active transport

Active transport involves the movement of solutes from an area of lower concentration to an area of higher concentration with the use of ATP energy. ATP energy is used by either a vesicle or carrier protein to move substances across the cell membrane.

Vesicular transport

Click here for the vesicular transport tutorial

Vesicular transport is the active movement of substances across the cell membrane via a vesicle.
In exocytosis, materials in the intracellular fluid are engulfed by a vesicle, which then secretes (empties) the contents of the vesicle into the extracellular fluid.
In endocytosis, materials in the extracellular fluid are engulfed by a vesicle, which then empties its contents in the intracellular fluid.
Phagocytosis is a special type of endocytosis in which large particles in the extracellular fluid are engulfed by the cell. The vesicle resulting from phagocytosis is then joined by a lysosome, which breaks down the contents of the vesicle into simpler chemical substances. The broken down contents of the vesicle are then released to the intracellular space.
Pinocytosis is a special type of endocytosis in which extracellular fluid is taken into the cell. The fluid is then emptied into the intracellular space.
In receptor-mediated endocytosis, specialized surface receptors on the cell membrane bind with a specific chemical in the extracellular space. A vesicle then forms, taking in only the chemical detected by the receptors.