Can Glucose Diffuse Through the Cell Membrane by Simple Diffusion?

Sugars need assistance to pass through a cell membrane.
••• Comstock/Stockbyte/Getty Images

Glucose is a six-carbon sugar that is directly metabolized by cells to provide energy. The cells along your small intestine absorb glucose along with other nutrients from the food you eat. A glucose molecule is too large to pass through a cell membrane via simple diffusion. Instead, cells assist glucose diffusion through facilitated diffusion and two types of active transport.

Cell Membrane

A cell membrane is composed of two phospholipid layers in which each molecule contains a single phosphate head and two lipid, or fatty acid, tails. The heads align along the inner and outer boundaries of the cell membrane, while the tails occupy the space in between. Only small, nonpolar molecules can pass through the membrane through simple diffusion. The lipid tails reject polar, or partially charged, molecules, which include many water-soluble substances such as glucose. However, the cell membrane is peppered with transmembrane proteins that provide passage to molecules that the tails would otherwise block.

Facilitated Diffusion

Facilitated diffusion is a passive transport mechanism in which carrier proteins shuttle molecules across the cell membrane without using the cell’s energy supplies. Instead, the energy is provide by the concentration gradient, which means that molecules are transported from higher to lower concentrations, into or out of the cell. The carrier proteins bind to glucose, which causes them to change shape and translocate the glucose from one side of the membrane to the other. Red blood cells use facilitated diffusion to absorb glucose.

Primary Active Transport

The cells along the small intestine use primary active transport to ensure that glucose only flows one way: from digested food to the inside of cells. Active transport proteins use adenosine triphosphate (ATP), the cell’s energy storage molecule, to pump glucose into the cell, either with or against the concentration gradient. The transport proteins are known as ATPase enzymes because they can free a phosphate group from ATP and utilize the resulting energy to do work. Active transport ensures that glucose won’t leak out of small intestine cells during periods of glucose starvation.

Secondary Active Transport

Secondary active transport is another method by which cells import glucose. In this method, a transmembrane protein known as a symporter imports two sodium ions for every glucose molecule it imports. The method doesn't use ATP, but instead relies on the higher concentration gradient of sodium outside the cell relative to the cell interior. The positively charged sodium ions provide electrochemical energy to import glucose with or against the glucose concentration gradient. Secondary active transport is used by cells in the small intestine, heart, brain, kidneys and certain other organs.

Related Articles

Where Does Glucose Reabsorption Occur?
What Are Concentration Gradients in Microbiology?
What Are Three Things That Determine if a Molecule...
How the Plasma Membrane Controls What Goes Into & Comes...
The Most Common Organic Molecules in Cells
Why Do We Need Enzymes for Digestion?
Definition of Cell Surface Proteins
How Do Bacteria Feed?
How Does the Plasma Membrane Maintain Homeostasis?
What Kinds of Molecules Can Pass Through the Plasma...
Difference Between Aerobic & Anaerobic Cellular Respiration...
Where Is Starch Stored in Plant Cells?
Why Can the Surface of the Plasma Membrane Be Described...
How Does Alcohol Kill Bacteria?
10 Facts on Photosynthesis
Examples of Diffusion in Organs
Similarities & Differences Between Osmosis & Diffusion