The cell membrane is the border security of a cell. Understanding transport across the cell membrane is crucial for grasping how cells interact with their environment. Use this resource to learn about the fluid mosaic model and the different transport methods.
The fluid mosaic model
The structure of the cell membrane can be described using the fluid mosaic model. This model represents the membrane as a dynamic, flexible layer composed of a phospholipid bilayer with embedded proteins.
An interactive three-dimensional model of the cell membrane.
Cell membrane model:
Phospholipid bilayer: Represented by the two layers of yellow spheres (hydrophilic heads) with yellow tails (hydrophobic tails) in between.
Integral proteins: Shown as large, varied-coloured structures (green, blue) spanning the membrane.
Peripheral proteins: Depicted as smaller coloured blobs (orange, grey, purple) on the membrane surface.
Glycoproteins: Illustrated by the red hexagon chains attached to a protein.
Glycolipids: Shown as blue and red chains on the lipid surface.
Cholesterol: Depicted as grey ovals integrated into the bilayer.
Phospholipids are molecules that have a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail. "Hydro" means "water", "philos" means loving and "phobos" means "hating".
They are arranged in a way that creates a semi-permeable barrier. This means that the membrane selectively allows substances to move in and out of the cell. Within this bilayer, proteins float like boats on a sea, facilitating communication and transport of molecules.
It is called the "fluid" because the lipid and protein molecules can move sideways within the layer, giving the membrane flexibility and allowing it to self-heal. The "mosaic" part refers to the patchwork of proteins that drift in or out of the bilayer, providing a range of functions like transport, structural support and cell recognition.
The nature of the cell membrane lets the cell interact with the environment, adapt to changes and perform functions that are essential for life.
Transport across the cell membrane
There are many different ways for materials to be transported into and out of a cell. Let’s look at simple diffusion, facilitated diffusion and active transport.
Simple diffusion
Simple diffusion is a passive transport process, meaning that it doesn’t use any energy from the cell. In this process, molecules move from an area of higher concentration to an area of lower concentration until they are evenly spread out, or when "equilibrium" is reached.
An interactive three-dimensional animated model of the simple diffusion.
Simple diffusion model
Phospholipid bilayer: Represented by the two layers of yellow spheres (hydrophilic heads) with yellow tails (hydrophobic tails) in between.
Molecules moving: Green spheres present on either side of the bilayer representing substances moving across the cell membrane. The region above the bilayer has more green spheres than the region below. Green spheres from the region above are shown passing through the bilayer to the region below.
This type of diffusion relies on a difference in concentration on the inside and outside of a cell. It happens without help from proteins.
Examples of substances that are transported across the cell membrane by simple diffusion are the gases oxygen and carbon dioxide.
Facilitated diffusion
Facilitated diffusion is also a type of passive transport. But unlike simple diffusion, it requires some help from special proteins. These transport proteins act as gateways (or channels) to let molecules move from an area of higher concentration to an area of lower concentration.
As well as protein channels, diffusion can be facilitated by carrier proteins, which bind to molecules and release them on the other side of the membrane.
An interactive three-dimensional animated model of the facilitated diffusion.
Facilitated diffusion model
Phospholipid bilayer: Represented by the two layers of yellow spheres (hydrophilic heads) with yellow tails (hydrophobic tails) in between.
Protein channels: Depicted as pink, orange and blue structures spanning the bilayer, forming channels.
Molecules moving: Green, purple and red spheres present on either side of the bilayer representing substances moving across the cell membrane.
The region below the bilayer has more green spheres than the region above. Green spheres from the region below are shown passing through the blue protein channel to the region above.
The region above the bilayer has more red spheres than the region below. Red spheres from the region above are shown passing through the pink protein channel to the region below.
The region above the bilayer has more purple spheres than the region below. Purple spheres from the region above are shown passing through the orange protein channel to the region below.
This process allows substances to move in and out of a cell efficiently, even if they cannot pass through the bilayer on their own.
Examples of substances that are transported across the cell membrane by facilitated diffusion are glucose and ions.
Active transport
What about when substances need to move against a concentration gradient to cross the cell membrane? How do substances move from a region of lower concentration to a region of higher concentration? This is where active transport comes in.
Active transport requires energy, usually in the form of adenosine triphosphate (ATP), which is the energy currency of a cell. It works against the natural flow of molecules. Specialised carrier proteins in the cell membrane, called pumps, facilitate this movement.
An interactive three-dimensional animated model of the active transport through a uniporter protein.
Active transport model
Phospholipid bilayer: Represented by the two layers of yellow spheres (hydrophilic heads) with yellow tails (hydrophobic tails) in between.
Carrier protein: Depicted as a green structure spanning the bilayer, forming a channel.
Molecule moving: Shown as a blue sphere moving through the green structure.
This animation shows one type of active transport where one molecule is moved across the cell membrane in one direction. The type of carrier protein that facilitates this is called a uniporter ("uni" means "one" and "porter" means "to carry"). There are also symporters which transport two molecules across the cell membrane at a time in one direction, and antiporters which transport two molecules across the cell membrane in opposite directions.
Examples of substances transported across the cell membrane by active transport are sodium and potassium ions.
Exercise
See how well you understand transport across the cell membrane with a quick quiz.
