Cell Membranes


All living things are made of cells, and cells are the smallest units that can be alive. Life on Earth is classified into five kingdoms, and they each have their own characteristic kind of cell. However the biggest division is between the cells of the prokaryote kingdom (the bacteria) and those of the other four kingdoms (animals, plants, fungi and protoctista), which are all eukaryotic cells. Prokaryotic cells are smaller and simpler than eukaryotic cells, and do not have a nucleus.

We'll examine these two kinds of cell in detail, based on structures seen in electron micrographs (photos taken with an electron microscope). These show the individual organelles inside a cell.


Eukaryotic Cells


Prokaryotic Cells



Summary of the Differences Between Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

Eukaryotic cells

small cells (< 5 mm)

larger cells (> 10 mm)

always unicellular

often multicellular

no nucleus or any membrane-bound organelles

always have nucleus and other membrane-bound organelles

DNA is circular, without proteins

DNA is linear and associated with proteins to form chromatin

ribosomes are small (70S)

ribosomes are large (80S)

no cytoskeleton

always has a cytoskeleton

cell division is by binary fission

cell division is by mitosis or meiosis

reproduction is always asexual

reproduction is asexual or sexual



Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years - 2.5 billion years longer than eukaryotic cells. It is thought that eukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells that became incorporated inside larger prokaryotic cells. This idea is called endosymbiosis, and is supported by these observations:



The Cell Membrane

The cell membrane (or plasma membrane) surrounds all living cells. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown in this diagram.

The phospholipids form a thin, flexible sheet, while the proteins "float" in the phospholipid sheet like icebergs, and the carbohydrates extend out from the proteins.

The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane.

The proteins usually span from one side of the phospholipid bilayer to the other (intrinsic proteins), but can also sit on one of the surfaces (extrinsic proteins). They can slide around the membrane very quickly and collide with each other, but can never flip from one side to the other. The proteins have hydrophilic amino acids in contact with the water on the outside of membranes, and hydrophobic amino acids in contact with the fatty chains inside the membrane. Proteins comprise about 50% of the mass of membranes, and are responsible for most of the membrane's properties.

The carbohydrates are found on the outer surface of all eukaryotic cell membranes, and are usually attached to the membrane proteins. Proteins with carbohydrates attached are called glycoproteins. The carbohydrates are short polysaccharides composed of a variety of different monosaccharides, and form a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition, and antigens such as the ABO antigens on blood cells are usually cell-surface glycoproteins.

Remember that a membrane is not just a lipid bilayer, but comprises the lipid, protein and carbohydrate parts.


Transport Across The Membrane

Cell membranes are a barrier to most substances, and this property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment. This is compartmentalization is essential for life, as it enables reactions to take place that would otherwise be impossible. Eukaryotic cells can also compartmentalize materials inside organelles. Obviously materials need to be able to enter and leave cells, and there are five main methods by which substances can move across a cell membrane:

1. Simple Diffusion

A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

2. Osmosis

Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells (its concentration is about 50 M), the diffusion of water has its own name - osmosis. The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.

Water Potential. Osmosis can be quantified using water potential, so we can calculate which way water will move, and how fast. Water potential (Y, the Greek letter psi, pronounced "sy") is a measure of the water molecule potential for movement in a solution. It is measured in units of pressure (Pa, or usually kPa), and the rule is that water always moves by osmosis from less negative to more negative water potential (in other words it's a bit like gravity potential or electrical potential). 100% pure water has Y = 0, which is the highest possible water potential, so all solutions have Y < 0 (i.e. a negative number), and you cannot get Y > 0.

Cells and Osmosis. The concentration (or OP) of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:

The diagram below shows what happens when 2 fresh raw eggs with their shells removed with acid are placed into sucrose solution (hypertonic) and distilled water (hypotonic). Water enters the egg in water (endosmosis) causing it to swell and water leaves the egg in sucrose causing it to shrink (exosmosis).

These are problems that living cells face all the time. For example:


3. Facilitated Diffusion.

Facilitated diffusion is the transport of substances across a membrane by a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. As the name suggests, this is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient. There are two kinds of transport protein:

The rate of diffusion of a substance across a membrane increases as its concentration gradient increases, but whereas lipid diffusion shows a linear relationship, facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate being limited by the number of transport proteins. 

4. Active Transport (or Pumping).

Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP into ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.

The Na+K+ Pump. This transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps. We look at it in more detail in module 4 (A2 course)


5. Vesicles

The processes described so far only apply to small molecules. Large molecules (such as proteins, polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.

Endocytosis is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane, which then pinches shut to form a closed vesicle. Strictly speaking the material has not yet crossed the membrane, so it is usually digested and the small product molecules are absorbed by the methods above. When the materials and the vesicles are small (such as a protein molecule) the process is known as pinocytosis (cell drinking), and if the materials are large (such as a white blood cell ingesting a bacterial cell) the process is known as phagocytosis (cell eating).

Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. Materials to be exported must first be enclosed in a membrane vesicle, usually from the RER and Golgi Body. Hormones and digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands.

Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in by endocytosis at one end of a cell and passing out by exocytosis at the other end.

Summary of Membrane Transport



uses energy

uses proteins



Simple Diffusion N N N N
Osmosis N N Y N
Facilitated Diffusion N Y Y Y
Active Transport Y Y Y Y
Vesicles Y N Y Y

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