Chapter 3 Transport across plasma membranes

Review Questions: Chapter 3 Q1, 3-10, 12, 14-18

Basic Diffusion

The Chemical & Physical Structure of Cell Membranes

The cell membrane (or plasma membrane) surrounds all living cells, and is the cell's most important organelle. 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,

Cell membrane structure 1.gif

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 (integral proteins), but can also sit on one of the surfaces (peripheral 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.
  • Proteins that span the membrane are usually involved in transporting substances across the membrane
  • Proteins on the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape, or in cell motility. They may also be enzymes, catalysing reactions in the cytoplasm.
  • Proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding then triggers other events in the cell. They may also be involved in cell signalling and cell recognition, or they may be enzymes, such as maltase in the small intestine.
  • The carbohydrates are found on the outer surface of all eukaryotic cell membranes, and are attached to the membrane proteins or sometimes to the phospholipids. Proteins with carbohydrates attached are called glycoproteins, while phospholipids with carbohydrates attached are called glycolipids. 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.

Cut up the diagrams in the following document and paste onto an A4 sheet to illustrate the key components of a plasma cell membrane. Annotate this diagram .

membrane proteins diag.JPG

Fns of Plasma Proteins.JPG

Diffusion, Osmosis and Selectively Permeable Membranes

Petes Class Results Diffusion Across a SPM


Most people had results as in the above photo.
In the Starch condition : a blue black colour developed within the bag, none developed outside the bag.
In the Glucose condition: after a period of time glucose was detected outside the bag as indicated by the brown colour on the glucose test strip.
In both conditions you may have detected a change in the level of the respective solutions in the thistle funnel.This is mainly due to the movement of water molecules into and out of the bag. If you made such observations please include these in your results and discuss the reasons for any movement in your discussion.

Movement across cell membranes

Cell Membrane animation

Cellular Respiration Animation

Cell Division : Mitosis


Mitosis online quiz