Cell Biology

Membrane is any delicate sheet that separates one region from other blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered a membrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as membrane too.

Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable.

An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvent, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are still selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent let only some specific solutes to pass blocking others.

Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to other region where it is less concentrated. For example, during the boiling of water in a kitchen gaseous water particles tend to uniformly spread in the air by diffusion.

Concentration gradient is the difference of concentration of a substance between two regions.

Concentration is a term used to designate the quantity of a solute divided by the total quantity of the solution. Since water in general is the solvent in this situation it is not correct to refer to “concentration of water” in a given solution.

Osmosis is the phenomenon of movement of solvent particles, in general water, from a region of lower solute concentration to a region of higher solute concentration. Diffusion, in the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.

One can consider osmosis as movement of water (solvent) and diffusion as movement of solutes, both concentration gradient-driven.

The osmotic pressure of a solution does not depend on the nature of the solute, it depends only on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles even containing different solutes exert same osmotic pressure.

Even when the solution contains a mixture of different solutes its osmotic pressure depends only on its total particle concentration regardless the nature of the solutes.

The cell membrane is a selectively permeable membrane, i.e., it allows the passage of water and some selected solutes.

Cell Membrane Review - Image Diversity: cell membrane

Comparatively to other a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).

When a solution is less concentrated than other the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have same concentration both receives the adjective isotonic. So this classification makes sense only for comparison of solutions.

The plant cell when placed under hypertonic medium loses a great amount of water and its cell membrane detaches from the cell wall. In that situation the cell is called plasmolysed cell. When the plasmolysed cell is placed under hypertonic medium it absorbs water and becomes a turgid cell. This phenomenon is called deplasmolysis.

Substances that maintain highly hypertonic environment, like sugar and salt, are used in the production of dried meat, fish or fruits (for example, cod) because the material to be conserved is then dehydrated and the resulting dryness prevents the growth of populations of decomposer beings (since these beings also lose water and die).

Cytoskeleton is the cytoplasmic structure that supports the cell, keeps its shape and fixates and moves the cell organelles. It is made of an extensive network of fibers dispersed in the cytoplasm and anchored in the plasma membrane. Its components are microtubules, microfilaments and intermediate filaments.

Cell Skeleton and Cell Movement - Image Diversity: the "cell skeleton"

Microtubules are made of consecutive dimers of the protein tubulin (each dimer has an alpha and a beta tubulin associated). Microtubules participate in cell division, they are constituents of cilia and flagella and they also form the centrioles.

Cytoskeleton and Cell Movement - Image Diversity: microtubules tubulin

Passive transport is the movement of substances across membranes in favor of their concentration gradient, i.e., from a more concentrated region to a less concentrated region. Active transport, in the other hand, is the transport of substances across membranes against their concentration gradient, from a less concentrated to a more concentrated region. In passive transport, because it is spontaneous, there is no energy spending; the active transport however requires energy (work) to occur.

Active transport is a work to maintain or increase the concentration gradient of a substance between two regions while passive transport acts in a manner to reduce the concentration gradient.

The three main types of passive transport are simple diffusion, osmosis and facilitated diffusion.

Cell Membrane Review - Image Diversity: passive transport

Simple diffusion is the direct passage of substances across the membrane in favor of their concentration gradient. In facilitated diffusion the movement of substances is also in favor of their concentration gradient but the substances move bound to specific molecules that act as “permeabilizers”, i.e., facilitators of their passage through the membrane.

Cell Membrane Review - Image Diversity: facilitated diffusion

One of the main examples of facilitated transport is the entrance of glucose from the blood into cells. Glucose from blood binds to specific permeases (hexose-transporting permeases) present in the cell membrane and by diffusion facilitated by these proteins it enters the cell to play its metabolic functions.

Facilitated diffusion resembles chemical catalysis because the transported substances bind to permeases like substrates bind to enzymes and in addition after one transport job is concluded the permease is not consumed and can perform successive other transports.

The higher the concentration gradient of a substance the more intense its simple diffusion will be. If the concentration gradient diminishes the intensity of simple diffusion diminishes too.