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  Functions of the Plasma Membrane, Biology tutorial

Functions of cell membrane:

Cell membrane (also called as plasma membrane or cytoplasmic membrane) is biological membrane which separates interior of all cells from outside environment. Cell membrane is selectively permeable to ions and organic molecules and manages movement of substances in and out of cells.

Cell Transport:

Cell's plasma membrane doesn't simply form the sack in which to keep all cytoplasm and other cellular organelles. Plasma membrane is very significant structure that functions to permit certain substances to enter or leave cell. It can pump other substance in cell against concentration gradient or pump other wastes etc. out of cell. Some of transport process occurs passively without cell needing to expend any energy to make them take place.

Passive Transport Processes:

a) Diffusion is movement of ions or molecules from regions of higher concentration to regions of lower concentration.

Passive procedures - need no expenditure of energy by the cell:

i) Simple diffusion = net movement of the substance from area of high concentration to area of low concentration. Rate of diffusion is influenced by:

  • concentration gradient
  • cross-sectional area by which diffusion occurs
  • temperature
  • molecular weight of a substance
  • distance through which diffusion occurs

ii) Osmosis = diffusion of water across the semipermeable membrane (like cell membrane) from area of low solute concentration to the area of high solute concentration.

Diffusion and Plasma Membrane:

Plasma membrane will permit certain substances to cross it but not others. Such a membrane is referred to as selective permeable. Plasma membrane's permeability depends on the large part on its makeup. Both protein portion and phospholipid portion of membrane are engaged in permeability.

Thus three characteristics of a molecule that determine the permeability of the membrane to that species are:

1) Polarity - (Hydrophobic vs Hydrophylic)

2) Charge - (charged vs uncharged)

3) Size - (large vs small)

Active Transport:

Active Transport pumps materials across membrane against concentration gradient. I.e. from low concentration to high concentration thus needs energy. Movement of the substance across the cell membrane from area of low concentration to area of high concentration using carrier molecule

Animal Cells:

Red Blood Cell (RBC) that has internal solute concentration of approx 0.9% salt (equivalent) and place it in different solutions of varying salt and concentrations.

The solution which loses water is Hypotonic that solution which gains water is Hypertonic. One solution is always compared to other solution. Inside of cell is Hypotonic to outside if cell (is cell shrinks). Outside of cell is Hypertonic to inside (i.e. cell shrinks) etc. Isotonic is a special equilibrium case where there is no net movement of water.

Plant Cell Turgor Pressure:

Plant cells contain one extra structure enclosing it, that animal cells lack cell wall. If plant cells are exposed to same situations as RBC's osmotic reactions are same but cell wall avoids swelling and rupture.

Facilitated Diffusion:

This is like simple diffusion in sense that it is diffusion (across membrane) from the high concentration to lower concentration. Though, this time rate of diffusion is really accelerated by action membrane proteins which act as carrier molecules and help in diffusion. These "carrier proteins" are called as Permeases.  Simple diffusion can also be achieved by passage of solutes by tunnel-like transmembrane proteins known as channel proteins.

Endomembrane System in Eukaryotic Cells:

Cytoplasm of the cell is enclosed by the cell membrane or plasma membrane. Plasma membrane in plants and prokaryotes is generally covered by cell wall. This membrane serves to divide and protect the cell from its surrounding environment and is prepared generally from the double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Therefore, layer is known as phospholipid bilayer. It may also be known as fluid mosaic membrane. Embedded within this membrane is the range of protein molecules which act as channels and pumps which move different molecules in and out of cell. Membrane is said to be semipermeable, in that it can either allow a substance (molecule or ion) pass through freely, pass by to limited extent or not pass by at all. Cell surface membranes also have receptor proteins which permit cells to detect external signaling molecules like hormones.

Cytoskeleton:

Cytoskeleton organize and maintains cell's shape; anchors organelles in place; assists during endocytosis, uptake of external materials by the cell, and cytokinesis, separation of daughter cells after cell division; and moves parts of cell in procedures of growth and mobility. Eukaryotic cytoskeleton is made up of microfilaments, intermediate filaments and microtubules. There is great number of proteins related with them, each controlling cell's structure by bundling, directing, and aligning filaments. Prokaryotic cytoskeleton is less well-studied but is involved in maintenance of cell shape, polarity and cytokinesis.

Genetic material:

Two different types of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but few viruses (e.g., retroviruses) contain RNA as their genetic material. Biological information contained in the organism is encoded in DNA or RNA sequence. RNA is also utilized for information transport (like mRNA) and enzymatic functions (like ribosomal RNA) in organisms which use DNA for the genetic code itself. Prokaryotic genetic material is ordered in the simple circular DNA molecule (bacterial chromosome) in nucleoid region of cytoplasm. Eukaryotic genetic material is separated into different, linear molecules known as chromosomes inside discrete nucleus, generally with extra genetic material in few organelles like mitochondria and chloroplasts.

The human cell contains genetic material contained in cell nucleus (the nuclear genome) and in mitochondria (the mitochondrial genome). In humans nuclear genome is separated in 23 pairs of linear DNA molecules known as chromosomes. Mitochondrial genome is the circular DNA molecule distinct from nuclear DNA. Though mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins engaged in mitochondrial energy production and specific tRNAs.

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