Cell, or Plasma, membrane

Cell, or Plasma, membrane

Cell, or Plasma, membrane

  • Structure - 2 primary building blocks include

protein (about 60% of the membrane) and lipid, or

fat (about 40% of the membrane).

The primary lipid is called phospholipids, and molecules of phospholipid form a 'phospholipid bilayer' (two layers of phospholipid molecules). This bilayer forms because the two 'ends' of phospholipid molecules have very different characteristics: one end is polar (or hydrophilic) and one (the hydrocarbon tails below) is non-polar (or hydrophobic):

  • Functions include:
    • supporting and retaining the cytoplasm
    • being a selective barrier .
    • transport
    • communication (via receptors)

Related Questions General Physiology

AdenosineTriphosphate (ATP)

  • Animal cells cannot directly use most forms of energy
    • Most cellular processes require energy stored in the bonds of a molecule, adenosine triphosphate (ATP)
    • ATP is referred to as the energy currency of the cell

It is a nucleotide, formed from:

  • the base adenine (the structure with 2 rings),
  • the 5 carbon sugar deoxyribose (one ring)
  • 3 phosphates

Energy is stored in the bonds between the phosphates and is released when the bonds are broken

Biological Functions are Extremely Sensitive to pH

  • H+ and OH- ions get special attention because they are very reactive
  • Substance which donates H+ ions to solution = acid
  • Substance which donates OH- ions to solution = base
  • Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
    • pH = - log [H+]
    • [H+] is the H ion concentration in moles/liter
    • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
    • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
      • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times
  • Human blood pH is 7.4
    • Blood pH above 7.4 = alkalosis
    • Blood pH below 7.4 = acidosis
  • Body must get rid of ~15 moles of potential acid/day (mostly CO2)
    • CO2 reacts with water to form carbonic acid (H2CO3)
    • Done mostly by lungs & kidney
  • In neutralization H+ and OH- react to form water
  • If the pH changes charges on molecules also change, especially charges on proteins
    • This changes the reactivity of proteins such as enzymes
  • Large pH changes occur as food passes through the intestines.

Nucleic Acids:

  • Two major types: DNA
  • RNA (including mRNA, tRNA, & rRNA) 
    • Both types have code which specifies the sequence of amino acids in proteins
    • DNA = archival copy of genetic code, kept in nucleus, protected
    • RNA = working copy of code, used to translate a specific gene into a protein, goes into cytoplasm & to ribosomes, rapidly broken down
  • Nucleic acids are made of 5 nucleotide bases, sugars and phosphate groups
  • The bases make up the genetic code ; the phosphate and sugar make up the backbone
  • RNA is a molecule with a single strand
  • DNA is a double strand (a double helix) held together by hydrogen bonds between the bases
    • A = T; C= G because:
      • A must always hydrogen bond to T

C must always hydrogen bond to G

DNA (Deoxyribonucleic acid) - controls cell function via transcription and translation (in other words, by controlling protein synthesis in a cell)

Transcription - DNA is used to produce mRNA

Translation - mRNA then moves from the nucleus into the cytoplasm & is used to produce a protein . requires mRNA, tRNA (transfer RNA), amino acids, & a ribosome


tRNA molecule

  • sequence of amino acids in a protein is determined by sequence of codons (mRNA). Codons are 'read' by anticodons of tRNAs & tRNAs then 'deliver' their amino acid.
  • Amino acids are linked together by peptide bonds (see diagram to the right)
  • As mRNA slides through ribosome, codons are exposed in sequence & appropriate amino acids are delivered by tRNAs. The protein (or polypeptide) thus grows in length as more amino acids are delivered.
  • The polypeptide chain then 'folds' in various ways to form a complex three-dimensional protein molecule that will serve either as a structural protein or an enzyme.

Lipids:

  • about 40% of the dry mass of a typical cell
  • composed largely of carbon & hydrogen
  • generally insoluble in water
  • involved mainly with long-term energy storage; other functions are as structural components (as in the case of phospholipids that are the major building block in cell membranes) and as "messengers" (hormones) that play roles in communications within and between cells
  • Subclasses include:
    • Triglycerides - consist of one glycerol molecule + 3 fatty acids (e.g., stearic acid in the diagram below). Fatty acids typically consist of chains of 16 or 18 carbons (plus lots of hydrogens).
    • phospholipids - Composed of 2 fatty acids, glycerol, phosphate and polar groups , phosphate group (-PO4) substitutes for one fatty acid & these lipids are an important component of cell membranes

steroids - have 4 rings- cholesterol, some hormones, found in membranes include testosterone, estrogen, & cholesterol