Introduction to Cell Structure & Function

I. Introduction

• Cell biology is a young science
• resolution of investigative "instruments"
  
• Cell fractionation

II. Cell Size and Shape

A. Most cells lie in a certain size range

1. Lower limit example:

Dialister pneumosintes

free-living bacterium, 0.5 X 0.5 X 1.6 um (may be smaller under certain conditions)

probably near lower size limit for independent biological activity-

Each cell: 2.8 X 10^-14 g dry weight (assume 75% water):

Of this mass: 3.9% is DNA, 10.1% is RNA, 43% is protein, 16.8% is carbohydrate and 21.8% is lipid.

Rule of Thumb:

You need 20X the mass (expressed in Daltons) of DNA for the protein for which it provides information (explain later where this comes from)

So, 3.9% of the dry weight is 1.09 X 10^-15 g or 6.5 X 10⁸ Daltons of DNA (1 dalton = mass of a hydrogen atom or 1.67 X 10^-24 g)

Use the rule of thumb and get 0.33 X 10⁸ Da of different proteins

Assume that the average MW of a bacterial protein is approx 40,000 Da

So, 0.33 X 10⁸/4 X 10⁴ = 813 different proteins in the cell.

If you assume that a few of these proteins are structural, this means that Bacteroides has approximately 800 enzymes to catalyze an equal number of chemical reactions.

The Rule of Thumb Explained:

Average protein = 40,000 Da

MW of amino acid = 100 Da

So, each protein has 400 amino acids

Each amino acid requires 3 nucleotides to code for it in DNA

So, 400 amino acids X 3 = 1200 dinucleotides of DNA

(Why is it dinucleotides?)

MW of a dinucleotide of DNA = 660 Da

So, 1200 X 660 = 792,000 Da X 813 = 6.5 X 10⁸ Da

• Exception to lower limit rule:

Mycoplasma has 5 X 10⁸ Da of DNA (How many 40,000 Da proteins does this code for?)

• Vaccinia: 1.6 X 10⁸ Da
• Lambda phage: 0.32 X 10⁸ Da
• Viroids: 0.0013 X 10⁸ Da
  

2. Upper limit examples:

• relationship between the nucleus and the rest of the cell
  
• characteristic of large cells?
  

a. multinucleate cells

• Pelomyxa or Chaos (the giant amoeba) 5mm in diameter
• Nitella (a green algae) 10,0000 um in diameter, protoplasts 0.3 mm diameter
  

b. active cytoplasmic streaming

c. surface to volume ratio

• as cells get larger, the smaller the s/v ratio
• Why might smaller s/v ratios be a problem for a cell?
  
• Solutions (microvilli; neuron)
• s/v ratios and cell division.
• Practise exercises
  

B. Procaryotes and Eukaryotes

1. eukaryotic cells compared with prokaryotic cells

Summary Chart

III. Brief Review of Eukaryotic Cell Structure and Function

A. Overall organization of plant and animal cells

B. Structure/Function of Cell Components

1. Membranes & Cell Coverings

• plasma membrane function
  
• What are the main components of cell membranes?
  
• What does the composition of a membrane tell you in general about permeability into a cell?
  
• Animal cell glycocalyx

• What is the purpose of the microvilli?
  
• specialized cell junctions
  

-adhesive junctions (desmosomes)

-tight junctions

-gap junctions

• Cell Coverings in Plants

-cell wall

-plasmodesmata
-middle lamella

2. Endoplasmic reticulum (ER)

• Part of an extensive internal membrane system characteristic of eukaryotic cells.
  
• Types of ER

        -rough (RER)
        -smooth (SER)

• Functions

3. Golgi Apparatus

• Part of the internal membrane system
• Distinctive structure
• Functions
  

4. Nuclear envelope

• Two membrane surrounding chromosomes
  
• Comes from ER
• Contains "pores"
  
• What sort of molecules move between nucleus and cytoplasm?
  

5. Cytoplasmic matrix

What is the matrix?

What does the matrix do?

The cytoplasmic matrix has both a viscous flow like a liquid and elastic deformation like a solid (matrix near plasma membrane is more solid and in the cell center more liquid). Why?

6. Cytoskeleton

What is the cytoskeleton made of?

What does it do?

Examples

Membrane Bound Organelles

7. Nucleus

• site of storage & replication of most cells hereditary material
• contains chromosomes
• contains nucleoli 
  
• the degree of DNA packaging in chromatin depends on the state of the cell
  
• single large DNA molecule in each chromosome many times longer than the chromosome
  
• How is this compaction accomplished?
  

8. Lysosomes

• single membrane bound organelle containing digestive enzymes
• produced from the Golgi Complex
  
• Functions
  

9. Peroxisomes

• single membrane bound organelle produced from vesiculation of the ER; most enzymes added through post-translational translocation.
  
• Functions

10. mitochondria

• organelles has distinctive system of inner and outer membranes
• associated with ATP synthesis through oxidative phosphorylation
• bacteria-like properties
  

11. Chloroplast

• Organelles found in plants involved with ATP synthesis through photophosphorylation and the creation of reduced carbon compounds through the Calvin Cycle (photosynthesis)
• Energy for photosynthesis comes from thylakoid membranes which contain specialized structures designed to maximize the absorption and retention of light energy
• Bacteria-like properties
  
• Do you think that plants have mitochondria?