7.2 Cell StructureLearning Outcomes:2. Describe basic cell structure and explain their functions.3. Distinguish among bacteria, viruses and eukaryotic cells.

Readings: 2: 42-46, 3: 66-69, 18: 467-476Recall the observations of Anton van Leeuwenhoek and Robert Hooke who first observed microscopic organisms. They gave us the cell theory: all livings things are composed of cells.

Every cell consists of a plasma membrane surrounding internal fluids or cytoplasm and a form of DNA. All cells also have a mechanism for making proteins involving tiny ribosomes.Prokaryotic and eukaryotic cells share these characteristics in common but have many more differences.

Archaea Bacteria Eukarya Cell structure Prokaryotic cells

Domains Bacteria and Archaea

Eukaryotic cellsDomain Eukarya

size 1- 10 m 10 – 100 m

Location and in cytoplasm contained inside nucleus

"In the morning I used to rub my teeth with salt and rinse my mouth with water and after eating to clean my molars with a toothpick.... I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving. The biggest sort had a very strong and swift motion, and shot through the water like a pike does through the water; mostly these were of small numbers." -Anton van Leeuwenhoek

arrangement of genetic information

arranged in a circular chromosome and small circular plasmids

DNA strands are coiled around histone proteins and highly condensed into chromosomes

Internal structuresribosomes

MicrotubulesEndoplasmic

reticulumGolgi apparatus

Mitochondria

Chloroplasts

√someNoNoNo, but may have same enzymes in cytoplasm

No, if photosynthetic, perform in cytoplasm

√√ for cytoskeleton√ for processing proteins√ for further processing proteinsMost have except few anaerobic protists Perform energetic rxns in cytoplasm involving oxygenPlantae, some protists

External structures 1. Flagella some some

2. Plasma membrane

Protein carriers/channels in membranes Water enters cells through “aquaporins” (small pores in the plasma membrane made of proteins).Other small molecules (eg. glucose) can also be transported by protein carriers through membranes.

All cells have a plasma membrane made of phospholipids. In membranes, lipids are arranged in a double layer and only allow some small uncharged molecules to pass through (eg. CO2) (Fig 3.11a).

Charged molecules (eg. Na+, sodium ions) must pass through ion channels which are also composed of proteins sitting in the membrane and have specificity to only allow specific ions through.Facilitated diffusion (Fig 3.11b) means no energy is required to move some molecules as movement goes along the molecules’ concentration gradient (from an area of high concentration where there are many of these molecules to an area of low conc.) Active transport (Fig 3.12) means energy must be expended to move a molecule against its concentration gradient.Large molecules enter or exit cells through vesicles by endo or exocytosis. (endo = in

Cell structure Prokaryotic cells Eukaryotic cells3. Cell wall

4. External Outer membrane

-differing compositions of carbohydrate in wall

Found in Gram - bacteria (in Domain Bacteria)

Found in Plantae, Fungi, some protists, with differing major components (eg. cellulose, chitin) Not in animal cells

none

Bacterial membranes and cell walls:

Note: Peptidoglycan is a carbohydrate. Lipopolysaccharides are, as the name suggests, lipids with sugars (carbohydrates) linked to them. In pathogenic bacteria these lipolysaccharides often contain toxins.Prokaryotes divide by binary fission

Circular chromosome is copied into two.

Two copies separate

Cell grows larger and divides in two with one identical chromosome in each new cell.

Also shown in Fig 18.2

Bacteria can have a form of sex by sharing plasmids

Viruses Fig 18.4

-consist of genetic material (DNA or RNA) and a protein coat (capsid) They may or may not have an additional covering called an envelope made of more protein.-require a living cell to make more of themselves

Viral infection ends when cells of the immune system are able to recognize and degrade viruses.Viruses can sometimes incorporate into a cell’s DNA and stay for many cell divisions/years (lysogenic cycle). Such viruses are latent. A certain environmental signal can cause virus

production to suddenly begin Eg.: chickenpox virus can become active in nerve cells of adults (who once had chicken pox) causing “shingles” decades later-Viruses usually only recognize very specific cell types egs:

1. inject DNA

2. take over cell’s machinery for making DNA and proteins

3. make many copies of viral DNA and viral coat proteins assemble into viruses

4. cell lyses (breaks open) viruses are released and spread to nearby cells

Note: This form of genetic transfer allows bacteria (often of different species) to share genes. E coli 0157 (pathogenic strain of E. coli that killed the people in Walkerton Ontario) make virtually the same toxin as found in Shigella dysenteriae, the organism which causes dysentery. It is thought that S. dysenteriae transferred the gene to make the toxin to E. coli through a plasmid.

- bacteriophage specific sp. of bacteria. -adenoviruses cells of lungs and respiratory systemPrions (Fig 18.6)

Normally when proteins in a cell are misfolded they are either refolded by proteins called chaperones or destroyed (in lysosomes).What makes prions different from other pathogens (viruses, bacteria, eukaryotic pathogens) is that they do not carry any genetic material (DNA or RNA) with them. Previously it was thought that all pathogens needed to have their genetic material with them in order to multiply. The normal form of prions (PrPc) is found in many nerve cells and like other proteins it is coded for by a gene. QUESTION: Where is the PrP gene found?

Questions to Consider:1. Although mutations are rare they happen frequently enough for species to evolve. Because bacterial species reproduce so quickly, a mutation which provides some advantage (i.e. allows a bacterium to resist an antibiotic, become more infectious or pathogenic etc) will increase quickly in a population. Besides the quick reproduction rate of bacteria what other feature assists their quick evolution (gaining new advantageous inherited characteristics)?

2. Why could it be advantageous for a virus to remain dormant for some time rather than reproducing and causing cells to lyse rapidly?

3. Antibodies in your immune system recognize specific shapes of foreign macromolecules (proteins, carbohydrates, lipids and combinations of all three) that are different from any shape of self macromolecules (normally found in the host’s body). Based on what you know now about bacterial cells, what components of bacterial cells may be recognized by antibodies. Hint: what structures or molecules do bacteria have that are different from eukaryotic cells? (FYI: we will talk about how antibodies work in greater detail next week)

4. In order to evade the immune system some pathogens get inside the cells of the host organism. They usually do not initially kill the cells they enter. Considering what you know about how molecules enter cells, how might pathogens get inside?

5. When you get a cold, you have been infected with a virus that infects cells of your airways (bronchial passages and lungs). A virus which infects a lung cell will have the cell make many

A prion is a misfolded protein which in brain cells causes spongiform encephalopathy (holes in the brain tissue) The word comes from proteinaceous infectious particle. It seems to be one specific protein which, when misfolded, can cause spongiform encephalopathy. The normal form of the protein is PrPC and the misfolded disease-causing form is PrPSC. It is not known what the normal protein does, although it is found in many nerve cells.

One misfolded protein can stimulate normal proteins to become misfolded themselves (Fig 18.6). This is the only protein known to have this infectious capability. Misfolded proteins build up in nerve cells causing the cells to stop functioning

copies of the virus and then the cell bursts and the viruses spread to nearby cells. Aside from mucus, what is in a tissue when you blow your nose?