2.1c Prokaryotic & Eukaryotic Cells

28/08/2012 § Leave a comment

When scientists started studying the cell structure in detail back in the 1950s, they discovered the ultrastructure of the cell, or the fine detailed structure of the cell. Cells can be divided into two types based on their structure, and that there is the topic of our blog post today – the prokaryotic and eukaryotic cell.

First we have the prokaryotes or prokaryotic cells. They were the first organisms to ever evolve on this planet and to this day have the simplest cell structure amongst all organisms. Prokaryotes are small in size, unicellular and are everywhere, including the soil, our skin, water and intestines. Yes, our intestines. Note that bacteria are prokaryotes (single-celled organisms!) and yup, that means there are bacteria living in our intestines. The good part is most of those prokaryotes (usually the bacterium Escherichia coli or E. coli) are harmless. The bad news is not all of them are.

Prokaryotic cells have no nucleus; their name actually means before the nucleus. Prokaryotes divide through binary fission, a process that copies the cell’s DNA and splits it with both copies in the daughter cells. I trust we’ll be studying it later.

Parts of a prokaryotic cell include the following:

  • cell wall: a protective outer layer that protects the cell, is always present, made up of peptidoglycan, maintains its shape, keeps the cell from bursting and general prevents damage to the cell
  • plasma membrane: a layer after the cell wall made up almost entirely of phospholipids, the membrane controls the substance traffic going in and out of the cell by active transport, is partially permeable (allows liquids/gases to pass through it) and can produce ATP by aerobic cell respiration
  • cytoplasm: the fluid filling the space inside the plasma membrane – the filings of the prokaryote – that holds dissolved substances, enzymes and ribosome, does not hold any membrane-bound organelles, takes care of the chemical reactions of the metabolism, and holds DNA in a region called the nucleoid
  • pill: like the hands of the prokaryote; hair-like structures made of protein that is used for cell-to-cell adhesion as well as forming aggregations of cells; can be pushed into or out of the cell wall from which it protrudes; also like a telephone line that cells use when exchanging DNA during conjugation
  • flagella: like the legs of the prokaryote; also structures made of protein in the shape of a corkscrew; moves the prokaryote; is embedded in the cell wall; solid and inflexible
  • ribosomes: small organelles 70S in size; responsible for synthesizing proteins by reading messenger RNA; proteins produced can stay in the cell or leave it
  • nucleoid: the region of the cytoplasm in the prokaryote that holds naked DNA (essentially unguarded DNA unlike the nucleus); circular; total amount vs amount in eukaryotes is less; compared to the rest of the cytoplasm, there are fewer ribosomes – less protein – therefore the nucleoid is stained less

Eukaryotic cells have a far more complicated ultrastructure than prokaryotic cells. In contrast to prokaryotic cells, eukaryotes have nuclei and organelles in its cytoplasm with single or double membranes. Such organelle are the following:

  • nucleus: with a double membrane and pores; holds uncoiled chromosomes called chromatin; holds the genome of the cell; here, DNA is replicated/transcribed and where the messenger RNA pick up its information before moving back into the cytoplasm
  • rough endoplasmic reticulum (rER): made up of flat membrane sacs called cisternae; synthesizes protein through its ribosomes which later is carried off in small sacs that bud off and move to the Golgi apparatus
  • Golgi apparatus: again made up of flat membrane sacs called cisternae; don’t have ribosomes attached to it; processes the proteins brought by the rER’s small sac deliveries; most of these proteins are secreted out of the plasma membrane
  • lysosomes: spherical; single membraned; formed via the Golgi vesicles; under a microscope, are dark shapes because of its high protein concentrations; hold digestive enzymes that can break down anything from food to organelles to the cell itself
  • mitochondria: the power house of the cell; produces ATP through aerobic cell respiration; double membraned; turned and folded into itself to form structures called cristae; fluid inside is called its matrix
  • free ribosomes: lacks membrane; 20 nm in diameter; also synthesize protein for the cytoplasm to use as enzymes; ribosomes are formed in the area of the nucleus called the nucleolus – the brain of the nucleus

That’s a lot to cover and it’s a lot to memorize – to know. The thing I think is most fundamental in this part of the unit is that these cells, whether prokaryotic or eukaryotic, are all so systematic and organized that it’s a wonder how unconsciously intelligent cells are.

Also, it’s important to point out the difference between prokaryotes and eukaryotes:

  • while prokaryotes have naked loops of DNA, eukaryotes have chromatin and chromosomes that hold its DNA
  • while prokaryotes hold their genetic material in the nucleoid, eukaryotes hold theirs inside the nucleus, behind the protective double membrane nuclear envelope
  • while prokaryotes don’t have the power houses (mitochondria), eukaryotes always do
  • while prokaryotes have smaller ribosomes at 70s, eukaryotes‘ ribosomes are 80S (S = the unit of the size of organelles, Svedberg)
  • while prokaryotes have little to no membranes, eukaryotes have countless membranes that protect its organelles

There are also differences between plant and animal cells, both of which are eukaryotes:

  • animal cells have no cell wall but instead a plasma membrane while plant cells greatly rely on their rigid cell walls
  • animal cells don’t require or contain chloroplasts while plant cells again rely on chloroplasts to photosynthesize for food
  • animal cells’ choice of polysaccharides is glycogen while plant cells‘ choice is starch
  • animal cells don’t have vacuoles while plant cells do
  • animal cells are rounded and can change shape while plant cells are rigid and very fixed

That covers the basics of cells. There we have it. I guess.

DATA-BASED QUESTIONS for CHAPTER 1

(1)

  • a. i) eukaryotic, ii) root tip, iii) interphase
  • b. i) 16µm, ii) 12.5mm
  • c. Through osmosis, the cell loses its water because the water moves to the outside solution (the salt water). This decreases the volume of the cytoplasm and as a result, the plasma membrane shrivels up and starts to move away from the cell wall.

(2)

  • a. I: mitochondria, II: nucleus, III: lysosome
  • b. 100x magnification
  • c. 1000µm —> 1mm
  • d. 0.75µm

(3)

  • a. 0.15mm —> 150µm
  • b. actual diameter = 5µm
  • c. It is not an animal because the structure of the siphon clearly indicates, with its chloroplasts and especially with organelles and structures like the vacuole and the cell wall, that Bryopsis pennata is a plant and not an animal.
  • d. Thanks to the textbook answers – the organism is not multicellular because the nuclei are not dividing. It can be unicellular with simply multiple nuclei. It can also be acellular as a small cell with only one nucleus. I may need to clear this up first.
  • e. Having interconnected, pressurized vacuoles, such as the one the Bryopsis pennata possesses, allows the organism to stay completely upright. All the parts of the organism can stand rigid and no parts will wilt and weaken. A disadvantage of having interconnected, pressurized vacuoles is the risk it takes to be just one main system. If some part of the siphons is punctured, then the entire structure will lose its stability.
  • f. Through osmosis, Bryopsis pennata will absorb more water because of the fresh water’s density and will risk bursting the organism. It can also cause the organism to lose its ability to take in mineral ions.

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