Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1
Kinds of Cells and Their Measurement.
An ostrich egg is a single cell. So is Mycoplasma. One is the largest cell there is, the other the smallest. Between them, how do we sort cells into kinds, and in which units do we measure them? That is what this note is about.
1. Sorted by job: somatic and reproductive cells
On the basis of physiological function, cells are of two kinds: somatic cells and reproductive cells (gametes). Cells that build the organs and organ systems of the body are somatic. In higher organisms they are usually diploid. Cells of root, stem and leaf, nerve cells and blood cells are all somatic.
For sexual reproduction, a diploid organism makes haploid cells by meiosis in its reproductive organs. These are the reproductive cells, or gametes. Sperm and egg are examples. One thing to hold on to: a gamete is always haploid.
2. Sorted by nucleus: the prokaryotic cell
Now sort by how the nucleus is organised. On this basis cells again fall into two kinds: the prokaryotic (primitive) cell and the eukaryotic (true) cell.
A cell with no membrane-bound nucleus, and no other membrane-bound organelle either, is a prokaryotic cell. The name is Greek: Pro = before, karyon = nut, or nucleus. It means the stage before a nucleus has formed.
A prokaryotic cell has a single circular DNA, with non-histone protein, lying free in the cytoplasm. This free DNA is called the nucleoid.
Its ribosome is 70S, and the cell divides by binary fission, also called amitosis. Respiration is anaerobic. Nutrition is mostly by absorption, and in some cells by photosynthesis.
Prokaryotes include Mycoplasma, bacteria (Escherichia coli) and cyanobacteria (BGA = Blue Green Algae). Every organism of Kingdom Monera is prokaryotic.
3. The eukaryotic cell
A cell with a membrane-bound nucleus and other membrane-bound organelles (mitochondria, chloroplast, Golgi body, lysosome) is a eukaryotic cell. Greek eu = good, karyon = nucleus.
The nucleus of a eukaryotic cell is wrapped in a two-layered nuclear envelope. Inside lie the nucleoplasm, a nucleolus and several chromosomes. The chromosomes are rod-shaped, two-ended, and made of DNA and histone protein.
The ribosome is 80S. Division is by mitosis and meiosis. Respiration is aerobic. Nutrition is by absorption, ingestion and photosynthesis.
Eukaryotes include algae, fungi, bryophytes, pteridophytes, gymnosperms, angiosperms and all animals. Now sort ten clues into the two boxes yourself.
sort themTap a chip, then pick a bin
Which kind of cell?
Pick up a chip, then drop it in a box. Ask yourself: a cell from before the nucleus formed, or a cell with a membrane-bound nucleus?
Right. Next one.
Not yet. Check the pairs: 70S against 80S, circular against rod-shaped.
All in place. 70S, free circular DNA (the nucleoid), binary fission, anaerobic respiration and cyanobacteria (Kingdom Monera) belong to the prokaryote. 80S, rod-shaped chromosomes with histone, mitosis and meiosis, aerobic respiration and algae belong to the eukaryote.
4. Units for measuring cells
Cells are small, so the units must be small too. The book's table gives these relations: 1 cm = 0.4 inch 1 mm = 0.1 cm 1 μm (micron, μ) = 0.001 mm 1 nm = 0.001 μm 1 Å = 0.1 nm
Each unit goes with one way of seeing. cm: cells visible to the naked eye, such as an egg. mm: cells seen clearly under a microscope. μm: most cells and parts seen under a light microscope. nm: parts seen under an electron microscope. Å: parts seen by X-ray work with the electron microscope. Now match the pairs yourself.
match themPick a use, drop it in a unit
What is measured in which unit?
Pick up a use, then drop it in the unit that measures it.
Right. Next one.
Not yet. Think: the smaller the thing, the more powerful the instrument you need to see it.
All matched. As you step down from cm to Å, the way of seeing goes from the naked eye to the electron microscope.
Turn the table around and you get a ladder: 1 mm = 1000 μm, 1 μm = 1000 nm, 1 nm = 10 Å. So 1 μm = 10,000 Å, and 1 cm = 10 mm = 10,000 μm. Going to a smaller unit multiplies the number; going to a bigger unit divides it. Climb the ladder yourself.
calculatorStep by 1000 each time
The unit ladder
None of this goes anywhere. The notebook is yours.
5. The smallest, the largest and the longest
The smallest cell is Mycoplasma, also called PPLO (Pleuro Pneumonia Like Organism). The largest cell is the ostrich egg, measuring 17 × 12.5 cm.
The longest cell in the human body is the motor neuron, about 1.37 m. The fibres of cotton, jute and palm are long too, and can be seen with the naked eye.
Climb down the ladder of units: how many fit in each step, and what you need to see it.
zoomdrag the handle
From cm to Å
6. Check yourself
Three questions in the exam's style.
check yourselfpick one answer
Question 1
How many nanometres (nm) are there in 1 micrometre (μm)?
1 μm is 1000 nm, not 100; this answer is a factor of ten too small.
Right. 1 μm is 0.001 mm and 1 nm is 0.001 μm, so 1 μm equals 1000 nm.
10 000 is the value of 1 μm in angstroms, not in nanometres.
10 nm is 100 angstroms, a tiny fraction of 1 μm.
Give the second one a moment's thought.
check yourselfpick one answer
Question 2
In a prokaryotic cell there are:
i. Circular DNA ii. Ribosomes iii. Lysosomes
Right. i and ii are right but iii is wrong: prokaryotes have no lysosomes.
i and iii: iii is wrong.
ii and iii: iii is wrong.
Not all three, because prokaryotes have no membrane-bound organelles.
The last is straightforward, but do not rush it.
check yourselfpick one answer
Question 3
What is the main feature of the eukaryotic cell?
A cell without a nucleus is a prokaryotic trait.
A nucleus without a membrane belongs to prokaryotes.
Right. The eukaryotic cell has a well-organised, envelope-bound nucleus.
Small size is not a defining eukaryotic feature.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is a prokaryotic cell? What do Pro and karyon mean?
AnswerA cell without a membrane-bound nucleus or other membrane-bound organelles. Pro = before, karyon = nucleus/nut.
QuestionWhat are the ribosome sizes of prokaryotic and eukaryotic cells? How does a prokaryotic cell divide?
Answer70S in prokaryotes, 80S in eukaryotes. A prokaryotic cell divides by binary fission (amitosis).
QuestionWhat is a eukaryotic cell? What do eu and karyon mean?
AnswerA cell with a membrane-bound nucleus and other membrane-bound organelles. eu = good, karyon = nucleus.
QuestionGive the relations among cm, mm, μm, nm and Å.
Answer1 cm = 0.4 inch; 1 mm = 0.1 cm; 1 μm = 0.001 mm; 1 nm = 0.001 μm; 1 Å = 0.1 nm.
QuestionWhich unit suits parts seen under a light microscope and which under an electron microscope?
Answerμm for cells and parts seen with a light microscope; nm for parts seen with an electron microscope.
QuestionWhich is the smallest cell and which the largest?