Prokaryotes have a simple cell structure without compartmentalization.
Eukaryotes have a compartmentalized cell structure.
Application: Structure and function of organelles within exocrine gland cells of the pancreas and within palisade mesophyll cells of the leaf.
Application: Prokaryotes divide by binary fission.
Skill: Drawing of the ultrastructure of prokaryotic cells based on electron micrographs.
Skill: Drawing of the ultrastructure of eukaryotic cells based on electron micrographs.
Skill: Interpretation of electron micrographs to identify organelles and deduce the function of specialized cells.
Prokaryotic Cells
- No compartmentalization, all the materials flow in the cytoplasm
Compartmentalization = organelles are separated by the membrane to perform specific function independently and contemporarily within a cell
- The prokaryotes replicate and divide itself by binary fission, a process which the chromosome replicates itself to form 2 sets of chromosome. The cell membrane then moves inwards, so the cell is divided into 2 individuals. Each receives a set of chromosome, so the 2 cells are genetically identical.
binary fission = asexual reproduction for prokaryotic cells, the DNA replicate and separate contemporarily
The Structure always present:
- Cell wall: protects the cell from bursting, major components are peptidoglycan (amino acid + carbohydrate)
- Plasma membrane: double-layered membrane encloses the cell and controls materials exchange
- Cytoplasm: gel-like fluid holds enzymes for chemical reaction and other cell materials
- Nucleotide: a region in the cytoplasm where chromosome densely located
- Chromosome: the chromosome has no protein with it so it is "naked"
- Ribosomes: 70S ribosome for protein synthesis
you can see the 70S or 80S in front of the ribosome, actually "S" is Svedberg unit, a description of ribosome size. The "S" represents the time to sediment the ribosome under centrifugation.
The additional structure (present sometimes):
- Capsule: extra protection to the cell
- Plasmid: a small circle of DNA apart from the chromosome, the replication is independent to chromosome
- Flagellum: projecting from the cell wall, for movement
- Pili: project from the cell wall, for pulling other prokaryotes during genetic materials exchange
Animal Eukaryotic Cells
- have compartmentalization, organelles (materials packed within a membrane) flow in the cytoplasm
- replicated and divided by mitosis (you will learn in the IB Biology chapter 1.6)
- Eukaryotes and prokaryotes have something in common, including they both have the cell membrane, cytoplasm, and ribosomes but the eukaryotes are much more complex
- eukaryotes do not have cell wall
- Nucleus:
- chromosome is bounded by double-layered nuclear envelope membrane
- dark spot called nucleolus for ribosome production inside the nucleus
- small holes called nuclear pores on the membrane for material move in or out - Chromosome: chromosome combines with histone protein, forming chromatin
- Ribosomes: 80S ribosomes for protein synthesis, made of RNA and protein
- Rough endoplasmic reticulum (rER):
- ribosome attached to the ER, the site of protein synthesis
- the space between the ER membrane called cisternae where the protein is collected - Golgi apparatus:
- protein from rER is brought to here by vesicles for processing
- the protein is then transported to the cell membrane by secretory vesicle and secreted - Smooth endoplasmic reticulum (sER):
- enzyme but NOT ribosome attached to the ER
- producing phospholipid for cell membrane and lipids
- it also performs different functions in the different cell for example:
liver: break down toxins
ovaries: estrogen (a lipid hormone) production - Mitochondria:
- double-layered membrane organelle
- have own set of DNA, replicated independently
- the inner membrane folds to form cristae where the ATP (energy to the cell) is generated under aerobic
condition (with oxygen)
- abundant in muscle - Lysosomes:
- made by Golgi apparatus
- contain hydrolytic enzyme to break down components e.g. old organelle in the cell, bacteria in the white blood cell
- Paramecium and Chlorella that mentioned in the previous IB Biology chapter 1.1 are also eukaryotes!
Plant Eukaryotic Cells
- The plant eukaryotic cells are similar to animal eukaryotic cells except
- The plant cell normally does not have lysosomes and have 3 more organelles
- Chloroplast:
- double-layered membrane organelle,
- have own set of DNA, replicated independently
- site of photosynthesis (using light to produce food starch) - Vacuole: located at the centre of the cell, large in size, filling the whole cell to keep the cell turgid and firm
- Cell wall:
- composes of cellulose and carbohydrates lignin and pectin
- maintain the rigidness of the cell so the leaves can hold in place to absorb light
Summary of Prokaryotes and Eukaryotes
The list summarizes the differences and similarities of the prokaryotes and eukaryote
Some concepts you need to clarify:
- nucleoid: in prokaryotes only, the region in the cytoplasm where chromosome densely located
- nucleus: eukaryotes exclusive, the chromosome is bounded by the nuclear envelope, a distinguished organelle when compared to the nucleoid
- nucleolus: it appears as a dark spot inside the nucleus, region in the nucleus where ribosomes are produced
Drawing of Cells
- under the electron microscope, the cell details are revealed more detailedly
- When drawing the image in the microscope,
1. need to use "dot" to fill the cytoplasm in drawing
2. labelled the structure, the pointing cannot cross the cell, placing the labels in the two sides
3. never shading in the drawing even the organelles are dark in the microscopy image
Prokaryotic cells
Under Light microscope:
- shows the 70S ribosome, nucleotide, plasma membrane, cell wall, pili, and flagella
Eukaryotic cells
Under Light microscope:
- plasma membrane, nucleus, 80S ribosome and mitochondria can be revealed but not very detailed under the light microscope
- the nucleus is darker than chloroplast in the plant cell
- the large space in the plant cell is the vacuole (don't miss it)
Under Electron microscope:
- the Golgi apparatus, ER, the detail of the mitochondria can be shown
- remember no shading in the drawing, even though you see the secretory vesicle is dark, you don't need to colour the vesicle black in the drawing
In this IB biology topics, you have to
- remember the structure (e.g. which organelles are double-layered membrane bounded) and the functions of the organelles
- what is compartmentalization
- compare the differences of the prokaryotes and eukaryotes
- how to draw the microscope image (remember! never add shading in the drawing!)
That is the end of this topic!
Photo references:
1. Openstax, https://openstax.org/books/microbiology/pages/3-3-unique-characteristics-of-prokaryotic-cells
2. Biology dictionary, https://biologydictionary.net/eukaryotic-cell/
3. Pngitem, https://www.pngitem.com/middle/imoihoi_ultrastructure-of-a-eukaryotic-cell-a-plant-cell/
5. Wiki, https://simple.wikipedia.org/wiki/Binary_fission
6. Brenda Walpole (2014). Biology for the IB Diploma (Second Edition). Cambridge University Press.