IB Biology - Curriculum Notes
Ultrastructure of Cells
∑ = Understandings:
The next two understandings point are basically asking to compare and contrast prokaryotes and eukaryotes (previous syllabus) but with a focus on compartmentalization. The following goes with the comparison in the table below.
∑ Prokaryotes have a simple cell structure without compartmentalization.
All prokaryotes have a cell membrane and a cell wall surrounding the outside membrane. The cell wall is made from peptidoglycan. The entire interior of the cell is filled with cytoplasm (not compartmentalized) as no membrane-bound nucleus is present.
∑ Eukaryotes have a compartmentalized cell structure.
Eukaryotes have a much more complicated cellular structure. The inside of the cell also contains cytoplasm but it is separated by compartments that allow for specialization. The compartments are membrane-bound organelles such as the nucleus and the mitochondria. Some advantages of compartmentalization are
1) Enzymes that serve a specific function or catalyze a specific reaction can be concentrated within the compartment instead of being spread throughout the cytoplasm.
2) Ideal conditions or particular processes can be maintained within the compartments such as pH
3) Organelles with their content can be moved around the cell
4) Damaging substances such as digestive enzymes (lysosome) can be contained within their organelle.
Comparison between prokaryotic cells and eukaryotic cells
|Prokaryotic Cells||Eukaryotic Cells|
|Small - less than 10µm||Larger cells – greater than 10 µm|
|DNA free within the cytoplasm||DNA contained within the nucleus|
|DNA in a ring free of proteins (naked DNA)||DNA associated with proteins|
|No membrane bound organelles||Membrane-bound organelles|
|No mitochondria||Mitochondria present|
|70s ribosomes||80s ribosomes|
|No internal compartmentalization to form organelles||Internal membranes that compartmentalize their functions|
|Reproduction through binary fission||Reproduction through mitosis and meiosis|
∑ Electron microscopes have a much higher resolution than light microscopes.
- The limit of resolution is the minimum distance that can be observed before two objects merge together to form one object. The smaller the limit of resolution the higher the resolving power.
- Electron microscopes have a greater resolution (about .001 µm) when compared to a light microscope (about 0.2 µm)
- The resolution of light microscopes is limited by the wavelength of light (400-700 nm). If the magnification becomes too great the image becomes blurry
- Electrons have a much shorter wavelength so they have a much greater resolution (about 200x greater than a light microscope)
∑ Prokaryotes divide by binary fission.
- Binary fission is the form of asexual cell division that results in the reproduction of two genetically identical prokaryotic cells.
- All prokaryotic cells divide by binary fission.
ß - Skill: Drawing of the ultrastructure of prokaryotic cells based on electron micrograph
Micrograph on the right
- The nucleoid region is the dark region in the middle (in other micrographs with a light background, the nucleoid region could possibly be a light region without ribosomes).
- You can see the cell wall surrounding the cell.
- Ribosomes are the black dots within the cytoplasm.
- The pili are not visible on this cell.
Applications and skills: ß
ß - Application: Structure and function of organelles within exocrine gland cells of the pancreas and within palisade mesophyll cells of the leaf.
Exocrine Gland Cells of the Pancreas
- These are animal cells that are specialized to secrete large quantities of digestive enzymes.
- They will have all the organelles of an animal cell but will have many ribosomes and rough ER to create the enzymes which are proteins and transport them outside the cell.
- They have many mitochondria to supply the ATP needed for these processes.
Here are the specific details of the functions of a eukaryotic animal cell
- Produce proteins.
- Free ribosomes within the cytoplasm produce a protein that will be used internally within the cell.
- Ribosomes attached to the rER produce proteins that will be secreted.
- Organelle found in most eukaryotic cells that processes and packages macromolecules such as proteins.
- Proteins are usually modified for secretion out of the cell.
- Known as the control center of the cell.
- The nucleus regulates cell activities through gene expression.
- Contains the majority of the cell’s DNA.
- It is surrounded by a double membrane called the nuclear envelope, which has small nuclear pores to allow molecules to move in and out of the nucleus.
- Membrane-bound organelles that carry out aerobic cellular respiration to create ATP.
- Contain highly folded inner membranes called cristae which increase the surface area to enhance the mitochondrion’s ability to produce ATP (oxidative phosphorylation takes place here).
- Space enclosed by the inner membrane is called the matrix. This is where the Kreb’s cycle takes place.
- Contain their own DNA genome.
Rough endoplasmic reticulum (rER)
- Site of protein synthesis (attached ribosomes) for secretion out of the cell.
- rER tubules are continuous with the outer layer of the nuclear envelope.
- Contains hydrolytic enzymes that digest worn-out organelles, food particles, and viruses or bacteria.
- Formed from the Golgi apparatus.
- Bounded by a single membrane.
ß - Palisade Mesophyll cells carry out most of the photosynthesis in the leaf.
- They have many chloroplasts to allow the cell to carry out the maximum levels of photosynthesis.
- The cells are surrounded by a cell wall to hold the shape of and protect the cell and a plasma membrane to allow substances in and out of the cell.
- They also have mitochondria which are membrane-bound organelles that carry out aerobic cellular respiration to create ATP.
- They have vacuoles which are a large cavity in the middle of the cell that stores water and dissolved substances, e.g. sugars and metabolic by-products
- They are basically plant cells with many chloroplasts.
ß - Skill: Drawing of the ultrastructure of eukaryotic cells based on electron micrographs.
- The diagram to the right shows an animal cell like a liver cell which contains many ribosomes, rough endoplasmic reticulum (rER), lysosomes, Golgi apparatus, many mitochondria, and the nucleus.
- Liver cells contain many mitochondria for energy and rough endoplasmic reticulum with ribosomes for secretion purposes.
ß - Skill: Interpretation of electron micrographs to identify organelles and deduce the function of specialized cells.
- Identify as many structures and organelles you can from the two micrographs below. Also, practice with the 3 micrographs in your book on page 25