1.2 Ultrastructure of cells
Lesson one: Prokaryotic cells
Objectives: understand what is meant by a prokaryotic cell. Know some key differences between prokaryotic and eukaryotic cells. Know the structure of a prokaryotic cell.
- Prokaryote: one of the three domains of life, along with eukaryote and archaea (each of the three domains covers multiple kingdoms including eubacteria, or ‘true bacteria’ ) Prokaryotic cells have certain characteristics, such as the lack of a true nucleus.
- Flagella: a tail like structure that allows prokaryotes to move
- Mesosome: an infolding of the cell membrane that has metabolic functions eg.respiration
- Nucleoid: a region of the prokaryotic cell where naked DNA can be found (not with archaea, their DNA is found with histone proteins)
- Peptidoglycan: a compound composed of protein and carbohydrate like elements, that forms the cell wall of a prokaryotic cell (not in archaea bacteria, their cell wall is made from carbohydrates)
- Organelle: part of a cell with a recognised function
- Ribosome: a non-membrane bound organelle, which occurs differently in all three domains (in eukaryotes they are larger 80S size, in prokaryotes they are smaller 70S size, in archaea they are smaller 70S size and a different shape)
The content of a Prokaryotic cell is quite different from that of a eukaryotic cell. Having seen your own cells (from the lab on cell theory), you can remember that ‘you’ have eukaryotic cells.
Task: make a high quality drawing of a prokaryotic cell on paper. Annotate the labels to add functions to the organelles, based on the last slide.
Q) Mesosome is missing, what is the function?
Lesson two: Why don’t we have giant cells?
Objective: understand that cell size is limited by decreasing surface area to volume ration
- Surface area :volume ratio – a mathematical expression derived by dividing the surface area by the volume
- Diffusion distance: the distance a molecule travels by diffusion eg. from the cell membrane to the nucleus.
image credit (US National department of wildlife and fisheries)
Image credit: the alien project
This is unusual, why aren’t there giant man -eating cells running around?
Cells are limited by their surface area to volume ratio. As a cell get’s bigger, it’s surface area to volume ratio get’s smaller until they can no longer absorb the things they require by simple diffusion
Activity: Work out the surface area to volume ratio for a hypothetical Streptococcus bacteria of differing sizes.
View this excellent slideshow on cells, including surface area the aspect of surface area – to – volume ratio
this lesson uses slides 41-51.
- Larger cells have difficulty obtaining sufficient nutrients, as their surface volume ratio : volume ratio is too low.
- Larger cells also suffer from greater diffusion distances, from the cell membrane to the centre.
- Overall the effect is to limit cell size to 1-10 micrometres in Prokaryotic cells, and 10-100 micrometres in Eukaryotic cells.
Lesson three: Plant and Animal cells
Objectives: Understand the differences between plant and animal cells.
- Organelle: Part of a cell with a specialised function. This improves the efficiency of a cell as multiple processes happen simultaneously in different parts of the cell, in specially adapted areas eg. mitochondria
A generalised animal cell is meant to represent all animal cells. This means the chosen cell can’t have specialisations (eg. axons in nerve cells). A liver cell is often used to represent a generalised animal cell. A liver cell does have large amounts of mitochondria (to fuel metabolic processes like de-toxifying poisons), and Rough Endoplasmic Reticulum (to make enzymes for the metabolic processes).
The function of these organelles can be understood through analogy. I would recommend viewing a cell as a city.
Cell City idea:
Summary: Unique features found only in animal cells: Lysosomes, Centrioles. The extra-cellular matrix is a collection of molecules including collagen (protein) and polysaccharides (carbohydrate) which are secreted externally to the cell, which also help to support the cell structurally.
The general features of a plant cell are often represented using a palisade mesophyll cell. These contain a large number of chloroplasts for photosynthesis, and are located on near the upper surface of a leaf to maximise exposure to light (light usually comes from above, and this also avoids shading by other tissues).
Plant cells have a lot of features in common with animal cells. They have a regular shape, are often larger, and have some unique organelles. The cell wall is an extra-cellular component.
Lesson four: Stem cell therapy
Objectives: understand how stems cells can be used to treat certain conditions. Categorise stem cells
- Stem cell: An undifferentiated cell eg. embryonic cell
- Totipotent, Pluripotent, and Multipotent: decreasing degrees of potency, respectively; can become any cell, can become most cells, can become only two or three types of cell.
- Embryonic Stem Cell Therapy: Therapy which replaces damaged cells with cells from an embryo.
- Therapeutic cloning: a process of creating an artificial embryonic cell with the nucleus of an adult cell, to avoid the problem of tissue rejection.
- Adult Stem Cell therapy: therapy which replaces damaged cells with cells from adult stem cell tissue such as bone marrow.
Stem cells are cells which are undifferentiated. This means they have not yet specialised to form particular cells. The term potency, refers to their ability to form different kinds of cells.
Early embryonic cells (fewer than four divisions) are totipotent. They can become any cell. Older embryonic cell care pluripotent (they can become any kind of cell except placental cells). Adult stem cells tend to be mutipotent (they can become only a few kinds of cells).
Stem cell therapy involves using stem cells to replace damaged or poorly functioning cells in a patient. So far, the most progress has been made with adult stem cell therapy. However, the potential exists to use embryonic stem cell therapy (ESC), but the ethical implications of using human embryos has tended to hinder this research.
One of the diseases which has been treated using embryonic stem cell theory, is Statgard’s disease; a disease which involves progressive loss of vision (through degeneration of the retinal cells). Vision is restored by surgically placing a patch of correctly functioning retinal cells behind the retina. The retinal cells are grown from embryonic stem cells.
Task, read the link below and discuss.
The problem of tissue rejection
One of the problems with using Embryonic Stem Cells, is that the surface membrane proteins of the retinal cells may not match the proteins of the recipient. This will cause the immune system to reject the transplanted cells.
This can be avoided by using therapeutic cloning. In therapeutic cloning, an artificial embryonic cell is created by replacing the true nucleus of an embryonic cell, with the nucleus of an adult cell from the recipient.
Stem cells may also be taken from the umbilical cord blood of a new-born baby, maintained in tissue culture, as a reserve of cells that could replace faulty cells eg. in the case of leukaemia. This would not be effective in preventing genetic diseases, as these cells would carry the same faulty genes.