Cell Biology Intro
In cell biology we look at structures and try to define their function in the cell.
When we cut the microvilli from the top (parallel) we can see a structure of microtubules
within them.
The 3 essential features of a cell:
It has a limiting membrane (inside/outside compartments)
Contains biomolecules (e.g. protein/RNA/DNA)
Is an autonomous unit in performing a function, hence can respond and adapt to
stimuli and can (often) reproduce itself (not true for neurons for example, but holds
true for most)
Robert Hooke first came up with the term “cell”
Cells are very diverse; the oocyte is ±150 µm in diameter, while the sperm cell is ±1,5 µm in
diameter. So, two very important cells in life look completely different while containing
similar “ingredients”: DNA, RNA and proteins.
DNA structure also tells you a lot about its function, as DNA that gets used very often differs
from DNA that is being stored for later use.
How cells organize function within it is through organelles, hence divided by a limiting
membrane. However, not all cells contain organelles. Bacteria for example, contain no
nucleus or organelles. Archaea do not have a nucleus or organelles, but a large group of
extremophiles (cells that live in volcanoes or cells on the bottom of sea beds under heavy
pressure) belong to this group. Eukaryotes do have organelles, and a nucleus, and can form
multicellular organisms.
To study cells, we can use an assortment of microscopes:
The first microscope was made by Robert Hooke, used to study pond water, using only one
lens.
➔ Light microscope: Turn on light, which will go through lenses that turns it into a beam
which goes through the specimen. The objective lens is moved, which controls the
magnification and the focus. The light goes through mirrors/reflectors and goes into
the eyepiece and into the eyes
➔ Fluorescent microscope: This revolutionized cell biology as it filters out one light and
reflects it through the light. If the specimen contains fluorescent protein, the light
being reflected back will be a bit different as some energy is lost. Through the
objective we can see the fluorescent light. This was a good discovery since GFP were
discovered due to this. Using GFPs we can visualize subpopulations of cells in tissue
in living organisms. The problem with this is however, that when looking at synaptic
vesicles, the wavelength of the light is 500nm while the diameter of the vesicle is
40nm. Meaning that when illuminated, all the neurotransmitters are going to emit
green light, and we don’t know which photon these large wavelengths came from.
You therefore need electron microscopes.
➔ Electron microscope: Electrons have wavelengths in picometers, so are much smaller,
hence precision is much higher. How does it work? An electron gun at the top emits
electrons (instead of photons), and put a strong + plate to pull the electrons down to
, the electron magnetic lenses, which passes the electron through a tunnel and to the
specimen. Below the specimen we have the objective lens, which determines the
magnification and focus. The electron beam falls on a fluorescence screen in the end,
so that we can then see it. The specimen is stained with heavy metals to give them a
+ charge.
Advantages: Great resolution (all the way up to atoms), visualization of the whole cell
not just a fluorescent probe, huge magnification range
Disadvantages: requires fixation of cells (microscope operates under vacuum so
otherwise, the cell dries out), heavy metals can be toxic to work with, only small
pieces of tissue can be imaged at a time, time-consuming method.
How are form and shape related in organelles?
You need to be able to recognize certain organelles, as are shown in the figure above
Let’s looks at the organelles in detail:
Nucleus:
Structure and function: stores DNA and does the transcription of DNA to RNA. Inside there
are light and dark parts; the dark part is the nucleolus, which makes ribosomes. On the
outside there is a clear envelope wrapping around it. The nucleus is therefore double-
membraned which allows specificity in which genetic material enters/leaves.
, DNA morphology in relation to function:
Endoplasmic reticulum:
Structure and Function: Translation of RNA to protein and protein folding. It is connected to
the nuclear envelope, and has a tubular structure with
dots on it (ribosomes) if it is a RER (to produce proteins).
It is in tubules to increase the surface area. More
membrane is needed to have room for all these
ribosomes to make enough proteins. The ER makes
secretory vesicles at the ends to secrete proteins to the
outside, so the lumen of the ER is continuous with the
lumen of the Golgi, which matures the proteins and
secretes vesicles too, to release them to the outside or to
give to lysosomes to digest (if too much protein is made).
Uptake of proteins is also possible (so the reverse), where
the lumen of endosomes is continuous with the outside, and proteins can be taken up to
either be given back to the ER (via Golgi) or lysosome.
Golgi complex:
Structure and function: Needed for protein modification (post-translational modifications by
adding or removing sugar groups) and sorting (all the layers in the Golgi are used to sort the
proteins). Structure can be compared with a “stack of pancakes”.
Mitochondria:
In the cell there is a lot of cellular transport, as from the Golgi we have a lot of exchange,
which is expensive in energy (consumes ATP), so we need the Mitochondria. Have a double
membraned structure, also inside to increase surface area as ATP is
transmembrane/membrane bound proteins, so the membrane is needed to house enough
ATP. Since they were derived from another organism, they have their own DNA, which is
useful for studying genetic relations.
The chloroplast is mitochondria’s counterpart in plants. (Plants need both mitochondria and
chloroplasts. Mitochondria is needed to burn the glucose provided by photosynthesis).
The cytoplasm:
