BIO120H1 Lecture Notes - Membrane Potential, Antiporter, Amphiphile
Tuesday, January 6, 2008
- We have 1 midterm, 1 final exam and 6 labs. Labs are 27% with 8% from
quizzes, best 2 out of 3 (4% each). 65% is from exams: the midterm is 25%
and covers section 1 of lectures and labs 1 to 2. The final exam is worth 40%
and covers all lectures and labs 1-6.
- Midterm is different from course manual: it is actually on Friday February
27 from 2 to 4.
- There will be lab tutorials that will be held in 3 weeks, there will be 3
tutorials, one before the midterm and two before then, they appear on
alternating weeks on Mondays from 3 to 4 and will be posted next lecture.
- If you have any questions, asked the professor after lecture, come to
tutorials or ask on the course website on Blackboard. He doesn’t take
questions during the lecture.
- This is a picture of a cell and the central dogma of biology is that DNA
encodes mRNA which gets translated into proteins. You go from DNA to
proteins and proteins are functional components of living cells.
- We’re used to seeing pictures like this where the cell is static and doesn’t
move but even in a simple diagram like this, you can see there will be lot of
movement – first to transcribe DNA into mRNA there needs to be proteins on
the DNA moving along the DNA & transcribing the DNA into mRNA. Then
the mRNA needs to be transported out of the nucleus and into the cytosol. In
the cytosol is where ribosomes bind to mRNA and starts the translation into
proteins. That is movement again, movement of mRNA outside of nucleus
and into the cytosol where ribosomes will move onto the mRNA, move along
it and translate it into proteins and the proteins will move around the cell,
some will be secreted outside the cell going through the ER.
- This is all to emphasize that there is a lot of movement within a cell & from
this picture, one can appreciate that there are numerous compartments within
the cell so the cell is compartmentalized into different functional sub
compartments like the nucleus that stores DNA & is also the main site for
transcription. Then you have the cytosol & different organelles in the cytosol
such as ER that all have specialized functions. The cell is compartmentalized
in order to provide functional specialization & there is a lot of movement in
the cell that is not captured in images such as the one in the slide.
- There is a lot of movement in the cell whether it's proteins moving, whether
it's signals being transmitted from the outside of the cell to the inside of the
cell or whether it's the movement of proteins or lipids within the membranes
that surround the cell or the organelles within the cell.
- A movie is playing emphasizing movement within the cell.
- We will appreciate how active it is inside the cell eventually and we’ll
appreciate more the little subtleties we saw in the movie as we go through this
part of the course.
- Here is an animal cell which has a number of compartments in it called
organelles and we’re familiar with these compartments such as lysosome
which is the site for the degradation of what the cell doesn’t need anymore,
proteins or structural components, that it doesn’t need will go to the lysosome
and be degraded.
- The mitochondria are the energy source for the cell where ATP is produced.
- The nucleus is where the genetic material DNA is stored and site of
transcription from DNA to RNA.
- The Golgi is the site of protein modifications and sorting.
- The endoplasmic reticulum – you have two types: you have the rough ER
that is transcribing transmembrane proteins and proteins that will be secreted
outside the cell surrounding these little specks called ribosomes that are
actively translating proteins from mRNA & delivering them into the ER. You
also have smooth ER which is a major site of lipids in the cell.
- You can see all these compartments that are separated from each other have
specialized roles in the cell.
- He has highlighted 2 compartments or properties of the cell: the
extracellular matrix and the lysosomes that are specific to animal cells – the
extracellular matrix plays an important role in the adhesion of the cell to a
specific location & also plays an important role in maintaining cell shape &
another role in cell development.
- In contrast, plant cells have some specific organelles that aren’t mainly
found in animal cells – a major one are chloroplasts that are the site of
photosynthesis and are found only in plants and not animal cells.
- A vacuole plays a prominent role in a plant cell – they are mainly composed
of this large vacuole and there are 2 types of vacuoles: one acts like a
lysosome that is found in animal cells and the other ones are for storage of
proteins and certain small molecules and they can actually be storage spaces
for antimicrobial compounds so when the cell gets punctured, the compounds
ooze out of the cell and will kill whatever is trying to invade it.
- Okay so you have a vacuole, chloroplast and importantly a cell wall
surrounds plant cells and not animal cells – the cell wall is a rigid structure
and makes the cell often look square so it is involved in maintaining cell
shape as well as help protect from external stresses such as temperature,
mechanical stress, insect invasion or bacterial invasion so the cell wall
surrounds the plant cell.
- Importantly, of course plant cells also have structures such as a nucleus,
endoplasmic reticulum, Golgi apparatus, peroxisomes (the site of degradation
of many fatty acids), mitochondria and plasma membranes and are also found
in animal cells – the ones listed in the slide are in both animal and plant cells.
- There are lots of different compartments with functional specialization.
- Cytoplasm is the space outside the nucleus, everything outside the nucleus
in the cell shown in the slide. The cytoplasm would include the organelles
- The cytosol is the aqueous component of the cytoplasm so it doesn’t include
- The lumen is the aqueous part inside organelles – so inside the mitochondria
there for example, inside the organelles is the lumen.
- In section 3 we will cover the membrane components, so the membranes
that define the different organelles within the cell & actually make the
boundary of the cell by the plasma membrane. So what are the components of
these membranes that delimit the organelles & the cell itself? – that is the 1st
- Then there is a conundrum b/c if you separate your cell from the
environment, you need ways of transporting small molecules or getting
signals across from one side of the membrane to the other so we are going to
look at how cells can transport small molecules across these membranes in
both organelles & the plasma membrane (Lectures 2-4).
- Lectures 5-6 we’re going to look at protein sorting so how do proteins get to
different places in the cell – they all start out on ribosomes in the cytoplasm
& then get shuttled to different compartments of the cell. Some proteins will
end up in the membrane, some will be secreted outside the cell & others may
be retained within the cell. Just to emphasize, it all starts in the nucleus DNA
mRNA cytoplasm where it is translated into proteins and that is where
- Vesicle trafficking will be lectures 7-8 so how do things get around the cell
in vesicles – so basically one important thing is things that are destined for
membranes or outside the cell must be translocated through vesicles – they
will start out in the ER, so these are transmembrane proteins or things
destined for the outside of the cell, & will be translocated into the ER,
through the Golgi apparatus & then shuttled to their appropriate compartment
- Exocytosis & endocytosis is the final part of vesicle trafficking. One of the
destinations of vesicle trafficking is exocytosis, translocation of things
outside the cell & we’ll also cover how things get into the cells through
endocytosis, so how do cells take up molecules from the outside?
- Finally we’ll go into a different type of movement which is the transduction
of signals from the outside of the cell to the inside of the cell – this should
read lectures 10, 11, 12 & not 13 in the slide, so lectures 10 – 12 will be
signal transduction or transmission of signals from outside of the cell to the
inside of the cell.
- You see there is movement that will occur at all these different parts of the
section 1 lectures.
- We start with membrane structure – they delimit the cells & organelles.
- Cell membranes enclose the cell and the organelles – it is an important thing
to keep in mind because this is what allows the different compartments to
maintain their functional distinction from the rest of the cell so you have
division of the cell into compartments and this defines the boundaries of these
compartments and maintains differences b/w the cytosol, the organelles & the
extracellular environment. These differences are what’s required to maintain
different functions in each of these different compartments.
- It is important to realize that it is the cell membranes that allow the cell and
the organelles to maintain these differences.