Cells are the smallest self-contained unit. Cells can live on their own occasionally: single cell
organisms. Human cells cannot live on their own. There are 10 trillion cells in the human body.
Maybe there is the same number of bacteria too in our body.
In a cell you need to diffuse molecules. It takes a long time for molecules to travel. We subdivide
everything into small things so that it takes less time to travel.
If the cell is big, the volume is bigger than the surface area. Problem: it would take too long for the
molecules to find what they are looking for.
If the cell is small, the surface is bigger than the volume: bigger surface to volume ratio. Therefore
molecules can easily diffuse into and out of the cell.
Exception: eggs are a single cell and they are huge. In an egg, we do not need diffusion because it is
just there to store stuff. This is why it can be so big.
Prokaryotes: bacteria and archaea. They provide the greatest amount of diversity. Eukaryotes are all the organisms (plants, animals, etc) that we encounter in daily life. Eukaryotes
can be larger than prokaryotes because they are further subdivided (with organelles).
Prokaryotes (ex: E. Coli bacteria) : DNA sits in the middle without a membrane. They have a cell wall
without cellulose. Sometimes flagella.
Strengths of prokaryotes: Prokaryotes can oxidise all kinds of chemicals. So they can live in all kind
of weird places. You can find them everywhere. Animals (eukaryotes) can eat very few things
(oxidise fats and sugars).
Eukaryotes: they are heavily compartmentalized to carry out chemical reactions quickly and
efficiently. Prokaryotes are generally 10 times smaller in diameter and 1000 times smaller in volume
that eukaryotes cells. The divisions of the eukaryotic cell are similar in size to the prokaryotic entire
Animal and plant cells are pretty similar but there are some differences. Cell wall is made up of
cellulose in plant cells so they do not need osmoregulation and the sodium potassium pump. The plant cell has a vacuole (lysosome) that stores the water that comes into a cell. Also, plants have
chloroplasts where the energy is made.
The endomembrane system: all the internal membranes that we find inside the eukaryotic cell. We
have two nuclear membranes surrounding the nucleus. We have the Endoplasmic reticulum (ER)
which connects with the nuclear outer membrane. The Golgi apparatus. Vesicles that travel
between these organelles. The cell membrane itself belong to the endomembrane system.
Evolutionarily, there was first the cell membrane and then the internal membranes formed.
Lumen: it is the interior of the ER and the Golgi. It is equivalent to the outside of the cell.
The vesicle buds off an organelle and it later fuses with the plasma membrane to release the
content of the lumen into the extracellular fluid. So the interior of the Golgi or the ER, can get to the
outside of the cell. The dark red membrane would be the leaflet of the lipid bilayer that faces the cytosol and it always
faces the cytosol. So the orientation remains the same at all times. This is important because these
lipid bilayer are different in the cytosolic side than the one facing the outside of the cell.
Nucleus: all the DNA is here. It is in the center of the cell.
What happens in the nucleus? DNA is replicated. DNA is transcribed into mRNA or rRNA by RNA
polymerases. The mRNA is transported out into the cytoplasm where it is translated. The r RNA
stays behind and directly folds into a 3D structure (similar to a protein). Ribosomes are made of 4
rRNAs and about 80 proteins in the nucleus. RNA is supposed to have come first in evolution than
A lot of stuff must get in and out of the nucleus and that is why nuclear pores are there. Proteins
get in and rRNA gets out. Protein translation happens always in the cytosol. All of the RNA is transported out into the cytoplasm and then they are translated into proteins by ribosome and if
they are needed in the nucleus they have to get back into the nucleus by the nuclear pore.
The rough endoplasmic reticulum: it is called rough because it has the ribosomes on its outside.
Ribosomes are very big compared to normal proteins. Ribosome