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Cell Biology Summary
Summary for the course Cell Biology, covering book chapters and lectures
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CHAPTER 1 – CELLS: THE FUNDAMENTAL UNITS OF LIFECells are membrane-enclosed units filled with an aqueous solution of chemicals and with the ability to
make copies of themselves by dividing in two. The simplest forms of life are solitary cells, while higher
organisms are groups of cells.
Cells can vary in size, shape, chemical requirements (oxygen, light, water to live) and function.
All cells are composed of the same kinds of molecules, which are involved in the same chemical reactions,
and contain DNA molecules that carry genetic information.
Cells were made visible in the seventeenth century when the microscope was invented. The first
microscope was a light microscope, which uses visible light to illuminate specimens.
The development of the light microscope depended on the production of glass lenses, and led to the
discovery of cells by Robert Hooke in 1665, who examined a piece of cork.
Light microscope
The minimum resolvable is 200nm. (cells have a diameter of 5-20µm)
The most important characteristic of an objective lens is the resolving power (or resolution), which is the
least distance between two points that can be distinguished as individuals. It is calculated as
d=0.61λ/n*sinα, where λ is the wavelength of the light used; n is the refractive index of the medium
between the objective lens and the coverslip; α is the half-angle of the cone of light that enters or exits the
cell. (“n sinα” is the same as N.A., a number written on the objective lens.)
Seeing the internal structure of a cell is hard because its components are small and transparent, so we need
to stain cells with dyes that colour their components differently (DAPI stains DNA, while DIOC 6 stains lipid
membranes).
Fluorescence microscope
The minimum resolvable is 20nm, so they allow to see cell components more in detail.
Fluorescent molecules (fluorophores) absorb light of a specific colour (they get excited) and produce light
of a different colour that highlights a specific cell component.
Electron microscope
The minimum resolvable is 0.2nm.
It uses beams of electrons instead of light as source of illumination. There is no possibility to look at living
cells, since this type of microscopes work at low air pressure, but they allow to recognize different
organelles.
THE PROKARYOTIC CELL
They are unicellular organisms that don’t have a nucleus and no organelles except ribosomes. They are
divided in two domains: bacteria and archaea. Most of the prokaryotes are bacteria.
The components of a bacterial cell are:
Plasma membrane, made of a bilayer of phospholipids and proteins. It allows to transport
substances in and out of the cell, and surrounds the cytoplasm containing the DNA.
, Cell wall, it surrounds the plasma membrane.
DNA, circular and free in a region of the cytoplasm called nucleoid.
Ribosomes, they are free in the cytoplasm, and synthetize proteins by translating the mRNA.
Flagellum, a sort of tail that allows the movement of bacteria.
Most prokaryotes live as single-celled organisms, but others join together forming chains or clusters. Some
are aerobic (they need oxygen to oxidise food molecules), other are anaerobic.
Some bacteria make photosynthesis (cyanobacteria) and are thought to be the precursors of chloroplasts.
THE EUKARYOTIC CELL
They are bigger and more elaborate. Some live as single-celled organisms, such as yeasts; others form more
complex organisms, such as plants, fungi and animals (fungi and plant cells have a cell wall, animal cells
don’t). They all have a cellular membrane that surrounds the cytoplasm, in which are the organelles. The
organelles are surrounded by membranes, so different chemical processes can take place in each organelle.
Components:
Nucleus: contains DNA. It is surrounded by a double membrane (nuclear envelope) with many
nuclear pores, which allow the transportation of substances in and out of the nucleus. DNA is
compacted by proteins to fit in the nucleus: these proteins are euchromatin (less condensed) and
heterochromatin (highly condensed). In the nucleus we find the nucleolus, where rRNA is
synthetized and combined with proteins to form pre-ribosomes, which are then transported out of
the nucleus.
Mitochondria: enclosed in two different membranes and contain their own DNA and ribosomes.
The function of mitochondria is to generate energy for the cell. They exploit the energy from the
oxidation of food to produce ATP. Mitochondria are thought to have been derived from bacteria
that were engulfed by an ancestor of the eukaryotic cell. This created a symbiotic relationship in
which the eukaryote and the bacterium helped each other.
endoplasmic reticulum (ER): enclosed by a membrane connected to the nuclear envelope. It is
formed by the smooth ER (SER) and the rough ER (RER; because there are ribosomes attached to its
membrane). Functions of the ER are synthesis of phospholipids and folding and modifications of
proteins.
Golgi apparatus is made sacs surrounded by a membrane (cisternae). It modifies and packages
molecules made in the ER, that are destined to be transported out of the cell or to other organelles.
Endosomes, it receives the molecules that are transported into the cell via endocytosis. Early
endosomes will develop into late endosomes and then lysosomes.
Lysosomes, contain enzymes to break down molecules. In plant cells the breakdown of macro
molecules takes place in the central vacuole.
Peroxisomes, break down toxic molecules.
Organelles only found in plant cells:
Chloroplasts perform photosynthesis. They are surrounded by two membranes and contain a third
folded membrane (thylakoid membrane, it contains chlorophyll), which is stacked into grana
(singolare granum). Grana are interconnected and surrounded by the stroma. Chloroplasts contain
their own DNA. They are thought to have been evolved from bacteria when a cyanobacterium was
engulfed by an eukaryotic cell.
, Vacuole: it occupies the 80-90% of the cell, it stores water and other components.
No lysosomes
many Golgi stacks per cell (only one in animal cells).
Chloroplasts are members of a family called plastids. Other plastids are amyloplasts, that contain starch;
chromoplasts, that contain pigments of other colours; chloro-amyloplasts: chloroplasts that partly turned
into amyloplasts.
MODEL ORGANISMS
All cells are thought to be descended from a common ancestor, so the study of one organism can help us
understand others. For this reason, biologists chose to study a few representative species called model
organisms, e.g. E. coli, Saccharomyces cerevisiae (yeast), Drosophila, C. elegans (nematode) and mice.
CHAPTER 2 - CHEMICAL COMPONENTS OF CELLS
Sugars (or carbohydrates)
The simplest sugars are called monosaccharides, and they are compounds with the general formula
(CH2O)n.
The most known monosaccharide is glucose, a C6 sugar. It can exist as a linear or a cyclic form (like other
sugars). The cyclic ring of glucose is made of 5 C atoms and 1 O atom. The 6th C atom is a side group of the
ring. Glucose also has 5 hydroxyl groups.
Two monosaccharides can form a disaccharide (such as sucrose=glucose+fructose). Oligosaccharides are
larger polymers (usually from 2 to 10), while polysaccharides are even larger. The bond is formed between
an OH group on one sugar and an OH group on another by a condensation reaction, in which a water
molecule is expelled. The bonds can be broken by the reverse process of hydrolysis.
Fatty acid
A fatty acid molecule consists of a long hydrophobic hydrocarbon tail combined with a hydrophilic
carboxyl (-COOH) head. Molecules such as fatty acids that have both a hydrophobic and a hydrophilic part
are called amphipathic.
Saturated fatty acids don’t have double bonds between C atoms. The unsaturated ones have one or more
double bonds.
Fatty acids can form triacyglycerol molecules = 3 fatty acids + glycerol; phospholipids = glycerol + 2 fatty
acids + phosphate group; glycolipids = sugar instead of phosphate.
Fatty acids are a food reserve in cells: they are stored in the cytoplasm in the form of fat droplets
composed of triacylglycerol molecules (compounds made of 3 fatty acids and a glycerol molecule).
Amino acids
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