Download File - need help with revision notes?

Biology: Unit F211: Cells, Exchange and Transport
Module 1: Cells
1.1.1
– Cell Structure
When looking at a cell under a microscope, its ultra structure is revealed. Ultra structure is the detail
inside of the cell. There are many organelles with specific functions a – this is the division of labour;
all of the organelles working together, each contributing to the cell’s survival. All of the organelles
below are found in all Eukaryotic cells (plants and animals). Eukaryotic cells have a true nucleus
surrounded by a nuclear envelope.
The nucleus houses nearly all of the cell’s genetic material and
has 4 components: the Nucleolous is the tightly wrapped DNA
making ribosomes and RNA. It is the dense, spherical structure at the
centre of the nucleus. Chromatin is the diffuse DNA in use in the cell
at the time. It is made up of DNA and proteins. The nuclear envelope
protects the DNA from mutation. The nuclear pores are gaps in the
envelope which allow mRNA to leave the nucleus, permitting
communication between the nucleus and the rest of cell.
The endoplasmic
reticulum is made up of flat
membrane bound sacs called cisternae, which are continuous with
the outer nuclear membrane. Rough endoplasmic reticulum is
studded with ribosomes for making proteins, and the RER acts as a
method of storage and transportation of the proteins that have
been synthesised on the attached ribosomes. Smooth
endoplasmic reticulum is involved in the synthesis of lipids, used
to make hormones.
The golgi
apparatus is a stack of membrane bound sacs. The
golgi receives proteins from the rough endoplasmic reticulum and
modifies them, perhaps adding sugar, folds them into enzymes, and
sorts them according to their destination. Enzymes are pinched off
into secretary vesicles which can then travel to the plasma
membrane to be secreted outside of the cell, or to other organelles
inside the cell itself.
The mitochondria have two membranes separated by a fluid filled
space. The inner membrane is highly folded to form christae. Mitochondria
are the site when adenosine triphosphate (ATP) is produced during
aerobic respiration.
The ribosomes are tiny organelles found in both eukaryotic and
prokaryotic cells. They are the site of protein synthesis – an assembly line
which reads coded instructions (mRNA) from the nucleus to join amino
acids to create proteins.
The lysosome is a spherical sac surrounded by a membrane which
contains digestive enzymes. Anything in the cell which needs to be
destroyed, such as invading bacteria, misfolded proteins or worn out
ribosomes, are sent to the lysosome to be destroyed. Broken up products
are reused and recycled to make new compounds and organelles.
The centrioles are small protein microtubules which are found next to
the nucleus of animal cells. They form the spindle fibres which move the
chromosomes during mitosis cell division.
The chloroplast is found in most plant cells, and is the site of
photosynthesis. Stacks of membrane sacs called thylakoids make up
piles of granum. Chlorophyll molecules are present on the thylakoid
membranes so that light energy can be used to form carbohydrate
molecules from water and carbon dioxide.
The cell
wall is found in only plant eukaryotic cells. They are made from cellulose
(a carbohydrate polymer made from glucose sub-units). The cellulose forms a sieve
like network of strands that make the wall strong. The cell wall is held rigid by the
pressure of the fluid inside the cell – turgor pressure – supporting the cell, and so
supporting the whole plant.
The plasma
membrane is selectively permeable. It is a phospholipid bilayer
interspersed with proteins. It separates the cell from the outside environment, gives
physical protection and allows import and export of selected chemicals. It prevents the
cell from bursting, or lysis.
The cytoskeleton is the network of protein fibres found within cells.
These protein tubes are called microtubules, and are about 25nm in diameter.
The cytoskeleton has 2 functions; to give particular shape and structure (the
shape of a red blood cell for example) and to enable organelles to move around
in the cell. Chromosomes are also moved by the cytoskeleton during mitosis.
The proteins which move organelles or chromosomes are called protein
motors. They pull the organelle around using the network of microtubules in
the cytoskeleton.
The flagella (undulipodia) are almost identical in structure to the cilia, but
there are often fewer of them, and they are longer. They are extensions of the
cytoskeleton and plasma membrane, and are used for the propulsion of the cell
itself, for example, the tail of the sperm cell. The microtubules use energy form
ATP in order to move.
The cilia are extensions of the plasma membrane and cytoskeleton. They waft in one
direction. They are found on ciliated epithelial cells in the trachea, and in the oviduct.
They sweep mucus away from the lungs and move the egg out of the oviduct and
towards the uterus.
Eukaryotic Cells are between 20 – 40 µm in length.
True nucleus with a nuclear envelope
Mitochondria,
Endoplasmic
Reticulum, Golgi,
DNA
Lysosome, Vacuole,
wrapped in
Cilia, Flagella,
histone
Chloroplasts
proteins
forming
chromosomes
.
Complex cytoskeleton made from
protein microtubules
Large Ribosomes – 30 nm
Prokaryotic Cells have a different structure to eukaryotes. Bacteria are an example of a
prokaryote. They do not have a true nucleus with nuclear envelope. DNA mutates easily because it
is not protected by a nuclear envelope or proteins.
Small ribosomes – 20nm
DNA nucleoid – Not bound
into chromosomes by histone
proteins- “naked” Not bound
by a nuclear envelope- “free”.
May also contain a plasmid
loop of DNA which is useful
for genetic engineering.
Cell wall made from
peptidoglycan (protein and sugar)
Pilli – for bacterial
conjugation. Tubs allow
bacteria to exchange plasmid
DNA.
Infolds of plasma
membrane are the site of
respiration, there are no
mitochondria. Mesosomes
help cell division.
1-5 micrometres (µm) long
Capsule – a jelly like coating
which sticks to things and
protects the cell from destruction.
Microscopy
Magnification: The degree to which the size of an image is larger than the object itself.
Resolution: The degree to which it is possible to distinguish two points that are very
close together as separate.
Light Microscopes
 Maximum magnification: x1500
 Maximum resolution: 200 nm
Advantages:
 Small, portable, cheap, living cells can be viewed,
real colour images
Disadvantages:
 Low resolution, low magnification
Preparing a specimen – Staining
Coloured stains are chemicals that bind to chemicals or specific organelles in the
specimen. This allows them to be seen under the microscope and enables us to
highlight particular organelles:




Light Green – Plant cell Walls
Aceitic Orcein – DNA red
Iodine – very dark blue starch grains
Methylene Blue – Dark blue nucleus, light blue cytoplasm
Electron Microscopes
Electron microscopes use electrons rather than visible light as the radiation source.
Transmission Electron Microscopes





500,000 x Magnification
High resolution
2D black and White image
Works in a similar way to a light microscope – passes
a beam of electrons through a thinly cut specimen.
You must prepare the sample by dehydrating the
specimen (replacing water with ethanol), coating in
heavy metals, mounting on a copper grid and placing
it in a vacuum within the microscope.
Electron Microscopes use electromagnetic lenses and electron guns with computer
detectors and screens.
Scanning Electron Microscope




Advantages
High resolution
High Magnification
Scanning created 3D image
100,000 x magnification
High resolution
3D black and white image
Scanning Electron Microscopes work by bouncing
electrons off the surface of an object to build up a 3D
image.
Disadvantages
Large, Expensive, Immovable
Must be trained to use them
Specimens must be dead, and coated in very expensive metals
Artefacts (things created or destroyed during preparation which do not give an
accurate portrayal of the thing in real life) are created.