OCR F211 Biology Notes

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Cells, Exchange and Transport
(a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron
microscope and a scanning electron microscope;
Microscope Resolution Magnification
Light 200nm X1400
Transmission 0.5nm X300000
Scanning 0.5nm X300000
SEM gives a 3d picture and cell surface structures can be seen. TEM gives a 2d picture and allows details of
organelles to be seen. However, expert preparation of sample is required.
Light microscope is easy to use and allows living processes to be seen.
(b) explain the difference between magnification and resolution;
Magnification: number of times larger the image is compared to the object
Resolution: the ability to distinguish between two separate points
(c) explain the need for staining samples for use in light microscopy and electron microscopy;
A lot of biological material inside a cell is transparent. Stains give contrast, enabling detail to be seen inside the cell.
Iodine; stains starch grains black and other structures yellow
Methylene blue; stains nuclei dark blue and cytoplasm light blue
In TEM, electron dense materials are used as stains, eg. salts of lead/uranium.
In SEM, heavy metals such as gold cover the object. The electrons are scattered by the surface of the object, giving
an image.
(d) calculate the linear magnification of an image;
mm >> um: multiply by 1000
um >> nm: multiply by 1000
(i) Compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of
plant cells and animal cells.
(e) describe and interpret drawings and photographs of eukaryotic cells as seen under an electron microscope
and be able to recognise the following structures:

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Nucleus,
Contains genetic information in DNA of chromosomes.
DNA is organised in genes which code for the production of proteins which control the activities of
the cell.
Nucleolus,
Makes ribosomes
Within the nucleus
Nuclear envelope,
A double membrane with nuclear pores to allow mRNA and ribosomes and other material out of the
nucleus.
Rough endoplasmic reticulum,
Ribosomes on surface
Transports polypeptides to Golgi apparatus
Smooth endoplasmic reticulum (ER),
Lipid and steroid synthesis.…read more

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Helps draw water into cell by osmosis to keep it turgid
Cell wall (plant);
Mechanical support and protection
Prevents osmotic bursting
Chloroplasts (plant);
Contain chlorophyll
Absorb sunlight during photosynthesis to make glucose
Starch grains (plant);
Made up of many glucose molecules bonded together
Storage carbohydrate
(f) outline the interrelationship between the organelles involved in the production and secretion of proteins (no
detail
of protein synthesis is required);
The gene containing the instructions for the production of the protein is copied onto a piece of
mRNA…read more

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Some have Flagella
Cell Membranes
(a) outline the roles of membranes
at the surface of cells;
o Separate cell contents from the outside environment
o Control entry and exit of molecules
o Permit movement of substances by exocytosis and endocytosis
o Cell recognition via cell surface antigens
o Provide receptors for signalling molecules such as hormones
o Microvilli to increase surface area of cell
within cells
o Surrounds organelles
o Prevents disruption of reactions (eg.…read more

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Cholesterol,
Stabilises the phospholipid bilayer by binding to polar heads and non-polar tails of phospholipids
Controls fluidity by preventing phospholipids solidifying at low temperatures and become too fluid at high
temperature
Reduces permeability and makes the barrier more complete, preventing molecules like water and ions from
passing through the membrane.…read more

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Insulin and glucagon do not cross cell membranes as they are large and polar. Glycoprotein receptor
molecules on the cell surface detect insulin
o Steroid are fat-soluble, so they cross the phospholipid bilayer and interact with receptors inside cells.
(g) explain the role of membrane-bound receptors as sites where hormones and drugs can bind;
Hormones are used in cell signalling.…read more

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Endocytosis
Membrane folds in, engulfing molecules
Triggered by binding of molecule to receptor site
Fuses with itself and pinches off
Formation of vesicle
Vesicle moves through cytoplasm via network of microtubules
Requires ATP
Exocytosis;
Substance packaged by Golgi apparatus are delivered to the cell surface in vesicles
Vesicle fuses
With cell surface membrane
Releasing enzyme contents to exterior
(i) explain what is meant by osmosis, in terms of water potential.…read more

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