Concise and coloured notes for Pearson Edexcel's International A Level Biology sat in January, June or October. This document follows the IAL specification and textbook order, covering all the topics in Unit 2, specifically Chapter 3: Cell Structure, Reproduction and Development.
Unit 2 Summary Edexcel International A Level Biology - Chapter 4
Unit 1 Summary Edexcel International A Level Biology - Chapter 2
Unit 1 Summary Edexcel International A Level Biology - Chapter 1
All for this textbook (5)
Written for
IAL Edexcel Biology
All documents for this subject (4)
1
review
By: shahnazvohra28 • 7 months ago
Seller
Follow
cherrydino
Reviews received
Content preview
AS Level Biology Notes for Topic 3 – Unit 2 ; Development, Biodiversity & Conservation
3A Cell Structure —-
Observing Cells
➢ Magnification → a measure of how much bigger the image you see is than the
real object.
➢ A light microscope has 2 types of lenses; an eyepiece lens of x10 and a series
of 3 objective lenses with different magnifications each.
○ Total magnification = Eyepiece lens magnification x Objective lens
magnification
➢ Resolution (resolving power) → a measure of how close 2 objects should be
before they’re seen as one. This is the ability to distinguish between two different
points.
➢ The resolution of a microscope limits the magnification that it’s capable of.
○ The resolution of a light microscope is limited by the wavelength of light.
○ Electron microscopes have a higher resolution as electrons have a much
smaller wavelength than visible light.
Light Microscopes: used for specimens larger than 200nm
➢ They shine light through the specimen – they’re useful for looking at whole cells,
small organisms and tissues within organs.
○ They’re cheap & portable, easy to use, can view whole & living specimens.
○ However, it only shows 2D images, requires staining which can introduce
artefacts, has a resolving power of 200nm and a magnification of x1500.
Electron Microscopes: used for specimens larger than 0.5nm
➢ The fired electrons are picked up by an electromagnetic lens to produce an image
– it requires the specimen to be dead, providing a snapshot. There are 2 types:
➢ Transmission Electron Microscopes (TEM) → transmit a beam through a thin
specimen and used to produce 2D images of smaller sub-cellular ultrastructures.
○ Can see ultrastructures in detail, has a resolving power of 0.1nm and a
magnification of x2,000,000
➢ Scanning Electron Microscopes (SEM) → scans a fire beam of electrons and
collects the electrons scattered on the surface, used to produce 3D images of
the specimens.
○ Has a resolving power of 20nm and a magnification of x200,000.
○ However, specimens must be fixed in plastic and viewed in a vacuum (dead),
specimens may get damaged by the beam, specimens must be stained with
heavy metals which introduced artefacts, they are expensive and large.
, AS Level Biology Notes for Topic 3 – Unit 2 ; Development, Biodiversity & Conservation
➢ Sometimes, specimens must be stained as the cytoplasm/cell structures are
transparent or difficult to distinguish.
○ Haematoxylin — stains plant/animal cells’ nuclei purple/blue.
○ Methylene blue — stains animal cells' nuclei blue.
○ Acetocarmine — stains chromosomes in dividing nuclei of cells.
○ Iodine — stains starch-containing material in plant cells blue-black.
○ Toluidine blue — stains tissues that contain DNA/RNA blue.
Cell Theory:
➢ All living organisms are made up of one or more cells.
➢ Cells are the basic functional unit in living organisms.
➢ New cells are produced from pre-existing cells.
➢ All cells share common features; cell surface membranes, cytoplasm, DNA and
ribosomes.
➢ Ultrastructure → internal structures of the cell.
Eukaryotic Cells – Common Cellular Structures
All cells are surrounded by a membrane which controls the
Cell Surface
exchange of materials between the internal and external
Membrane
environment.
Relatively large and separated from the cytoplasm by a
double membrane (nuclear envelope) which has many
pores that allow mRNA and enzymes to enter/exit.
Nucleus The nucleus also has chromatin (material for chromosomes),
chromosomes are sections of linear DNA tightly wound
around histones – darkly stained regions of the nucleus are
the nucleolus, which are sites of ribosome production.
The site of aerobic respiration, they’re surrounded by a
double membrane with the inner membrane folded into
cristae. The matrix contains enzymes needed for aerobic
Mitochondria
respiration, producing ATP; mitochondrial DNA and
ribosomes are also found in the matrix (needed for the
replication of mitochondria before cell division).
Found on the RER/cytoplasm and are not surrounded by a
membrane, they’re made up of ribosomal RNA and
Ribosomes proteins. In prokaryotes, they’re 70s (50s large/30s small) &
in eukaryotes, they’re 80s (60s large/40s small)
They’re the site of protein synthesis (translation).
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller cherrydino. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £6.65. You're not tied to anything after your purchase.
