- A clean, no-nonsense summary for Edexcel AS/A level Biology B Topic 2
Cells, Viruses and Reproduction of Living Things
- Written using specification and contains everything needed for an A/A*
- Linked set of flashcards also available
,Microscopy
Cell Theory
Cell theory: All organisms are composed of 1 or more cells, all processes of life take place in
cells, cells are the smallest units, new cells are formed by the division of old cells, cells first
evolved 4 billion years ago but life happened gradually.
Magnification and Resolution
Magnification: How big the image is compared to the original object and is measured by a
factor. By using more lenses, magnification can be increased but the image becomes more
blurred. For magnification equation, image and actual must be in the same units.
Resolution: The smallest distance at which two separate objects can be distinguished and is
a distance measured in cm. Resolution is limited by the wavelength used so a shorter
wavelength improves resolution.
Light Microscopes
Specimens are illuminated with light and focused using glass lenses. There are 3 lenses:
Eyepiece lens (magnify image), Objective lens (magnify image) and Condenser lens (focus
light on specimen).
Magnification = objective lens x eyepiece lens.
A regular light microscope has a resolution of 200nm but with techniques such as
fluorescence, it can be reduced to 1nm.
Specimens need to be thin for light to pass through and should be stained. Common stains:
Iodine (starch in plant cells), Methylene Blue (DNA in nuclei), Toluidine Blue (lignin in xylem
cells) and Phalloidin (cytoskeleton).
Transmission Electron Microscope
Uses beams of electrons instead of light waves. Electrons are produced using hot wire,
focuses using electromagnets and detected on a phosphor screen or photographic film.
Specimens need to be thin and embedded in plastic. They also need to be stained with an
electron dense heavy metal (uranium and lead), making the images monochrome.
Conditions must be in a vacuum to prevent electrons scattering so it’s not suitable for living
organisms. Specimens can also be damaged by the electron beam and artefacts can occur.
Scanning Electron Microscope
Projects a fine beam of electrons onto a specimen and detects the electrons scattered back
by the surface. This is converted to a 3D image.
Internal structures cannot be seen.
Eukaryotic Cells
Features
Larger (> 10 μm), multicellular, nucleus with membrane bound organelles, linear DNA with
proteins, large 80S ribosomes, cytoskeleton, cell division by mitosis or meiosis, asexual or
sexual reproduction. Plant, animal and fungi cells are examples of eukaryotic cells.
Ultrastructure is the structure of cells that is only visible through an electron microscope like
organelles found in the cytoplasm which carry out specific roles. A tissue is a group of similar
cells performing a similar function. An organ is a group of linked tissues working together as
a functional unit. An organ system is a group of organs working together to carry out a
specific complex function.
, Nucleus
Structure: Largest organelle (10-20 μm). Surrounded by double membrane called nuclear
envelope. There are pores on the envelope called nuclear pores. Fluid inside the nucleus is
called the nucleoplasm.
Function: Stores DNA in the form of chromatin (DNA bound to proteins called histones)
when not dividing and chromosomes when dividing, controls activity of the cell, brings cell
division.
Nucleolus
Structure: Dense darker stained area inside the nucleus
Function: Site of ribosome and RNA production
Ribosomes
Structure: Made of rRNA and proteins. In eukaryotic cells, there are 80S ribosomes (made
from 60S subunit and 40S subunit). Ribosomes are found free in cytoplasm or attached to
rough endoplasmic reticulum.
Function: Used in the process of protein synthesis in translation to assemble polypeptide
chains.
Rough Endoplasmic Reticulum
Structure: Single membrane structure made of flattened sacs of membrane called cisternae
with ribosomes on its surface. The rough endoplasmic reticulum is found next to the nucleus
or joined to the nuclear envelope
Function: After a protein is made, the membrane forms vesicles to transport it around the
cell and carry out exocytosis.
Smooth Endoplasmic Reticulum
Structure: Same structure as the rough endoplasmic reticulum without the ribosomes.
Function: Manufactures lipids and steroids such as hormones.
Mitochondria
Structure: Double membrane structure that is capsule shaped and filled with a fluid called
the matrix. The inner membrane is folder into cristae for larger surface area. The surface of
cristae is studded with stalked enzymes (ATP synthase) which produce ATP. Contains
ribosomes to translate DNA code and make proteins. Contains their own genetic material as
mitochondrial DNA.
Function: Powerhouse of the cell by producing ATP through aerobic respiration which is used
as energy. Cells that require more ATP have more mitochondria.
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