Module 2: Cell Structure Flashcards
Define Magnification
The number of times larger an image appears, compared with the size of the object.
magnification = image size / actual size
Define Resolution
The clarity of an image; the higher the resolution, the clearer the image.
Why are light microscopes good?
- cheap
- easy to use
- portable and able to use in the field as well as in labs
- able to use to study whole living specimens
How do light microscopes work?
Light passes through a condenser lens then through the specimen
The light is then focused through the objective lens and then through the eyepiece lens
Light microscopes have several objective lenses that can be rotated to five different magnifications. Usually it
is x4, x10, x40, x100 (oil immersion lens)
The eyepiece lens is usually x10 and magnifies the image more.
Total magnification is eyepiece magnification x objective magnification
State the resolution and magnification that can be achieved by a light microscope
Magnification ‐ most light microscopes can magnify up to x1500 maximum. (There is actually little limit to how much they can magnify but after x1500, the image becomes blurred due to low resolution.
Resolution ‐ the maximum resolving power using light is 200nm . This means that is 2 objects are closer than 200nm they will be seen as 1 object. The limit is due to the size of the wavelength of light ‐ 2 objects can only be distinguished if light waves can pass between them.
Explain the need for staining samples in light microscopy
Some of these can be seen directly but many need to be stained as a lot of biological material is not coloured so you can’t see details.
Staining ‐ these are coloured chemicals that bind to chemicals on or in the specimen and allow it to be seen. Some cells bind to specific cell structures e.g. acetic orcein stains DNA dark red.
Sectioning ‐ specimens are embedded in wax and then thin sections are cut without distorting the structure. This is very useful for making sections of soft tissue like brain or lung.
What is an eyepiece graticule?
A microscope eyepiece can be fitted with a graticule.
This is transparent with a ruler etched on it.
When looking at the specimen the eyepiece graticule scale it superimposed onto it and the specimen can be measured in eyepiece units (epu).
Eyepiece units are arbitrary (not of fixed length) - they represent different lengths at different magnifications.
The eyepiece scale has to be calibrated (its value worked out) for each different objective lens.
A stage micrometer (tiny ruler) is placed on the microscope stage. Each division on the ruler is usually 0.01mm (10µm)
The stage micrometer is compared with the eyepiece graticule scale to see how many eyepiece units are a length of the the stage micrometer
Then the size of each epu is calculated.
example:
40 epu = 1mm
(1000µm).
Therefore 1epu = 1000µm/40epu
How do electron microscopes work?
• Electron microscopes generate a beam of electrons.
• Electromagnets are used to focus the electrons onto a specimen (instead of lenses).
• The image produced from the electron beam is projected onto a screen to make a black and white image (electron
micrographs).
Why do electron microscopes have a higher resolution than light microscopes?
We can achieve higher resolutions with electron microscopes (than with light microscopes) because electron beams have a shorter wavelength than light so the resolution is greater.
Transmission electron microscope (TEM)
Maximum magnification: X 500 000
Maximum resolution: 0.1nm
- A beam of electrons transmitted through the specimen.
- Denser parts absorb more electrons and makes them look darker on the image.
+ Highest resolution
- Can only be used with very thin prepared samples
- Produce 2D images
Scanning electron microscope (SEM)
Maximum magnification: X 100 000
Maximum resolution: 10nm
• A beam of electrons is directed onto the sample and the electrons bounce off the surface to give a final 3D image
+ Can be used with 3D (dead) specimens
- Lower resolution (than TEM)
Advantages and disadvantages of electron microscopes
Advantages:
+ SEM can produce 3D images that show shapes and cellular or tissue arrangement.
+ High resolution.
+ Can produce very detailed images of structures inside cells.
Disadvantages:
- Extremely expensive
- Electron beams are deflected by molecules in the air ‐ samples have to be placed in a vacuum and so specimens must be dead
Compare and contrast light microscopes and electron microscopes
INSERT PICTURE
What is the structure and function of a nucleus?
Structure:
The nucleus is surrounded by nuclear envelope (double membrane) studded with pores to allow mRNA and nucleotides through.
Contains genetic material in form of chromatin.
Contains nucleolus (ribosome factory)
Function:
Controls cell activities
Stores the organism’s genome.
Provides instructions for protein synthesis.
What is the structure and function of a nucleolus?
Structure:
The nucleolus does not have a membrane around it. It contains RNA.
Function:
This is where ribosomes are made.
What is the structure and function of a nuclear envelope?
Structure:
The nuclear envelope consist of a double membrane which is wrapped around the nucleus.
Function:
The nuclear envelope separates the contents of the nucleus from the rest of the cell.
In some regions the inner and outer membranes fuse together. At these points some dissolved substances and ribosomes can pass through.
What is the structure and function of Smooth Endoplasmic Reticulum?
Structure:
A system of membrane bound sacs, containing cisternae.
There are no ribosomes on its surface.
Function: SER contains enzymes that catalyse reactions involved with lipid synthesis, such as: - synthesis of cholesterol - synthesis of lipids/phospholipids - synthesis of steroid hormones
It is involved with absorption, synthesis and transport of lipids from the gut.
What is the structure and function of Rough Endoplasmic Reticulum?
Structure:
A system of membrane bound sacs, containing cisternae.
It is studded with ribosomes.
Function:
It provides a large SA for ribosomes, which assemble amino acids into proteins. These proteins then actively pass through the membrane into the cisternae and are transported to the golgi apparatus for modification and packaging.
What is the structure and function of the golgi apparatus?
Structure:
Consists of a stack of membrane-bound flattened sacs.
Function:
To modify and package proteins for secretion in vesicles.
To form lysosomes.
What is the structure and function of mitochondria?
Structure:
mitochondria are formed from 2 membranes separated by a narrow inter-membrane space.
the inner membrane is folded to increase its surface area into extensions called cristae. The central part is called the matrix.
Function:
aerobic respirations take place in the mitochondria and releases chemical energy in the form of ATP.w
What is the structure and function of chloroplasts?
Structure:
- Green organelle with double-membrane (envelope)
- Inside is a colourless matrix – the stroma
- Contains flattened membrane sacs called thylakoids.
- These stack together to form a granum (pl. grana).
- The grana can be interconnected by tubular extensions called intergranal lamella
- Chlorophyll molecules are found on the thylakoid membranes and in the intergranal membranes
Function:
Chloroplasts are the site of photosynthesis.
What is the structure and function of lysosomes?
Structure:
These are small bags formed by the golgi apparatus. Each is surrounded by a single membrane.
They contain powerful digestive enzymes.
They are abundant in phagocytic
Function:
Lysosomes keep powerful digestive enzymes away from the rest of the cell.
Lysosomes can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse.
What is the structure and function of centrioles?
Structure:
Centrioles consist of two bundles of microtubules at right angles to each other. The microtubules are made of tubulin protein subunits and are arranged to form a cylinder.
Animal cells only
No membrane
Function:
Before a cell divides, the spindle, made of threads of tubulin, forms from the centriole. Chromosomes attach to the spindle and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell.
What is the structure and function of vacuole?
Structure:
The vacuole is surrounded by a membrane called the tonoplast and contains fluid.
Function:
It is filled with water and solutes to maintain cell stability, because when fill it pushes against the cell wall, making it turgid.
If all the plant cells are turgid then this helps to support the plant.
