Cell Structure Flashcards
Light microscope overview
Gathers light from a tiny area of a thin, well-illuminated specimen mounted on glass slides. The image is magnified by a system of lenses (ocular lens or eyepiece).
Image seen = photomicrograph
Observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts.
Max. magnification + resolution of light microscope
×1500 (useful) - 2000
~ 0.2 micrometres (µm) or 200 nm
Advantages of light microscope
- Relatively cheap
- Easy to use/prepare samples
- Portable
- Study whole living specimens or individual living cells or dead cells
- Colour
Disadvantage of light microscope
- Limited resolution so cannot magnify any higher while still giving a clear image
- Cannot see smaller organelles e.g. ribosomes
- Cannot see in 3D
Laser scanning (confocal) overview
Used to scan an object point by point, as a single spot of focused light moves across the specimen with the help of scanning mirrors, causing fluorescence from the components with a dye.
The emitted light from the specimen is filtered through a pinhole aperture, so only radiated light from very close to the focal plane is detected. Computer complies the image for analysing.
When is laser scanning used
- used to clearly observe whole living specimens, as well as cells.
- in the medical profession, e.g. to observe fungal filaments within the cornea of the eye of a patient with a fungal corneal infection,
- used in many branches of biological research.
- thick section of tissue or small living organisms
- structure of the cytoskeleton in cells
Max. magnification + resolution of laser scanning confocal
X 2000
500 nm axially and 150 nm laterally
Advantages of laser scanning confocal
- Depth selectivity - focus on structures at different depths within a specimen (3D)
- Rapid, non-invasive technique allowing early diagnosis and effective management
- High resolution images compared to CT scan, MRI and USG for dermatological use.
- Used on thick specimens
- Living specimens
Disadvantages of laser scanning confocal
- High cost
- Limited number of excitation wavelengths available with common lasers
- Slow process and takes a long time to obtain an image - uses computer
- Laser can cause photodamage to cells
- Designed for better quality NOT high mag
TEM overview
Use electromagnets to focus a beam of electrons - transmitted through the specimen (stained with metal salts). Some e pass through and are focused on the photographic plate.
Denser parts of the specimen absorb more e and appear darker on the final image (contrast between different parts of the object). Photograph - e micrograph.
Max. magnification + resolution of TEM
2-50 million
0.2nm
Advantages of TEM
- High magnification
- High resolution
- Seen in 2D - simple structures
- Allows the internal structures within cells (or even within organelles) to be seen
Disadvantages of TEM
- Very expensive, large, difficult to move
- Specimens must be dead - vacuum
- Can’t be viewed in 3D
- Image is in black/white
- Complex, lengthy preparation of slides - artefacts can be introduced
- Sample preparation may result in distortion
- Need very thin specimens
- Specimen must be chemically fixed by dehydrated and stained
SEM overview
SEMs scan a beam of electrons across the specimen. This beam bounces off the surface of the specimen and the electrons are detected, forming an image.
This means SEMs can produce three-dimensional images that show the surface of specimens. Computer software programmes can add false colour
Max. magnification + resolution of SEM
x 15-200,000
0.5-4 nm
Advantages of SEM
- High magnification
- High resolution
- 3D image
- Used on thick or 3-D specimens
- Allow the external, 3-D structure of specimens to be observed
Disadvantages of SEM
- Very expensive, large, difficult to move
- Specimens must be dead - vacuum
- Specimen is often coated with a fine film of metal
- Image is in black/white
- Complex preparation of slides
- Sample preparation may result in distortion
- They give lower mag images (less detail) than TEMs
- Need a great deal of skill and training to use
Photomicrographs
Light - 2D colour
Laser - 3D colourful
SEM - 3D black and white
TEM - 2D black and white
What is differential staining
When stains bind to specific cell structures to identify different cellular components and cell types on a single preparation
What are stains
Coloured chemicals that bind to molecules in/on the specimen, making it easy to see.
Methylene blue use
All purpose stain for brilliant colour
Methylene blue colour
Deep blue colour
Acetic orcein use
Binds to DNA - chromosomes
Acetic orcein colour
Dark red
Eosin use
Cytoplasm, RBCs, collagen, and muscle fibres for histological examination
Eosin colour
Pink
Sudan use
Lipids
Sudan colour
Red
Iodine in potassium iodide solution use + colour
Cellulose in plant cell walls - yellow
Starch granules - blue/black (looks violet under the microscope)
Magnification formula
I = AM
Magnification = Image size / Object size
Magnification definition
The number of times larger an object appears, compared to the real object.
Resolution definition
The ability to distinguish between 2 points. The higher the resolution, the greater detail you see.
Resolution of eye
100 micrometers
How to work out magnification from a microscope
Objective lens x eyepiece lens (usually x10)
Microscope drawing rules
- title
- record magnification
- clear, single lines (no shading)
- take up half the space
- label lines should not cross or have arrowheads
- labels connect directly to the labelled part
EPG
eyepiece graticule units (small ruler)
1sm (stage micrometer)=
0.1mm, 100 μm
Calibration
1) Number of divisions at which EPG & SM match up
2) Length of stage micrometer section (mm) - SM/0.1
3) Length of eyepiece graticule division (EPQ) (mm) (2)/Number of EPG division
4) Length of EPG division (micrometer) - (3) x 1000
Nucleus structure
- Surrounded by a nuclear envelope to protect it from damage.
- Pores let substances in and out.
- Largest single organelle in the cell
- Contains coded genetic information in the form of DNA molecules.
- DNA associates with proteins called histones to form a complex called chromatin. Chromatin coils and condenses to form structures known as chromosomes.
Nucleus function
- Control centre of the cell
- Stores the genome
- Transmits genetic information
- Provides instructions for protein synthesis
Nucleolus structure
Area in the nucleus
Is not surrounded by a membrane
Contains RNA
Nucleolus function
The RNA is used to produce rRNA, which combines with proteins to form ribosomes
Nuclear membrane function
to act as a barrier that separates the contents of the nucleus (eg chromosomes, DNA) from the cytoplasm and chemical reactions in cytoplasm
Rough endoplasmic reticulum (RER) structure
A system of membranes containing cisternae (fluid filled cavities), coated with ribosomes.
Membrane is continuous with the nuclear membrane
Rough endoplasmic reticulum (RER) function
Intracellular transport system: the cisternae provide channels to transport substances to different parts of the cell
Provides a large SA for ribosomes to carry out protein synthesis
Smooth endoplasmic reticulum (SER) structure
A system of membranes containing cisternae (fluid filled cavities), but with no ribosomes.
The membrane is continuous with the nuclear membrane
Smooth endoplasmic reticulum (SER) function
Contain enzymes involved in lipid metabolism
- Synthesis of cholesterol
- Synthesis of phospholipids
- Synthesis of steroid hormones
Involved in absorption, synthesis and transport of lipids from the gut
Golgi apparatus structure
Stack of membrane-bound flattened sacs
Often seen with secretory vesicles that transport things to and from the Golgi
Golgi apparatus function
Modification of proteins
- Adding sugar to make glycoproteins
- Adding lipids to make lipoproteins
- Folding proteins into tertiary structure
Once modified the proteins can be packaged into secretory vesicles that are pinched off. These can be stored/moved to the plasma membrane for fusion or exocytosis
Mitochondria structure
Around 2-5 micrometres long
Double membrane structure with fluid filled space between.
Inner membrane is folded into cristae
Within the inner membrane is a fluid-filled matrix
Mitochondria function
ATP production during aerobic respiration
Most common in areas of high metabolic activity.
Self-replicating so can increase in number when there is high demand.
Also contain a small amount of mitochondrial DNA (mtDNA)
Chloroplast structure
Around 4-10 micrometres long
Double membrane structure
Inside are stack of membrane sacs called thylakoid membranes containing chlorophyll. Each stack is called a granum.
Surrounded by fluid called the stroma
Also contain loops of DNA and starch grains.
Chloroplast function
Site of photosynthesis
- Light is used to make ATP and water is split
- ATP is used to reduce carbon dioxide to make carbohydrates
Vacuole structure
Contains fluid (water and solutes)
Surrounded by a membrane called a tonoplast
Vacuole function
Maintains cell stability as it pushes against the cell wall. Cell is turgid
If all cells are turgid, it helps to support the plant
Lysosome structure
‘Bags’ surrounded by a single membrane and formed by the Golgi apparatus
Contain hydrolytic enzymes which aid in phagocytosis
Lysosome function
- Stores hydrolytic enzymes safely so they do not mix with other cell contents
- Engulf pathogens, foreign material and old organelles. Digest them and recycle useful substances back to the cell for reuse.
Cilia and undulipodia structure
Protrusions from the cell but still surrounded by the surface membrane
Made from centrioles
Contain microtubules
Cilia and undulipodia function
Epithelial cells in airways use cilia to waft mucus
Nearly all cells have at least one cilia to detect signals from its environment
Undulipodia helps sperm cells to move
Ribosome structure
Around 20nm wide
Made of rRNA and proteins
Consist of 2 subunits made in the nucleolus that combine in the cytoplasm
Some are free in the cytoplasm, some attach to the RER
Ribosome function
RER ribosomes – synthesising proteins that are exported out of the cell
Free ribosomes – synthesising proteins that are used inside the cell
Centrioles structure
Absent from (most) plants.
Centrioles come in pairs. Each centriole is made of a bundle of microtubules (protein subunits).
Each centriole aligns at a right angle to the other to form the pair.
Centrioles function
Forms the spindle fibres right before cells division. Spindle fibres attach to the chromosomes and pull chromosomes to opposite ends of the cell
Divide under the cell surface membrane to form cilia. Microtubules extend out of the centrioles to form the cilia or undulipodia on the surface
Cellulose cell wall structure
Found on the outside of the plasma membrane
Made of fibres of a carbohydrate polysaccharide – cellulose
Cellulose cell wall function
Provide strength and support
Maintain cell shape
Prevent cell bursting
Are permeable to solutions to pass through
Flagella function
a motility organelle that enables movement and chemotaxis
Plasma membrane function
provides protection for a cell, provides a fixed environment inside the cell, one is to transport nutrients into the cell and also to transport toxic substances out of the cel
Preparation and examination of microscope
https://www.savemyexams.co.uk/a-level/biology/ocr/17/revision-notes/2-foundations-in-biology/2-1-cell-structure/2-1-2-using-a-microscope/
Similarities between prokaryotes and eukaryotes
- Plasma membrane
- Cytoplasm
- Ribosome for protein synthesis
- DNA and RNA
Differences between prokaryotes and eukaryotes
- Smaller than eukaryotes
- Less developed cytoskeleton with no centrioles
- No nucleus
- No membrane-bound organelles e.g. Golgi or mitochondria
- Peptidoglycan cell wall
- Smaller ribosomes (70S, not 80S)
Prokaryotes also have …
- Protective capsule
- Small loops of DNA called plasmids and large loop of DNA (found within the nucleoid). No linear chromosomes
- Flagella for movement
- Pili – hair-like projections for adhesion and to exchange plasmids
Features of bacterial cell
- Flagella
- Plasma membrane
- Cell wall, made of peptidoglycan
- Cytoplasm
- Ribosome
- Pili
- Plasmid
- Nucleoid
- Mesosome
- Capsule
What is the slime capsules for
protection/adhesion
Plasmid
extra loop of DNA, involved in resistance - we’ve used it for genetic engineering
Mesosome
nucleoid replication
Invagination
in cell membrane - increases SA for transport
Production of proteins
- mRNA copy of the instructions (gene) for insulin is made in the nucleus. -> Transcription
- mRNA leaves the nucleus through a nuclear pore.
- mRNA attaches to a ribosome, in this case attached to rough endoplasmic reticulum. Ribosome reads the instructions to assemble the protein (insulin).
Secretion of proteins
- Insulin molecules are ‘pinched off in vesicles and travel towards Golgi apparatus.
- Vesicle fuses with Golgi apparatus.
- Golgi apparatus processes and packages insulin molecules ready for release.
- Packaged insulin molecules are ‘pinched off in vesicles from Golgi apparatus and move towards plasma membrane. -> Along the cytoskeleton
- Vesicle fuses with plasma membrane.
- Plasma membrane opens to release insulin molecules outside. = exocytosis
Cytoskeleton structure
Network of proteins consisting of
- microtubules made of tubulin
- intermediate filaments
- acting microfilaments
Cytoskeletal motor proteins
Myosin, kinesis, dyneins acts as molecular motors. They hydrolyse ATP as an energy source
Provides mechanical strength to cells
- Push against membrane to provide mechanical strength and keep the shape stable
- intermediate filaments anchor certain organelles e.g. nucleus in position
- can extend between tissues, so cells can adhere to one another and allow cell signalling
Aids transport within cells
Microtubules form the track over which motor proteins ‘walk’ and drag organelles
Microtubules make spindle fibres, which pull chromosomes to poles
Enables cell movement
Microtubules make up the cilia and undulipodia, which enable cell movement
Size of cell
0.01 – 0.10 mm
Width of cell membrane
7.5 - 10 nm
Size of nucleus
5-10 μm
Size of ribosome
