Cell structure and Microscopes Flashcards
define resolution
resolution is the minimum distance apart that two objects can be in order for them to appear as separate items
what is the equation for magnification
image size/object size
define magnification
the increase in the size of an object when viewed through a microscope or other optical device
How do laser scanning confocal microscopes work?
they use a laser to only visualise a very narrow region of the sample
how do light microscopes work?
light (source of image) passes through the specimen and focused on an eyepiece which magnifies the image
How do electron microscopes work?
the electron (source of image) beam passes though the specimen and are focused using electromagnetic lenses. the electron beam has a very short wavelength and so has a high resolving power
what is the ability to reproduce detail known as?
the resolving power or resolution
define the details of a light microscope
- focused through a lens
- x1500 magnification
- 0.2 micrometers resolution
- live specimen can be used
- can use both section and external view of specimen
- cheap
- image viewed directly
- staining is required
define details of a transmission electron microscope (TEM)
- focused through EM condenser
- 50 x 10^6 magnification
- 0.1 nm resolution
- live specimen not used
- section of specimen used
- expensive (1.5 million)
- computer screen to see image
- staining is required
define details of a scanning electron microscope (SEM)
- focused through EM condenser
- 1.5 x 10^6 magnification
- 0.02 micrometers resolution
- live specimen not used
- external view of specimen use
- cost in between
- computer screen to see image
- staining is required
what is a stain?
used to increase contrast so that individual subcellular structures can be identified more easily e.g. methylene blue
what is an artefact?
things that you can see down the microscope that aren’t part of the cell or specimen that you’re looking at e.g. dust, fingerprints (created from a stain or the incorrect preparation of a slide)
wavelength - resolution
the shorter the wavelength, the higher the resolution
Limitations of TEM
- must be in a vacuum so no living species
- complex staining process
- not in colour
- section needs to be very thin
- image may contain artefacts
why can’t 0.1 nm always be used as the resolving power for the TEM?
- difficulties preparing specimen limits resolution
- higher energy electron beam required and may destroy specimen
what do we use to mesaure the size of objects when using a light microscope?
Eyepiece Graticule
What is an eyepiece graticule?
glass disc in the eyepiece with scale etched on it (usually 10 mm long with subdivision)
Why can’t a graticule be used to directly measure objects under the microscope?
With different objects, the objective lens will magnify to a different degree so it needs to be calibrated for a particular objective lens
How do we calibrate an eyepiece graticule?
Using a stage micrometer
What is a stage micrometer?
It has a scale etched onto it and it is placed on the stage of the microscope (usually 2 mm long and smallest subdivisions are 0.01 mm)
How do you calculate the scale for different objective lenses (graticule and micrometer)?
Dividing the differences in magnification
What are all the sub cellular structures in a eukaryotic cell?
Nucleus
Mitochondria
Ribosomes
Golgi apparatus
Endoplasmic reticulum
lysosomes
Centrioles
What extra sub cellular structures do plant cells have?
Permanent Vacuole
Chloroplast
Cell wall
What is the shape and size of the nucleus?
Spherical in shape and between 10 and 20 micrometers in diameter
What is the nuclear envelope?
- It’s a double membrane that surrounds the nucleus
- Its outer membrane is continuous with the endoplasmic reticulum and it often has ribosomes on the surface
- Controls the entry and exit of materials in the nucleus
- Contains the reactions taking place within the nucleus
What are nuclear pores?
- Allows the passage of large molecules (messenger RNA)out of the nucleus
- around 3000 pores in each nucleus (40-100 nm diameter)
What is the nucleoplasm?
Granular, jelly-like material that makes up the bulk of the nucleus
What are chromosomes?
They consist of protein-bound, linear DNA (the ends of the DNA are not joined to form a loop)
What is the nucleolus?
- A small spherical region in the nucleoplasm
- Manufactures ribosomal RNA and assembles the ribosomes
- May be more than one nucleolus in a nucleus
What are the functions of the nucleus?
- To act as the control centre through production of mRNA and tRNA (so protein synthesis)
- Retain genetic material of cell in form of DNA and chromosomes
- Manufacture ribosomal RNA and ribosomes
What are chromatins?
- They are the dark patches and they consist of DNA coiled around proteins called histones
- Together they form chromosomes
What is the endoplasmic reticulum?
- An elaborate 3D system of sheet-like membranes, spreading throughout the cytoplasm
- Continuous with the outer nuclear membranes
- The membranes enclose a network of tubules and flattened sacs called cisternae
What are the two types of Endoplasmic Reticulum?
Rough endoplasmic reticulum
Smooth endoplasmic Reticulum
What are the functions of endoplasmic reticulum?
- It has ribosomes present on the outer surface of the membranes
- It provides a large surface area for synthesis of proteins and glycoproteins
- Provides a pathway for transport of materials, especially proteins, throughout the cell
What are the functions of smooth endoplasmic reticulum?
- No ribosomes on surface and often more tubular in appearance
- It synthesises, stores and transports lipids and carbohydrates
What is the Golgi apparatus?
Consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles
What happens to the proteins and lipids produced by the ER?
- They pass though the Golgi apparatus in a strict sequence
- They are modified by the Golgi and often have non-protein components added to them (e.g. carbohydrates)
- They are also labelled so they can be sorted and sent to correct destinations
What happens to the proteins and lipids once they are sorted in the Golgi?
- They are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae
- The vesicles may move to the cell surface, where they fuse with the membrane and release the contents outside
What are the functions of the Golgi apparatus?
- add carbohydrates to proteins to make glycoproteins
- produce secretory enzymes
- secrete carbohydrates
- transport, modify and store lipids
- form lysosomes
What are mitochondria?
- They are the sites of the aerobic stages of respiration and so are responsible for the production of ATP
- They are usually rod shaped and 1-10 micrometers in length
What is the structure of a mitochondria?
- There is a double membrane around the organelle that controls the entry and exit of material
- The inner membrane folds to form Cristae
What are cristae?
- Extensions of the inner membrane
- They provide a large surface area for the attachment of enzymes and other proteins involved in respiration
Apart from the cristae and the membranes, what makes up the remainder of the mitochondria?
The matrix
What does the matrix of the mitochondria contain?
- Proteins, lipids, ribosomes and DNA that allow the mitochondria to control the production of some of their own proteins
- Many enzymes involved in respiration are found in the matrix
When is the number and size of mitochondria high in cells?
When they have a high level of metabolic activity and need a lot of ATP
What is a chloroplast?
The site of photosynthesis (disc-shaped and 2-10 micrometers long, 1 micrometer in diameter)
What is the chloroplast envelope?
A double plasma membrane that surrounds the organelle. Highly selective in what leaves and enters
What are the grana?
- They are stacks of up to 100 disc-like structures called thylakoids
- Some thylakoids have tubular extensions (lamellae) that join up with adjacent grana
- First stage of photosynthesis takes place in grana
What is the stroma?
A fluid-filled matrix where second stage of photosynthesis takes place. There are a number of structures in stroma(starch grains)
How are chloroplasts adapted to their function?
- Granal membranes
- The fluid in the stroma
- The chloroplasts
How are granal membranes adapted to their function?
They have a large surface area for attachment of chlorophyll, electron carriers and enzymes (all of which are attached in a highly ordered fashion)
What does the fluid of stroma possess?
All the enzymes needed to make sugars
What do chloroplasts contain?
DNA and ribosomes to easily manufacture some of the proteins needed for photosynthesis
What are lysosomes?
- Formed when vesicles from the Golgi apparatus contain enzymes
- They contain lysozomes, enzymes that hydrolyse the cell walls of certain bacteria
- They isolate these enzymes from the rest of the cell before releasing them, to the outside or into a phagocytic vesicle in the cell
- Up to 1 micrometer in diameter
What are the functions of lysosomes?
- To hydrolyse material ingested by phagocytic cells
- Release enzymes to outside of cell (exocytosis) to destroy material around cell
- To digest worn out organelles so useful chemicals can be re-used
- Completely break down cells after they have died (autolysis)
What are ribosomes?
Small cytoplasmic granules found in all cells. Many occur in cytoplasm or associated with RER
What are the 2 types of ribosomes?
80S and 70S
Where are 80S found?
Eukaryotic cells, around 25 nm in diameter
Where are 70 S found?
Prokaryotic cells, mitochondria and chloroplasts, they are slightly smaller
What are ribosomes made up of?
2 subunits, one large and one small, each of which contain ribosomal RNA and protein
How much of the dry mass of a cell can they account up to for?
25%
What are Centrioles?
-Small protein tubes of microtubules
- They form fibres in cell division known as spindles which separate chromosomes
What do cell walls consist of?
Microfibrils of the polysaccharide cellulose, embedded in a matrix
How strong are microfibrils?
They have considerable strength and contribute to the overall strength of the cell wall
What are the features of a cell wall?
- They consist of a number of polysacharrides
- There is a thin layer, middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together
What are the functions of the cellulose cell wall?
- Provide mechanical strength in order to prevent the cell from bursting under the pressure by the entry of water by osmosis
- Give mechanical strength to the plant as a whole
- To allow water to pass along it and so to contribute to the movement of water along the plant
What are cell walls of algae made up of?
Either cellulose, glycoprotein or mixtures of both
What are cell walls of fungi made up of?
They don’t contain cellulose but comprise a mixture of a nitrogen-containing polysaccharide called chitin, a polysaccharide called glycan and glycoproteins
What are vacuoles?
A fluid-filled sac bounded by a single membrane, called the tonoplast
What do vacuoles contain?
A solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocymins
What do vacuoles support?
Herbaceous plants and herbaceous parts of woody plants, by making cells turgid
How are the sugars and amino acids in a vacuole beneficial?
They may act as a temporary food store
What can the pigments in a vacuole do?
Colour petals to attract pollinating insects
What are the size of prokaryotic cells?
They are small, ranging from 0.1-10 micrometers in length
What is the cell wall made up of in a prokaryote?
Murein - polymer of polysaccharides and peptides
(may also be known as peptidoglycan)
How do some bacteria further protect themselves?
By secreting a capsule of mucilaginous slime around the cell wall
Apart from the cell wall, what also surrounds a prokaryotic cell?
The cell-surface-membrane (made from phospholipids)
What type of ribosomes are found in the cytoplasm in a prokaryote?
70s - smaller than in eukaryotic cells but still synthesise proteins
How do bacteria store food reserves?
Glycogen granules and oil droplets
How is genetic material stored in a prokaryote?
In the form of a circular strand of DNA
How is DNA also stored in a prokaryote?
- As smaller, circular pieces of DNA, plasmids
- These can reproduce themselves independently and may give the bacterium resistance to harmful chemicals (antibiotics)
- They are used extensively as vectors in genetic engineering
What is the role of the cell wall in a prokaryote?
A physical barrier that excludes certain substances and protects against mechanical damage and osmotic lysis
What is the role of the capsule in a prokaryote?
It protects bacterium from other cells and helps groups of bacteria to stick together for further protection
What is the role of the cell-surface-membrane in a prokaryote?
It acts as a differentially permeable layer, which controls the entry and exit of chemicals
What is the role of the circular DNA in a prokaryote?
Posesses the genetic information for the replication of bacterial cells
What is the role of a plasmid in a prokaryote?
Posesses genes that may aid the survival of bacteria in adverse conditions (produces enzymes that break down antibiotics)
Are there any membrane-bound organelles in a prokaryote?
No
Are there any chloroplasts in a prokaryotic cell?
No - only bacterial chlorophyll associated with the cell - surface membrane in some bacteria
What are viruses?
Acellular, non-living particles
How small are viruses?
20-300 nm
How do viruses store their genetic material?
They contain nucleic acids such as DNA or RNA
Where is the only place viruses can multiply?
All viruses are parasitic in that they can only reproduce by infecting living cells and using their ribosomes to produce new viral particles (can only multiply inside living host cells)
Where is the nucleic acid in a virus located?
It is enclosed within a protein coat called a capsid
What is on the outer surface of a capsid or outer membrane of a virus?
Attachment proteins which are essential to allow the virus to identify and attach to a host cell
What do viruses sometimes have?
An outer layer called an envelope formed usually from the membrane - phospholipids of a cell they were made in
