Stuff I missed for Cells

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How does a compound light microscope work?
Two simple convex glass lenses are used in a compound light microscope.
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What are the disadvantages of a light microscope and how do we overcome this?
The relatively long wavelength of light means that a LM has a resolution of 0.2µm. This can be overcome by using beams of electrons instead of beams of light - the beam of electrons have a resolution of 0.1nm because of their shorter wavelength.
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What is the resolution/resolving power of a microscope?
The minimum distance apart that two objects can be in order for them to appear as separate items.
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What does resolution/resolving power depend on?
The wavelength or the form of radiation used.
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What is cell fractionation?
This is the process where cells are broken up and the different organelles they contain are separated out.
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Before cell fractionation begins, what type of solution is the tissue placed in?
Cold, buffered, isotonic solution.
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Why is the solution that the tissue is placed in before cell fractionation cold?
To reduce enzyme activity that may break down the organelles.
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Why is the solution that the tissue is placed in before cell fractionation isotonic?
To prevent organelles bursting or shrinking as a result of osmotic gain or loss of water.
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Why is the solution that the tissue is placed in before cell fractionation buffered?
So that the pH does not fluctuate. Any change in pH could alter the structure of the organelles or affect the functioning of enzymes.
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What are the two stages to cell fractionation?
Homogenation and ultracentrifugation.
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What is the homogenisation of a cell?
The cells are broken up by a homogeniser which releases the organelles from the cell. The resultant fluid (homogenate) is then filtered to remove any complete cells and large pieces of debris.
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What is ultracentrifugation?
Ultracentrifugation is the process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge. This spins tubes of homogenate at very high speed in order to create a centrifugal force.
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How does the process of ultracentrifugation work?
The tube of filtrate is placed in the centrifuge and spun at low speeds; the heaviest organelles are forced to the bottom of the tube, where they form a pellet. The supernatant is removed, transferred to another tube & spun faster than before.
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What is the speed of centriguation needed to separate: a) nuclei; b) mitochondria; and c) lysosomes.
a) 1,000; b) 3,500; and c) 16,500.
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Why do electron microscopes need a near vacuum in their chamber?
Because electrons are absorbed or deflected by the molecules in air.
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What are the two main advantages to electron microscopes?
It has a high resolving power and as electrons are negatively charged the beam can be focused using electromagnets.
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What are the limitations of the TEM and the SEM?
The whole system must be in a vacuum - can't observe living things. A complex staining process is needed - image still not in colour. The specimen must be extremely thin - TEM only. Image may contain artefacts.
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What are artefacts?
They are things that result from the way the specimen is prepared. They may appear on the finished photomicrograph but are not part of the natural specimen. It is therefore not always easy to be sure that what we see really exists in that form.
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Why can't we always achieve the maximum resolution?
Difficulties preparing the specimen limit the resolution that can be achieved. Also a higher energy electron beam is required which may destroy the specimen.
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What does a TEM consist of?
It consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.
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How does a TEM work to produce an image?
The beam passes through a thin section of the specimen. Some parts absorb the electrons and appear dark; others allow them to pass through and appear bright. An image is produced on a screen and this can be photographed to produce a photomicrograph.
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What is a limitation of the TEM's photomicrographs and how do we overcome it?
In the TEM, the specimens must be extremely thin to allow electrons to penetrate which results in a flat 2D image. We can partly get over it by taking a series of sections through a specimen and putting them together, but this is slow and complicated
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How does an SEM work?
A beam of electrons is directed onto the specimen from above and is then passed back and forth across a portion of the specimen in a regular pattern. The specimen scatters the electrons in a pattern determined by the contours of the specimen surface.
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How is an image produced by an SEM?
Computer analysis of the pattern of scattered electrons and secondary electrons produced can be used to build up a 3D image.
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What are the resolving powers of a light microscope, a TEM and an SEM?
LM: 0.2µm. TEM: 0.1nm. SEM: 20nm.
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What is an eyepiece graticule?
It is a glass disc with a scale etched on it that is placed in the eyepiece of a microscope.
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Why do we have to calibrate the graticule for a particular objective lens
The scale on the eyepiece graticule cannot be used directly to measure the size of objects under a microscope's objective lens because each objective lens will magnify to a different degree.
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What is a stage micrometer and what is its normal scale?
It is a special microscope slide which also has a scale etched onto it. The scale is usually 2mm long and its smallest subdivisions are 0.01mm (10µm).
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What is the ultrastructure of a cell?
Each cell type has an internal structure that suits it for its job.
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What are eukaryotic cells?
They have a distinct nucleus and possess membrane-bound organelles.
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What are prokaryotic cells?
These lack membrane-bound organelles and do not have a nucleus or a nuclear envelope.
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What is the nuclear envelope?
It is a double membrane that surrounds the nucleus. Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface.
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What is the function of the nuclear envelope?
It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it.
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What is the function of nuclear pores?
They allow the passage of large molecules, such as messenger RNA, out of the nucleus. There are typically around 3000 pores in each nucleus.
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What is the size of a nuclear pore?
40-100nm in diameter.
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What is the nucleoplasm?
This is the granular, jelly-like material that makes up the bulk of the nucleus.
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What are chromosomes?
These consist of protein-bound, linear DNA.
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What is the nucleolus?
It is a small spherical region within the nucleoplasm.
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What is the function of the nucleolus?
It manufactures ribosomal RNA and assembles the ribosomes. There may be more than one nucleolus in a nucleus.
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What is the size of a mitochondrion?
1-10µm in length (rod-shaped).
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What are the cristae?
They are extensions of the inner membrane, which in some species extend across the whole width of the mitochondrion.
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What is the matrix?
It contains proteins, lipid, ribosomes and DNA that allows the mitochondria to control the production of some of their own proteins. Many enzymes involved in respiration are found in the matrix.
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Why do cells that have a high level of metabolic activity contain high numbers of mitochondria?
Mitochondria are responsible for the production of the energy-carrier molecule ATP from respiratory substrates such as glucose. Because of this, the number and size of the mitochondria and the number of their cristae are high in cells that need ATP.
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What is the size of a chloroplast?
They are normally disc-shaped, 2-10µm long and 1µm in diameter.
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How are chloroplasts adapted to their function?
The granal membranes provide a large surface area, the fluid of the stroma possesses all enzymes needed, and chloroplasts contain both DNA and ribosomes.
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How does the granal membranes having a large surface area mean that chloroplasts are adapted to their function?
The membranes provide a large SA for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis. These chemicals are attached to the membrane in a highly ordered fashion.
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How does the stroma containing all needed enzymes mean that chloroplasts are adapted to their function?
It possesses all the enzymes needed to make sugars in the second stage of photosynthesis.
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How does containing both DNA and ribosomes mean that chloroplasts are adapted to their function?
They can quickly and easily manufacture some of the proteins needed for photosynthesis.
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What is the structure of the smooth ER?
It is an extensive membrane system made up of cisternae, like the rough ER, but is more tubular in appearance than the rough ER. It lacks ribosomes on its outer membrane.
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How do Golgi transport the modified proteins and lipids?
They are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae. These vesicles may move to the cell surface, where they fuse with the membrane and release their contents to the outside.
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Where do the proteins and lipids modified by the Golgi come from?
The ER.
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What are the functions of lysosomes?
To hydrolyse material ingested by phagocytic cells, to release enzymes outside the cell to destroy material around the cell (exocytosis), to digest worn out organelles so we can recycle them, and completely break down cells after they have died.
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What is the size of a lysosome?
1µm in diameter.
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What features do cell walls contain?
1) They consist of a number of polysaccharides, such as cellulose. 2) There is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together.
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What do the cell walls of algae consist of?
Either cellulose or glycoproteins, or a mixture of both.
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What do the cell walls of fungi consist of?
They don't contain cellulose but comprise a mixture of a nitrogen-containing polysaccharide called chitin, a polysaccharide called glycan and glycoproteins.
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Examples of cells that contain lots of mitochondria are...
muscle and epithelial cells (e.g. intestine epithelial cells for active transport).
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Examples of cells that may contain an extensive ER are...
liver and secretory cells (e.g. intestine secretory cells) for making and storing lots of carbs, proteins and lipids.
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Examples of cells that may contain a well developed Golgi apparatus are...
secretory cells (e.g. intestine).
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Examples of cells that contain lots of lysosomes are...
secretory (e.g. epithelial) and phagocytic cells.
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All cells have the same genes. So how are cells specialised?
Every cell contains the genes needed for it to develop into any one of the many different cells in an organism. But only some of these genes are switched on (expressed) in any one cell, at any one time.
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What is a specialised cell?
This is a cell that have adapted to a particular function to perform it more effectively, either by changing its shape or by changing the numbers of each of their organelles.
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What is a tissue?
This is a collection of similar cells that perform a specific function.
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What are epithelial tissues?
They consist of sheets of cells and line the surfaces of organs. They often have a protective or secretory function.
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What are xylem tissues?
They are made up of a number of similar cell types and is used to transport water and mineral ions throughout the plant and also gives mechanical support.
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What is an organ?
An organ is a combination of tissues that are coordinated to perform a variety of functions, although they often have one predominant major function.
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What is an organ system?
This is when organs work together as a single unit. These organ systems may be grouped together to perform particular functions more efficiently.
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There are a number of organ systems in humans. Give three examples.
Digestive system, respiratory system and circulatory system.
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What is the size of a bacteria?
0.1 to 10µm in length.
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What is the structure of a bacterial cell?
It has a cell wall surrounded by a capsule; inside the cell wall is the cell-surface membrane. The genetic material is in the form of a circular strand of DNA, and separate from this are smaller circular pieces of DNA called plasmids.
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What is the function of the capsule?
The capsule of mucilaginous slime protects bacterium from other cells and helps groups of bacteria to stick together for further protection.
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What is within the cytoplasm of a bacterium?
70S ribosomes which synthesise proteins and food reserves stored as glycogen granules and oil droplets.
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What is the role of plasmids in a bacterium?
It possesses genes that may aid the survival of bacteria in adverse conditions, e.g. produces enzymes that break down antibiotics.
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What is the role of the circular DNA in a bacterium?
It possesses the genetic information for the replication of bacterial cells.
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What are viruses?
They are acellular, non-living particles.
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What is the size of a virus?
20-300nm.
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What is the structure of a virus?
They contain nucleic acids such as DNA or RNA as genetic material which is enclosed within a protein coat called a capsid. Some viruses, like HIV, are further surrounded by a lipid envelope. The lipid envelope or the capsid have attachment proteins.
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What is the purpose of the attachment proteins on the surface of a virus?
They are essential to allow the virus to identify and attach to a host cell.
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Are centrioles essential to spindle fibre formation?
No because plant cells don't have centrioles but do develop a spindle apparatus.
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How do prokaryotic cells divide?
By a process called binary fission.
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How does binary fission occur?
The circular DNA molecule replicates & both copies attach to the cell membrane. The plasmids also replicate. The cell membrane begins to grow between the two DNA molecules & begins to pinch inward, dividing the cytoplasm into two. A cell wall forms.
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What is the end result of binary fission?
Two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids.
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How do viruses replicate?
They replicate by attaching to their host cell with their attachment proteins. They then inject their nucleic acid into the host cell. The genetic information in this acid then provides the instructions for the host cell's metabolic processes to
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What are the three stages of the cell cycle?
Interphase (also known as resting phase), nuclear division and the division of the cytoplasm (cytokinesis).
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Roughly what percentage of the cell cycle is interphase?
90%
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What is the problem with cancer-treating drugs?
They also disrupt the cell cycle of normal cells. However, the drugs are more effective against rapidly dividing cells. Hair producing cells divide rapidly and so are vulnerable to damage (causes hair loss commonly seen in cancer patients).
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Card 2

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What are the disadvantages of a light microscope and how do we overcome this?

Back

The relatively long wavelength of light means that a LM has a resolution of 0.2µm. This can be overcome by using beams of electrons instead of beams of light - the beam of electrons have a resolution of 0.1nm because of their shorter wavelength.

Card 3

Front

What is the resolution/resolving power of a microscope?

Back

Preview of the front of card 3

Card 4

Front

What does resolution/resolving power depend on?

Back

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Card 5

Front

What is cell fractionation?

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