Cell Structures (OCR GCSE Combined Science A (Gateway): Biology): Flashcards
Exam code: J250
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Define magnification.
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Define magnification.
How many times larger an image appears compared with the actual size of the specimen.
What is the function of the objective lenses on a light microscope?
They magnify the image of the specimen.
Different objective lenses give different magnifications, so the specimen can be viewed at low or high power.
What is the stage of a light microscope used for?
It holds the glass slide in position under the objective lens.
Why does a light microscope need a lamp?
Light must pass through the thin specimen so that an image can be seen through the eyepiece lens.
What is the difference between the coarse and fine focus?
The coarse focus makes large adjustments to bring the specimen roughly into focus.
The fine focus makes small adjustments to produce a sharp image.
Why is a coverslip placed over a specimen on a slide?
It holds the specimen flat and protects it.
It is lowered gently to avoid trapping air bubbles, which would obscure the image.
Stains are used to view ＿＿＿＿＿＿＿＿ specimens or to highlight different structures and tissues.
Stains are used to view colourless specimens or to highlight different structures and tissues.
Which stain is commonly used when preparing a slide of onion cells?
Iodine solution.
Which stain is commonly used when preparing a slide of cheek cells?
Methylene blue.
When using a light microscope you should always start with the ＿＿＿＿＿＿＿＿ power objective lens.
When using a light microscope you should always start with the lowest power objective lens.
Why should you start with the lowest power objective lens?
It is easier to find the specimen in the field of view.
It also reduces the risk of the lens damaging the slide or coverslip.
Why is a drop of water added beneath the coverslip?
The thin layer of cells would otherwise dry out and be damaged.
Write the equation used to calculate magnification.
Magnification = image size ÷ actual size
True or False?
The eyepiece and objective magnifications are added together.
False.
They are multiplied: total magnification = magnification of eyepiece lens × magnification of objective lens.
An image of a cell is 30 mm wide at ×3000. Calculate the actual size.
Actual size = image size ÷ magnification
30 ÷ 3000 = 0.01 mm (10 µm)
Why must both measurements be in the same unit before calculating?
Magnification compares two sizes, so mixing units gives the wrong answer.
Convert first, remembering that 1 mm = 1000 µm.
True or False?
Magnification has no units.
True.
It is a comparison of two sizes, so it is written simply as ×10 or ×5000.
Define eukaryotic cell.
A cell in which the genetic material is enclosed within a nucleus.
Plants and animals are made of eukaryotic cells.
Define prokaryotic cell.
A cell with no nucleus, whose genetic material floats free in the cytoplasm.
Bacteria are prokaryotic cells.
What is the function of the nucleus?
It contains the genetic material (DNA), packaged into chromosomes, which controls the activities of the cell.
What is the function of the mitochondria?
They are the site of aerobic respiration.
They contain the enzymes needed for the reactions of cellular respiration.
What is the function of the ribosomes?
They are the site of protein synthesis.
What is the function of the chloroplasts?
They are the site of photosynthesis.
They contain the green pigment chlorophyll, which absorbs light.
How does the cell membrane control what enters and leaves a cell?
It acts as a selective barrier, allowing some molecules through but not others.
Why does the cell membrane contain receptor molecules?
They allow the cell to detect and respond to signals from outside, such as hormones.
Define plasmid.
A small loop of extra DNA found in a prokaryotic cell, separate from the main chromosome.
Why are plasmids an advantage to bacteria?
They can be exchanged between bacteria and can carry useful genes, for example those giving antibiotic resistance.
In a prokaryotic cell the genetic material is one long ＿＿＿＿＿＿＿＿ chromosome.
In a prokaryotic cell the genetic material is one long circular chromosome.
＿＿＿＿＿＿＿＿ is the gel-like substance in which most of the cell's chemical reactions take place.
Cytoplasm is the gel-like substance in which most of the cell's chemical reactions take place.
Where is the genetic material found in a eukaryotic cell?
Inside the nucleus, packaged into linear chromosomes.
Name a sub-cellular structure found in plant cells but not animal cells.
Chloroplasts, found in the cells of leaves and stems.
True or False?
Prokaryotic cells are much larger than eukaryotic cells.
False.
Prokaryotic cells are much smaller, often only about 1 µm across, and have a much simpler structure.
True or False?
Both prokaryotic and eukaryotic cells have a cell membrane.
True.
Both are enclosed by a cell membrane that acts as a selective barrier.
Define resolution.
How well two points that are close together can be told apart.
Why do electron microscopes have a higher resolution than light microscopes?
They use a beam of electrons, whose wavelength is far shorter than that of visible light.
An electron microscope uses a beam of ＿＿＿＿＿＿＿＿ instead of light to form an image.
An electron microscope uses a beam of electrons instead of light to form an image.
How has electron microscopy increased our understanding of cells?
Its much higher resolution and magnification allow sub-cellular structures to be seen in far greater detail, such as the internal structure of mitochondria.
What does TEM stand for?
Transmission electron microscope.
Which type of microscope has the highest resolution?
The transmission electron microscope (TEM), the most powerful type of electron microscope.
Name two structures understood better because of the electron microscope.
The nucleus and the cell membrane.
The internal structure of mitochondria and chloroplasts can also now be seen.
Why can a light microscope not show the inside of a mitochondrion?
Its resolution is too low to separate structures that are very close together.
The ＿＿＿＿＿＿＿＿ of an electron beam is much shorter than that of visible light.
The wavelength of an electron beam is much shorter than that of visible light.
Give two limitations of using an electron microscope.
They are very expensive and are not portable.
Preparing the samples is also a complicated process.
Give one advantage of a light microscope over an electron microscope.
It is cheap and easy to use, and stained specimens can be viewed in colour.
True or False?
Electron micrographs are produced in natural colour.
False.
They are black and white; false colour can be added afterwards.
True or False?
A TEM has a higher magnification than a light microscope.
True.
It has both a much higher magnification and a much higher resolution.
Why are stains often needed with a light microscope but not a TEM?
Many sub-cellular structures are colourless under light, so stains are needed to make them visible.
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