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Biology A Level > Cell Structure > Flashcards

Cell Structure Flashcards

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1
Q

Why are cells important to scientists ?

A

All life on Earth exists as cells which have basic features in common, providing indirect evidence for evolution (extra features cause cell differences)

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2
Q

What are the common components of an animal cell ?

A
  1. Phospholipid bilayer plasma membrane
  2. Cyctoplasm
  3. Mitochondrion
  4. Ribosomes
  5. Golgi apparatus
  6. Golgi vesicle
  7. Lysosome
  8. Nucleus
  9. Nucleolus
  10. Nuclear pores/nuclear envelope
  11. Rough endoplasmic reticulum
  12. Smooth endoplasmic reticulum
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3
Q

Describe the structure of the nucleus ?

A

The nucleus is surrounded by a clear nuclear envelope which is a double membrane, with nuclear pores. It encloses a fluid called nucleoplasm which contains nucleotides and enzymes for DNA and RNA synthesis

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4
Q

What is the function of the nucleus

A

The nucleus controls cell activities by regulating gene expression. It contains the majority of the cell’s genetic information in the form of chromosomes which are made out of chromatin

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5
Q

What is chromatin composed of ?

A

Proteins, RNA, DNA

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6
Q

Describe the structure of chromosomes ?

A

Linear DNA tightly wound around histones

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7
Q

Why does the nucleus contain nuclear pores ?

A

To allow MRNA and ribosomes to travel out of the nucleus. It also allows enzymes, RNA nucleotides signalling molecules to travel in

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8
Q

Describe the structure of the nucleolus ?

A

It is a densely stained structure that is not enclosed in a membrane

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9
Q

What is the function of the nucleolus ?

A

It is the site where ribosomes are assembled and ribosomal RNA is synthesised

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10
Q

Describe the structure of a mitochondrion ?

A

Oval shaped. It is enclosed in a double membrane with the inner layers folded inwards to form cristae with a large surface area. It also contains matrix which is a liquid that the cristae projects onto

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11
Q

What is the size range of mitochondria ?

A

2-5 micrometers

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12
Q

What does matrix contain ?

A

DNA, ribosomes and enzymes that help with the mitochondria’s function (aerobic respiration)

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13
Q

What is the function of mitochondria ?

A

It is the site of aerobic respiration, which is an exothermic process that releases energy in the form of ATP

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14
Q

Describe the structure of ribosomes ?

A

A very small organelle that freely floats in the cytoplasm or found attached to rough endoplasmic reticulum. It is made up proteins and ribosomal RNA. It is made up of a large and small subunit

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15
Q

What is the function of ribosomes ?

A

It is the site where the primary structure of proteins are made (translation phase)

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16
Q

Describe the structure of the Golgi apparatus ?

A

It is a stack of fluid-filled, membrane-bound, flattened sacks (cisternae)

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17
Q

What is the function of the Golgi apparatus ?

A

The Golgi apparatus receives proteins from the rough endoplasmic reticulum and modifies them further. It then packages these modified proteins into transportable vesicles (exocytosis) as some go to the cell surface to be secreted

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18
Q

Describe the structure of a Golgi vesicle ?

A

It is a small, fluid filled sac in the cyctoplasm surrounded by a membrane.

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19
Q

What is the function of the Golgi vesicles ?

A

It stores proteins made by the Golgi apparatus and transports them out of cells via the cell membrane

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20
Q

Describe the structure of the rough endoplasmic reticulum (RER) ?

A

It is a series of flattened membranes (cisternae) enclosing a fluid-filled space. It is studded with ribosomes and is continuous with the nucleus

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21
Q

What is the function of the rough endoplasmic reticulum (RER) ?

A

It folds (forms the tertiary structure) and processes proteins (may involve modification) made on attached ribosomes to be transported

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22
Q

Describe the structure of the smooth endoplasmic reticulum (SER) ?

A

A series of flattened membranes enclosing a fluid-filled space (sacs). They are different from rough endoplasmic reticulum as they do not have ribosomes on their surface

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23
Q

What is the function of smooth endoplasmic reticulum ?

A

Synthesises and processes lipids

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24
Q

Describe the structure of a lysosome ?

A

A lysosome is a large organelle surrounded by membrane. It has no clear internal structure but contains hydrolytic enzymes (which make an acidic internal environment)

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25
Q

What is the function of lysosomes ?

A

They contain hydrolytic (digestive) enzymes to break down waste, worn out material or invading cells in the cell (via phagocytosis)

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26
Q

Describe the structure of centrioles ?

A

Hollow fibres made out of microtubules.

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27
Q

Where are centrioles not found ?

A

In flowering plants and fungi

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28
Q

What is the function of centrioles ?

A

They are self-replicating organelles that help organise the spindle fibres during cell division. Two centrioles at a right angle form a centrisome

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29
Q

What is the function of the cyctoskeleton ?

A

It is made up of microtubules to provide support during cell division to prevent collapse, and movement of the cell

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30
Q

What additional structures are present in a plant cell that are not found in an animal cell ?

A

cell wall
chloroplast
permanent vacuole

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31
Q

Describe the structure of the chloroplast ?

A

Chloroplast are double membrane bound organelles. Within them they have stacked-up thykaloid membranes forming structures called grana. The grana are held together by structures called lamellae (thin, flat thykaloid membranes). There is a fluid in the chloroplast called the stroma

32
Q

What is the function of chloroplasts ?

A

The site of photosynthesis. The light-dependant stages occur in the thykaloid membrane parts, and the light-independant stages occur in the stroma

33
Q

Describe the structure of the permanent vacuole ?

A

Membrane bound organelle in the cytoplasm containing cell sap (weak solution of sugar and salts). The surrounding membrane is called the tonoplast.

34
Q

What is the function of the permanent vacuole ?

A

It helps to maintain cell pressure to keep the cell rigid and prevent wilting

35
Q

Describe the structure of the cell wall ?

A

Made out of beta glucose polysaccharide called cellulose. This means that adjacent beta glucose monomers in a cellulose chain must be inverted for glycosidic bonds to form. This also causes hydrogen bonds to form between parallel cellulose chains, forming microfibrils

36
Q

What is the function of the cell wall ?

A

It is freely permeable to allow substances to enter and exit out of the cell. It also has a high tensile strength due to microfibrils meaning that it can withstand turgor pressure and provide structural support to the cell

37
Q

Name the organelles that make up a prokaryotic cell ?

A

Single circular DNA chromosome
plasma membrane
ribosomes (70s)
cytoplasm
cell wall
flagella *
plasmids*
slime capsule *
pili (protein strands that help bacteria to join onto one another and transfer DNA)

38
Q

What is the difference between prokaryotic and eukaryotic cells ?

A

Eukaryotic cells have their DNA enclosed in a nuclear membrane, whereas prokaryotic cells do not. Their DNA floats freely in the cytoplasm and is not associated with proteins (is not tightly wound around histones but is still coiled)

39
Q

What is the function of the flagella ?

A

Causes a locomotion movement to help the cell to move.

40
Q

What is the function of the slime capsule ?

A

It surrounds the cell to provide protection, especially from immune system attacks

41
Q

What is the function of plasmids ?

A

They contain a small number of genes for antibiotic resistance etc. which can be transferred between bacteria

42
Q

What other secondary differences are there between prokaryotic and eukaryotic cells

A

Prokaryotic cell wall is made out of peptidoglycan (polymer of peptides and polysaccharides) whereas eukaryotic cell wall is made out of cellulose

Prokaryotic cells have 70s ribosomes, which are smaller than eukaryotic ribosomes

Prokaryotic cells have no membrane-bound organelles in their cytoplasm due to them being smaller than eukaryotic cells

43
Q

Describe viruses ?

A

They are acellular, non-living particles. This means that they cannot carry out the seven life processes

Movement
Respiration
Sensitivity
Growth
Reproduction (they have to invade a host cell and use its enzymes to reproduce)
Excretion
Nutrients

44
Q

Describe the structures found in a virus ?

A

Attachment proteins (to attach to and enter host cells)
Lipid envelope *
Matrix *
Capsid
Genetic material (DNA/RNA)
reverse transcriptase enzyme *

45
Q

Define magnification ?

A

How much larger an image is compared to the specimen being observed

46
Q

What is the formula for magnification ?

A

Magnification = image size/actual size

47
Q

Define resolution ?

A

How well a microscope distinguishes between two points that are close together (resolutions tell us minimum distance between two points for them to be distinguishable)

48
Q

What is the difference between resolution and magnification ?

A

Magnification is about size and how much larger a specimen is, but resolution is about how much detail can be observed (e.g. something can be very magnified but can be blurry)

49
Q

What are the three types of microscope ?

A

Optical (light) microscope
Transmission electron microscrope (TEM)
Scanning electron microscope (SEM)

50
Q

How does an optical microscope work ?

A

It uses light to form an image, focussed by a glass lens

51
Q

Describe the resolution of an optical microscope ?

A

Optical microscopes have the lowest resolution of 200nm because they use light to form an image. Light has a greater wavelength, so this means that the minimum distance it can distinguish between is greater, and hence a lower resolution

52
Q

What are the advantages of a light microscope ?

A

The specimen can be viewed alive as a vacuum is not required.

The preparation process and stains needed are also very simple (the specimen does not need to be as thin).

We can view the image directly and in colour with rarely any artefacts

53
Q

What are the disadvantages of a light microscope ?

A

They have the lowest resolution and magnification (x1500).

54
Q

Describe the resolution of an electron microscrope ?

A

Electron microscopes have a greater resolution because electrons have a much smaller wavelength, meaning that they can distinguish between points that are closer together.

55
Q

How does a transmission electron microscope work ?

A

They use an electromagnet to focus a beam of electrons, which is transmitted through the specimen. Denser parts of the specimen absorb more electrons, so appear darker on the micrograph

56
Q

What are the advantages of a transmission electron microscope ?

A

Highest magnification (x50,000,000) and resolution (0.1nm). This means that we can see sub-cellular structures in more detail than if we were to use an SEM or an OM

57
Q

What are the disadvantages of a transmission electron microscope ?

A

Slide preparation is very complex as a very thin sample is needed for electrons to pass through, and because heavy metal stains must be used. This increases the likelihood of artefacts being present

Using electron microscopes also requires. a vacuum as any air particles can cause interference with the micrograph due to collisions. This means specimens must be dead.

The image cannot be viewed directly, but it must be produced on a fluorescent screen and there is no colour

58
Q

How does a scanning electron microscope work ?

A

It uses an electromagnet to focus a beam of electrons that is scanned across the specimen. They beam bounces off of the surface of the specimen, and these are detected. This produces a 3D image of the external structure of the sample.

59
Q

What are the advantages of a scanning electron microscope ?

A

It has a high resolution (20nm) and it has a high magnification (x500,000). However, these aren’t as high as a transmission electron microscope.

SEMs allow us to view the 3D structure of a specimen

60
Q

What are the disadvantages of a scanning electron microscope ?

A

Preparing a sample is very complex as a thin sample must be used and it must be stained with a heavy metal dye. This means that artefacts are more likely to be produced

The specimen being observed cannot be alive as a vacuum is needed (to prevent collision interference with air particles)

The specimen cannot be viewed directly but is instead shown on a fluorescent screen. The micrograph has no colour.

61
Q

What is an artefact ?

A

Things that you view under a microscope that are not part of the actual specimen. They are produced during slide preparation

62
Q

Name some examples of how artefacts are formed ?

A

Fingerprints, dust, air bubbles, squashing the sample, staining the sample

63
Q

How would we prepare a microscopic slide for viewing ?

A
  1. Add a drop of water to the slide using a pipette
  2. Use tweezers to place a thin piece of the sample onto the water drop
  3. Add a drop of stain so that the sample can be viewed
  4. Add a cover slip on top by using a mounting needle (carefully tilt and lower to prevent air drops or the sample folding)
64
Q

Name some stain examples ?

A

Iodine dissolved in potassium iodide (plants)

Methyline blue (animal cells)

65
Q

What do we use cell fractionation and ultracentrifugation for ?

A

To separate organelles from other organelles so that we can study them

66
Q

Briefly describe the stages of cell fractionation ?

A
  1. Chop up sample using scissors
  2. Add sample to a solution that is cold, isotonic to the sample and buffered
  3. Place sample in a homogeniser to break down the cell membrane and release the organelles
  4. Filter the homogenate to remove any large cell debris using a gauze
67
Q

Describe the sample solution needed for cell fractionation and why each condition is necessary ?

A

Cold - to reduce enzyme activity (as that may break down organelles)

Isotonic to solution (so osmotic water movement does not occur and cause damage to the cells)

Buffered - so that enzymes do not denature

68
Q

Describe the stages for ultracentrifugation if we wanted to obtain a sample of chloroplasts ?

A
  1. Pour the filtrate into a tube and place the tube in the centrifuge
  2. Spin the tube at low speeds so that pellets of nuclei (most dense sub-cellular structure) form at the bottom
  3. Drain of the supernatent and place into another tube
  4. Spin at a higher speed in the centrifuge so that chloroplast pellet forms
69
Q

Order organelles from the most to least dense ?

A

Nuclei
Chloroplast
Mitochondria
Lysosomes
Endoplasmic Reticulum
Ribosomes

70
Q

How can we improve the homogenisation stage ?

A

Add an abrasive substance such as sand to the homogeniser so that its rough texture helps to further break down structures

Homogenise for longer

71
Q

What may happen if we do not filtrate the homogenate before placing it into a centrifuge ?

A

The first pellet formed will be of the remaining cell debris from the homogenate

72
Q

Outline a method to prepare a microscope in order to view a specimen ?

A
  1. Switch on the microscope light source
  2. Turn the objective lens to the lowest magnification (x40)
  3. Place the slide on the stage and secure using stage clips
    4 . Turn the coarse adjustment so that the stage is directly below the eyepiece (but so that the objective lens is not touching the slide)
  4. Look through the eyepiece and use the coarse focus to produce a clearer image of the specimen
  5. Turn the fine focus to place the sample into focus
  6. If necessary, repeat readjusting the resolution at higher magnifications to view the specimen more closely
73
Q

How else can we adjust a microscrope ?

A

We can readjust the condenser to change the light intensity. We can also move the microscope slide so that it is in the center of the field of view.

74
Q

How do we insert a graticule into the eyepiece of a microscopic lens ?

A

Unscrew the top lens, rest the graticule halfway down on the rim, and replace the top lens

75
Q

How do we calibrate an eyepiece graticule ?

A
  1. Place the stage micrometer on the stage of the microscope
  2. Using the low power objective, focus the image and count how many divisions of the eyepiece graticule are equal to one square on the stage micrometer
  3. One square on a stage micrometer is equal to 1000 micrometers
  4. Do 1000 micrometers divided by the number of divisions to find the length of one division
  5. Repeat at medium and high power objectives
76
Q

Describe how lysosomes play a role in phagocytosis ?

A

A white blood cell forms a vacuole around the cell, and the lysosome then fuses with the vacuole. The lysosome secretes digestive enzymes, which breaks down the invading cell. The soluble digestive products are released into the cytoplasm.

Biology A Level (3 decks)

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