Cell Structure (1)

Question 1

What is viewed under a light microscope and why

Answer 1

Specimens can be viewed under a light microscope; this allows some details of cellular material to be observed

Question 2

What are permanent slides

Answer 2

Such slides are produced by cutting very thin layers of tissue which are stained and permanently mounted on a glass slide for repeated use

Question 3

How to prepare a temporary slide for liquid specimen, purpose of coverslip

Answer 3

Add a few drops containing the liquid sample to a clean slide using a pipette

Lower a coverslip over the specimen and gently press down to remove air bubbles

Coverslips protect the microscope lens from liquids and help to prevent drying out

Question 4

How to prepare a temporary slide for solid specimen

Answer 4

Use scissors or a scalpel to cut a small sample of tissue, and peel away or cut a very thin layer of cells from the tissue sample

The preparation method always needs to ensure that samples are thin enough to allow light to pass through

Place the sample onto a slide

A drop of water may be added at this point

Apply iodine stain

Gently lower a coverslip over the specimen and press down to remove any air bubbles

Question 5

How to prepare a slide using human cells

Answer 5

Brush teeth thoroughly with normal toothbrush and toothpaste

This removes bacteria from teeth so they don’t obscure the view of the cheek cells

Take a sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5-10 seconds

Smear the cotton swab on the centre of the microscope slide for 2-3 seconds

Add a drop of methylene blue solution

Methylene blue stains negatively charged molecules in the cell, including DNA and RNA

This causes the nucleus and mitochondria to appear darker than their surroundings

Place a coverslip on top

Lay the coverslip down at one edge and then gently lower the other edge until it is flat

This reduces bubble formation under the coverslip

Absorb any excess solution by allowing a paper towel to touch one side of the coverslip

Question 6

Why are specimens stained

Answer 6

The cytoplasm and other cell structures may be transparent or difficult to distinguish; stains allow them to be viewed clearly under a light microscope

Question 7

What are the stains and respective uses

Answer 7

Iodine - Stains starch blue-black, and colours nuclei and plant cell walls pale yellow

Crystal violet - Stains cell walls purple

Methylene Blue - Stains animal cell nuclei blue

Congo red - Is not taken up by cells and stains background red to provide contrast with cells present

Question 8

What are biological drawings

Answer 8

Line drawings that show specific features that have been observed when the specimen was viewed

Question 9

What are the rules and conventions when drawing biological drawings

Answer 9

The drawing must have a title

The magnification under which the observations shown by the drawing are made should be recorded if possible

A scale bar may be used

A sharp pencil should be used

Drawings should be on plain white paper

Lines should be clear, single lines without sketching

No shading should be used

The drawing should take up as much of the space on the page as possible

Well-defined structures should be drawn

Only visible structures should be drawn, and the drawing should look like the specimen

The drawing should be made with proper proportions

Structures should be clearly labelled with label lines that:

Do not cross

Do not have arrowheads

Connect directly to the part of the drawing being labelled

Are on one side of the drawing

Are drawn with a ruler

Question 10

Difference between plan drawings and drawings of cells

Answer 10

Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram)

Question 11

What is magnification

Answer 11

Magnification is the number of times that a real-life specimen has been enlarged to give a larger view/image

Question 12

What is the eyepiece graticule

Answer 12

The eyepiece graticule is an engraved ruler that is visible when looking through the eyepiece of a microscope

Eyepiece graticules are often divided into 100 smaller divisions known as graticule divisions, or eyepiece units

Question 13

What does each eyepiece graticule division represent and how is it calibrated

Answer 13

The values of the divisions in an eyepiece graticule vary depending on the magnification used, so the graticule needs to be calibrated every time an object is viewed

The calibration is done using a stage micrometer; this is a slide that contains a tiny ruler with an accurate known scale

1 graticule division = number of micrometers divided by number of divisions
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Question 14

What are the purpose of stage micrometer and eye piece graticule

Answer 14

An eyepiece graticule and stage micrometer are used to measure the size of an object when viewed under a microscope

Question 15

How to calculate the size of a specimen

Answer 15

The calibrated eyepiece graticule can be used to measure the length of an object

The number of graticule divisions covered by an object need to be multiplied by the magnification factor:

graticule divisions covered by object x magnification factor = length of object (µm)

Question 16

What is a light microscope made up of

Answer 16

A light microscope has two types of lens which allow it to achieve different levels of magnification:

An eyepiece lens, which often has a magnification of x10

A series of objective lenses, each with a different magnification, e.g. x4, x10, x40 and x100

Question 17

How do I calculate the total magnification of a specimen being viewed

Answer 17

To calculate the total magnification of a specimen being viewed, the magnification of the eyepiece lens and the objective lens are multiplied together:

total magnification = eyepiece lens magnification x objective lens magnification

Question 18

What is resolution and what happens if it is too low

Answer 18

The resolution of a microscope is its ability to distinguish two separate points on an image as separate objects; this determines the ability of a microscope to show detail

If resolution is too low then two separate objects will be observed as one point, and an image will appear blurry, or an object will not be visible at all

The resolution of a microscope limits the magnification that it can usefully achieve; there is no point in increasing the magnification to a higher level if the resolution is poor

Question 19

How is the resolution of a light microscope limited

Answer 19

The resolution of a light microscope is limited by the wavelength of light

Visible light falls within a set range of light wavelengths; 400-700 nm

The resolution of a light microscope cannot be smaller than half the wavelength of visible light

The shortest wavelength of visible light is 400 nm, so the maximum resolution of a light microscope is 200 nm

E.g. the structure of a phospholipid bilayer cannot be observed under a light microscope due to low resolution:

The width of the phospholipid bilayer is about 10 nm

The maximum resolution of a light microscope is 200 nm, so any points that are separated by a distance of less than 200 nm, such as the 10 nm phospholipid bilayer, cannot be resolved by a light microscope and therefore will not be distinguishable as separate objects

Question 20

What is the max resolution of electron microscope and why

Answer 20

0.5nm since electrons have much smaller wavelength than light

Question 21

How does a light microscope operate

Answer 21

ght microscopes are used for specimens larger than 200 nm

Light microscopes shine light through the specimen

The specimens can be living, and therefore can be moving, or dead

Light microscopes are useful for looking at whole cells, small plant and animal organisms, and tissues within organs such as in leaves or skin

Question 22

How does an electron microscope operate

Answer 22

Electron microscopes, both scanning and transmission, are used for specimens larger than 0.5 nm

Electron microscopes fire a beam of electrons at the specimen

The electrons are picked up by an electromagnetic lens which then shows the image

Electron microscopy requires the specimen to be dead, meaning that they can only be used to capture a snapshot in time, and not active life processes as they occur

Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail

Question 23

Difference between transmission and scanning electron microscopes

Answer 23

Transmission electron microscopes (TEM) fire electrons through a specimen

Scanning electron microscopes (SEM) bounce electrons off the surface of a specimen

Question 24

Differences between electron and light microscopes

Answer 24

1. Electron microscope
   Large machines that are permanently installed in laboratories

Need to create a vacuum for electrons to travel through

Specimen preparation is complex

Maximum magnification of x500 000

Maximum resolution of 0.5 nm

Specimens are always dead

2. Light Microscopes
   Small and portable

No vacuum required

Specimen preparation can be simple

Maximum magnification of x2000

Maximum resolution of 200 nm

Specimens can be living or dead

Question 25

What can cells be divided into

Answer 25

Eukaryotic and Prokaryotic

Question 26

Difference between Eukaryotic and Prokaryotic cells in their untrastructure

Answer 26

Eukaryotic cells have a more complex ultrastructure than prokaryotic cells

Question 27

What is ultrastructure

Answer 27

Internal structure of cells

Question 28

What does the cytoplasm of eukaryotic cells contain

Answer 28

The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles

Question 29

What is the cell membrane

Answer 29

All cells are surrounded by a cell surface membrane which separates the inside of cells from their surroundings Cell surface membranes controls the exchange of materials between the internal cell environment and the external environment The membrane is described as being partially permeable as it allows the passage of some substances and not others The cell membrane is formed from a phospholipid bilayer and spans a diameter of around 10 nm Cell surface membranes separate cell contents from the surrounding environment and control the passage of substances into and out of cells

Question 30

What is the nucleus

Answer 30

Present in all eukaryotic cells, the nucleus is a large organelle that is separated from the cytoplasm by a double membrane The nucleus contains the DNA, which is arranged into chromosomes Chromosomes contain DNA and proteins, which are collectively referred to as chromatin The nuclear membrane is known as the nuclear envelope, and contains many pores Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to move in The nucleus contains a region known as the nucleolus, which is the site of ribosome production The nucleus of a cell contains the genetic material

Question 31

What are the rough and smooth endoplasmic membrane

Answer 31

The endoplasmic reticulum (ER) is made up of a series of membranes that form flattened sacs within the cell cytoplasm The ER is linked with the nuclear envelope There are two distinct types of ER, with different roles within the cell 1. The rough endoplasmic reticulum (RER) Continuous folds of membrane that are linked with the nuclear envelope The surface of the RER is covered in ribosomes The role of the RER is to process proteins that are produced on the ribosomes 2. The smooth endoplasmic reticulum (SER) The SER does not have ribosomes on the surface It is involved in the production of lipids, and of steroid hormones such as oestrogen and testosterone The RER has ribosomes on its outer surface and is continuous with the nuclear envelope, while the SER lacks ribosomes

Question 32

What is the golgi body

Answer 32

The Golgi body is often referred to as the Golgi apparatus or the Golgi complex It consists of a series of flattened sacs of membrane It can be clearly distinguished from the ER, as it is not connected to other membrane-bound compartments, and it has a distinctive 'wifi symbol' appearance Its role is to modify proteins and package them into vesicles The Golgi body processes proteins and packages them into vesicles

Question 33

What is the mitochondria

Answer 33

Mitochondria (singular mitochondrion) are relatively large organelles surrounded by a double-membrane They are smaller than the nucleus and chloroplasts, but can be seen with a light microscope The inner membrane is folded to form cristae Mitochondria are the site of aerobic respiration within eukaryotic cells The mitochondrial matrix contains enzymes needed for aerobic respiration Small, circular pieces of DNA, known as mitochondrial DNA, and ribosomes are also found in the matrix This allows the production of proteins required for respiration Mitochondria have a highly folded inner membrane; this provides a large surface area for embedded proteins that are involved with aerobic respiration

Question 34

What is a ribosome

Answer 34

Ribosomes are found in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells Each ribosome is a complex of ribosomal RNA (rRNA) and proteins 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells Smaller, 70S ribosomes (composed of 50S and 30S subunits) are found in prokaryotes, mitochondria and chloroplasts Ribosomes are the site of translation during protein synthesis Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein

Question 35

What are vesicles

Answer 35

Vesicles are small, membrane-bound sacs used by cells for transport and storage They can be pinched off the ends of the Golgi body; these are known as Golgi vesicles They can fuse with the cell surface membrane to allow exocytosis, or bud from the membrane during endocytosis Vesicles carry out transport and storage of substances within cells

Question 36

What is a lysosome

Answer 36

Lysosomes contain hydrolytic enzymes for the breakdown of biological molecules Lysosomes are specialised vesicles which contain hydrolytic enzymes Hydrolytic enzymes break down biological molecules, e.g. Waste materials, such as worn-out organelles Engulfed pathogens during phagocytosis Cell debris during apoptosis (programmed cell death)

Question 37

What are centrioles and where are they not found

Answer 37

Centrioles are hollow fibres made of microtubules Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division Note that centrioles are not found in flowering plants and fungi Centrioles are involved with the movement of chromosomes during cell division

Question 38

What are microtubules

Answer 38

Microtubules are hollow tubes made of tubulin protein α and β tubulin proteins combine to form dimers, which are then joined into protofilaments Thirteen protofilaments in a cylinder make a microtubule Microtubules make up the cytoskeleton of the cell The cytoskeleton is used to provide support and movement of the cell Microtubules are tubes of protein that are involved with the structure of cell cytoskeletons

Question 39

What are cilia

Answer 39

Cilia are hair-like projections made from microtubules They can be found of the surface of some cells where they Allow the movement of substances over the cell surface E.g. ciliated epithelial cells in the airways waft mucus away from the lungs

Question 40

What are microvilli

Answer 40

Microvilli are cell membrane projections that increase the surface area for absorption Microvilli are found in parts of the body that carry out absorption, e.g. The lining of the small intestine The kidney tubules Microvilli increase the surface area of the cell surface membrane

Question 41

What is the cell wall

Answer 41

Cell walls are outside cell surface membranes and offer structural support to some types of cell Structural support is provided by the polysaccharide cellulose in plants, and by chitin in fungi Cell walls are freely permeable and do not play a role in controlling the movement of substances into and out of cells Plant cell walls contain cellulose

Question 42

What are chloroplasts

Answer 42

Chloroplasts are larger than mitochondria, and are also surrounded by a double-membrane Membrane-bound compartments called thylakoids stack together to form structures called grana Grana are joined together by lamellae Photosynthetic pigments such as chlorophyll are found in the membranes of the thylakoids, where their role is to absorb light energy for photosynthesis Chloroplasts contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Question 43

What are plasmodesmata

Answer 43

Plasmodesmata are bridges of cytoplasm between neighbouring plant cells They allow the transfer of substances between plant cells Plasmodesmata mean that the cytoplasm of neighbouring plant cells is continuous; this allows substances to move easily between cells, e.g. sucrose can move easily from the surrounding cells into the phloem

Question 44

What is a large permanent vacuole

Answer 44

Large permanent vacuoles are found in plant cells, where they store cell sap and provide additional structural support to cells Vacuoles are sometimes found in animal cells, but these will be small and temporary Vacuoles are surrounded by the tonoplast, which is a partially permeable membrane Large permanent vacuoles store cell sap inside plant cells

Question 45

What are images taken by microscopes called

Answer 45

Micrographs

Question 46

How to identify cells in animal cell micrographs

Answer 46

t is possible to identify organelles in micrographs of animal cells on the basis of their shape, location, and size relative to other organelles, e.g. The nucleus will always be the largest organelle Mitochondria are the next largest, and are often cylindrical with a folded inner membrane Note that mitochondria are not always cylindrical, but can also be circular; their shape will depend on their age, and on the angle at which they were sliced during specimen preparation RER will be near the nucleus, and ribosomes can sometime be seen Lysosomes and vesicles will be smaller than mitochondria

Question 47

How to interpret plant cell micrographs

Answer 47

Plant cell micrographs can be interpreted using the same techniques as animal cells Large, seemingly empty spaces inside cells will be vacuoles The nucleus will be the largest dark region in the cell Chloroplasts are the next-largest organelles, and grana are often visible

Question 48

What are the common structures between plant and animal cells

Answer 48

Cell surface membrane Cytoplasm Nucleus Mitochondria Rough and smooth endoplasmic reticulum Golgi bodies Vesicles and lysosomes Ribosomes Microtubules

Question 49

What structures are exclusive to plant cells compared to animal cells

Answer 49

Cellulose cell wall Large permanent vacuoles Chloroplasts Plasmodesmata

Question 50

What are the structures found in animal cells and not plant cells

Answer 50

Centrioles Microvilli

Question 51

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Question 52

What do cells need to survive

Answer 52

All organisms require a constant supply of energy to maintain their cells and stay alive

Question 53

Where is energy required in organisms

Answer 53

In anabolic reactions to build larger molecules from smaller molecules To move substances across the cell membrane in active transport, or to move substances within the cell In animals, energy is required For muscle contraction In the conduction of nerve impulses

Question 54

What does ATP do

Answer 54

ATP from respiration is used to transfer energy in all energy-requiring processes in cells Energy is released from ATP when it is broken down to ADP and inorganic phosphate This process is reversed during respiration to make ATP and maintain a supply of energy

Question 55

What is ATP

Answer 55

Adenosine Triphosphate (ATP) is a nucleotide (DNA and RNA are nucleotides)

Question 56

What common organism has prokaryotic cells

Answer 56

Bacteria

Question 57

What is difference in cellular structure between prokaryotes and eukaryotes

Answer 57

Their genetic material is free in the cytoplasm and is circular Eukaryotic genetic material is packaged as linear chromosomes in the nucleus Prokaryotes lack membrane-bound organelles This means that they do not have any internal structures surrounded by membrane, e.g. a nucleus or mitochondria They are many times smaller than eukaryotic cells Prokaryotic cells are usually 1-5 μm in diameter, while eukaryotic plant cells can be 10-100 μm across Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S) Their cell walls are made of peptidoglycan rather than cellulose or chitin

Question 58

Are prokaryotes unicellular

Answer 58

True

Question 59

Are eukaryotes unicellular

Answer 59

False - can function together in multicellular organisms

Question 60

What structures are always present in prokaryotes

Answer 60

Cell wall (peptidoglycan) Cell surface membrane Cytoplasm Circular DNA Ribosomes

Question 61

What structures are sometimes present in prokaryotes and uses

Answer 61

Flagellum - motion Capsule - protection Infolding of cell surface membrane - allow photosynthesis or carry out nitrogen fixation Plasmid - small circle of DNA Pili - for attachment to other cells or surfaces for sexual reproduction

Question 62

Features of prokaryotes in size, genetic material, cell division, ribosomes, cell wall and organelles

Answer 62

1. Size 0.5-5 μm 2. Genetic material Circular chromosome in the cytoplasm Not associated with proteins 3. Cell division Binary fission No spindle fibres involved 4. Ribosomes 70S 5. Organelles No membrane-bound organelles 6. Cell wall Made of peptidoglycan

Question 63

Features of eukaryotes

Answer 63

1. Size Up to 100 μm 2. Genetic material Linear chromosomes in the nucleus Associated with histone proteins 3. Cell division Mitosis or meiosis Chromosomes are separated by spindle fibres 4. Ribosomes 80S 5. Organelles Multiple membrane-bound organelles, e.g. a nucleus, mitochondria, chloroplasts 6. Cell wall Made of cellulose in plants, or chitin in fungi

Question 64

What are viruses and are they cells

Answer 64

Viruses are non-cellular particles that infect living cells Note that viruses are not cells, and they are not considered to be living organisms, so are referred to as 'particles'

Question 65

Size of a virus

Answer 65

20-300nm

Question 66

What do viruses have structurally

Answer 66

A nucleic acid core made of either DNA or RNA A protein coat called a capsid

Question 67

What do viruses sometimes have structurally

Answer 67

Some viruses have an outer layer called an envelope; this forms from the membrane phospholipids of the host cell in which they were produced

Question 68

How do viruses reproduce

Answer 68

Viruses can only reproduce by infecting living cells and using their protein-building machinery to produce new viral particles Viruses use attachment proteins on their surface to bind to and infect their host cells