A2.2. Cells

Question 1

What are the advantages of freeze fracture electron microscopy?

Answer 1

Freeze fracture microscopy provides a planar view of membrane structure with high resolution details. The distribution and organization of membrane proteins and other molecules is rendered visible.

Question 2

Describe one use of fluorescent microscopy

Answer 2

Immunofluorescence uses antibodies chemically labelled with fluorescent dyes which bind to & mark target cells which is useful in cancer diagnosis.

Question 3

Which cell structures are found in both animal and plant cells?

Answer 3

Cytoplasm, genetic material and plasma membrane.

Question 4

Describe features of prokaryotic cell structure

Answer 4

Prokaryotes are smaller than eukaryotes, about the size of organelles of eukaryotic cells. Prokaryotes contain no true nucleus, and instead have a nucleoid DNA loop part of their circular DNA structure. A prokaryotic cell wall is made from peptidoglycan. It has 70s ribosomes for protein synthesis.

Question 5

What are some examples of unicellular organisms?

Answer 5

The paramecium and the chlorella

Question 6

Draw a eukaryotic cell (plant cell)

Reference A2.2 Slide deck 2, slide 17

Answer 6

.

Question 7

Compare and contrast the structure and function of cilia and flagella, with examples

Answer 7

Both are made up of microtubules.
Both are microtubules, and comprise a ring of 9 microtubule doublets surrounding a central pair of microtubules (9+2 arrangement). Both contain a basal body at the base, which is identical in structure to centrioles that anchor the cilia and flagella. Contrasting cilia and flagella, while flagella can be singular and long, cilia are shorter but numerous. Cilia can be found in certain protists such as the Euglena and Paramecium. They can also be found in the epithelium of the respiratory system to move mucus across the cell surface or in the oviduct to help move ovum from oavry to uterus. Flagella occur single as small motile cells such as sperm to aid movement.

Question 8

Compare and contrast eukaryotic cells with prokaryotic cells

Answer 8

Both prokaryotic and eukaryotic cells have a plasma membrane, ribosomes and genetic material. Prokaryotic cells do not have a true nucleus while eukaryotic cells do. The prokaryotic cell is made of peptidoglycan while the eukaryotic cell wall for plants is made from cellulose. Prokaryotic cells mostly have 70s ribosomes while eukaryotic cells have 80s ribosomes.

Question 9

State the structure and function of the mitochondria, and where it can be found

Answer 9

The mitochondria is a double membraned organelle that is the site of aerobic respiration. The exterior is called the matrix and contains aqueous solution of metabolites and enzymes needed for the respiration process. The interior membrane is called the cristae and is folded. The mitochondria s found in all cells, and abundant in muscle and liver cells.

Question 10

State structure function and location of ribosomes

Answer 10

Ribosomes consist of protein and RNA. Ribosomes are found anywhere in the plasma membrane, but can also be found within the mitochondria and chloroplast as 70s ribosomes. Ribosomes can also be bound to the rough endoplasmic reticulum. They are utilised as the site for protein synthesis in the translation process.

Question 11

State the location, structure and function of the RER

Answer 11

The rough endoplasmic reticulum is a network of interconnected folded membranes. It touches the nuclear envelope. The RER has many metabolically active cells. The RER in particular has cisternae which are flattened membrane sacs. Vesicles are formed at its margins and pinch off. site of synthesis, folding & post-translational modification of secretory proteins, e.g. digestive enzymes

Question 12

State the location, structure and function of the SER

Answer 12

The smooth ER manufactures lipids, phospholipids & steroids. In muscle cells, it stores calcium ions. Its structure consists of a branched network of tubular membranes. Not studded with ribosomes

Question 13

State the location, structure and function of the Golgi apparatus

Answer 13

The Golgi Apparatus synthesizes hormones and enzymes which it packages into vesicles (GA, SHE, PIV acronym). Consists of stack-like collection of flattened membranous sacs (cisternae)
One side of the membranes is formed by fusion of membranes of vesicles from ER. Opposite side is formed from swellings at the margin that pinches off. Abundant in metabolically active cells, i.e. secretory cells. It has no specific area within the cell, but it is encased within the plasma membrane.

Question 14

State the location, structure and function of lysosomes.

Answer 14

Lysosomes are temporary vacuoles, hence they are not found in plant cells that already have a large central vacuole. Lysosomes contain hydrolytic enzymes to break down foreign particles through phagocytosis. They can escape into the cell membrane. They can self digest, and are involved in programmed cell death.

Question 15

What makes up the cytoplasm?

Answer 15

Cytoskeleton is a network of fibers extending throughout the cytoplasm
It is made up of microtubules, microfilaments and intermediate filaments (FYI). Microtubules 1) provide mechanical support and maintain cell shape. 2) They also control the movement of chromosomes in cell division, guide the movement of secretory vesicles from Golgi apparatus to plasma membrane. Microfilaments- Each filament consists of a twisted double chain of actin (globular proteins)
1) It resists tension to maintain & change cell shape, involved in cleavage furrow formation during cell division and 2)regulates cell motility particularly during muscle contraction

Question 16

What is the function of the centriole?

Answer 16

It consists of 9 sets of three microtubules each arranged in a short, hollow cylinder.
Two centrioles occur at right angles outside the nucleus, forming the centrosome.

Centrioles replicate & grow spindle fibres to organize the movement of chromosomes during nuclear division.

Question 17

State the structure and function of chloroplasts.

Answer 17

Chloroplasts are membrane bound organelles. Thylakoids are loosely arranged in grana in the stroma (stack of coins).

Question 18

Permanent vacuole

Answer 18

A fluid-filled space surrounded by a single membrane called tonoplast.
When filled with water, it pushes the cytoplasm against the cell wall, creating turgor pressure to maintain plant tissue rigidity.
Chemicals such as pigments (betalains), carbohydrates, fats & proteins can be stored.
The interior is slightly acidic (pH 5.0) contains hydrolase enzymes to break down waste material

Question 19

State location, structure and function of cell wall.

Answer 19

Found in the exterior of plant cells, not considered an organelle.
Made up of cellulose arranged in bundles called microfibrils.
Allows plant cells to develop high internal pressure due to water influx & remain turgid.

NOT an organelle.

Question 20

State location, structure and function of glycoproteins.

Answer 20

Large molecules of proteins to which sugar molecules (oligosaccharides) are bonded together.
Enable animal cells to adhere together and to a basement membrane to form tissues & organs.

Question 21

Is a fungus cell a prokaryotic and eukaryotic cell?

Answer 21

Eukaryotic organisms that obtain food by absorbing nutrients from the external environment.
Do not have chloroplasts, thus cannot photosynthesize.
Have a cell wall made of chitin.
Mostly multicellular, although some are unicellular (yeast).
Reproduces asexually by budding.

Question 22

What are some examples of exceptions to eukaryotic cell structure? Hint: Sieve tub element, Mucos, red blood cells

Answer 22

Atypical cells are multi-nucleate but not divided into distinct cells.
Examples: The mould Mucor in which the body consists of fine-like structures called hyphae, striated muscle fibres in the skeletal muscles.

Red blood cells do not have nucleus so that more haemoglobin can be packed within & can adopt a biconcave shape for larger surface area for the exchange of oxygen.

Sieve tubes are made up of narrow, elongated sieve tube elements connected end-to-end, which have no nucleus & organelles.
The end walls (sieve plates) are perforated. Each sieve tube is connected to a companion cell by strands of cytoplasm (plasmodesmata) that passes through narrow gaps (pits) in the walls. Companion cells – contain membrane-bound organelles such as mitochondria to provide ATP for the transport of sucrose along the phloem.

Question 23

What is cell theory (key: 3 points)

Answer 23

Cell theory states that firstly, 1) Cells can only arise from pre-existing cells 2) Living organisms are composed of cells 3) Organisms consisting of only one cell carry out all functions of life in that cell’ cell perform life functions at some point in their existence

Question 24

What are the units of length in microscopy which we must remember?

Answer 24

1 mm= 1000 micrometres, 1 micrometre = 1000 nanometres

Question 25

Distinguish between resolution and magnification.

Answer 25

Resolution is the ability to separate visually small objects that are very close together. However, magnification is just the amount of times larger an image is than the specimen.

Question 26

List the features of cells that can be observed by electron microscopy that are not visible by the light microscope

Answer 26

The cell’s ultrastructure- its individual organelles and plasma membrane.

Question 27

State two problems that arise in electron microscopy because of the nature of an electron in relation to the living cell.

Answer 27

Firstly, it cause significant damage to biological specimens, especially living cells. The interaction of electrons with the sample can lead to ionization, molecular bond breakage, and other forms of radiation damage, which can kill or alter the cells.

Secondly, the equirement for Vacuum Conditions. However, living cells cannot survive in a vacuum, as it leads to dehydration and collapse of the cell structure.

Question 28

Describe cryogenic Microscopy and its advantages.

Answer 28

Technique—>Flash-freezing of cell, then exposes electrons to high resolution images, producing a 3D shape.

Advantages–> Some materials such as biological molecules are not comaptible with high-vacuum conditions and high intensity electron beams. Hence advantages of CRYOGENIC EM, include-

1) There is no evaporation of water around the molecule as cryogenic EM uses frozen samples.

2) Specimen is not destroyed and is alive and frozen, all possible with a lower intensity electron beam.

Question 29

Explain the method of freeze-fracture microscopy.

Answer 29

FIRSTLY, biological material i INSTANTLY FROZEN in LIQUID Nitrogen.

Then, it is BROKEN UP IN A VACUUM.

Secondlly, the EXPOSED SURFACE LOSES SOME ICE, hence being described as ETCHED.

Thirdly, a CARBON REPLICA of surface is made, COATED with HEAVY METAL to strenghen before examination under an electron micrograph

Lastly, resulting electron micrograph is produced by freeze etching.

1) FROZE< LIQUID NITROGEN, VACUUM 2) ETCHED 3) CARBON REPLICA, HEAVY METAL COATING. 4) FREEZE ETCHING PRODUCES FINAL PRODUCT

Question 30

Describe the ADVANTAGES of feeze-fracture electron microscopy.

Answer 30

It provides a PLANAR VIEW OF THE MEMBRANE with VERY HIGH RESOLUTION DETAILS.

Secondly, The DISTRIBUTION and organization of membrane proteins and other molecules are rendered visible.

(Therefore, when we are shown an image where we can bsee the location and distribution of protein, it is a freeze fracture technique. )

Question 31

Describe what cryogenic microscopy is often used for.

Answer 31

Cryogenic microscpy, where we flash freeze biological material without killing the specimen, is often used to reveal HOW PROTEINS FUNCTION, how they MALFUNCTION with disease and HOW to target them with drugs.

Question 32

Describe some uses of cryogenic electron microscopy.

Answer 32

Used in X-Ray crystallography, the structure of DNA was discovered in this way by Rosalind Franklin.

Question 33

Distiguish between cryogenic and flash freezing electron microscopy (not tested just for personal understanding)

Answer 33

Cryogenic electon microscopy uses flash-freezing methods while freeze fracture methods employ instant freezing in liquid nitrogen

Question 34

Describe the techique of fluorescence microscopy (3m)

Answer 34

Fluorescent dyes absorb & then emit light at a longer wavelength.
They can be used to bind to biomolecules (DNA, proteins) to reveal their cellular locations.

There are two sets of filters,
The FIRST SET filters the light BEFORE it reaches the specimen. It passes ONLY those wavelengths that excite the SPECIFICALLy chosen fluorescent dye.

The SECOND SET of filters BLOCS OUT LIGHT and passes ONLY those wavelengths emitted WHEN THE FLUORESCENT DYE FLUORESCES. Dyed objects show up in bright colours on a dark background.

Question 35

Applications of fluorescent microscipy?

Answer 35

.

Firstly, Other dyes can be used with ANTIBODY molecules which then BIND SELECTIVELY to targeted biomolecules e.g. DNA showing the distribution in the cell. This is used to explin e.g. structure of cell’s plasma membrane and mechanisms by which muscles contract.

Secondly, Can be used in IMMUNOFLUORESCENCE.
Immunofluorescence uses ANTIBODIES which are CHEMICALLY LABELLED with FLUORESCENT DYES to be visualised under a lught microscope. Antibodies ATTACH to AFFECTED CELLS AND DYES INDICATE WHICH CELLS ARE MARKED THIS WAY. This is used for CANCER DIAGNOSIS when the cancer is present only at a small sampe of tissue.

Question 36

Describe the structure and function of the smooth-endoplasmic reticulum.

Answer 36

The smooth endoplasmic reticulum consists of a tubular membrane network that is branched. The smooth-endoplasmic reticulum manufactures lipids, phospholipids and steroids (acronym LPS). Furthermore, in muscle cells, the smooth endoplasmic reticulum stores calcium ions.

Question 37

Describe the structure and function of microtubules and microfilaments in the eukaryotic cell.

Answer 37

Microtubules are straight, unbranched, thickest, hollow cylindrical fibres (25 nm in diameter). They are made up of tubulin (globular protein), capable of reassembling & dismantling

Functions: control the movement of chromosomes in cell division, guide the movement of secretory vesicles from Golgi apparatus to plasma membrane
Provide mechanical support, maintains cell shape.

Microfilaments- each filament consists of a twisted double chain of actin (globular proteins)
Function: resist tension to maintain & change cell shape, involved in cleavage furrow formation during cell division, regulates cell motility particularly during muscle contraction

Question 38

Outline the roles of the lysosomes, the Golgi Apparatus and the endoplasmic reticulum in the eukaryotic cell (4 marks)

Answer 38

Lysosomes contains hydrolytic enzymes to breakdown foreign particles (phagocytosis) or worn out organelles (auto-phagy). They are also involved in programmed cell death. The golgi apparatus synthesizes hormones and enzymes which it packages into vesicles (GA, SHE, PIV acronym). Abundant in metabolically active cells, i.e. secretory cells. The rough endiplasmic reticulum synthesizes proteins, The RER has many metabolically active cells. Site of synthesis, folding & post-translational modification of secretory proteins, e.g. digestive enzymes. The SER is involved in synthesizing, lipids, phospholipids and steroids.

Question 39

Outline how the electron microscope has increaded our knowledge of cell structure (2 marks)

Question 40

Describe purpose of the large central vacuole in plant cells.

Answer 40

When filled with water, it pushes the cytoplasm against the cell wall, creating turgor pressure to maintain plant tissue rigidity.
Chemicals such as pigments (betalains), carbohydrates, fats & proteins can be stored.
The interior is slightly acidic (pH 5.0) contains hydrolase enzymes to break down waste material

Question 41

List and describe the structures commonly found in a prokaryotic cell.

Answer 41

Nuceloid, plasmid, pilli and flagellum. A plasmid is a small circular DNA molecule found in bacteria and some other microscopic organisms. Plasmids are physically separate from chromosomal DNA. The pilli are for reproduction and the flagella propel movement of the cell