Topic 2A - Cell Structure And Division

Question 1

What are eukaryotic and prokaryotic cells?

Answer 1

Eukaryotic cells are complex and include all animal and plant cells, as well as cells in algae and fungi. Prokaryotic cells are smaller and simpler.

Question 2

What organelles do plant cells have?

Answer 2

Same organelles as animal cells but with:
A cellulose cell wall with plasmodesmata (channels for exchanging substances with adjacent cells)
A vacuole (compartment that contains cell sap)
Chloroplasts.

Question 3

What do algal and fungal cells contain?

Answer 3

Algal cells are like plant cells, with the same organelles including a cell wall and chloroplasts.
Fungal cells are also like plant cells, but their cell walls are made of chitin.
They don’t have chloroplasts.

Question 4

Describe the cell-surface membrane.

Answer 4

It regulates the movement of substances into and out of the cell. It also has receptor molecules which allow it to respond to chemicals like hormones.

Question 5

Describe the nucleus.

Answer 5

A large organelle surrounded by a nuclear envelope, which contains many pores. The nucleus contains chromosomes (which are made from protein-bound linear DNA) and one or more structure called a nucleolus. The nucleus controls the cell’s activities my controlling the transcription of DNA. The pores allow substances like RNA to move between the nucleus and the cytoplasm. The nucleolus makes ribosomes.

Question 6

Describe the mitochondrion.

Answer 6

They have a double membrane. The inner one is folded to form cristae. Inside is the matrix, which contains enzymes involved in respiration. It is the site of aerobic respiration where ATP is produced.

Question 7

Describe the chloroplast

Answer 7

A small, flattened structure found in plant and algal cells. It’s surrounded by a double membrane, and also has membranes inside called thylakoid membranes. They are stacked up to form grana, which are linked together by lamellae.

Question 8

Describe the Golgi apparatus.

Answer 8

A group of fluid-filled, membrane-bound flattened sacs. Vesicles are often seen at the edges of the sacs. It processes and packages new lipids and proteins. It also makes lysosomes.

Question 9

What is a golgi vesicle?

Answer 9

Stores lipids and proteins made by the golgi apparatus and transports them out of the cell via the cell-surface membrane.

Question 10

What is a lysosome?

Answer 10

A round organelle surrounded by a membrane. It contains hydrolytic/digestive enzymes which are kept separate from the cytoplasm and can be used to digest invading cells or to break down worn out components of the cell.

Question 11

What is a ribosome?

Answer 11

A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum. It’s made up of proteins and RNA. It’s not surrounded by a membrane. The site where proteins are made.

Question 12

What is RER

Answer 12

Covered with ribosomes, folds and processes proteins that have been made at the ribosomes.

Question 13

What is SER?

Answer 13

Synthesises and processes lipids.

Question 14

What does the cell wall do?

Answer 14

Supports cells and prevents them from changing shape.

Question 15

What is the cell vacuole?

Answer 15

A membrane-bound organelle found in the cytoplasm of plant cells. It contains cell sap - a weak solution of sugar and salts. The surrounding membrane is called the tonoplast. It helps to maintain pressure inside the cell and keeps the cell rigid. This stops plants wilting. Also involved in the isolation of unwanted chemicals inside the cell.

Question 16

What are specialised cells? How are epithelial cells in the small intestine specialised to absorb food efficiently?

Answer 16

In multicellular eukaryotic organisms, cells become specialised to carry out specific functions. A cell’s structure helps it to carry out its function. For example, the walls of the small intestine have lots of finger-like projections called villi. These increase surface area for absorption. The epithelial cells of the surface of villi have folds in their cell-surface membranes, called microvilli, which further increase the surface area.
They also have mitochondria to provide energy for the transport of digested food molecules into the cell.

Question 17

What are tissues, organs and organ systems?

Answer 17

Specialised cells are grouped together to form tissues. A tissue is a group of cells working together to perform a particular function. Different tissues work together to form organs. Different organs make up an organ system.

Question 18

What is the cell wall made up of in prokaryotes?

Answer 18

Murein, a glycoprotein.

Question 19

What is a capsule and a plasmid?

Answer 19

Slime capsule helps to protect bacteria from attack by cells of the immune system. Plasmids are small loops of DNA that aren’t part of the main circular DNA molecule. They are not always present in prokaryotic cells.

Question 20

What is a flagellum? Whats unique about the cytoplasm of a prokaryotic cell?

Answer 20

Tail to make the bacteria move. The cytoplasm has no membrane-bound organelles. It has ribosomes smaller than in eukaryotes.

Question 21

Describe viruses.

Answer 21

They are nucleic acids surrounded by protein. They’re smaller than bacteria. Viruses contain a core of genetic material - either DNA or RNA. The protein coat around the core is called the capsid. They have attachment proteins which let the virus cling onto a suitable host cell.

Question 22

How do prokaryotic cells replicate?

Answer 22

Binary fission. The cell replicates its genetic material, before physically splitting into two daughter cells.
The circular DNA and plasmids replicate. The main DNA loop is only replicated once, but plasmids can be replicated many times. The cell gets bigger and the DNA loops move to opposite poles of the cell. The cytoplasm begins to divide and new cell walls begin to form. The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but variable number of copies of the plasmids.

Question 23

How do viruses replicate?

Answer 23

They use their attachment proteins to bind to complementary receptor proteins on the surface of host cells. Different viruses have different attachment proteins and therefore require different receptor proteins on host cells. They inject their DNA or RNA into the host cell to use its machinery to replicate the viral particles.

Question 24

What is magnification and resolution?

Answer 24

How much bigger the image is than the sample you are looking at.
Magnification = size of image/size of real object
Resolution is how well a microscope distinguishes between two points that are close together.

Question 25

What are optical microscopes?

Answer 25

They use light to form an image. They have a max resolution of 0.2 micrometers which means you can’t view organelles smaller than that. E.g ribosomes, endoplasmic reticulum and lysosomes. Max useful magnification is x 1500.

Question 26

What are electron microscopes?

Answer 26

They use electrons to form an image. They have a higher resolution than optical microscopes so give a more detailed image. They have a maximum resolution 1000 times higher than optical microscopes. They also have a higher maximum useful magnification.

Question 27

Compare the two different electron microscopes.

Answer 27

Transmission electrons microscopes use electromagnets to focus a beam of electrons, which is then transmitted through the specimen. Denser parts of the specimen absorb more electrons, which makes them look darker on the image you end up with. They are good because they give high resolution images so you can see the internal structure of organelles like chloroplasts. But they can only be used on thin specimens. Scanning electrons microscopes scan a beam of electrons across the specimen. This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image. The images you end up with show the surface of the specimen and can be 3D. SEMs are good because they can be used on thick specimens. But they give lower resolution images than TEMs.

Question 28

How do you prepare a temporary mount of a specimen on a slide?

Answer 28

Pipette a small drop of water onto the slide (a strip of clear glass or plastic). Then use tweezers to place a thin section of your specimen on top of the water drop. Add a drop of a stain. They highlight parts of a cell. For example, eosin makes the cytoplasm show up. Iodine in potassium iodide solution is used to stain starch grains in plant cells. Add the cover slip by standing the slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles.

Question 29

Describe the first two stages in cell fractionation.

Answer 29

Homogenisation. Grind the cells in a blender to break up the plasma membrane and release the organelles into solution. The solution must be kept ice-cold to reduce the activity of enzymes that break down organelles. Solution should be isotonic to prevent damage to the organelles through osmosis. A buffer solution should be added to maintain the pH. Next, the homogenised cell solution is filtered through a gauze to separate any large cell debris or tissue debris, like connective tissue from the organelles.

Question 30

What happens in ultracentrifugation?

Answer 30

The cell fragments are poured into a tube. The tube is put into a centrifuge and is spun at a low speed. The heaviest organelles, like nuclei, move to the bottom of the tube by the centrifuge. They form a thick sediment at the bottom - the pellet. The rest of the organelles stay suspended in the fluid above the sediment - the supernatant. The supernatant is drained off, poured into another tube, and spun in the centrifuge at a higher speed. The next heaviest organelles, the mitochondria, form a pellet. The supernatant is drained off and spun and an even higher speed. The process is repeated at higher speeds until al the organelles are separated out. From bottom to top: nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum, ribosomes.

Question 31

What is mitosis?

Answer 31

A parent cell divides to produce two genetically identical daughter cells. Mitosis is needed for the growth of multicellular organisms and for repairing damaged tissues.

Question 32

Describe the cell cycle.

Answer 32

Consists of a period of cell growth and DNA replication called interphase. Mitosis happens after that. Interphase is divided into G1, S and G2. Mitosis first occurs, then gap phase 1 where the cell grows and new organelles and proteins are made. Then synthesis where the DNA is replicated. Then gap phase 2 where the cell keeps growing and proteins needed for cell division are made.

Question 33

What happens in interphase and prophase?

Answer 33

In interphase, the cells DNA is unravelled and replicated. The organelles are also replicated and the ATP content is increased. In prophase the chromosomes condense, getting shorter and fatter. Tiny bundles of protein called centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope breaks down and chromosomes lie free in the cytoplasm.

Question 34

What happens in metaphase, anaphase and telophase?

Answer 34

In metaphase, the chromosomes line up along the middle of the cell and become attached to the spindle by their centromere. In anaphase, the centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite poles of the spindle, centromere first. This makes the chromatids appear v shaped. In telophase, the chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again. They’re now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei. Division of the cytoplasm (cytokinesis) finishes in telophase. There are now two daughter cells that are genetically identical to the original cell and to each other. Each daughter cell starts the interphase part again to get ready for the next round of mitosis.

Question 35

How does cancer occur?

Answer 35

Mitosis and the cell cycle are controlled by genes. A mutation in a gene that controls cell division can cause the cells to keep dividing, forming a tumour. Cancer is a tumour that invades surrounding tissue.

Question 36

How do some cancer treatments work?

Answer 36

Some treatments control the rate of cell division by disrupting the cell cycle, which can affect normal cells but the treatment is more likely to kill tumour cells. For example, chemotherapy can prevent the synthesis of enzymes needed for DNA replication. If these aren’t produced, the cell is unable to enter the synthesis phase and will kill itself. Radiation and some drugs damage DNA. Just before and during the S phase, the DNA is checked for damage. If severe damage is detected, the cell will kill itself.

Question 37

What is the method for observing mitosis on a root tip?

Answer 37

Cut 1 cm from the tip of a growing root (because that’s where mitosis takes place). Prepare a boiling tube containing 1 M hydrochloric acid and put it in a water bath at 60°C. Transfer the root tip into the boiling tube and incubate for about 5 mins. Use a pipette to rinse the root tip well with cold water. Leave the tip to dry on a paper towel. Place the root tip on a microscope slide and cut 2 mm from the very tip of it. Get rid of the rest. Use a mounted needle to break the tip open and spread the cells out thinly. Add a few drops of stain and leave it for a few minutes. The stain will make the chromosomes easier to see under a microscope. E.g toluidine blue O. Place a cover slip over the cells and push down firmly to squash the tissue.

Question 38

How can you observe cells using an optical microscope?

Answer 38

Clip the slide you’ve prepared onto the stage. Select the lowest powered objective lens. Use the coarse adjustment knob to bring the stage up to just below the objective lens. Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards away from the objective lens until the image is roughly in focus. Adjust the focus with the fine adjustment knob until you get a clear image of what’s on the slide.

Question 39

What is the mitotic index?

Answer 39

Proportion of cells undergoing mitosis. A high mitotic index could mean that it’s a root tip, tissue repeat is taking place or there is cancerous growth in the tissue.

Question 40

How can you calculate the size of cells?

Answer 40

An eyepiece graticule is fitted onto the eyepiece. It has numbers but not units. The stage micrometer is placed on the stage - it is a microscope slide with an accurate scale (and units) and it’s used to work out the value of the divisions on the eyepiece graticule at a particular magnification. This means when you take the stage micrometer away and replace it with the slide containing your tissue slide, you’ll be able to measure the size of the cells. For example, line up the eyepiece graticule and the stage micrometer. Each division on the stage micrometer is 0.1mm long. At this magnification, 1 division on the stage micrometer is the same as 4.5 divisions on the eyepiece graticule, so you need to divide 0.1 by 4.5. This is 0.022 mm. So if you look at a cell and it’s 4 eyepiece divisions you know it’s 4 x 0.022.

Question 41

What are artefacts?

Answer 41

Bits of dust, air bubbles or fingerprints etc. or inaccuracies by squashing and staining your sample. Artefacts are especially common in electron micrographs because specimens need a lot of preparation before you can view them. To distinguish between artefacts and organelles you need to prepare specimens in different ways.

Question 42

Why is a nucleus not visible in the above image?

Answer 42

Nucleus not stained. A slice (so nucleus in another part of cell).

Question 43

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains.

Answer 43

Pipette a water droplet onto a slide. Cut a thin piece of paint tissue and float it on the slide. Add potassium iodine stain. Lower cover slip using mounted needle.

Question 44

What is the function of the nucleus?

Answer 44

Contains DNA

Question 45

Explain why these organelles cannot be seen with an optical microscope?

Answer 45

Low resolution. Wavelength of light too long.

Question 46

Why should the solution in which the liver tissue was ground be ice-cold?

Answer 46

Prevents enzymes from digesting the organelles. (Do not mention denaturing because this prevents enzymes being denatured).

Question 47

Why would the first pellet have to be removed first before removing the pellet with mitochondria?

Answer 47

To remove nuclei. Which are heavier/more dense.

Question 48

Why are lots of mitochondria needed in muscle cells?

Answer 48

Mitochondria site of respiration. Production of ATP. For contraction.

Question 49

Why do enzyme-secreting cells have lots of rER?

Answer 49

Enzymes are proteins. Proteins made on ribosomes.

Question 50

Why is HCl added in the method with root tips?

Answer 50

To separate cell walls allowing the stain to diffuse into the cells.

Question 51

Pressing the coverslip downwards enabled to student to observe the stages of mitosis clearly. Explain why.

Answer 51

To create a thin layer of cells. So that light could pass through.

Question 52

Suggest two explanations excluding student errors why students calculated different mitotic indices.

Answer 52

Garlic grown for different lengths of time. The root tips from different garlic plants. Single field of view is not representative of root tip.