Cell Structure

Question 1

Magnification Equation?

Answer 1

Image Size / Actual Size.

Magnification is how much bigger the image is than the specimen.

Always convert the units in the formula to the same units.

Question 2

Resolution?

Answer 2

Also knows as resolving power.

It is how well a microscope ca distinguish between two points that are close together.

If a microscope cannot separate two objects, then increasing the magnification will not help.

Question 3

Cell Fractionation Fluid?

Answer 3

Solution must be cold to reduce enzyme activity that could break organelles,

Solution must have the same water potential to prevent organelles from bursting or shrinking,

Must be buffered (ph the same) so the functioning of the enzyme stay the same.

Question 4

Units Of Measurement?

Answer 4

1 Kilometre = 1000 metres
1 Metre = 1000 mm
1 Millimetre = 1000um
1 Micrometre = 1000nm

Question 5

Light Microscopes?

Answer 5

Use light to form an image.

They have a maximum resolution of 0.2 um. So we cannot view organisms smaller than 0.2 um (including ribosomes, endplasmic reticulum and lysosomes).

They have a poor resolution as a result of the long wavelengths of light they use.

Maximum magnification is x1500.

Question 6

Electron Microscopes?

Answer 6

Use electrons to form an image.

The electron beam has a very short wavelength and so a much better resolution (high resolving power).

The electrons are negatively charged and so the beam can focus using electromagnets.

The best modern electron microscopes can resolve images that are just 0.1nm apart - 2000x better than a light microscope.

Because electrons are absorbed or deflected by the molecules in air, a near-vacuum has to be created within the chamber of an electron microscope in order for it to work effectively.

There are two types of electron microscope:

• transmission electron microscope (TEM),
• scanning electron microscope (SEM).

Question 7

TEM Micropscope?

Answer 7

Transmission electron microscope.

Uses electromagnets to focus a beam of electrons, which is then transmitted through the specimen to produce an image.

Denser parts of the specimen absorbs more electrons, which makes them darker on the image. The beam is penetrating the specimen from below. Other parts of the specimen allow the electrons to pass through and so appear bright.

They produce high resolution images, so the internal structure of the organelles can be seen, like chloroplasts.

They can only be used on thin filaments.

An imagine is produced on a screen and this can be photographed to give a photomicrograph.

The TEM produces 2D images. We can make up a 3D image by taking a series of sections of 2D images. However, this is a slow and complicated process. The development of the SEM has overcome this problem.

Question 8

Resolving Power Of TEM?

Answer 8

The resolving power of the TEM is 0.1 nm although this cannot always be achieved because:

• Difficulties preparing the specimen limit the resolution that can be achieved.
• A higher energy electron beam is required and this may destroy the specimen in some cases.

Question 9

Limitations Of The TEM?

Answer 9

• The whole system must be in a vacuum and therefore living specimens cannot be observed.
• Staining of the specimen is required.
• The image is not in colour.
• The specimen must be extremely thin.
• The image may contain artefacts (things that result from the way the specimen is prepared. They are not part of the natural specimen). What we see on the microphotograph is not always true.

Question 10

SEM Microscope?

Answer 10

The scanning electron microscope.

The SEM directs a beam of electrons onto the surface of the specimen from above, rather than from below (like TEM). This knocks off electrons, which are gathered into a cathrode ray tube to form an image.

The beam is then passed back and forth across a portion of the specimen in a regular pattern.

This builds up a 3-D image by computer analysis of the pattern of scattered electrons and secondary electrons produced.

Can be used on thick specimens.

The scattering depends on the contours of the specimen surface.

The basic SEM has a lower resolving power then TEM, around 20 nm. This is still better than light microscope.

Question 11

Limitations Of The SEM?

Answer 11

(Same as TEM except the specimen doesn’t need to be thin).

• The whole system must be in a vacuum and therefore living specimens cannot be observed.
• Staining of the specimen is required.
• The image is not in colour.
• The image may contain artefacts (things that result from the way the specimen is prepared. They are not part of the natural specimen). What we see on the microphotograph is not always true.

Question 12

Eukaryotic and prokaryotic organisms?

Answer 12

Eukaryotic - Comlex cells, including animals and lants, fungi and algae.

Prokaryotic - Smaller, single-celled organisms, including bacteria.

Question 13

Structure of eukaryotic cells?

Answer 13

Animal - Plasma membrane (cell-surface membrane),
Cytoplasm, 
Lysosomes, 
Ribosomes,
Nucleus,
Nucleolus (inside nucleus),
Nuclear envelope,
Rough endoplasmic reticulum,
Smooth endoplasmic reticulum,
Mitochondria,
Golgi apparatus, 

Plant - Plant cells have the same organelles as animal cells, but they have a few extra:

• Cellulose cell wall with plasmodesmata (channels for exchanging substances with adjacent cells),
• Vacuole (compartment that contains cell sap),
• Chloroplasts,
• Starch grains (to store excess sugars).

Algal - they can be single-celled or multicellular. They have the same organelles as plant cells.

Fungal - The same as plant cells, with two key differences:

• Their cell walls are made of chitin, not cellulose,
• They dont have chloroplasts (because they dont photosynthesise).

Question 14

Plasma membrane function?

Answer 14

Plasma membrane - mainly made of lipids and proteins. It regulates the movement of substances in and out of the cell.

It has receptor molecules on it, which allows it to respond to chemicals, like hormones.

Question 15

Nucleus function?

Answer 15

A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores.

The nucleus contains chromosomes (made from protein-bound linear DNA) and one or more nucleolus (plural = nucleoli).

Chromatin is also found in the nucleus.

Its function is to control the cells activities (by controlling DNA transcription).

DNA contains instructions to make proteins. The pores allow for substances to move between the nucleus and the cytoplasm.

The nucleolus makes ribosomes.

Question 16

Mitochondrion function?

Answer 16

Contain an outer-membrane, inner-membrane (double membrane), crista and a matrix.

The inner membrane is folded to form structures called cristae.

Inside is the matrix, which contains enzymes involved in respiration.

The mitochondria is the site of the aerobic respiration, where ATP is produced. They’re found in cells that are very active and require a lot of energy.

Question 17

Chloroplast function?

Answer 17

The chloroplast is a small, flattened organelle found in plant and algae cells.

It has a double membrane, and membranes inside called thylakoid membranes. These are stacked up to produce grana. Grana are linked together by lamellae - thin, flat pieces of thylakoid membrane.

Chloroplasts are the site where photosynthesis occurs i.

Some parts of photosynthesis take place in the grana, whilst some take place in the stroma (thick fluid in the chloroplasts).

Question 18

Golgi apparatus function?

Answer 18

A group of fluid-filled, membrane-bound, flattened sacs.

Vesicles are often seen at the edges of Golgi apparatus.

It processes and packages new lipids and proteins. It also makes lysosomes.

Question 19

Golgi vesicle function?

Answer 19

Small, fluid filled sac in the cytoplasm, surrounded by a membrane and produced by the golgi apparatus.

It stores lipids and proteins made by the Golgi apparatus and transports them out of the cell via the cell-surface membrane.

Question 20

Lysosome function?

Answer 20

A round organelle, surrounded by a membrane, with no clear internal structure. Its a type of golgi vesicle.

It contains digestive enzymes, called lyzsosomes. These are kept separate from the cytoplasm by the surrounding membrane, and are used to digest invading cells or to break down worn out components of the cell.

Question 21

Ribsome function?

Answer 21

Small organelle, either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum.

Made of proteins and RNA.

Not surrounded by a membrane.

This is where proteins are made.

Split into a small sub-unit and a large sub-unit. This can be seen on a diagram, because one side is smaller than the other.

Question 22

Rough endoplasmic reticulum function?

Answer 22

A system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes.

Folds and processes proteins that have been made at the ribosomes.

Question 23

Smooth endoplasmic reticulum function?

Answer 23

Similar to rough endoplasmic reticulum, but with no ribosomes.

It synthesises and processes proteins.

Question 24

Cell wall function?

Answer 24

A rigid structure that surrounds cells in plants, algae and fungi.

In plants and algae, its made mainly of carbohydrate cellulose. In fungi, its made from chitin.

It supports cells and stops them from changing shape.

Question 25

Cell vacuole function?

Answer 25

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 maintain pressure inside of the cell and keep the cell rigid. This stops the plants wilting. It is also involved in the isolation of unwanted chemicals inside the cell.

Question 26

Specialised cells?

Answer 26

In eukaryotic organisms, cells become specialised to carry out specific functions. A cells structure is adapted to carry out its function.

In an exam, you must apply knowledge of cell structure to explain how it carries out its job. For example, a respiring cell needs a lot of energy, so it will have a lot lf mitochondria. If a cell needs a lot of proteins, it will have a lot of ribosomes.

Question 27

Specialised cells are organised into?

Answer 27

Tissues, organs and organ systems.

In eukaryotic organisms, specialised cells are grouped together to form tissues.

A tissue is a group of working cells, they work together to carry out a particular function.

Different tissues work together to form organs. Different organs make up an organ system.

Question 28

Structure of prokaryotic cells?

Answer 28

They’re smaller and more simple. E.g. bacteria.
1. Plasma membrane - made of lipids and proteins. Controls movement of substances in and out of cell.

2. Cytoplasm - Has no membrane-bound organelles. It has ribsomes but they’re smaller than eukaryotic cell ribosomes.
3. Plasmids - Small loops of DNA that are not part of the main circular DNA molecule. They contain genes for things like antibiotic resistance, and can be passed between prokaryotes. They are not always present in prokaryotic cells, but some cells have several.
4. Flagellum - Long, hair-like structure that rotates to make the prokaryotic cell move. Not all cells have them, but some cells have more than 1.
5. Circular DNA - roams free in the cytoplasm. It is one, long, coiled strand. Not attached to histone proteins, or any other proteins.
6. Cell wall - Supports the cell and prevents it from changing shape. Made of the polymer called murein - a glycoprotein (protein with a carbohydrate attached).
7. Capsule - Some cells have thee. Made up of secreted slime. Helps to prevent the bacteria from attack by cells of the immune system.

Question 29

Viruses?

Answer 29

They’re acelluar - not cells.

A virus is a nucleic acid, surrounded by a protein - they’re not alive.

They are smaller than bacteria (HIV is around 0.1um across).

No plasma membrane, no cytoplasm and no ribsomes.

All viruses reproduce inside of the cells of other organisms. These are known as host cells.

Virus’ contain a core of genetic material - either DNA or RNA.

The protein coat around the core is called the capsid.

Attachment proteins stick out from the edge of the capsid. These let the virus cling onto the host cell.

Question 30

Binary fission?

Answer 30

In binary fission, prokaryotic cells replicate.

1. The circular DNA and the plasmids replicate. The main DNA loop is only replicated once, but plasmids can be replicated loads of times.
2. The cell gets bigger and the DNA loops move to opposite ‘poles’ (ends) of the cell.
3. The cytoplasm begins to divide (and new cell walls begin to form).
4. The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of plasmid DNA.

Question 31

Viruses binding to host cells?

Answer 31

Viruses 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 hot cells. Some viruses therefore, can only infect one type of cell (but some viruses can infect lots of different types of cells).

Because they’re not alive, viruses don’t undergo cell division. Instead, they inject their DNA or RNA into the host cell. The host cell will then use it’s own machinery (enzymes, ribosomes) to replicate the virus.

Question 32

Practical skill: using a slide?

Answer 32

Here is how to prepare a ‘temporary mount’ of a specimen on a slide:

1. Pipette a drop of water onto the slide (stripe of clear glass or plastic).
2. Use tweezers to place a thin section of a specimen onto of the water.
3. Add a drop of stain, used to highlight the objects in a cell. E.g. eosin is used to make the cytoplasm of a cell highlighted. Iodine can be used to make the starch grains in the plant cell show up.
4. Add the cover slip (a square piece of clear plastic). To do this, stand the slip up right on the slide, next to the water droplet. Then tilt the slide, and lower it, avoiding any air bubbles.

Question 33

Cell Fractionation Process?

Answer 33

Cell fractionation separates organelles.

For animal cells (only ones we study):

1. Homogenisation.
2. Filtration.
3. Ultracentrifugation.

Speed of centrifugation at which organelles are separated out:
Nuclei = 1000 (revolutions per min-1)
Mitochondria = 3500
Lysosomes = 16,500

Question 34

Homogenisation?

Answer 34

Step 1 of cell fractionation.

This can be done several ways, e.g. by vibrating the cells or grinding them in a blender.

This breaks up the plasma membrane and releases the organelles into solution.

The solution must be kept ice cold, to reduce the activity of the enzymes that break down the organelles.

The solution must also be isotonic - same concentration of chemicals as the cells being broken down so that the cells are not changed/damaged by osmosis.

A buffer solution should be added to maintain the pH.

Question 35

Filtration?

Answer 35

Step 2 of cell fractionation.

Next, the homogenised cell solution is filtered through a gauze to separate any large cell debris, like connective tissue, or any smaller debris.

Question 36

Ultracentrifugation?

Answer 36

Step 3 of cell fractionation.

After filtration, you’re left with a mixture of organelles.

1. The cell fragments are poured into a tube. The tube is put into a centrifuge (machine that separates material by spinning) and is spun at a low speed.
2. The heaviest organelles, like nuclei, get flung to the bottom of the tube. They form a thick sediment at the bottom - the pellet. The rest of the organelles are suspended in the fluid above the pellet - the supernatant.
3. The supernatant is drained off, poured into another tube and spun in the centrifuge at a higher speed. Again, the heaviest organelles are flug to the bottom of the tube - the pellet. The supernatant is drained off and spun again at an even higher speed.
4. The process is repeated until all the organelles are separated out. Each time, the pellet is made up of lighter and lighter organelles.

Question 37

What is mitosis?

Answer 37

The division of a parent cell to produce two, genetically identical daughter cells.

It is needed for growth of multicellular organisms, and for repairment.

Question 38

Cell cycle?

Answer 38

The cells in a multicellular organism that have the ability to divide, undergo a cell cycle.

The cell cycle consists interphase; a period of cell growth and DNA replication and mitosis, which happens after interphase.

Interphase is divided into G1, S and G2.

1. Mitosis (begins and ends with mitosis).
2. Gap phase 1 - cell grows and new organelles and proteins are made.
3. Synthesis - cell replicates its DNA, ready to divide by mitosis.
4. Gap phase 2 - cell keeps growing and proteins needed for cell division are made.

Question 39

Mitosis stages?

Answer 39

(Mitosis doesn’t include interphase, but I have included it here).

Interphase - cell carries out its normal functions, but also prepares to divide. The cells DNA is unravelled and replicated, to double the genetic material. The organelles are also replicated and ATP is increased.

1. Prophase - Chromosomes condense, getting shorter and fatter. Bundles of protein called centrioles start moving to opposite ends of the cell, forming a network of protein fibres called spindle. The nuclear envelope breaks down and chromosomes are free in the cytoplasm.
2. Metaphase - The chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere.
3. Anaphase - The centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite ends of the spindle, centromere first. This makes the chromatids appear v-shaped.
4. 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 nuclaur envelope forms around each group of chromosomes, so there are now two nuclei. Division of the cytoplasm (cytokenisis - which stars in anaphase), ends in telophase.

There are now two daughter cells, genetically identical to each-other.

Question 40

Calculating the length of mitosis stages?

Answer 40

You can calculate how long each stage of mitosis has taken.

E.g. “a scientist counts 100 cells. 10 are in metaphase. One complete cell cycle is 15 hours. How long do the cells spend in metaphase?”

So, 10/100 cells are in metaphase. So the proportion of time spent in metaphase mjst be 10/100th of the cell cycle.

15 x 60 = 900 minutes
10/100 = ans
Ans x 900 = 90 mins in metaphase.

Question 41

Practical: investigating mitosis?

Answer 41

Preparing and staining a root tip to investigate mitosis stages. Put safety goggles on and a lab coat. Also wear gloves when using stains.

1. Cut 1cm from the tip from a growing root (e.g. of an onion). It needs to be the tip because that is when growth occurs.
2. Prepare a boiling tube containing 1M hydrochloric acid. Put in a water bath of 60 degrees.
3. Transfer the root tip into the boiling tube and incubate for 5 minutes.
4. Use a pipette to rinse the root tip well with cold water. Leave the tip to dry on a paper towel.
5. Place the root tip on a microscope slide and cut 2mm from the very tip of it. Get rid of the rest.
6. Use a mounted needle to break the tip open and spread the cells out thinly.
7. Add a few drops of stain and leave it for a few minutes. The stain will make the chromosomes easier to see under the microscope. There are loads of different stains - toluidine blue O, ethano-orcein, feulgen stain (use an extra rinse if you use last one).
8. Place a cover slip over the cells and push down firmly to squash the tissue. This makes it thinner so that light passes through easier. Don’t smear the chromosomes sideways - you’ll damage the chromosomes.
9. Now use a microscope (optical).

Question 42

Using an optical microscope?

Answer 42

Using an optical microscope:

Start by clipping the slide onto the stage.

Select the lowest-powered objective lens.

Use the coarse adjustment knob to bring the stage downwards, away from the objective lens, until the image is in focus roughly.

Adjust the focus with the fine-adjustment knob until you get a clear image.

Use a higher-powered magnification lens and re-focus to get a higher magnification.

(P.36 - you should be able to label one of these microscopes).

Question 43

Cancer treatments involving mitosis?

Answer 43

Some cancer treatments work by controlling the rate of cell division by disturbing the cell cycle. This kills the tumour cells (made by uncontrolled mitosis).

These treatments, however, do not distinguish from normal cells and tumour cells. This means that normal cells are also killed. However, tumour cells divide much more frequently than normal cells and so the treatments are more likely to kill tumour cells.

Some cell cycle targets of cancer treatments include:
G1 - some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these are not produced, the cell is unable to enter the synthesis phase. This disrupts the cycle and so the cell kills itself.
S phase - radiation and some drugs damage the DNA, at several points in the cell cycle (including juts before and after the S phase). The DNA in the cell is checked for damage. If severe damage is detected, the cell will kill itself - preventing further tumour growth.

Question 44

Using an optical microscope?

Answer 44

Using an optical microscope:

Start by clipping the slide onto the stage.

Select the lowest-powered objective lens.

Use the coarse adjustment knob to bring the stage downwards, away from the objective lens, until the image is in focus roughly.

Adjust the focus with the fine-adjustment knob until you get a clear image.

Use a higher-powered magnification lens and re-focus to get a higher magnification.

(P.36 - you should be able to label one of these microscopes).

Question 45

Calculating mitotic index?

Answer 45

The mitotic index is the number of cells undergoing mitosis.

Mitotic index = number of cells with visible chromosomes / total number of cells observed.

This lets you work out how quickly the tissue is growing and if anything weird is going on.

A plant root tip is constantly growing, so the index would be high. In other tissues, a high index may suggest cancer.

Question 46

Practical: calculating the size of a cell

Answer 46

You can use an eyepiece graticule and stage micrometer to calculate the size of a cell.

An eyepiece graticule is fitted onto the eyepiece. Its like a transparent rules with numbers, but no units.

The stage microscope is placed on the stage. Its like a slide with an accurate scale (has units) and is used to work out the value of the divisions on the eyepiece graticule at a particular magnification.

This means that when you take the stage micrometer away and replace it with the slide containing your tissue sample, you’ll be able to measure the size of cells.
P. 37

This technique is the complicated version of actual = image/magnification.

Question 47

Artefacts?

Answer 47

Artefacts are things that you can see down the microscope that are not part of the cell or specimen that your looking at.

They can be anything from dust, air bubbles or fingerprints. Also include inaccuracies caused by squashing and staining your sample.

Artefacts are made usually during the preparation of your slides and shouldn’t be there at all. They are especially common when using electron microscopes because specimens need a lot of preparations before your able to view them.

The first scientists would distinguish from organelles and artefacts by repeating the prep and viewing of an organelle. If an image could be seen in one image and not another, it is likely that it is an artefact and not part of the specimen.