Module 2: Section 1 - Cell Structure

Question 1

Plant cells have the all the same organelles as animals cells, but with a few added extras. What are these extras?

Answer 1

• a cell wall with plasmodesmata (‘channels’ for exchanging substances with adjacent cells)
• a vacuole (compartment that contains cell sap)
• and of course good old chloroplasts

Question 2

What is the function of a plasma membrane?

Answer 2

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

Question 3

Describe a plasma membrane.

Answer 3

The membrane found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells. It’s made of mainly lipids and protein.

Question 4

What is the function of a cell wall?

Answer 4

Supports plant cells

Question 5

What is the function of the nucleus?

Answer 5

The nucleus controls the cell’s activities (by controlling the transcription of DNA). DNA contains instructions to make proteins. The pores allow substances (e.g) RNA to move between the nucleus and the cytoplasm. The nucleolus makes ribosomes.

Question 6

What is the function of a lysosome?

Answer 6

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

Question 7

What is the function of a ribosome?

Answer 7

The site where proteins are made.

Question 8

What is the function of RER?

Answer 8

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

Question 9

What is the function of SER?

Answer 9

Synthesises and processes lipids.

Question 10

What is the function of a vesicle?

Answer 10

Transports substances in and out of the cell (via the plasma membrane) and between organelles. Some are formed by the Golgi apparatus or the endoplasmic reticulum, while others are formed at the cell surface.

Question 11

What is the function of the Golgi apparatus?

Answer 11

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

Question 12

What is the function of the mitochondrion?

Answer 12

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

Question 13

What is the function of a chloroplast?

Answer 13

The site where photosynthesis takes place. Some parts of photosynthesis happen in the grana, and other parts happen in the stroma (a thick fluid found in chloroplasts).

Question 14

What is the function of a centriole?

Answer 14

Involved with the separation of chromosomes during cell division.

Question 15

What is the function of cilia?

Answer 15

The microtubules allow the cilia to move. The movement is used by the cell to move substances along the cell surface.

Question 16

What is the function of flagellum?

Answer 16

The microtubules contract to make the flagellum move. Flagella are used like outboard motors to propel cells forward. (e.g. when a sperm cell swims)

Question 17

Describe flagellum

Answer 17

Flagella on eukaryotic cells are like cilia but longer. They stick out from the cell surface and are surrounded by the plasma membrane. Inside they’re like cilia too - two microtubules in the centre and nine pairs around the edge.

Question 18

Describe cilia

Answer 18

Small, hair-like structures found on the surface membrane of some animal cells. In cross-section, they have an outer membrane and a ring of nine pairs of protein microtubules inside, with two microtubules in the middle.

Question 19

Describe centriole

Answer 19

Small, hollow cylinders, made of microtubules (tiny protein cylinders). Found in animal cells, but only some plant cells.

Question 20

Describe a chloroplast

Answer 20

A small, flattened structure found in plant cells. It’s surrounded by a double membrane, and also has membranes inside called thylakoid membranes. These membranes are stacked up in some parts of the chloroplast to form grana. Grana are linked together by lamellae - thin, flat pieces of thylakoid membrane.

Question 21

Describe a mitochondria

Answer 21

They’re usually oval-shaped. They have a double membrane - the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.

Question 22

Describe the Golgi apparatus

Answer 22

A group of fluid-filled, membrane-bound, flattened sacs. Vesicles are often seen at the edges of the sacs.

Question 23

Describe a vesicle

Answer 23

A small fluid-filled sac in the cytoplasm, surrounded by a membrane.

Question 24

Describe a SER

Answer 24

Similar to a RER, but with no ribosomes

Question 25

Describe a RER

Answer 25

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

Question 26

Describe a ribosome

Answer 26

A very small organelle that either floats free in the cytoplasm or is attached to the RER. It is made up of proteins and RNA. It’s not surrounded by a membrane.

Question 27

Describe a lysosome

Answer 27

A round organelle surrounded by a membrane, with no clear internal structure.

Question 28

Describe a nucleus

Answer 28

A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores. The nucleus contains chromatin (which is made from DNA and proteins) and a structure called the nucleolus.

Question 29

Describe a cell wall

Answer 29

A rigid structure that surrounds plant cells. It’s made mainly of the carbohydrate cellulose.

Question 30

Give six steps of how organelles work together to produce a protein

Answer 30

1) proteins are made at the ribosomes
2) the ribosomes on the RER make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.
3) new proteins produced at the rough ER are folded and processed (e.g sugar chains are added) in the rough ER
4) then they’re transported from the ER to the Golgi apparatus in vesicles
5) at the Golgi apparatus, the proteins may undergo further processing (e.g sugar chains are trimmed or more are added)
6) the proteins enter more vesicles to be transported around the cell. e.g. glycoproteins (found in mucus) move to the cell surface and are secreted

Question 31

The organelles in cells are surrounded by the cytoplasm. The cytoplasm is more than just a solution of chemicals though - it’s got a network of protein threads running through it. These protein threads are called the cytoskeleton. How are the protein threads arranged in eukaryotic cells?

Answer 31

In eukaryotic cells the protein threads are arranged as microfilaments (small solid strands) and microtubules (tiny protein cylinders)

Question 32

What are the four main functions of the cytoskeleton?

Answer 32

1) the microtubules and microfilaments support the cell’s organelles, keeping them in position
2) they also help to strengthen the cell and maintain its shape
3) as well as this, they’re responsible for the movement of materials within the cell. For example, the movement of chromosomes when they separate during cell division depends on contraction of microtubules in the spindle
4) the proteins of the cytoskeleton can also cause the cell to move. For example, the movement of cilia and flagella is caused by the cytoskeletal protein filaments that run through them. So in the case of single cells that have a flagellum (e.g sperm cells), the cytoskeleton propels the whole cell

Question 33

How can microtubules and microfilaments be prevented from functioning?

Answer 33

The assembly of microtubules and microfilaments, and the movement of materials along them, requires energy from respiration. So microtubules and microfilaments can be prevented from functioning using respiratory inhibitors.

Question 34

How do you view prokaryotic cells and why?

Answer 34

Prokaryotes like bacteria are roughly a tenth the size of eukaryotic cells. This means that normal microscopes aren’t really powerful enough to look at their internal structure. So, bacterial cells have to be seen under an electron microscope.

Question 35

What is the formula for calculating magnification?

Answer 35

magnification = image size/object size

Question 36

Define magnification

Answer 36

Magnification is how much bigger the image is than the specimen

Question 37

Define resolution

Answer 37

Resolution is how detailed the image is. More specifically, it’s how well a microscope distinguishes between two points that are close together. If a microscope lens can’t separate two objects, then increasing the magnification won’t help.

Question 38

Tell me three things about light microscopes

Answer 38

1) light microscopes use light
2) they have a lower resolution than electron microscopes - they have a maximum resolution of about 0.2 micrometres. So they’re usually used to look at whole cells or tissues
3) the maximum useful magnification of a light microscope is about x 1500

Question 39

Tell me four things about Laser Scanning Confocal Microscopes

Answer 39

1) Laser Scanning Confocal Microscopes use laser beams to scan a specimen, which is usually tagged with a fluorescent dye
2) The laser causes the dye to fluoresce - give off light. This light is then focused through a pinhole onto a detector. The detector is hooked up to a computer, which generate an image. The image can be 3D
3) The pinhole means that any out-of-focus light is blocked, so these microscopes produce a much clearer image that a normal light microscope
4) They can be used to look at objects at different depths in thick specimens

Question 40

Electron microscopes use electrons instead of light to form an image. They have a higher resolution than light microscopes so give more detailed images. What are the two kinds of electron microscope and how do they work?

Answer 40

1) transmission electron microscope (TEM) - 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. TEMs are good because they provide higher resolution images (so they can be used to look at a range of organelles) but they can only be used on thin specimens.
2) scanning electron microscope (SEM) - 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 produced show the surface of the specimen and can be 3D. But they give lower resolution images than TEMs.

Question 41

What is the maximum resolution and maximum magnification of a light microscope/TEM and SEM?

Answer 41

light microscope: max r = 0.2 micrometres / max m = x1500

TEM: max r = 0.0002 micrometres / max m = can be more than x1000000

SEM: max r = 0.002 micrometres / max m = usually less than x500000

Question 42

In light microscopes and TEMs, the beam of light (or electrons) passes through the object being viewed. An image is produced because some parts of the object absorb more light (or electrons) than others.

Sometimes the object being viewed is completely transparent. This makes the whole thing look white because the light rays (or electrons) just pass straight through. How would you get past this problem?

Answer 42

To get round this, the object can be stained:

• for the light microscope, this means using some kind of dye. Common stains include methylene blue and eosin.
• the stain is taken up by some parts of the object more than others - the contrast makes different parts show up.
• different stains are used to make different things show up. For example, eosin is used to stain cell cytoplasms. Methylene blue stains DNA.
• more than one stain can be used at once

Question 43

How do you stain a sample for an electron microscope?

Answer 43

For the electron microscope, objects are dipped in a solution of heavy metals (like lead). The metal ions scatter the electrons, again creating contrast - some parts of the object show up darker than others.

Question 44

How do you prepare a microscope slide for a dry mount?

Answer 44

• your specimen needs to let light through it for you to be able to see it clearly under the microscope. So if you’ve got a thick specimen, you’ll need to take a thin slice to use on your slide
• use tweezers to pick up your specimen and put it in the middle of a clean slide
• pop a cover slip on top

Question 45

How do you prepare a microscope for a wet mount?

Answer 45

• start by pipetting a small drop of water onto the slide. Then use the tweezers to place your specimen on top of the water drop
• to put the cover slip on, stand 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 under there - they’ll obstruct your view of the specimen
• once the cover slip is in position, you can add a stain. Put a drop of stain next to one edge of the cover slip. Then put a bit of paper towel next to the opposite edge. The stain will get drawn under the slip, across the specimen.

NB: wet mounts are good for looking at tiny organisms that live in water