Section 1: Cell Structure

Question 1

What is the function of a plasma membrane?

Answer 1

The function of the plasma membrane is to regulate 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 2

What is magnification?

Answer 2

Magnification is how much bigger the image is than the specimen (the sample you’re looking at).

Question 3

What is the formula for magnification?

Answer 3

Magnification = Image size
—————-
Actual size

Question 4

What is resolution and give an example.

Answer 4

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.
For example, when you look at a car in the dark that’s a long way away you see the two headlights as one light. This is because your eyes can’t distinguish between the two points at a distance - your eyes produce a low resolution image. When the car gets a bit closer you can see both headlights - a higher resolution image.

Question 5

What are the different types if microscope?

Answer 5

Light microscopes, laser scanning confocal microscopes and electron microscopes.

Question 6

Describe light microscopes.

Answer 6

They use light. They have a lower resolution than electron microscopes. They have a maximum resolution of about 0.2 micrometers. So they’re usually used to look at whole cells or tissues. The maximum useful magnification is about x1500.

Question 7

Describe laser scanning confocal microscopes.

Answer 7

These are a special type of light microscope that use laser beams (intense beams of light) to scan a specimen that’s usually tagged with fluorescent dyes.
A laser beam is focused through a lens which is aimed at a beam splitter. This splits the beam and some of the light is directed to the specimen. When the laser hits the dyes it causes them to give off fluorescent light. This light is then focused through a pinhole onto a detector. The detector is hooked up to a computer, which generates an image. The pinhole means that any out-of-focus light is blocked, so these microscopes produce a much clearer image than a normal light microscope.
These microscopes can be used to look at objects at different depths in thick specimens. Multiple images produced by the microscope can be combined by the computer to generate 3D images of a specimen.

Question 8

Describe electron microscopes.

Answer 8

Electron microscopes use electrons instead of light to form an image. They have a higher resolution than light microscopes so give more detailed images.

Question 9

What are the types of electron microscope?

Answer 9

Transmission electron microscope (TEM) and scanning electron microscope (SEM)

Question 10

Describe transmission electron microscopes.

Answer 10

TEMs use electromagnets to focus a beam of electrons, which is then transmitted through the specimen to produce 2D images. 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 high resolution images, so they can be used to look at very small organelles, e.g. ribosomes. They can also be used to look at the the internal structures of organelles in detail. But specimens viewed on TEMs need to he quite thinly sliced. The angle at which specimens are cut can affect how they appear.

Question 11

Describe scanning electron microscopes.

Answer 11

SEMs 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 than TEMs.

Question 12

Describe protein production

Answer 12

There are a variety of organelles involved in protein production. Each one has a different role. Proteins are made at the ribosomes - the ribosomes on the rough endoplasmic reticulum (RER) make proteins that are excreted or attached to the cell membrane, whereas the free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.
New proteins produced at the RER are folded and processed (e.g. sugar chains are added) in the RER. Then they’re transported from the RER to the Golgi apparatus in vesicles. At the Golgi apparatus, the proteins may undergo further processing (e.g. sugar chains are trimmed or more are added). The proteins enter more vesicles to be transported around the cell. E.g. glycoproteins (found in mucas) move to the cell surface and are secreted.

Question 13

What are prokaryotic and eukaryotic cells?

Answer 13

There are two main types of organism - eukaryotes and prokaryotes. Prokaryotic organisms are prokaryotic cells (i.e. they’re single celled organisms) and eukaryotic organisms are made up of eukaryotic cells. Both types of cells contain organelles. Eukaryotic cells are complex and include all animal and plant cells. Prokaryotic cells are smaller and simpler, e.g. bacteria.

Question 14

What are organelles?

Answer 14

Organelles are parts of cells. Each one has a specific function. If you examine a cell through an electron microscope, you can see its organelles and the internal structure of most of them - this is known as the cell ultrastructure.

Question 15

Describe the features of an animal cell.

Answer 15

An animal cell contains a plasma membrane, rough endoplasmic reticulum, a nucleolus, a nucleus, smooth endoplasmic reticulum, lysosomes, ribosomes, a nuclear envelope, a Golgi apparatus, cytoplasm and mitochondria.

Question 16

Describe the features of a plant cell.

Answer 16

A plant cell contains a plasma membrane, rough endoplasmic reticulum, a nucleolus, a nucleus, smooth endoplasmic reticulum, lysosomes, ribosomes, a nuclear envelope, a Golgi apparatus, cytoplasm, mitochondria, a vacuole, a cellulose cell wall and chloroplasts.

Question 17

How do animal cells and plant cells differ?

Answer 17

Plant cells have all the same organelles as animal cells, but with a few added extras;

• a cell wall with plasmodesmata (‘channels’ for exchanging substances between adjacent cells).
• a vacuole (compartment that contains cell sap).
• chloroplasts (the organelles involved in photosynthesis).

Question 18

Describe the plasma membrane and explain its function.

Answer 18

The plasma membrane is the membrane found on the surface of animal cells and just inside the cell wall of plant cells and prokaryotic cells. It’s made mainly of lipids and proteins.x
The function of the plasma membrane is to regulate 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 19

Describe the cell wall and explain its function.

Answer 19

The cell wall is a rigid structure that surrounds plant cells. It’s made mainly of the carbohydrate cellulose.
The function of the cell wall is to support plant cells.

Question 20

Describe the nucleus and explain its function.

Answer 20

The nucleus is 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 often a structure called the nucleolus.
The function of the nucleus it that it 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 21

Describe a lysosome and explain its function.

Answer 21

A lysosome is a round organelle surrounded by a membrane, with no clear internal structure.
The function of a lysosome is that it 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 22

Describe a ribosome and explain its function.

Answer 22

A ribosome is 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 function of the ribosome is that it is the site where proteins are made.

Question 23

Describe the rough endoplasmic reticulum (RER) and explain its function.

Answer 23

The RER is a system of membranes enclosing a fluid-filled space. The surface is covered with ribosomes.
The function of the RER is to fold and process proteins that have been made at the ribosomes.

Question 24

Describe the smooth endoplasmic reticulum (SER) and explain its function.

Answer 24

The SER is similar to the RER, but with no ribosomes.

The function of the SER is to synthesise and process lipids.

Question 25

Describe a vesicle and explain its function.

Answer 25

A vesicle is a small fluid-filled sac in the cytoplasm, surrounded by a membrane.
The function of vesicles are to transport 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 26

Describe the Golgi apparatus and explain its function.

Answer 26

A Golgi apparatus is a group of fluid-filled, membrane bound, flattened sacs. Vesicles are often seen at the edges of the sacs.
The function of the Golgi apparatus is to process and package new lipids and proteins. It also makes lysosomes.

Question 27

Describe a mitochondrion and explain its function.

Answer 27

A mitochondrion is usually oval-shaped. It has a double membrane - the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.
The function of mitochondria are that they are the site of aerobic respiration, where ATP is produced. Mitochondria are found in large numbers in cells that are very active and require a lot of energy.

Question 28

Describe a centriole and explain its function.

Answer 28

A centriole is a small hollow cylinder, made of microtubules (tiny protein cylinders). They are found in animal cells but only some plant cells.
The function of the centriole is that it is involved with the separation of chromosomes during cell division.

Question 29

Describe a chloroplast and explain its function.

Answer 29

A chloroplast is 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 grant. Grana are linked together by lamallae - thin, flat pieces of thylakoid membrane.
The function of the chloroplast is that it is the site where photosynthesis takes place. Some parts of photosynthesis happen in the grana, and other parts happen in the storms (a thick fluid found in chloroplasts).

Question 30

Describe a centriole and explain its function.

Answer 30

A centriole is small, hollow cylinders made of microtubules (tiny protein cylinders). Found in animal cells, but only some plant cells.
The function of the centriole is that it is involved with the separation of chromosomes during cell division.

Question 31

Describe cilia and explain their function.

Answer 31

Cilia are 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 a single pair of microtubules in the middle.
The function of the cilia is that the microtubules allow the cilia to move. This movement is used by the cell to move substances along the cell surface.

Question 32

Describe the flagellum and explain its function.

Answer 32

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

Question 33

Describe the cytoskeleton.

Answer 33

The organelles in cells are surrounded by the cytoplasm. They 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. In eukaryotic cells the protein threads are arranged as microfilaments (very thin protein strands) and microtubules (tiny protein cylinders).

Question 34

What are the four functions of the cytoskeleton and give examples

Answer 34

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 transport of organelles and materials within the cell.
   For example, the movement of chromosomes when they separate during cell division depends on contraction of microtubules in the spindle.
   Also, the movement of vesicles around the cell relies in cytoskeletal proteins.
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 35

How do you prepare a dry mount?

Answer 35

This is the simplest way of preparing a slide for examination under a microscope. This technique is particularly useful for observing specimens such as hairs, parts of insects, pollen, parts of flowers, etc.
1. Firstly, 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 quite a thick specimen, you’ll need to take a thin slice to use on your slide.
2. Use tweezers to pick up your specimen
and put it in the middle of a clean slide.
3. Pop a cover slip (a square of thin, transparent plastic or glass) on top. Your slide is now ready to use.

Question 36

How do you prepare a wet mount?

Answer 36

Wet mounts involve your specimen being in a liquid (usually water). They are more difficult to carry out than dry mounting but can produce slides that give a really clear view of the specimen. This technique can be used with a variety of specimens including living samples (e.g. tiny aquatic organisms).

1. Start by pipetting a small drop of water onto the slide then use tweezers to place your specimen on top of the water drop.
2. 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.
3. 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.

Question 37

What are the differences between prokaryotic and eukaryotic cells?

Answer 37

Prokaryotic cells are extremely small cells, they are less than 2nm in diameter, their DNA is circular, there is no nucleus - the DNA is free in the cytoplasm, the cell wall is made of a polysaccharide - but not cellulose or chitin, there are few organelles and no membrane-bound organelles (e.g. there are no mitochondria), they have a flagella (when present) made of the protein flagellin - arranged in a helix, they have small ribosomes (20nm or less). Examples include E. Coli bacterium, Salmonella bacteria.
Eukaryotic cells are