Topic 2A: Cell Structure And Division

Question 1

Define magnification:

Answer 1

How many times bigger an image appears compared to the objects actual size.

Question 2

Define resolution:

Answer 2

The minimum distance apart two objects need to be in order to appear as separate items.

Question 3

Optical (light) microscopes:

Answer 3

• Use light to form an image.
• Maximum resolution of around 0.2 micrometers.
• Maximum useful magnification of around x1500.

Question 4

Electron microscopes:

Answer 4

• Use electrons to form an image.
• Higher resolution and magnification than optical microscopes, so give more detailed images.
• Maximum resolution of around 0.0002 micrometers.
• Maximum useful magnification of around x1,500,000
• Give a black and white image (but is often coloured using a computer).

Question 5

Transmission electron microscopes (TEM):

Answer 5

• Use electromagnets to focus a beam of electrons, which is then transmitted through the specimen.
• Denser parts of the specimen absorb more electrons, so appear darker in the image you end up with.
• Give higher resolution images, so you can see the internal structures of organelles e.g. chloroplasts.
• Specimen must be viewed in a vacuum, so must be a non-living organism.
• TEM’s can only be used on thin specimens.
• 2-D image

Question 6

Scanning electron microscopes (SEM):

Answer 6

• Scan a beam of electrons across the specimen - knocking off electrons from the specimen, which are gathered in a cathode ray tube to form an image.
• Must be used on non-living organisms.
• Produces a 3-D image.
• Can be used on thicker specimens.
• Gives a lower resolution image than TEM’s.

Question 7

How to prepare a optical microscope slide:

Answer 7

1. Start by pipetting a small water droplet onto the centre of the microscope slide.
2. Use tweezers to place a thin section of your specimen on top of the water droplet, your specimen needs to be thin to allow light to pass through it.
3. Add a drop of stain - stains highlight objects in a cell.
4. Place your cover slip on top of the specimen. To do this: stand your coverslip upright on the slide, next to the water droplet, then carefully tilt and lower your specimen so it covers the specimen. Don’t get any bubbles under your cover slip, they’ll obstruct your view of the specimen.

Question 8

What are artefacts?

Answer 8

Things that you can see when you look down a microscope that aren’t part of the cell or specimen you are looking at.

Question 9

Examples of microscope artefacts:

Answer 9

• Dust particles
• Fingerprints
• Inaccuracies caused by squashing or staining your sample.

Question 10

When are artefacts most commonly made?

Answer 10

During the preparation of your specimen.

Question 11

Where are artefacts most common?

Answer 11

In electron micrographs because specimens require a lot of preparation when they are viewed under an electron microscope.

Question 12

How were the first scientists who used electron microscopes able to distinguish between artefacts and organelles?

Answer 12

By using several different sample preparation techniques. If an object can be found using one preparation technique but not another, it is more likely an artefact.

Question 13

Compare the advantages and disadvantages of TEM’s and SEM’s:

Answer 13

TEM’s
Advantages:
• Gives a high resolution image, so shows small objects.
Disadvantages:
• Can only be used on thin specimens and non-living organisms.
• Only produces a 2-D image.
SEM’s:
Advantages:
• Can be used on thicker organisms.
Disadvantages:
• Gives a lower resolution image than TEM’s.
• Can only be used on non-living organisms.

Question 14

What are the 4 examples of eukaryotic cells?

Answer 14

Animal, plant, algal and fungal cells.

Question 15

Describe the structure and function of the mitochondrion:

Answer 15

• Structure - Oval shape, contains a double membrane: an outer and inner membrane. The inner membrane is folded to create a structure called crista, inside the inner membrane is the matrix which contains enzymes for respiration.
• Function - Site for aerobic respiration. Aerobic respiration produces ATP (a common energy source in the cell). Mitochondria are found in large numbers in cells that are very active and require a lot of energy.

Question 16

Describe the structure and function of the nucleus:

Answer 16

• Structure: A large organelle surrounded by a nuclear envelope, the nucleus contains many pores, a nucleolus and chromosomes (made from protein-bound linear DNA).
• Function: Controls the activities of the cell by controlling the transcription of DNA. The DNA contains instructions which help it make proteins. The pores allow substances to move between the nucleus and cytoplasm, and the nucleolus makes ribosomes.

Question 17

Describe the structure and function of the cell membrane:

Answer 17

• Structure: The membrane found on the surface of animal cells and just inside the cell wall of other cells.
• Function: Regulates the movement of molecules in and out of the cell. Contains receptor molecules which allow it to respond to chemicals like hormones.

Question 18

Describe the structure and function of the chloroplasts:

Answer 18

• Structure: A small, flattened organelle found in plant and algal cells. Surrounded by a double membrane and has membranes inside called the thylakoid membranes which are stacked up in some parts of the chloroplast to form grana. The grana are linked together by lamellae (thin, flat pieces of thylakoid membrane).
• Function: Site of photosynthesis, some parts of photosynthesis take place in the grana, other parts take place in the stroma (a thick fluid found in chloroplasts).

Question 19

Describe the structure and function of the Golgi apparatus:

Answer 19

• Structure: A group of fluid-filled membrane-bound flattened sacs, containing Golgi vesicles which are seen at the edge of the sacs
• Function: Processes and packages proteins and lipids and makes lysosomes.

Question 20

Describe the structure and function of the Golgi Vesicles:

Answer 20

• Structure: A small fluid-filled sac in the cytoplasm surrounded by a membrane and produced by the Golgi apparatus.
• Function: Stores proteins and lipids produced by the Golgi appartus and transports them out of the cell via the cell-surface membrane.

Question 21

Describe the structure and function of the lysosome:

Answer 21

• Structure: A round organelle surrounded by a membrane with no clear internal structure.
• Function: Contains hydrolytic enzymes kept separate from the cytoplasm by the surrounding membrane, they digest invading cells and break down worn out components of the cell.

Question 22

Describe the structure and function of ribosomes:

Answer 22

• Structure: A very small organelle that floats free in the cytoplasm or is attached to the rough endoplasmic reticulum. It’s made up of proteins and RNA, and is not surrounded by a membrane.
• Function: site of protein synthesis.

Question 23

Describe the structure and function of the rough endoplasmic reticulum (RER):

Answer 23

• Structure: A system of membranes enclosing a fluid-filled space. Its surface is covered with ribosomes.
• Function: Folds and processes proteins made at the ribosomes.

Question 24

Describe the structure and function of the smooth endoplasmic reticulum (SER):

Answer 24

• Structure: A system of membranes enclosing a fluid-filled space.
• Function: Processes and synthesises lipids.

Question 25

Describe the structure and function of the cell wall:

Answer 25

• Structure: A rigid structure that surrounds the cell in plants, algae and fungi. In plants and algae, its mainly made up of the carbohydrate chitin.
• Function: Supports the cell and prevents it from changing shape.

Question 26

Describe the structure and function of the cell vacuole:

Answer 26

• Structure: A membrane-bound organelle found in the cytoplasm. It contains cell-sap (a weak solution of salt and sugars). Its surrounding membrane is called the tonoplast.
• Function: Helps maintain pressure inside the cell and keep the cell rigid. Also involved in the isolation of unwanted chemicals inside the cell.

Question 27

When is the start and end of the cell cycle?

Answer 27

• Start: When the cell is produced be cell division.
• End: When the cell divides to produce two identical cells.

Question 28

When does the cell cycle begin and end?

Answer 28

Start: When cell is produced by cell division.
End: When cell divides to produce two new daughter cells.

Question 29

What happens during gap phase 1 (period of interphase) of the cell cycle?

Answer 29

The cell grows and new organelles and proteins are made.

Question 30

What happens during the synthesis phase (period of interphase) of the cell cycle?

Answer 30

The cells DNA replicates ready for mitosis.

Question 31

What happens during gap phase 2 (period of interphase) of the cell cycle?

Answer 31

The cell continues to grow and produce proteins necessary for cell division.

Question 32

What happens during the interphase of a cell?

Answer 32

The cell prepares for cell division by:
• unravelling and replicating its DNA to double the genetic content
• replicating its organelles.
• Increasing its ATP content.

Question 33

What is mitosis?

Answer 33

• A form of cell division thats part of the cell cycle, where the original parent cell produces two new identical daughter cells.

Question 34

State 3 reasons why mitosis is important?

Answer 34

1. Growth of multicellular organisms.
2. Reparation of damaged tissue.
3. Asexual reproduction

Question 35

Describe what happens during the prophase of mitosis?

Answer 35

• Chromosomes condense; getting shorter and fatter
• Bundles of proteins called centrioles move to opposite ends of the cell, forming a network of proteins called the spindle.
• Nuclear envelope breaks down, the chromosomes lie free in the cytoplasm.

Question 36

What happens during metaphase of mitosis?

Answer 36

• Chromosomes line up along the middle and attach to spindle fibres by their centromeres.