Unit 1.2 Cells

Question 1

How can cells be seen with high magnification?

Answer 1

Cells can be seen with much higher magnification using an electron microscope rather than a light microscope.

Question 2

What organelles can be seen under an electron microscope?

Answer 2

With an electron microscope many membranous organelles can be seen within the cell.

Question 3

Why is the cell separated into compartments?

Answer 3

The cell is separated into compartments so that metabolic processes with specific conditions can be isolated.

Potentially harmful chemicals like enzymes can be isolated and molecules with specific functions (e.g. chlorophyll) can be concentrated in one area.

Internal membranes also provide a transport system and provide a large surface area for enzyme attachment.

Question 4

What is the size of a mitochondrion?

Answer 4

A mitochondrion is approximately 0.5 micrometers in diameter.

Question 5

What is an organelle?

Answer 5

An organelle is a specialised structure with a specific function within a cell.

Question 6

What is the defining feature of an eukaryotic cell?

Answer 6

The nucleus.

Question 7

Explain the structure of a nucleus.

Answer 7

1. It is a spherical structure and is the largest organelle (10-20 micrometer in diameter).
2. It is surrounded by a double membrane forming a nuclear envelope.
3. The outer membrane is continuous with the endoplasmic reticulum and has ribosomes attached.
4. The envelope has tiny nuclear pores to allow substances (mRNA, ribosomes) to leave the nucleus and others to enter (nucleotides).
5. The jelly-like substance within the nucleus is called the nucleoplasm. It contains chromatin which is made up of coiled of DNA wound around histone proteins.
6. The chromatin condenses during cell division to form visible chromosomes.
7. Within the nucleus is one or more spherical bodies each called a nucleolus.

Question 8

State the function of a nucleus.

Answer 8

The site of DNA/chromosomes.

Question 9

State the function of the nucleolus.

Answer 9

Makes ribosomal RNA, which is a constituent of ribosomes (rRNA)

Question 10

Explain the structure of mitochondria.

Answer 10

Mitochondria are rod-shaped organelles 1-10 micrometers in length and 0.5 micrometers in diameter.

They are surrounded by two membranes forming an envelope.

Between the two membranes is a fluid-filled inter-membranal space.

The inner membrane is folded to form extensions called cristae.

Within the inner membrane is the matrix which contains enzymes, organic molecules such as lipids and a single circle of DNA which allows mitochondria to make its own proteins (on small ribosomes) and replicate itself.

Question 11

State the function of mitochondria.

Answer 11

They are the site of aerobic respiration in cells.

Question 12

Where does the formation of ATP occur?

Answer 12

The reactions which produce the energy carrying molecule ATP occur in the matric and on the cristae. The folds provide a large surface area for the attachment of enzymes.

Question 13

Where are mitochondria most concentrated at?

Answer 13

Metabolically active cells, e.g. muscle cells have many mitochondria.

Question 14

What is the endoplasmic reticulum?

Answer 14

This is a complex system of membrane-bound flattened sacs or cisternae containing fluid.

It is a dynamic system.

The ER is connected to the nucleus and may link to other bodies.

There are two types:
Rough ER
Smooth ER

Question 15

Describe the structure of Rough ER.

Answer 15

The Rough ER looks very regular.

It has structures called ribosomes on its outer surface.

Proteins are synthesised on the ribosomes and the proteins are transported within the RER and finally secreted from the cell.

Question 16

Describe the structure of the Smooth ER.

Answer 16

The Smooth ER looks less regular and lacks ribosomes.

Lipids and steroids are synthesised and transported in the SER.

Question 17

Tell me the structure of ribosomes.

Answer 17

They consist of a large and a small component.

They are made in the nucleolus from protein and ribosomal RNA.

They may be free in the cytoplasm or attached to ER to form RER.

There are two types - large (80S found in eukaryotes, or small (70S) found in prokaryotes/mitochondria/ chloroplasts

Question 18

State the function of ribosomes.

Answer 18

They are the site of protein synthesis in cells, i.e. the assembly of amino acids in a particular sequence to make a specific polypeptide chain using the code within the DNA.

This is called translation.

Free ribosomes make proteins for use within cells.

RER make proteins for secretion (‘export’) by a process called excytosis.

Question 19

Describe the structure of the golgi body.

Answer 19

A structure similar to SER but more compact.

It is a stack of dynamic flattened membrane bound sacs with associated vesicles.

It is formed from RER being pinched off at the ends to form small vesicles.

These coalesce to form the Golgi body.

At the other end of the Golgi body, vesicles pinch off, move towards the cell membrane and fuse with it - exocytosis.

Question 20

State the function of the Golgi body.

Answer 20

Transports proteins in the cell, modifies them and packages the,

It also secretes carbohydrate such as cellulose to form cell walls, as well as transporting and storing lipids.

The vesicles produced by the Golgi body may then travel to the cell membrane and secrete their contents by exocytosis.

The Golgi also forms small temporary vacuoles called lysosomes.

Question 21

What are lysosomes?

Answer 21

Small temporary vacuoles surrounded by a single membrane formed from the Golgi body.

Question 22

What do lysosomes contain?

Answer 22

They contain hydrolytic (digestive) enzymes. The enzymes are isolated from the rest of the cell in these vacuoles.

Question 23

What do lysosomes do?

Answer 23

They fuse with other vacuoles containing invading bacteria or worn-out organelles and then release their contents so breaking down the bacteria/old organelle.

Question 24

Where are centrioles found?

Answer 24

In animal and protoctista cells only.

They are found in the cytoplasm just outside the nucleus.

Question 25

What does a centriole consist of?

Answer 25

A ring of microtubules (protein) making a hollow cylinder.

The two cylinders are positioned at right angles to one another and together are called a centrosome.

Question 26

State the function of a centriole.

Answer 26

They replicate just before nuclear division and then each pair moves to opposite poles of the cell where they synthesise the microtubules of the spindle apparatus.

Question 27

State the function of chloroplasts.

Answer 27

Chloroplasts contain photosynthetic pigments which trap light energy for photosynthesis (not all do).

Chlorophyll molecules are found on the very large surface area of the thylakoid membranes.

Question 28

Describe the structure of the permanent vacuole.

Answer 28

Plant cells have a large permanent vacuole that is fluid filled and bound by a membrane called the tonoplast.

It contains cell sap, a solution which stores glucose, amino acids, minerals, vitamins and other solutes.

Question 29

State the function of the permanent vacuole.

Answer 29

It controls osmosis and is important in supporting soft plant tissues/young (non-woody tissues.

It draws in water by osmosis so that the cells become turgid and have mechanical support.

Question 30

Describe the structure of the cell wall.

Answer 30

The cell wall of a plant consists of cellulose microfibrils, aggregated into fibres, and embedded in a polysaccharide matrix called pectin.