CBS - Cells and Organelles

Question 1

What are the fundamental units of life, and why is that important to know?

Answer 1

Cells are the fundamental units of life.

All living organisms are made of cells. This is important to know because nothing less than a cell can truly be called living (e.g. viruses).

Question 2

Define a cell.

Answer 2

A semi-independent, living unit within the body, (in unicellular organisms, it is completely independent) in which are sited the mechanisms for metabolism, growth and replication (by division).

It consists of an aqueous solution of organic molecules surrounded by a membrane.

Question 3

Define an organelle.

Answer 3

A sub-unit within a cell, with a defined structure and performing specific, integrated activities.

Some are bounded by membranes, others are non-membranous (e.g. ribosomes).

Question 4

Define a tissue.

Answer 4

An organised assembly of cells and their extracellular products which carry out similar and coordinated activities within the body (e.g. connective, lymphoid system).

Question 5

Define an organ.

Answer 5

An assembly of tissues coordinated to perform specific functions within the body (e.g. eye, ear, heart, lungs, liver).

Question 6

Define a system.

Answer 6

An assembly of organs with specific, related activities, sharing regulatory influences (e.g. respiratory).

OR

A diffuse functional network of cells situated in many parts of the body, sharing specific activities (e.g. immune).

Question 7

Define a prokaryote.

Answer 7

A single-celled organism in which the chromosome is a circular strand lying free in the cell (i.e. no nucleus) and has no membranous organelles (e.g. bacteria).

Question 8

Define a eukaryote.

Answer 8

One or more cells in which chromosomes are enclosed in a nucleus; typically have cytoplasmic, membrane-bound organelles, DNA divided into a series of linear chromosomes, and considerable differences occur between cells within the same organism.

All complex organisms (plants, fungi, animals, protozoa, algae) are eukaryotes.

Question 9

Define a virus.

Answer 9

An assemblage of nucleic acid (DNA or RNA) and proteins (and often other molecules) which is parasitic on prokaryotes/eukaryotes. Viruses invade cells, subvert their protein synthesis machinery to make more viruses instead of normal cell proteins, then escape to infect other cells.

Question 10

Why are viruses not considered ‘living’?

Answer 10

Viruses are not cells or organisms in the strict sense because:

• they lack a plasma membrane
• only operate chemically within host cells

Question 11

What is the difference in how chromosomes are presented in prokaryotic and eukaryotic cells?

Answer 11

PROKARYOTIC: single circular

EUKARYOTIC: paired linear

Question 12

Where are the chromosomes located in prokaryotic and eukaryotic cells?

Answer 12

PROKARYOTIC: in the nuclear region

EUKARYOTIC: in a membrane-bound nucleus

Question 13

Is the nucleolus and histones absent or present in prokaryotic and eukaryotic cells?

Answer 13

PROKARYOTIC: absent

EUKARYOTIC: present

Question 14

Where is the extra-chromosomal DNA in prokaryotic and eukaryotic cells?

Answer 14

PROKARYOTIC: in plasmids

EUKARYOTIC: in mitochondria (and plasmids)

Question 15

What size are the ribosomes subunits in prokaryotic and eukaryotic cells?

Answer 15

PROKARYOTIC: 70S

EUKARYOTIC: 80S cytoplasmic, and 70S mitochondrial

Question 16

How does cell division occur in prokaryotic and eukaryotic cells?

Answer 16

PROKARYOTIC: through binary fission

EUKARYOTIC: through mitosis and meiosis

Question 17

Is the mitotic spindle present or absent in prokaryotic and eukaryotic cells?

Answer 17

PROKARYOTIC: absent

EUKARYOTIC: present

Question 18

Is the endoplasmic reticulum present or absent in prokaryotic and eukaryotic cells?

Answer 18

PROKARYOTIC: absent

EUKARYOTIC: present

Question 19

Is the mitochondria present or absent in prokaryotic and eukaryotic cells?

Answer 19

PROKARYOTIC: absent

EUKARYOTIC: present

Question 20

Are the lysosomes present or absent in prokaryotic and eukaryotic cells?

Answer 20

PROKARYOTIC: absent

EUKARYOTIC: present

Question 21

Is the Golgi present or absent in prokaryotic and eukaryotic cells?

Answer 21

PROKARYOTIC: absent

EUKARYOTIC: present

Question 22

Are the peroxisomes present or absent in prokaryotic and eukaryotic cells?

Answer 22

PROKARYOTIC: absent

EUKARYOTIC: present

Question 23

Is the cytoskeleton present or absent in prokaryotic and eukaryotic cells?

Answer 23

PROKARYOTIC: absent

EUKARYOTIC: present

Question 24

Is the cell wall present or absent in prokaryotic and eukaryotic cells?

Answer 24

PROKARYOTIC: present

EUKARYOTIC: absent (apart from some fungi)

Question 25

Are the sterols in plasma membranes present or absent in prokaryotic and eukaryotic cells?

Answer 25

PROKARYOTIC: absent

EUKARYOTIC: present

Question 26

Are the internal membranes present or absent in prokaryotic and eukaryotic cells?

Answer 26

PROKARYOTIC: only for photosynthetic organisms

EUKARYOTIC: present - there are numerous membrane-bound organelles

Question 27

How would one visualise the subcellular details of a cell?

Expand on the topic.

Answer 27

Only Electron Microscope (EM) imaging can reveal the subcellular details in a cell.

There are two types of electorn microscopes we need to know, and they have different functions:

• Transmission Electron Microscope (TEM): to look inside a cell (electrons go through the specimen)
• Scanning Electron Microscope (SEM): to see the cell surface(electrons scattered off cell surface by heavy metal coating e.g. Au)

Both involve elaborate preparation and can only evaluate dead cells.

Question 28

What limits the maximum size of a cell?

Answer 28

The limitation for the size of a cell is the distance by which it can diffuse nutrients across it. Diffusion at distant less than 50μm is efficient, and is reduced above that.

Note that
‘Surface Area (SA) : Volume (V) Ratio’

As cell increases in size the volume increases more than the surface area.
As a consequence, as the cell gets bigger, it’s increasingly inefficient for things to diffuse across a cell.

Question 29

How do specialised cells overcome this

problem?

Answer 29

Thin processes:
- directed transport of substances around cell via the cytoskeleton e.g. neurones and oligodendrocytes

“Giant” multinucleate cells:
- so that gene expression can occur in more than one place e.g. skeletal muscle cells (happens when cells join together)

Gap junctions:
- there are channels between cells to allow movement of substances between cells

Question 30

What are some advantages of having membrane-bound cellular organelles?

Answer 30

• it allows for specialised environments to exist within them
• it allows different functions to operate under different conditions
  e. g. pH
• a single cell can have numerous different functional compartments

Question 31

Describe the cytoskeleton.

Answer 31

The cytoskeleton is made up of protein filaments:

• actin: the thinnest (found in muscle)
• intermediate filaments: the mechanical strength of the cell
• microtubules: the thickest (they pull daughter cells apart)

They all contribute to the mechanical strength of the cell, control the shape, and drive and guide movement of materials around.

Question 32

What are some physical features of the nucleus?

Answer 32

• nucleolus: where the rDNA is transcribed and the ribosome units are assembled
• nuclear envelope: surrounded by two layers of membrane
• nuclear pores: allows transport in and out

Question 33

Describe the nucleus.

Answer 33

It is the largest organelle in the cell (diam. 3-10μm). It is also the only organelle clearly visible by light microscopy.

It contains genetic material that is packaged. The DNA is organised as chromosomes.

Chromatin is the complex of DNA/histone and non-histone proteins. DNA winds round histones, creating nucleosomes.

Question 34

Describe the smooth and rough endoplasmic reticulum.

Answer 34

SMOOTH ENDOPLASMIC RETICULUM:

• where the biosynthesis of membrane lipids and steroids takes place
• the start of N- linked glycosylation
• detoxification of xenobiotics (e.g. P450 system)

ROUGH ENDOPLASMIC RETICULUM:

• coated with ribosomes – for translation of proteins for secretion or insertion into the cell membrane
• this is where proteins are folded, and Cys-Cys (cysteine) bridges form
• vesicles are budded from RER and transported to the Golgi body

The SER and RER are continuous set of endosomes - they are interconnected.

Question 35

Describe the Golgi apparatus.

Answer 35

It is 4-8 closely-stacked, membrane-bound channels (cisterna).

It modifies proteins delivered from RER via vesicles:

• modifying N-linked carbohydrates
• glycosylation of O-linked carbohydrates and lipids

It also synthesises/ packages materials to be secreted. It directs new proteins in vesicles to their correct compartments, and transport membrane lipids around cell.

They also create lysosomes.

Question 36

Describe the secretory vesicles.

Answer 36

They are vesicles that bud off from the Golgi.

Vesicles fuse with the inner surface of the plasma membrane and release their contents through exocytosis (e.g. hormones and neurotransmitters).

Question 37

Describe peroxisomes.

Answer 37

They are large vesicles (0.5-1.5μm), which are not very electron-dense.

They are involved in:

• detoxification
• phospholipid synthesis e.g. plasmalogens
• oxidation of Very Long Chain Fatty Acids (VLCFA)

They contain enzymes which generate (and degrade) H2O2.

Question 38

Describe lysosomes.

Answer 38

They are electron-dense spheres in the EM.

They are membrane-bound, with >50 different hydrolytic enzymes which can hydrolyse all major cellular macromolecules. They all require a low pH (average 5.2).

It is involved in organelle turnover/replacement - known as autophagy.

Question 39

Describe mitochondria.

Answer 39

It is the second largest organelle in the cell. It has two layers of membrane, 0.5-2μm long. The number per cell reflects the metabolic activity of that cell.

It contain some mitochondrial DNA, which encode some of their proteins (~1%).

Here, sugars are oxidised – generating ATP (through the Krebs cycle).

It has inner membrane in folds - called cristae – which increase the surface area.

Krebs cycle enzymes/ Electron Transport Chain enzymes are located in different parts of the membrane structure.