Cells and Organelles

Question 1

What are “communities of cells”

Answer 1

Higher organisms (e.g. humans) 
Individual cells perform specialised functions

Question 2

What is a cell?

Answer 2

• All living organisms are made of cells
• A semi-independent living unit within the organism/independent living unit in a unicellular organism which has the mechanisms for metabolism, growth and replication (by meiosis/mitosis/binary fission)
• Consists of an aqueous solution (cytoplasm) of membrane bound/non-membrane bound organic molecules (organelles)

Question 3

What is an organelle?

Answer 3

• Sub-unit within a cell, with a defined structure and performing specific, integrated activities
• Either membrane-bound/non-membrane bound

Question 4

What is tissue?

Answer 4

-assembly of cells and their extracellular products (extracellular matrix, connective tissue and lymphoid) co-ordinated to carry specific related functions in the body

Question 5

What is an organ?

Answer 5

-Assembly of related tissues coordinated to perform specific related functions within the body

Question 6

What is a system?

Answer 6

-Assembly of organs with specific related functions that are regulated the same way (e.g. respiratory) OR be diffused functional network of cells located around the body, with specific related functions (e.g. immune)

Question 7

What is the extracellular matrix?

Answer 7

-Extracellular matrix (collagen, enzymes and glycoprotein) provides strength and structural integrity to cells

Question 8

What is a prokaryote?

Answer 8

• uni-cellular organism with the chromosome (genetic material) lying free in the cell as a circular strand
• no membrane-bound organelles
• bacteria

Question 9

What is a eukaryote?

Answer 9

• uni-cellular/multi-cellular organism with chromosomes enclosed in the nucleus
• have cytoplasmic membrane-bound organelles
• DNA divided into a series of linear chromosomes
• large differences between cells in the same organism
• Animals, plants, fungi, protozoa (unicellular eukaryotes) and algae

Question 10

Why are the differences between prokaryotes and eukaryotes significant in terms of pathoogen site detection?

Answer 10

• Prokaryotes are usually pathogens
• Differences between prokaryotes and eukaryotes indicate target sites against the prokaryote pathogen without harming the infected eukaryote (human)

Question 11

What is a virus?

Answer 11

• assembly of nucleic acid (DNA/RNA) and proteins which is parasitic on prokaryotes/eukaryotes.
• Invade host cells, subvert protein synthesis mechanism to reproduce before escaping host cell to invade other cells.
• NOT CELLS OR ORGANISMS– lack plasma membrane and requires a host to survive

Question 12

What is the drawback of EM?

Answer 12

Image produced may not be a 100% reality
Preperation of sample and the fact that the sample is deceased means that the image may not be 100% accurate to a normal living cell.
Preparation is labour intensive

Question 13

What is a Transmission Electron Microscope (TEM) used for?

Answer 13

• To look inside cell specimen
• electrons pass through the specimen (based on density)
• Osmium tetroxide stabilsises lipids unlike in light microscope sample prep

Question 14

What is a Scanning Electron Microscope (SEM) used for?

Answer 14

• To see the specimen cell surface

- electrons deflected off cell surface by heavy metal coating (which is applied during prep)

Question 15

At what distance is diffusion efficient?

Answer 15

50μm

Question 16

What limits the maximum size of a cell?

Answer 16

Diffusion is inefficient >50μm
As SA:Vol decreases (cell gets bigger), distance from nuclues to periphery to increases, causing less efficient diffusion hence why most cells are approx similar size

Question 17

What do specialised cells with a large SA:Vol ratio have to overcome inefficient diffusion?

Answer 17

• Thin permeable membrane
• Transport of substances around cell via cytoskeleton and vesicles
• Multinucleate so gene expression/transport can occur at multiple sites
• Gap junctions which are channels between cells to allow movement of substances

Question 18

What is the benefit of membrane-bound organelles?

Answer 18

• Allows organelles to exist in a specialised environment so different cellular processes can operate under different conditions (e.g. a specific optimum pH for an enzyme)
• Cell can have numerous different functional compartments

Question 19

What is the cytoskeleton?

Answer 19

• Arrangement of protein filaments (actin, microtubules and intermediate)
• Control mechanical strength
• Control shape
• Guide movement of substances in vesicles
• Control location of organelles

Question 20

What is cytosol?

Answer 20

Aqueous environment within plasma membrane

Question 21

What is cytoplasm?

Answer 21

Cytosol along with cellular organelles

Question 22

What is the Nucleus?

Answer 22

• Site of genetic material of the cell which is organised as chromosomes that exist as chromatin
• Chromosomes only visible during cell division.

Question 23

What is chromatin?

Answer 23

• Chromosomes are condensed to form chromatin
• Chromatin is a complex of DNA/histone and non-histone proteins
• DNA packaged around histones to form nucleosome

Question 24

Why is DNA packaged around histones?

Answer 24

Non-packaged DNA = 1.8m

Nucleosomes= 95mm

Question 25

What is the nucleolus?

Answer 25

-Site of rDNA (hence rRNA synthesis) and ribosome assembly

Question 26

What is the function of the nuclear envelope?

Answer 26

-Double membrane that surrounds the nucleolus that regulates the transport of substances in/out

Question 27

What is the function of Smooth Endoplasmic Reticulum?

Answer 27

• Made up of cisternae
• Biosyntheses if membrane lipids and steroids, start of N-linked glycosylation and detoification of xenobiotics (toxin) via peroxisomes

Question 28

What is the function of Rough Endoplasmic Reticulum?

Answer 28

• Made of cisternae
• SER + RER are continous set of endosomes
• Consists of ribosomes on surface
• Site of translation/protein synthesis/cleavage
• Proteins are folded via Cys-Cys disulfide bridges
• Vesicles consisting of newly synthesisied protein pinch of RER

Question 29

What is the Golgi Apparatus?

Answer 29

4-8 closely stacked cisternae

• Modifies protein (N-linked carbs + glycosylation of O-linked carbs and lipids) delivered from RER via vesicles
• Synthesise secretory products
• Direct newly synthesised protein to correct compartments
• Transport substances around cell
• produce lysosomes

Question 30

Why can lysosomes be a target site against pathogens?

Answer 30

Because it is membrane bound, it creates a specific environment for the survival of that cellular component
If that lyosome is destroyed, that cellular component wouldn’t be able to surivive which can cause the pathogen to die.

Question 31

What are secretory vesicles?

Answer 31

Membrane bound
Pinch off golgi apparatus and coalesces with the inside of plasma membrane before contents are secreted via exocytosis
Composition is similar to the plasma membrane hence allowing exocytosis

Question 32

What are peroxisomes?

Answer 32

Membrane-bound
Synthesised by SER
Involved in detoxification incl. hydrogen peroxide
Involved in phospholipid synthesis
Oxidation of Very Long Chain Fatty Acids (VLFCA)

Question 33

What are lysosomes?

Answer 33

Membrane-bound
Involved in organelle autophagy (turnover/replacement)
>50 different hydrolytic enzymes that can hydrolyse all major cellular macromolecules
Has a low pH due to acidic hydrolytic enzymes

Question 34

What is the mitochondria?

Answer 34

• Double membrane bound
• Contains some DNA to encode some of its own proteins (own genome)-1% of overall genome
• Site of Kreb’s cycle and Electron Transport Chain
• Inner membrane is tightlly folded to form cristae that extends into the matrix which increases SA for enzymes to embed in

Question 35

How does fitness influence number of mitochondria

Answer 35

The more active you are, the greater the number of mitochondria as your body has an increased metabolic activity.
This could explain why fitter people have better post-injury/op outcomes as their energy levels are not a limiting factor in recovery.

Question 36

What is the use of a light microscope?

Answer 36

Overall tissue organisation can be seen and individual cells distinguished