Cells and Organelles

Question 1

Why compartmentalise cells into organelles?

Answer 1

-Organelles are permissive environments for a set of biochemical functions
-Protects the cell by segregating destructive enzymes and chemicals
-Localise cellular processes for efficient function
-Separation of molecules required for specific functions

Question 2

What is the purpose of the plasma membrane?

Answer 2

-Encloses the cell through its ampithatic nature
-Provides a barrier between the external environment and internal environment of the cell
-Mediates any signals into and out of the cell

Question 3

Describe the structure of the plasma membrane

Answer 3

The plasma membrane is based on a phospholipid bilayer, with hydrophilic phosphate ‘heads’ pointing outwards and hydrophobic fatty acid ‘tails’ pointing inwards

Question 4

Describe the molecular organisation within the bilayer

Answer 4

• Membrane anchored and transmembrane proteins
• Phospholipid types (phosphatidylcholine tends to be on the outer layer while phospatidylserine tends to be on the inner layer of the bilayer)
• Cholesterol plays an important part in membrane fluidity, and is particularly concentrated in lipid rafts.

Question 5

What is the feature of cholesterol within the plasma membrane?

Answer 5

An ampipathic molecule that reduces the permeability of the cell membrane, by increasing packing of phospholipids, preventing water-soluble molecules from diffusing across the membrane.

Question 6

What is the function of lipid rafts within the plasma membrane?

Answer 6

Cholesterol-rich domains that compartmentalise function by concentrating molecules, e.g. act as ‘signalling platforms’ by concentrating signalling receptors.

Question 7

What is the function of carbohydrates within the plasma membrane?

Answer 7

Attached to the extracellular surface of the membrane, allowing cells to be recognised.
e.g. glycocalyx: a layer of carbohydrate that covers cells. many functions including cell recognition. cells lining the gut have a thick glycocalyx where digestive enzymes reside.
it also covers the membrane of endothelial cells, many bacteria and other cells.

Question 8

Where are proteins found?

Answer 8

Either attached peripherally to the membrane (receptors) or across the entire width of the membrane (integral proteins)

Question 9

What do plasma membrane receptors do?

Answer 9

Interact with specific chemical signals (ligands)

Question 10

What do integral plasma membrane proteins do?

Answer 10

Provide selective, regulated passage of molecules into and out of the cell.

Question 11

What is the purpose of the nucleus?

Answer 11

The control centre of the cell containing the cell’s genetic material and carrying out the essential functions of DNA replication and gene expression.

Question 12

Describe the structure of the nucleus

Answer 12

Surrounded by a double membrane, which separates the nucleoplasm from the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum, while the inner membrane is attached to a scaffold of intermediate filaments called the nuclear lamina.
Molecules move in and out of the nucleus via nuclear pores

Question 13

What is the purpose of the mitochondria?

Answer 13

Carrying out parts of aerobic respiration to produce ATP

Question 14

Describe the structure of the mitochondria

Answer 14

A double membrane, which creates a number of discreet compartments where aerobic respiration occurs.

Question 15

Describe the 4 compartments of the mitochondria

Answer 15

1. The outer membrane - selective permeability (e.g. pyruvate)
2. Intermembrane space- important in ETC and other enzymatic reactions (location of cytochrome C)
3. Inner membrane (cristae) - (folded into numerous cristae): electron transport chain
4. Matrix- enzymes for citric acid/Krebs cycle

Question 16

What is apoptosis? Give an example using cytochrome C

Answer 16

A form of programmed cell death, caused by a variety of toxic triggers. When cytochrome C, which usually resides in the mitochondria and participates in the electron transport chain, is released into the cytoplasm, it promotes a series of events that results in death.
These events activates caspases, which are a family of protease enzymes playing roles in apoptosis. These destroy nuclear lamins, activate DNases and destroy cell adhesion proteins.
Signifies how important it is to correctly localise molecules.

Question 17

What are conditions resulting from mitochondrial disfucntion?

Answer 17

These are clinically heterogenous groups of disorders.

Most cause multi organ dysfunction, with the brain and muscle (high energy demands) tending to be severely affected.
Recently, therapies have been designed to create IVF embryos where affected mitochondria can be replaced by a third donor - ‘three parent embryos’

Question 18

What is the Endoplasmic Reticulum and describe both types

Answer 18

A network of flattened membranes continuous with the nuclear membrane
Rough ER- is associated with ribosomes and carries out protein synthesis
Smooth ER- is the site of lipid synthesis and acts as a calcium store

Question 19

Describe the structure and purpose of Golgi apparatus

Answer 19

Consists of flattened membranes, sacs and vesicles
It can be thought of as the ‘sorting office’ of the cell, where proteins from the ER are carried in vesicles which fuse to become the cis cisterna, and are then sorted for their final destination in the cell.
The proteins move through the Golgi stack, and as they do, they undergo enzymatic modification which labels them for a specific cell destination.
Example: Adding mannose-6-phosphate will label molecules destined for the lysosome.

Question 20

What are lysosomes and describe an example of a lysosomal disorder?

Answer 20

Vesicles containing hydrolytic enzymes, which are used to degrade defective/old organelles, macromolecules and particles taken in from outside the cell. Optimal pH 5
An example: Tay-Sachs disease, a rare (and usually fatal) genetic disorder resulting from a failure to degrade lipids, causing them to accumulate in neuronal bodies, resulting in progressive damage to the nervous system, e.g. neurological regression, seizures and blindness.

Question 21

What are endosomes?

Answer 21

Vesicles involved in transport of molecules from the membrane, fusing with the lysosome to deliver their contents.

Question 22

What are peroxisomes?

Answer 22

Membrane-bound organelle that perform a large number of biochemical reactions. Small vesicles that contain oxidases and catalase and are involved in many processes such as fatty acid metabolism, biosynthesis of bile acids and detoxification.

Question 23

What is the purpose of the cytoskeleton and describe the structure?

Answer 23

• Organise cell structure; maintain correct shape of cell
• Supports fragile plasma membrane
• Provides mechanical linkages that let cell/tissue bear stress
• Facilitates movement of organelles
• Allows cell to adopt specific behaviours, e.g. growth, division, migration and motility.
  An interconnected network made up of three classes of filamentous proteins:
  1. Microtubules
  2. Microfilaments
  3. Intermediate filaments

Question 24

Describe intermediate filaments (Cytoskeleton)

Answer 24

Particularly found in cells that require a lot of strength, such as epithelial cells in skin. In skin they have a very high tensile strength, allowing stretching and pressure on the epithelial sheet, without which the epithelium would rupture.
Keratin is a family of intermediate filament proteins present in keratinocytes in the epidermis
Vimentin is family of intermediate filament proteins present in keratinocytes in the epidermis
Nuclear lamina is composed of lamins (intermediate filaments)
Purposes: - Regulating chromatin organisation
- Anchoring nuclear pore complexes
(supporting the nuclear membrane)
- Nuclear organisation

Question 25

Describe microtubules (cytoskeleton)

Answer 25

Highly dynamic cylindrical tubes that continuously grow and shrink, and thus push and pull associated structures. Hollow cylindrical tubes composed of tubulin heterodimers → protofilaments → tubes
Primarily responsible for the movement of vesicles and organelles around the cell, particularly important in neurons. Also involved in mitotic/meiotic chromosomal movement and ciliary and flagellar motility.
Intracellular organelle transport:
Kinesin motor proteins move cargo to the periphery (away from centrosome)
Dynein motor proteins move cargo towards the cell body (to centrosome)
Microtubules are a component of the mitotic spindle that attaches to chromosomes and separates chromosome pairs into daughter cells.

Question 26

Describe cilia and flagella (micotubules, cytoskeleton)

Answer 26

Cillia is found in cells that line the respiratory tract, and help move mucus and entrapped dust and microbes away from the lungs. Also found in fallopian tubes, where they assist the movement of the ovum towards the uterus.
Sperm cells use a flagella for propulsion
Defective cilia and flagella are associated with a condition called Kartagener’s syndrome, in which individuals will suffer frequent respiratory infections (ineffective mucus clearance) and infertility (immotile sperm)

Question 27

Describe cell junctions, name the types and why they are important in skin

Answer 27

Cells connect to each other, or to the underlying extracellular matrix by cell junctions. These are transmembrane protein complexes that interact with similar proteins on adjacent cells, linking not only the cells to each other, but to each other’s cytoskeletons as well.
3 types:
1. Anchoring Junction
2. Tight Junction
4. GAP Junction

Question 28

Describe anchoring cell jucntions

Answer 28

Provide mechanical stability to groups of epithelial cells so that they can function cooperatively.
Proteins involved:
C-C Adherens = cadherins
C-C Desmosomes = desmosomal cadherins
C-ECM Hemidesmosomes = integrins
C-ECM Focal adhesions = integrins
Epidermolysis bulose simplex- Defects in the hemidesmosome complex disrupts the dermo-epidermal layer, resulting in severe blistering.