Cells and Organelles Flashcards
(28 cards)
Why compartmentalise cells into organelles?
-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
What is the purpose of the plasma membrane?
-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
Describe the structure of the plasma membrane
The plasma membrane is based on a phospholipid bilayer, with hydrophilic phosphate ‘heads’ pointing outwards and hydrophobic fatty acid ‘tails’ pointing inwards
Describe the molecular organisation within the bilayer
- 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.
What is the feature of cholesterol within the plasma membrane?
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.
What is the function of lipid rafts within the plasma membrane?
Cholesterol-rich domains that compartmentalise function by concentrating molecules, e.g. act as ‘signalling platforms’ by concentrating signalling receptors.
What is the function of carbohydrates within the plasma membrane?
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.
Where are proteins found?
Either attached peripherally to the membrane (receptors) or across the entire width of the membrane (integral proteins)
What do plasma membrane receptors do?
Interact with specific chemical signals (ligands)
What do integral plasma membrane proteins do?
Provide selective, regulated passage of molecules into and out of the cell.
What is the purpose of the nucleus?
The control centre of the cell containing the cell’s genetic material and carrying out the essential functions of DNA replication and gene expression.
Describe the structure of the nucleus
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
What is the purpose of the mitochondria?
Carrying out parts of aerobic respiration to produce ATP
Describe the structure of the mitochondria
A double membrane, which creates a number of discreet compartments where aerobic respiration occurs.
Describe the 4 compartments of the mitochondria
- The outer membrane - selective permeability (e.g. pyruvate)
- Intermembrane space- important in ETC and other enzymatic reactions (location of cytochrome C)
- Inner membrane (cristae) - (folded into numerous cristae): electron transport chain
- Matrix- enzymes for citric acid/Krebs cycle
What is apoptosis? Give an example using cytochrome C
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.
What are conditions resulting from mitochondrial disfucntion?
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’
What is the Endoplasmic Reticulum and describe both types
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
Describe the structure and purpose of Golgi apparatus
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.
What are lysosomes and describe an example of a lysosomal disorder?
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.
What are endosomes?
Vesicles involved in transport of molecules from the membrane, fusing with the lysosome to deliver their contents.
What are peroxisomes?
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.
What is the purpose of the cytoskeleton and describe the structure?
- 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
Describe intermediate filaments (Cytoskeleton)
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
