Cell structure and function Flashcards
Compare and contrast eukarytotes and prokaryotes
Recall that the diversity of life can be divided into three domains: bacteria, eukarya and archaea. Bacteria and archaea are prokaryotes, while humans and other multicellular organisms are eukaryotes.
Similarities between prokaryotes and eukaryotes include that they are both enclosed by membranes, to separate the outside environment from the inside; follow the central dogma of biology and engage similar molecular machines in the process; and are filled with cytoplasm. ^[for interest look up Heinrich, 2021, Nature– visualisation of cell ultrastructure]
One key difference between eukaryotes and prokaryotes is the packaging of DNA. Bacteria, for example, do not possess a nucleus, and instead contain their genetic material in a more loosely packed nucleoid i.e. not membrane bound ^[recent literature would suggest some bacteria can encase their DNA in hsitones]. In other words, DNA is localised to the nucleus and is separated from the cytoplasm by a porous nuclear envelope. The eukaryotic cell also contains numerous membrane bound organelles (compartments dedicated to a specific task) in its cytoplasm, while the prokaryotic cell does not. Lastly, another feature unique to eukaryotic cells is that they can assemble together to form complex multicellular organisms.
Describe a multicellular organism
A multicellular organism consists of many cell types with varying degrees of specialisation. All cells of a multicellular organism originate from a single cell (i.e. cellular differentiation) during development ^[this is as a result of interplay between the genetics and epigenetics]. It is differentiation or specialisation of the cells that results in diversity both between and within organisms.
Describe the cellular theory of life
That all cells in an organisms must derive from a single cell, leads into the tenets of the cell theory, neamely:
- all living organisms are made of one or more cells
- cells are the basic units of life
- all cells arise from pre-existing cells
Describe the lipid structure of cell membranes
The cell membrane is a feature common to both eukaryotes and prokaryotes, and works to separate the inside of the cell from the outside. The cell membrane is composed of a phospholipid bilayer. **This bilayer is selectively permeable ([[Physiology Lecture 2]]); it allows in some molecules but restricts the access of others. **
The membranes of organelles have a similar structure to cell membranes.
Features of the internal and external environments:
- The extracellular space, which is filled with interstitial fluid ([[Physiology Lecture 3]]), composed of ~ 75% water
- The intracellular space, with is filled with cytoplasm and also composed of 75% water
- The extracellular matrix or ECM, which is a network of proteins that cells are anchored to in a tissue
Describe key features of the cell membrane
Cell membranes are composed of a phospholipid bilayer.
Phospholipids are a class of lipids ([[Biochemistry Lecture 4]]).
Their structure can be simply divided into a hydrophilic ‘head’: composed of a variable group i.e. choline, ethanolamine, serine, phosphate and glycerol; and hydrophobic fatty acid ‘tails’.
Phospholipids are able to self-assemble into a bilayer when submerged in water. Hydrophobic elements associated with each other, and hydrophilic elements associate with each other.
The cell membrane is composed of other molecules besides phospholipids. Proteins makes up 505 of the mass of the membrane.
There are two types of proteins foun within cellular membranes adn they are:
- integral membrane proteins, which are embedded into the lipid bilayer, although many traverse the lipid bilayer (as transmembrane proteins)
- peripherla membrane proteins, which are not embedded into tht elipid bilayer and instead adhere to integral membrane proteins
The broad functions of membrane proteins are: transportation, signalling and adhesion
Describing signalling and adhesion
-Membrane proteins facilitate signalling by biding molecules present in the EC space and activating a process in the cytoplasm ([[Pharmacology Lecture 3]])
-Membrane proteins facilitate adhesion by allowing a cell to bind another cell or to the ECM. Examples include occluding/tight junctions and adhering junctions between endothelial (epithelial) cells ([[Histology Lecture 1]], [[Histology Lecture 2]])
Medical relevance of cell membranes:
- treatment i.e. hypernatremia, dehydration
- receptors, transporters, enzymes, ion channels, GPCRs – make up 60% of the membrane
- endocytosed vaccines, protein replacement therapies, CRISPR-Cas9 mRNA gene editing therapeutics
