Cell Boundaries Flashcards
What are cell boundaries
Cells require a boundary with their environment
also have internal boundaries allowing compartmentalisation
What do cell boundaries allow
- Formation of suitable semi-fluid matrix (protoplasm) for cell functions (life).
- Formation of specialised microenvironments that can concentrate solutes/dedicated cell processes.
- Segregation of harmful activities
What are the main biological molecules within membranes
composed of lipids and proteins in varying ratios, also contain carbohydrates
Membrane lipids are Amphipathic, what does this mean?
contain hydrophilic (water-loving) and hydrophobic (water-hating) parts
Describe why the structure of a phospholipid leads to the formation of a lipid bilayer
hydrophilic head and hydrophobic tail
Effects how molecules orientate in an aqueous environment.
Phosphate heads point outwards because they are hydrophilic and fatty acid tails point inwards because they are hydrophobic
What is the name of the most common phospholipid in eukaryotic cell membranes
Phosphatidyl choline
There are various types of what in cell membranes
Phospholipids
In what compound are carbohydrates found within membranes
Glycolipids
What is the purpose of cholesterol in the cell membrane of Eukaryotes
increases barrier properties, less permeable to water
Fungi ergosterol, anti-tumor and antioxidant properties, and used Vit D synthesis
What is the name of the model used to describe the phospholipid bilayer
Fluid-Mosaic model’ of membrane structure -
contains integral proteins penetrating lipid bilayer, floating in ‘lipid sea’. Also peripheral proteins outside lipid bilayer linked by noncovalent bonds
What evidence is there for lipid-anchored membrane proteins
Evidence from freeze-fracture, see protrusions and depressions in different planes.
What are the key structural features of Fluid-mosaic model
- Lipid bilayers are self-assembling (repairs tears).
- Lipids layers are fluid. Lipids may move 2 µm in 1 sec!
However, proteins more restricted movement and some held in fixed position by cytoskeleton, ‘rafts’ and membrane ‘domains’. - Asymmetry - inner and outer lipid layers have different components and lipid composition. Lipids rarely cross monolayers unless under action of ‘scramblase’ or ‘flippase’enzymes.
- ‘Integral’ proteins pass through the lipid bilayer.
How does the structure of the cell surface membrane relative to function in terms of movement of substances
- Provides ‘semi-permeable’ barrier to movement of solutes/gases.
- Small nonpolar molecules (O2, N2, CO2) and lipid soluble pass through easily.
- Ions and larger polar molecules (amino acids, sugars) fail.
- Water fairly rapid diffusion(osmosis), can be assisted by ‘aquaporin’ channels.
- Allows maintenance of interior and compartmentalisation
- Transporting solutes. Integral proteins provide means of accumulating sugars/amino acids/ions, act as channels often via active processes.
In what other ways does the structure of the cell membrane relate to function
- Flexibility for cell movement and division, membrane fusion.
- Respond to external signals. Protein receptors in membrane respond to external stimuli.
- Intercellular interaction. Carbohydrates on membrane surface provide cell identity, used in self-recognition
How can cells adjust membrane fluidity
By altering lipid composition
E.g. cells grown at lower temperatures have more lipids with double bonds in tails to maintain fluidity
Why does membrane composition vary
according to biological function
E.g. inner mitochondrial membrane has a high protein content due to transport processes whereas nerve myelin sheath has a high lipid content has electrical insulation
Why may come cells have a cell wall
Plasma membranes are very thin - hence often need additional support/protection
What is the cell wall in bacteria made from
Peptidoglycan
What are the cell walls in fungi made from
Chitin
What are the cell walls in plants made from
Cellulose
What is the cell cortex
Framework of proteins under membrane
Attached to plasma membrane via transmembrane proteins
The best understood example of the cell cortex is in animal red blood cells - erythrocytes
How does it benefit the cell
A protein called spectrin forms a meshwork providing support and maintaining cell shape
Connected to plasma membrane via actin and anchor proteins, provides elasticity
What may cells have as extracellular components?
an extensive exterior cell coat based on sugar molecules (the slimy glycocalyx)
Attached to membrane proteins and/or glycolipids
What is the significance of a complex extracellular matrix
Organised extracellular material
Collagen (most abundant glycoprotein in animal external cell membranes) forms strong fibers outside the cells
Protects the cell surface from mechanical and chemical damage
Also has an important role in cell-cell recognition, and provides cell ‘clothing’
