Physiology Membrane Lecture 1 - What is a Membrane? Flashcards
Why are membranes important?
Plasma membrane forms an outer boundary of every cell which is essential for maintaining the environment inside and outside of the cell.
Selectively permeable.
Controls the entry of nutrients and exit of waste (and secretory) products.
Maintains differences in ion concentration inside and outside the cell.
Participates in joining of cells to form tissues and organs.
Enables a cell to respond to changes (signals) in the cell’s environment.
Membrane functions are crucial for cell survival
Many functional differences between cell types are due to variations in the composition of their plasma membranes - Different cells interact in different ways with essentially the same ECF.
Gives some example of secretory products!
Hormones
Neurotransmitters
Enzymes
What is a plasma membrane?
A fluid phospholipid bilayer embedded with proteins
What appearance does the plasma membrane have down an electron microscope?
Trilaminar - like a train tracks!
What makes up the lipid part of the plasma membrane?
Phospholipids and cholesterol
Describe the head of a phospholipid!
Phosphate group
Negatively charged
Polar
Hydrophilic
Describe the tail of a phospholipid!
Fatty chains
Uncharged
Non-polar
Hydrophobic
What does hydrophilic mean?
Water loving
What does hydrophobic mean?
Water hating
Describe the organisation of phospholipid in the plasma membrane.
They for a lipid bilayer where the hydrophilic heads face toward the outside of the plasma membrane and the hydrophobic tails face towards each other.
Describe the fluidity of the plasma membrane.
The phospholipids are constantly moving - they twirl, vibrate and move around their own half of the membrane millions of times per second.
Why can phospholipids move around their half of the plasma membrane?
The bonds between the phospholipids are not strong!
What is the cholesterol in the plasma membrane for?
Fluidity and Stability by helping to prevent the phospholipids from packing together. There are different amounts of cholesterol in the different cell types.
Name the types of membrane proteins!
Integral proteins
Transmembrane proteins
Peripheral proteins
Describe integral proteins!
They are embedded in the lipid bilayer e.g. receptors for hormones which bond to these integral proteins and activate a peripheral protein.
Describe Transmembrane proteins!
They extend through the membrane and can span the lipid bilayer multiple times. Examples are transporters and channels.
Describe peripheral proteins!
They do not penetrate the membrane and are more common intracellularly. An example is a receptor-associated enzyme.
Where are membrane carbohydrates located?
On the outer surface of the cell. Can be known as a sugar coating!
What are glycoproteins?
When short carbohydrate chains are bound to a membrane protein.
What are glycolipids?
When short carbohydrate chains are bound to a lipid.
What is more common, a glycoprotein or a glycolipid?
Glycoprotein
Together what do glycoproteins and glycolipids form?
A layer call the glycocalyx
What are the 3 important functions of the lipid bilayer as a basic structural barrier that encloses a cell?
- It forms the basic structure of the membrane.
- Its hydrophobic interior serves as a barrier. The cell can maintain differences in solute composition and concentrations inside/outside the cell.
- It is responsible for the fluidity of the membrane. Enables the cell to change shape (e.g. RBC squeezing through capillaries or a muscle cell)
Some transmembrane proteins form water-filled highly-selective ions channels. Name two!
Leak channels
Gated channels
Describe leak channels.
Channels that are always open!
Describe gated channels.
Channels that only open in response i.e. to a chemical message
Give an example of a disease that is caused by a genetic mutation in channels.
Cystic fibrosis
Give an example of a drug that targets channels.
Ca2+ channel blockers are used to manage hypertension an abnormal heart rhythms
Describe carrier or transport proteins?
Span the membrane
Exhibit substrate specificity - Accept only a particular molecule (or ion) or group of closely related molecules.
Different cell types express different kinds of carriers.
E.g. Thyroid gland cells and iodine
What are docking-marker acceptors?
Membrane proteins located on the inner surface of the membrane. They interact with secretory vesicles leading to exocytosis of the vesicle content.
What membrane protein is involved with exocytosis?
Docking-marker acceptors
Describe membrane bound enzymes!
Peripheral proteins that can be located on either side of the membrane but are most commonly found intracellularly e.g. protein kinase A
Describe receptor in relation to the plasma membrane!
Are proteins most commonly found on the outer surface of the cell. They bind specific molecules (lock and key) e.g. hormones
What are cell adhesion molecules?
They are membrane proteins also know as CAMs
Give two example of cell adhesion molecules!
- Cadherins (helps hold cells within tissues together)
2. Integrins (span the plasma membrane acting as a link between extra- and intra-cellular environments)
What do membrane carbohydrates serve as?
Self-identity markers
Describe self-identify markers!
Short carbohydrate chains on the outer membrane surface serve as self-identity markers that enable cells to identify and interact with one another
Why is it important that different types of cells have different markers (self-identify markers)?
It is important in cell-to-cell interactions, especially during embryonic development
What is the role of membrane carbohydrates in tissue growth?
So that cells do not overgrow their own territory.
What does abnormal surface markers (membrane carbohydrates) cause?
Cancer cell growth or excessive cell growth
What are specialised cell junctions?
When some cells are directly linked
What are the three types of specialised cell junctions?
- Desmosomes
- Tight junctions
- Gap junctions
Describe the specialised cell junction: Desmosomes!
Adhering junctions that anchor cells together, especially in tissues subject to stretching. E.g. skin, heart, uterus
Describe the specialised cell junction: Tight junction!
Join the lateral edges exclusively of epithelial cells near their lumenal (apical) membranes. (tight or leaky)
They are protein junctions.
Describe the specialised cell junction: Gap junctions!
‘Communicating’ junctions that allow the movement of charge carrying ions and small molecules between two adjacent cells.
