2.5- sTRUCTURE OF CELL MEMBRANE Flashcards
Why do cell membrane form a barrier?
As cell membrane forms a barrier and separates the cell contents from the cell’s exterior environment, they need to allow some molecules through into or out of the cell. Some organelles also have membranes within them, these form barriers too.
What does permeability refer to?
Permeability refers to the ability to let substances pass through.
What can pass through the membrane?
- Some very small molecules can diffuse through the cell membrane, in between structural molecules.
- Some substances dissolve in the lipid layer and pass through.
- Other substances pass through special protein channels or are carried by carrier proteins.
What is the role of the membrane at the surface of cells?
The plasma membrane:
- It separates the cell’s components from its external environment. (in single-celled organisms the environment is its external surroundings; in multicellular organism, such as humans each cell’s environment is the tissue fluid )
- Regulates transport of materials in and out of the cell.
- May contain enzymes involved in specific metabolic pathways.
- Has antigens, so that the organism’s immune system can recognise the cell and not attack it.
- May release chemicals to signal to other cells.
- Contains receptors for chemical signals and so is a site for cell communication or signalling; hormones and drugs may bind to membrane bound receptors
- May be the site of chemical reactions
Why are the organelles separated?
The membrane separates the organelle content form the cytoplasm so that each organelle is a discrete entity able to perform its function.
What is the role of the membrane within the cell?
- Mitochondria have folded inner membrane called cristae. This gives large surface area for some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur.
- The inner membrane of chloroplast, called thylakoid membranes, house chlorophyll. On these membranes some of the reactions of photosynthesis occur
- There’re some digestive enzymes on the plasma membrane of epithelial cells that line the small intestine and these enzymes catalyse some of the final stages in the breakdown if certain sugars
Who came up with the fluid mosaic model and what is it?
In 1972,SInger and Nicolson proposed a model that allowed the passage of molecules through the membrane. Their structure explained how cell membranes could be more dynamic and interact more with the cell’s environment.
What did the FMM propose?
It proposed that the fabric of membrane consisted of a phospholipid bilayer(double layer) with proteins floating in it making a mosaic pattern. Lipids molecules can change places with each other and some proteins my move giving fluidity.
WHat is the bilayer made up of?
Its made up of two layers of phospholipid molecules. Their hydrophilic heads are in contact with the watery exterior (cytoplasm). The hydrophobic tail regions are in the centre of the membrane away from water.
What are there in the membrane?
Spanning the membrane there are:
- Some of them have pores and act as channels to allow ions, which have an electrical charge and are surrounded by water molecules, to pass through.
- Some proteins are carriers and, by changing their shape, carry specific molecules across the membrane.
- Other proteins may be attached to the carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals like hormones.
Why is cholesterol important in eukaryotic cell membranes?
It helps to regulate the fluidity of the membrane, maintain mechanical stability and resist the effects of temperature changes on the structure of the membrane.
What is the fluid mosaic model?
The theory of cell membrane structure with proteins embedded in a sea of phospholipids.
What is a glycolipid?
A lipid/phospholipid with a chain of carbohydrate molecules attached.
What is a glycoprotein?
A protein with a chain of carbohydrate molecules attached.
What is a plasma membrane?
A cell surface membrane.
