plasma membranes Flashcards
1
Q
What is the plasma
membrane?
A
All the membranes of cells, which have the same basic structure described by the fluid-mosaic model The cell-surface membrane which separates the cell from its external environment
2
Q
What is the phospholipid
bilayer?
A
Arrangement of phospholipids found in cell membranes; the hydrophilic phosphate heads form both the inner and outer surface of a membrane, sandwiching the fatty acid tails to form a hydrophobic core
3
Q
Why are phospholipid
bilayers suited as
membranes?
A
• Cells normally exist in aqueous environments • The inside of cells and organelles are also usually aqueous environments • Phospholipid bilayers are suited because the outer surfaces of the hydrophilic phosphate heads can interact with water
4
Q
What is the fluid-mosaic
model?
A
Model of the structure of a cell membrane in which phospholipids within the phospholipid bilayer are free to move and proteins of various shapes and sizes are embedded in various positions • A model proposed by American scientists Singer and Nicolson in 1972 • Explains how cell membranes are dynamic and interact with the cells environment
5
Q
Describe cell membrane
components
A
a. Glycoprotein - branching carbohydrate portion of a protein which acts as a recognition site for chemicals e.g. hormones b. Glycolipid - acts as a recognition site e.g. for cholera toxins c. Cholesterol - for stability/flexibility d. Hydrophilic heads of phospholipid molecules - point outwards e. Protein molecule lying on the surface (extrinsic protein) f. Pore g. Protein molecule spanning the phospholipid layer (intrinsic protein) h. Hydrophobic tails of phospholipid molecules - point inwards i. Protein molecule partly embedded (extrinsic protein
6
Q
What a membrane proteins?
A
Protein components of cell-surface
membranes
7
Q
What are intrinsic proteins?
A
Also known as integral proteins • Transmembrane proteins that are embedded through both layers of a membrane • Have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place • Channel and carrier proteins (both involved in transport across the membrane
8
Q
What are channel proteins?
A
Membrane proteins that provide a hydrophilic channel through a membrane, which allows passive movement of polar molecules and ions down a concentration gradient through membranes • Held in position by interactions between the hydrophobic core of the membrane, and the hydrophobic R-groups on the outside of the proteins
9
Q
What are carrier proteins?
A
Membrane proteins that have in important role in both passive transport and active transport into cells • This often involves the shape the protein changing
10
Q
What are glycoproteins?
A
Extrinsic membrane proteins with attach carbohydrate molecules of varying lengths and shapes that are embedded in the cell-surface membrane • Play a role in cell adhesion and as receptors for chemical signals
11
Q
What is cell signalling?
A
A complex system of intercellular communication • When the chemical binds to the receptor, it elicits a response from the cell • This may cause a direct response or set off a cascade of events inside the cell
12
Q
Give examples of cell
signalling
A
Receptors for neurotransmitters e.g. acetylcholine at nerve cell synapses. Binding of the neurotransmitters triggers or prevents an impulse in the next neurone • Receptors for peptide hormones inc. insulin and glucagon, which affect the uptake and storage of glucose by cells • ß-blockers are used to reduce the response of the heart to stress
13
Q
What are glycolipids?
A
Cell-surface membrane lipids with attached carbohydrate molecules of varying lengths and shapes • Called cell markers or antigens, and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
14
Q
What are extrinsic proteins?
A
Also called peripheral proteins • Present in one side of the bilayer • Normally have hydrophilic Rgroups on their outer surface and interact with the polar heads of the phospholipids or with intrinsic proteins • Can be present in either layer, and some move between layers
15
Q
What is cholesterol?
A
A lipid with a hydrophilic end and a
hydrophobic end, like a
phospholipid
16
Q
What is the role of
cholesterol molecules in a
membrane?
A
• Regulates the fluidity of membranes • Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads, and the hydrophobic end interacting with the tails, pulling them together • Adds stability to membranes without making them too rigid • Prevent the membranes becoming too solid by stopping the phospholipid molecules rom grouping too closely and crystallising (at low temps)
17
Q
What is the role of
membranes within cells?
A
Cristae in mitochondria give a large SA for the some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur • Thylakoid membranes in chloroplasts have chlorophyll, and one these membranes some of the reaction of photosynthesis occur • Digestive enzymes on the plasma membranes of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
18
Q
What factors affect
membrane structure?
A
- Temperature
* Presence of solvents
34
Q
What is bulk transport?
A
A form of active transport where large molecules or whole bacterial cells are moved into or out of a cell by endocytosis or exocytosis Energy in the form of ATP is required for: • Movement of vesicles along the cytoskeleton • Changing the shape of cells to engulf materials • The fusion of cell membranes as vesicles form or as the meet the cell-cell-surface membrane
37
Q
What is osmosis?
A
Diffusion of water through a partially
permeable membrane down a water
potential gradient. A passive
process and energy is not required
47
Q
What happens if a plant cell is
placed in a solution with a
lower water potential than their
own?
A
• Water is lost from the cells by osmosis • This leads to a reduction in the volume the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall • The cell is said to be plasmolysed
