cell membrane structure Flashcards
How is a cell membrane bilayer formed?
*The cell membrane is made up of phospholipids
*Phospholipids contain a hydrophilic
polar phosphate head and
hydrophobic non-polar fatty acid tails.
*Cells contain an aqueous cytoplasm
and are surrounded by aqueous
solution (tissue fluid)
*The hydrophilic polar phosphate
heads face the aqueous solutions
*The hydrophobic fatty acid tails shield
away and face each other.
Thus forms a bilayer.
What elements are present in a
phospholipid?
Carbon
Hydrogen
Oxygen
Phosphorus
Why is the cell surface membrane
described as a fluid mosaic model.
- Fluid - The phospholipid molecules
sway from side to side and proteins
within it - Mosaic - there proteins e.g intrinsic
proteins staggered in the bilayer
producing a mosaic pattern - Model- The agreed structure is based
upon experimental and chemical
evidence and so is classed as a
model.
What is the role of the cell surface
membrane (external membrane)?
(1) Control what comes in and out
(2) Cell recognition (antigens)
(3) Cell communication (cell signalling)
Define diffusion?
- Diffusion is the net movement of particles from a high concentration to
a low concentration.
*Particles move down the concentration gradient - Passive process
What kind of molecules diffuse easily through the bilayer?
- Non-polar molecules
- Fat soluble/ steroid
- Small polar molecules e.g water
Why can oxygen and carbon dioxide
diffuse through the bilayer?
Oxygen and carbon dioxide are non-
polar molecules. They diffuse through
the polar fatty acid tails and are not
rejected.
Why cant large polar molecules e.g
glucose diffuse through the bilayer?
- Glucose is large
Phospholipids, (fatty acid tails) form an
impermeable barrier to large molecules - It is polar, containing an OH group
- It is rejected by the large hydrophobic fatty acid tails and would need a channel protein to diffuse through.
What is the definition of facilitated diffusion?
The movement of large polar particles
from a high concentration to a low concentration
Down a concentration gradient.
Using carrier/ channel proteins.
It is a passive process
Why is facilitated diffusion a passive
process
Particles already have kinetic energy
Particles are moving down a concentration gradient.
What is the role of intrinsic proteins?
*Proteins span the entire bilayer.
*Hydrophobic R groups facing outwards interact with the hydrophobic fatty acid region of the bilayer and allow for stability.
*Hydrophilic R groups face in wards allowing polar molecules to move through via facilitated diffusion or
active transport.
* There are two types of channel and carrier proteins.
What elements are definitely present in a intrinsic and extrinsic protein’s
Carbon
Hydrogen
Oxygen
Nitrogen
What is cell signalling?
*Cells communicate with each other to
control processes inside the body and to respond to changes in the
environment.
*One cell releases a messenger molecule (hormone), molecule travels in the blood to another cell.
*Messenger detected by the cell because it binds to a specific
complimentary receptor
*This creates a cascade of chemical reactions within the specific cell
What are carrier proteins and their roles
large polar molecules e.g amino acids/
glucose must be carried across the
membrane by carrier proteins
(1) molecule binds to binding site carrier protein
(2) carrier changes shape (allosteric change in shape)
(3) molecule released on the other side of the membrane
(4) Moving down the concentration gradient – facilitated diffusion (passive process)
-Carrier proteins can also be involved in active transport.
What are channel protein’s and their
roles?
charged ions must pass through ion
channel proteins
They bind to a specific bonding site down their concentration gradients – they can never diffuse directly through
the phospholipid bilayer
This is facilitated diffusion (passive process)
What is the role of extrinsic proteins?
Receptor sites where drugs, hormones
and antibodies bind triggering chemical reaction within the cell as well as cell signalling.
What is a glycoprotein and glycolipid?
Glycoprotein= protein + carbohydrate
Glycolipid = phospholipid +
carbohydrate
What are the roles of glycolipid and
glycoproteins?
Function:
*Receptor sites where drugs, hormones and antibodies bind,
triggering chemical reaction within the
cell as well as cell signalling.
*They also act as antigens- cell
surface molecules that act as recognition markers.
*They have OH and H groups that can form hydrogen bonds with surrounding molecules.
What is the role of cholesterol
Cholesterol binds to the fatty acids
tails, packing them closer together.
It maintain the stability and fluidity of
the membrane
How does cholesterol maintain the stability of membranes at different temperatures?
*Body temperature- cholesterol holds
the fatty acid tails together, it makes
the cell more rigid, less fluid
*Icy areas cholesterol prevents crystallisation hence allows the cell to
survive and function
Bacteria do not have cholesterol how do
they maintain their shape
They have a murein cell wall which
helps them to maintain shape.
why would a single cell be more likely need cholesterol but not cells in a tissue
A tissue is made up of a group of similar/ same cell types.
These cells can support the shape of
each other.
Single cell unable to do this. - it would
need more cholesterol.
What can increase the fluidity of the cell
surface membrane at low temperatures?
*Double bonds between carbon atoms in the fatty acid chains
* Fatty acids having shorter chains
* Decrease in cholesterol
What is the role of internal membranes
(Membranes within cells)?
(1)Membrane around organelles-
compartmentalisation (keep specific conditions needed e.g. in the RER)
(2)They form vesicles to transport substances to different areas of the
cell e.g. transport vesicle RER to Golgi
apparatus
(3) Membranes can be sites for chemical reactions ( e.g. inner
membrane of mitochondria contains
enzymes needed for respiration. RER
attachment of enzymes or ribosomes.
(4) barrier between organelle and cell,
controlling movement of substances
in and out of the organelle
