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Chapter 4: cell membranes and
transport
Phospholipids:
Phospholipids: are lipid molecules that consist of a polar head (hydrophilic) and nonpolar tail
(hydrophobic) because one of the fatty
acid groups is replaced with a phosphate
group, making the head polar
...
Micelle: Ball-like structure formed
by phospholipids, where the polar
heads are on the exterior, shielding
the hydrophobic tails which point in
towards each other
...
Basic structure of
membranes
...
respiration and excretion) can be controlled
Structure of membranes:
●
The phospholipid bi-layer is visible using the electron microscope on a very high
magnification (x100,000 -One hundred thousand)
●
Around 7 nm thick
The fluid mosaic model:
●
Fluid: Because phospholipids and proteins can move around by diffusion -
Phospholipids mainly move sideways within their own layer (monolayer) , while some
proteins can move within the bi-layer
...
●
●
Mosaic: Pattern of scattered proteins when the membrane is viewed from above
...
●
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Phospholipid tails point inward, forming non-polar hydrophobic interior
Phospholipid heads face outwards, into the aqueous (water-containing) medium that
surrounds the membranes
...
cholesterol)
...
Tail length also affects fluidity: the l onger the tail, the more fluid the membrane is
...
Some organisms, such as bacteria and yeasts, who cannot regulate their own body
temperature, respond by increasing the proportion of unsaturated fatty acids in their
membranes to maintain fluidity
...
●
Some phospholipid molecules can be chemically modified to act as s
ignalling
molecules; these may move around the bi-layer activating other molecules eg
...
●
Some may be hydrolysed into smaller, water-soluble molecules(vesicles)which
diffuse through the cytoplasm and bind to special receptors -eg
...
●
Cholesterol increases fluidity of membrane at low temperatures
Done by preventing it from becoming too rigid by prohibiting close packing of
the phospholipid tails
...
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●
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Glycolipid: Attached to a lipid molecule
Glycoprotein: Attached to a protein molecule
Chains project into watery exterior surrounding cell
●
Form hydrogen bonds with water and stabilize membrane structure
●
Glycocalyx: Sugary coating formed by carbohydrate chains
●
The glococalyx in animal cells is formed mainly from proteins, while in plants it is
mainly from glycolipids
...
Different cells have different receptors depending on their function
...
Signalling receptors: Part of the signalling system that coordinates the activities of
cells
...
When messenger molecules bind to the receptor, it triggers a series of chemical
reactions inside the cell
...
Receptors involved in Endocytosis: Bind to molecules that are parts of structures
to be engulfed by the cell surface membrane
...
proteins) into the cell by engulfing them
...
Cell adhesion: Binding cells to other cells
● Some glycolipids and glycoproteins act as cell markers or antigens, allowing cell to
cell recognition
...
ABO blood group proteins all
have small differences in their carbohydrate chains
...
Found in inner and outer
layer, or most commonly spanning the entire membrane, which are called
trans-membrane proteins
...
●
Extrinsic (peripheral) proteins: Entirely outside the membrane, found on the inner
and outer surface
...
Intrinsic (integral) proteins
●
●
Have hydrophobic and hydrophilic regions
Stay in the membrane because the hydrophobic regions, made from non-polar amino
acids are next to the hydrophobic phospholipid tails and get repelled by the watery
environment on either side of the membrane
The hydrophilic regions, made up of polar amino acids, are repelled by the non-polar
interior of the membrane and therefore face the aqueous environment inside and
outside the cell, or line the hydrophilic pores which pass through the membrane
...
Many have short carbohydrate chains attached to the outer side that protrude into the
aqueous environment, called glycoproteins
Protein functions in cell membrane:
Transport proteins
●
Provide hydrophilic channels or passageways for ions and polar molecules to pass
through membrane
...
Each transport protein is specific for a particular type of ion or molecule
...
Enzymes
●
Some proteins on the inside of the cell surface are attached to the cytoskeleton
(system of protein filaments inside the cell)
...
●
Involved in changes of shape when cells move
...
●
Complex range of signalling pathways which coordinate activities of the cell so they
respond appropriately to the environment, even if the cells are large distances apart
in the body
...
2
...
4
...
●
●
Stimuli can be from inside (hormones) as well as outside (light)
...
●
In a typical signalling pathway, molecules must cross or interact with the cell surface
membrane
...
steroid hormones can diffuse directly through
the cell surface membrane and bind to receptors in the cytoplasm or nucleus
...
Signal arrives at a protein receptor in the cell surface membrane
...
Only cells with this receptor
can recognize the signal
...
Signal changes the shape of the receptor, and since this spans the membrane,
the message is in effect passed to the inside of the cell (signal transduction)
...
3
...
G-proteins got their name because the switch mechanism involves binding to
GTPmolecules - similar to ATP, but with guanine instead of adenine
...
After 1 receptor molecule is stimulated, m
any second messenger molecules can be
made in response - amplification, a k ey feature of signalling
...
Signalling cascade: A
mplification triggered by G-protein
...
Response is produced
Alternative ways receptors change activity of the cell:
●
Opening an ion channel, resulting in change of membrane potential
●
Acting directly as a membrane-bound enzyme
●
Act as a intracellular (inside the cell) receptor when initial signal passes through
Movement of substances in and out of the cell:
There are 5 main ways in which movement is done:
1
...
Facilitated diffusion
3
...
Active transport
5
...
Move down a concentration gradient
Random movement caused by natural kinetic energy
To reach equilibrium
Factors that affect diffusion:
●
Steepness of concentration gradient: The greater the difference in concentration,
the faster the rate of diffusion
...
-Surface area can be increased by folding
-The larger the cell, the smaller it's surface area is in relation to it's volume -volume
increases more rapidly than surface area as size increases
...
-Time it takes for a molecule to reach it's destination by diffusion increases rapidly
with distance traveled
...
Nature of molecules or ions: S
mall, non-polar molecules diffuse faster through the
membrane (eg
...
-Non-polar molecules are not repelled by the hydrophobic interior of the phospholipid
bi-layer
...
-Water molecules, despite being polar, diffuse directly through the bi-layer because
they are small enough to not be
repelled by hydrophobic tails
Facilitated diffusion:
Is the diffusion of a substance through
transport proteins in a cell membrane; the
proteins provide hydrophilic areas that allow
the molecules or ions to pass through the
membrane which otherwise would be less permeable to them
...
Channel proteins
2
...
●
Usually 'gated' - Part of the protein molecule on the inside surface of the membrane
can move to close or open the pore, like a gate
...
●
eg
...
●
Some channels occur in a single protein while others are formed by several proteins
combined
...
●
The molecules will move down the concentration gradient across the membrane like
in normal diffusion
...
●
In the case of carrier proteins, the rate of diffusion also depends on whether they are
open or not
...
●
Special type of diffusion involving only water molecules
...
●
●
Remember: Solute + Solvent = Solution
Two solutions separated by a partially permeable membrane - only allows certain
molecules through
Water potential:
●
Tendency of water to move out of a solution
...
A solution containing a lot of water (dilute) has a higher water potential than a
solution containing a little water (concentrated)
...
● Water potential of pure water at atmospheric pressure is 0
...
Solute potential and pressure potential:
●
Solute potential: Extent to which solute molecules decrease the water potential of
the solution
...
Adding more solute to a solution decreases it's water potential
...
Increasing pressure increases water potential
Osmosis in animal cells:
●
If the water potential surrounding the cell is too high - cell swells and bursts(lysis)
●
If it is too low - cell shrinks
●
Essential to maintain a constant water potential in the bodies of animals
●
Hypotonic: Water potential surrounding cells too high
●
Isotonic: Normal water potential surrounding cells
●
Hypertonic: Water potential surrounding cells too low
Osmosis in plant cells:
●
●
●
●
●
●
Unlike animal cells, plants have a cell wall
...
When water potential is higher outside than inside the cell, water will enterthe cell
...
This is the pressure potential and it increases the water potential until the water
potential inside and outside the cell are equal - equilibrium is reached
...
●
Cell wall prevents cell from bursting
...
●
Water potential = Solute potential + Pressure potential
●
Hypotonic: Fully turgid
●
●
Isotonic: No net movement of water
Hypertonic: Water leaves cell and protoplast gradually shrinks until it exerts no
pressure on the cell wall - pressure potential 0
...
This is called plasmolysis
...
When the pressure potential has just reached 0 and plasmolysis is
about to occur is called incipient plasmolysis
Active transport:
●
●
Movement of molecules or ions through carrier proteins across a cell membrane
against their concentration gradient using energy from ATP
...
●
●
●
●
●
Can occur either into or out of the cell
...
For each ATP molecule used it pumps 3 sodium ions out of the cell while at the same
time allowing 2 potassium ions into the cell
...
d) is created across membrane
...
The receptor acts as an
ATPase enzyme in bringing about the hydrolysis of ATP to ADP(adenosine
diphosphate) and phosphate to release energy
...
In
plants, active transport is used inorganic ions from soil to the root hairs and load
sugars into the phloem tissues
Bulk transport:
●
Bulk transport of large quantities of materials into (endocytosis) and out of
(exocytosis) of cells
...
Two forms:
-Phagocytosis (cell eating): Bulk uptake of solid material
...
Example: White blood cells engulfing bacteria
...
-Pinocytosis (cell drinking): Bulk uptake of liquid
...
● Protein receptors on the outer cell surface membrane detect the molecules that need
to be transported and binds to them
...
Usually
involves golgi body
...