L: 1.4 Membrane transport
 Water, oxygen, carbon dioxide, excretory products, nutrients, ions are continuously
passing across plasma membrane, many cells also secrete products such as
hormones, enzymes.
 Concentration gradient – is a difference in concentration of substance between two
regions. Diffusion will always take place where such a gradient exists, until
equilibrium is reached, so particles are evenly distributed.
 OSMOLARITY – concentration of solutes in a solution etc. (ex: water – 0 osmolarity as
it has nothing in it)
 WATER POTENTIAL – tendency of water molecules to move from an area of high
solute concentrations to an area of lower solute concentrations
 NORMAL SALINE – solution of 0.90% w/v (weight by volume) of SODIUM CHLORIDE
and is isotonic with human cells. Used frequently in intravenous drips when patient
cannot take liquids orally and is in danger of dehydration. (???)
PASSIVE TRANSPORT
 SIMPLE DIFFUSION – the simplest way in which molecules can get in or out of the cell
through plasma membrane. Diffusion is a PASSIVE process, which takes place AS
MOLECULES MOVE RANDOMLY. NO ENERGY INPUT IS REQUIRED, movement occurs
by way of a SIMPLE CONCENTRATION GRADIENT.
Simple diffusion occurs where the membrane is fully permeable to the
substance/channel proteins in the membrane are large enough for the substance to
pass through
(E.g. cell respiration)
 CHANNEL PROTEINS (not a way of transport!!!) – large molecules and charged
particles such as 𝐶𝑙 − (chloride ions) and 𝐾 + (potassium ions) pass through certain
proteins that form channels for them. NO ENERGY INPUT IS REQUIRED. Transport
relies on the kinetic energy of the particles moving down their concentration
gradient.
Channel proteins have HYDROPHILIC INTERIOR, which means that water-soluble
substances can pass through them. They are also SPECIFIC – only allow particular
substances to pass through.
Some of those channels are PERMAMENTLY OPEN, and some are GATED – only open
when they are stimulated to do so and to allow certain ions to pass through.
 FACILITATED DIFFUSION – allow transport of substances which are polar and cannot
diffuse through the lipid layer of the membrane such as glucose and amino acids
CARRIER PROTEIN first combines with the diffusing molecules on one side of the
membrane, carries them through the CHANNEL PROTEIN (helps to facilitate the
process) and then releases them on the other side.
It allows faster diffusion rate for molecules that particular cell need.
NO ENERGY INPUT IS REQUIRED AS MOLECULES MOVE DOWN THEIR
CONCENTRATION GRADIENT.
 OSMOSIS – special case of diffusion, which is a passive movement of water across a
partially permeable membrane from an area of a LOW SOLUTE CONCENTRATION to
an area OF HIGH SOLUTE CONCENTRATION (to a ‘saltier’ place)
- When solute concentrations inside and outside of a cell are the same, the
number of water molecules which will leave the cell is the same as those
entering it (ISOTONIC SOLUTION)
- Animal cell placed in pure water will take in water by osmosis until it eventually
may burst (HYPOTONIC SOLUTION)
- Cell (doesn’t matter if animal/plant) placed in a solution with high concentration
of solutes the cell will shrink or ‘crenate’ as water leaves by osmosis.
(HYPERTONIC SOLUTION)
In both situations above the cell will NOT FUNCTION PROPERLY, its metabolism will
be affected
-
In medical procedures human tissues/organs are bathed in a solution of
‘NORMAL SALINE’ which has the same OSMOLARITY as human cell cytoplasm
(ISOTONIC WITH THE HUMAN CELL CYTOPLASM), so the osmosis does not occur,
and cells are not damaged.
-
Plant cell wall protects plants from being damaged, when plant cell is placed in
water it will take it in, but plant cell wall will RESIST THE ENTRY OF FURTHER
WATER ONCE THE CELL IS FULL – Plant cell that is full becomes rigid and firm –
condition known as TURGOR
ACTIVE TRANSPORT
Many substances a cell needs occur in low concentrations in the surroundings outside the
plasma membrane
To move this substances into the cell against the concentration gradient the cell must use
metabolic energy released from breakdown of ATP to ADP and 𝑷𝒊 , this is what we call an
active transport
 THE SODIUM-POTASSIUM PUMP – maintains concentration of sodium and
potassium ions inside the cell and in extracellular space.
Cells are able to exchange sodium ions for potassium ions against the concentration
gradient using energy released from the breakdown of ATP.
Sodium ions are pumped out of the cell (Sodium – Na-na zewnątrz) and Potassium
ions are pumped into the cell
Sometimes cell need to transport large molecules or materials in bulk across the
plasma membrane, cell can release or take in such a materials in vesicles.
FLEXIBILITY and FLUIDITY of the plasma membrane are essential to enable both
endo and exocytosis.
 ENDOCYTOSIS – Uptake of materials into the vesicles. REQUIRES ENERGY FROM
ATP.
Part of the plasma membrane is pulled inward and surrounds the liquid/solid that is
to be moved from the extracellular space into the cell. The material becomes
enclosed in a vesicle and, which pinches off from the plasma membrane and is
drawn into the cell.
 PHAGOCYTOSIS – substance taken in are particles (e.g. bacteria)
 PINOCYTOSIS – substance taken in is in a solution (e.g. end product of digestion)
 EXOCYTOSIS – Export of materials. REQUIRES ENERGY FROM ATP.
Materials for export are made in the rER and then transported to the Golgi
apparatus to be processed – here they are enclosed in membrane-bound vesicles
and moved to the plasma membrane along microtubules.
Arrangement of molecules in membrane of vesicle is very similar to the one in
plasma membrane, so as the vesicle approaches the plasma membrane it enables it
to fuse with it, and as it is doing so to release contents to the extracellular space.
Nerve impulses are able to pass across synapses due to the EXO and ENDOCYTOSIS.
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VESICLES – help also to transfer and organize substances inside the cell, involved
in metabolism, transport and enzyme storage, some chemical reactions also
occur inside them
-
ATP-> ADP + 𝑷𝒊
-
ATP –adenosine triphosphate/ ADP – adenosine diphosphate/ 𝑃𝑖 – inorganic P
Facilitate – ułatwiać/umożliwiać
Expenditure – wydatkowanie/rozchód (eks pen di cza)
Normal saline – roztwór izotoniczy soli kuchennej
Intravenous – dożylny (intczra wi nys)
Intravenous drip – kroplówka dożylna
Dilute – rozcieńczony
Plasmolysed - Shrinking of plant cells by osmotic loss of cytoplasmic water
In bulk – in large quantities – hurtowo
Uptake – pobranie
Export – wywozić coś/wyrzucić