This document contains summarised notes for Topic 4 Exchange and transport, taken using the Pearson Edexcel B biology activate textbook. Notes taken with referencing to specification.
THE NEED FOR TRANSPORT
Cells require a supply of chemicals (glucose and O2) for cellular respiration
Respiration supplies energy for the other reactions and produces waste product, CO 2
Waste products need to be removed before they cause damage
Cell transport of raw materials cross the surface cell membrane
FLUID MOSAIC MODEL
Cell surface membrane acts as the gatekeeper, controls transport of materials in and
out and within the cell
ER and Golgi apparatus are membrane-bound structures that move substances
which are inside the cell
THE MAIN TYPES OF TRANSPORT IN
CELLS
PASSIVE TRANSPORT MECHANISMS ACTIVE TRANSPORT MECHANISMS
Diffusion Endocytosis
Facilitated diffusion Exocytosis
Osmosis Active transport
DIFFUSION – movement of the molecules of a liquid or gas from an area where there’s a
high conc to an area of low conc
Moves down the conc gradient
Occurs because of the random motion of molecules due to the energy they have,
which is dependent on temp
Spread out evenly until they reach uniform distribution, equal numbers move in all
directions
O2 and CO2 diffuse freely across
Larger hydrophobic molecules and ions cannot move across
,FACILITATED DIFFUSION – is diffusion that takes place through carrier proteins or protein
channels
Substances with strong positive or negative charge and large molecules can’t pass
through by simple diffusion so this is used
Specialised form of diffusion
Proteins only allow specific
substances to move through
passively
Channel proteins form pores
Each type of channel protein
allows one particular type of
molecule or ion through,
dependent on its shape and
charge
Protein carriers are specific for particular molecules
Once a carrier has picked up a molecule it changes shape, moves the molecule
through membrane within the carrier, and then releases the molecule
Doesn’t use energy
OSMOSIS – a specialised form of diffusion, involves movement of
solvent molecules (water) down a conc gradient through a partially
permeable membrane
OSMOTIC CONC – measure of conc of solutes in a solution that
have an osmotic effect
ISOTONIC SOLUTION – osmotic conc of solutes in solution is the
same as that in cells
HYPOTONIC SOLUTION – osmotic conc of the solutes is lower
than that in cytoplasm of the cells. (water moves in)
PLANTS
Cytoplasm swells and presses on cell wall, generates
hydrostatic pressure
Inward pressure of cell wall increases until it cancels out tendency for water to move
in
Cell wall prevents cell bursting
Reaches turgor state when pressures balance (TURGID)
HYPERTONIC SOLUTION – osmotic conc of the solutes in the solution is higher than that in
the cytoplasm (water moves out)
PLANTS
Turgor is lost
Cell membranes pull away from cell wall, protoplasm shrinks (incipient plasmolysis)
Vacuole reduces (PLASMOLYSIS)
ANIMALS
When too much water moves in, cell bursts (HYPO)
When too much water moves out, cell shrivels as the concentrated cytoplasm loses
its internal structure. (HYPER)
, WATER POTENTIAL – measure of potential for water to move out by osmosis (max=0)
TURGOR PRESSURE – measure of the inward pressure exerted by the cell wall on the
protoplasm as the cell contents expand and press outwards, force which opposes entry of
water by osmosis (usually +)
OSMOTIC POTENTIAL – measure of the potential of a solution to cause water to move in
across a partially permeable membrane as a result of dissolved solutes
ACTIVE TRANSPORT
Involves a carrier protein
May be specific, picking up only one type of ion or molecule or may work for several
similar substances that have to compete for a place on the carrier
Requires energy, provided by molecules of ATP
Cells that carry out lots of AT have lots of mitochondria to supply ATP needed
Carrier system involves enzyme ATPase, it catalyses the hydrolysis of ATP, breaking 1
bond and forming 2 more to provide ATP needed by system or release transported
substances
One-way system
May move out through open
channels
Substances move in faster than
out by diffusion
SODIUM POTASSIUM PUMP – actively
moves K ions in and Na ions out
Vital for working of nervous
system, each nerve impulse
depends on an influx of Na ions
through axon membrane
Ions have to be actively pumped out of neurone again so another impulse can pass
ENDOCYTOSIS – the movement of large molecules into cells through vesicle formation
Can occur during ingestion of bacteria during phagocytosis
EXOCYTOSIS – the movement of large molecules out of cells by the fusing of a vesicle
containing molecules with the surface cell membrane
Requires ATP
GAS EXCHANGE
THE NEED FOR GAS EXCHANGE SURFACES
Organisms respire – for aerobic respiration they need O2 and produce waste CO2
Exchange of gases happens by diffusion, high to low, down conc gradient
GE IN SMALL ORGANISMS
Nutrients and O2 they need can diffuse directly into cell and waste substances out
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