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Topic 4 summary notes (A Level Biology Edexcel B) CA$10.04   Add to cart
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Summary

Topic 4 summary notes (A Level Biology Edexcel B)

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  • Course
  • AS Level Biology Edexcel B
  • Institution
  • PEARSON (PEARSON)
  • Book
  • Edexcel A level Biology B Student Book 2 ActiveBook

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.

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Document information

  • No
  • Topic 4
  • May 22, 2021
  • 16
  • 2020/2021
  • Summary

Subjects

  • biology
  • exchange and transport
  • edexcel b
  • a level
  • topic 4
  • cell transport mechanisms
  • gas exchange
  • circulation
  • transport in plants

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  • PEARSON (PEARSON)
  • Biology Edexcel B
  • AS Level Biology Edexcel B
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CELL TRANSPORT AND GAS EXCHANGE

TRANSPORT IN CELLS

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

WATER POTENTIAL () = TURGOR PRESSURE (P) + OSMOTIC POTENTIAL ()
(usually negative) (usually positive) (always negative)
(Water Tastes Outstanding)

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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