CHAPTER 8
SELECTIVITY
C4:- The Intracellular environment of
cells differs in composition from the
extracellular environment of cells
C4.1 Know that the proportions of chemicals in the
intracellular environment of cells are different from
those in the extracellular environment of cells
• Matter is any substance that makes up living
and non living things-solid, liquid or gas.
• Elements are the units of matter that cannot
be converted into simpler substances-made of
one type of atom
• In nature matter is arranged in molecules-one
or two types of atoms i.e. water
• Also includes compounds-atoms of two
elements combined.
Organisms• Made of mostly water
• Most common atoms in the body-hydrogen and
oxygen
• Carbon and hydrogen and oxygen-very important
as account for most of the living matter on Earth
• Carbon important as forms strong covalent bonds
by sharing electrons with other atoms-especially
each other and to hydrogen and oxygen.
• Remember these form the major groups of
macromolecules-lipids, proteins, carbs, nucleic
acids.
Organic Molecules
• Remember! Organisms built from 35 smaller
units-an arrangement of 20 amino acids,
lipids, phospholipids, sugars, bases and
phosphate groups that make up nucleic acids.
• Living organisms-living cells-large amount of
chemicals continually reacting with each other
• There are differences between the
proportions of the elements on earth and in
your body
MOLECULES- manufactured by cells
are called organic
• See pg 71 table
• These molecules constantly involved in reactions
• Making new molecules –transforming energy
• To proceed efficiently must control
amount of chemicals used
I.E respiration needs just the right amount of
glucose and oxygen-and waste products must be
removed to maintain the correct intracellular
environment.
• The environment is maintained by the cellular
membrane-outside the cell varies greater than
inside the cell
C4.2 Understand why the internal
composition of the cell is regulated
• For cellular metabolic processes to function
correctly the internal environment must remain
stable
• Enzymes in the cell convert one or more
substrates to other substances constantly
• Remember it’s the 3 dimensional shape of
proteins/enzymes that assist in this reaction
control-and the factors which influence the shape
of these molecules-temp/pH/Concentration etc
All internal conditions must be regulated
for reactions to proceed efficiently
• Efficiency depends on the relative concentrations
of substrates and products
• i.e if a cell runs out of oxygen- respiration slows
down
• This may then mean conditions are suitable for
FERMENTATION• Similarly if waste products like Urea or CO2
accumulate the reaction slows down
• It is the cell membrane which controls the ins and
outs in order to ensure all reactions proceed
correctly and efficiently- ensures enzymes can
operate effectively under the right conditions
C4.3 Explain how selectivity exchange
occurs at the cell membrane
• Selective exchange is when the membrane restricts
some substances and freely allows others in or out=
differential permeability -Or SEMI PERMEABLE
• Some molecules pass by diffusion- some require
energy = active transport-e.g mineral ions like
potassium and sodium- charged molecules have more
difficulty passing through
• Some facilitated diffusion-no energy
• proteins in the cell membrane – allow the passagesometimes by having receptors that recognise the
molecule , cause structural change in the protein
membranes that facilitates its passage across.
• Eg, insulin see pg 73
C5 Movement of soluble materials across
membranes may be passive or require the
expenditure of energy
• C5.1 Understand that the movement of
substances by diffusion and osmosis is passive
• Unless molecules at -273 degrees Celsius
molecules are in constant motion
• Kinetic energy of these particles is also heat
energy
• The amount depends on the temp of the
substance and its mass
• Gas and liquid- freer particles- more collisionsoccurs until the particles fill a space evenlypassive- no energy required
Diffusion
• Movement of particles from a region of high
concentration to low concentration with the
concentration gradient
• The greater the difference in concentrationsteeper the gradient-faster the diffusion
process
• Small molecules diffuse quicker than large
• Water, oxygen and carbon dioxide diffuse
easily
• No energy required
Diffusion
• Simple diagram-copy if you must!
Osmosis
• Diffusion of water- movement of water molecules
form a region of high concentration to low
concentration or defined as movement of water
from a region of low solute concentration to high
solute concentration to reach equilibrium.
• Osmosis has a more dramatic effect on animal
cells than plant cells due to animal cells having
NO cell wall and therefore can burst or shrivel
easily!
OSMOSIS
C5.2 Understand that the active transport of
substances against the concentration gradient
requires energy. This energy is supplied by ATP.
• Active transport is movement of solutes against
the gradient from high to low
• Transport proteins in the cell membrane actively
pump specific substances across the membrane
moving them against the gradient with energy
supplied
• Energy is supplied by ATP-Adenosine TriPhosphate
• Nerve cells provide a good example:– Transmission of nerve signals requires the exchange of
sodium and potassium ions across the membranes by
diffusion but they become fatigued
• They need to be re-charged by pumping them
against the gradient back to the other side of
the membrane
– This is why you need regular study breaks- to
pump the ions back!