# 2.7-1 size and surface area

```These organisms need to take in oxygen and expel waste gases.
What process is involved and how is each organism adapted?
2.7-1
SIZE AND SURFACE AREA
*
Key Words:
surface area to volume
ratio – diffusion – Fick’s
Law
Objectives:
• To describe the effect of increase in size on surface area
• To describe the factors influencing exchange between
organisms and their environment
• To explain the significance of surface area to volume
ratio in organisms
Objective One:
Effect of increase in size
on surface area
For each of the ‘organisms’ above work out the surface area,
volume and then surface area to volume ratio. Q1 on handout.
SA=6
Vol=1
SA/V=6
SA=24
Vol= 8
SA/Vol=3
SA=96
Vol=64
SA/Vol=1.5
What is the consequence of this? Q2 on handout
The larger an organism, the more exchange has to take place to
meet the organism’s needs (larger volume) – but each unit of its
volume is supplied by a proportionally decreasing surface area.
How can an organism increase in volume while still
managing to exchange enough nutrients by diffusion?
Answer questions 3 & 4 on handout.
D
SA= 34
V=8
SA:V= 4.25
E
SA=28
V= 8
SA:V=3.5
To obtain sufficient oxygen for the demands of the cells in the
body organisms need to greatly increase the surface area of the
gas exchange surface without significantly increasing volume.
Extension Question on handout:
Dessication/dehydration problems – surface also has a
protective function.
WHY DO WE BREATHE? (handouts)
respiration creates a constant demand to obtain
Cell .........................
oxygen for aerobic respiration and release .............................
carbon dioxide
..................
from aerobic respiration.
gases (oxygen and carbon
exchange is the transfer of ...............
Gas ....................
dioxide between the organism and the environment.
lungs across the cell
Gas exchange takes place in the ..............
membrane
................... by simple diffusion
...............
concentration gradient across their
Animals need to maintain a .........................................
ventilation
exchange surfaces. Mammals achieve this through ...............
circulation
and blood ........................
carbon dioxide - lungs – concentration gradient – ventilation gases - respiration – oxygen –– membrane – diffusion exchange – concentration gradient – circulation
Compare gas percentages in inspired and expired air.
Extension –
Why is there a difference between alveolar and expired air
percentages?
Objective Two:
Factors influencing gas exchange rate.
oxygen required for respiration is determined by:
The amount of ................
size
• the amount of living cells in the organism (i.e. ..........)
metabolic rate and
• the rate at which the cells are respiring
.............. (.............................
activity)
so the amount of oxygen required depends (partly) on the
volume of the organism.
................
The amount of oxygen
............ which can enter the organism by
diffusion is governed by:
surface area of the organism
• the ......................
efficiency of gas exchange depends on the
Thus the ...................
surface area:volume ratio.
– oxygen – volume – metabolic rate – surface area –
– efficiency – size – respiring
If temperature is constant – other than surface area, what other
factors may influence rate of diffusion of gases between the air
and the organism?
CONCENTRATION GRADIENT and DISTANCE
This can be expressed as Fick’s Law :
rate of diffusion
is proportional to:
surface area × difference in conc.
length of diffusion path
(membrane thickness)
WHY AM I SHOWING YOU A PICTURE OF A CAULIFLOWER?
Wall of
alveolus
alveol
bronchiol
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