A Level Biology
SURFACE AREA AND VOLUME OCR A Module 3
Surface area : volume ratio
• Smaller organisms have a larger surface area : volume (SA:V) ratio ratio Surface area
-
volume
• As size increases, SA:V ratio decreases
3. 5cm
surface area
surface area
2cm S 7
< > 2cm
=(2x2) x6 2m2
=
S 3. 5cm =
(3.5 x
3.5) x6 7m2=
S
C
.W
2cm volume - 1
-
volume
m
.
E 3.5 x 3.5 3.5
=
=
= x
2 x 2x2
WS
8. ...
-
=
3. 5cm
*
...,
...
3
SA:V 73.5:42.9
=
=
SA:V 24:8
= =
-
Exchange and transport
• Organisms need to exchange substances with the environment In plants, carbon
→ waste products (e.g. carbon dioxide, urea) need to be excreted dioxide is needed for
→ oxygen and glucose are needed for aerobic respiration photosynthesis.
→ heat is also exchanged
• Unicellular organisms → can rely on diffusion to exchange substances
→ short diffusion pathway from cell surface to centre of cell
→ larger SA:V ratio
• Multicellular organisms → often need exchange and transport systems
→ long diffusion pathway from body surface to centre of body so diffusion
would take much too long to deliver oxygen
→ smaller SA:V ratio so cannot exchange enough substances through surface
• Very active organisms have a higher metabolic rate
→ metabolic rate = the amount of energy used up by an organism in a given amount of time
→ active organisms use up more ATP in a given amount of time so must respire quickly
→ need more oxygen per unit of body mass
→ need to deliver oxygen and glucose rapidly and constantly to tissues via a transport system
I"race
• Trachea → windpipe from mouth
• Bronchi → branch off from trachea, one bronchus to each lung
&
*
bronchi↑f=
IeO li
E
• Bronchioles → branch off from bronchi, smaller tubes ↓
• Alveoli → tiny air sacs at the end of bronchioles where gas
.* IW
↳
~
exchange occurs
4
W
• Surrounded by the ribcage and intercostal muscles W so sign
Elastic fibres mean the lungs stretch and
bronchioles
recoil during inhalation and exhalation.
Smooth muscle can relax during exercise to Trachea C-shaped cartilage pieces, smooth
decrease resistance and increase air flow. muscle, elastic fibres, ciliated
epithelium with goblet cells
Bronchioles Elastic fibres, non-ciliated epithelium
Goblet cells secrete mucus into the (smaller)
trachea which traps microorganisms and
dust. The cilia on the columnar epithelial Alveoli Elastic fibres, non-ciliated epithelium
cells move the mucus towards mouth.
Gas exchange in the alveoli
• Oxygen diffuses from the air space into the blood across
the alveolar epithelium then the capillary endothelium
(carbon dioxide diffuses in the opposite direction)
• Alveoli are adapted for efficient gas exchange
→ have a large surface area (lots of spheres) &8 D
→ alveolar epithelium is one cell thick so there is a short capillary 0: ⑧
I
-
alveolar
epithelium
diffusion pathway endothelium
8 i
ar
space
8
"↓ ↑0
....
oooooo
→ alveolar epithelium is squamous epithelium so cells are & 02 CO2
flattened which gives a short diffusion pathway & & - 0
D
⑧
&
C&-
0
→ elastic fibres allow stretch and recoil to help force air ⑧ E! O
0
out and increase surface area blood
-
→ layer of surfactant reduces cohesion between water
molecules which prevents the alveoli collapsing
• Oxygen and carbon dioxide concentration gradients are
maintained by ventilation and blood flow
• Good blood supply means blood is always close to the alveoli
Ventilation
• Inspiration = breathing in, expiration = breathing out
Inspiration (inhalation) Expiration (exhalation)
↓ thoracic I ↑
caviy
I
* -B /a
⑧ !
↳
where
-space lungs ↳
ODi
-
!diaphragm
are contained
Il
iP ribs and -
0
⑧
↑
0
flattened 11 Il ⑧
external intercostal 11
O O curved
diaphragm IlIl muscles
O - Il
-0
- -
• Active process (requires energy) • Passive process
• External intercostal muscles and diaphragm • External intercostal muscles and diaphragm relax
contract • Ribcage moves down and in
• Ribcage moves up and out • Volume of thoracic cavity decreases
• Volume of thoracic cavity increases • Pressure in thoracic cavity increases above
• Pressure in thoracic cavity decreases below atmospheric pressure
atmospheric pressure • Air moves down a pressure gradient out of the
• Air moves down a pressure gradient into the lungs lungs
Forced expiration is active
because it requires contraction of
the internal intercostal muscles.
Measurements of breathing
• Spirometer → used to take measurements involving volume of air in the lungs
• Tidal volume → volume of air in a normal breath at rest
• Breathing rate → number of breaths per minute
• Vital capacity → maximum volume of air that can be breathed in (or out)
• Oxygen consumption → rate at which an organism uses oxygen (normally in dm3/min)
5 5
oxygen consumption
em L
=volume decrease I
To mrtn
over min
e
4
unfurn
⑪
es -
tidal
-
volume
tidal
volume
vital
I vital - I
capacity
capacity -residual volume 0
- I S
⑧ 0
I 2
0 10 20 30 40 50 60
time (mins)
time(s)
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