These notes use information from two textbooks, the internet and extra information from my lessons, and cover each specification point for this topic. They are fully comprehensive and include diagrams etc. Helped me achieve an A* in Biology A level.
All living cells need to be able to take up substances to their environment
- Water, minerals, oxygen, glucose, fats, proteins
And excrete substances to their environment
- Carbon dioxide, oxygen, ammonia
Exchange surface: place that substances are moved across membranes in an organism
Single-celled organisms can exchange all the substances they need to via their outer surfaces as
diffusion alone is enough to supply their needs. Organism’s surface must be permeable to
oxygen + carbon dioxide, and moist (moist surface allows the gases to dissolve - can only diffuse
through membranes in solution, not in their gaseous form) - simple organisms usually live in
water so this isn’t a problem. Diffusion = slow but because distance between centre of a small
organism + the outside is small, diffusion is adequate - smaller organism → decr diffusion
distance
HOWEVER most (large) multicellular organisms require specialised exchange surfaces
because without them gases wouldn’t be exchanged fast enough or in large enough quantities
for multicellular organisms to survive. This is due to:
SA:V
● Single-celled organisms have a large SA:V
● Bigger organism → smaller SA:V
- TIB as organisms get larger, their SA incr but their vol incr by a greater factor
● + incr distances substances need to travel from outside to reach the cells at centre of
body - central cells (has large vol of cells) → can’t absorb enough oxygen through
available SA to meet the needs of the body
Metabolic activity
● Metabolic activity of single-celled organism is usually low → oxygen demands + carbon
dioxide production of cell are relatively low
, ● Bigger organism → incr metabolic activity → incr oxygen demand + carbon dioxide
production - demand for oxygen is higher than supply
- Mammals have high metabolic rate because they’re active + maintain their body
temp independent of the environment
Organisms that evolved to colonise land developed waterproof surface → avoid excess water loss.
These surfaces are impermeable to oxygen + carbon dioxide
The features of an efficient exchange surface
All effective exchange surfaces have certain features in common:
1. Incr SA - provides area needed for exchange, overcomes limitations of larger organism’s
SA:V eg. RHC in plants + villi in small intestine
2. Thin layer - short diffusion distance → fast + efficient eg. alveoli in lungs + villi in small
intestine
3. Gd blood supply - ensures substances are constantly delivered tp + removed from
exchange surface → maintains steep conc gradient → incr rate of diffusion (more efficient)
eg. alveoli in lungs, fish gills + villi in small intestine
4. Ventilation - maintains steep conc gradient for gases → incr rate of diffusion (more
efficient) eg. alveoli in lungs + gills of fish (where ventilation means flow of water
carrying dissolved gases)
The structures and functions of the components of the mammalian gaseous exchange
system
● Animals that live on land face constant conflict between need for gaseous exchange and
need for water
- Gaseous exchange surfaces = moist → oxygen dissolves into water → then water
diffuses into body tissues
- TMT conditions needed to successfully take in oxygen = also ideal for water evap
● Mammals have evolved complex systems - allow them to exchange gases efficiently but
minimise amount of water lost from body
, Human gaseous exchange system:
(Looking at this system as example of specialised systems common to all mammals)
Tissues of the gaseous exchange system
Cartilage
- Form of connective tissue that provides strengthening + support
- Composed of cells surrounded by mucopolysaccharides (cont amino groups)
- Not as armstrong or rigid as bone but resistant to tension + compression - more flexible
- Larger tubes in gas exchange system have a relatively large diameter + thin walls →
cartilage keeps trachea, bronchi + larger bronchioles open - would collapse without this
strengthening
Ciliated epithelium
- Distinguished by presence of cilia
Goblet cells
Smooth muscle
- Maintains the tone in the airways + allows expansion in conditions where extra oxygen
needed eg. during exercise - found in the trachea, bronchi + large bronchioles
Elastic fibres
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