7. Principles of Exchange and Transport

examples of essential substances

Oxygen, glucose (respiration), plants need nitrates to make protein for growth

toxic products / waste

Carbon dioxide, urea

Where does this exchange take place in unicellular organisms?

Through the cell surface membrane

What do multicellular organisms have to facilitate this exchange?

Specialised exchange surfaces e.g. alveoli

What is meant by the surface area of an organism?

Total number of cells in direct contact with the environment

What is the volume?

3D space occupied by metabolically active tissues

What happens to this ratio as the cube gets larger?

Increasing size = decreases S.A:V

Small organisms can gain their metabolites directly across their surface due to large SA:V

Larger organisms require specialised exchange

surfaces to maximise efficiency due to small S.A:V

1. Increasing the surface area of the exchange surface

They have a large surface area and very small

volume = large S.A:V

Respiratory substances can diffuse through the body surface in sufficient quantities to meet their metabolic needs.

When are specialised exchange surfaces needed? What effect does this have on the surface area?

When the surface area to volume ratio is not large enough to enable enough respiratory gases to enter or leave

Specialised exchange surfaces significantly increase the surface area for exchange

example of external strucutre

folded external membranes of external gills of tadpoles = increase S.A for gases to diffuse

example Internal: structure

fish gills and alveoli of mammalian lungs increase surface area.

Thin separating surface

Exchange surfaces that are thin increase the rate of diffusion

Large concentration gradient diffusion occur

When there is a concentration gradient

How is this gradient created in respiring organisms?

Oxygen being used by respiring cells creates low levels of oxygen which enables oxygen to move into the lungs from the environment`

What additional feature do larger mammals require?

Ventilation system – to bring oxygen in and push carbon dioxide out.

What is the importance of mass flow?

Transport of substances within large organisms

Explain how mass flow works.

Brought about by pressure differences. Material moved from where the

pressure is generated (source) to where there is less pressure (sink)

To maximise the rate of exchange, gas exchange surfaces have:

 Large surface area

 Moist surface

 Diffusion gradient

 Permeable to oxygen and carbon dioxide

 Leaves are thin – short diffusion distance

flicks law

Where does photosynthesis take place in a leaf?

Chloroplasts in the palisade mesophyll layer

What does photosynthesis require and what does it produce?

Uses carbon dioxide and produces oxygen

explain graph

Y – during these hours there is no light therefore no photosynthesis. Only respiration occurs = more carbon dioxide produced than used

X – more/bright light = Rate of photosynthesis > rate of respiration. More carbon dioxide used than produced

Compensation points = rate of photosynthesis = rate of respiration

No net gain or loss of carbon dioxide

Leaves are thin facilitates gas exchange

Short diffusion distance – higher S.A:V

All cells close to an exchange surface

Large surface area and moist gas exchange surface of SML facilitates gas exchange

Loose arrangement of the cells = large surface area

Gases involved in respiration and photosynthesis can easily diffuse between cells of SML and PML.

Intercellular air spaces of the SML facilitates gas exchange

Facilitate diffusion of gases within the leaf

Stomata (singular = stoma) facilitates gas exchange

Allow gases to enter or leave. Open during the day and close at night. Closing at night prevents water loss.

Controlled by guard cells – when turgid (guard cells) =stomata open, flaccid = stomata closed.

Very short diffusion distance in mammals

Squamous epithelial cells lining the alveoli = very thin

Alveoli surrounded by blood capillaries - which are one squamous endothelial cell thick

Capillaries are narrow which means RBC have the squeeze through and are in contact with the endothelial wall = further reduces diffusion distance.

Large surface area in mammals

Millions of saccular shaped alveoli and a large network of capillaries = increase S.A

A steep concentration gradient

Created by ventilation of the lungs (bringing oxygen in and releasing Carbon dioxide out). Flow of blood through capillaries maintains steep conc gradient as it is low in oxygen but rich in carbon dioxide when arriving

Surfactant is secreted in the alveolar wall.

What is its role?

Reduces surface tension in the moisture coating of alveoli – prevents their collapse (prevents the alveoli sticking together) which would reduce S.A.

What role do macrophages play?

Protect against infection by digesting microbes

through phagocytosis.

Inspiration (or inhalation) -

intercoastal muscles contact, ribs pulled up and out. Diaphragm contracts and move down. Volume increases, pressure decreases. This creates a difference in pressure between lungs and atmosphere – air enters the lungs (mass flow)

Expiration (exhalation) -

Intercostal muscles relax, ribs move down and in. Diaphragm relaxes and moves up. Volume decreases, pressure increases. Thorax has a higher pressure that the atmosphere, air forced out of lungs.

smoking causes cancer

Tar is carcinogenic (causes cancer). Can cause damage to DNA and cause cancer. Cancer cells divide to form a tumour which can block the airways and/or damage large sections of the lungs. This can reduce surface area.

Emphysema

Break down of the alveolar wall. Less alveoli which reduces the surface area = less diffusion of gases.

Cigarette smoke breaks down the elastic lining of the alveoli reducing their ability to stretch and recoil. If recoil is affected exhalation may not be efficient – residual air remains in the alveoli – less oxygen can enter.

Shortness of breath, tired due to less respiration.

Bronchitis

Inflammation of the bronchial tubes. Narrower bronchial tubes and mucus production = reduce air flow into and out of the lungs (harder to maintain a steep concentration gradient)

What does tar do to the cilia?

Shortens cilia. Paralyses them which makes it harder to remove mucus and microbes from respiratory tract – increasing risk of infection.