Module 3, section 1: Exchange and Transport

Why do organisms need to exchange substances with their environment?

Obtain nutrients needed for biological processes, and to remove waste products.

How is substance exchange different in unicellular organisms?

Substances can directly diffuse in/out across the cell membrane, due to high SA:V ratio.

Why can’t simple diffusion be used for substance exchange in multicellular organisms?

1. Cells are deep within the body: diffusion pathway too long to be effective.

2. Low SA:V ratio: too many substances needed, not enough space to receive them.

3. Higher metabolic rate: higher demand for metabolites, so quicker solution needed.

Define diffusion.

Net movement of particles down a concentration gradient from an area of higher concentration to an area of lower concentration.

How does a multicellular organism exchange substances then?

Via specialised exchange surfaces.

What are some important features of specialised exchange surfaces?

1. Large surface area: Increases the efficiency of absorption, i.e lots can happen at the same time.

2. Thin: Reduces diffusion pathway length. Means that substances have to travel less distance.

3. Good blood supply/ventilation: Ensures that concentration gradients are maintained.

How are alveoli a good example of effective exchange surfaces?

There are millions of them in each lung. This ensures a very large surface area, this increasing efficiency. Alveoli are very thin, only about one cell thick. This massively reduces the diffusion pathway, ensuring breathed in oxygen can quickly move into the required blood vessel. Each alveolus is surrounded by a capillary bed, ensuring high blood supply, meaning concentration gradients are achieved and maintained.

How does air enter the alveoli in mammal lungs?

Air enters the trachea, which splits into the bronchi. The bronchi then splits into several bronchioles. Further along, the bronchioles split into alveoli, the main site of gas exchange.

Goblet cell function?

Secrete mucus that trap pathogens and dust particles, preventing them from reaching the alveoli.

Cilia function?

Hair like structures on the surface of the airway linings. They waft the mucus away from the alveoli into the throat where its swallowed and destroyed in the stomach. Reduces the occurrences of lung infections.

Elastic fibre function?

Cover the walls of trachea, bronchi, bronchioles and alveoli. Stretch during inspiration, and relax during exhalation.

Function of smooth muscle?

Widen tubes during exercise, making tubes wider, with less resistance to airflow.

Function of cartilage rings?

Maintain structure in the trachea. Prevent airway from collapsing when air pressure drops.

Describe the process of inspiration.

Intercostal muscles and diaphragm contract. Causes diaphragm to flatten, increasing volume of the thorax. This means pressure of the lungs is less than atmospheric pressure, causing air to move into the lungs.

Why is inspiration an active process?

Because the intercostal muscles and diaphragm are contracting, which requires energy.

Describe the process of expiration.

Diaphragm and intercostal muscles relax, decreasing volume of the thorax and increasing internal pressure. This forces air out of the lungs.

Conversely, why is expiration a passive process?

Because the diaphragm and intercostal muscles relax, thus increasing internal pressure.

What does a spirometer do?

Investigates breathing.

Why does the volume of gas in a spirometer decrease?

All gases are kept within the system, meaning that oxygen used for respiration is being recycled. Due to lack of replenishment, amount of gas in the machine decreases as the patient carries on breathing.

What is the purpose of soda-lime in the spirometer?

Absorbs carbon dioxide formed by the person respiring.

What is tidal volume?

Volume of air in each breath. (around 0.4dm³)

What’s vital capacity?

Max. volume of air that can be breathed in/out by the subject.

Oxygen uptake?

The volume of oxygen used up by the subject. (measured in dm³/min)

How does oxygen enter the fish’s system?

Water enters via the mouth, and passes out of the gills.

What are gills made of?

Gill filaments. Means high surface area. These are covered in secondary lamellae, further increasing surface area. (Think bronchioles and alveoli)

What is a gill arch?

Supports the gill filaments, and contains arteries and vessels.

Describe the counter-current system.

High oxygen water always passes next to low oxygen blood, meaning there is a net movement of oxygen from water to blood, due to a steep concentration gradient being formed.

Describe ventilation in fish.

Fish opens its mouth and the floor of the buccal cavity (area in mouth) drops. Water is then sucked in, and the buccal cavity is raised again. Due to pressure increase, water is forced through the gills, and leaves via the operculum.

What’s the structure and function of the operculum?

Bony flap located outside the gill of the fish, which protects the gill filaments and allows water to exit when oxygen has been absorbed.

What mechanism do insects use for gas exchange?

Tracheae, microscopic holes that cover the insect’s body. Moves in via spiracles, which are pores on the surface.

What do tracheae split into?

They split into tracheoles, which have thin permeable walls and go into individual cells. They contain fluid that O2 diffuses into.