Marine Science (9693) | Gas exchange

🌊 A-Level Marine Science 9693 Study Notes: Gas Exchange (Topic 6.3)

Welcome to the exciting world of marine physiology! This chapter looks at one of the most fundamental processes for life: **Gas Exchange**. Whether you are a gigantic blue whale or a tiny planktonic copepod, every cell in your body needs oxygen to perform aerobic respiration, and needs to get rid of carbon dioxide waste.
In the ocean, getting oxygen is tricky because its solubility is low and varies with temperature and depth. Therefore, marine organisms need clever adaptations to efficiently capture this vital gas.

6.3.1 The Purpose of Gas Exchange

Gas exchange is simply the movement of gases across a surface. In living organisms, this process supports **aerobic respiration**.

Aerobic respiration is summarized as:
\(Glucose + Oxygen \rightarrow Carbon\ Dioxide + Water + Energy\ (ATP)\)

• Raw material needed: Oxygen (\(O_{2}\)). This must move into the organism from the environment.
• Waste product: Carbon Dioxide (\(CO_{2}\)). This must move out of the organism into the environment.

This movement happens via diffusion, moving gases down their concentration gradient (from high concentration to low concentration).

Quick Review: Diffusion is passive—it requires no energy from the organism.

6.3.2 The Challenge of Size: Surface Area to Volume Ratio (SA:V)

Why can’t all marine organisms just absorb oxygen across their skin? The answer lies in the relationship between size and surface area.

Understanding SA:V Ratio

The **Surface Area to Volume Ratio (SA:V)** compares the amount of outer surface available for diffusion (SA) to the internal space (Volume) that needs the gas.

• As an organism gets bigger, its volume increases much faster than its surface area.
• Small organisms have a **high SA:V ratio**. (Lots of surface area relative to their internal volume).
• Large organisms have a **low SA:V ratio**. (Not enough surface area to supply the massive internal volume).

Analogy: Think of a house. A small hut has lots of wall area compared to the small volume inside (high SA:V). A giant skyscraper has a huge internal volume, and the wall surface area simply isn't large enough to let enough heat or air diffuse efficiently into the centre (low SA:V).

SA:V and Specialisation (6.3.2)

If an organism has a low SA:V (i.e., it’s large), simple diffusion across its outer surface is far too slow to supply the core cells with enough oxygen. The diffusion distance is too long.

Therefore, large, active animals require two major adaptations:

1. Specialised Gaseous Exchange Surfaces: Structures like gills, which are highly folded and provide a massive surface area specifically for gas exchange.
2. Transport Systems: Systems (like blood/circulatory system) that rapidly move the captured oxygen from the specialised surface (gills) to all the deep-lying cells that need it.

Key Takeaway: High SA:V = Simple diffusion works. Low SA:V = Need for gills and a transport system.

6.3.3 Gaseous Exchange by Simple Diffusion

The simplest form of gas exchange relies on diffusion across the entire body surface. This is only possible for organisms that meet certain criteria related to their size and metabolic needs.

Example: Coral Polyps

Coral polyps are an excellent example of a marine organism that relies solely on **simple diffusion** (6.3.3).

• They are typically small, thin, and possess a relatively **high SA:V ratio**.
• They are mostly sessile (non-moving), meaning they have a relatively low metabolic rate and thus a low oxygen demand.
• The distance oxygen needs to diffuse is very short, allowing diffusion across the polyp's outer membrane to be efficient enough to meet their needs.

6.3.3 & 6.3.4 Specialized Ventilation Methods in Fish

Fish, being highly mobile and large (low SA:V), need vast, constantly renewed exchange surfaces. Gills provide the surface area, but they must be continuously ventilated (a process where water is moved over the gills).

1. Pumped Ventilation (Buccal Pumping)

This method involves actively using muscles in the mouth (**buccal cavity**) and the **operculum** (gill cover) to create a continuous, one-way flow of water over the gills.

Step-by-Step Mechanism

1. The fish opens its mouth and lowers the floor of the buccal cavity (increasing volume, lowering pressure). The operculum valve remains closed. Water rushes in.
2. The fish closes its mouth and raises the floor of the buccal cavity (decreasing volume, increasing pressure).
3. This increased pressure forces the water out over the gill lamellae, causing the operculum valve to open.

The key is that steps 1 and 3 overlap slightly, maintaining a near-constant flow of water (and thus oxygen) over the gills.

Example: Grouper (6.3.3)
The Grouper is a large, typical bony fish that uses pumped ventilation.

Relation to Habitat and Motility (6.3.4):

• Habitat: Often found near reefs or rocks, they are ambush predators.
• Motility: They are not continuously swimming.
• Advantage: Pumped ventilation allows the grouper to remain stationary (or move slowly) while ensuring adequate water flow over its gills. This is essential for maintaining their position in the reef environment.

Did you know? Bony fish like Grouper rely on pumped ventilation because if they simply waited for water to flow over their gills, they would often suffocate due to insufficient oxygen supply.

2. Ram Ventilation

This method is much simpler mechanically but requires high speed and constant movement. The fish holds its mouth slightly open while swimming, forcing water to "ram" (rush) across the gills.

Step-by-Step Mechanism

1. The fish swims at high speed with its mouth open.
2. The water is continuously forced into the mouth, through the gills, and out the operculum (or gill slits).

Example: Tuna (6.3.3)
Tuna, many sharks (like the Blue Shark), and other oceanic pelagic fish use ram ventilation.

Relation to Habitat and Motility (6.3.4):

• Habitat: Open ocean (pelagic), requiring continuous, high-speed movement.
• Motility: High metabolic rate requires vast amounts of oxygen constantly.
• Advantage: Ram ventilation is highly energy-efficient *at high speeds* because the movement of the fish does the work, reducing the need for buccal muscles.
• Limitation: Organisms that use ram ventilation are often obligate ram ventilators, meaning they must keep swimming to breathe. If they stop, they suffocate.