Marine Science (9693) | Movement of substances

Movement of Substances (Physiology of Marine Organisms)

Welcome to one of the most fundamental chapters in Marine Science! Understanding how things move into, out of, and around marine organisms is key to grasping how they survive in the ocean. This process, called transport, dictates everything from how a microscopic diatom gets its nutrients to how a giant tuna regulates its salt balance.

Don't worry if terms like 'water potential' sound complicated—we will break them down using clear steps and relatable marine examples!

6.2.1 Mechanisms of Transport Across Membranes

All movement of substances must cross the cell surface membrane, which is selectively permeable. This means it controls which substances enter or leave the cell (6.1.3). The movement mechanisms fall into two main categories: Passive (no energy needed) and Active (energy needed).

A. Passive Transport (Movement Down the Gradient)

Passive transport relies entirely on the natural kinetic energy of particles, moving them from an area of high concentration to an area of low concentration (down the concentration gradient).

There are three main types you must know:

1. Diffusion

• Description: The net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, resulting from random movement.
• Energy required: None (no ATP).
• Marine Example: Oxygen moving directly from the surrounding seawater into the cells of small marine invertebrates, like coral polyps, or into the gills of larger fish.

2. Facilitated Diffusion

• Description: Movement of particles down the concentration gradient, but requiring specific membrane proteins (channel proteins or carrier proteins) to help them cross the hydrophobic lipid bilayer.
• Energy required: None (no ATP).
• Why is it needed? Substances that are charged (ions like Na+ or Cl-) or relatively large (like glucose) cannot pass directly through the lipid bilayer, so they need a 'helper' protein.

3. Osmosis

• Description: The net movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.
• Key distinction: It is always about the movement of water, and it is a special type of diffusion.
• (We cover water potential in detail below!)

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B. Active Transport (Movement Against the Gradient)

Active transport is necessary when an organism needs to accumulate a substance (e.g., nutrients) that is scarce in the environment, or expel a waste product that is concentrated inside the cell.

• Description: The movement of particles (molecules or ions) across a membrane against their concentration gradient (from low concentration to high concentration).
• Energy required: Yes, it requires metabolic energy in the form of ATP.
• Mechanism: Requires specific carrier proteins (often called 'pumps') which change shape using ATP energy to push the substance across the membrane.
• Marine Example: Marine algae actively pumping nitrates (NO₃⁻) from the low-concentration seawater reservoir into their cells to support protein synthesis, even when the internal concentration is already higher.

6.2.2 Water Potential ($\Psi$)

Water potential ($\Psi$) is a concept central to understanding osmosis. It determines the direction in which water moves.

• Definition: Water potential ($\Psi$) is the measure of the relative number of water molecules and their freedom to move in a solution. It is measured in units of pressure (usually kilopascals, kPa).
• Pure water has the highest water potential, which is defined as zero kPa ( \(\Psi = 0\) kPa).

The Effect of Dissolved Solutes

Adding any dissolved substance (a solute) to water will reduce the water potential, making the value more negative.

• A solution (like seawater) will always have a negative water potential (\(\Psi < 0\)).
• The more solutes present, the more negative (lower) the water potential is.

The Rule of Movement: Water always diffuses from a region of higher water potential (less negative) to a region of lower water potential (more negative).

Analogy: Think of water molecules as free dancers on a dance floor. Adding salt (solutes) is like adding bouncers that grab the water molecules, reducing their freedom to move. The more bouncers (solutes), the lower (more negative) the water potential.

6.2.4 & 6.2.5 The Surface Area to Volume Ratio (SA:V)

The relationship between an organism's surface area (SA) and its volume (V) is critical for determining how effectively substances can move into and out of the body, particularly via diffusion.

Calculating SA:V (6.2.4)

To demonstrate this principle, you need to understand how SA and Volume relate for simple shapes (e.g., in PA investigations using agar blocks):

For a Cube (Side length \(L\)):
Surface Area (SA) = \(6 \times L^2\)
Volume (V) = \(L^3\)
SA:V Ratio = \(\frac{6 L^2}{L^3}\)

The Principle:

As an organism (or a cell) increases in size, its volume increases much faster than its surface area. Therefore, SA:V ratio decreases with increasing size.

Implications for Marine Organisms

This mathematical principle has huge biological consequences (6.3.2):

• Small organisms (High SA:V): Microscopic organisms like phytoplankton or tiny larvae have a very high SA:V. This allows them to rely on simple diffusion across their entire body surface for gas exchange, nutrient uptake, and waste removal. They don't need complex organs.
• Large organisms (Low SA:V): Large marine animals like tuna or whales have a low SA:V. Diffusion alone cannot meet the massive metabolic demands of their central tissues. They require specialized exchange surfaces (like gills or lungs) and internal transport systems (like circulatory systems) to distribute substances efficiently.

6.2.7 The Movement of Water and Cellular Effects

Osmosis dictates how cells gain or lose water, depending on the water potential of the surrounding solution. The effect is dramatically different in animal cells (which lack a cell wall) compared to plant cells (which have a rigid cell wall).

1. Animal Cells (e.g., Fish Blood Cells)

Animal cells are highly susceptible to changes in external water potential because they lack a rigid cell wall.

• Isotonic Solution: \(\Psi_{external} = \Psi_{cell}\). Water moves equally in and out. The cell is normal and healthy. (e.g., Most marine invertebrates live in seawater that is isotonic to their body fluids.)
• Hypotonic Solution: \(\Psi_{external} > \Psi_{cell}\). Water moves into the cell. The cell swells and may eventually burst (lysis) because there is no cell wall to withstand the pressure.
• Hypertonic Solution: \(\Psi_{external} < \Psi_{cell}\). Water moves out of the cell. The cell shrivels up (crenation) because of dehydration.

2. Plant Cells (e.g., Marine Algae or Seagrass)

Plant cells, including those of marine plants and algae, have a strong, rigid cell wall that limits swelling.

• Isotonic Solution: The cell is flaccid (limp). There is no net movement of water, and the cell is not firm.
• Hypotonic Solution (Ideal Environment): Water moves into the cell. The cell swells, and the plasma membrane pushes against the cell wall. This builds up turgor pressure, making the cell firm (turgid). The cell wall prevents lysis.
• Hypertonic Solution: Water moves out of the cell. The vacuole shrinks, and the plasma membrane pulls away from the cell wall. This process is called plasmolysis. The plant or algae wilts.

6.2.3 & 6.2.6 Practical Investigations (PA)

Experimental work is crucial for understanding transport. You must be familiar with investigations involving diffusion, osmosis, and SA:V.

Investigating Diffusion and SA:V (Agar Blocks)

In these experiments, agar blocks containing a substance (like phenolphthalein indicator) are soaked in a solution (like dilute acid). The time taken for the acid to diffuse fully into the block shows the rate of diffusion.

By comparing blocks of different sizes (and therefore different SA:V ratios), we find:

• Result: Smaller blocks (high SA:V) complete diffusion much faster than larger blocks (low SA:V).
• Conclusion: A larger SA:V ratio facilitates faster and more efficient movement of substances, supporting the physiological needs of smaller organisms.

Investigating Osmosis (Plant Tissue and Visking Tubing)

In these experiments, plant tissues (like potato cylinders) or non-living membranes (Visking tubing or dialysis tubing) are immersed in solutions of varying water potentials (e.g., different concentrations of NaCl or sucrose).

1. Visking Tubing:

• A bag of Visking tubing containing a known solution (e.g., 20% sugar) is placed in water.
• Since the sugar solution has a lower \(\Psi\) than the pure water, water moves into the tubing by osmosis, causing the bag to gain mass or swell.

2. Plant Tissue (Estimating Tissue Water Potential):

• Plant tissues (e.g., potato strips) are weighed and placed in a series of external solutions with known, different water potentials.
• After incubation, the strips are reweighed, and the percentage change in mass is calculated.
• Result Interpretation: If the strip loses mass, the external solution had a lower \(\Psi\) than the tissue. If it gains mass, the external solution had a higher \(\Psi\).
• Estimation: The water potential of the tissue is estimated by finding the external concentration at which there is zero net change in mass (the point where the solution is isotonic to the tissue).

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You've navigated the complexities of cellular transport! Remember that diffusion, osmosis, and active transport are all essential for marine life to interact with the constantly changing chemical environment of the sea.