🌊 Marine Science (9693) A Level Study Notes: General Cell Structure (6.1)

Welcome to the start of your A Level journey into marine physiology! Don't worry if cell biology seems daunting; we’re going to look at the fundamental building blocks of all marine life, from tiny plankton to giant kelp.
Understanding how these cellular "factories" operate is essential because it explains all the amazing physiological adaptations (like osmoregulation and gas exchange) that marine organisms use to survive in the ocean. Let's dive in!

1. The Basics: Cells as the Units of Life

All living organisms in the ocean (and everywhere else) are made of cells. Cells share many common features, regardless of whether they belong to a shark or a microscopic diatom.

Key Definitions
  • Cell Surface Membrane: The boundary separating the cell's contents from the surrounding environment (seawater or other tissues).
  • Cytoplasm: The jelly-like substance filling the cell, where most metabolic reactions occur. It contains the organelles.
  • Organelles: Specialized sub-units within a cell that perform specific functions (like tiny organs).
  • Genetic Material (DNA): The instructions for building and operating the cell.
1.1 Visualising Cells: Magnification (6.1.5)

Since most marine cells are microscopic (we study them using photomicrographs or electron micrographs), you must be able to calculate how much an image has been enlarged.

The magnification formula is a core skill:

$$ \text{Magnification} = \frac{\text{Image Size}}{\text{Actual Size}} $$

Crucial Tip: When using this formula, the Image Size (what you measure on the drawing/photograph) and the Actual Size (the real-life size of the cell) must be in the same units (e.g., both micrometres ($\mu$m) or both millimetres (mm)).

Example: If you measure a micrograph of a marine algae cell at 10 mm, and you know its actual size is 0.01 mm, the magnification is \( 10 \text{mm} / 0.01 \text{mm} = 1000 \times \).

Key Takeaway (Section 1)

Cells are the fundamental units of marine life. Remember the magnification formula and ensure your units are consistent when calculating actual size or magnification from images.

2. The Eukaryotic Marine Cell: Structure and Function (6.1.1)

Marine organisms are mostly eukaryotic (cells containing a nucleus and membrane-bound organelles). Below are the essential structures and their specific jobs.

2.1 The Control Center and Factories
  • Nucleus: The "Command Centre."

    Function: Contains the cell's genetic material (DNA) and controls all cell activities (growth, metabolism, reproduction).

  • Ribosomes: The "Protein Builders."

    Function: The site of protein synthesis (translation). These are found floating freely in the cytoplasm or attached to the RER.

  • Rough Endoplasmic Reticulum (RER): The "Protein Highway."

    Structure: A network of membranes covered in ribosomes.
    Function: Synthesis, folding, and transport of proteins, especially those destined for secretion (export) or insertion into membranes.

  • Smooth Endoplasmic Reticulum (SER): The "Detox and Lipid Workshop."

    Structure: A network of membranes without ribosomes.
    Function: Synthesis of lipids (fats), steroids, and membrane phospholipids; detoxification of drugs and poisons.

  • Golgi Body (or Golgi Apparatus): The "Post Office."

    Function: Modifies, sorts, and packages proteins and lipids received from the ER into vesicles for transport, secretion, or use within the cell.

2.2 Energy and Structure Organelles
  • Mitochondria: The "Powerhouse."

    Function: Site of aerobic respiration, where organic molecules (like glucose) are broken down using oxygen to release energy in the form of ATP (Adenosine triphosphate). Important for active marine animals like tuna or filter feeders.

  • Chloroplasts: The "Solar Panels" (Found in marine producers like phytoplankton and macroalgae).

    Function: Site of photosynthesis, converting light energy, carbon dioxide, and water into glucose (organic compounds).

  • Cell Wall (Plant/Algae cells only): The "Outer Skeleton."

    Function: Provides structural support, protection, and maintains the cell's shape. It is typically made of cellulose (or silica in diatoms).

  • Large Permanent Vacuole (Plant/Algae cells only): The "Water Tower/Storage."

    Function: Stores water, nutrients, and waste products. It maintains turgor pressure, which helps support the plant structure (e.g., maintaining the shape of a kelp blade).

Did you know? Many tiny marine animals (zooplankton) are nearly transparent. Their cells contain far fewer pigments or dense structures compared to large fish cells, which helps them camouflage in the open ocean!

Key Takeaway (Section 2)

Eukaryotic cells are highly compartmentalised. Match the organelles (e.g., Mitochondria, Chloroplast, RER) precisely to their functions (e.g., Respiration, Photosynthesis, Protein Synthesis).

3. The Cell Surface Membrane and Transport (6.1.2, 6.1.3)

The cell membrane is arguably the most critical structure in marine physiology, as it controls what enters and leaves the cell, allowing organisms to maintain stable internal conditions despite fluctuating external seawater conditions.

3.1 The Fluid Mosaic Model (6.1.2)

We describe the cell membrane using the Fluid Mosaic Model.

  • Fluid: The components (especially the phospholipids) can move laterally (side-to-side), giving the membrane flexibility.
  • Mosaic: The membrane is studded with various proteins scattered throughout, like tiles in a mosaic pattern.
Structure of the Membrane

The backbone of the membrane is the Phospholipid Bilayer.

Each phospholipid molecule has two parts:

  1. Hydrophilic Head: Phosphate group, "water-loving," faces the aqueous (watery) cytoplasm inside the cell and the aqueous environment outside (seawater).
  2. Hydrophobic Tail: Fatty acid chains, "water-fearing," face inwards towards the center of the bilayer, away from water.

The proteins embedded in this bilayer play crucial roles in communication and transport:

  • Channel Proteins: Act like tunnels or pores, allowing specific ions or molecules (often small and charged) to pass straight through the membrane passively (without energy).
  • Carrier Proteins: Bind to specific molecules (like glucose or amino acids) and change shape to ferry them across the membrane. They can be used for both passive transport (down a concentration gradient) or active transport (against a concentration gradient, requiring ATP energy).
3.2 Selectively Permeable Membranes (6.1.3)

The fluid mosaic structure makes the membrane selectively permeable (or semi-permeable).

  • What gets through easily? Small, non-polar molecules like oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$).
  • What needs help (proteins)? Larger molecules, polar molecules (like water), and ions (charged particles like $\text{Na}^+$ or $\text{Cl}^-$).

This selective nature is vital for maintaining the correct internal concentration of salts and water—a core necessity for marine animals coping with highly saline or changing estuarine environments.

Quick Review: Membrane Transport

We relate the selectively permeable membrane to three main types of transport (details covered fully in 6.2):

  1. Diffusion: Passive movement of substances (e.g., $\text{O}_2$) from high concentration to low concentration.
  2. Facilitated Diffusion: Passive movement using specific membrane proteins (channels or carriers).
  3. Active Transport: Movement against the concentration gradient, requiring energy (ATP) and carrier proteins.

Don't worry if this seems tricky at first: The key distinction is Active needs energy (ATP), Passive does not.

Key Takeaway (Section 3)

The Fluid Mosaic Model describes the bilayer structure of phospholipids and the embedded proteins (channels and carriers). This structure results in the membrane being selectively permeable, which controls all cellular transport.

4. Comparing Marine Cell Types (6.1.4)

In Marine Science, we frequently compare cells from marine producers (phytoplankton, algae, seagrasses) and consumers (fish, crustaceans).

4.1 Typical Animal vs. Plant/Algae Cells

The organelles found in both cell types are essentially the same (nucleus, RER, mitochondria, etc.), but the key differences relate to structure and energy capture.

  • Marine Animal Cell (e.g., Fish Muscle Cell):

    Generally small, flexible shape (due to only having a cell surface membrane), and possess many mitochondria to meet high energy demands for swimming or osmoregulation.

  • Marine Plant/Algae Cell (e.g., Kelp Blade Cell):

    Often larger, rigid shape (due to the cell wall), contains chloroplasts (for photosynthesis), and usually has a large permanent vacuole (for support and storage).

4.2 Interpreting Micrographs (6.1.4)

When you see a photomicrograph (taken with a light microscope) or an electron micrograph (taken with an electron microscope), you need to be able to identify the key structures (6.1.1).

  • Photomicrographs: Show less detail; you can usually see the cell wall, nucleus, and sometimes the shape of the vacuole or chloroplasts.
  • Electron Micrographs (EMs): Show much greater internal detail, including the double membrane of the mitochondria and the internal structure of the ER or Golgi. These are often used to show the fluid mosaic structure in cross-section.

Memory Aid: If you see a thick, defined outer layer and green internal dots (chloroplasts), you are definitely looking at a producer, like a phytoplankton or macroalgae cell!

Key Takeaway (Section 4)

Be prepared to recognise and differentiate between animal and producer cells based on the presence of key features like the cell wall, chloroplasts, and large vacuole. Use magnification calculations to determine the actual size of the organism shown in any micrograph.