Science (Double Award) | Structure and functions in living organisms

🔬 Structure and Functions in Living Organisms: Study Notes

Hello future biologists! This chapter is the foundation of understanding how life works. We are going to explore the tiny units that make up everything alive—cells—and discover how their structure is perfectly suited to the job they do (their function). Don't worry if this seems tricky at first; we will break down the complex processes into easy, bite-sized pieces!

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Section 1: The Basic Units of Life – Cells and Organization

1.1 Levels of Organization

Living organisms are highly organized, much like a well-structured building. Understanding how small parts build up into large systems is crucial.

Think of it like building a house:

1. Cell: The smallest unit of life. (The single brick)
2. Tissue: A group of similar cells working together to perform a specific function. (A wall built of many similar bricks)
3. Organ: A structure made of different tissues working together. (A room, made of walls, floors, and ceilings)
4. Organ System: A group of organs working together to carry out a major life function. (The entire plumbing system or electrical system of the house)
5. Organism: A complete living thing. (The finished house)

Example: In humans, muscle cells form muscle tissue, which is part of the stomach (an organ), which is part of the digestive system (an organ system).

1.2 Animal Cell Structure (Eukaryotic)

All eukaryotic cells (cells with a nucleus, like those in animals, plants, and fungi) share several key components.

• Nucleus: This is the control center or the brain of the cell. It contains the genetic material (DNA) which controls the cell’s activities.
• Cytoplasm: A jelly-like substance where most chemical reactions occur. It fills the cell and holds the organelles.
• Cell Membrane: The outer layer that controls which substances can enter or leave the cell. It is partially permeable (or selectively permeable).
• Mitochondria: The powerhouses of the cell! This is where aerobic respiration takes place, releasing energy (ATP) for the cell.
• Ribosomes: Tiny structures where protein synthesis happens (they build proteins).

1.3 Plant Cell Structure (Eukaryotic)

Plant cells have all the parts listed above (nucleus, cytoplasm, membrane, mitochondria, ribosomes) PLUS three essential extra components:

• Cell Wall: A rigid outer layer made of cellulose. It gives the plant cell support and a fixed shape. Think of it as a strong, protective exoskeleton.
• Chloroplasts: Found mainly in the green parts of plants. They contain the green pigment chlorophyll and are the site of photosynthesis (making food using light).
• Permanent Vacuole: A large sac filled with cell sap (water, salts, sugars). It helps keep the cell rigid by pushing the contents against the cell wall (this is called turgor pressure).

1.4 Key Takeaway: Comparing Plant and Animal Cells

This comparison is frequently tested!

Feature	Animal Cell	Plant Cell
Cell Wall	Absent	Present (Made of cellulose)
Chloroplasts	Absent	Present (In green parts)
Vacuole	Small, temporary, or absent	Large, permanent central vacuole
Shape	Irregular/Flexible	Fixed/Regular

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Section 2: Specialization: Structure Matches Function

2.1 Why Specialized Cells?

In multicellular organisms (like you!), not all cells are the same. Cells become specialized (differentiated) to perform a unique job very efficiently. Their structure changes dramatically to help them carry out this function.

Analogy: A general-purpose tool can do many jobs poorly, but a specialized tool (like a screwdriver or wrench) can do one job perfectly. Cells are the same way!

2.2 Examples of Specialized Cells

We must know how the structure of specific cells helps them function:

1. Sperm Cell (Function: Reproduction)

• Structure Feature: Possesses a long tail (flagellum).
• Functional Advantage: Allows the cell to swim towards the egg quickly.
• Structure Feature: Packed with mitochondria (in the middle section).
• Functional Advantage: Provides the large amount of energy needed for swimming.

2. Root Hair Cell (Function: Absorption of Water and Minerals)

• Structure Feature: Has a long, thin projection called a root hair.
• Functional Advantage: Greatly increases the surface area for absorption from the soil.
• Structure Feature: Thin cell wall and membrane.
• Functional Advantage: Allows easy passage of water and minerals.

3. Red Blood Cell (Function: Transport Oxygen)

• Structure Feature: Biconcave shape (indented disc shape).
• Functional Advantage: Maximizes surface area for efficient oxygen absorption and release.
• Structure Feature: Lacks a nucleus (when mature).
• Functional Advantage: Leaves maximum space available to carry the oxygen-transporting pigment, haemoglobin.

4. Palisade Mesophyll Cell (Function: Photosynthesis)

• Structure Feature: Packed with chloroplasts.
• Functional Advantage: Maximizes the capture of sunlight needed for photosynthesis.

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Section 3: Moving Materials Across Membranes (Transport)

The cell membrane controls the movement of substances. There are three main ways materials move in and out of cells.

3.1 Diffusion (Passive Transport)

Diffusion is the spreading out of particles from an area where they are in high concentration to an area where they are in low concentration.

• This process is passive—it requires no energy (ATP).
• It continues until the particles are evenly spread out (a state called equilibrium).
• This is how oxygen moves into blood from the lungs, and carbon dioxide moves out.

Analogy: If you spray deodorant in one corner of a room, the smell eventually spreads everywhere. The deodorant molecules are moving by diffusion.

The rate of diffusion can be increased by:

• Increasing the temperature (particles move faster).
• Increasing the concentration gradient (a bigger difference between high and low concentration).
• Increasing the surface area (e.g., the alveoli in the lungs).

3.2 Osmosis (Special Case of Diffusion)

Osmosis is one of the trickiest concepts, so read carefully!

Osmosis is the movement of water molecules across a partially permeable membrane from an area of high water potential (dilute solution) to an area of low water potential (concentrated solution).

Key Points to Remember for Osmosis (The "Big Three"):

1. It must involve WATER molecules only.
2. It must happen across a PARTIALLY PERMEABLE MEMBRANE (like the cell membrane).
3. Water moves from dilute (more water) to concentrated (less water).

What happens in cells?

• Animal Cells: If placed in pure water, water rushes in, and the cell swells and bursts (lysis). If placed in very salty water, water leaves, and the cell shrinks (crenation).
• Plant Cells: If water leaves, the vacuole shrinks, and the cell membrane pulls away from the cell wall (plasmolysis). The strong cell wall prevents bursting if water rushes in; the cell becomes stiff and firm (turgid).

3.3 Active Transport

Sometimes, a cell needs to move substances *against* the concentration gradient—from where they are low concentration to where they are high concentration. This is like pushing a boulder uphill!

Active Transport is the movement of substances from a low concentration to a high concentration (against the concentration gradient), requiring energy (ATP), supplied by the mitochondria.

• It is the opposite of diffusion.
• It is Active because it uses stored Energy (ATP).
• Example: Root hair cells use active transport to absorb essential mineral ions from the soil, even when the concentration of those ions is much higher inside the root cell than in the surrounding soil.

Key Takeaways Summary

You have successfully reviewed the building blocks of life! Remember these crucial links:

Structure = Function. (E.g., Root hair cells have a large surface area to maximize absorption.)
Energy Required. Diffusion and Osmosis are Passive. Active Transport requires ATP.