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

Welcome to Structure and Functions in Living Organisms!

Hi there! This chapter is the foundation of Biology. Everything you learn about how animals, plants, and microorganisms work starts right here. Don't worry if some of the names sound complicated; we will break them down into simple, easy-to-understand chunks.

What we will learn: We’ll look at the smallest unit of life—the cell—and discover how tiny structures are perfectly designed to carry out massive jobs, from fighting infections to growing tall trees. Understanding structure helps us understand function!

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1. The Basic Unit of Life: Cells

Every living thing is made of cells. Think of cells as the tiny Lego bricks that build everything from a single bacterium to an elephant!

1.1 Common Cell Components (Animal and Plant)

Both animal and plant cells share three essential parts.

• Cell Membrane: This is the boundary of the cell, like a security gate. It controls which substances can move into and out of the cell. It is partially permeable.
• Cytoplasm: A jelly-like substance where most of the cell’s chemical reactions happen. It fills the cell and contains the organelles.
• Nucleus: This is the control center, like the brain of the cell. It contains the cell's genetic material (DNA) and controls all cell activities.

1.2 Differences Between Plant and Animal Cells

Plant cells have three extra features that animal cells do not, which allow them to stand up, make their own food, and store lots of water.

Plant Cell Extras

1. Cell Wall: A strong outer layer made of cellulose. It supports the plant cell and gives it a fixed shape. (Analogy: A sturdy wooden fence around the security gate.)
2. Chloroplasts: Contains the green pigment chlorophyll. This is the site of photosynthesis (where plants make food using sunlight).
3. Permanent Vacuole: A large sac filled with cell sap (water and dissolved substances). It helps maintain turgor (pressure) to keep the plant rigid.

Quick Review: How to remember the three plant-only parts?

C Works C Very Perfectly. (Cell Wall, Chloroplasts, Vacuole - Permanent).

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2. The Organization of Living Organisms

Cells don't work alone. They are organized into a strict hierarchy to make sure the organism functions smoothly. Think of it like a sports team, where individual players (cells) form specialized groups (tissues) to achieve a goal.

Step-by-Step Levels of Organization

This hierarchy moves from simple, specialized units to complex, functioning bodies:

1. Cells: The basic unit (e.g., a single muscle cell).

2. Tissues: A group of similar cells working together to perform a specific function (e.g., muscle tissue, xylem tissue in plants).

3. Organs: A structure made of several different tissues working together (e.g., the heart, the leaf, the stomach).

4. Organ Systems: A group of organs working together to perform major life functions (e.g., the digestive system, the circulatory system, the nervous system).

5. Organism: The complete living thing (e.g., a human, a cat, a sunflower).

Mnemonic Aid: Use the first letters to remember the order:
Can Tiny Outside Strangers Organize? (Cell, Tissue, Organ, System, Organism)

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3. Transport of Substances in and out of Cells

Cells need to constantly exchange materials with their surroundings. They need oxygen and glucose in, and waste products out. This happens through three main processes.

3.1 Diffusion

Diffusion is how particles spread out naturally.

• Definition: The movement of particles (atoms, molecules, or ions) from an area of high concentration to an area of low concentration.
• Energy required? No. It is a passive process.
• The Rule: Particles move down the concentration gradient (like rolling a ball down a hill).

Example: When you spray deodorant in one corner of a room, the scent eventually spreads to the whole room. The scent particles diffuse from where they are highly concentrated (the spray area) to where they are lowly concentrated (the rest of the room).

This process is essential for gas exchange (moving oxygen into the blood and carbon dioxide out) in the lungs.

3.2 Osmosis

Osmosis is a special case of diffusion, specifically involving water.

• Definition: The movement of water molecules from a region of high water potential (dilute solution) to a region of low water potential (concentrated solution) across a partially permeable membrane.
• Energy required? No. It is also passive.

Key Point for Osmosis: It MUST happen across a partially permeable membrane (like the cell membrane), and it is ONLY about the movement of water.

Analogy: Imagine two sides of a door separated by a security guard (the membrane). On one side, there are 10 people and 90 buckets of water (high water potential). On the other, 50 people and 50 buckets of water (low water potential). The people can’t cross, but the water buckets can. Water will move across to try and even out the concentration.

3.3 Active Transport

Sometimes, cells need to move substances *against* the natural flow (uphill). This requires effort!

• Definition: The movement of particles across a cell membrane against the concentration gradient (from low concentration to high concentration).
• Energy required? Yes. This process requires energy (ATP), hence the name Active Transport.

Example: Root hair cells in plants use active transport to suck up essential minerals (like nitrates) from the soil, even when the concentration of minerals is much higher inside the root cell than in the soil water.

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4. Specialized Cells and Their Functions

Just like in a city, different cells have different jobs. Their structure is specifically adapted (changed) to help them do that job extremely well.

4.1 Specialized Animal Cells

1. Red Blood Cells (RBCs): Transporting Oxygen

• Adaptation: They are biconcave discs (pushed in on both sides). This shape increases the surface area to volume ratio, allowing oxygen to be absorbed and released quickly.
• Adaptation: They have no nucleus when mature. This leaves maximum space for the transport protein, haemoglobin, which binds to oxygen.

2. Nerve Cells (Neurons): Sending Electrical Messages

• Adaptation: They have long, thin fibers called axons which allow impulses to travel long distances very quickly (e.g., from your toe to your spinal cord).

3. Sperm Cells: Fertilization

• Adaptation: They have a long tail (flagellum) to swim towards the egg.
• Adaptation: They have lots of mitochondria (energy generators) to provide the energy needed for swimming.

4.2 Specialized Plant Cells

1. Root Hair Cells: Absorbing Water and Minerals

• Adaptation: They have a long, thin projection (the root hair) that penetrates the soil. This massively increases the surface area for absorbing water (by osmosis) and minerals (by active transport).

2. Xylem Cells: Water Transport

• Adaptation: Xylem forms continuous hollow tubes (vessels) with no cytoplasm or end walls. This allows water to flow smoothly and quickly from the roots up to the leaves.

3. Phloem Cells: Sugar Transport

• Adaptation: Phloem transports sucrose (sugar) produced in the leaves to all other parts of the plant. They have perforated end walls (sieve plates) to allow the sugary solution to pass through.

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Summary and Encouragement

Well done! You have covered the building blocks of life. Remember, the key concepts here are how cells are organized, and the three ways substances move across membranes. Go back and practice defining Diffusion, Osmosis, and Active Transport until you know them perfectly. You've got this!