Biology | Transport

Welcome to the World of Transport in Biology!

Hello everyone! In this chapter, we are diving into one of the most fundamental processes of life: Transport. Whether you are a microscopic bacterium, a giant oak tree, or a human being, every living organism needs to move stuff around—nutrients in, waste out, water up, and oxygen everywhere!

Don't worry if some concepts seem tricky at first. We will break down everything into simple steps. By the end of this chapter, you will understand how substances move into and out of cells, how plants move massive amounts of water up against gravity, and how our own amazing circulatory system works!

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Section 1: Movement Across Membranes

Every cell is surrounded by a cell membrane, which acts like a security guard, deciding what enters and what leaves. Substances move across this barrier mainly through two processes: Diffusion and Osmosis.

1.1 Diffusion

Diffusion is the easiest way substances move around. It relies purely on random particle movement.

Definition: Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

Think of it this way: Imagine spraying perfume in one corner of a room. Initially, the perfume particles are highly concentrated there. Soon, they spread out randomly until they are evenly distributed throughout the room. That's diffusion!

• Concentration Gradient: This is the difference in concentration between two areas. Substances naturally move 'down' this gradient (from high to low).
• Biological Importance: Diffusion is vital for gas exchange (like oxygen moving from the lungs into the blood, and carbon dioxide moving out).

Factors Affecting the Rate of Diffusion

How fast diffusion happens depends on a few key things:

1. Temperature: Higher temperature means particles have more kinetic energy and move faster, increasing the rate.
2. Surface Area: A larger surface area (like the many small air sacs in your lungs) means more space for diffusion to happen.
3. Concentration Gradient: A steeper gradient (a big difference between high and low concentrations) means faster diffusion.

1.2 Osmosis

Osmosis is a special type of diffusion, and it’s always about water. This is often a tricky concept, but we will simplify it!

Definition: Osmosis is the net movement of water molecules from an area of higher concentration of water (dilute solution) to an area of lower concentration of water (concentrated solution) across a partially permeable membrane.

• Partially Permeable Membrane: This membrane has microscopic holes that are large enough for tiny water molecules to pass through, but too small for larger dissolved particles (solutes like sugar or salt) to pass.

Analogy: Imagine a crowd of people (water molecules) trying to balance out two rooms separated by a door that only allows the small people (water) to move, but blocks the big people (salt). The small people will naturally move to the side that has fewer small people until the water levels are balanced.

Osmosis and Cells

What happens when cells are placed in different solutions?

1. Animal Cells (e.g., Red Blood Cells):

• In Pure Water (High Water Concentration Outside): Water rushes in. The cell swells and bursts (lysis), as it has no strong cell wall to hold it in place.
• In Very Salty Solution (Low Water Concentration Outside): Water rushes out. The cell shrinks and shrivels (crenation).

2. Plant Cells:

• In Pure Water (High Water Concentration Outside): Water rushes in, pressing the cytoplasm against the rigid cell wall. The cell becomes stiff and firm (turgid). This is healthy for plants!
• In Very Salty Solution (Low Water Concentration Outside): Water rushes out. The vacuole and cytoplasm shrink away from the cell wall. The cell is plasmolysed, and the plant wilts.

Important Note: Always remember, in osmosis, only water moves, and it moves to the area where there are more dissolved substances (to try and dilute them).

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Section 2: Transport in Plants

Plants are experts at moving materials. They have a brilliant system for transporting water all the way up to their leaves, and moving sugars down to their roots.

2.1 Vascular Bundles: Xylem and Phloem

The plant transport system is made up of tubes called vascular bundles (veins). The two main types of transport tubes are the Xylem and the Phloem.

A. Xylem Vessels (Water Transport)

Xylem vessels transport water and mineral ions from the roots, through the stem, and up to the leaves.

• Structure: Xylem vessels are hollow, dead tubes strengthened by lignin (a tough material). They form continuous, narrow pipelines.
• Direction: Transport is always upwards (unidirectional).

Memory Aid: Xy-lem sounds a bit like Hydro, which means water!

B. Phloem Vessels (Sugar Transport)

Phloem vessels transport sucrose (sugars) and amino acids—the products of photosynthesis—to all parts of the plant, especially growing regions and storage organs (like fruits or tubers).

• Process: This movement is called translocation.
• Direction: Transport is upwards and downwards (bidirectional).

Memory Aid: Ph-loem starts with ‘P’ and carries the Products of Photosynthesis.

2.2 Transpiration and the Transpiration Stream

How does water get pulled all the way up a tall tree? Through a process called transpiration.

Definition: Transpiration is the loss of water vapour from the plant leaves, mainly through pores called stomata.

When water evaporates from the leaves, it creates a ‘pull’ or suction force, which draws the column of water molecules up the xylem vessels from the roots. This movement of water is called the transpiration stream.

Why is Transpiration Necessary?

1. It transports water and essential mineral ions to the leaves for photosynthesis.
2. It helps cool the plant (like sweating in humans).

Factors Affecting the Rate of Transpiration

The rate of water loss is controlled by external factors, mainly through their effect on the stomata:

• Temperature: Higher temperature means faster evaporation, so transpiration increases.
• Humidity: High humidity means the air is already saturated with water vapour, slowing down evaporation, so transpiration decreases.
• Wind/Air Movement: Wind blows away the humid air surrounding the leaf, maintaining a concentration gradient, so transpiration increases.
• Light Intensity: Higher light intensity causes the stomata to open wider for photosynthesis, increasing the rate of water loss.

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Section 3: Transport in Animals – The Circulatory System

In complex animals, diffusion alone is too slow to transport substances over long distances. We use a dedicated system: the circulatory system, which is a closed circuit powered by the heart.

3.1 The Composition of Blood

Blood is often called a tissue, composed of a liquid matrix (plasma) and various types of cells.

• 1. Plasma: The straw-coloured liquid component (about 55% of blood volume). It transports everything: blood cells, digested food (glucose, amino acids), hormones, carbon dioxide, and urea (waste product).
• 2. Red Blood Cells (RBCs):
  • Function: Transport oxygen.
  • Structure: Contain the red pigment haemoglobin, which binds to oxygen. They are biconcave discs and uniquely lack a nucleus to maximize space for haemoglobin.
• 3. White Blood Cells (WBCs):
  • Function: Crucial part of the immune system; fight pathogens (disease-causing agents).
  • Types: Include phagocytes (engulf bacteria) and lymphocytes (produce antibodies).
• 4. Platelets:
  • Function: Involved in blood clotting to prevent excessive blood loss when a vessel is cut.

3.2 Blood Vessels

There are three main types of blood vessels, each designed for a specific job:

1. Arteries:
   • Function: Carry blood AWAY from the heart.
   • Structure: Have thick, muscular, elastic walls to withstand the high pressure created by the pumping heart.
   • Exception: Pulmonary Artery carries deoxygenated blood.
2. Veins:
   • Function: Carry blood TOWARDS the heart.
   • Structure: Have thin walls and a wide lumen (internal space). Because pressure is low, veins have valves to prevent blood from flowing backward.
   • Exception: Pulmonary Vein carries oxygenated blood.
3. Capillaries:
   • Function: Form vast networks within tissues to allow exchange of materials (oxygen, glucose, CO2, waste).
   • Structure: Walls are only one cell thick, allowing for very rapid diffusion.

Did you know? If you laid out all your capillaries end-to-end, they would stretch over 60,000 miles! This huge network ensures every cell is close to an exchange point.

3.3 The Human Heart and Double Circulation

The heart is a muscular pump responsible for maintaining blood flow around the body. Humans have a Double Circulatory System, meaning blood passes through the heart twice for every complete circuit of the body.

Structure of the Heart

The heart has four chambers:

• Atria (Singular: Atrium): Upper chambers that receive blood.
• Ventricles: Lower, muscular chambers that pump blood out. The Left Ventricle has the thickest wall as it must pump blood to the entire body.
• Valves: Ensure blood flows in the correct direction and prevents backflow.

The Double Circuit Explained

This system is highly efficient because it ensures the blood returning from the lungs (full of oxygen) is kept separate from the blood returning from the body (low in oxygen).

1. Pulmonary Circuit (Heart to Lungs):

Deoxygenated blood enters the Right Atrium, moves to the Right Ventricle, and is pumped via the Pulmonary Artery to the lungs to pick up oxygen.

2. Systemic Circuit (Heart to Body):

Oxygenated blood returns to the Left Atrium, moves to the Left Ventricle, and is pumped via the Aorta (the body’s largest artery) to the rest of the body.

Common Mistake to Avoid: The Right side of the heart handles Deoxygenated blood (from the body); the Left side handles Oxygenated blood (from the lungs).

Well done! You have completed the intensive study of transport systems in Biology. Remember to use these notes and revisit the analogies to reinforce your understanding. Keep up the great work!