Transport Systems

Multicellular organisms require specialized transport systems because simple diffusion is too slow to move essential substances over long distances. These systems ensure that every cell receives nutrients and oxygen while having waste products efficiently removed.

### Transport in Humans: The Circulatory System

Humans possess a closed, double circulatory system, which is highly efficient. It consists of the heart, blood vessels, and blood.

1. Blood: Blood is a fluid connective tissue composed of four main components suspended in **plasma** (a straw-coloured liquid containing water, proteins, glucose, and hormones).

* Red Blood Cells (Erythrocytes): These biconcave discs lack a nucleus to maximize space for haemoglobin, the protein that binds with oxygen to form oxyhaemoglobin for transport from the lungs to body tissues.

* White Blood Cells (Leucocytes): These are key to the immune system. Phagocytes engulf and digest pathogens, while lymphocytes produce antibodies to neutralize specific pathogens.

* Platelets (Thrombocytes): These are small cell fragments that initiate blood clotting at a wound site, preventing blood loss and pathogen entry.

2. Blood Vessels: There are three main types of blood vessels.

* Arteries: Carry high-pressure, oxygenated blood (except the pulmonary artery) away from the heart. They have thick, muscular, and elastic walls to withstand and maintain high pressure, and a narrow lumen.

* Veins: Carry low-pressure, deoxygenated blood (except the pulmonary vein) towards the heart. They have thinner walls, a larger lumen, and contain valves to prevent the backflow of blood.

* Capillaries: These are microscopic vessels forming vast networks (capillary beds) that infiltrate tissues. Their walls are only one-cell thick, providing a short diffusion path for the efficient exchange of gases, nutrients, and waste between blood and body cells.

3. The Heart: The heart is a muscular organ that functions as a **double pump**. Its four chambers are separated by a **septum**, which prevents the mixing of oxygenated and deoxygenated blood.

* The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs (pulmonary circuit).

* The left side of the heart receives oxygenated blood from the lungs and pumps it to the rest of the body (systemic circuit).

* The cardiac cycle describes the sequence of events in one heartbeat. Systole is the contraction phase, where the heart muscle pumps blood out. Diastole is the relaxation phase, where the chambers fill with blood. Atrioventricular valves (between atria and ventricles) and semi-lunar valves (in the aorta and pulmonary artery) ensure one-way blood flow.

### Transport in Plants: The Vascular System

Plants have a vascular system composed of two main transport tissues: xylem and phloem, found together in vascular bundles.

1. Xylem: This tissue transports water and dissolved mineral ions from the roots to the rest of the plant.

* Structure: Xylem vessels are hollow, continuous tubes formed from dead cells. Their walls are strengthened with a waterproof polymer called lignin, which also provides structural support to the plant.

* Process: The Transpiration Stream: Water moves up the xylem through a continuous column. This movement is driven by transpiration, the evaporation of water from the leaf surface. The forces of cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to the xylem walls) help pull the water column upwards.

2. Phloem: This tissue transports dissolved sugars (like sucrose) and amino acids from where they are made (**source**, e.g., leaves) to where they are needed for growth or storage (**sink**, e.g., roots, fruits, flowers).

* Structure: Phloem consists of living cells, primarily sieve tube elements (which form the tube) and companion cells (which provide metabolic support for the sieve tubes).

* Process: Translocation: This is the movement of sugars in the phloem. It is an active process requiring energy, allowing the plant to move substances against a concentration gradient.

3. Transpiration: This is the loss of water vapour from plant leaves, primarily through pores called **stomata**. The opening and closing of stomata are controlled by **guard cells**. While transpiration drives the movement of water, excessive water loss can be harmful.

* Factors Affecting Transpiration Rate:

* Light Intensity: Higher light intensity increases the rate as stomata open for photosynthesis.

* Temperature: Higher temperatures increase the kinetic energy of water molecules, speeding up evaporation.

* Wind Speed: Wind removes the humid layer of air around the leaf, steepening the water potential gradient and increasing evaporation.

* Humidity: High humidity reduces the water potential gradient between the leaf and the air, slowing down transpiration.