Mass transport

Welcome to Mass Transport!

In this chapter, we are looking at how organisms move "stuff" (like oxygen, glucose, and water) over long distances. In tiny organisms, diffusion is enough. But once you get bigger, diffusion is too slow—imagine trying to deliver mail across the country by walking! Mass transport systems are the biological "motorways" that speed things up. We will look at how animals use blood and hearts, and how plants use specialized tubes called xylem and phloem.

1. Mass Transport in Animals: Haemoglobin

Haemoglobins are a group of chemically similar molecules found in many different organisms. Their job is simple: pick up oxygen where there’s lots of it (the lungs) and drop it off where it’s needed (the tissues).

Structure and Binding

Haemoglobin is a protein with a quaternary structure (it's made of four polypeptide chains). Each chain has a "haem" group containing an iron ion, which is what actually binds to the oxygen.
Cooperative Binding: This is a clever trick! When the first oxygen molecule binds, it changes the shape of the haemoglobin molecule. This change in shape makes it much easier for the second and third oxygen molecules to bind.
Analogy: Think of a crowded bus. The first person to get on has to find a seat and get settled (tricky), but once they have, they make it look more inviting for the next few people to jump on!

The Oxyhaemoglobin Dissociation Curve

This graph shows how "saturated" the haemoglobin is with oxygen at different partial pressures of oxygen (\(pO_2\)).
- Loading (Association): Happens in the lungs where \(pO_2\) is high. Haemoglobin has a high affinity for oxygen here.
- Unloading (Dissociation): Happens in the tissues where \(pO_2\) is low. Haemoglobin has a low affinity and lets go of the oxygen.

The Bohr Effect

When cells respire, they produce carbon dioxide (\(CO_2\)). High concentrations of \(CO_2\) make the environment more acidic, which changes the shape of haemoglobin slightly. This causes it to release oxygen more readily.
Quick Review: The Bohr effect shifts the curve to the right. This is great because it means more oxygen is delivered to muscles that are working hard and producing lots of \(CO_2\).

Environmental Adaptations

Different animals have different types of haemoglobin depending on where they live:
- Low oxygen environments (e.g., high altitudes or muddy burrows): Animals here have haemoglobin with a higher affinity for oxygen (the curve is to the left) so they can load oxygen more easily.
- Very active animals (e.g., small birds): They have haemoglobin with a lower affinity (the curve is to the right) so that oxygen is unloaded very easily to their hardworking muscles.

Key Takeaway: Haemoglobin's ability to change shape allows it to be an efficient delivery system, loading oxygen in the lungs and dumping it at the tissues, especially when \(CO_2\) levels are high.

2. The Circulatory System and the Heart

Mammals have a closed, double circulatory system. "Closed" means the blood stays inside vessels, and "double" means the blood passes through the heart twice for every complete circuit of the body.

The Structure of the Heart

You need to know the gross structure of the heart. Remember: "Left is Right and Right is Left" when looking at a diagram!
- Atria: Thin-walled upper chambers that receive blood.
- Ventricles: Thick-walled lower chambers that pump blood out. The left ventricle has much thicker muscle than the right because it has to pump blood all the way around the body, not just to the lungs.
- Major Vessels: Aorta (to body), Pulmonary Artery (to lungs), Pulmonary Vein (from lungs), Vena Cava (from body), and Coronary Arteries (supply the heart muscle itself with blood).

The Cardiac Cycle

This is the sequence of events in one heartbeat. Don't worry if the names sound fancy; they just describe whether the muscle is contracting or relaxing.
1. Atrial Systole: Atria contract, pushing blood into the ventricles.
2. Ventricular Systole: Ventricles contract. The pressure shuts the atrio-ventricular (AV) valves (to prevent backflow to the atria) and opens the semi-lunar valves, pushing blood into the arteries.
3. Diastole: Both atria and ventricles relax. The high pressure in the arteries shuts the semi-lunar valves. Blood flows into the atria from the veins.

Maths Moment: Cardiac Output
You can calculate how much blood the heart pumps per minute using this formula:
\(CO = stroke\ volume \times heart\ rate\)
- Stroke volume is the volume of blood pumped in one beat.
- Heart rate is the number of beats per minute.

Did you know? The "lub-dub" sound of your heart is actually the sound of your heart valves slamming shut!

Blood Vessels

Each vessel is "built" for its specific job:
- Arteries: Carry blood at high pressure. They have thick muscle layers and elastic tissue that stretches and recoils to maintain pressure.
- Arterioles: Smaller arteries that can constrict to control blood flow to specific tissues.
- Veins: Carry blood at low pressure. They have wider lumens and valves to prevent blood from flowing backward.
- Capillaries: The "exchange" vessels. Their walls are only one cell thick (short diffusion distance) and they form massive networks called capillary beds to provide a large surface area.

Tissue Fluid

Blood doesn't touch your cells directly. Instead, substances move into tissue fluid first.
1. At the start of the capillary bed (arteriole end), hydrostatic pressure (blood pressure) is high. This pushes water and small molecules out of the blood.
2. Large proteins stay in the blood because they are too big to fit through the gaps.
3. At the end of the capillary bed (venule end), the water potential of the blood is lower than the tissue fluid (because the proteins are still there). Water moves back into the capillary by osmosis.
4. Any leftover fluid is drained away by the lymphatic system.

Key Takeaway: The heart creates pressure to move blood through specialized vessels, while pressure differences allow for the exchange of nutrients via tissue fluid.

3. Mass Transport in Plants: Xylem

Plants don't have hearts, but they still need to move water from their roots to their leaves (sometimes over 100 meters up!). They use the xylem.

The Cohesion-Tension Theory

This explains how water moves up against gravity:
- Transpiration: Water evaporates from the leaves through stomata. This creates "tension" (a pulling force).
- Cohesion: Water molecules are "sticky" because of hydrogen bonds. They stick together in a continuous column.
- Adhesion: Water molecules also stick to the walls of the xylem.
Analogy: It's like drinking through a straw. When you suck at the top (transpiration), the "stickiness" of the water (cohesion) pulls the whole liquid column up.

Memory Aid: Cohesion = Comrades (water sticking to water). Adhesion = Apart (water sticking to something else/the wall).

4. Mass Transport in Plants: Phloem

The phloem transports organic substances (like sucrose) from where they are made (sources, like leaves) to where they are used or stored (sinks, like roots or growing fruits). This process is called translocation.

The Mass Flow Hypothesis

This is the leading theory for how translocation works:
1. At the Source: Sucrose is actively transported into the phloem. This lowers the water potential, so water moves in by osmosis. This creates high hydrostatic pressure.
2. At the Sink: Sucrose is used up or stored. This increases the water potential, so water moves out by osmosis. This creates low hydrostatic pressure.
3. The Flow: The pressure gradient pushes the sap from the source to the sink.

Evidence for Mass Flow

Students often find this tricky, but just remember these two experiments:
- Ringing Experiments: If you remove a ring of bark (containing phloem) from a tree, a bulge of sugar-rich fluid forms above the ring. This shows sugars move downwards in the phloem.
- Tracer Experiments: If you give a plant radioactive carbon dioxide (\(^{14}CO_2\)), it will make radioactive sugars. You can then use X-ray film to track these sugars moving through the phloem.

Common Mistake to Avoid: Don't confuse Xylem and Phloem! Xylem is for Water (alphabetically close to X), and Phloem is for Food/Sugars (sounds like F).

Key Takeaway: Plants use physical forces (transpiration/cohesion) for water transport in the xylem and pressure gradients (mass flow) for sugar transport in the phloem.