# The Need for Transport Systems in Animals ## Why Organisms Need to Exchange Substances - Every living organism must exchange materials with its surroundings - **Oxygen and nutrients** need to be taken in from the environment - **Waste products** generated by metabolism need to be removed - These exchanges occur at specific locations called **exchange sites** - In humans: lungs (for gases), small intestine (for nutrients) - In plants: roots (for water and minerals), leaves (for gases)

A substance has only truly entered or left an organism once it crosses the cell surface membrane

## Small Organisms: Diffusion is Sufficient - Single-celled organisms like **amoeba** can exchange substances directly with their environment - They possess a **large surface area to volume ratio** - The **diffusion distances** are very short, so molecules reach all parts of the cell quickly - They have **low metabolic demands** due to relatively low activity levels - Diffusion alone is fast enough to meet all their metabolic needs ## Why Large Organisms Need Transport Systems Larger, multicellular organisms require specialised **mass transport systems** for three key reasons: ### 1. Increasing Transport Distances - In complex organisms, **exchange sites are located far from other cells** - Example: human lungs are distant from leg muscle cells - This creates **large transport distances** that diffusion cannot cover efficiently - Diffusion alone would be far too slow to meet cellular metabolic requirements - Cells would die waiting for oxygen or nutrients to arrive ### 2. Decreasing Surface Area to Volume Ratio

Surface area refers to the total area exposed to the environment; volume refers to the total internal space of an organism

- As organisms increase in size, their **SA:V ratio decreases** - This is because volume increases much faster than surface area | Dimension | Surface Area | Volume | SA:V Ratio | |-----------|--------------|--------|------------| | 1 cm cube | $6 \text{ cm}^2$ | $1 \text{ cm}^3$ | $6:1$ | | 2 cm cube | $24 \text{ cm}^2$ | $8 \text{ cm}^3$ | $3:1$ | | 3 cm cube | $54 \text{ cm}^2$ | $27 \text{ cm}^3$ | $2:1$ | The relationship between organism size and SA:V ratio is clearly shown below:

- A decreasing SA:V ratio means: - **Less surface area** available for absorption of nutrients and gases - **Longer diffusion distances** to reach internal cells and tissues - Simple diffusion becomes inadequate for meeting cellular needs

Calculate SA:V ratio by dividing surface area by volume: $\text{SA:V} = \frac{\text{surface area}}{\text{volume}}$

### 3. Increasing Levels of Activity - Larger organisms are typically **more physically active** than smaller ones - They contain vastly more cells than single-celled organisms - More cells means a **higher level of metabolic activity** overall - Greater demand for oxygen and glucose for respiration - More waste products (like carbon dioxide) that need removing - This increased demand means diffusion cannot supply or remove substances quickly enough ## Mass Transport Systems: The Solution - Larger organisms have evolved **specialised mass flow transport systems** - These enable efficient transport of nutrients and waste products

Mass flow is the bulk movement of materials in a specific direction, involving some form of force or pressure

### Key features of mass transport systems: - **Diffusion still occurs** but only at specific exchange sites - At the start of the transport route (e.g. lungs) - At the end of the route (e.g. body tissues) - Between cells and their immediate fluid surroundings ### Functions of mass transport systems: 1. **Bring substances quickly** from one exchange site to another - Example: oxygen from lungs to muscle tissue in seconds rather than hours 2. **Maintain diffusion gradients** at exchange sites and between cells and their fluid surroundings - By constantly delivering fresh substances and removing waste - This keeps concentration gradients steep for efficient diffusion 3. **Ensure effective cell activity** by keeping the immediate fluid environment of cells within a suitable metabolic range - Prevents buildup of toxic waste products - Maintains optimal pH and nutrient levels ## Example: The Mammalian Circulatory System - The circulatory system in mammals is a prime example of a mass transport system - **One-way flow of blood** within blood vessels carries: - Essential nutrients (glucose, amino acids, fatty acids) - Oxygen for respiration - Hormones for cell signalling - Removes carbon dioxide and other waste products - The system connects exchange sites: - **Lungs** (gas exchange with atmosphere) - **Small intestine** (nutrient absorption) - **Kidneys** (waste removal) - **All body tissues** (via capillary networks)

Mass transport systems in plants (xylem and phloem) are equally essential for their survival as large, multicellular organisms

## Comparing SA:V Ratios for Different Shapes The SA:V calculation differs depending on the shape of the cell or organism: ### Cube - Surface area: $(l \times l) \times 6$ sides - Volume: $l^3$ - Example: If $l = 4$ cm - SA = $(4 \times 4) \times 6 = 96 \text{ cm}^2$ - V = $4^3 = 64 \text{ cm}^3$ - SA:V = $96:64 = 1.5:1$ ### Cuboid - Surface area: $2(lh + lw + hw)$ - Volume: $l \times w \times h$ - Example: If $l = 4$ cm, $w = 2$ cm, $h = 1$ cm - SA = $2((4 \times 1) + (4 \times 2) + (1 \times 2)) = 28 \text{ cm}^2$ - V = $4 \times 2 \times 1 = 8 \text{ cm}^3$ - SA:V = $28:8 = 3.5:1$ ### Cylinder - Surface area: $2\pi r^2 + 2\pi rh$ (two circles + curved rectangle) - Volume: $\pi r^2h$ - Example: If $r = 2$ cm and $h = 6$ cm - SA = $2\pi(2)^2 + 2\pi(2)(6) = 8\pi + 24\pi = 32\pi \text{ cm}^2$ - V = $\pi(2)^2(6) = 24\pi \text{ cm}^3$ - SA:V = $32:24 = 1.33:1$

Different cell shapes have evolved to maximise surface area for their specific function, such as the biconcave disc shape of red blood cells

## Summary: Why Size Matters - **Small organisms** (high SA:V ratio) - Short diffusion distances - Low metabolic demands - Can rely on simple diffusion - **Large organisms** (low SA:V ratio) - Long diffusion distances - High metabolic demands - Require specialised mass transport systems The evolution of transport systems was crucial for allowing organisms to grow larger and become more complex and active.