Adaptations of Exchange Surfaces in Plants and Animals

Step 1: Calculate the surface area of a 2 cm cube.

• $6 \times (2 \times 2) = 24 \text{ cm}^2$

Step 2: Calculate the volume of the 2 cm cube.

• $2 \times 2 \times 2 = 8 \text{ cm}^3$

Step 3: Calculate the ratio (Surface Area $\div$ Volume).

• $24 \div 8 = 3$. This is expressed as a ratio of 3:1.
• Note: AQA questions may require you to express measurements of very small cells in standard form (e.g., $1.2 \times 10^{-3} \text{ m}$).

Adaptations of the Human Lungs

Every time you take a breath, oxygen travels down into your lungs to reach millions of microscopic air sacs called alveoli. This is where gas exchange occurs.

The lungs contain 300 to 600 million alveoli, which creates a massive surface area of roughly $75$ to $100 \text{ m}^2$. The walls of the alveoli and the surrounding capillaries are both exactly one cell thick, providing a very short diffusion distance.

The alveoli are surrounded by a rich network of capillaries. Constant blood flow carries away oxygen and brings in carbon dioxide, while ventilation moves fresh air in and out.

Together, these two processes ensure there is always a steep concentration gradient. A moist internal lining also allows gases to dissolve before diffusing, increasing efficiency.

Adaptations of the Small Intestine

To get energy from your meal, digested food molecules must cross the gut wall and enter your bloodstream. The small intestine, measuring 5 to 6 metres long, is specially adapted for this massive absorption task.

The internal lining is folded into millions of tiny, finger-like projections called villi. Each epithelial cell on a villus is further covered in microscopic folds called microvilli, increasing the surface area by a factor of roughly 500.

The wall of each villus is only one cell thick, providing a short diffusion pathway into the dense capillary network inside. This rich blood supply rapidly sweeps absorbed nutrients away to maintain a steep concentration gradient.

Some nutrients, like glucose, must be absorbed against their concentration gradient when gut levels fall. To do this, epithelial cells contain numerous mitochondria to release the energy needed for active transport.

Each villus also contains a central vessel called a lacteal, which specifically absorbs fatty acids and glycerol.

Adaptations of Fish Gills

How do fish breathe underwater when water holds far less oxygen than air? They use gills, which are highly efficient exchange surfaces protected under a bony flap called the operculum.

Gills are made of long stalks called gill filaments, which are covered in tiny, flat plates called lamellae. This creates a massive surface area for gas exchange.

The lamellae are one cell thick and packed with capillaries. Water flows over the lamellae in the opposite direction to the blood flowing through the capillaries.

This arrangement is called a counter-current flow system. It ensures the oxygen concentration in the water is always higher than in the adjacent blood, maintaining a steep concentration gradient across the entire length of the gill.

Adaptations of Plant Roots

Unlike animals, plants cannot walk around to hunt for water or nutrients; they must pull everything they need directly from the soil. They do this using specialised root hair cells.

These single-celled extensions of the root epidermis have a long "hair" projection that massively increases their SA:V ratio. They have thin cellulose cell walls to provide a short pathway for diffusion and osmosis.

Root hair cells contain a large permanent vacuole filled with concentrated cell sap. This maintains a low water potential inside the cell, ensuring water constantly moves in from the soil via osmosis across a partially permeable membrane.

Because roots are underground in the dark, they do not contain chloroplasts. However, they are packed with mitochondria to release energy for the active transport of essential mineral ions (like nitrates) from the soil.

Adaptations of Plant Leaves

If you look closely at a typical leaf, you will notice it is incredibly flat and surprisingly thin. This shape maximises the surface area exposed to sunlight and keeps the diffusion distance for gases very short.

Inside the leaf, the spongy mesophyll tissue contains interconnecting air spaces. This creates a large internal surface area, allowing carbon dioxide to quickly reach the photosynthesising cells.

The lower surface of the leaf contains tiny pores called stomata, which allow carbon dioxide to enter and oxygen to exit.

These pores are controlled by pairs of guard cells. Guard cells become turgid (swollen) to open the stomata during the day, and become flaccid (limp) to close them at night to prevent excessive water loss.