2.1.2 Features of exchange surfaces which aid passive and active transport

• Biconcave - the biconcave shape of erythrocytes 
• Large Number - the millions of alveoli in the lungs
• Long Extension - the long extension of the Root Hair Cell
• Air Spaces in Spongy Mesophyll - increases surface area in contact with gases

A thin separating surface speeds up the rate of passive or active transport by reducing the distance molecules have to move to pass through the membrane.

• Squamous epithelia cells - root hair cells one cell thick
• Vaculole extending into extension - root hair cells. Speeds up osmosis

The higher the difference in concentration gradiants, the faster the rate of transport.

• CIrculation - rich blood supply to alveoli maintains diff in conc of O2 between alveolus and capillary
• Ventilation - air flow continuously moving into lungs and alveoli maintains diff in conc
• Air Spaces - in spongy mesophyll increases difference in concentration of cell and air space
• Capillaries One Erythrocyte in Diameter - enhances diffusion gradients by reducing the distance molecules have to move. 
  Also the fact erythrocytes are constantly moving through capillaries maintains conc gradients

Palisade Mesophyll

• Tightly Packed
• Top of leaf
• Many Mobile Chloroplasts

Spongy Mesophyll

• Loosly packed
• Air Spaces (^ conc grad)
• Xylem within layer (^ SA in contact with gases)

Root hairs

• Long cell extension (^ SA)
• One Cell Thick (V distance for diff)
• Vacuole extends into extension to speed up osmosis

Capillaries

• Diameter of an erythrocyte - single file erythrocytes (V distance for erythrocytes to move)
• Wall is 1 cell thick
• Erythrocytes constantly moving through (maintains conc grad)

Erythrocytes

• Biconcave (^ SA)
• No nucleus (^ volume to store haemoglobin)
• Diameter close to diameter of capillary (one at a time)

Alveoli

• Large Number (^ SA)
• Squamous Epithelial Cell lining (thin)
• Rich blood supply (maintain conc grad)
• Continuous Airflow - Ventilation (maintain conc grad)