The challenges of size
What you'll learn
- Why larger multicellular organisms need exchange surfaces and transport systems.
- How to calculate and use surface area : volume ratio.
- How the heart, blood vessels, blood, xylem and phloem are adapted for transport.
- How a potometer can investigate factors affecting water uptake in plants.
Why size creates a challenge
All cells need substances to move in and out. For example, cells need oxygen and glucose for respiration, and they must remove waste products such as carbon dioxide and urea.
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
In very small organisms, diffusion across the cell surface can be enough because every part of the organism is close to the outside. As organisms get larger, many cells are buried deep inside the body, so diffusion alone is too slow.
Surface area : volume ratio
Surface area : volume ratio
Surface area : volume ratio compares how much outside surface an organism has with how much internal volume it has. It is often written as surface area : volume.
Surface area controls how much material can cross the outside at once. Volume is linked to how many cells need supplying. As an organism gets bigger, its volume increases faster than its surface area, so its surface area : volume ratio decreases.
The size problem
Large multicellular organisms need specialised exchange surfaces and transport systems because they have a low surface area : volume ratio and longer diffusion distances.
Calculating surface area : volume ratio
A cube-shaped organism has sides of 2 cm. Work out its surface area : volume ratio.
- Find the surface area. A cube has 6 faces, each with area side ×\times× side, so:
- Find the volume. For a cube:
- Compare surface area with volume:
So the surface area : volume ratio is 3 : 1.
Ratio sanity check
If the organism gets bigger, expect the surface area : volume ratio to get smaller, not larger.
Exchange surfaces and transport systems
An exchange surface is a specialised surface where substances move between an organism and its environment. Good exchange surfaces often have:
- a large surface area;
- a thin barrier, giving a short diffusion distance;
- a good blood supply or transport link;
- movement of air or water to maintain a steep concentration gradient.
Examples include the alveoli in the lungs, the small intestine for digested food absorption, and root hair cells in plants.
A transport system moves substances around an organism. In animals, this is mainly the circulatory system. In plants, it is mainly the xylem and phloem.
Substances transported in organisms include:
- oxygen to cells for aerobic respiration;
- carbon dioxide away from respiring cells, or into leaves for photosynthesis;
- water for chemical reactions, support and photosynthesis;
- dissolved food molecules such as glucose and amino acids;
- mineral ions such as nitrates and magnesium ions in plants;
- urea, a waste product made in the liver and carried to the kidneys.
The human circulatory system
The human circulatory system is a double circulatory system. This means blood passes through the heart twice for each complete circuit of the body.
- The pulmonary circulation carries blood between the heart and lungs.
- The systemic circulation carries blood between the heart and the rest of the body.
This is useful because blood can be pumped to the lungs at lower pressure, protecting the delicate exchange surfaces, then returned to the heart and pumped to the body at higher pressure.
Heart structure
The heart is made of cardiac muscle, a muscle tissue adapted to contract rhythmically without getting tired quickly.
The four chambers are:
- right atrium — receives deoxygenated blood from the vena cava;
- right ventricle — pumps deoxygenated blood to the lungs through the pulmonary artery;
- left atrium — receives oxygenated blood from the lungs through the pulmonary vein;
- left ventricle — pumps oxygenated blood to the body through the aorta.
The septum separates the two sides of the heart, preventing oxygenated and deoxygenated blood from mixing. The left ventricle has a thicker muscular wall because it pumps blood all around the body.
Valves
Valves prevent blood flowing backwards. The main heart valves are:
- tricuspid valve — between the right atrium and right ventricle;
- bicuspid valve or mitral valve — between the left atrium and left ventricle;
- semilunar valves — at the exits of the ventricles into the pulmonary artery and aorta.
Blood vessels are adapted to their functions
| Blood vessel | Main job | Key adaptations |
|---|---|---|
| Arteries | Carry blood away from the heart | Thick muscular and elastic walls to withstand high pressure; relatively narrow lumen |
| Veins | Carry blood back to the heart | Wider lumen; thinner walls; valves to prevent backflow |
| Capillaries | Exchange substances with cells | Walls one cell thick; very narrow lumen; huge network close to cells |
Why blood is slow in capillaries
Blood is not slow in capillaries simply because each capillary is narrow. It slows because there are very many capillaries, giving a very large total cross-sectional area. Slow flow gives more time for diffusion.
Blood as a transport tissue
Blood contains plasma, red blood cells, white blood cells and platelets. For this topic, focus on plasma and red blood cells.
Plasma is the liquid part of blood. It carries dissolved substances, including carbon dioxide, urea, glucose, amino acids, hormones and mineral ions. It also transports heat around the body.
Red blood cells are adapted to carry oxygen because they:
- contain haemoglobin, a protein that binds to oxygen;
- have a biconcave disc shape, giving a large surface area;
- have no nucleus, leaving more space for haemoglobin;
- are flexible, so they can squeeze through capillaries.
Plant transport: roots, xylem and phloem
Plants also face the challenge of size. Roots take up water and mineral ions, while leaves make sugars by photosynthesis. These substances must be moved around the plant.
Root hair cells
Root hair cells are adapted to absorb water and mineral ions from the soil.
They have a long hair-like extension, which gives a large surface area. Water enters by osmosis, the movement of water from a more dilute solution to a more concentrated solution through a partially permeable membrane. Mineral ions may be taken up by active transport, which uses energy from respiration to move substances against their concentration gradient.
Xylem
Xylem carries water and mineral ions from roots to leaves. Xylem vessels are adapted because they are:
- long hollow tubes, so water can flow through them;
- made from dead cells with no end walls;
- strengthened with lignin, which helps stop them collapsing.
Water movement through xylem is linked to transpiration from the leaves.
Phloem
Phloem carries dissolved sugars, mainly sucrose, from where they are made to where they are used or stored. This movement is called translocation.
Phloem is made of living cells. Sieve tube cells allow sap to flow, and companion cells provide energy for transport.
Transpiration and translocation
Transpiration is the loss of water vapour from plant leaves, mainly through stomata. Translocation is the movement of dissolved sugars through phloem from sources to sinks.
A source is a part of the plant that releases sugar, such as a leaf doing photosynthesis. A sink is a part that uses or stores sugar, such as roots, fruits or growing shoots.
Stomata
Stomata are tiny pores, usually on the underside of leaves. They allow carbon dioxide to enter for photosynthesis, but water vapour can also diffuse out. Each stoma is controlled by two guard cells, which can open or close the pore.
Factors affecting water uptake
A plant’s rate of water uptake usually increases when transpiration increases.
Important factors include:
- Light intensity — more light usually opens stomata for photosynthesis, increasing transpiration.
- Air movement — moving air removes moist air from around the leaf, keeping a steep diffusion gradient.
- Temperature — higher temperature gives water molecules more kinetic energy, increasing evaporation and diffusion.
Very high temperatures
At very high temperatures, a plant may close its stomata to reduce water loss. In that case, transpiration may decrease instead of continuing to rise.
Using a potometer
A potometer estimates the rate of water uptake by a leafy shoot. As the shoot takes up water, an air bubble moves along a capillary tube. You measure the distance moved in a set time.
A potometer can test factors such as light intensity, wind speed or temperature. Keep other variables controlled, such as leaf surface area, time measured and starting bubble position.
What a potometer measures
A potometer measures water uptake, not transpiration directly. It is used as an estimate because most water taken up by a plant is lost by transpiration.
Calculating rate of water uptake
In a potometer, an air bubble moves 48 mm in 6 minutes. Calculate the rate of water uptake in mm per minute.
- Choose the rate equation:
- Substitute the values:
- Calculate the rate:
You may also be asked for percentage change in mass, especially if a plant shoot or leaf is weighed before and after.
percentage change=change in massoriginal mass×100\text{percentage change} = \frac{\text{change in mass}}{\text{original mass}} \times 100percentage change=original masschange in mass×100In the exam
- Link size to surface area : volume ratio and diffusion distance when explaining why transport systems are needed.
- For heart questions, state the route clearly: body → vena cava → right side → lungs → left side → aorta → body.
- In potometer questions, say which factor is changed, which variables are controlled, and calculate rate using change divided by time.
Check yourself
- Why does a large organism need a transport system even if diffusion still happens?
- How are arteries, veins and capillaries each adapted to their functions?
- How would increasing air movement affect water uptake in a potometer?
