Module Three 13.1 How do dissolved materials get into and out of animals and plants?

Specialised organ systems

To make transporting materials more efficient, organ systems have become specialised. This usually involves increasing the surface area which allows more molecules to touch the cell membrane at one time. Examples in the human body include the alveoli in the lungs (see above) and the villi in the small intestine.

Most of the water and minerals which enter a plant root are absorbed by root hair cell.

Carbon dioxide enters the plant through holes in the leaves called stomata. It enters the leaves and cells by simple diffusion. Diffusion is the movement of liquids or gases from a higher concentration to a lower concentration, the molecules move along a concentration gradient. There is a higher concentration of carbon dioxide gas outside the plant so it moves into the plant where the concentration is lower.

Leaves are well designed for diffusion of gases because they are flat and thin so that gases can move easily across the leaf. One advantage of them being flat is that this increases their surface area; this means that there is more surface area for carbon dioxide to enter. The leaves surface area is further increased by internal air spaces.

Plants need the tiny holes called stomata in their leaves to allow carbon dioxide to diffuse into the plant. The stomata are tiny pores on the underside of a leaf. There are very few on these holes on the topside or upper surface of the leaves. Stomata only open during the day, this is because plants need carbon dioxide during the day to carry out photosynthesis, and they also allow oxygen to escape (the by-product of photosynthesis).

Flowering plants have two separate transport systems, one for water and minerals and the other for nutrients.

The xylem tissue transports water and minerals from the roots to the stem and leaves.

The phloem tissue carries nutrients such as sugars made in the leaves by photosynthesis to the rest of the plant.

Water enters the plant through the roots by a process called OSMOSIS. Water which enters the plant enters through the root hair cells. The root cells are designed so that water absorbed from the soil passes into the XYLEM. Root hair cells on the outside of the root have long arms which grow between soil particles to find water. They also have thin cell walls so that water can enter more easily. They are very delicate cells that are easily destroyed, so new cells continuously replace old ones as the root grows. The diagram on page 1 shows the passage of water from the soil to the XYLEM which is found running through the centre of roots. Remember the surface area of the roots is increased by root hairs, this means that there is more surface area to pick up water.

Xylem tissue is responsible for transporting water throughout the plant. It transports the water from the roots to the stem and the leaves. The water inside the plant cells gives support for young plants, if the plants wilt this is because the plants cells are short of water.

The definition for OSMOSIS is: the movement of water molecules from a less concentrated to a more concentrated solution through a partially permeable membrane. A partially permeable membrane is a barrier that will only allow small molecules through e.g. water molecules. An example of a partially permeable membrane is the cell membrane.

Water molecules will move from a less concentrated to a more concentrated solution until the number of water molecules is balanced each side of the membrane, i.e. there is the same concentration on each side.

The water molecules in the diagram above will move from left to right, until each side has exactly the same concentration.

When water moves into plant cells by osmosis it increases the pressure inside the cell. The cell walls are very rigid and are strong enough to prevent cells in plants from bursting. It is the pressure of the water which keeps the cell rigid; this provides support especially in young plants, where the water pressure in the cells actually keep the plant upright. If the cells are full they are TURGID. If the cells lose water then they are FLACCID. If the cells in a plant are flaccid then the plant will wilt.

Plants store excess carbohydrates as starch in starch grains. The reason for this is that starch is insoluble and sugars are soluble. If the plants stored carbohydrates as soluble sugars the solution would draw a lot of water inside the cells by the process of osmosis. Starch is insoluble so does not cause large amounts of water to accumulate in storage cells as a result of osmosis.

Diffusion - movement of molecules from a high to low concentration, i.e. they move along a concentration gradient. It is a passive process - no energy is needed. Substances are sometimes absorbed against a concentration gradient - they move from a low to high concentration. This requires energy from respiration and is called active uptake. It is important in the roots of plants because it enables the plant to absorb ions from the soil from very dilute solutions. If there was no active uptake then the materials would move in the opposite direction and the plants would be unhealthy.

How are the surfaces of plants adapted for the efficient exchange of gases and solutes?

Leaves. They are flat and wide to give them a large surface area to absorb plenty of light and carbon dioxide, this also makes it easy for gases to diffuse across the leaf and get to all the cells. It has air spaces which increases the surface area so that the gases can get to all the cells. Stomata open during the day when carbon dioxide is needed for photosynthesis, and close at night to prevent the loss of water vapour. Stomata can also close in emergencies when the weather is very hot and dry.

Roots. The roots surface is very thin; this allows easy entry of water and minerals. Every plant has an extensive network of roots so that there is a large surface area for water to enter the plant. To increase the surface area further they are covered with tiny delicate root hairs.

Active transport

Active transport is the movement of a substance across a cell membrane AGAINST the concentration gradient or AGAINST diffusion. The substance moves from a LOW concentration to a HIGH concentration. This is against what should naturally happen and hence needs ENERGY.

Glucose can move from a LOW concentration (on the left) to a HIGH concentration (on the right) by active transport. An example of this happening is in the small intestines where glucose moves from the small intestines and into the blood.

Aerobic respiration takes place in the mitochondria of cells (singular = mitochondrion).


What is the job of the breathing system and why is it important?

The job of the breathing system is to take oxygen into the body and to get rid of carbon dioxide from the body. Cells need the oxygen to be able to release energy for the jobs they do. All cells produce carbon dioxide as a waste product.

The breathing system includes the ribs, rib muscles, diaphragm, lungs, trachea, bronchi, bronchioles and alveoli.

You should be able to label these on a diagram.

Air enters the breathing system through the mouth or nose.

Air then travels down a large tube called the trachea or windpipe.

The trachea then divides into two smaller branches called bronchi (singular = bronchus). Each bronchus leads to each lung.

Each bronchus splits into smaller branches called bronchioles. These bronchioles branch smaller and smaller.

At the end of the bronchioles there are a large number of air sacs called alveoli.

We need to breathe because all of our cells carry out respiration. Respiration is how we make energy for ourselves. Aerobic respiration is respiration using oxygen. Respiration is a chemical reaction where glucose (sugar) and oxygen react together to release energy, carbon dioxide and water.

The breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream and carbon dioxide can pass out of the bloodstream and into the air (see diagram on previous page).

The lungs are in the chest cavity (upper part of the body or thorax). They are protected from damage by the ribcage. They are separated from the lower part of the body (abdomen) by the diaphragm. To make air move into the lungs the ribcage moves out and the diaphragm flattens. This is reversed to make air move out of the lungs. Breathing in and out is called ventilation. (see diagram below)

Why have alveoli?

Alveoli are excellent at doing their job of gas exchange. Their special features include –

A large surface area to ensure maximum gas exchange. If you squashed down the alveoli in a pair of adult lungs and spread them out over a flat surface, they would cover over half a tennis court!

A moist surface. The lining of the alveoli secrete a special fluid. This speeds up the diffusion of oxygen and carbon dioxide.

A rich blood supply. The blood capillaries wrap closely around the alveoli, ensuring rapid gas exchange.

Thin walls. The walls of the alveoli are just one cell thick to make sure the gases only have to pass through membranes easily into the blood.

Ventilation in detail

To breathe in or INHALE –

Muscles between the ribs CONTRACT.

This pulls the ribcage up and outwards.

At the same time, the diaphragm muscles contract.

This causes the diaphragm to flatten.

The movement of the ribs and diaphragm cause an increase in the volume of the chest area (thorax).

This increased volume causes a decrease in the pressure in the thorax.

This decrease in pressure results in air rushing in from HIGH pressure outside of the body to the LOWER pressure inside of the lungs.