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1.8 Transport of Substances in Plants

Learning Objective :
Synthesising the concept of transport of substances in plants.

Learning Outcomes :
A student is able to :

state what translocation is,
explain the importance of translocation in plants,
describe the process of transpiration,
explain the importance of transpiration,
state external conditions affecting transpiration.

1. Translocation

The two-directional transport of dissolved organic solutes in the phloem is known as translocation.
Translocation is important to plants because organic substances such as sugars and amino acids are transported from the leaves to the storage organs , to the growing regions for growth and development and to the cells for metabolism. This is important for the survival of the plant.

2. Transpiration

Transpiration is the loss of water vapour through the evaporation in plants (almost 90% of water contained in a plant).
Water is lost through the stomata of leaves.

Stomata control the rate at which transpiration occurs. The opening of the stomata is controlled by guard cells that gain and loss water, which close and open the stomata respectively.
When the water pressure in the guard cells becomes greater than in the surrounding cells, the stomata open allowing transpiration.

Transpiration is important to the plant because :

it helps in the absorption and transport of water and mineral ions from the roots to different parts of the plant.
it helps to cool the plant.
it helps to supply water to all plant cells for metabolic processes.
it helps to prevent plants from wilting by helping them to maintain cell turgidity.

3. The process of transpiration

Water is lost from the external surfaces of the mesophyll cells of the leaves by evaporation.
The air spaces in the mesophyll are saturated with water vapour.
The air in the atmosphere outside the stomata is less saturated with water.
As a result, water vapour in the air spaces of the leaf diffuses from the plant cells into the atmosphere through the stomata.
The movement of air outside the leaf carries water vapour away from the stomata.
The loss of water from a mesophyll cell makes the cell hypertonic to an adjacent cell.
Water from the adjacent cell diffuses into the mesophyll cell by osmosis and this in turn draws water from another adjacent cell into this cell.
Water continues to diffuse from neighbouring cells into the adjacent cells.
Finally, water is drawn from the xylem vessels in the veins.
A pulling force is created to pull water up the xylem vessels as a result of the evaporation of water vapour and this is known as transpirational pull.

4. Factors affecting the rate of transpiration

Air movement : An increase in air movement increases the rate of transpiration. Thus, transpiration is higher on a windy day.

Temperature : An increase in temperature increases the rate of transpiration.

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Light intensity : An increase in light intensity increases the rate of transpiration. Thus, transpiration is high during the day and radically drop at night.

Relative humidity : High humidity surrounding the leaves reduces the rate of transpiration. The higher the humidity of the surrounding atmosphere, the lower is the rate of transpiration.

The various types of immunity

Immunity is defined as the ability of the body to defense against pathogens.
There are two different types of immunity. They are natural immunity and acquired immunity.
Acquired immunity can be naturally acquired or artificially acquired immunity.
Naturally acquired or artificially acquired immunity is further divided into two different types : active immunity and passive immunity.

Differences between active immunity and passive immunity

Artificially acquired active immunity

Immunity obtained through an injection of vaccine

Artificially acquired passive immunity

Immunity obtained through an injection of serum which contains specific antibody.

1.4 Lymphatic System

Learning Objective :
Synthesising the concept of lymphatic system

Learning Outcomes :
A student is able to :

describe the formation of interstitial fluid,

state the composition of interstitial fluid,
state the importance of interstitial fluid,
describe the fate of interstitial fluid,
describe the structure of the lymphatic system,
explain how the lymphatic system complements the circulatory system.

1. How the interstitial fluid is formed :

The higher hydrostatic pressure at the arterial end of the capillaries forces some fluid out through the capillary walls into the interstitial spaces which are found between the cells.
This fluid is known as interstitial or tissue fluid.
The interstitial fluid has the same composition as the blood plasma but does not contain erythrocytes, platelets and large protein molecules.

2. Importance of the interstitial fluid

It forms the internal environment of the body.
It bathes the cells and supplies them with their requirements such as oxygen and nutrients.
It receives excretory waste products such as carbon dioxide and urea from the cells.
It helps to keep the internal environment of the body within a normal range.

3. What happens to the interstitial fluid?

90% of the interstitial fluid flows back into the veins.
10% of the interstitial fluid enters the lymphatic capillaries and becomes lymph.
Lymph is similar to blood plasma and has no erythrocytes, platelets and protein molecules but it contains a higher number of lymphocytes.
Lymph is carried by the lymphatic vessels and finally by the thoracic duct.
The thoracic duct finally drains the lymph into the left subclavian vein and right vein and back into the bloodstream.

Role of the lymphatic system

Collects the interstitial fluid and returns it to the circulatory system.
Transports fat from the villi of the ileum to the blood circulatory system.
Lymph nodes filter out bacteria and other foreign particles. Phagocytes engulf and destroy foreign particles.
Lymphocytes in the lymph nodes produce antibodies.

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1.3 Mechanism of Blood Clotting

Learning Objective :
Understanding the mechanism of blood clotting

Learning Outcomes :
A student is able to:

explain the necessity for blood clotting at the site of damaged blood vessels,
explain the mechanism of blood clotting,
predict the consequences of impaired blood clotting mechanism in an individual.

Necessity of blood clotting :

to prevent excessive blood loss from the body
to prevent blood pressure from falling to a low level in order to maintain proper blood circulation
to prevent entry of microorganisms and foreign particles into the body

Mechanism of blood clotting :

When an injury causes a blood vessel wall to break, platelets are activated. They change shape from round to spiny, stick to the broken vessel wall and each other, and begin to plug the break. They also interact with other blood proteins to form fibrin. Fibrin strands form a net that entraps more platelets and blood cells, producing a clot that plugs the break.

When a wound starts bleeding on our bodies, an enzyme called thromboplastin that is released from damaged tissue cells combines with the calcium and prothrombin in the blood. As a result of the chemical reaction, the resulting mesh of threads form a protective layer, which solidifies eventually. The top layer of cells eventually die, becoming cornified, so forming the scab. Underneath the scab, or protective layer, new cells are being formed. When damaged cells are completely replaced, the scab drops off.

Consequences of impaired blood clotting mechanism :

Haemophiliacs are people who suffer from a condition of impaired clotting mechanism known as haemophilia due to lack of certain clotting factors. This causes serious bleeding and can bring about death.

A blood clot (thrombus) that forms in a artery may cause blockage of the artery, a condition known as thrombosis.
When the thrombus gets dislodged and is carried away by blood circulation, it is known as an embolus. The condition is known as embolism.

A stroke is caused by an embolus blocking flow of blood to the brain cells.

Angina is a specific type of pain in the chest caused by inadequate blood flow through the blood vessels (coronary vessels) of the heart muscle (myocardium).

Figure A shows a normal artery with normal blood flow. Figure B shows an artery containing plaque buildup

An angina is caused by partial blockage of the coronary artery while a heart attack or myocardial infarction is caused by a total blockage of the coronary artery.

1.2 Concept of the Circulatory System

Learning Objective :
Synthesising the concept of circulatory system

Learning Outcomes :
A student is able to:

state what a circulatory system is,

state the three components of circulatory system in humans and animals,
state the three components of of circulatory system in humans and animals,
state the medium of transport in humans and animals,
state the composition of human blood
explain the function of blood and haemolymph in transport,
describe the structure of human blood vessels,
explain how blood is propelled through the human circulatory system,
explain briefly how blood pressure is regulated,
compare and contrast the circulatory systems in the following: humans, fish and amphibians,
conceptualise the circulatory system in humans.

The Human Circulatory System consists of :

Heart

Blood
Blood Vessels (arteries / veins / capillaries)

Human blood consists of :

55% blood plasma (a clear, pale yellow liquid), and

45% blood cells

The composition of blood plasma is :

90% water, and
10% dissolved substances such as plasma protein (albumin, globulin, fibrinogen and antibodies), digested food (glucose, amino acids) vitamins, minerals, waste substances, hormones and respiratory gases.

The blood cells are made up of three types, namely,

Erythrocytes (red blood cells)
Leucocytes (white blood cells) which are further divided into granulocytes and agranulocytes, and
Blood platelets (thrombocytes)

Comparison of the three types of blood cells :

Functions of blood in transport :

Transport of oxygen from the lungs to the cells in all parts of the body
Transport of carbon dioxide from the cells to the lungs to be removed.
Transport of absorbed food materials
Transport of excretory waste products
Transport of heat
Transport of hormones
Transport of water to tissues

Structures of human blood vessels :

The blood vessels are made up of three types:

Arteries are blood vessels that carry blood away from heart.
Veins are blood vessels that carry blood toward the heart.
Capillaries are small, thin-walled blood vessels that connect arteries to veins and their purpose is to allow oxygen to diffuse from the blood into the cells and carbon dioxide to diffuse from the cells into the blood

Differences between arteries, capillaries :

Structures of the human heart :

The heart consists of 4 Chambers which makes up 2 pumps :

2 atrium chambers (relatively thin walls / lower pressure)
2 ventricle chambers (relatively thick walls / higher pressure)

Blood Pressure in Humans

Systole vs. Diastole phases
Systolic vs. Diastolic Pressure
Healthy Blood Pressure (120/80)
Hypertension - Consistent High Blood Pressure (140/90 or above)
Blood pressure is measured using a device called a sphygmomanometer

Circulatory systems in insects, fish, amphibians and humans :

Closed circulatory system

Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities.

Open circulatory system

The open circulatory system is common to molluscs and arthropods. Open circulatory systems (evolved in crustaceans, insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood.

1.1 Importance of Having A Transport System in Some Multicellular Organisms

Learning Objective :
Understanding the importance of having a transport system in some multicellular organisms

Learning Outcomes :
A student is able to :

identify the problem that could be faced by multicellular organisms in obtaining their cellular requirements and getting rid of their waste products,
suggest how the problem is overcome in multicellular organisms.

Humans and large organisms possess an efficient circulatory system which is made up of the heart, blood vessels and blood as the medium.
Large organisms need a medium to transport materials around the body. The medium or blood is carried in blood vessels which consist of arteries, veins and capillaries.
This is because large organisms have an increased need for oxygen and nutrients to be supplied to the cells and for waste products to be removed.
At the same time they have a small TSA/V ratio which causes the exchange of substances by direct diffusion to be too slow to meet the requirements of the organism.
The heart functions as a pump that forces blood through the blood vessels through out the whole organism.