The cells of multicellular organisms may differentiate and become adapted for specific functions. Tissues are aggregations of similar cells; organs are aggregations of tissues performing specific physiological functions.
Organs are organised into organ systems, which work together to form organisms.
Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.
Animal tissues and organs (1)
A tissue is a group of cells with similar structure and function. Examples of tissues include:
■ muscular tissue, which can contract to bring about movement
■ glandular tissue, which can produce substances such as enzymes and hormones
■ epithelial tissue, which covers some parts of the body.
Organs are made of tissues. One organ may contain several tissues. The stomach is an organ that contains:
■ muscular tissue, to churn the contents
■ glandular tissue, to produce digestive juices
■ epithelial tissue, to cover the outside and the inside of the stomach.
Animal tissues and organs (2)
Organ systems are groups of organs that perform a particular function. The digestive system is one example of a system in which humans and other mammals exchange substances with the environment. The digestive system includes:
■ glands, such as the pancreas and salivary glands, which produce digestive juices
■ the stomach and small intestine, where digestion occurs
■ the liver, which produces bile
■ the small intestine, where the absorption of soluble food occurs
■ the large intestine, where water is absorbed from the undigested food, producing faeces
Plant tissues and organs
Plant organs include stems, roots and leaves.
Examples of plant tissues include:
■ epidermal tissues, which cover the plant
■ mesophyll, which carries out photosynthesis
■ xylem and phloem, which transport substances around the plant.
Unlike the human digestive system, plants create their own "food" via photosynthesis. The finished product of photosynthesis gives glucose (as well as oxygen released). Glucose turns into starch which is insoluble so it cannot be directly digested. Starch is carried around the plant to do various functions.
Green plants and algae use light energy to make their own food. They obtain the raw materials they need to make this food from the air and the soil. The conditions plants are grown in can be changed to promote growth.
Photosynthesis is summarised by the equation:
light energy is the source of energy for reaction
carbon dioxide + water ------------------------------------------------------------------> glucose + oxygen
b) During photosynthesis:
■ light energy is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and algae
■ this energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugar (glucose)
■ oxygen is released as a by-product.
The rate of photosynthesis may be limited by: SHORTAGE OF LIGHT, LOW TEMPERATURE ,SHORTAGE OF CARBON DIOXIDE OR LACK OF WATER.
What also must be beared in mind is even though by increasing the factors above increasing the factors will only increase photosynthesis to an extent and too much heat will actually kill the plant. All of these factors are required at a level for photosynthesis to take place.
The glucose produced in photosynthesis may be converted into insoluble starch for storage. Plant cells use some of the glucose produced during photosynthesis for respiration.
Some glucose in plants and algae is used:
■ to produce fat or oil for storage
■ to produce cellulose, which strengthens the cell wall
■ to produce proteins.
To produce proteins, plants also use nitrate ions that are absorbed from the soil.
aggregation: Several things grouped together to make a whole.