Lesson 5: Properties and Functions of the Living Cells

Another important characteristic of cell/living things is ability to root or respond to and detect changes in their environment. .

Unit 1: Irritability
Irritability is simply response to stimulus. Stimulus represents environmental changes that affect the normal functioning of the cell organism. The Irritability can be defined as the ability of the cell to detect and respond (react) to external stimuli.

Unit 2: Types of Responses
(i) Tropism
This response involves the movement of some part of the organism in response to a stimulus. The stimuli include; light, gravity, touch, water and chemical. Tropism is a directional movement of some part of a non-motile organism in response to a stimulus. If the movement is towards the source of stimuli, it is called positive but if the movement is away from source of the stimulus it is termed negative.

Examples of tropic responses made by plant include;
(i) Phototropism — Response to light
(ii) Hydrotropism — Response to water
(iii) Geotropism — Response to force of gravity
(iv) Chemotropism — -Response to (chemical, acid, base)

(ii) Taxis: is the directional movement of a motile organism in response to a stimulus. This always involves the movement of a whole organism in response to external stimulus like water, gravity, light and chemical. Example can be seen in Euglena, Paraniecium moving away from light effect.

The stimuli that bring about tactic movement are:
(a) Light — phototaxis
(b) Chemical — chemotaxis

(iii) Nastism: this is a non-directional movement made by part of a stationary (non-motile) plant in response to an external stimulus. Movement of part is not controlled by the stimulus. Example is found in sensitive plant (Mimosa pudica) which rapidly closes its leaf at a touch.

Other examples are sunflower that follows the movement of the sun, morning glory plant etc.

Examples of nastic movements are:
(i) Thermonasty — response, to changes in temperature.
(ii) Photonasty : response to changes in light

Unit 3: Movement
As a characteristic of living things, movement involves change of position by the cell or organism. Movement is a form of activity that results in change of form. There are many kinds of movement and ways bringing about these movements are known.

Some of the movements are explained below:
(i) Cyclosis
In some unicellular organism, it is possible to observe cytoplasm flowing from one part of the cell to the other. This is called cyclosis or protoplasmic streaming. This movement helps to move or circulate materials within a cell. e.g. movement of food vacuole in paramecium.

(ii) Amoeboid movement
This impact the way and manner amoeba moves. This kind of movement is also found in white blood corpuscles. It involves change of state of the cytoplasm from gel to the sol. The formation of pseudopodium follows this trend.

(iii) Organelles for movement
Single celled organisms possess specialised organelles that helps them to move.

The organelles include;
(i) Pseudopodia.
(ii) Cilia.
(iii) Flagella.

Scroll Down to Select Page 7 for the next topic – Lesson 6: Reproduction

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