Comprehensive Study Notes on Conditions of Movement, Skeletal Systems, and Plant Movements

Conditions of Movement and Structural Skeletal Systems

Movement in living organisms requires specific structural and functional prerequisites. For an organism to perform coordinated movement, two fundamental conditions must be met. First, the living organism must possess a skeleton composed of articulated parts or segments. Second, the organism must be capable of maintaining its balance during movement.

Skeletons are categorized based on their position relative to the organism's body into endoskeletons and exoskeletons.

An endoskeleton (الهيكل الداخلي) is an internal support structure located inside the body. Endoskeletons are further subclassified into two main structural types:

1. Bony Skeleton: Formed of hard bone tissue, found in organisms such as humans.

2. Cartilaginous Skeleton: Formed of flexible cartilage tissue, found in organisms such as sharks.

An exoskeleton (الهيكل الخارجي) is an external supporting structure that covers the outside of the body. Exoskeletons are characteristic of the phylum Arthropoda (المفصليات / مفصليات الأرجل). Typical examples of organisms possessing an exoskeleton include shrimps, crabs, and spiders.

Overview of the Five Types of Plant Movement

Plants exhibit specialized forms of movement in response to internal physiological cues and external environmental stimuli. The five primary types of movement observed in plants are:

1. Movement by Touch (e.g., Mimosa).

2. Sleeping and Awakening Movement (e.g., Mimosa and legumes).

3. Tropism (الانتحاء), which includes Phototropism (response to light), Geotropism (response to gravity), and Hydrotropism (response to water / $H_2O$).

4. Haptotropism or Pulling Movement (البركة بالشد), which operates either by tendrils or by roots.

5. Cytoplasmic Streaming (الانسياب السيتوبلازمي).

Movement by Touch and Sleeping and Awakening Movement

Both touch movement and sleeping/awakening movements rely on specialized anatomical structures within compound leaves. A typical compound leaf structure consists of leaflets (وريقات), rachises (including a primary rachis and secondary rachises), and swollen hinge-like bases called pulvini (singular: pulvinus). The leaf features a primary swollen structure (primary pulvinus) at the base of the primary rachis, as well as secondary pulvini at the bases of secondary rachises and individual leaflets.

Touch movement occurs when external contact or mechanical stimulation is applied directly to the leaflets, causing them to fold rapidly.

Sleeping and awakening movement is a diurnal rhythm seen in plants like Mimosa and various legumes, where leaves respond to light and darkness.

Physiological Mechanism of Pulvinar Movement:

The physiological mechanism underlying both touch and sleeping/awakening movements depends directly on changes in turgor pressure driven by the diffusion of water ($H_2O$). When stimulated, water ($H_2O$) diffuses out from the cells of the pulvini into the neighbouring tissues. The loss of water causes the pulvini to lose turgidity and shrink. As a result of this localized shrinkage, the plant structures experience structural collapse, causing the leaflets and rachises to droop, hang downwards, or collapse, making the plant appear wilted.

Plant Tropisms and Hormonal Regulation by Auxins

Tropism (الانتحاء) is defined as the occurrence of curvature in specific parts of a plant—namely the stem and the root—in response to asymmetrical environmental stimuli.

Tropisms are chemically directed and regulated by plant hormones known as auxins (أوكينات). Auxins are produced and secreted primarily at the apex or tip of the coleoptile, which corresponds to the terminal bud (القمة النامية).

Phototropism

Phototropism is the directional growth curvature of plant organs in response to a directional light stimulus.

When a plant grown in a glass beaker receives equal distribution of light from all surrounding sides, auxins remain uniformly distributed across both sides of the stem and root apex. Because auxin concentration is equal on all sides, uniform growth occurs and no curvature is produced.

When light shines unilaterally (from one side only), auxins diffuse away from the lighted side and accumulate ($+$) on the darkened side of both the stem and the root.

Effect on the Stem: The stem is positively phototropic ($+$). In stems, a high accumulation ($+$) of auxins speeds up the rate of cell growth and elongation. Consequently, the cells on the darkened side elongate much faster than those on the lighted side, causing the stem to curve and bend directly towards the light source.

Effect on the Root: The root is negatively phototropic ($-$). In roots, a high accumulation ($+$) of auxins produces an inhibitory effect, stopping or slowing down cell growth on the darkened side. Meanwhile, the lighted side, having a lower auxin concentration suitable for root elongation, continues to grow normally. As a result of this differential growth, the root curves away from the light source.

Geotropism

Geotropism is the growth movement of plant parts in response to the force of gravity.

When a plant is positioned vertically upright, the stem grows straight upwards away from gravity, and the root grows straight downwards towards gravity.

When a plant is placed horizontally, gravity causes auxins to migrate and accumulate ($+$) along the lower side of both the stem and the root.

Stem Response (Negative Geotropism, $-$): The lower side of the horizontal stem contains an accumulation ($+$) of auxins. High auxin concentration in stem tissue stimulates cell elongation, speeding up the rate of growth on the lower side relative to the upper side (which grows normally at a lower auxin level). This causes the stem to bend and grow upwards, away from gravity.

Root Response (Positive Geotropism, $+$): The lower side of the horizontal root also accumulates ($+$) auxins. However, in root tissues, high auxin concentration leads to growth inhibition, effectively stopping cell elongation on the lower side. The upper side, possessing a lower, non-inhibitory auxin concentration, grows normally. This uneven growth forces the root to curve and grow downwards, toward gravity.

Hydrotropism

Hydrotropism is the growth movement of plant roots in response to a moisture or water ($H_2O$) gradient in the surrounding soil.

The root is positively hydrotropic ($+$ Hydrotropic).

When soil moisture is distributed unevenly, auxins accumulate ($+$) on the side of the root that directly faces the water ($H_2O$) source.

Because elevated auxin concentrations inhibit cell elongation in root tissue, cell growth is stopped or significantly reduced on the side facing the water. Conversely, the side away from the water ($H_2O$) contains lower auxin levels and grows normally. This differential growth rate causes the root to bend and extend directly toward the moisture source in the soil.

Haptotropism and Cytoplasmic Streaming

Haptotropism (Pulling Movement)

Haptotropism (البركة بالشد), commonly referred to as pulling movement, is a tactile growth movement where plant parts attach to or pull against external surfaces or substrates. It is divided into two primary modes:

1. Pulling by Tendrils: Exhibited by climbing plants where specialized tendrils coil around solid supports, pulling the stem upright.

2. Pulling by Roots: Exhibited by corms and bulbs, where specialized contractile roots contract vertically to pull the underground storage organs down to an optimal soil depth.

Cytoplasmic Streaming

Cytoplasmic streaming (الانسياب السيتوبلازمي) is an intracellular movement characterized by the continuous, unidirectional movement or flowing of the cytoplasm within living plant cells. This movement is essential for transporting nutrients, metabolites, organelles, and chemical signals throughout the cellular environment.