โครงสร้างและหน้าที่ของพืช

• Plant structure and function are intricately linked
• Plants are multicellular eukaryotes with tissues and organ systems

Plant Cells

• Plant cells have unique structures
• Cell wall: Rigid outer layer made of cellulose; provides support and protection
• Chloroplasts: Site of photosynthesis; contain chlorophyll
• Central vacuole: Large storage organelle; regulates turgor pressure
• Plasmodesmata: Channels through cell walls that connect adjacent cells

Plant Tissues

• Plants have three main tissue systems: dermal, vascular, and ground

Dermal Tissue

• Outer protective layer
• Epidermis: Single layer of cells that covers the plant; secretes a waxy cuticle to prevent water loss
• Guard cells: Specialized epidermal cells that regulate the opening and closing of stomata for gas exchange
• Trichomes: Hair-like outgrowths that protect the plant and reduce water loss

Vascular Tissue

• Transports water, minerals, and sugars throughout the plant
• Xylem: Transports water and minerals from the roots to the rest of the plant; composed of dead cells called tracheids and vessel elements
• Phloem: Transports sugars from the leaves to the rest of the plant; composed of living cells called sieve-tube elements and companion cells

Ground Tissue

• Fills the space between the dermal and vascular tissues
• Parenchyma: Thin-walled cells that perform various functions, including photosynthesis, storage, and secretion
• Collenchyma: Elongated cells with thick cell walls that provide support and flexibility
• Sclerenchyma: Cells with very thick cell walls that provide rigid support

Plant Organs

• Roots, stems, and leaves are the main organs of plants

Roots

• Anchor the plant and absorb water and minerals from the soil
• Root hairs: Extensions of epidermal cells that increase the surface area for absorption
• Root cap: Protective layer of cells that covers the root tip
• Vascular cylinder: Central core of the root that contains the xylem and phloem
• Cortex: Ground tissue that surrounds the vascular cylinder

Stems

• Support the plant and transport water, minerals, and sugars between the roots and leaves
• Nodes: Points on the stem where leaves are attached
• Internodes: Regions of the stem between nodes
• Buds: Structures that can develop into new leaves, stems, or flowers
• Vascular bundles: Strands of xylem and phloem that run the length of the stem

Leaves

• Main site of photosynthesis in plants
• Blade: Flat, expanded part of the leaf
• Petiole: Stalk that attaches the leaf to the stem
• Veins: Vascular bundles that run through the leaf
• Mesophyll: Ground tissue of the leaf that contains chloroplasts
• Stomata: Pores on the surface of the leaf that allow for gas exchange

Plant Functions

• Photosynthesis, transport, and reproduction are essential plant functions

Photosynthesis

• Process by which plants convert light energy into chemical energy
• Occurs in chloroplasts
• Requires carbon dioxide, water, and light
• Produces glucose and oxygen

Transport

• Water and minerals are transported from the roots to the rest of the plant through the xylem
• Sugars are transported from the leaves to the rest of the plant through the phloem
• Transpiration: Evaporation of water from the leaves that helps to pull water up the xylem

Reproduction

• Plants reproduce sexually and asexually
• Sexual reproduction involves the fusion of gametes (sperm and egg)
• Asexual reproduction involves the production of new plants from vegetative parts of the parent plant

Plant Development

• Plants have indeterminate growth, meaning they can continue to grow throughout their lives
• Meristems: Regions of actively dividing cells that allow for plant growth
• Apical meristems: Located at the tips of roots and shoots; responsible for primary growth (elongation)
• Lateral meristems: Located in the vascular and cork cambium; responsible for secondary growth (thickening)

Plant Hormones

• Plant hormones regulate plant growth and development
• Auxins: Promote cell elongation and apical dominance
• Cytokinins: Promote cell division and delay senescence
• Gibberellins: Promote stem elongation, seed germination, and flowering
• Abscisic acid: Promotes dormancy and closes stomata
• Ethylene: Promotes fruit ripening and leaf abscission

Plant Responses to the Environment

• Plants respond to various environmental stimuli, such as light, gravity, and touch
• Phototropism: Growth in response to light; mediated by auxins
• Gravitropism: Growth in response to gravity; roots grow downwards, shoots grow upwards
• Thigmotropism: Growth in response to touch; allows plants to climb