Lecture 02 Components of plant cell

Components of Mature Cell

A typical plant cell is formed of protoplasmic (living) and non- protoplasmic contents.

protoplasmic contents

Plastids

There are several different types of plastids,

organelles unique to plants.

Three main types are:

•           Chloroplasts 

•           Chromoplasts 

•           Leucoplasts 

Mature plastids develop from small, green protoplastids in growing, embryonic plant cells.

Chloroplasts

The word chloroplast is derived from the Greek words chloros, which means green, and plast, which means form. Contain chlorophyll-bering units grana embedded in stroma. Found in tissues exposed to light. Responsible for photosynthesis.

Chromoplasts

Contain color due to carotenoid (yellow, orange, red) and xanthophyll (yellow) pigments stored in their vacuoles and oil droplets. Highly concentrated in the petals of animal-pollinated flowers and the skins of animal-dispersed fruit, function in providing color necessary to attract pollinators and seed dispersers.

Leucoplasts

Category of plastid found in plant cells. They are non-pigmented, in contrast to other plastids. Lacking pigments, leucoplasts are not green, so they are located in roots and non-photosynthetic tissues of plants. They may become specialized for storage of starch, lipid or protein and are then known as amyloplasts, elaioplasts, or proteinoplasts respectively.

Special Forms of Plastids

A. Etioplasts

Chloroplasts that have not been exposed to light. They are usually found in flowering plants (Angiosperms) grown in the dark. If a plant is kept out of light for several days, its normal chloroplasts will actually convert into etioplasts. Etioplasts lack active pigment and can technically be considered leucoplasts. High concentrations of etioplasts will cause leaves to appear yellow rather than green.

B. Statoliths

Specialised forms of amyloplast used by plants in detecting and responding to gravity. Statoliths are denser than the cytoplasm and tend to move towards the bottom of the cell, indicating which direction is 'down'. They are found mainly in root tissues.

Stromules

Plastids inside plant cells are often interconnected by a network of microscopic tubules known as stromules.

Non-Prptoplasmic contents

The ergastic substances formed as a result of metabolic activity may be waste products or stored food materials.

A. starch grains

Starch or amylum is a carbohydrate consisting of a large number of glucose units. In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially amyloplasts.

B. Crystals

      Solid waste products that are deposited in plant tissues in the form of calcium salts and silica. In some taxonomic groups the type, morphology and location of these structures is of systematic significance.

Calcium salts are mostly deposited as:

I. Calcium Oxalate Crystals

      Calcium oxalate crystals in plants are formed from endogenously synthesized oxalic acid and calcium from the soil solution in contact with plant roots. Calcium oxalate is a poisonous substance. The poisonous plant (Dieffenbachia) contains crystals and on ingestion can prevent speech and be suffocating. Kidney stone sufferers should not eat plants high in oxalates.

II. Calcium Carbonate Crystals

Deposited in the form of Cystolith ("cavity" and "stone") it is a botanical term for the inorganic concretions, usually of calcium carbonate, formed in a cellulose matrix in special cells, generally in the leaf of plants of certain families.

Meristem

   A meristem is the tissue in all plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place. Differentiated plant cells generally cannot divide. The term meristem was derived from the Greek word merizein, meaning to divide. Meristematic cells are analogous in function to stem cells in animals.

Characteristics of  Meristematic Cells

•           Incompletely or not at all differentiated,

•           Capable of continued cellular division (youthful).

•           Small and protoplasm fills the cell completely.

•           The vacuoles are extremely small.

•           The cytoplasm does not contain differentiated plastids, although they are present in rudimentary form (proplastids).

•           Meristematic cells are without intercellular cavities.

•           The cell wall is a very thin primary cell wall.

Classification of Meristems According to Origin (Promeristem theory)

A. Primary Meristems

•           Protoderm - Lies around the outside of the stem and develops into the epidermis.

•           Procambium - Lies just inside of the protoderm and develops into primary xylem and primary phloem. It also produces the vascular cambium, a secondary meristem.

•           Ground meristem - Develops into cortex and pith. It produces the cork cambium, another secondary meristem.

These meristems are responsible for primary growth, or an increase in length.

B. Secondary Meristems

•           Vascular Cambium - Produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. .

•           Cork Cambium - Gives rise to the periderm which replaces the epidermis.

Classification of Meristems According to Position

A.        Apical Meristems - (Shoot apex) - The source of all above-ground organs, (Root apex) - It is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens and (Floral meristem) - When plants begin flowering, the shoot apical meristem is transformed into an inflorescence meristem which goes on to produce the floral meristem which produces the familiar sepals, petals, stamens, and carpels of the flower.

B. Lateral Meristems - They are involved in lateral growth e.g. Vascular cambium and Cork cambium.

C. Intercalary Meristems - Occur only in monocot (particularly grass) in stems at the base of nodes and leaf blades. Intercalary meristems at the nodes allow for rapid stem elongation, while those at the base of leaf blades allow damaged leaves to rapidly regrow.

Theories of Development and Differentiation of Meristems

Apical Cell Theory

Solitary apical cells occur in many bryophytes and pteridophytes formed all tissues and organs of the plant. The apical cell theory was proposed as the basis for an understanding of the method of growth and morphology in many groups. But the theory was not applicable to seed plants.

Histogen Theory

Under this theory the more or less distinct major regions of the stem and root apex were called histogens (tissue builders). This theory, in contrast to the apical cell theory, placed the origin of axis apices in a group of initials. The histogens were:

The dermatogen: a uniseriate, external layer; formed the epidermis.

The periblem: the region between plerome and dermatogen. Formed the cortex.

The plerome: a central core; formed the pith and primary vascular tissues.

Tunica and corpus Theory

The growing apex of the stem is differentiated into:

Corpus: in the middle of apical meristem, divide to form the central vascular cylinder and cortex.

Tunica: outer enveloping layer around the corpus, divide to form the epidermis and part of the cortex in some cases.

Promeristem theory

The promeristem region is differentiated into:

Protoderm: formed the epidermis in stem and piliferous layer in the root.

Procambium: formed primary phloem and primar xylem.

Ground meristem: formed the cortex and the pith.