Plant Anatomy and Photosynthesis

Stems: Monocots vs. Dicots

• Outline of types of stems, primarily distinguished by their internal vascular tissue arrangement and other characteristics:

  • Monocots (Monocotyledoneae): Plants with a single cotyledon (seed leaf), parallel leaf venation, and fibrous root systems.

  • Dicots (Dicotyledoneae): Plants with two cotyledons, net-like leaf venation, and usually taproot systems.

• A diagram illustrating a longitudinal section (a cut along the length) through the apex of the stem (the tip where growth occurs) will reveal primary growth. This will lead to a cross-section (transverse section) analysis, showing the internal arrangement of tissues perpendicular to the stem's length.

• Definition of a "cross section" as synonymous with "transverse section," both referring to a cut made horizontally through an organ.

Dicot Stems

• Observations on dicot stem structure reveal distinct tissue organization:

  • Presence of epidermis: The outermost protective layer of cells, covered by a waxy cuticle to reduce water loss.

  • Vascular tissue organized in bundles: These bundles, comprising xylem and phloem, are responsible for long-distance transport of water, minerals, and sugars.

  • Cortex: Defined as the tissue located between the epidermis and the vascular tissue, typically composed of parenchyma cells involved in storage and sometimes photosynthesis.

  • Vascular bundles arranged in a ring: This characteristic arrangement forms a "eustele," a central cylinder of vascular tissue, with regions of parenchyma (medullary rays) separating the bundles.

  • Central region identified as pith: Also composed of parenchyma cells, the pith is located at the very center of the stem, often involved in storage of water and nutrients. While functionally similar to cortex, its central location distinguishes it structurally.

• Contrast with dicot roots: Dicot roots typically do not have a centrally located pith; instead, their vascular tissue forms a central core (stele) often star-shaped.

• Transition noted between root (below ground organ for absorption and anchorage) and stem (above ground organ for support and transport) involves significant reorganization of vascular and nonvascular tissues as the plant grows from primary root to primary stem structure.

Vascular Bundle Components

• Discussion of vascular bundle organization within dicots and their primary tissue types:

  • Three main tissue types identified within each vascular bundle:

  1. Sclerenchyma (S), specifically sclerenchyma fibers or sclereids.

  2. Xylem (X).

  3. Phloem (P).

• Characteristics of these tissues:

  • Sclerenchyma and xylem are typically thick-walled cells; sclerenchyma cells and the vessel elements/tracheids of xylem are often lignified (contain lignin), making them robust and providing structural support. Both tend to stain dark red due to the lignified secondary cell walls, indicating dead cells at maturity.

  • Differentiation in size: Xylem tissue contains larger diameter cells (vessel elements) compared to the more uniform and smaller diameter of sclerenchyma fibers, which are often found as a cap over the phloem.

  • Phloem cells are described as thin-walled with only primary cell walls (e.g., sieve tube elements and companion cells). They are living cells at maturity and therefore do not take up the dark, lignified stain.

• Location of the sclerenchyma is always external to the phloem, typically forming a protective cap or sheath, providing additional protection and structural support to the sensitive vascular tissues.

Dicots vs. Monocots

• Dicot stems possess a distinct central pith, clearly demarcated from the vascular ring and cortex, aiding in storage. Monocot stems, in contrast, generally do not exhibit a clearly defined pith and cortex; their ground tissue is often undifferentiated and interspersed with scattered vascular bundles.

• Example: Corn (Zea mays) represents a typical monocot plant. Its stem structure lacks a distinct pith and cortex, with vascular bundles instead being scattered throughout the ground tissue, further distinguishing it from dicots.

• Notable differences also exist in root structures: monocot roots typically possess a large, central pith, while dicot roots usually have a central vascular bundle that may appear star-shaped without a central pith.

Visual Aid of Vascular Bundles

• Informal mnemonic to remember the arrangement of vascular tissues based on a facial analogy:

  • Sclerenchyma as hair or beard (external, protective).

  • Phloem as forehead (upper part of the vascular bundle).

  • Xylem as facial features (eyes, nose, mouth - internal, with larger elements).

Examination Preparation

• Anticipated exam questions may involve identification of specific structures from microscopic images of vascular tissues, requiring precise knowledge of cellular arrangements and tissue types.

• Potential for matching terms with highlighted anatomical structures in exam questions, emphasizing the importance of visual recognition coupled with functional understanding.

Leaves: Focus on Dicot Leaves

• Examination of dicot leaves typically highlights their dorsiventral (upper and lower surfaces differ) structure and optimized internal arrangement for photosynthesis.

• Importance of upper and lower epidermis: These are protective layers. The upper epidermis, often covered by a thick cuticle, minimizes water loss and protects against radiation. The lower epidermis contains most of the stomata for gas exchange.

• Middle layer of the leaf called mesophyll, which splits into two distinct regions:

  1. Palisade layer: Composed of elongated, column-shaped parenchyma cells, tightly packed side-by-side beneath the upper epidermis. These cells are rich in chloroplasts and are thus the primary site of photosynthesis, optimally positioned for light absorption.

  2. Spongy layer: Consists of irregularly shaped parenchyma cells with large intercellular air spaces between them. These spaces are crucial for the efficient diffusion of gases (like CO2 and O2) within the leaf, facilitating gas exchange for photosynthesis and respiration.

• Adaptation of leaf structure for optimal photosynthetic efficiency: The arrangement of palisade cells maximizes light capture, while the spongy layer facilitates gas movement. Chloroplasts within palisade cells are strategically positioned to absorb maximum light energy.

Veins in Leaves

• Leaf veins are extensions of the vascular system into the leaf blade, forming a network for transport and support. They consist of:

  • Xylem: Responsible for the transport of water and dissolved mineral nutrients from the stem to the leaf cells. It moves unidirectionally.

  • Phloem: Transports sugars (produced during photosynthesis) from the leaves to other parts of the plant where they are needed for energy or storage. It moves bidirectionally.

  • Bundle sheath: These are parenchyma cells that surround the vascular tissue in larger veins. They provide structural support and, in some plants (like C4 plants), play a vital role in specialized photosynthetic pathways by concentrating CO_2 around Rubisco.

• Discussion of stomata:

  • Stomata are microscopic pores, largely on the leaf surface, regulated by a pair of guard cells. They facilitate gas exchange (O2 released, CO2 uptake) and serve as the primary site for transpiration (the release of water vapor).

  • The opening and closing of stomata are controlled by the turgor pressure within the guard cells, which changes in response to light, CO_2 concentration, and water availability. Increased turgor makes guard cells bow outwards, opening the pore.

  • Stomatal distribution is typically more prevalent on the lower epidermal surface. This adaptation minimizes water loss through transpiration, as the lower surface is generally cooler and less exposed to direct sunlight, thus reducing the evaporative potential.

Secondary Growth in Plants

• Transition to secondary growth concepts, which involves an increase in the girth or thickness of a plant axis, contrasting with primary growth.

• Distinguishing between primary and secondary meristems:

  • Primary meristems (apical meristems): Located at the tips of shoots and roots, responsible for primary growth, which is growth in length.

  • Secondary meristems (lateral meristems): Responsible for secondary growth, which is growth in thickness, essential for woody plants. The two main types are the vascular cambium and cork cambium.

• Key players in secondary growth:

  • Vascular cambium: A cylindrical meristematic tissue located between the primary xylem and primary phloem. It produces secondary xylem (wood) to the inside and secondary phloem to the outside, contributing significantly to the stem's diameter.

  • Cork cambium (phellogen): A lateral meristem that arises in the cortex (or sometimes epidermis or phloem). It produces cork cells (phellem) toward the outside and phelloderm cells toward the inside, forming the periderm (bark).

Characteristics of Secondary Growth

• Secondary growth is exclusive to dicots (and gymnosperms); monocots typically do not display secondary growth in the described forms due to their scattered vascular bundles and lack of a continuous vascular cambium.

• Functionality of secondary growth:

  • Wood (secondary xylem) and bark (all tissues external to the vascular cambium, including secondary phloem, cork cambium, and cork) are direct products of cambium activity. Wood provides structural support and water transport, while bark offers protection to the stem.

Visual Representation of Secondary Growth

• Examination of growth rings (annual rings) in wood is a common method to determine tree age. These rings represent the layers of secondary xylem produced by the vascular cambium in one growing season.

  • Earlywood (springwood) forms during favorable conditions (spring) with large-diameter vessel elements for efficient water transport.

  • Latewood (summerwood) forms during less favorable conditions (late summer/fall) with smaller, thicker-walled cells for structural support.

• Descriptions of different types of tree bark and varied growth patterns due to secondary growth, reflecting the activity of the cork cambium and the formation of layers of periderm.

Function of Plant Structures

• Transition focus from anatomical structure to function, emphasizing how plant tissues and organs perform vital processes.

  • Photosynthesis described as a crucial function primarily carried out by leaves. This process converts light energy into chemical energy in the form of glucose sugars.

  • Overall process requires inorganic inputs: carbon dioxide (CO2) from the atmosphere, water (H2O) absorbed by roots, and light energy captured by chlorophyll. It yields glucose sugars (C6H{12}O6) for energy and growth, and oxygen (O2) as a byproduct.

Photosynthesis Overview

• Description of essential structures involved:

  • Chloroplasts: The organelles within plant cells where photosynthesis occurs. They possess a double membrane.

  • Thylakoids: Flattened sacs within chloroplasts, arranged into stacks called grana. The light-dependent reactions take place on the thylakoid membranes.

  • Chlorophyll types (a and b): Green pigment molecules embedded in the thylakoid membranes that absorb light energy, primarily in the blue-violet and red regions of the spectrum, reflecting green light.

• Photosynthesis process simplified into two main stages:

  • Light-dependent reactions: Occur on the thylakoid membranes, converting light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

  • Light-independent reactions (Calvin cycle): Occur in the stroma (the fluid-filled space within the chloroplast, outside the thylakoids). These reactions use the ATP and NADPH generated in the light reactions to convert atmospheric CO_2 into glucose sugars.

Photosynthesis Process

• Key reactions and detailed steps:

  • Water (H2O) is split (photolysis) in the thylakoid membranes by light energy. This releases electrons, protons (H^+), and molecular oxygen (O2) as a byproduct. The electrons replenish those lost by chlorophyll.

  • Light-dependent reactions involve an electron transport chain and chemiosmosis to generate ATP and NADPH. ATP provides energy, and NADPH provides reducing power (electrons) for the subsequent Calvin cycle.

  • Calvin cycle methods emphasize carbon fixation, where CO_2 is incorporated into an organic molecule. This cycle efficiently uses the generated ATP and NADPH to reduce fixed carbon into glyceraldehyde-3-phosphate (G3P), which can then be converted into glucose and other organic compounds.

Action Spectrums in Photosynthesis

• An action spectrum plots the rate of photosynthesis versus the wavelength of light. It shows which wavelengths are most effective for driving the process, often mirroring the absorption spectrum of chlorophyll but also including contributions from accessory pigments.

• Light absorption characteristics of chlorophyll: Chlorophyll strongly absorbs blue-violet and red light but poorly absorbs green wavelengths. This is why plants appear green, as green light is reflected or transmitted.

• Historical experiment showing relationship between light wavelengths and photosynthesis rates via Spirogyra (a filamentous alga) and the role of aerobic bacteria (bacteria that need oxygen): Theodor W. Engelmann's experiment in 1883 demonstrated that the highest concentration of aerobic bacteria gathered around the parts of the Spirogyra filament illuminated by blue-violet and red light, indicating maximum oxygen production (and thus photosynthesis) at these wavelengths.

Rubisco Enzyme in Photosynthesis

• Described as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an enzyme with dual functionality, essential for carbon fixation.

  • Fixes both CO2CO2 (carboxylation): In the Calvin cycle, Rubisco catalyzes the addition of CO2CO2 to ribulose-1,5-bisphosphate (RuBP), initiating the production of sugars. This is the desired photosynthetic pathway.

  • Fixes O2O2 (oxygenase): Rubisco can also bind to O2O2 instead of CO2CO2. This leads to photorespiration, a process generally considered inefficient because it consumes ATP and NADPH, releases CO2CO2, and does not produce sugar, thereby reducing photosynthetic yield.

• Noteworthiness of its promiscuous nature (ability to bind to both CO2CO2 and O2O2) impacting photosynthetic efficiency is significant, particularly in hot, dry conditions where stomata close, and O2O2 levels rise inside the leaf. Adaptations like C4 and CAM photosynthesis have evolved in some plants to minimize photorespiration by concentrating CO2CO2 around Rubisco.

Glossary of Key Terms

• Longitudinal Section: A cut made along the length of an organ.

• Cross Section (AKA Transverse Section): A cut made horizontally through an organ, perpendicular to its length.

• Epidermis: The outermost protective layer of cells in plants, often covered by a waxy cuticle to reduce water loss.

• Cortex: The tissue located between the epidermis and the vascular tissue, typically composed of parenchyma cells involved in storage and sometimes photosynthesis.

• Vascular Cylinder (Stele): The central core of a plant root or stem, containing vascular tissue (xylem and phloem); also referred to as a eustele in dicot stems where bundles are in a ring, or atactostele implied for scattered bundles in monocots.

• Pericycle: a cylinder of parenchyma or sclerenchyma cells that lies just inside the endodermis and is the outermost part of the stele (vascular cylinder) in plant roots. It is the site of origin for lateral roots

• Lateral Roots: Branch roots that extend horizontally from the primary root, aiding in further absorption and anchorage.

• Endodermis: A cylinder of cells that encloses the vascular tissue (stele) in plant roots, regulating the movement of water and dissolved solutes into the stele. It contains the Casparian strip, a waxy barrier that forces water and minerals to pass through the endodermal cells rather than between them.

• Pith: The central region of a stem, composed of parenchyma cells, often involved in storage of water and nutrients. Distinctly present in dicot stems.

• Vascular Bundle: A strand of vascular tissue in plants, comprising xylem and phloem, responsible for long-distance transport of water, minerals, and sugars.

• Xylem: Plant vascular tissue responsible for the unidirectional transport of water and dissolved mineral nutrients from the roots to the rest of the plant.

• Phloem: Plant vascular tissue that transports sugars (produced during photosynthesis) from the leaves to other parts of the plant, moving bidirectionally.

• Sclerenchyma: A plant tissue composed of thick-walled, often lignified cells that provide structural support, commonly found as a protective cap or sheath over the phloem in vascular bundles.

• Atactostele: A characteristic arrangement of vascular tissue in monocot stems where vascular bundles are scattered throughout the ground tissue, lacking a distinct pith or cortex.

• Eustele: A characteristic arrangement of vascular tissue in dicot stems where vascular bundles are arranged in a distinct ring around a central pith.

• Primary Root: The initial root developed from the plant embryo, primarily responsible for anchorage and absorption of water and minerals.

• Primary Stem: The initial stem developed from the plant embryo and growing in length from apical meristems, primarily responsible for support and transport.

• Upper/Lower Epidermis: The protective outermost layers of cells on the top and bottom surfaces of a leaf; the upper minimizes water loss with a thick cuticle, while the lower typically contains most stomata for gas exchange.

• Cuticle: A waxy layer covering the epidermis of leaves and stems, primarily to reduce water loss.

• Mesophyll: The middle layer of a leaf, located between the upper and lower epidermis, which is the primary site of photosynthesis and gas exchange.

• Palisade (Palisade Layer): A layer of elongated, column-shaped parenchyma cells, rich in chloroplasts, located beneath the upper epidermis of a leaf, optimally positioned for light absorption and primary site of photosynthesis.

• Spongy (Spongy Layer): A layer of irregularly shaped parenchyma cells with large intercellular air spaces, located below the palisade layer in a leaf, crucial for efficient diffusion of gases such as CO2CO2 and O2O2.

• Guard Cells: Specialized epidermal cells that regulate the opening and closing of stomata, controlling gas exchange and transpiration.

• Stomata: Microscopic pores, largely on the leaf surface, regulated by a pair of guard cells, facilitating gas exchange (O2O2 release, CO2CO2 uptake) and serving as the primary site for transpiration.

• Bundle Sheath: Parenchyma cells that surround the vascular tissue in larger leaf veins, providing structural support and, in some plants, playing a role in concentrating CO2CO2​ (e.g., in C4 plants).

• Vein: Extensions of the vascular system (xylem and phloem) into the leaf blade, forming a network for transport of water, minerals, and sugars, and providing structural support.

• Vascular Cambium: A cylindrical lateral meristematic tissue located between the primary xylem and primary phloem, responsible for producing secondary xylem (wood) to the inside and secondary phloem to the outside, increasing a plant's girth.

• Secondary Xylem (Wood): The secondary vascular tissue produced by the vascular cambium towards the inside, providing structural support and water transport.

• Secondary Phloem: The secondary vascular tissue produced by the vascular cambium towards the outside, responsible for transporting sugars throughout the plant.

• Cork Cambium (Phellogen): A lateral meristem that arises in the cortex (or sometimes epidermis or phloem), producing cork cells (phellem) externally and phelloderm cells internally, forming the periderm.

• Periderm (Bark): The collective term for cork, cork cambium, and phelloderm, forming a protective outer layer in woody stems and roots.

• Bark: All tissues external to the vascular cambium in a woody plant, including secondary phloem, cork cambium, and cork.

• Chloroplast: The organelles within plant cells where photosynthesis occurs, possessing a double membrane and containing thylakoids and chlorophyll.

• Thylakoid: Flattened sacs within chloroplasts, arranged into stacks called grana, where the light-dependent reactions of photosynthesis take place on their membranes.

• Absorption Spectrum: A graph plotting the wavelengths of light absorbed by a particular pigment, such as chlorophyll.

• Action Spectrum: A graph plotting the rate of photosynthesis versus the wavelength of light, showing which wavelengths are most effective for driving the process.

• ADP+P->ATP (ATP): Adenosine Triphosphate (ATP) is the primary energy currency of the cell, generated during the light-dependent reactions of photosynthesis by combining adenosine diphosphate (ADP) with an inorganic phosphate (P) via chemiosmosis.

• ATP Synthase: (Though not explicitly named, its function is implied.) An enzyme complex that catalyzes the synthesis of ATP from ADP and inorganic phosphate, driven by the movement of protons across a membrane during chemiosmosis.

• Proton Gradient: A difference in the concentration of protons (H+H+) across a membrane (e.g., thylakoid membrane), which stores potential energy used to drive ATP synthesis during chemiosmosis.

• NADP->NADPH (NADPH): Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is a reducing agent (electron carrier) generated during the light-dependent reactions of photosynthesis, providing reducing power (electrons) for the Calvin cycle.

• Light Reactions (Light-Dependent Reactions): The first stage of photosynthesis, occurring on the thylakoid membranes, where light energy is converted into chemical energy in the form of ATP and NADPH, and molecular oxygen is released.

• O2O2 from H2OH2O: Molecular oxygen (O2O2) is released as a byproduct when water (H2OH2O) is split (photolysis) by light energy in the thylakoid membranes during the light-dependent reactions.

• H+H+ from H2OH2O: Protons (H+H+) are released when water (H2OH2O) is split (photolysis) by light energy in the thylakoid membranes during the light-dependent reactions, contributing to the proton gradient.

• Synthetic Reactions (Calvin Cycle / Light-Independent Reactions): Also known as the Calvin cycle, these reactions occur in the stroma of the chloroplast, using the ATP and NADPH generated in the light reactions to convert atmospheric CO2CO2​ into glucose sugars.

• Calvin Cycle (C3): The light-independent reactions of photosynthesis, occurring in the stroma, where CO2CO2​ is fixed and converted into glucose sugars using ATP and NADPH. It is also referred to as C3 because the first stable organic intermediate formed is a three-carbon compound.

• RUBISCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase): An enzyme essential for carbon fixation in the Calvin cycle, having dual functionality: it catalyzes the addition of CO2CO2 to ribulose-1,5-bisphosphate (RuBP) (carboxylation), but can also bind to O2O2 instead of CO2CO2​ (oxygenase), leading to photorespiration.