Plant Anatomy and Photosynthesis

A set of flashcards covering key concepts in plant anatomy and photosynthesis, focusing on structures, functions, and processes relevant to the exam.

What are the two primary types of stems compared in plant anatomy, and how do their fundamental structures differ?

The two types are monocot stems and dicot stems. They differ significantly in their vascular bundle arrangement: monocot stems have vascular bundles scattered throughout the ground tissue, lacking a distinct pith or cortex, while dicot stems possess vascular bundles arranged in a ring, clearly separating a central pith from an outer cortex.

In dicot stems, what is the cortex, and what is its anatomical position and typical composition?

The cortex is a region of ground tissue located between the epidermis (outermost protective layer) and the vascular tissue (which is arranged in a ring). It typically consists of parenchyma cells that may store food, and sometimes collenchyma cells providing support, occupying the outer part of the stem before the vascular bundles.

In the context of dicot stems, what is the pith, and what is its structural and functional role?

The pith is a central region of nonvascular ground tissue in dicot stems, enclosed by the ring of vascular bundles. Composed primarily of large, thin-walled parenchyma cells, its primary role is storage of water and nutrients, and it can also provide some structural support.

What are the three main tissues found within a vascular bundle, and what is the primary function of each?

The three main tissues in a vascular bundle are: 1. Sclerenchyma: Provides structural support and protection to the vascular tissues, often forming a cap or sheath. 2. Xylem: Responsible for the transport of water and dissolved minerals from the roots to other parts of the plant. 3. Phloem: Responsible for the transport of sugars (produced during photosynthesis) from leaves to other parts of the plant where they are needed for energy or storage.

How can one morphologically distinguish between sclerenchyma and xylem within a plant stem, particularly based on cell characteristics?

You can distinguish them by cell size and presence of lumens: Xylem typically has larger diameter cells (vessel elements and tracheids) which are adapted for efficient water transport, forming open tubes. Sclerenchyma cells, such as fibers or sclereids, generally have smaller diameters with very thick, lignified cell walls and often a very small lumen, primarily serving a supportive and protective function rather than transport.

What is the defining characteristic of vascular bundle arrangement in monocot stems, and how does this contrast with dicots?

The defining characteristic of vascular bundles in monocot stems is their scattered arrangement throughout the ground tissue of the stem, without forming a distinct ring. This contrasts sharply with dicot stems, where vascular bundles are typically arranged in a single, organized ring that clearly separates the cortex from the pith.

What biological process is specifically accountable for the increase in girth or thickness of woody plants, and what type of tissues are involved?

Secondary growth, achieved through secondary meristem development, is responsible for the increase in girth of woody plants. Secondary meristems, specifically the vascular cambium and cork cambium, produce new cells that contribute to the widening of the stem and root, forming wood and bark.

What are the two primary types of cambia involved in facilitating secondary growth in plants, and what does each tissue produce?

The two main types of cambia involved in secondary growth are: 1. Vascular cambium: A cylindrical lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem to the outside, increasing the plant's diameter. 2. Cork cambium: Another lateral meristem that produces periderm, which includes cork (phellum) cells to the outside and phelloderm to the inside, forming part of the plant's outer bark for protection.

Explain why wood is technically defined as secondary xylem, detailing its origin and formation process.

Wood is considered secondary xylem because it is produced by the vascular cambium during secondary growth. The vascular cambium is a meristematic tissue that forms a cylinder running the length of the stem and roots. It continuously divides, adding new xylem cells (wood) inwards towards the center of the stem and new phloem cells outwards. This accumulation of secondary xylem is what constitutes the bulk of woody stems.

What is the overarching biochemical function of photosynthesis in plants and other photoautotrophs, and what is its significance for life on Earth?

The primary function of photosynthesis is to convert light energy into chemical energy, stored in the form of organic compounds, principally glucose (C6H{12}O_6). This process utilizes carbon dioxide and water to synthesize sugars, releasing oxygen as a byproduct. It is fundamentally significant as it forms the base of most food webs, directly or indirectly sustaining nearly all life on Earth, and is responsible for maintaining atmospheric oxygen levels.

Which two main pigments are indispensable for capturing light energy during photosynthesis, and where are they located?

The two main pigments involved in photosynthesis are chlorophyll a and chlorophyll b. Both are located within the thylakoid membranes of chloroplasts. Chlorophyll a is the primary photosynthetic pigment, directly involved in light reactions, while chlorophyll b acts as an accessory pigment, broadening the range of light wavelengths that can be absorbed and passed to chlorophyll a.

What are the two crucial energy-carrying molecules produced as the primary outputs of the light-dependent reactions of photosynthesis, and what are their roles?

The primary outputs of the light reactions of photosynthesis are ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). ATP is an energy currency molecule, providing the energy needed for subsequent metabolic processes. NADPH is a reducing agent, carrying high-energy electrons, which are both essential for driving the carbon fixation and sugar synthesis reactions of the Calvin cycle (light-independent reactions).

What three key inputs does the Calvin cycle specifically utilize to ultimately synthesize glucose?

The Calvin cycle uses carbon dioxide (CO2), ATP, and NADPH to produce glucose. Carbon dioxide serves as the carbon source, while the ATP and NADPH generated during the light reactions provide the necessary energy and reducing power, respectively, to convert (CO2) into sugars.

Name the specific enzyme that initiates carbon fixation in the Calvin cycle, and explain its full name and crucial function.

The enzyme responsible for fixing carbon dioxide in the Calvin cycle is Rubisco, which stands for ribulose-1,5-bisphosphate carboxylase/oxygenase. Its crucial function is to catalyze the first major step of carbon fixation, where (CO_2) is combined with a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), to form an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate.

Describe photorespiration, including why and under what conditions it occurs, and its impact on photosynthetic efficiency.

Photorespiration is a metabolic process where the enzyme Rubisco binds to oxygen (O2) instead of carbon dioxide (CO2). This occurs when (O2) concentrations are high and (CO2) concentrations are low, often in hot, dry conditions when stomata close to conserve water. Instead of producing two molecules of 3-phosphoglycerate (as in normal carbon fixation), photorespiration produces one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate, which must be converted back to 3-phosphoglycerate at considerable energy cost, thus wasting energy and reducing the efficiency of photosynthesis as it doesn't yield ATP or sugar.

Within a plant leaf, where does the majority of photosynthetic activity take place, and why is this location optimized for the process?

The majority of photosynthesis occurs in the palisade mesophyll layer of a leaf. This layer is optimally situated just beneath the upper epidermis, allowing efficient capture of sunlight. Its cells are typically elongated, tightly packed, and contain a very high concentration of chloroplasts, enabling maximum light absorption and biochemical conversion.

What specialized structural features on plant leaves enable the essential exchange of gases required for photosynthesis and respiration?

Stomata (singular: stoma) are the specialized structural features that allow for gas exchange in plant leaves. These are microscopic pores, primarily located on the epidermal surface, flanked by two guard cells. They regulate the intake of carbon dioxide (CO2) for photosynthesis and the release of oxygen (O2) as a byproduct, as well as the release of water vapor during transpiration.

Explain the ecological and physiological reasons why dicot leaves typically have their stomata predominantly on the lower epidermal surface.

Dicot leaves have stomata primarily on the lower epidermal surface to minimize water loss through transpiration. The lower surface is generally cooler and less exposed to direct sunlight and wind compared to the upper surface. Placing stomata here reduces the rate of water evaporation, helping the plant conserve water while still allowing for necessary gas exchange (intake of (CO2) and release of (O2)). This adaptation is crucial for survival in various terrestrial environments.

What specific internal adaptations within chloroplasts allow them to efficiently facilitate the light absorption and energy conversion stages of photosynthesis?

Chloroplasts contain internal membrane-bound sacs called thylakoids, which are often stacked into structures called grana. The thylakoid membranes contain chlorophyll and other pigments that absorb light energy. This light energy is then used to energize electrons and pump protons, establishing an electrochemical gradient across the thylakoid membrane. This gradient drives the synthesis of ATP through ATP synthase, while the energized electrons are used to reduce NADP^+ to NADPH. The fluid-filled space surrounding the thylakoids, called the stroma, is where the Calvin cycle reactions occur.

What significant gas is produced as a crucial byproduct of photosynthesis, and from which specific reactant molecule does it originate?

The significant gas produced as a byproduct of photosynthesis is oxygen (O2). It originates from the splitting of water (H2O) molecules during the light-dependent reactions. This process, known as photolysis, releases electrons, protons, and oxygen gas. The oxygen is then released into the atmosphere, which is vital for aerobic respiration in most living organisms.

Elaborate on the mechanism of how a proton gradient within a chloroplast directly contributes to the synthesis of ATP.

In chloroplasts, light energy absorbed by pigments drives the pumping of protons (H^+) from the stroma into the thylakoid lumen, creating a high concentration of protons within the lumen and thus forming an electrochemical proton gradient (also known as a proton-motive force) across the thylakoid membrane. These accumulated protons then diffuse back out of the thylakoid lumen into the stroma through a specialized enzyme complex called ATP synthase. The flow of protons through ATP synthase causes its rotor and catalytic sites to spin, driving the phosphorylation of ADP (adenosine diphosphate) to ATP (adenosine triphosphate), a process known as chemiosmosis.

What is the primary anatomical and functional role of bundle sheath cells in plant leaves, especially in relation to plant physiology?

The primary role of bundle sheath cells in plant leaves is to surround and protect the vascular tissue (xylem and phloem), forming a sheath around the vascular bundles. Functionally, they play a significant role in photosynthesis, particularly in C4 plants, where they concentrate carbon dioxide (CO_2) to minimize photorespiration. In C3 plants, they are less metabolically active in photosynthesis but still provide structural support and aid in short-distance transport between the mesophyll and vascular tissues.