The Plant Cell 5
Origin and Definition of the Cell
Robert Hooke (1635-1703):
English microbiologist.
Observed structures he called "cells" in 1665.
Defined cells as the structural and functional units of life.
Other Key Players in Cell Theory Development:
1669 Robert Hooke: English polymath, coined the term "cells" (from "little rooms").
1838 Matthias Schleiden: German botanist, proposed that all plant tissues consist of cellular masses.
1839 Theodor Schwann: German zoologist, extended Schleiden's observation to animals.
1858 Rudolf Virchow: German pathologist, generalized that cells can arise only from pre-existing cells.
Cell Theory
Cell theory is a fundamental concept in biology with three core principles:
All organisms are composed of cells: This principle is based on Hooke's observations and further supported by Schleiden and Schwann's work on plants and animals.
The cell is the basic unit of life: This also stems from Hooke's initial findings.
All cells come from pre-existing cells: This crucial principle was established by Virchow, refuting the idea of spontaneous generation.
Prokaryotic vs. Eukaryotic Cells
Karyon: A Greek word meaning "kernel," referring to the nucleus in this context.
Prokaryotes ("before a nucleus"):
Lack a true, membrane-bound nucleus.
Possess circular DNA, which is located in a region called the nucleoid.
Lack other membrane-bound organelles.
Eukaryotes ("with a true nucleus"):
Possess a true, membrane-bound nucleus.
DNA is bound to proteins called histones and is located inside a lipid bilayer (the nuclear envelope).
Generally larger than prokaryotes and typically include plant, animal, fungi, and protist cells.
Contain various membrane-bound organelles.
The Plant Cell: General Structure
The plant cell is distinct from animal cells due to several key features:
Consists of a rigid cell wall and a protoplast.
Protoplast: Encompasses the cytoplasm and the nucleus.
Cytoplasm: The entire content within the cell membrane, excluding the nucleus.
Cytosol (Cytoplasmic matrix): The jelly-like substance where various cellular entities and membrane systems are suspended.
Membrane-bound entities (Organelles): Include chloroplasts, mitochondria, and lysosomes (cytosomes).
Membranous systems: Examples include the Golgi apparatus and endoplasmic reticulum.
Non-membranous entities: Include ribosomes, actin filaments, and microtubules.
The Nucleus
Often called the "blueprint" of the cell, the nucleus is a central control hub:
Primary Functions:
Controls if and when a protein is created within a cell.
Stores the cell's genetic information (DNA), known as the nuclear genome, and passes it to daughter cells during cell division.
Plays a central role in cellular biosynthesis by directing RNA synthesis for protein production.
Nuclear Envelope:
A porous bilayer membrane that surrounds the nucleus.
Some pores are continuous with the endoplasmic reticulum, allowing for material exchange and structural integration.
Chromatin:
Describes the thin threads and grains visible within the nucleoplasm (the internal nuclear material).
Made up of hereditary DNA tightly bound to proteins called histones.
During nuclear division (mitosis or meiosis), chromatin condenses and organizes to form distinct chromosomes.
Chromosome Number:
Differs significantly among different species.
In plants, chromosome number can vary widely, from 2n = 4 to 2n = 1260.
Angiosperms (flowering plants) generally have fewer chromosomes than older plant lineages like ferns, suggesting an evolutionary trend towards reduction over time.
Ploidy Levels:
Haploid (n): Represents the number of unique chromosomes in a single set, typically found in gametes (e.g., pollen or egg cells).
Diploid (2n): Represents two sets of chromosomes, one from each parent, found in somatic (body) cells.
Polyploid (xn): Describes cells or organisms with more than two complete sets of chromosomes, common in many plant species.
Nucleolus:
A dense, non-membranous structure within the nucleus.
Characterized by high concentrations of RNA, proteins, and specific DNA loops.
The DNA loops are known as nuclear organizer regions, which contain genes for ribosomal RNA (rRNA) and are responsible for forming ribosomal subunits.
Ribosomes
Ribosomes are essential for protein synthesis:
Structure: Small particles composed of ribosomal RNA (rRNA) and proteins.
Subunits: Consist of a small subunit and a large subunit, both produced in the nucleolus and then exported to the cytoplasm.
Function: Link together amino acids in a specific sequence to form proteins, a process known as translation (protein synthesis).
Location: Can be found freely suspended in the cytosol or attached to the endoplasmic reticulum (ER) and the outer surface of the nuclear envelope, where they synthesize proteins destined for secretion or insertion into membranes.
Polysomes (Polyribosomes): Aggregates of ribosomes actively engaged in protein synthesis, translating the same messenger RNA (mRNA) molecule simultaneously, allowing for efficient production of multiple copies of a protein.
Plastids
Plastids are characteristic components of plant cells, involved in vital processes like photosynthesis and storage.
General Characteristics:
Surrounded by a double bilayer membrane.
The inner membrane can fold inward to form flattened sacs called thylakoids.
The homogeneous matrix surrounding the thylakoids is called the stroma.
Principal Types of Plastids (classified by pigments):
Chloroplasts: Contain chlorophylls and carotenoids, responsible for photosynthesis.
Chromoplasts: Contain carotenoid pigments, giving rise to red, orange, and yellow colors.
Leucoplasts: Lack pigments and elaborate internal structure, primarily for storage.
Chloroplasts
Pigments: Contain chlorophylls (which impart the green appearance) and carotenoid pigments (yellow/orange, typically masked by chlorophyll abundance).
Function: Responsible for photosynthesis, the process by which light energy is converted into chemical energy.
Structure and Abundance:
Disc-shaped and highly abundant in photosynthetic tissues (e.g., 40-50 in a single leaf cell).
Generally orient their broad surface parallel to the cell wall to maximize light absorption but can move to avoid excessive light.
Complex Internal Structure:
Grana: Dense stacks of thylakoids (specifically called grana thylakoids), resembling stacked coins.
Stroma Thylakoids: Thylakoids that traverse the stroma and interconnect individual grana stacks.
Chlorophylls and carotenoids are embedded within the thylakoid membranes.
Semi-Autonomous Nature:
Possess their own circular DNA, located in nucleoids (similar to prokaryotes), and lack associated histones.
Can produce some of their own polypeptides (proteins).
Endosymbiotic Origin:
Share many structural and genetic similarities with bacteria (e.g., circular DNA, size, type of ribosomes).
This supports the theory that chloroplasts originated from ancestral photosynthetic bacteria (cyanobacteria) that were engulfed by a eukaryotic host cell.
Overall Control from Nucleus: While semi-autonomous, the majority of chloroplast proteins are encoded by nuclear DNA, synthesized in the cytosol, and then imported into the chloroplasts.
Chromoplasts
Pigmentation: Pigmented like chloroplasts but lack chlorophylls.
Pigments: Primarily use carotenoids, which are responsible for the red, orange, and yellow colors of many flowers, autumn leaves, ripe fruits, and some roots (e.g., carrots).
Development: Can develop directly from existing chloroplasts, where the internal thylakoid structure disappears, and carotenoids accumulate.
Function: While not fully understood, they are believed to play a role in attracting insects and animals for pollination and seed dispersal.
Leucoplasts
Differentiation: Are the least differentiated type of plastid, lacking both pigments and elaborate internal structures.
Amyloplasts: A common type of leucoplast that specifically synthesizes and stores starch.
Other Roles: Other leucoplasts may be specialized for synthesizing or storing oils or proteins.
Proplastids
Undifferentiated Form: These are small, colorless to pale green, undifferentiated plastids found in actively dividing cells (meristematic cells).
Development:
Require light for their proper development into mature plastids.
In the absence of light, they form elaborate semi-crystalline, tubular membranes known as prolamellar bodies, leading to the formation of etioplasts.
When etioplasts are subsequently exposed to light, their prolamellar bodies develop into thylakoids, and they differentiate into chloroplasts.
Mitochondria
Often called the "powerhouses of the cell," mitochondria are critical for cellular respiration.
Structure:
Enclosed by a double bilayer membrane: an outer and an inner membrane.
The inner membrane features numerous invaginations called cristae, which significantly increase its surface area.
Size: Much smaller than chloroplasts.
Site of Respiration: The primary site of aerobic respiration, where energy from organic molecules (like glucose) is converted into adenosine triphosphate (ATP), the cell's main energy currency.
Abundance: More abundant in cells with high ATP demand, reflecting their energy-producing role.
Other Biosynthesis Roles: Also involved in the biosynthesis of crucial molecules such as amino acids, vitamin cofactors, and fatty acids.
Apoptosis (Programmed Cell Death):
Play a key role in initiating programmed cell death.
During apoptosis, mitochondria swell and release cytochrome c (normally involved in the electron transport chain).
The release of cytochrome c activates cellular proteases (enzymes that break down proteins) and nucleases (enzymes that break down RNA and DNA), leading to the systematic dismantling of the cell.
Matrix: The enclosed inner compartment, housing numerous proteins, RNA, its own circular DNA, small ribosomes, and various solutes (e.g., enzymes for the Krebs cycle).
Peroxisomes
Peroxisomes are small, single-membraned organelles with diverse metabolic roles.
Structure: Surrounded by a single bilipid membrane.
Interior: Possess a granular interior, sometimes containing a crystalline core composed of protein (often catalase).
Replication: Self-replicating, meaning they can grow and divide to produce more peroxisomes.
Import of Components: Import all necessary proteins and enzymes from the cytosol, as they do not have their own genetic material.
Roles in Photorespiration: In plants, peroxisomes participate in photorespiration, a process that consumes oxygen and releases carbon dioxide, acting in concert with chloroplasts and mitochondria.
Glyoxysomes: A specialized type of peroxisome found in the storage tissues of oil-rich seeds.
They house enzymes crucial for converting stored fats into sucrose during seed germination, providing the energy and carbon skeletons needed to kick-start early seedling growth.
Vacuoles
Vacuoles are prominent, versatile organelles in plant cells, often occupying a large portion of the cell volume.
Membrane: Surrounded by a single lipid bilayer membrane called the tonoplast.
Origin: Originate primarily from the endoplasmic reticulum (ER).
Protein and Component Transport: Many components of the tonoplast and various vacuolar proteins are processed and transported from the Golgi apparatus.
Cell Sap: Most vacuoles are filled with a solution called "cell sap."
Predominantly water, it also contains a variety of dissolved components, the composition of which depends on the cell type.
These components can include inorganic ions, sugars, organic acids, and amino acids.
High concentrations of certain substances can lead to the formation of crystals, such as calcium oxalate crystals, which are a common type.
Volume: A single large central vacuole can take up to 90\% of the total cellular volume in mature plant cells.
Storage Functions:
Serve as storage sites for primary metabolites, such as water, sugars, and ions, contributing to turgor pressure.
Act as a repository for toxic secondary metabolites (e.g., nicotine, tannins, and some waste products), effectively sequestering them away from the active cytoplasm. When the cell or tissue is ruptured, these compounds can be released, acting as defense mechanisms.
Accumulate pigments like anthocyanins, which are responsible for the red and blue colors observed in many fruits and flowers.
Key Discussion Points
Why is it important for chloroplasts to be able to rearrange?
Chloroplast rearrangement allows plant cells to regulate the amount of light they absorb.
They can position themselves to maximize light capture under low-light conditions.
Conversely, they can move away from direct, intense light to avoid photodamage and prevent overwhelming the photosynthetic machinery, which could kill the tissue.
This is particularly important for plants utilizing different modes of photosynthesis (e.g., C4 and CAM plants).
Chloroplasts and Mitochondria: Evidence for Endosymbiosis?
Both organelles share compelling similarities with bacteria, supporting the endosymbiotic theory:
Own Circular DNA: Both possess their own circular DNA molecules, similar to the genomes of bacteria.
Lack of Histones: Their DNA is not associated with histones, characteristic of prokaryotic DNA.
Double Bilayer Membrane: Each is enclosed by a double membrane, where the inner membrane is thought to be derived from the ancestral engulfed prokaryote's membrane, and the outer membrane from the host cell's engulfing vesicle.
Similar Ribosomes: They contain ribosomes that are structurally more similar to bacterial ribosomes than to eukaryotic cytoplasmic ribosomes.
Self-Replication: Both reproduce by fission, a process similar to bacterial cell division.
Why is it beneficial to have large vacuoles?
Turgor Pressure: A large central vacuole filled with cell sap exerts turgor pressure against the cell wall, providing structural support to the cell and plant, preventing wilting.
Storage: Efficiently store water, nutrients, ions, and beneficial secondary metabolites.
Waste and Detoxification: Isolate and store toxic secondary metabolites and metabolic waste products, keeping them away from critical metabolic processes in the cytoplasm.
Growth: Can expand rapidly, allowing for significant cell enlargement with minimal allocation of metabolic resources, contributing to overall plant growth.
Surface Area to Volume (SA:V) Ratio: By occupying a large volume of the cell, the vacuole pushes the cytoplasm and its organelles to the periphery, increasing the effective surface area for exchange with the external environment relative to the cytoplasmic volume (though this aspect is more indirect than direct SA:V enhancement of the organelle itself).