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This section is all about the plant cell and what plants need for growth. There is also a bit of an intro to photosynthesis but that will be covered in detail in the next section. We start out with understanding plant cells with a focus on what makes them different from animal cells. For example - a cell wall and chloroplasts are extremely notable! We then dive into a history of how we came to know what plants need and how they grow. It seems obvious now but it was not always so! We finish up with some great time lapse videos showing plant growth in action. Plants do some amazing things - just not at the same speed that we live in!
Central Questions and Topics
Central Questions and Topics
How do the structure and contents of plant cells translate into the properties of plants we see today?
central vacuole --> turgor pressure --> wilting
cellulose & hemicellulose (long chain carbohydrate) --> cell wall --> rigidity....also, ruminants can eat it
lignin (carbon ring) --> connects cellulose & provides relative impermeability to water....also, ruminants can't eat it
chloroplast --> contains chlorophyll a & b --> central in photosynthesis --> can be moved around the cell via cytoplasmic streaming --> makes plants green (why green?)
Plants seem to "know" which way is up and where the sun is. You can see this if you tip a plant over or move the source of light. How does this work? How do they "know" and respond accordingly?
How do plants grow? More specifically - be able to explain the science AND the history, including what plants "eat".
Plant Cells - what makes them special and what they contain
Plant Cells - what makes them special and what they contain
Crash Course - the Plant Cell (differences from animal cell)
Nucleus (animal and plants)
Organelles (animal and plants)
Cytoplasm (animal and plants)
Rigid Cell Wall (gives structure to leaves, roots, stems)
Cellulose - we cannot digest this - look to ruminants & bacteria
Lignin
Organelles formed via endosymbiosis
Mitochondria
Chloroplasts (a plastid)
Central Vacuole - rigidity controlled by water
Turgor pressure (cells made turgid)
Reinforces the plant
Chloroplasts moving within plant cells (notice cell wall) possibly to become properly oriented to light. This is just a quick and cool video for you to see real life chloroplasts moving around in a dynamic plant cell!
Cytoplasmic streaming, also known as cyclosis or protoplasmic streaming, is a fascinating phenomenon in plant cells. It's the movement of the cytoplasm, the jelly-like fluid that fills the cell interior, within the cell wall. This movement creates a flowing current, transporting various cellular components throughout the cell.Â
The driving force behind cytoplasmic streaming is the action of motor proteins. These specialized proteins, like myosin, "walk" along protein filaments (actin filaments) within the cytoplasm, dragging organelles and other cellular components along with them.Â
Chloroplasts on the Move:
Efficient Positioning: Cytoplasmic streaming plays a key role in positioning chloroplasts within the cell. By constantly moving them around, streaming ensures they reach different areas of the cytoplasm, maximizing their exposure to sunlight. By moving chloroplasts throughout the cell, streaming allows them to capture light from different angles, increasing the overall efficiency of photosynthesis.
Concentration Gradients: Photosynthesis uses carbon dioxide (CO2) from the air. Streaming helps maintain concentration gradients within the cell. As CO2 is depleted near a chloroplast actively photosynthesizing, streaming brings fresh CO2-rich cytoplasm from other areas, replenishing the supply for continued photosynthesis.
Removal of Waste Products: Photosynthesis produces oxygen (O2) as a waste product. Streaming helps remove this oxygen from the vicinity of the chloroplast, preventing it from accumulating and potentially hindering the process.
This is an idealized diagram of the plant cell (not to scale). This is meant to be an aid to what is discussed in the Crash Course video above.Â
Microscope Picture of Plant Cell
This is here to get you a non-idealized look at the real plant cell. Again it is here to reinforce the learning from the Crash Course video
Diagram of Primary Plant Cell Wall Structure
This diagram of the primary plant cell wall is just to give you an idea of how rigid molecules like cellulose and hemicellulose give a plant cell its rigid cell wall. Â
Pectin:
Structure: A complex polysaccharide (sugar molecule chain) primarily composed of galacturonic acid units. These units can be linked together in various ways and may have side chains with other sugars attached. Imagine a flexible network of sugar molecules with some variations.
Function: Pectin is found mainly in the middle lamella (glue layer) and primary cell wall of plant cells. It acts as a cementing agent, holding plant cells together and providing some structural support, especially in young and growing tissues. It's like the mortar between bricks, creating a connected structure. Pectin also plays a role in cell signaling and plant defense mechanisms.
Figure showing primary (A) and secondary (B) plant cells walls. Â
Primary Cell Wall:
Timing: Formed first, during the initial growth and expansion of plant cells.
Structure: Composed mainly of cellulose microfibrils (long, unbranched chains of sugar molecules) embedded in a matrix of hemicellulose (shorter, branched sugar molecules) and pectin (glue-like polysaccharides). Imagine a flexible mesh of cellulose fibers held together by hemicellulose and pectin, allowing for cell growth.
Function: Provides initial structural support and shape to the cell, allows for cell expansion and growth, and facilitates communication between neighboring cells.
Lignin: Generally absent or present in very low amounts in the primary cell wall.
Secondary Cell Wall:
Timing: Deposited after the cell has reached its mature size, on the inside of the primary cell wall.
Structure: More complex than the primary wall. It contains cellulose microfibrils like the primary wall, but in a more rigid and ordered arrangement. Additionally, it has a significant amount of lignin, a complex aromatic polymer that fills the spaces between cellulose microfibrils. Think of it as additional reinforcement with cellulose fibers bundled together and "glued" by lignin, creating a much stiffer structure.
Function: Provides additional strength, rigidity, and impermeability to the cell wall. This is crucial for supporting mature tissues like wood and xylem (water transport) vessels. Lignin also helps resist pathogens and harsh environmental conditions.
what a plant needs to grow, growth behavior, and intro to photosynthesis
what a plant needs to grow, growth behavior, and intro to photosynthesis
Botany:Â A Blooming History (Part 2) - How a Plant Grows and Introduction to Photosynthesis
Jean-Baptiste van HelmontÂ
experiment detailing mass of plant & mass of soil it is growing in - plants don't eat soil! - conclusion - plants add mass and grow by drinking water (incorrect - weight is added by carbon not water)
Jan IngenhouszÂ
experiment with submerged leaves in a jar - accidental exposure to sunlight leads to bubbles of gas (later found to be oxygen) - experiment repeated with many plants - plants need sunlight!
Julius von SachsÂ
wrote treatise on plant growth
experiment to figure out how sunlight plays a role in the production of starch - strips green color - iodine applied and starch is turned black - used on plants with differing exposure to sunlight - sunlight is used to make starch!
used microscope to see chloroplasts within plant cells
Sachs's method of staying awake to get work done...cocaine
Plants produce sugar which is stored as starch - this is how plants grow
Stomata - openings through which plants take in carbon dioxide for photosynthesis
Andrew Benson & Melvin Calvin
experiment - How do plants use carbon dioxide to fuel their growth? - use of cyclotron (particle accelerator) to produce radioactive carbon atoms - this provides an isotopic label that allows you to follow carbon as it moves through a plant - chromatogram shows which compounds (i.e. sugars) contained the radioactive carbon - demonstrates HOW plants use carbon from the atmosphere to grow!
Calvin Cycle (now called Calvin-Benson Cycle)
Feud between Calvin & Benson
Manipulating photosynthesis for food production
adjusting light timing in a greenhouse allows peppers to grow to maturation
carbon dioxide is elevated in greenhouses (harvested from factories) to increase fruit yields and sugars in tomatoes
Synthetic Photosynthesis could provide us with oxygen and hydrogen gases for fuel - this could also draw down carbon dioxide in the atmosphere
Photosynthesis and seasonal variation on a global scale
half of photosynthesis happens in oceans
plants are significant drivers of Earth's atmospheric constituents
Really Cool Time-lapse demonstrating how plants sense which way is up by actually sensing gravity itself
Gravitropism (also known as geotropism) is the ability of a plant to direct its growth in response to gravity. It's how a plant knows which way is up and which way is down.
Positive Gravitropism
Roots: Roots exhibit positive gravitropism, meaning they grow in the direction of gravity. This ensures that the roots grow downward into the soil where they can anchor the plant and absorb water and nutrients.
Negative Gravitropism
Shoots:Shoots exhibit negative gravitropism, meaning they grow against the direction of gravity. This ensures that they grow upwards towards sunlight which is needed for photosynthesis.
How Gravitropism Works
Plants sense gravity through specialized cells called statocytes. Inside these cells are starch-filled organelles called amyloplasts.
Sensing Gravity: Amyloplasts settle to the bottom of the cell due to gravity. Their position within the statocytes signals the plant's orientation relative to gravity.
Hormonal Response: The settling of the amyloplasts triggers an uneven distribution of the plant hormone auxin.
In roots, auxin builds up on the lower side, inhibiting cell elongation on that side and causing the root to curve downward.
In shoots, auxin builds up on the lower side, promoting cell elongation and causing the shoot to curve upward.
It's All About Concentration
Optimal Levels: Auxin acts as a growth hormone at certain concentrations, but at higher levels, it can become an inhibitor. Roots and shoots have different sensitivities and optimal concentrations of auxin for proper growth.
Roots: Roots are highly sensitive to auxin. Even the small amounts of auxin that accumulate from gravitropism reach a concentration that inhibits cell elongation, leading to the downward curve.
Shoots: Shoots are less sensitive to auxin. The accumulation on the lower side leads to a concentration that falls within a growth-promoting range, causing cells to elongate and the shoot to bend upwards.
Differential Gene Expression
The way cells in roots and shoots respond to auxin is also influenced by their genetic makeup. Roots and shoots express different sets of genes that regulate how they respond to this hormone. These differences lead to:
Cellular Responses: The same concentration of auxin may trigger different cellular processes and gene activation in root cells compared to shoot cells, leading to opposite effects on growth.
Evolutionary Adaptation
The opposing growth responses to auxin are an evolutionary advantage for the plant:
Roots: The inhibitory effect of auxin ensures that roots grow downward for optimal water and nutrient uptake and for strong anchorage.
Shoots: The growth-promoting effect of auxin ensures that shoots grow upwards towards sunlight, which is essential for photosynthesis and survival.
Really Cool Time-lapse demonstrating how plants sense where light is grow towards it.
Phototropism
Definition: The growth of a plant in response to a light stimulus.
Mechanism: Plants have light-sensitive proteins called phototropins. When light hits one side of a plant, these phototropins trigger the movement of the growth hormone auxin to the shaded side of the plant. Increased auxin on that side causes the cells to elongate more, leading to bending.
Positive Phototropism
Direction: Growth towards the light source.
Example: Stems and leaves of a plant bending towards a window.
Benefits: Allows plants to maximize light capture for photosynthesis, which is vital for their growth and survival.
Negative Phototropism
Direction: Growth away from the light source.
Example: Roots growing downward, away from sunlight.
Benefits:
Helps roots anchor the plant in the soil.
Guides roots deeper to find water and nutrients.
Phototropins: The Light Sensors
Phototropins are a type of photoreceptor protein found in plants.
They consist of two main parts:
A protein structure.
A light-absorbing molecule called a chromophore.
The Key is the Chromophore:
The chromophore in phototropins is specifically designed to absorb light in the blue wavelength range (around 400-500 nm).
This blue light is particularly effective for signaling plant growth responses.
When a specific blue wavelength photon strikes the chromophore, it gets absorbed.
Light Absorption Triggers a Shape Change:
This absorption of light energy triggers a change in the shape of the chromophore.
This shape change activates the protein portion of the phototropin.
Activated Phototropin Initiates Downstream Responses:
The activated phototropin can then set off a cascade of events within the plant cell.
This includes changes in gene expression, protein activity, and ultimately, plant growth patterns.
Nuances of Blue Light Perception:
It's important to note that some phototropins can be sensitive to slightly different blue light sub-ranges. This allows plants to potentially distinguish between the quality and direction of the light source.
What About Other Wavelengths?
While phototropins are the primary players in phototropism, plants have other photoreceptors for different wavelengths:
Cryptochromes: Sensitive to blue and ultraviolet light, involved in regulating plant development and circadian rhythms.
Phytochromes: Sensitive to red and far-red light, involved in responses like seed germination and shade avoidance.
Combined Light Perception:
These different photoreceptors work together to give plants a more comprehensive picture of their light environment.
The combined information from these receptors allows plants to fine-tune their growth and development strategies for optimal survival and reproduction.
Really Cool Time-lapse demonstrating the process of growth and development from a seed.
Really Cool Time-lapse demonstrating the process of growth and development from a seed.
Really Cool Time-lapse demonstrating the process of growth and development from a seed.
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