Plant Death

Light or Death: Parallels Between Plants & People

Understanding Light Energy
Reading Time: 9 - 11 minutes (2517 words)

Both plants and humans require energy to carry out life-sustaining processes. For plants, this energy is harnessed from light through photosynthesis, a complex set of reactions that convert light energy into chemical energy stored in glucose. Us humans, obtain energy from the food we consume, measured in calories. Both systems are fundamental to survival, growth, cellular repair, metabolic maintenance, and reproduction. While plants transform light energy directly into what they need to survive and thrive, humans ingest food, which has its own original energy source—often from plants or animals that have consumed plants and/or other protein. Despite the differences in energy acquisition, both mechanisms serve as the basis for complex biochemical pathways that sustain life.

Plant Light Requirements

Photosynthesis is a critical process for plants that allows them to convert light energy into chemical energy stored as glucose. This complex biochemical process primarily occurs in the chloroplasts, specialized organelles that contain the pigment chlorophyll. The process can be divided into two main stages: the light-dependent reactions and the light-independent reactions, commonly known as the Calvin Cycle. The light-dependent reactions capture light energy to produce high-energy molecules like ATP and NADPH. The Calvin Cycle utilizes these molecules to fix carbon dioxide into glucose. Besides glucose, the process also produces oxygen as a by-product, contributing to Earth's atmospheric composition. Understanding photosynthesis not only illuminates plant physiology but also has implications for agriculture, biofuels, and climate science.

Components of Photosynthesis


Chloroplasts are like tiny factories inside the cells of plants and some other organisms, where the magic of photosynthesis happens. Here's a simple breakdown:

  1. What They Do: Chloroplasts turn sunlight into food for the plant. This process is called photosynthesis.
  2. What's Inside: These mini-factories contain a green substance called chlorophyll. Chlorophyll is like a solar panel; it captures sunlight to start the process of making food.
  3. Structure: Chloroplasts are wrapped in a double layer (imagine a double-skinned bubble) for protection and efficiency. Inside these, there's another special layer called the thylakoid membrane.
  4. Thylakoid Membrane: Think of the thylakoid membrane as the main workshop inside this tiny factory. It's where sunlight, water, and air are turned into energy during the first part of photosynthesis. This membrane is crucial because it's where sunlight is captured and the process of making plant food begins.

Light-Dependent Reactions

  1. Where It Happens: These reactions occur in the thin, flat parts inside the plant cell's chloroplasts, called the thylakoid membranes, which we mentioned above.
  2. Role of Light and Chlorophyll: When sunlight hits the green pigment in plants (chlorophyll), it triggers a reaction. Think of chlorophyll as tiny solar panels on these membranes.
  3. Exciting Electrons: The light's energy "excites" or gives energy to electrons (tiny particles) in chlorophyll. This is a bit like the sun's rays giving energy to a solar panel, causing it to generate power.
  4. Electron Transport Chain: These energized electrons travel through a series of structures (protein complexes) in the thylakoid membrane. Imagine it as a mini electric circuit inside the plant cell.
  5. Pumping Protons: As the electrons move through this circuit, they help move tiny particles called protons across the membrane. This creates a difference in proton concentration across the membrane, much like having more water on one side of a dam than the other.
  6. Making ATP: This difference (or gradient) drives the production of ATP (a molecule that stores energy for the cell) from ADP (a lower-energy molecule) and an inorganic phosphate. ATP is like a charged battery that stores energy.
  7. Forming NADPH: Meanwhile, another molecule, NADP+, gets converted to NADPH. This is like packing energy into a container for use later.

Both ATP and NADPH (the packed energy containers) are then used in the next part of photosynthesis to make food for the plant.

Calvin Cycle (Light-Independent Reactions)

The Calvin Cycle is a part of photosynthesis, the process by which plants make their own food. It's named after Melvin Calvin, the scientist who discovered it. To put it simply, the Calvin Cycle is like a factory line inside plant cells that turns air and energy into food for the plant. Here's a breakdown of how it works:

The Starting Point: Ingredients

  • Carbon Dioxide (CO₂): This is the gas that plants take in from the air.
  • Energy Carriers (ATP and NADPH): These are like tiny rechargeable batteries made in the first part of photosynthesis. They store energy from sunlight and are used to power the Calvin Cycle.

The Steps of the Calvin Cycle

  1. Fixation: The cycle starts when a molecule from the air (CO₂) is attached to a five-carbon sugar already in the cycle. This is a bit like attaching a car to the back of a train.
  2. Reduction: This step uses energy from the little batteries (ATP and NADPH) to change the CO₂, now attached to the sugar, into a form the plant can use. Think of it like a factory line, where the raw material (CO₂) is gradually changed into something useful.
  3. Regeneration: Finally, some of these molecules go on to become glucose (sugar), which is the end product and like the final product coming off the factory line. This sugar is what the plant uses for energy and growth. The remaining molecules stay in the cycle to help process more CO₂.

Energy Storage and Usage

Plants are pretty smart when it comes to managing their energy. They make their own food through photosynthesis and then decide how to use and store it. Let's break it down:

  1. Making Food: During photosynthesis, plants use sunlight to make a type of sugar called glucose. This glucose is the plant's main energy source. This is the same quick energy store humans use.
  2. Using Energy Right Away: Just like we eat food for energy to walk or think, plants use some of this glucose immediately. They break it down in a process called respiration (not the same as breathing in humans), which gives them the energy to grow, repair and perform other vital functions.
  3. Storing Energy for Later: Plants also think ahead. They convert any extra glucose into starch—a more complex sugar that's like a long-term energy storage molecule. Starch is stored in different parts of the plant, like the roots (think of carrots or potatoes) and fruits (like apples or berries).
  4. Tapping Into Stored Energy: When there's reduced or no sunlight (like at night, during winter, indoors, in the shade, or when some cheap Amazon light doesn't deliver enough), plants can't photosynthesize (enough, or at all), and need to rely on their stored energy. During these times, they use their stored starch. At this point, the plant breaks down the starch back into glucose, which it can use for energy when needed.

The Bottom Line

Just as we might use a Snickers Bar for some quick energy and store extra calories as fat on your thighs and belly for later (does later ever actually come?), plants use glucose immediately for energy and convert extra into starch for storage. This smart energy management helps them survive in different conditions, whether there's plenty of light or none at all.

Environmental Factors

Various environmental factors can influence the rate of photosynthesis. Light intensity, wavelength, temperature (both in Fahrenheit and Celsius), and the availability of water and carbon dioxide all play roles. Optimal conditions vary among plant species and even among different stages of a single plant's life cycle. The whole thing is very complex and somewhat confusing.

Caloric Intake in Humans

Humans obtain their energy through the consumption of food, which is supposed to provide us with essential nutrients and calories. Calories are a unit of energy that measures the amount of heat produced when the food is metabolized. The human digestive system breaks down food into its basic components—proteins into amino acids, carbohydrates into sugars, and fats into fatty acids and glycerol. These are then metabolized to provide energy for cellular functions, growth, and maintenance. Unlike plants, humans do not have the ability to directly harness energy from light; we are dependent on external sources for their caloric intake.

Similarities and Differences

  1. Form of Energy: Both systems convert one form of energy to another—light to chemical energy in plants and chemical to mechanical/thermal energy in humans.
  2. Biochemical Reactions: Both rely on complex biochemical pathways to convert, store, and utilize energy.
  3. Growth and Maintenance: Energy is used for growth, repair, and maintenance in both plants and humans.
  4. Energy Storage: Excess energy is stored for future use—starch in plants and fat in humans.
  5. Dependency: Humans are generally dependent on plants (either directly or indirectly) for their energy needs, making them secondary consumers, while plants are primary producers.
  6. Sustainability: Plants need a continuous supply of light, water, and carbon dioxide to sustain photosynthesis, while humans require a balanced diet to meet their energy and nutritional needs.

Light as Energy for Plants

Light serves as the essential ingredient for plant growth, primarily because it fuels photosynthesis—as we've already discussed, the process where plants convert light energy into chemical energy in the form of glucose. This glucose is then stored as starch, a polysaccharide that the plant can tap into for future energy needs.

Photosynthetically Active Radiation (PAR)

I've written extensively about light before, bur for photosynthesis to occur effectively, plants require light within a specific wavelength range, usually between 400-700 nm. This spectrum is known as Photosynthetically Active Radiation (PAR). A deficiency in PAR leads to an energy shortfall in the plant, hampering its metabolic activities and, by extension, growth.

Human Caloric Intake and Metabolism

Similarly, humans need calories to fuel metabolic processes that maintain life. These calories come from carbohydrates, proteins, and fats. Carbohydrates are often the primary source of quick energy, stored as fat for future use when in excess. Much like how plants convert available light into glucose and subsequently starch, humans convert caloric intake into glycogen or fat for storage.

Coping Mechanisms During Energy Deficit

When subjected to an energy deficit, both plants and humans exhibit survival mechanisms. Plants, when experiencing a lack of adequate light, will slow down their growth, drop foliage, and even enter dormancy. They may also demonstrate etiolation—a condition where they elongate their stems and leaves in a desperate attempt to absorb more light to sustain photosynthesis, and energy production.

Humans, use the glucose in our cells and bloodstream for energy, when we exhaust this stream of quick access fuel, we start using stored fat for energy. At this point, the body will starts to break down fat which will get processed by our liver to form ketones, a secondary energy source. This is essentially a survival strategy, much like how plants use starch during a light deficit.

The Deceptive Nature of Survival Mechanisms

Interestingly, survival mechanisms for plants suffering a light deficit can often be misleading. Plants will willingly expend extra energy to produce new foliage in a light-deficient environment in an attempt to gather more light. The plant doesn't understand that the sunlight its genetics have evolved to expect isn't same when it comes to grow lights. This flurry of new foliage, often accompanied with flowering or inflorescence, often gives plant parents the false impression of a happy and healthy plant. In reality this typically healthy plant behaviour is an attempt to increase the plant's light-absorbing surface area to increase photosynthesis.

For us humans, especially when on extreme diets, our weight loss which most of us hope is from fat loss, is often our body sacrificing muscle mass to conserving fat stores for the ketones our brain needs to stay functioning at an optimal level. The term 'skinny fat' is often used to describe people who do far too much cardio, and ingest too few calories to maintain a healthy level of lean muscle. They are skinny, but they are still carrying around unhealthy levels of fat. Take a look at the cardio section at your local gym, usually there are a few dedicated examples of cardio queens in this kind of shape.

The Inevitability and Consequences of Energy (light) Deficits

The bottom line remains the same for both plants and humans: A consistent and chronic energy deficit will lead to deteriorative conditions and, ultimately, death. Plants that constantly receive less light than they need will eventually die. For humans, a prolonged calorie deficit without appropriate supplementation leads to malnourishment and can be fatal over time.

Think of light intensity/density (PPFD) like calories for a human. If you need 2,000 calories to maintain your existing body weight without tapping into my fat stores, and you eat 2,000 calories a day, you'll be a happy camper. If you only take in 1,500 calories by example, you'll suffer a 500 calorie deficit and steadily start to lose weight since that required energy will cone from your body using energy from fat. The same kind of thing happens with a plant. If it doesn't get the exposure to the light volume it needs (PPFD/DLI) to maintain itself then it will begin to use its sore of starch/carbs.

For both humans and plants the process at this point is very similar. As the stored energy is depleted biological processes ?

For a human, we can easily go to the fridge and gain the energy we need by eating more. A plant doesn't have this luxury and when it it feels 'hungry' it will try to consume more light. This is why it will often push out new foliage, or drop old, typically lower foliage, to try and be able to absorb more light. It will also often try to stretch towards the light (etiolation) to capture more photons from being closer to the light source.

For the human if we continually only are able to eat 85% of what our body needs to sustain itself, at some point it will drop below a safe level as our fat and muscle are depleted. At this point systems start to shut down and fail, we are more prone to disease and illness, we can't repair tissue, etc. If you've had the painful experience of watching a family member waste away from cancer then you know what this physical deterioration looks like. This process is similar for plants. They wither, drop foliage, are more prone to disease, pests, etc. They can't maintain basic system processes like water uptake, or transpiration as they die, this speeds up the problems they face since all things within the plant's control are internal. As these systems fail the plant has no ability to correct anything and without adequate light, and/or foliage they simply, often slowly, starve to death.

By understanding the parallel mechanisms of energy management in plants and humans, we can better appreciate the delicacy and complexity of biological systems. Light for plants is not just a preference; it's a necessity—much like how calories are indispensable for human survival.

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