Photosynthesis and Cellular Respiration on the farm

All life processes are supported by the simple sugar glucose. Glucose is both the primary source of energy for these processes and an important building block for many other compounds. Plants capture light energy through their leaves and use it to convert carbon dioxide and water into glucose. The process is called ‘photosynthesis’. Photosynthesis is the source of all of the organic compounds and most of the energy used to sustain life on Earth. Oxygen is a by-product of photosynthesis. Photosynthesis  produces most of the oxygen in our atmosphere.

Plant leaves contain pigments called chlorophyll that absorb red and blue wavelengths of light and reflect green light, which is why leaves appear green to our eyes. When a chlorophyll molecule absorbs a single photon of light, it releases a single electron that is used to drive the reactions that create glucose.

Chlorophyll is contained in organelles (Structures within cells e.g. chloroplasts) known as chloroplasts. Inside the chloroplasts, the chlorophyll is stored (A milk pattern which involves a series of lines applied to the surface of a drink, using the pushing technique; this design harnesses the effect of the eddies to stretch each line around the edges of the cup. This pattern is an extension of the tulip design.) in the membrane of little green pancake-like stacks known as thylakoids(A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria). The light-dependent reactions take place at the thylakoid membrane that surrounds these little pancakes. It is also here at the that the oxygen we breathe is created.

• What Other Ingredients Do Plants Need to Produce Glucose?

Plants need carbon dioxide and water to produce glucose, which is made of carbon, hydrogen, and oxygen. 6 CO2 + 6 H2O → C6H12O6 + 6 O2. (→ = Light energy plus chlorophyll).

A plant’s roots take up water from the soil. Xylem, a woody tissue containing bundles of capillaries, transports water and minerals throughout the plant. The carbon dioxide that is needed for photosynthesis comes directly from the atmosphere. It is taken into the leaves of plants through small pores known as stomata(The pores in a leaf that open and close when the plant requires to allow gases and moisture to diffuse in and out of the leaf when needed). The stomata allow the CO₂ to diffuse into the leaf.

In this image, you can see the leaves outer layer called the epidermis, peeled away to reveal the plant cells inside the leaf which contain the chloroplasts

Light-dependent Reactions

The first step in photosynthesis is the light-dependent reaction. In this step, chlorophyll molecules absorb photons and use this energy to release an energized electron. The electron is passed to a chain of molecules and enzymes that use it to create two energy-carrying molecules, ATP and NADPH. To replace the lost electrons, the chlorophyll splits water molecules, absorbing electrons and giving off oxygen gas and hydrogen ions: 2H2O → O2 + 4H+ + 4e–. ATP and NADPH then go on to take part in the light-independent (or ’dark’) reactions, of the Calvin cycle.The Calvin cycle is a circular series of reactions that use the energy-carrying molecules (ATP and NADPH) created in the light-dependent reactions to ‘fix’ carbon dioxide into organic molecules.In the Calvin cycle, one CO2 molecule reacts with ribulose bisphosphate (RuBP), a molecule with five carbon atoms, adding one carbon atom to create two molecules of glyceraldehyde 3-phosphate (GA3P), containing three carbon atoms each. Five of every six molecules of GA3P produced in the cycle are used to regenerate RuBP. Five molecules of GA3P (with three carbon atoms each) create three molecules of RuBP (with five carbon atoms each). The sixth molecule of GA3P is used to create glucose. Two GA3P molecules create a single molecule of glucose containing six carbon atoms. The energy-carrying molecules ATP and NADPH drive the series of reactions forward.

The Calvin Cycle: this diagram shows you where each ingredient in the production of glucose enters the cycle

How Do Plants Make Use of Their Glucose Supply?

The glucose created during photosynthesis provides the energy for all the other cellular processes in the plant. Plants can transport glucose to where it is needed via a second type of vascular tissue called phloem. Xylem carries water and minerals up to the leaves to be used in photosynthesis, and phloem carries glucose back to other parts of the plant. The xylem and phloem together form the ‘veins’ in the leaves and stems of plants.When glucose reaches the cells, it can be used to release energy, taking in oxygen and releasing carbon dioxide and water in a process known as respiration. This process takes place in organelles called mitochondria. Mitochondria are found in nearly all cells in both plants and animals. Respiration generates more of the energy-carrying molecules ATP and NADPH, which are then used to drive other cellular reactions. Because respiration consumes the glucose and oxygen created during photosynthesis and releases carbon dioxide, water, and energy, it can be thought of as the ‘opposite’ of photosynthesis.

Carbohydrate Synthesis and molecules effect

As well as functioning as an energy source used for respiration, plants can use glucose to create more complex carbohydrates, such as starch and cellulose, and a range of other molecules. Both starch and cellulose are made of long chains of glucose molecules joined together.

Cellulose: Cellulose is made of long, straight chains of glucose molecules. Cellulose chains join together to make long, strong fibers. These fibers form the cell walls that surround plant cells, which make them rigid and strong. Cellulose can’t be digested by animals, and so it forms a large part of the fiber content of your food. Although cellulose is carbonized during roasting, much of the structure of the cell walls remains intact. This structure determines the way coffee beans shatter during grinding. If the individual cells in the seeds are smaller, the beans will be harder and denser as a result. Some cellulose also breaks down during roasting to create citric acid. (T Nakabayashi, 1978.)

Starch: When glucose molecules are joined together in a different way, they form starch. Starch consists of branched chains of glucose molecules. At the end of each branch, glucose molecules can be added or removed as needed, which means starch can function as a glucose storage molecule. Starch is stored in all plant cells but especially in fruits, seeds, rhizomes, and tubers, and near the tips of branches, to prepare for the next growing season. Little if any starch is present in coffee beans.

Proteins: Plants use glucose, combined with nitrates from the soil, to create amino acids, the ‘building blocks’ of proteins. Nitrates in the soil are thus essential to plant growth. Nitrates occur naturally in soil; they are created by bacteria and during lightning strikes. Adding extra nitrates to the soil, in the form of organic matter (compost) or chemical fertilizer, can speed up plant growth considerably. The sulfur atoms found in proteins are an important part of many molecules responsible for the aromas of coffee. For example, certain mercaptans have a characteristic smell of roasted coffee.

Sucrose: Glucose can be converted to fructose, which joins to another molecule of glucose to create sucrose. Sucrose is the sugar that makes fruits taste sweet, enticing animals to eat the ripe fruit and thereby spread the mature seed. Sucrose in coffee seeds breaks down during roasting, taking part in caramelization and Maillard reactions to create many of the molecules responsible for coffee’s complex flavor. The sucrose also factors in the production of organic acids, including acetic and lactic acids.

Lipids: Glucose is also converted into lipids (fats). Lipids are a concentrated form of energy storage in plant seeds, and they support the growth of the seedling. Lipids in coffee include terpenes, which are responsible for some highly desirable flavor attributes (for example, limonene) and are thought to be responsible for some of coffee’s health benefits.

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The most intense solar radiation reaches a plant growing in the tropics when the sun is directly overhead. As the Earth tilts, the solar intensity is reduced because the radiation is spread over a larger area.With some simple trigonometry, you can see that a shift in the angle of the sun of 60° reduces the available sunlight by approximately half. A coffee farm located on the Tropic of Capricorn will receive direct sunlight on the 21st of June. But by the 22nd of December, the angle of the sun will have shifted from 23° 26′ south to 23° 26′ north. this change results in direct sunlight being spread over roughly 36 percent more land area, reducing the intensity of light where it strikes the earth. However, the times when the sun is directly overhead roughly corresponds with the rainy season, which can result in significant cloud cover obscuring the sun’s rays in an effect known as albedo. Based on observations on Reunion Island, sited in the Indian Ocean, east of Madagascar, Bertrand et al., 2012 reported that solar radiation was negatively correlated with elevation, due to the frequent cloudy weather in the highlands, and positively correlated with temperature.’ In other words, increased solar radiation and temperature occurred in areas of lower elevation.

• During the season when the sun is directly overhead in the tropics, the combination of a lot of rain and a lot of light represents an intense growing phase for coffee plants. At this time of year, coffee plants tend to flower. Coffee farms located closer to the equator have some complexity regarding this seasonal fluctuation, however, because they can experience two rainy seasons per year. The biological clock of a coffee plant usually works on an annual cycle, but in some places on the equator (such as Colombia), some regions can have trees flowering while in a neighboring valley the farmers are harvesting.

Albedo

• Sunlight can reach a plant from below as well as from above. One means of this is by reflection from the Earth’s surface. In wine growing this is known as albedo. The extreme form of albedo comes from fresh snow, which will reflect over 80 percent of the solar radiation. High up, clouds in the stratosphere can reflect 70 percent of the solar radiation. Dark-colored, wet soil (typical of most coffee farms) will reflect only around 10 percent of the sun’s radiation. This is very similar to the amount that will be reflected from forest cover.The availability of sunlight is the major rate-limiting factor in the process of photosynthesis by coffee plants. For this reason, coffee farmers may thin the forest canopy as their plants mature.

Aspect

• Aspect refers to the orientation of a hillside relative to its compass bearing. It is an established principle of winemaking that the angle and orientation of a hillside can alter the flavour of a wine. The classic example of this is the difference between north- and south-facing slopes. If your coffee farm were on the Tropic of Capricorn in your rainy season, the sun will be aligned with the Tropic of Cancer. In this situation, a north-facing slope receives the most direct sunlight and a south-facing slope receives the least possible amount of light exposure.(Bertrand et al., 2012)concluded that the terroir of the coffee plant determined the sensory characteristics and chemical contents of its beans. They also found that the plant’s altitude and slope exposure created nuances in the sensory characteristics of coffees grown within a terroir.
• What’s the difference between Eastern or Western Aspect?

Plants with an east-facing aspect receive the first morning light, making them drier than plants with a west-facing aspect. The dew and rain begin to evaporate sooner in the day than they would on a west-facing aspect, so they have a head start. Plants with a west-facing aspect are usually warmer than those on a south-facing aspect, so ripening tends to occur more quickly. The last two decades have seen increases in the land area devoted to shade-grown coffee, but at the same time, non–shade-grown production has increased almost exponentially. ‘Shade-grown’ now describes around 24 percent of the land used for coffee. This amount is down from 43 percent in 1996. Yield-focused government incentives have been the driver for the widespread adoption of full-sun farming over the past two or three decades. Coffee research institutes created in the 1970s and 1980s , promoted the reduction or removal of shade cover.

• There is some controversy around the subject of shade. A disconnect exists between conservationists looking to maintain biodiversity and the viewpoint of yield-driven government incentives, aimed at increasing farmer prosperity. However, the literature points towards a happy medium here. Studies … have predominantly revealed that intermediate shade levels (approximately 35%–50%) produce the highest coffee yield, which is probably because of the balance maintained between optimal temperatures in shaded environments and optimal photosynthetic rates in unshaded environments … Because coffee yields are typically assessed independently of yield from timber, other crops, or ecosystem services, it may be difficult for governments and conservation institutes to weigh the benefits of diversified farming approaches. High yields don’t always equate with high quality.

Does Shade Grown Coffee Taste Better?

• it is clear that shade coverage is able to reduce average temperatures for coffee plants. using 45 percent shade netting found a significant difference between inner and outer leaf temperatures of coffee plants and a significant overall temperature drop. we measured differences of 4◦ C for inner leaves (measure from the trunk up to the sixth leaf) and 2◦ C for outer leaves. The same experiment accumulated sensory impressions of coffee grown under differing levels of shade cover. The chart below records the findings of their sensory panel. In addition, to testing full sun and shade, they also tested fruit load by removing a quarter and a half of the fruit from certain trees. The reason for this is that full sun plants tend to overbear and so the experiment sought to test if pruning could counteract this issue whilst still yielding good tasting coffee under full sun. The panel showed a clear preference for the shade-grown coffee over two growing seasons. A scientific study conducted on Reunion Island (the site of the Typica variety’s famous mutation into the Bourbon variety) collected sensory and chemical data from sixteen microclimates across the island. This research found a correlation between a cooler climate and positive sensory performance. Positive quality attributes such as acidity, fruity character and flavour quality were correlated and typical of coffees produced at cool climates.’
• One theory to explain why coffee may taste better in shade is the slower maturation of the fruit. In the case of the Reunion Island sensory trials, In a warmer micro-environment with high irradiance, coffee berries ripened faster in full sun than in shade. Therefore the harvest peak was delayed by about 1 month owing to shade. The slowed-down ripening process of coffee berries at higher elevations (lower air temperatures), or under shading, allows more time for complete bean filling Vast, yielding beans that are denser and far more intense in flavour than their neighbors grown at lower altitudes. Tropical climates are characterized by a reduced seasonal temperature variation. Where large changes in temperature do occur, altitude is usually the main modulating factor. But shade can give the farmer the ability to ‘micro-adjust’ the climate. The Reunion Island study confirmed that temperature during seed development has a major effect on the flavour of roasted coffee. The sensory trials found multiple correlations between coffee quality and lower temperatures. Coffees produced in regions with a cool climate (more elevated) are more acidic, have a better aroma quality and display fewer flavor defects than those produced in warmer regions (less elevated). Conversely, coffees grown under the hottest temperature conditions have lower acidity, lower aromatic quality, as well as the presence of green and earthy off-flavors … Aroma quality, acidity, fruitiness and overall quality were favored by cool climates, whilst the undesirable earthy and green tastes were increasingly present as the temperature increased. It therefore appears that the quality was weakest under warm climates.

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