Photosynthesis and Cellular Respiration

Photosynthesis vs. Cellular Respiration

• Photosynthesis is performed by plants, while cellular respiration is performed by plants and animals.
• Photosynthesis converts radiant energy into chemical energy by storing glucose, whereas cellular respiration breaks down the chemical energy stored in glucose into ATP.
• Photosynthesis occurs in chloroplasts, while cellular respiration occurs in mitochondria.
• Photosynthesis is an anabolic process, and cellular respiration is catabolic.

Anabolic and Catabolic Processes

• Anabolic processes synthesize larger molecules from smaller ones and require energy (endothermic). Photosynthesis is anabolic because it constructs glucose.
• Catabolic processes break down larger molecules into smaller ones, releasing energy (exothermic). Cellular respiration is catabolic because it breaks down glucose into ATP.

Endosymbiotic Theory

• Proposes that an ancestor of cyanobacteria was engulfed by an ancestor of today's eukaryotic cells, resulting in a mutually beneficial association.
• The cyanobacterium was protected from a harsh environment, and the eukaryotic host obtained food molecules.
• This association eventually led to the evolution of plant cells.
• Like chloroplasts, cyanobacteria contain chlorophyll a and carry out photosynthesis.
• Unlike plant cell chloroplasts, cyanobacteria contain chlorophyll b and phycobilin.
• Cyanobacteria lack membrane-bound organelles like nuclei.

Photosynthesis Basics

• Almost all plants, some bacteria, and protists are photosynthetic autotrophs.
• Autotrophs generate their own organic matter through photosynthesis.
• Radiant energy is transformed into chemical energy.

Leaves: The Photosynthetic Organ of Plants

• Leaves maximize surface area for sunlight exposure and limit the distance for gases to reach chloroplasts.
• The cuticle is a waxy layer above the epidermis that provides moisture retention and protection.
• Stomata are openings between guard cells that allow gas exchange, while guard cells control the opening and closing of stomata.
• The mesophyll is the middle tissue of the leaf, palisade mesophyll is the site of photosynthesis, and spongy mesophyll absorbs gases and some water.
• Vascular bundles are found in the vein: xylem transports water and minerals, and phloem transports sugars.

Chloroplasts

• Chloroplasts are the location of photosynthesis in plants and algae, containing an outer and an inner membrane.
• Stroma is the semi-liquid material within the chloroplast, used to create glucose.
• Thylakoids are membrane-bound sacs that stack to form a granum, and lamellae are thin membranes that connect grana.
• Photosynthesis occurs in the stroma and thylakoid membrane.
• The thylakoid membranes enclose a space known as the lumen.
• Thylakoids with a greater surface area significantly increases the efficiency of photosynthesis.
• Chloroplasts contain their own DNA and can replicate.

Pigments

• A pigment is an organic compound that gives living things their color, including skin, hair, eyes, and plants.
• Pigments absorb and reflect different wavelengths of light, causing things to appear certain colors.

Light Energy

• Light comes in the form of photons, which are packets of electromagnetic radiation
• Photons have different wavelengths and energy: shorter wavelengths have more energy, and longer wavelengths have less energy.
• The visible light spectrum ranges from 380 nm (violet) to 750 nm (red).
• Photosystems are clusters of photosynthetic pigments that absorb light energy and initiate photosynthesis.
• Different pigments respond to different wavelengths.
• Only 40% of the sun's energy is intercepted by plants, and only 5% of that is used in photosynthesis.

Chlorophyll, Accessory Pigments, and Leaf Colors

• Chlorophylls a and b absorb photons in the blue-violet and red regions and reflect green light.
• Chlorophyll a is the primary photosynthetic pigment.
• Chlorophyll b and carotenoids act as accessory pigments, absorbing photons that chlorophyll a misses.
• In spring and summer, leaves appear green due to high chlorophyll concentrations.
• Xanthophylls (yellow) and carotenoids (orange) are accessory pigments in thylakoid membranes.
• Anthocyanins (red, violet, blue) are located in plant cell vacuoles and formed in autumn.
• In cooler temperatures, plants break down chlorophyll, revealing yellow, red, and brown colors.
• The combination of chlorophylls a and b with accessory pigments covers the entire visible spectrum.
• Wavelengths from 400 nm to 700 nm are photosynthetically active radiation (PAR).

Summary

• Photosynthesis employs chlorophyll and is opposite of respiration.
• The endosymbiotic theory explains the origins of plants.
• Chloroplasts convert light energy into chemical energy.
• Chlorophyll a and b, along with other pigments, facilitate photosynthesis.
• Light includes photons of electromagnetic radiation.