Cells, Photosynthesis, Respiration, and Osmosis

Cell Organization and Cellular Organelles

All living things are composed of cells, but cells are organized in different ways. Scientists divide cells into two primary classifications: prokaryotic and eukaryotic. Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles. In contrast, eukaryotic cells contain membrane-bound organelles and a true nucleus. In all cells, the cell membrane serves as a selectively permeable barrier that regulates what enters and leaves the cell. Inside the cell membrane lies the cytoplasm, which is the material that includes the jelly-like cytosol and, in eukaryotic cells, suspended organelles.

Within eukaryotic cells, specialized structures carry out essential life processes. The nucleus contains genetic material (DNA) and directs cellular activities. Protein synthesis is carried out by ribosomes. Ribosomes on the rough endoplasmic reticulum make many proteins, while the smooth endoplasmic reticulum makes lipids; the endoplasmic reticulum also helps transport these materials. The Golgi apparatus then modifies, sorts, and packages proteins for secretion or intracellular use. For waste management, lysosomes contain digestive enzymes to break down waste materials and cellular debris. In addition, cells rely on vacuoles to store water, nutrients, and waste products.

Plant and animal cells share many common features, but they also have key differences. Plant cells have cell walls, and many plant cells have chloroplasts, whereas animal cells have neither. The plant cell wall provides structural support, rigidity, and protection outside the cell membrane. Chloroplasts contain chlorophyll and are the sites of photosynthesis in plant and algae cells. Furthermore, plant cells typically contain a large central vacuole that maintains internal turgor pressure, keeping the cell firm.

Photosynthesis: Converting Light into Chemical Energy

Photosynthesis is the fundamental biological process that converts light energy from the sun into chemical energy stored in glucose. This process provides the primary source of food energy for terrestrial and aquatic food webs. Without photosynthesis, energy from sunlight could not be captured and converted into a usable form of food for living systems.

To carry out photosynthesis, autotrophic organisms use carbon dioxide and water as reactants and require light energy. Plants absorb water from the soil primarily through their roots. At the same time, carbon dioxide enters plant leaves through small openings called stomata. Once these starting materials are gathered within the plant, chemical reactions take place inside specialized organelles.

The actual site of photosynthesis in plant and algae cells is the chloroplast. Chloroplasts contain chlorophyll, which is the green pigment that absorbs light energy needed to drive the reaction. The primary products of photosynthesis are glucose and oxygen gas. The glucose provides stored chemical energy, while the oxygen produced during photosynthesis can be released into the air or surrounding water as a byproduct.

Cellular Respiration and the Energy Cycle of ATP

Cells must convert the chemical energy in food into a form they can directly use for work. Cellular respiration breaks down glucose to produce usable cellular energy in the form of ATP. ATP stores energy within the chemical bonds between its phosphate groups. When a cell requires energy to perform a task, energy is released from ATP when a phosphate group is removed, converting ATP into ADP.

Cellular respiration begins in the cytoplasm with glycolysis, which is the first stage of cellular respiration and occurs without requiring oxygen. Following glycolysis, if oxygen is available, eukaryotic cells continue through aerobic cellular respiration. In eukaryotic cells, the aerobic stages of cellular respiration occur in the mitochondria. Aerobic cellular respiration requires glucose and oxygen as reactants. Mitochondria carry out most stages of aerobic cellular respiration and produce most of the cell's ATP. The primary products of aerobic cellular respiration are carbon dioxide, water, and ATP. The carbon dioxide produced by cellular respiration is exhaled or released as metabolic waste.

When oxygen is absent or limited, cells rely on anaerobic pathways. Fermentation, illustrated by the muscle-cell and yeast examples below, allows glycolysis to produce a small amount of ATP without oxygen; anaerobic respiration is a distinct process that uses an electron acceptor other than oxygen. For example, lactic acid fermentation occurs in animal muscle cells during intense exercise when oxygen is limited. In contrast, in the absence of oxygen, yeast break down glucose through glycolysis and alcoholic fermentation, yielding ATP, ethanol, and carbon dioxide.

Photosynthesis and aerobic cellular respiration help cycle matter through ecosystems, but energy flows through living systems and is eventually dissipated as heat. The products of photosynthesis (glucose and oxygen gas) serve as the primary reactants for aerobic cellular respiration. In return, the carbon dioxide and water produced by cellular respiration are reactants for photosynthesis.

Passive Transport: Diffusion and Osmosis

To stay alive, cells must continuously exchange molecules with their external environment through the cell membrane. Because the cell membrane is selectively permeable, it allows certain substances to cross while restricting others. Movement across the membrane is categorized into passive transport and active transport. Passive transport, including diffusion and osmosis, does not require cellular energy (ATP). On the other hand, active transport uses cellular energy, directly or indirectly, to move substances against their concentration gradients.

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration. A familiar everyday example occurs when a drop of food coloring is placed into a glass of plain water: over time, the dye particles spread naturally from the concentrated drop throughout the entire glass without any stirring.

Osmosis is a specific category of passive transport. Specifically, osmosis is the diffusion of water across a selectively permeable membrane. In osmosis, water moves toward regions of higher solute concentration. Because osmosis is a passive process, water molecules move naturally without any expenditure of cellular energy (ATP).

Osmotic Environments: Cell Responses in Different Solutions

The concentration of solutes outside a cell compared to inside determines the direction of water movement during osmosis. Scientists classify solutions as isotonic, hypertonic, or hypotonic. An isotonic solution has an equal solute concentration compared to the interior of a cell. In an isotonic solution, water moves in and out of the cell at equal rates, resulting in no net volume change.

A hypertonic solution has a higher solute concentration than the interior of a cell. When placed in a hypertonic solution, water leaves the cell, causing the cell to shrink. In plant cells, this water loss causes plasmolysis, a condition that occurs when a plant cell loses water in a hypertonic environment and the cell membrane pulls away from the cell wall.

A hypotonic solution has a lower solute concentration than the interior of a cell. When placed in a hypotonic solution, water enters the cell, causing animal cells to swell and potentially burst. Plant cells respond differently because plant cell walls prevent plant cells from bursting when placed in hypotonic environments. Instead, incoming water fills the large central vacuole, generating turgor pressure. Turgor pressure is the pressure of a plant cell's contents against its cell wall, helping keep nonwoody tissues firm and upright.