Cell Structure and Function

The cell is the smallest unit that can carry out all the functions of life. Every living thing is built from one or more cells, and everything a cell does (growing, using energy, responding to its surroundings, reproducing) happens inside that cell or across its boundary. The cell theory states three ideas that hold true across all known life: 1. All living things are made of one or more cells. 2. The cell is the basic unit of structure and function in an organism. 3. All cells come from pre-existing cells.

All cells fall into one of two broad categories based on whether their genetic material is enclosed in a nucleus. A prokaryotic cell has no nucleus and no membrane-bound organelles. Its DNA floats freely in a region of the cytoplasm called the nucleoid. Prokaryotic cells belong to the domains Bacteria and Archaea, and they are generally simpler and smaller. A eukaryotic cell has a true nucleus that encloses its DNA inside a nuclear envelope, and it contains membrane-bound organelles that divide the cell into specialized compartments. Eukaryotic cells make up protists, fungi, plants, and animals.

Every cell is wrapped in a plasma membrane, a thin, flexible boundary built mainly from a double layer of phospholipids (the phospholipid bilayer) with proteins embedded in it. The membrane controls what enters and leaves the cell, so it is described as selectively permeable: small molecules like O₂ and CO₂ pass through easily, while larger or charged molecules need help from membrane proteins to cross. Inside the membrane is the cytoplasm, the jelly-like fluid that fills the cell. The cytoplasm holds the organelles in place and is where many chemical reactions of the cell take place. The fluid portion of the cytoplasm, not counting the organelles, is called the cytosol.

Plant and animal cells are both eukaryotic and share most of the same organelles, but plant cells have three structures animal cells lack, built for a life that makes its own food and does not move around.

The nucleus stores the cell's DNA and directs the cell's activities, so it is often called the control center of the cell. It is enclosed by the nuclear envelope, a double membrane with pores that let RNA and proteins pass between the nucleus and the cytoplasm. Inside the nucleus, DNA is packaged with proteins into a material called chromatin, which condenses into visible chromosomes when a cell divides. Most nuclei also contain a nucleolus, a dense region where ribosomal RNA is made and ribosomes are assembled before being sent out into the cytoplasm. Example: A skin cell needs to make a protein. The instructions for that protein are copied from DNA inside the nucleus, and the copy exits through a pore in the nuclear envelope to reach the ribosomes in the cytoplasm, where the protein is actually built.

Several organelles work together as a production line to build and ship proteins. Ribosomes are small structures, not enclosed by a membrane, that read genetic instructions and link amino acids together to build proteins. Ribosomes may float free in the cytoplasm or be attached to the endoplasmic reticulum. The endoplasmic reticulum (ER) is a network of folded membranes that extends from the nuclear envelope through the cytoplasm. Rough ER is studded with ribosomes on its surface and folds and processes the proteins those ribosomes make. Smooth ER has no ribosomes and instead builds lipids and helps break down toxins. The Golgi apparatus is a stack of flattened membrane sacs that receives proteins and lipids from the ER, finishes modifying them, packages them into vesicles, and ships them to their final destination inside or outside the cell.

Cells need a steady supply of usable energy, stored in a molecule called ATP. The mitochondrion (plural mitochondria) is the organelle where cellular respiration takes place, breaking down glucose in the presence of oxygen to release energy and store it in ATP. Mitochondria have their own small circular DNA and a double membrane, with the inner membrane folded into ridges called cristae that increase the surface area for this process. Nearly all eukaryotic cells have mitochondria, since nearly all eukaryotic cells need ATP. The overall equation for cellular respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP) The chloroplast is found only in plant cells and some protists, and it is the site of photosynthesis, capturing light energy and using it to build glucose from carbon dioxide and water. Like mitochondria, chloroplasts have their own DNA and a double membrane. The overall equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with light energy) Example: A leaf cell contains both chloroplasts and mitochondria. During the day, its chloroplasts use sunlight to build glucose. The cell's mitochondria then break down some of that glucose, day or night, to release usable energy as ATP.

Vacuoles are membrane-bound sacs used for storage. Plant cells typically have one large central vacuole that stores water and dissolved substances and presses outward against the cell wall to keep the cell rigid, a state called turgor pressure. Animal cells may have several small vacuoles instead. Lysosomes are membrane-bound organelles filled with digestive enzymes. They break down worn-out organelles, waste, and material taken in from outside the cell. Lysosomes are common in animal cells and rare in plant cells. The cytoskeleton is a network of protein fibers that runs through the cytoplasm. It gives the cell its shape, holds organelles in place, and allows the cell (or structures inside it) to move.

Using cell-observations.csv and the classification rule above: 1. Specimen 1 has a cell wall but no nucleus. Is it prokaryotic or eukaryotic, and how do you know? 2. Which specimen(s) classify as prokaryotic? 3. Specimen 3 has a nucleus and membrane-bound organelles, but no wall, chloroplasts, or vacuole. Classify it, and explain why it isn't a plant cell. 4. Specimen 6 has a nucleus but no chloroplasts. Does it classify as plant or animal? Which single missing structure decides it? 5. Which specimen(s) classify as plant?