Biology Practice Test: Cell Structure and Function

Seeing one cell divide into two shows new cells arising from an existing cell, which is the third idea of the cell theory. The claim that cells form from nonliving matter is the old idea of spontaneous generation, which the cell theory rejects.

Mitochondria carry out aerobic cellular respiration and make most of a cell's ATP. Cells with a high energy demand, like working muscle cells, have many of them. Lysosomes break down wastes and do not supply energy.

Every cell needs ribosomes to make proteins, so prokaryotes and eukaryotes both have them. Prokaryotes have no nucleus and no membrane-bound organelles such as mitochondria or endoplasmic reticulum; their DNA lies in the cytoplasm.

A cell wall made of cellulose is typical of plant cells, and the nucleus shows the cell is eukaryotic. Bacteria do have cell walls, but they are made of peptidoglycan, and bacteria have no nucleus.

Proteins meant for export are built by ribosomes on the rough ER, then modified and packaged in the Golgi apparatus, and carried in vesicles that fuse with the cell membrane. The Golgi apparatus cannot come first, because it processes proteins that have already been made.

The membrane is two layers of phospholipids. The hydrophilic heads face the watery fluid inside and outside the cell, and the hydrophobic tails point toward each other in the middle. Proteins are embedded in this bilayer (the fluid mosaic model).

Distilled water is hypotonic to the cells, so water moves by osmosis into the cells, where the solute concentration is higher. Animal cells have no cell wall to resist the pressure, so they swell and can burst. Shrinking happens in a hypertonic solution, the opposite case.

The salt solution is hypertonic, so water leaves the cells and the central vacuole shrinks. The membrane pulls away from the rigid cell wall (plasmolysis), while the wall itself keeps its shape. Plant cells do not burst, because the wall protects them.

Moving a substance from low to high concentration requires energy, so this is active transport (the sodium-potassium pump uses ATP). Facilitated diffusion also uses proteins, but it only moves substances down their gradient, from high to low concentration.

Movement down a concentration gradient with the help of a membrane protein and no energy input is facilitated diffusion. Simple diffusion would mean glucose slips directly through the lipid bilayer, which large polar molecules cannot do well.

Surface area = 6 × (2 cm × 2 cm) = 24 cm² and volume = 2 × 2 × 2 = 8 cm³, so the ratio is 24 : 8 = 3 : 1. A 1 cm cube would have 6 : 1; as a cell grows, its ratio drops, which is one reason cells stay small and divide.

Similar cells form a tissue, several tissues form an organ (such as the stomach), organs work together in an organ system (such as the digestive system), and organ systems make up the organism.

DNA is replicated during the S (synthesis) phase of interphase, before mitosis begins. In prophase the already copied chromosomes condense, and in anaphase the sister chromatids are pulled apart.

Enzymes are proteins; too much heat breaks the weak bonds that hold their shape, so the active site no longer fits the substrate (the enzyme is denatured). Molecules actually move faster when it is hotter, and enzymes are not used up in the reactions they speed up.

The response (lower blood glucose) counteracts the stimulus (high blood glucose) and then shuts off the signal, which is negative feedback. This is how the body maintains homeostasis. Positive feedback would push glucose even higher.