2.1 Cell Structures and Membranes
Key Takeaways
- Regents cell questions usually test structure-function reasoning: identify what a structure does, then explain how that function supports a larger system.
- The cell membrane is selectively permeable, so diffusion, osmosis, and active transport are central to cell homeostasis.
- Plant and animal cells share core organelles, but chloroplasts, large central vacuoles, and cell walls change how plant cells store, make, and maintain materials.
- In cluster questions, read diagrams and data first; a correct answer must match the evidence, not just a memorized organelle name.
Cell Structures and Membranes
NYSED's Life Science: Biology educator guide emphasizes evidence about interacting living systems, so cell questions are rarely just vocabulary checks. A cluster may show a microscope image, a membrane model, a data table from a saltwater experiment, or a passage about a tissue. Your job is to connect structure and function: what part is shown, what it does, and how that function supports a larger cell, tissue, organ, or organism.
A cell is the smallest unit that can carry out life processes. A multicellular organism is organized as cells, tissues, organs, organ systems, and the whole organism. Regents traps often come from stopping at the wrong level. A red blood cell shape supports transport in blood, but blood transport also depends on circulation, respiration, and the cell membrane allowing exchange with body fluids.
Core Organelles to Know
| Structure | Main Regents Function | Common Evidence Clue |
|---|---|---|
| Cell membrane | Selective boundary; controls movement in and out | A model with phospholipids, channels, receptors, or concentration differences |
| Nucleus | Stores DNA; directs gene expression through instructions | A diagram pointing to chromosomes or instructions for proteins |
| Ribosome | Builds proteins | A question about enzymes, receptors, antibodies, or structural proteins |
| Mitochondrion | Releases usable energy from food molecules as ATP | Oxygen use, cellular respiration, active cells, muscle work |
| Chloroplast | Converts light energy into chemical energy in sugars | Green plant cells, light, carbon dioxide uptake, oxygen release |
| Vacuole | Stores water, wastes, or materials | Plant cell turgor, storage, large central space |
| Cell wall | Rigid support outside plant, fungal, or bacterial membranes | A plant cell staying firm in water or a shape that does not burst easily |
Do not treat the nucleus as a brain or mitochondria as a place where energy is created from nothing. Energy is transformed, stored, and transferred. Mitochondria release energy stored in food molecules into ATP, and chloroplasts store light energy in sugar. Also remember that most body cells in an organism contain the same DNA, but different genes are used in different cell types.
The Membrane as a Homeostasis Tool
A cell membrane is selectively permeable, meaning some substances cross more easily than others. Small nonpolar molecules may pass through the lipid part. Water can move by osmosis, often through channel proteins. Ions, glucose, and larger polar substances usually require proteins, vesicles, or energy. That selectivity lets cells keep internal conditions different from the surrounding fluid.
| Process | Energy Needed? | Direction | Regents Example |
|---|---|---|---|
| Diffusion | No | High concentration to low concentration | Oxygen moving into cells where oxygen is lower |
| Osmosis | No | Water moves toward higher solute concentration | Plant cells becoming firm in fresh water |
| Facilitated diffusion | No | High to low through a protein | Glucose crossing through a carrier |
| Active transport | Yes | Low to high through a pump | Root cells taking minerals from dilute soil water |
| Endocytosis/exocytosis | Yes | Vesicle movement across membrane | A white blood cell engulfing material or a gland secreting protein |
Science-Data Example
Suppose potato strips are placed in three solutions for 30 minutes. In distilled water, average mass rises from 10.0 g to 10.8 g. In 5 percent salt solution, mass falls to 9.1 g. In 1 percent salt solution, mass remains near 10.0 g. The evidence shows water entered cells in distilled water, left cells in concentrated salt solution, and had little net movement in the 1 percent solution. A strong Regents answer would not simply say "osmosis happened." It would identify the concentration difference and connect water movement to cell volume or turgor.
This style also appears with diagrams of red onion cells, dialysis tubing, or animal cells. If the outside solution is hypertonic, water leaves the cell and the cell shrinks. If the outside solution is hypotonic, water enters. An animal cell can swell because it lacks a cell wall; a plant cell often becomes turgid because the wall resists expansion.
Regents Traps
- Trap: all movement across membranes needs energy. Diffusion, osmosis, and facilitated diffusion do not require cellular energy. Active transport and vesicle transport do.
- Trap: water moves toward more water. For exam reasoning, phrase it as water moves toward the side with higher solute concentration when the membrane allows water through.
- Trap: plant cells do photosynthesis instead of respiration. Plant cells with mitochondria also carry out cellular respiration.
- Trap: organelles work alone. Proteins may require DNA instructions in the nucleus, assembly at ribosomes, modification or transport through membranes, and energy from mitochondria.
How to Answer a Cluster Item
First identify the system scale: molecule, cell, tissue, organ, or organism. Next read the diagram labels, axes, and units. Then choose the answer that links the structure to the measured function. For constructed responses, use this sentence frame: "The data show ___; this supports ___ because the cell structure ___ allows ___." That frame keeps the response evidence-based, which matters because NYSED scoring uses the administration's rating guide rather than a general impression of whether an answer sounds reasonable.
Membrane Models in Stimulus Questions
When a diagram shows a membrane as two layers with embedded proteins, treat it as a model with limits. It can show selective transport and receptor binding, but it may not show actual scale, motion, or every molecule present. If an item asks for a limitation of the model, choose the answer that names something the model leaves out rather than an answer that contradicts membrane function.
For comparison questions, ask whether the structure increases surface area, controls materials, stores instructions, releases ATP, or gives support. That question is faster than memorizing every label separately, and it protects you when the cluster uses an unfamiliar cell type such as a root hair cell, nerve cell, or immune cell.
A plant cell is placed in a solution with a much higher solute concentration than the cell cytoplasm. What membrane process best explains why the cell loses volume?
A student observes many mitochondria in muscle cells from an animal. Which explanation best connects structure and function?
Which evidence would most directly support the claim that a membrane protein is needed for a substance to enter a cell?