Cells and transport across membranes
Connect common cell structures with their roles, then reason about diffusion and osmosis under stated conditions.
In this guide
A cell is an organized working system
Cells have a boundary that separates their internal environment from their surroundings. Typical prokaryotic and eukaryotic cells contain a plasma membrane, cytoplasm, genetic material and ribosomes; specialized cells may lose some components as they mature. Eukaryotic cells characteristically organize many functions in membrane-bound compartments, including a nucleus. Prokaryotic cells do not have a membrane-bound nucleus. Do not turn that distinction into 'prokaryotes have no DNA' or 'prokaryotes cannot make proteins.' Both statements would confuse the location or organization of a component with its existence.
Match structures with functions
Ribosomes assemble proteins. In a typical eukaryotic cell, mitochondria carry out major steps of aerobic cellular respiration, while the nucleus contains most of the cell's DNA. Chloroplasts carry out photosynthesis in photosynthetic plant cells and algae. A plant cell can contain both chloroplasts and mitochondria: photosynthesis does not eliminate the need for cellular respiration. Not every plant cell contains chloroplasts, so a claim about a leaf's photosynthetic cells should not automatically be applied to every root cell.
Worked example: diffusion has a net direction
Imagine a divider with openings that allow a particular dissolved substance to pass. Initially, the concentration is higher on the left than on the right. Under otherwise comparable conditions, random particle motion produces a net transfer down that concentration gradient. Individual particles may move in either direction; net movement describes the difference between those flows. At equilibrium, particles continue moving even when there is no overall concentration-driven transfer. Saying that diffusion ends because all molecular motion stops is therefore incorrect.
Osmosis depends on what can cross
Consider a simple model with a membrane permeable to water but not to the dissolved solute. Both sides begin at the same pressure and temperature. One side contains a more concentrated solution. Initially, water tends to move toward that side. This is osmosis: net movement of water across a selectively permeable membrane. The model's conditions matter because pressure differences and solute permeability affect the outcome. A statement that 'water always moves toward more solute' without specifying the system is too broad.
Passive does not mean motionless
Passive transport does not require the cell to supply metabolic energy directly for the transport process. Facilitated diffusion can use membrane proteins while remaining passive. Active transport uses an energy source to drive movement against an electrochemical gradient, directly or through coupling. For charged particles, electrical as well as concentration differences matter. At this foundation level, distinguish the transport mechanism and gradient before predicting movement; the presence of a protein alone does not prove that transport is active.
- A cell wall and a plasma membrane are different structures.
- Ribosomes occur in both major cell types; a membrane-bound nucleus does not.
- State membrane permeability and starting conditions in an osmosis explanation.
Try it yourself
A cell description includes DNA and ribosomes but no membrane-bound nucleus. Does the missing nucleus mean the cell has no genetic material? In a second conceptual model, water can cross a membrane but sugar cannot. Pressure and temperature are equal on both sides, but the right side has the higher sugar concentration. Predict the initial net movement of water and explain.
Reveal the worked answer
A diagram description says that a cell has DNA and ribosomes but no membrane-bound nucleus. Is the absence of a nucleus evidence that it has no genetic material? No; the description explicitly includes DNA and is consistent with a prokaryotic cell. In a second model, water can cross a membrane but sugar cannot; the two sides have equal pressure and temperature, and the right side is more concentrated. The initial net water movement is toward the right. These are conceptual models, not instructions for preparing solutions for biological or clinical use.
A diagram description says that a cell has DNA and ribosomes but no membrane-bound nucleus. Is the absence of a nucleus evidence that it has no genetic material? No; the description explicitly includes DNA and is consistent with a prokaryotic cell. In a second model, water can cross a membrane but sugar cannot; the two sides have equal pressure and temperature, and the right side is more concentrated. The initial net water movement is toward the right. These are conceptual models, not instructions for preparing solutions for biological or clinical use.
Put one idea into practice.
Try a short science session, then use the explanations to choose what to review next.
Start a short sessionSources & further reading
OpenStax Biology 2e: Passive transportOpenStax Biology 2e: Prokaryotic cellsOpenStax Biology 2e: Eukaryotic cellsOpenStax Anatomy and Physiology 2e: Erythrocyte specializationSomething unclear or incorrect? Suggest a correction to this guide.
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