State the cell theory and the evidence for it, and distinguish prokaryotic from eukaryotic cells and plant from animal cells (Virginia 2018 Biology SOL BIO.3.a).

What this topic is asking

Virginia Biology SOL standard BIO.3.a begins cell biology with the cell theory and the types of cells. The Biology EOC expects you to state the cell theory, know that it rests on evidence gathered with microscopes, and tell apart prokaryotic and eukaryotic cells, and plant and animal cells. This is foundational: nearly every later topic (organelles, transport, division) assumes you know what kind of cell you are dealing with.

The cell theory

The cell theory is a scientific theory, meaning it is a broad explanation supported by a large body of evidence. That evidence came from improvements in the microscope: Robert Hooke first described "cells" in cork, and later scientists observed living cells, leading to the conclusion that all organisms are cellular and that cells come only from other cells. This is a good example of how technology (the microscope) advances science and how a theory is built from many observations.

Prokaryotic and eukaryotic cells

The single biggest difference to remember is the nucleus: prokaryotes lack one, eukaryotes have one. Both cell types do share some features, a cell membrane, cytoplasm, ribosomes, and DNA, because all cells need to control what enters, hold their contents, make proteins, and store information. The EOC often gives a description or diagram and asks you to classify the cell.

Plant and animal cells

Both plant and animal cells are eukaryotic, so both have a nucleus and membrane-bound organelles. The differences are what plant cells have in addition:

• A cell wall outside the membrane, made of cellulose, giving the plant cell its rigid, boxy shape and support.
• Chloroplasts, the organelles of photosynthesis (animal cells cannot photosynthesize).
• A large central vacuole that stores water and helps keep the cell firm (turgid).

Animal cells lack all three. They tend to have a rounder shape and may have features such as centrioles. Knowing which structures are plant-only is a frequent EOC item, often as a drag-and-drop labeling task.

Try this

Q1. State the three parts of the cell theory. [3]

• Cue. All living things are made of cells; the cell is the basic unit of life; all cells come from pre-existing cells.

Q2. A cell has ribosomes, a cell membrane, and DNA, but no nucleus. Is it prokaryotic or eukaryotic, and how do you know? [2]

• Cue. Prokaryotic, because it lacks a nucleus (and membrane-bound organelles), even though it shares the membrane, ribosomes, and DNA common to all cells.