Free trial session

1. The Cell: Life's Basic Unit

GED® Science: Life Science · preview lesson

Every living thing — a bacterium, an oak tree, you — is built from cells. The cell theory sums up three big ideas:

All living things are made of one or more cells.
The cell is the basic unit of structure and function in life.
All cells come from existing cells.

There are two broad types. Prokaryotic cells (bacteria) are small and have no nucleus — their DNA floats freely. Eukaryotic cells (plants, animals, fungi) are larger and keep their DNA inside a nucleus, along with other specialized parts called organelles.

An animal cell. Each organelle has a specific job, like rooms in a tiny factory.

Key organelles and what they do:

Nucleus — the control center; holds DNA, the cell's instructions.
Mitochondria — the 'powerhouse'; release energy (ATP) from food.
Cell membrane — the gatekeeper; controls what enters and leaves.
Cytoplasm — the jelly-like fluid where everything sits and reactions happen.
Ribosomes — build proteins.

Plant cells add two parts animal cells don't have: a rigid cell wall for support, and chloroplasts that capture sunlight to make food.

⚠️ Common misconception: bacteria are not 'simple animals'. They are prokaryotes — a completely separate, nucleus-free kind of cell.

💡 Tip: remember 'mitochondria = powerhouse' and 'nucleus = control center'. Those two come up constantly.

Lesson Discussion

Ask a question about this lesson. A teacher or admin can answer here.

nanaroy49 asked on Aug 17, 2026

explain more about prokaryotes can consume food?

Teacher answer

Great question! While our lesson doesn't go into detail about how prokaryotes feed, we can look at general biology to see how they do it.

Because prokaryotes (like bacteria) are tiny, single-celled organisms, they don't have mouths or digestive organs. Instead, they obtain nutrients through their cell membrane in a couple of main ways:

Absorbing nutrients from their surroundings: Most bacteria release special digestive chemicals (enzymes) into their environment. These enzymes break down dead plant/animal matter or other organic material into tiny molecules, which the bacteria then absorb directly through their cell membrane.
Making their own food: Some bacteria are like plants—they can capture sunlight to produce their own food through photosynthesis (such as cyanobacteria). Others use chemical reactions from minerals around them to create energy.

Key takeaway for your GED®:
Remember that the cell membrane is the "gatekeeper" that controls what enters and leaves any cell, including letting nutrients in and waste out!

Answered by admin on Aug 18, 2026

nanaroy49 asked on Aug 17, 2026

explain more about prokaryotes can consume food?

Teacher answer

Great question! While this isn't covered in detail in this specific lesson, we can look at general biology to see how prokaryotes (like bacteria) obtain nutrients.

Because prokaryotes are single-celled and do not have mouths, digestive tracts, or specialized internal organs for digestion, they obtain food in a few unique ways:

Direct absorption: They absorb small nutrient molecules (like dissolved sugars and minerals) directly through their cell membrane from their surroundings.
External digestion: For larger food sources, many bacteria release digestive enzymes outside of their cell. These enzymes break the food down into tiny pieces outside, and the bacteria then absorb the nutrients through their membrane.
Making their own food (Photosynthesis or Chemosynthesis): Some prokaryotes don't "eat" external food at all. Instead, they make their own energy using sunlight (similar to plants) or by breaking down surrounding chemical compounds.

For your GED® test, the most important takeaway about prokaryotes from this lesson is that they are simple, single-celled organisms that lack a nucleus.

Keep up the great curiosity!

Answered by admin on Aug 18, 2026