Cell Structure & Organelles

Cells are the basic units of life, and their structures are closely tied to the jobs they perform. In eukaryotic cells, different organelles divide labor so the cell can make energy, build molecules, process information, and maintain internal balance. Understanding cell structure helps explain how organisms grow, respond to their environment, and stay alive. It also provides the foundation for genetics, physiology, and modern medicine.

A cell membrane surrounds the cell and controls what enters and leaves, while internal membranes create specialized compartments for different chemical reactions. Organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus work together like an organized production system. The cytoskeleton gives shape and helps move materials inside the cell. When these parts fail or are damaged, normal cell function breaks down and disease can result.

Key Facts

Cell theory states that all living things are made of cells, cells are the basic unit of life, and all cells come from preexisting cells.
The plasma membrane is a phospholipid bilayer with selective permeability that regulates transport into and out of the cell.
Surface area to volume ratio decreases as a cell gets larger, which limits cell size because exchange becomes less efficient.
Mitochondria carry out cellular respiration to produce ATP, often summarized as C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP.
Ribosomes build proteins by linking amino acids according to genetic instructions from mRNA.
Diffusion moves particles from high concentration to low concentration, while osmosis is the diffusion of water across a selectively permeable membrane.

Vocabulary

Organelle: An organelle is a specialized structure inside a cell that carries out a specific function.
Plasma membrane: The plasma membrane is the thin outer boundary of the cell that controls what enters and leaves.
Cytoplasm: Cytoplasm is the gel-like material inside the cell where organelles are suspended and many reactions occur.
Nucleus: The nucleus is the membrane-bound organelle that stores most of the cell's DNA and directs cell activities.
Selective permeability: Selective permeability is the property of a membrane that allows some substances to pass through more easily than others.

Common Mistakes to Avoid

Confusing cell membrane with cell wall, because animal cells have a membrane but do not have a cell wall. The membrane is flexible and regulates transport, while a cell wall is a rigid outer layer found in plants, fungi, and many prokaryotes.
Thinking all organelles are surrounded by membranes, because ribosomes are not membrane-bound. This matters because ribosomes still perform a major function by making proteins.
Assuming diffusion requires energy, because diffusion is a passive process driven by concentration differences. Energy is required for active transport, not for simple diffusion or osmosis.
Mixing up rough ER and smooth ER, because rough ER has ribosomes and helps make and process proteins, while smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification.

Practice Questions

1 A cell has 8 mitochondria and each mitochondrion produces 250 ATP molecules in a given time period. How many ATP molecules are produced in total during that time period?
2 A substance has concentration 18 units inside a cell and 5 units outside the cell. If it moves by diffusion, in which direction will the net movement occur, and what is the concentration difference?
3 Explain why a eukaryotic cell benefits from having membrane-bound organelles instead of carrying out all reactions in one undivided space.