Biology · High school and introductory college biology

Cell Structure and Function Study Guide

Every living thing is made of cells, and every cell is a set of parts that each do one job. Prokaryotic cells (bacteria) have no nucleus, while eukaryotic cells (animals, plants, fungi and protists) keep their DNA in a nucleus and divide their work among membrane-bound organelles. The cell membrane decides what gets in and out, and almost every question on this topic comes back to matching a structure to its job.

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Key terms

Cell: The basic unit of life. All living things are made of one or more cells, and new cells come only from existing cells.
Prokaryotic cell: A cell with no nucleus and no membrane-bound organelles. Its DNA sits in a region called the nucleoid. Bacteria and archaea are prokaryotes.
Eukaryotic cell: A cell with a nucleus and other membrane-bound organelles. Animals, plants, fungi and protists are made of eukaryotic cells.
Cell membrane: Selectively permeable boundary controlling what enters and leaves a cell. It is made of a phospholipid bilayer with embedded proteins.
Cytoplasm: Everything inside the cell membrane except the nucleus: the watery cytosol plus the organelles suspended in it. Many chemical reactions happen here.
Nucleus: Membrane-bound organelle that stores most of a eukaryotic cell’s DNA and controls the cell’s activities by directing which proteins are made.
Ribosome: Structure that assembles proteins from amino acids using messenger RNA. Found in every kind of cell, free in the cytoplasm or attached to the rough endoplasmic reticulum.
Endoplasmic reticulum (ER): A network of membranes. Rough ER is studded with ribosomes and helps make and fold proteins; smooth ER makes lipids and helps detoxify chemicals.
Golgi apparatus: Stack of flattened membranes that modifies, sorts and packages proteins and lipids into vesicles for delivery inside or outside the cell.
Mitochondrion: Organelle that produces most cellular ATP through aerobic respiration. Plant and animal cells both have them.
Chloroplast: Organelle in plant cells and algae where photosynthesis converts light energy into chemical energy stored in sugar.
Lysosome: Organelle containing digestive enzymes that break down worn-out parts, food particles and invaders. Common in animal cells.
Central vacuole: Large water-filled sac in most plant cells that stores water and other substances and helps keep the cell firm.
Cell wall: Rigid layer outside the cell membrane that supports and protects the cell. Plants, fungi, algae and most bacteria have one; animal cells do not.
Cytoskeleton: Network of protein fibers that gives the cell its shape, anchors organelles and allows movement inside and of the cell.
Diffusion: Movement of particles from where they are more concentrated to where they are less concentrated. It needs no energy from the cell.
Osmosis: Passive movement of water across a selectively permeable membrane, toward the side with more dissolved solute.
Homeostasis: Maintenance of stable internal conditions despite external change. The cell membrane is one of the structures that makes it possible.

Structure fits function

Nearly every organelle question is really a question about job and shape. A cell that needs a lot of ATP has a lot of mitochondria, which is why muscle cells have many of them. A cell that makes and exports proteins has plenty of rough ER and Golgi. A plant leaf cell packed with chloroplasts is built to photosynthesize.

When you meet an unfamiliar cell on a test, ask what it does all day, then predict which organelles it has most of.

Prokaryotes versus eukaryotes

The first split in every cell diagram is whether there is a nucleus. Prokaryotic cells are smaller and simpler: their DNA floats in the nucleoid, and they have ribosomes, a cell membrane and (usually) a cell wall, but no membrane-bound organelles. Eukaryotic cells have a nucleus and organelles that each work in their own compartment.

•Prokaryotes: no nucleus, no membrane-bound organelles, DNA in the nucleoid (bacteria, archaea).
•Eukaryotes: nucleus plus membrane-bound organelles (animals, plants, fungi, protists).
•Both: a cell membrane, cytoplasm, ribosomes and DNA.

Plant, animal and bacterial cells compared

•Plant cells have a cell wall, chloroplasts and a large central vacuole, in addition to the organelles animal cells have, including mitochondria.
•Animal cells have no cell wall or chloroplasts. They often have lysosomes and many small vacuoles.
•Bacterial cells have a cell wall and ribosomes, but no nucleus and no membrane-bound organelles.

How a protein travels through the cell

A protein that leaves the cell follows a route worth memorizing in order. The nucleus holds the DNA and a copy of the instructions (messenger RNA) goes out to a ribosome on the rough ER. The protein is made and folded there, travels in a vesicle to the Golgi apparatus to be modified and packaged, and a vesicle then carries it to the cell membrane to be released.

1Nucleus: DNA is copied into messenger RNA.
2Ribosome on the rough ER: the protein is built.
3Golgi apparatus: the protein is modified, sorted and packaged.
4Vesicle to the cell membrane: the protein leaves the cell.

Getting across the membrane

The cell membrane is selectively permeable, so only some things cross, and how they cross depends on whether the cell has to spend energy.

•Passive transport needs no energy from the cell: diffusion, osmosis and facilitated diffusion (through channel or carrier proteins), all moving with the concentration gradient.
•Active transport uses energy (ATP) to move substances against their concentration gradient.
•Large particles can enter or leave in vesicles (endocytosis and exocytosis).

Osmosis is the movement of water, not of the dissolved stuff. Water moves toward the side with more dissolved solute.

Common mistakes

Mistake: Mitochondria create energy.
Energy cannot be created. Mitochondria transform the energy stored in food molecules into ATP, the form of energy the cell can use.
Mistake: Only animal cells have mitochondria.
Plant cells have mitochondria too. They have chloroplasts as well, and use both.
Mistake: The cell wall and the cell membrane are the same thing.
The membrane is inside the wall and controls what crosses. The wall is a rigid outer layer for support and is not selective.
Mistake: Ribosomes are organelles bound by a membrane.
Ribosomes have no membrane around them. That is why even prokaryotes have them.
Mistake: Osmosis moves the solute.
Osmosis moves water across the membrane. The dissolved particles are what water moves toward.
Mistake: Bacteria are too simple to have a cell wall or ribosomes.
Bacteria have both. What they lack is a nucleus and membrane-bound organelles.

Common questions

What are the main parts of a cell and what do they do?↓

The cell membrane controls what enters and leaves. The nucleus stores DNA. Ribosomes build proteins. The endoplasmic reticulum and Golgi apparatus fold, modify and ship them. Mitochondria produce most of the cell’s ATP. Plant cells add chloroplasts for photosynthesis, a central vacuole for storage and a cell wall for support.

What is the difference between plant and animal cells?↓

Plant cells have a cell wall, chloroplasts and a large central vacuole. Animal cells have none of those, and are more likely to have lysosomes. Both have a nucleus, mitochondria, ribosomes, ER, a Golgi apparatus and a cell membrane.

What is the difference between prokaryotic and eukaryotic cells?↓

A prokaryotic cell has no nucleus and no membrane-bound organelles, and its DNA sits in the nucleoid. A eukaryotic cell has a nucleus and membrane-bound organelles. Bacteria are prokaryotes; animals, plants, fungi and protists are eukaryotes.

How do things get into and out of a cell?↓

Small or fat-soluble molecules can diffuse across the membrane, and water crosses by osmosis; neither costs the cell energy. Other substances cross through channel or carrier proteins, and active transport pumps substances against their gradient using ATP. Large particles move in vesicles.

What is the easiest way to memorize cell organelles?↓

Pair each organelle with its job in one short phrase, then test yourself by covering one side. Retrieving the answer from memory works better than rereading. A matching or flash card game made from the term list is a fast way to do it, and the free demo below uses these same terms.