Describe the major organelles of prokaryotic and eukaryotic cells and explain how each cell structure corresponds to its function (Ohio's Learning Standards for Science, Biology, B.C.3).

What this topic is asking

Ohio standard B.C.3 states that "each cell has a specific structure that corresponds to its function." Ohio's Biology EOC turns this into items where you read a cell from its parts: if a diagram shows abundant rough ER, the cell makes a lot of protein; if it shows many mitochondria, it has a high energy demand. The recurring crosscutting concept is structure and function, so almost every organelle item rewards linking the shape or contents of a structure to the job it does.

The organelles, as structure-and-function pairs

Each organelle is a worked example of structure fitting function. Learn them as a table in your head: structure, then job.

• Nucleus. A membrane-bound control center that stores the cell's DNA and directs activity by controlling which proteins are made. The defining feature of a eukaryotic cell.
• Ribosomes. Tiny structures, free in the cytoplasm or stuck to the rough ER, that build proteins by joining amino acids. Found in every cell, including prokaryotes.
• Rough endoplasmic reticulum (rough ER). A folded membrane studded with ribosomes; it folds and transports proteins. Cells that export a lot of protein have abundant rough ER.
• Smooth endoplasmic reticulum (smooth ER). A folded membrane without ribosomes; it makes lipids and helps detoxify substances.
• Golgi apparatus. A stack of membranes that modifies, sorts, and packages proteins and lipids into vesicles for delivery.
• Mitochondria. The site of aerobic cellular respiration, releasing energy as ATP. A double membrane with a folded inner membrane gives a large surface area for the reactions. Cells with a high energy demand have many.
• Chloroplasts (plants only). The site of photosynthesis, using light to build glucose. They contain the green pigment chlorophyll.
• Lysosomes. Membrane sacs of digestive enzymes that break down waste and worn-out parts.
• Vacuole. A storage sac; in plant cells, one large central vacuole stores water and keeps the cell firm (turgid).
• Cell membrane. The selectively permeable boundary that controls transport in and out of the cell (covered in the cell membrane and transport).
• Cell wall (plants, fungi, many prokaryotes). A rigid outer layer outside the membrane that gives support and shape. In plants it is made of cellulose.

Prokaryotic versus eukaryotic cells

All cells share a membrane, cytoplasm, ribosomes, and DNA, but the two cell types differ in organization.

• Prokaryotic cells (bacteria and archaea) are usually small and simple, with no membrane-bound nucleus and no membrane-bound organelles; their single loop of DNA floats free in a region of the cytoplasm.
• Eukaryotic cells (plants, animals, fungi, protists) are larger and more complex, with a true membrane-bound nucleus and many membrane-bound organelles.

The advantage of the eukaryotic design is compartmentalisation: membranes wall off reactions so the cell can run many different processes at once without them interfering. Ohio standard B.C.4 builds on this by linking cells to tissues, organs, organ systems, and the whole organism, so the organization that starts inside the cell continues up the levels of life.

Plant versus animal cells

Among eukaryotes, the EOC loves the plant-versus-animal comparison. Both have a nucleus, a membrane, cytoplasm, ribosomes, mitochondria, ER, and a Golgi apparatus. Plant cells add three structures that animal cells lack:

• a cell wall for support and shape,
• chloroplasts for photosynthesis,
• a large central vacuole for storing water and maintaining turgor.

Try this

Q1. Put these structures in the order a protein passes through them on its way to being exported: Golgi apparatus, ribosome, vesicle, rough ER. [2]

• Cue. Ribosome, rough ER, Golgi apparatus, vesicle.

Q2. A cell has many mitochondria and almost no chloroplasts. State one conclusion you can draw about this cell. [1]

• Cue. It has a high energy demand and carries out little or no photosynthesis, so it is likely an animal cell or a non-photosynthetic plant cell.