Use models to explain how the structure of cell organelles determines their function and supports the processes of the cell (North Carolina Standard Course of Study, Biology, LS.Bio.1).

What this topic is asking

North Carolina LS.Bio.1 asks you to explain how the structure of each organelle determines its function. For the Biology EOC you need to know the main organelles, what each does, and the recurring crosscutting concept of structure and function: the shape and parts of an organelle suit the job it performs. Items often describe what a cell does (releases lots of energy, makes lots of protein, photosynthesises) and ask which organelle is involved, so learn organelles by their jobs.

The control center and the protein line

The nucleus is the control center: it holds the cell's DNA (the instructions) inside a double membrane (the nuclear envelope) with pores that let materials pass. By controlling which genes are used, the nucleus directs the cell's activities. The nucleolus inside it makes ribosomes.

Proteins are built and shipped along a connected line. Ribosomes (free in the cytoplasm or attached to membranes) read the genetic message and assemble proteins. The rough endoplasmic reticulum (rough ER) is a network of membranes covered in ribosomes; it folds and transports the proteins they make. The smooth ER has no ribosomes and makes lipids and breaks down toxins. The Golgi apparatus receives proteins from the ER, then modifies, packages, and ships them in vesicles to where they are needed. This is a classic structure-and-function story: the rough ER is rough because it is covered in protein-making ribosomes.

Energy, digestion, and storage

The mitochondrion is the site of cellular respiration, releasing energy from glucose as ATP. Its inner membrane is folded into cristae, which increase the surface area for the reactions, so a cell that uses a lot of energy (a muscle cell, a nerve cell) has many mitochondria. This is the most-tested structure-and-function link on the EOC.

Lysosomes contain digestive enzymes that break down worn-out parts and waste, recycling materials. The vacuole stores water, nutrients, and waste; in plant cells it is a single large central vacuole that, when full, presses outward and keeps the cell firm (turgor).

Plant cells versus animal cells

Plant and animal cells share most organelles, but the EOC often tests the three structures plants have that animals do not:

• A cell wall of cellulose outside the membrane, giving support, shape, and protection.
• Chloroplasts, which carry out photosynthesis, capturing light energy to make glucose (they contain the green pigment chlorophyll).
• A large central vacuole that stores water and maintains turgor pressure.

Animal cells lack all three. Both cell types have a nucleus, mitochondria, ribosomes, ER, and a Golgi apparatus, so those are not the difference.

Try this

Q1. State the function of the rough endoplasmic reticulum and explain how its structure suits that function. [2]

• Cue. It makes and transports proteins; it is studded with ribosomes (which build the proteins), giving it its rough appearance and linking structure to function.

Q2. A cell is photosynthesising. Name the organelle responsible and state one other plant-only structure. [2]

• Cue. Chloroplast (photosynthesis); plus a cell wall or a large central vacuole.