Develop and use models to relate the structure of cell organelles to their function in plant and animal cells (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to relate structure to function for the parts of plant and animal cells, often by developing or interpreting a model of a cell. For the Biology I EOC that means knowing each major organelle, the job it does, and why its structure suits that job. The recurring move on the exam is to give you a striking feature (a cell crowded with mitochondria, a cell full of rough ER) and ask what function it points to. Learn the organelles as structure-and-function pairs, not just names.

The control center and the protein pathway

Most of the busiest organelles work together on one job: making and shipping proteins.

• Ribosomes are the sites where proteins are assembled from amino acids. They float free in the cytoplasm or sit on the rough ER.
• The rough endoplasmic reticulum (rough ER) is a network of membranes covered in ribosomes; it makes and folds proteins and ships them onward. A cell that exports a lot of protein (such as a gland cell) has abundant rough ER.
• The smooth endoplasmic reticulum (smooth ER) has no ribosomes; it makes lipids and helps detoxify substances.
• The Golgi apparatus is a stack of flattened membranes that modifies, sorts, and packages proteins and lipids into vesicles for delivery inside or outside the cell.

The EOC loves the order: ribosome and rough ER, then Golgi apparatus, then vesicle. Know this pathway as a sequence, because it can appear as a drag-to-order technology-enhanced item.

The energy organelles

Chloroplasts, found only in plant cells and algae, capture light energy to make glucose by photosynthesis. They contain the green pigment chlorophyll, which is why an item describing a cell full of chloroplasts points to a photosynthetic plant cell.

Waste, storage, and boundaries

• Lysosomes contain digestive enzymes that break down worn-out parts and waste; they are sometimes called the cell's recycling centers, and their enclosing membrane keeps those enzymes from harming the rest of the cell.
• The cell membrane is the selectively permeable boundary that controls what enters and leaves (covered fully in the cell membrane and transport).
• The cytoskeleton gives the cell shape and helps move materials inside it.

Plant-only additions:

• The cell wall of cellulose lies outside the membrane and gives the plant cell shape and support.
• The large central vacuole stores water and dissolved substances and keeps the cell firm (turgid). When it loses water, the plant wilts.

Why structure matches function

The whole standard is the recurring crosscutting concept structure and function. The inner folds of a mitochondrion increase surface area for respiration; the ribosome-studded rough ER suits a protein-exporting cell; the rigid cell wall suits a cell that needs support without a skeleton. When an EOC item shows an unusual amount of one organelle, the answer connects that structure to the function it serves.

Try this

Q1. Name the organelle that is the site of cellular respiration and explain why its inner membrane is folded. [2]

• Cue. The mitochondrion; the folded inner membrane (cristae) gives a large surface area for the respiration reactions.

Q2. Put these organelles in the order a protein passes through them on its way out of the cell: Golgi apparatus, ribosome, vesicle, rough ER. [2]

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