Explore the nucleus, ribosomes, membranes, endomembrane system, and the structures that organize a cell.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
As a cell grows, its volume increases faster than its surface area. The cell then has less membrane area available per unit of cytoplasm for moving materials in and out.
For a cube, surface area is 6s² and volume is s³, so surface-area-to-volume ratio is 6/s. Smaller cells and folded exchange surfaces help solve this limitation.
A cell has DNA, ribosomes, cytoplasm, and a plasma membrane, but no nucleus. Is it prokaryotic or eukaryotic?
Prokaryotic.
Bacteria and archaea lack a membrane-enclosed nucleus and other typical membrane-enclosed organelles. They still carry out complex metabolism and have ribosomes.
The nucleus contains most of a eukaryotic cell’s DNA. Its nuclear envelope is a double membrane. Nuclear pores regulate traffic such as RNA and proteins between nucleus and cytoplasm.
The nucleus is not a sealed container. Messages must leave, and proteins needed for DNA maintenance and gene expression must enter. The outer envelope connects with the ER.
How are DNA, chromatin, and chromosomes related?
Chromatin is DNA associated with proteins. Chromosomes are organized units of that material; they become especially condensed and visible during cell division.
Chromosomes do not appear only when a cell divides. Their DNA is present throughout the cell cycle, but its packing changes.
The nucleolus makes ribosomal RNA and helps assemble ribosomal subunits. Ribosomes use mRNA instructions to join amino acids into proteins.
Ribosomes are not membrane-enclosed. Free ribosomes generally make proteins used in the cytosol; ER-bound ribosomes make many secreted, membrane, and endomembrane-system proteins.
A cell produces many secreted digestive enzymes. Which ER type would you expect to be abundant, and why?
Rough ER, because its attached ribosomes make proteins entering the secretory pathway.
New proteins enter the ER, where they begin folding and processing. Smooth ER has roles such as lipid synthesis, detoxification, and calcium storage.
The Golgi apparatus receives many products from the ER, modifies them, and sorts them for delivery. Vesicles are small membrane sacs that carry cargo between compartments or to the cell surface.
A cargo molecule’s destination is not always outside the cell. Golgi products can go to the plasma membrane, lysosomes, or other destinations.
Put these in order for a typical secreted protein: Golgi, rough ER ribosome, transport vesicle, secretory vesicle, cell exterior.
Rough ER ribosome → transport vesicle → Golgi → secretory vesicle → cell exterior.
The protein enters the ER as it is made. Vesicles carry it to and from the Golgi. A secretory vesicle fuses with the plasma membrane in exocytosis.
Lysosomes contain digestive enzymes that break down macromolecules, worn cell parts, and some engulfed material. They recycle useful small molecules back to the cell.
Their membrane separates digestive conditions from the cytosol, and their acidic interior suits many of their enzymes. Digestion is controlled compartmental chemistry.
What does a large central vacuole do in a mature plant cell?
It stores water and other materials, and water inside it helps generate turgor pressure that supports the cell.
Turgor is pressure against the cell wall. A plant can wilt when its cells lose water. Some vacuoles also digest substances; contractile vacuoles in some protists pump out excess water.
Mitochondria help harvest chemical energy during cellular respiration. Chloroplasts capture light energy during photosynthesis. Both have double membranes and some of their own DNA and ribosomes.
Plant cells generally have mitochondria as well as chloroplasts. Photosynthesis makes organic fuel; respiration helps transfer energy from fuel into ATP.
Does a plant cell have a plasma membrane, a cell wall, both, or neither?
Both.
The cellulose-rich cell wall provides support outside the selectively permeable plasma membrane. Animal cells have a plasma membrane but no cell wall.
The cytoskeleton includes microtubules, microfilaments, and intermediate filaments. It supports shape, positions organelles, helps move cargo, and participates in cell movement and division.
Motor proteins can move along cytoskeletal tracks. Cilia and eukaryotic flagella use microtubules, while actin microfilaments take part in muscle contraction and cell shape changes.
A cell still builds proteins, but it cannot properly modify and sort many proteins after they leave the ER. Which organelle is most likely impaired?
The Golgi apparatus.
Choose the organelle by the failed job: ribosomes build chains, the ER begins processing, and the Golgi modifies and directs many products to their destinations.