Cells, organelles and membrane transport

Find the features shared by cells

A cell is a bounded system that carries out chemical reactions and exchanges matter and energy with its surroundings. Typical cells have a plasma membrane, cytoplasm, genetic material and ribosomes. Some specialised cells lose components during maturation: mature mammalian red blood cells, for example, lack a nucleus and ribosomes. Ribosomes build proteins by translating information carried in messenger RNA. A cell wall, chloroplast or membrane-bound nucleus is not universal. When comparing cell types, begin with the typical cellular foundation and then add the structures and exceptions that distinguish the particular cells described.

Prokaryotic cells lack a membrane-bound nucleus. Their DNA occupies a nucleoid region, and additional small DNA molecules called plasmids may be present. Bacteria and archaea are prokaryotes. They still have complex biochemical organisation and can reproduce and obtain energy. The absence of a nucleus does not imply absence of DNA, and the absence of mitochondria does not prevent cellular respiration: relevant processes can occur at the plasma membrane and in the cytoplasm.

Eukaryotic cells contain a nucleus and membrane-bound organelles. Animals, plants, fungi and diverse protists have eukaryotic cells. Plant cells typically have cellulose walls, chloroplasts in photosynthetic tissues and a large central vacuole. Animal cells lack cell walls and chloroplasts. A root cell can be a plant cell without photosynthesising, so do not turn a characteristic of a tissue into a claim about every cell of the organism. Both plant and animal cells can contain mitochondria.

Use compartments to follow a cellular product

The nucleus contains most eukaryotic DNA, and the nuclear envelope separates it from the cytoplasm. Messenger RNA can pass through nuclear pores to be translated by ribosomes. Free ribosomes make many proteins used in the cytosol. Ribosomes associated with rough endoplasmic reticulum make proteins entering the secretory pathway, including many membrane proteins and proteins exported from the cell. Roughness refers to attached ribosomes rather than a special kind of membrane material.

For a typical secreted protein, the rough endoplasmic reticulum supports synthesis and initial processing. Transport vesicles carry the protein to the Golgi apparatus, which modifies, sorts and packages it. A vesicle can then fuse with the plasma membrane and release its contents by exocytosis. Lysosomes contain enzymes involved in intracellular digestion. Mitochondria contribute to aerobic ATP production, while chloroplasts capture light energy for photosynthetic processes. These roles are related through cell metabolism but are not interchangeable.

The cytoskeleton helps organise a cell, move components and support division. It is a dynamic network rather than a solid internal skeleton. Large plant vacuoles can store substances and contribute to pressure against the cell wall. Think about a compartment's contents, membrane and connections. This approach lets you reason through unfamiliar diagrams: a set of flattened sacs receiving and exporting vesicles has a different role from a double-membraned organelle associated with aerobic energy metabolism.

Predict movement across membranes

Diffusion produces net movement down a concentration gradient through random molecular motion. Individual particles still move in both directions, even when the net movement is one way. At equilibrium there is no net movement, not a complete stop in molecular motion. Some small non-polar molecules pass through the membrane's lipid interior. Ions and many polar molecules need membrane proteins. Facilitated diffusion uses channels or carriers while remaining passive when movement follows the relevant electrochemical gradient.

Active transport moves a substance against its electrochemical gradient by coupling transport to an energy source. Some pumps use ATP directly; other transporters use the gradient of a second substance. Therefore, protein-mediated transport is not automatically active. Ask both whether a protein participates and whether energy is required to oppose a gradient. These are separate questions.

Osmosis concerns water movement across a selectively permeable membrane. In a simple comparison with non-penetrating solute, water tends to move towards the side with higher solute concentration. An animal cell in a sufficiently dilute external solution can swell; a plant cell's wall resists expansion and supports turgor. State the permeability assumptions before predicting an outcome. A solute that freely crosses the membrane cannot maintain the same long-term osmotic difference as an impermeant solute.