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Unit 2
10–13% of examCells are the smallest units of life, and each of their parts is built for a specific job. In this unit you'll tour the organelles, find out why cells stay small, and learn how the plasma membrane controls what gets in and out, from simple diffusion to pumps that spend ATP. You'll also see how internal membranes, which likely began through endosymbiosis, let eukaryotic cells split up their work.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Ribosomes, found in every living cell, build proteins by reading mRNA, while the endomembrane system (ER, Golgi complex, lysosomes, vacuoles and vesicles) makes, modifies and ships molecules. Mitochondria and chloroplasts each have a double membrane: chloroplasts carry out photosynthesis, and mitochondria carry out most of cellular respiration.
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As a cell grows, its volume increases faster than its surface area, so its surface area-to-volume ratio drops and the membrane can't exchange materials fast enough. That's why cells stay small or add folds and projections, and why small animals lose heat faster and tend to have higher metabolic rates per gram of body mass.
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The membrane is a phospholipid bilayer: the hydrophilic (water-loving) phosphate heads face the watery inside and outside of the cell, and the hydrophobic fatty acid tails face each other in the middle. Proteins, cholesterol (in animal cells) and molecules with sugar chains attached sit in this layer and can drift around, which is why it's called the fluid mosaic model.
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Because the middle of the membrane is hydrophobic, small nonpolar molecules like O₂ and CO₂ slip straight through, but ions and large polar molecules need channel or transport proteins to cross. Cell walls in plants, fungi, bacteria and archaea add a tough outer layer that supports the cell and keeps it from bursting when water rushes in.
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Passive transport moves substances down their concentration gradient, from high to low, without the cell spending energy, while active transport uses energy and can move substances from low to high. To move very large molecules or large amounts of material, cells use vesicles: endocytosis brings material in and exocytosis sends it out.
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Facilitated diffusion is passive transport through membrane proteins: ions such as Na⁺ and K⁺ need channel proteins, and large polar molecules use transport proteins, all moving down their gradient with no energy input. Water crosses in large amounts through special channels called aquaporins.
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Osmosis is the movement of water across a membrane from higher water potential (Ψ) to lower water potential, which usually means toward the side with more dissolved solutes. Whether the outside is hypotonic, hypertonic or isotonic to a cell tells you if the cell will gain water, lose water or stay the same, and organisms use tools like contractile vacuoles and central vacuoles to control their water balance (osmoregulation).
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Active transport uses membrane proteins powered by energy, usually from ATP, to build and maintain gradients across the membrane. The sodium-potassium pump is the classic example: each cycle moves 3 Na⁺ out and 2 K⁺ in, which helps create the voltage across the membrane called the membrane potential.
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Internal membranes divide a eukaryotic cell into compartments, so different reactions can run in their own spaces without getting in each other's way. Folded membranes, like the inner membrane of a mitochondrion, also add surface area where reactions can happen.
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Mitochondria and chloroplasts most likely descend from free-living prokaryotes that were taken in by a host cell and stayed, an idea called endosymbiotic theory; evidence includes their double membranes, their own circular DNA and their bacteria-like ribosomes. Prokaryotes usually lack membrane-bound organelles but still have specialized regions inside, while eukaryotes use internal membranes to divide the cell into compartments.
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