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Week 3 · Chapter

Chapter 3 — The Cell & Membrane Transport

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 2 (relate the chemistry of the body to its function — atoms and the three bond types, water's properties, the logarithmic pH scale with the blood's buffered range, and the four classes of organic compounds — and explain the cell's membrane, organelles, and transport (diffusion, osmosis and tonicity, active transport, and bulk transport), the overview of how a cell makes ATP, and how a gene is read into a protein), Objective 1 (use the language of the body correctly — the anatomical position, the directional-term pairs, the three body planes, and the body cavities and abdominal regions — distinguish anatomy from physiology, climb the levels of structural organization, list the functions and requirements of human life, and trace a homeostatic feedback loop through its receptor, control center, and effector, telling negative feedback from positive feedback) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Every cell in your body is wrapped in a thin border, the plasma membrane (the cell's outer skin, which decides what gets in and out). Inside, small working parts called organelles (tiny structures, each with its own job) build, ship, digest, and power the cell. This week you learn the traffic rules at that border: some things drift across downhill for free, and others are pumped uphill at a cost in energy. You also learn to predict which way water (the body's solvent) moves when a cell sits in a salty or a watery bath, and why a bag of fluid in a clinic is mixed to match your cells. One protein in that border, a pump, spends 1 ATP to move 3 sodium ions out and 2 potassium ions in, and your nerve and muscle weeks later in the course run on exactly this traffic.

Why you already care about this

Toss a green salad with salt and leave it on the counter for a while. The leaves go limp and a puddle forms in the bowl. Drop a raisin into a glass of plain water overnight, and by morning it has plumped up. Glance at a hanging bag of fluid in a clinic: that fluid is mixed to match the inside of your cells. You have seen those three moments, and they are the same physics, played out at the border of a cell.

Water crosses your cell borders all the time, and it always heads toward the side with more dissolved stuff. Salt on the leaves pulls water out; the raisin, full of sugar, pulls water in. The clinic's fluid is mixed so that water has no reason to rush either way. This chapter gives you the rule behind all three, and a way for you to call the direction every time.

The cell itself has a short, named history. Robert Hooke named the "cell" in Micrographia (1665), after the small box-like compartments he saw in a slice of cork. In the late 1830s, Matthias Schleiden and Theodor Schwann proposed that all living things are made of cells, and Rudolf Virchow later added that every cell arises from a cell that already existed.

By the end of this chapter you will be able to:

describe the plasma membrane as a phospholipid bilayer and say why it is selectively permeable;
match each organelle to its job, reading structure to function;
sort a transport as passive, active, or bulk by its direction and its cost;
read a bath's tonicity from the cell's side in three moves and state which way the water goes;
use C₁V₁ = C₂V₂ to find a diluted concentration and then name its tonicity;
state the sodium-potassium pump as 3 Na⁺ out and 2 K⁺ in per ATP, in that order.

Carry this line with you: water follows solute — compare, name, then point the water.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the levels of organization and homeostasis. Week 2, The Chemistry of Life, gave you polar molecules, ions, water as the solvent, lipids, proteins, and ATP.
This week: the plasma membrane, the organelle tour, passive transport and osmosis, tonicity read from the cell's side, active transport and the sodium-potassium pump, and bulk transport.
How they connect: Week 1's cellular level is where this chapter lives, and Week 2's polar heads, nonpolar tails, ions, and ATP become the membrane, the traffic across it, and the price of uphill moves.
Where we're going next: Week 4, Cellular Energy & Protein Synthesis, opens two organelles you meet here, the mitochondrion and the ribosome, and shows how each one does its job.

In Week 1 you climbed a ladder from the chemical level to the whole organism, and the second rung was the cell. You also met homeostasis, the body keeping its inside steady while the outside changes. In Week 2 you met polar molecules, which carry slight charges, and nonpolar ones, which do not. You learned that a lipid is built from glycerol and fatty acids, that a protein's shape sets its job, and that ATP releases energy when a phosphate group is split off. This chapter puts all of that to work for you. Your membranes are built from lipids that are part polar and part nonpolar. Their gates are proteins. And the pump that moves ions uphill spends the ATP you met last week.

1. Build the border: the phospholipid bilayer

Picture a crowd of people standing in a shallow pool, each holding a beach ball overhead and keeping their legs dry by tucking them up. That is close to how your cells' borders are built. Each building block has a head that likes water and two tails that avoid it.

The building block is a phospholipid. Its head carries a phosphate group and is polar, so it is attracted to water. Its two tails are fatty-acid chains and are nonpolar. Hydrophobic means water-avoiding; it describes the nonpolar tails of the phospholipids, which turn away from water toward the middle of the membrane.

Each of your cells has water on both sides of its border: the fluid inside the cell and the fluid outside it. So the phospholipids line up in two rows, tail to tail. The phospholipid bilayer is two rows of phospholipids arranged with their phosphate heads facing the water on both sides and their nonpolar, hydrophobic tails tucked inward; it is the basic structure of the plasma membrane.

Now picture the labeled figure in words, the way you would see it drawn. Along the top runs a row of round heads, facing the watery fluid outside the cell. Along the bottom runs a second row of heads, facing the watery fluid inside. Between the two rows, the tails point toward each other and fill the middle with an oily layer. Here and there, large proteins sit in the bilayer. Some span it from one side to the other.

The plasma membrane is the border of every cell, a phospholipid bilayer studded with proteins that decides what enters and leaves the cell.
A membrane protein is a protein set into the plasma membrane; a channel protein forms a passage, a carrier protein ferries a substance across, and a receptor protein binds a signal.

Structure determines function here, as everywhere you go in this course. The oily middle is the reason the membrane is choosy. A substance that is charged or strongly polar does not pass easily through a layer of nonpolar tails. That substance needs a protein to get across.

Selectively permeable describes a membrane that lets only certain substances cross it without help from a membrane protein and holds others back.

Worked example — read the border. For each substance, you use the structure of the bilayer to say whether it needs a membrane protein to cross.

A sodium ion, Na⁺, carries a charge. The middle of the bilayer is nonpolar, and charged particles do not pass easily through it. So Na⁺ needs a protein, such as a channel, to cross.
A potassium ion, K⁺, also carries a charge. By the same reasoning, it needs a protein to cross.
A large molecule too big to slip between the phospholipids needs a protein too, because size is one of the three reasons a substance cannot cross the bilayer on its own: size, charge, or polarity.

In words: the bilayer's oily core is the gatekeeper, and the proteins are the gates.

Misconception check. Students often draw the bilayer with the tails pointing out into the water and the heads hiding in the middle, because the tails look like the "outside" of the drawing. Reverse it. The heads are the water-attracted part, so they face the water on both sides. The tails are hydrophobic, so they hide in the middle, away from the water.

Memory hook: heads out to the water, tails tucked in.

2. Tour the organelles: structure to function

Think of a busy restaurant kitchen at dinner. There is a recipe book locked in an office, cooks at small stations, a counter where dishes are finished and boxed, a cleanup crew, a power supply, and a frame of shelves holding it all in place. Each of your cells runs on the same kind of division of labor, and each station is an organelle.

An organelle is a small structure inside a cell with its own job, such as the nucleus, a ribosome, or a mitochondrion.

Read each one in the course's fixed order, the way you will all term: what it is, then what it does.

The nucleus is the organelle that holds the cell's DNA, enclosed by a nuclear envelope made of two lipid bilayers. Human DNA carries roughly 20,000 protein-coding genes on 46 chromosomes.
A chromosome is one of the DNA packages in the nucleus; human body cells carry 46 of them.
The nucleolus is a region inside the nucleus that makes the RNA used to build ribosomes.
A ribosome is a small organelle that builds proteins.
The rough ER is the part of the endoplasmic reticulum studded with ribosomes; it makes and modifies proteins headed for the membrane or for export. ER stands for endoplasmic reticulum, a folded membrane network, rough with ribosomes or smooth without them.
The smooth ER is the part of the endoplasmic reticulum without ribosomes; it makes phospholipids and steroid hormones, stores and regulates Ca²⁺, and detoxifies some substances.
The Golgi apparatus is a stack of membrane sacs that modifies, sorts, and packages products from the rough ER and ships them in vesicles.
A lysosome is a membrane sac of enzymes that breaks down worn-out cell parts.
A mitochondrion is the organelle that converts the energy in nutrients into ATP; the plural is mitochondria.
The cytoskeleton is a network of protein fibers that gives the cell its shape and supports movement and transport inside it.
Cilia are short hair-like projections on a cell surface that beat to move material across that surface.
A flagellum is a single long whip-like projection that propels a cell.

Two counts deserve a moment. The nucleus holds 46 chromosomes, which is 46 ÷ 2 = 23 pairs. Honesty note: the figure of roughly 20,000 protein-coding genes is an estimate, so read it as "about," not as an exact count.

Worked example — follow one protein out of the cell. A cell needs to send a protein out into the fluid around it. You name each station in order and its job.

The ribosomes on the rough ER build the protein, because ribosomes build proteins and the rough ER makes proteins headed for export.
The Golgi apparatus receives the product, modifies it, sorts it, and packages it into a vesicle for shipment.
The vesicle carries the protein to the plasma membrane, where it is released outside. You will meet that last step by name in Section 6.

In words: build at the ribosome, finish and box at the Golgi, ship in a vesicle.

Misconception check. Students often swap three jobs: the ribosome, the Golgi apparatus, and the mitochondrion. It happens because all three sound like "the part that makes things." Pin each to one verb. The ribosome builds proteins. The Golgi apparatus sorts and ships. The mitochondrion makes ATP. If an answer says the Golgi makes energy or the mitochondrion packages proteins, two verbs have traded places.

Limitation: this week each organelle gets a one-line job. How the mitochondrion actually makes ATP, and how the ribosome reads a gene, are next week's story.

Memory hook: ribosome builds, Golgi ships, mitochondrion powers.

3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis

Put one drop of food coloring into a still glass of water and wait while you watch. The color spreads out from the drop until the whole glass is evenly tinted. No one stirred it. The particles drifted from where they were crowded to where they were sparse.

That drift has a name, and it costs the cell nothing.

A concentration gradient is a difference in how much of a substance is dissolved in one place compared with another; moving from higher to lower concentration is moving down the gradient.
Passive transport is movement of a substance across a membrane from higher to lower concentration that costs the cell no ATP.
Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated.
Facilitated diffusion is diffusion through a membrane protein, for substances that cannot cross the lipid bilayer because of their size, charge, or polarity.

Facilitated diffusion is still downhill. The protein is a door, not a motor. The substance moves from higher to lower concentration either way; the protein only gives it a way through the oily middle.

Water has its own version of diffusion, and it is the most important one this week. First, one more word.

A solute is a substance dissolved in a fluid, such as salt dissolved in water.
Osmosis is the diffusion of water across a selectively permeable membrane toward the side with more solute.

The short form is worth your memorizing: water follows solute. Wherever more stuff is dissolved, water moves toward it.

Worked example — call the water. In each kitchen moment, you find the side with more solute, then you say which way the water moves.

Salt is sprinkled on salad leaves. The outside of each leaf cell now has more solute than the inside. Water follows solute, so water moves out of the leaf cells, and the leaves go limp.
A raisin, packed with sugar, sits in plain water. The inside of the raisin has more solute than the water around it. Water follows solute, so water moves into the raisin, and it plumps up.

In words: in both cases, nothing pushed the water. It moved toward the side with more dissolved stuff.

Misconception check. Students often say "osmosis moves the salt." It is tempting because the salt is what you can see changing. But osmosis moves water. In the salad, the salt stays mostly where it was sprinkled; the water is what crosses the cell borders and pools in the bowl. Diffusion is the general word for particles moving downhill; osmosis is the word for water.

4. Read tonicity from the cell's side: compare, name, point the water

Picture a lab bench with three beakers, each holding a bath of a different strength, and a model cell ready to drop into each one. You want to predict, before you drop it, whether the cell will swell, shrink, or stay the same. That prediction has three moves, and you make them in the same order every time.

The unit for "how much is dissolved" is mOsm/L — milliosmoles per liter, a unit for how much dissolved solute a liter of fluid carries. A bigger number means more dissolved stuff.

Tonicity is a description of a bath by its effect on a cell's water, read from the cell's side by comparing the solute outside with the solute inside.
A hypotonic bath has less solute than the cell, so water moves into the cell and the cell swells and may burst.
An isotonic bath has the same solute concentration as the cell, so there is no net movement of water.
A hypertonic bath has more solute than the cell, so water moves out of the cell and the cell shrinks.

The three moves: compare the bath to the cell, name the bath, then point the water.

Worked example — Dataset C, the tonicity baths (illustrative). The model cell is 300 mOsm/L inside. The three baths are 100, 300, and 500 mOsm/L. These are classroom model values; the three cases are the standard definitions.

The table lines up each bath with its three moves and the cell's fate.

Dataset C (illustrative): a 300 mOsm/L model cell in three baths, in mOsm/L
Bath	Compare	Tonicity	Water moves	Cell fate
100 mOsm/L	100 < 300	hypotonic	in	swells and may burst
300 mOsm/L	300 = 300	isotonic	no net movement	stays the same
500 mOsm/L	500 > 300	hypertonic	out	shrinks

Now you walk each row in words.

Read the 100 mOsm/L bath: compare 100 mOsm/L with the cell's 300 mOsm/L; 100 < 300, so the bath has less solute and is hypotonic; water moves in, and the cell swells and may burst.
Read the 300 mOsm/L bath: compare 300 mOsm/L with 300 mOsm/L; 300 = 300, so the bath is isotonic; there is no net water movement, and the cell stays the same.
Read the 500 mOsm/L bath: compare 500 mOsm/L with 300 mOsm/L; 500 > 300, so the bath has more solute and is hypertonic; water moves out, and the cell shrinks.

You can check each row against the short rule. In the 100 bath, the cell has more solute, so water follows solute into the cell. In the 500 bath, the bath has more solute, so water follows solute out. The rule and the table agree.

Dilute a solution: C₁V₁ = C₂V₂

When you pour a solution into a bigger container and top it up with plain water, the same amount of solute now spreads through more fluid. The dilution rule tracks that.

C₁V₁ = C₂V₂ is the dilution rule: the starting concentration times the starting volume equals the new concentration times the new volume, so doubling the volume halves the concentration.

C₁ is the starting concentration, in mOsm/L; V₁ is the starting volume; C₂ is the concentration after dilution, in mOsm/L; and V₂ is the volume after dilution.

Worked example — dilute the isotonic bath. You take the 300 mOsm/L solution and dilute it into twice the volume.

Write the rule with the four values on one line: 300 mOsm/L × 1 volume = C₂ × 2 volumes.
Solve for C₂: C₂ = 300 ÷ 2 = 150 mOsm/L.
Read the new bath from the cell's side: 150 < 300, so a 150 mOsm/L bath is hypotonic to the model cell; water moves in, and the cell swells.

In words: adding plain water turned an isotonic bath into a hypotonic one. Doubling the volume halved the concentration: 300 mOsm/L versus 150 mOsm/L is a 2-fold drop, and that was enough to send water into the cell. You would decide, from that one comparison, that a cell dropped in the diluted bath swells.

Why an IV fluid is isotonic

Fluid given into your vein meets your red blood cells and every other cell it bathes. If the fluid were hypotonic, water would rush into those cells and they would swell. If it were hypertonic, water would leave them and they would shrink. An IV fluid is therefore mixed to be isotonic, so that the body's cells neither swell nor shrink. That is homeostasis at the level of the cell: the inside of each cell stays steady because the outside matches it. The clinical connection stops there.

Misconception check. Students often flip hypotonic and hypertonic, calling the 100 mOsm/L bath hypertonic because "the cell has more, so the cell is hyper." The cure is to read from the cell's side and to name the bath. Hypo means less: a hypotonic bath has less solute than the cell. Hyper means more: a hypertonic bath has more. Then let water follow solute. A cell in a hypotonic bath swells, never shrinks.

Caveat: the 300 mOsm/L model cell is a classroom model, a simplification you can hold in one hand. Real cells differ, and this course keys only the model's values, never the exact concentrations inside a real cell.

Memory hook: hypo, water in — the cell swells like a hippo.

5. Pay to go uphill: active transport and the sodium-potassium pump

Walking down a hill takes you no effort; carrying your full backpack up it takes a lot. Moving a substance against its gradient is the uphill walk, and the cell has to pay for it.

Active transport is movement of a substance across a membrane against its concentration gradient, paid for with ATP.

The best-known example sits in the membrane of nearly every cell this course studies.

The sodium-potassium pump is a membrane protein that uses one ATP to move three sodium ions out of the cell and two potassium ions in.

You write it the same way every time, in this order: 3 Na⁺ out, 2 K⁺ in, per ATP. Both moves are uphill. The pump keeps more Na⁺ outside the cell and more K⁺ inside it. That difference is a stored gradient, and the nerve and muscle weeks will spend it. The exact concentrations on each side are taught in words in this course and never keyed.

Worked example — count the pump. You count the ions moved for one ATP, then for two.

For one ATP: 3 Na⁺ out and 2 K⁺ in.
For two ATP: 3 × 2 = 6 Na⁺ out and 2 × 2 = 4 K⁺ in.

In words: every ATP the pump spends moves three sodium ions out and two potassium ions in, and two ATP move twice as many of each, so your cells pay one ATP for every round. Sodium always leaves; potassium always enters.

Misconception check. Two slips travel together here. First, students often think "active transport means fast" or "active transport means it goes through a protein." Neither is the definition. Facilitated diffusion also uses a protein, and it is passive. Active means against the gradient and paid in ATP. Second, students often write the pump as 2 Na⁺ out and 3 K⁺ in. Fix the order with the sentence: three sodium out, two potassium in.

Memory hook: downhill is free; uphill costs ATP.

6. Move it in bulk: endocytosis and exocytosis

Some loads are too big for any channel or carrier. Think of the difference between passing a note through your mail slot and moving your sofa into a house: the sofa needs the front door opened wide. Your cells' version of the open door is the membrane itself, which can fold around a load you could never fit through a channel.

Bulk transport is movement of large particles or volumes of fluid across the plasma membrane inside membrane vesicles.
A vesicle is a small sac of membrane that carries material inside a cell or into and out of it.
Endocytosis is bulk transport into the cell, in which the membrane wraps material in a pocket that pinches off as a vesicle.
Phagocytosis is endocytosis of large particles, sometimes called cell eating.
Pinocytosis is endocytosis of droplets of fluid and the solutes dissolved in them, sometimes called cell drinking.
Exocytosis is bulk transport out of the cell, in which a vesicle fuses with the plasma membrane and releases its contents outside.

Now you can finish the trip of the protein from Section 2. The Golgi apparatus packed it into a vesicle; the vesicle fuses with the plasma membrane; the protein is released outside. That last step is exocytosis.

Worked example — sort the transport. For each move, you ask two questions in order: which direction, relative to the gradient, and what does it cost? Then name the family.

Water crosses into a raisin toward the sugar inside. Downhill, no ATP: passive transport, by osmosis.
Na⁺ is moved out of a cell where Na⁺ is already higher outside. Uphill, paid in ATP: active transport, by the sodium-potassium pump.
A cell wraps a large particle in a pocket of membrane and pulls it inside. A vesicle carries the load: bulk transport, by phagocytosis.
A vesicle fuses with the membrane and releases its contents outside. A vesicle carries the load: bulk transport, by exocytosis.

In words: downhill and free is passive; uphill and paid is active; a load in a vesicle is bulk.

Misconception check. Students often swap endocytosis and exocytosis, because both end in "cytosis." Read the first part of the word. Endo- means within, so endocytosis brings material in. Exo- means outside, so exocytosis sends material out. A vesicle that fuses with the membrane and empties outward is exocytosis, never endocytosis.

Key terms

Hydrophobic — Water-avoiding; describes the nonpolar tails of the phospholipids, which turn away from water toward the middle of the membrane. (first used: 1. Build the border: the phospholipid bilayer)
Phospholipid bilayer — Two rows of phospholipids arranged with their phosphate heads facing the water on both sides and their nonpolar, hydrophobic tails tucked inward; the basic structure of the plasma membrane. (first used: 1. Build the border: the phospholipid bilayer)
Plasma membrane — The border of every cell, a phospholipid bilayer studded with proteins that decides what enters and leaves the cell. (first used: 1. Build the border: the phospholipid bilayer)
Membrane protein — A protein set into the plasma membrane; a channel protein forms a passage, a carrier protein ferries a substance across, and a receptor protein binds a signal. (first used: 1. Build the border: the phospholipid bilayer)
Selectively permeable — Describes a membrane that lets only certain substances cross it without help from a membrane protein and holds others back. (first used: 1. Build the border: the phospholipid bilayer)
Organelle — A small structure inside a cell with its own job, such as the nucleus, a ribosome, or a mitochondrion. (first used: 2. Tour the organelles: structure to function)
Nucleus — The organelle that holds the cell's DNA, enclosed by a nuclear envelope made of two lipid bilayers. (first used: 2. Tour the organelles: structure to function)
Chromosome — One of the DNA packages in the nucleus; human body cells carry 46 of them. (first used: 2. Tour the organelles: structure to function)
Nucleolus — A region inside the nucleus that makes the RNA used to build ribosomes. (first used: 2. Tour the organelles: structure to function)
Ribosome — A small organelle that builds proteins. (first used: 2. Tour the organelles: structure to function)
Rough ER — The part of the endoplasmic reticulum studded with ribosomes; it makes and modifies proteins headed for the membrane or for export. (first used: 2. Tour the organelles: structure to function)
Smooth ER — The part of the endoplasmic reticulum without ribosomes; it makes phospholipids and steroid hormones, stores and regulates Ca²⁺, and detoxifies some substances. (first used: 2. Tour the organelles: structure to function)
Golgi apparatus — A stack of membrane sacs that modifies, sorts, and packages products from the rough ER and ships them in vesicles. (first used: 2. Tour the organelles: structure to function)
Lysosome — A membrane sac of enzymes that breaks down worn-out cell parts. (first used: 2. Tour the organelles: structure to function)
Mitochondrion — The organelle that converts the energy in nutrients into ATP; the plural is mitochondria. (first used: 2. Tour the organelles: structure to function)
Cytoskeleton — A network of protein fibers that gives the cell its shape and supports movement and transport inside it. (first used: 2. Tour the organelles: structure to function)
Cilia — Short hair-like projections on a cell surface that beat to move material across that surface. (first used: 2. Tour the organelles: structure to function)
Flagellum — A single long whip-like projection that propels a cell. (first used: 2. Tour the organelles: structure to function)
Concentration gradient — A difference in how much of a substance is dissolved in one place compared with another; moving from higher to lower concentration is moving down the gradient. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
Passive transport — Movement of a substance across a membrane from higher to lower concentration that costs the cell no ATP. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
Diffusion — The net movement of particles from where they are more concentrated to where they are less concentrated. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
Facilitated diffusion — Diffusion through a membrane protein, for substances that cannot cross the lipid bilayer because of their size, charge, or polarity. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
Solute — A substance dissolved in a fluid, such as salt dissolved in water. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
Osmosis — The diffusion of water across a selectively permeable membrane toward the side with more solute. (first used: 3. Let it flow downhill: diffusion, facilitated diffusion, and osmosis)
mOsm/L — Milliosmoles per liter, a unit for how much dissolved solute a liter of fluid carries. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
Tonicity — A description of a bath by its effect on a cell's water, read from the cell's side by comparing the solute outside with the solute inside. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
Hypotonic — Describes a bath with less solute than the cell, so water moves into the cell and the cell swells and may burst. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
Isotonic — Describes a bath with the same solute concentration as the cell, so there is no net movement of water. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
Hypertonic — Describes a bath with more solute than the cell, so water moves out of the cell and the cell shrinks. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
C₁V₁ = C₂V₂ — The dilution rule: the starting concentration times the starting volume equals the new concentration times the new volume, so doubling the volume halves the concentration. (first used: 4. Read tonicity from the cell's side: compare, name, point the water)
Active transport — Movement of a substance across a membrane against its concentration gradient, paid for with ATP. (first used: 5. Pay to go uphill: active transport and the sodium-potassium pump)
Sodium-potassium pump — A membrane protein that uses one ATP to move three sodium ions out of the cell and two potassium ions in. (first used: 5. Pay to go uphill: active transport and the sodium-potassium pump)
Bulk transport — Movement of large particles or volumes of fluid across the plasma membrane inside membrane vesicles. (first used: 6. Move it in bulk: endocytosis and exocytosis)
Vesicle — A small sac of membrane that carries material inside a cell or into and out of it. (first used: 6. Move it in bulk: endocytosis and exocytosis)
Endocytosis — Bulk transport into the cell, in which the membrane wraps material in a pocket that pinches off as a vesicle. (first used: 6. Move it in bulk: endocytosis and exocytosis)
Phagocytosis — Endocytosis of large particles, sometimes called cell eating. (first used: 6. Move it in bulk: endocytosis and exocytosis)
Pinocytosis — Endocytosis of droplets of fluid and the solutes dissolved in them, sometimes called cell drinking. (first used: 6. Move it in bulk: endocytosis and exocytosis)
Exocytosis — Bulk transport out of the cell, in which a vesicle fuses with the plasma membrane and releases its contents outside. (first used: 6. Move it in bulk: endocytosis and exocytosis)

Summary

The plasma membrane is a phospholipid bilayer: heads out to the water, tails tucked in.
The membrane is selectively permeable; charged or large substances need a protein.
The ribosome builds proteins, the Golgi apparatus ships them, and the mitochondrion makes ATP.
Passive transport runs downhill and costs no ATP.
Osmosis moves water, not salt, toward the side with more solute.
Read tonicity from the cell's side: compare, name the bath, then point the water.
A hypotonic bath swells the cell; a hypertonic bath shrinks it; an isotonic bath leaves it the same.
Doubling the volume halves the concentration: C₁V₁ = C₂V₂.
An IV fluid is isotonic so the body's cells neither swell nor shrink.
Active transport runs uphill and is paid in ATP.
The pump moves 3 Na⁺ out and 2 K⁺ in per ATP.
Endocytosis brings a load in a vesicle; exocytosis sends one out.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative setup made up for this question, a model cell at 300 mOsm/L is placed in a bath of 240 mOsm/L. Compare, name the bath, and say which way the water moves and what happens to the cell.
In a second illustrative setup, a stock solution of 900 mOsm/L is diluted into twice its volume. Use C₁V₁ = C₂V₂ to find the new concentration, then name its tonicity for a 300 mOsm/L model cell.
A study card made up for this question says: "For four ATP, the pump moves 8 Na⁺ out and 12 K⁺ in." Work the correct counts and say what went wrong on the card.
In an illustrative case made up for this question, a molecule too polar to cross the bilayer moves through a carrier protein from higher to lower concentration. A classmate says, "That is active transport, because it goes through a protein." Using this week's definitions, explain what is right and what is wrong about that statement.

Answers (worked reasoning — read after attempting)

Compare: 240 < 300, so the bath has less solute than the cell. Name it: a bath with less solute is hypotonic. Point the water: water follows solute, so water moves into the cell, and the cell swells.
Write the rule with the four values: 900 mOsm/L × 1 volume = C₂ × 2 volumes. Solve: C₂ = 900 ÷ 2 = 450 mOsm/L. Compare: 450 > 300, so the diluted bath is still hypertonic; water moves out of the cell, and the cell shrinks. Diluting lowered the concentration, but not below the cell's.
Each ATP moves 3 Na⁺ out and 2 K⁺ in. For four ATP: 3 × 4 = 12 Na⁺ out and 2 × 4 = 8 K⁺ in. The card multiplied correctly but swapped the ions, the same slip as writing the pump as 2 out and 3 in. Sodium leaves; potassium enters.
What is right: the molecule does cross through a protein, and the protein is a carrier. What is wrong: going through a protein does not make a transport active. The molecule moves from higher to lower concentration, downhill, with no ATP spent, so it is passive transport, by facilitated diffusion. Active transport is defined by moving against the gradient and paying in ATP.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz; next week the mechanism that makes the pump's ATP, cellular respiration, gets its ordered overview.

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