Untitled Flashcard Set

AP Biology Study Guide: Cell Membranes and Transport

Based on your four OpenStax files, Sections 5.1–5.4. This is a summarized version with the main concepts and important AP Biology details.

File 1 — Section 5.1: Structure and Function of Plasma Membranes

1. Plasma membrane functions

The plasma membrane surrounds the cell and separates its internal environment from the external environment.

Its functions include:

• Controlling which substances enter and leave the cell.

• Maintaining dynamic homeostasis by keeping internal conditions different from external conditions.

• Allowing cells to recognize one another.

• Receiving signals from hormones and other molecules through receptors.

• Helping cells attach to each other and form tissues.

• Changing shape when needed, such as red and white blood cells passing through narrow capillaries.

2. Fluid mosaic model

The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a flexible structure made of different components.

• Fluid: Phospholipids and many proteins can move sideways within the membrane.

• Mosaic: The membrane contains different components, including phospholipids, proteins, cholesterol, and carbohydrates.

• Plasma membranes are approximately 5–10 nm thick.

3. Phospholipids and the bilayer

Phospholipids form the main structure of the membrane.

Each phospholipid contains:

• A hydrophilic head that interacts with water.

• Two hydrophobic fatty acid tails that avoid water.

• A glycerol backbone and a phosphate-containing group.

Phospholipids are amphiphilic, meaning they have both hydrophilic and hydrophobic regions.

They arrange themselves into a phospholipid bilayer:

• Heads face the watery environments inside and outside the cell.

• Tails face inward, toward each other.

• The hydrophobic interior helps prevent many polar and charged substances from passing directly through the membrane.

Phospholipids can also form structures such as micelles and liposomes in aqueous solutions.

4. Membrane proteins

Integral proteins have hydrophobic regions that interact with the membrane interior and hydrophilic regions that interact with watery environments.

5. Membrane carbohydrates

Carbohydrates are found on the outer surface of the plasma membrane.

• Glycoprotein: A protein with an attached carbohydrate.

• Glycolipid: A lipid with an attached carbohydrate.

• Glycocalyx: The carbohydrate coating formed by carbohydrate components on the cell surface.

Functions include:

• Cell recognition.

• Cell attachment.

• Distinguishing self from non-self.

• Helping cells interact with their environments.

• Supporting tissue formation and immune responses.

Some viruses exploit cell-surface recognition sites to enter specific cells. Rapid changes in viral surface proteins can make recognition by the immune system more difficult.

6. Membrane fluidity

Membrane fluidity depends on fatty acid structure, temperature, and cholesterol.

Saturated fatty acids

• Have no carbon-carbon double bonds.

• Have relatively straight tails.

• Pack closely together.

• Generally make membranes less fluid, especially at low temperatures.

Unsaturated fatty acids

• Have one or more carbon-carbon double bonds.

• Double bonds create bends or kinks in the tails.

• Prevent phospholipids from packing tightly.

• Help maintain membrane fluidity at low temperatures.

Cholesterol

• Is found between phospholipids in animal cell membranes.

• Acts as a buffer against temperature-related changes in fluidity.

• Helps prevent membranes from becoming too rigid at low temperatures and too fluid at high temperatures.

7. Selective permeability

The membrane is selectively permeable, meaning some substances cross more easily than others.

AP exam point: A molecule's size, polarity, charge, and lipid solubility affect its ability to cross the membrane.

File 2 — Section 5.2: Passive Transport

1. Passive transport

Passive transport moves substances without directly requiring cellular energy.

Substances move down their concentration gradient, from higher concentration to lower concentration.

A concentration gradient is a difference in the concentration of a substance between two regions.

Dynamic equilibrium: Molecules continue moving randomly, but there is no net movement from one region to another.

2. Factors affecting diffusion

Diffusion is the net movement of particles from higher concentration to lower concentration.

The rate of diffusion is affected by:

• Concentration gradient: A larger difference generally increases the rate.

• Molecular mass: Lighter molecules generally diffuse faster than heavier ones.

• Temperature: Higher temperature increases molecular movement and diffusion rate.

• Solvent density: Greater density generally slows diffusion.

• Solubility: Lipid-soluble substances cross the membrane more easily than many polar substances.

• Surface area: Greater membrane surface area increases the rate of diffusion.

• Membrane thickness: Greater thickness reduces the rate.

• Distance: Longer distances slow diffusion.

These factors help explain why cells generally need to remain small or have shapes that allow substances to reach their interiors efficiently.

3. Filtration

Filtration is movement through a membrane driven or enhanced by pressure.

For example, blood pressure helps force water and dissolved substances from the blood into kidney tubules.

4. Facilitated diffusion

Facilitated diffusion moves substances down their concentration gradients with the help of membrane proteins. It does not directly require ATP.

It is needed when substances cannot easily cross the hydrophobic membrane interior.

Channel proteins

• Form hydrophilic pathways through the membrane.

• Transport specific substances, including certain ions.

• May be continuously open or gated.

• Aquaporins are channels that facilitate water movement.

Carrier proteins

• Bind particular substances.

• Change shape to move the substance across the membrane.

• Are specific to their transported substances.

• Can become saturated when all available transport proteins are occupied.

Important: Both channel proteins and carrier proteins can participate in facilitated diffusion. The transported substances move down their concentration gradients.

5. Osmosis

Osmosis is the net movement of water across a selectively permeable membrane.

Water moves from a region with a higher concentration of free water molecules toward a region with a lower concentration of free water molecules. When a solute cannot cross the membrane, water movement is commonly described as being toward the side with the higher solute concentration.

Aquaporins facilitate rapid water movement.

6. Osmolarity and tonicity

• Osmolarity: The total concentration of dissolved solute particles in a solution.

• Tonicity: The effect of an extracellular solution on cell volume due to water movement.

Remember that water still moves in both directions in an isotonic solution. There is simply no net movement.

Red blood cells

• Hypotonic solution → swelling and potentially hemolysis.

• Isotonic solution → normal volume maintained.

• Hypertonic solution → shrinking, called crenation.

7. Osmoregulation

Osmoregulation maintains water and solute balance.

Examples from the source:

• Plant cells: Cell walls limit expansion and help prevent lysis. Water entering cells produces turgor pressure, which supports nonwoody plants.

• Plasmolysis: In a hypertonic environment, the plant cell membrane pulls away from the cell wall as water leaves the cell.

• Paramecia and amoebas: Contractile vacuoles remove excess water.

• Freshwater fish: Take in salts and excrete dilute urine.

• Saltwater fish: Secrete excess salt through their gills and excrete concentrated urine.

• Kidneys: Regulate the body's water balance.

• Osmoreceptors: Monitor solute concentration in the blood and help regulate water balance through hormonal responses.

• Albumin: A blood protein that contributes to osmotic pressure.

File 3 — Section 5.3: Active Transport

1. Active transport

Active transport uses energy to move substances against their concentration gradient or electrochemical gradient.

It commonly involves specific membrane proteins called pumps or transporters.

Examples include moving Na⁺ and K⁺ across animal cell membranes.

2. Electrochemical gradient

An electrochemical gradient combines two gradients:

1. Concentration gradient: Difference in the concentration of an ion across the membrane.

2. Electrical gradient: Difference in electrical charge across the membrane.

The inside of many living cells is electrically negative relative to the outside.

For example:

• Na⁺ is usually more concentrated outside the cell, so its concentration gradient favors movement inward.

• K⁺ is usually more concentrated inside the cell, so its concentration gradient favors movement outward.

• The negative interior attracts positively charged ions electrically.

The net direction depends on the combined electrochemical gradient and whether an appropriate pathway is available.

3. Primary and secondary active transport

Secondary active transport does not directly hydrolyze ATP at the cotransporter, but it depends on gradients that were established using energy.

4. Three transporter types

• Uniporter: Moves one type of substance.

• Symporter: Moves two different substances in the same direction.

• Antiporter: Moves two different substances in opposite directions.

These terms describe what is transported and the direction of movement, not by themselves whether ATP is required.

5. Sodium-potassium pump

The sodium-potassium pump (Na⁺/K⁺-ATPase) is an example of primary active transport in animal cells.

For each cycle, it moves:

• 3 Na⁺ out of the cell.

• 2 K⁺ into the cell.

• Uses energy from ATP.

Six steps of the pump

1. Three Na⁺ ions bind to the pump on the cytoplasmic side.

2. ATP is hydrolyzed, and a phosphate group attaches to the pump.

3. The pump changes shape, faces outward, and releases the three Na⁺ ions.

4. Two K⁺ ions bind from outside; the phosphate group detaches.

5. The pump changes shape back toward the cytoplasm.

6. The two K⁺ ions are released inside, and the cycle repeats.

Why is the pump important?

• Maintains different Na⁺ and K⁺ concentrations inside and outside the cell.

• Helps maintain the membrane potential.

• Contributes to the electrical difference across the membrane.

• Establishes conditions needed for secondary active transport.

Because three positive charges leave while two enter, each cycle results in a net movement of one positive charge outward.

6. Secondary active transport and cotransport

Secondary active transport uses the energy stored in an electrochemical gradient.

For example:

1. The sodium-potassium pump uses ATP to maintain a sodium gradient.

2. Sodium accumulates at a higher concentration outside the cell.

3. Sodium moves back down its electrochemical gradient through a cotransporter.

4. The transporter couples this movement to the movement of another substance, such as glucose or an amino acid, against that substance's gradient.

Hydrogen-ion gradients are also important in cellular respiration and photosynthesis. As H⁺ moves through ATP synthase down its electrochemical gradient, the released energy supports ATP production.

File 4 — Section 5.4: Bulk Transport

1. Bulk transport

Bulk transport moves large particles, macromolecules, or substantial quantities of material using membrane-bound vesicles. It requires energy.

The two major categories are:

• Endocytosis: Brings material into the cell.

• Exocytosis: Releases material outside the cell.

2. Endocytosis

During endocytosis:

1. The plasma membrane folds inward around material.

2. The membrane encloses the material.

3. The membrane pinches off.

4. A vesicle forms inside the cell.

Three main types

Phagocytosis

• Engulfs large particles, such as bacteria.

• Can be performed by white blood cells, including neutrophils.

• The engulfed material can be enclosed in a vesicle.

• The vesicle can fuse with a lysosome, whose enzymes help digest the material.

• Clathrin can help stabilize membrane regions involved in vesicle formation.

Pinocytosis

• Takes in extracellular fluid and dissolved substances.

• Generally forms smaller vesicles than phagocytosis.

• Does not always require fusion with a lysosome.

Potocytosis is a related process involving caveolin and small membrane invaginations called caveolae. It can help transport small molecules into and through cells.

Receptor-mediated endocytosis

• Receptors on the cell surface bind specific substances.

• The membrane folds inward and forms a vesicle.

• This allows selective uptake of particular materials.

• Clathrin can be involved in forming the vesicle.

Example: Cells use receptor-mediated endocytosis to remove LDL particles from the blood. Defective or missing LDL receptors can cause familial hypercholesterolemia, resulting in elevated blood cholesterol.

Some pathogens and toxins can exploit receptor-mediated entry pathways.

3. Exocytosis

During exocytosis:

1. A vesicle containing material moves toward the plasma membrane.

2. The vesicle fuses with the membrane.

3. The contents are released outside the cell.

Examples include:

• Releasing neurotransmitters at synapses.

• Secreting proteins.

• Releasing cellular waste.

4. Comparing transport methods

Essential AP Biology comparisons to memorize

Passive vs. active transport

Passive transport

Moves substances down their gradient; does not directly require cellular energy.

Examples: simple diffusion, osmosis, facilitated diffusion.

Active transport

Uses energy directly or indirectly to move substances against their gradient.

Examples: sodium-potassium pump and secondary active cotransport.

Three common AP exam traps

• Facilitated diffusion still is passive. Using a protein does not automatically mean ATP is needed.

• Isotonic does not mean water stops moving. Water moves in both directions, with no net movement.

• Unsaturated fatty acids increase membrane fluidity at low temperatures because their kinks prevent tight packing.

Quick self-check

1. Which membrane component has hydrophilic heads and hydrophobic tails?

Cholesterol

Phospholipid

Carbohydrate

Peripheral protein

2. A cell is placed in a hypertonic solution. What happens?

Water enters and the cell swells

No net water movement

Water leaves and the cell shrinks

ATP production immediately stops

3. Which transport method moves glucose down its gradient through a carrier without directly using ATP?

Primary active transport

Facilitated diffusion

Phagocytosis

Exocytosis

4. How many Na⁺ ions are moved out by one cycle of the sodium-potassium pump?

One

Two

Three

Four

5. A cell takes in a specific substance after it binds to a surface receptor. What is this called?

Osmosis

Simple diffusion

Pinocytosis only

Receptor-mediated endocytosis

Reset quiz