AP Biology 2.3-2.4: Cell Size and Environment

What is the main role of the plasma membrane?

It separates the cell interior from the external environment and selectively controls the movement of substances into and out of the cell. It also participates in cell recognition, adhesion, and signal transduction.

What does it mean that a membrane is selectively permeable?

A selectively permeable membrane allows some substances to cross more readily than others. Its permeability depends on factors such as molecular size, polarity, charge, and the presence of transport proteins.

Fluid mosaic model

The plasma membrane is a dynamic phospholipid bilayer containing a mosaic of lipids, proteins, and carbohydrates. Its components can move laterally within the membrane, giving it fluidity.

Compare the fluid mosaic model with a rigid protein-coated lipid model.

The fluid mosaic model places proteins within or attached to a mobile lipid bilayer rather than in continuous rigid layers outside it. This better explains membrane flexibility, lateral movement, transport, and signaling.

What is the approximate thickness range of a typical plasma membrane?

A plasma membrane is approximately $5$–$10$ nm thick. This is much smaller than the width of a typical human red blood cell, which is about $8$ μm.

A typical human plasma membrane is approximately what percentage protein, lipid, and carbohydrate by mass?

Approximately $50\%$ protein, $40\%$ lipid, and $10\%$ carbohydrate by mass, although these proportions vary substantially among cell types and membrane locations.

How can membrane composition vary among different biological membranes?

Different membranes contain different proportions of protein and lipid according to their functions. For example, an inner mitochondrial membrane is protein-rich, whereas myelin is especially lipid-rich.

Phospholipid

A phospholipid contains a glycerol backbone, two fatty acid tails, and a phosphate-containing head group. Its polar, hydrophilic head and nonpolar, hydrophobic tails make it amphiphilic.

Amphiphilic molecule

A molecule with both a hydrophilic region and a hydrophobic region. Phospholipids are amphiphilic because their charged or polar heads interact with water while their nonpolar hydrocarbon tails avoid it.

Hydrophilic region

A polar or charged region that interacts favorably with water, often through hydrogen bonding or ion-dipole attractions. In a phospholipid, the hydrophilic region is the phosphate-containing head.

Hydrophobic region

A nonpolar region that does not interact favorably with water. In a phospholipid, the hydrophobic region consists of the two hydrocarbon fatty acid tails.

Lipid bilayer

A double layer of phospholipids with hydrophilic heads facing aqueous solutions and hydrophobic tails oriented toward the membrane interior. It forms the basic structural barrier of the plasma membrane.

Why do phospholipids spontaneously form a bilayer in water?

The hydrophilic heads interact favorably with water on both sides, while the hydrophobic tails cluster away from water and face one another. This arrangement lowers unfavorable interactions between nonpolar tails and water.

Micelle versus liposome

A micelle is typically a small sphere with hydrophilic heads outward and hydrophobic tails inward. A liposome is a vesicle enclosed by a phospholipid bilayer and can contain an aqueous interior.

How can a membrane self-seal after a small puncture?

Phospholipids rearrange so that hydrophobic tails remain shielded from water while hydrophilic heads remain exposed. This spontaneous reorganization closes small defects without requiring covalent bonds to be formed.

Cholesterol in a membrane

Cholesterol is a steroid lipid positioned among phospholipid tails. It buffers membrane fluidity by limiting excessive movement at high temperature and preventing tight packing at low temperature.

Integral membrane protein

A protein embedded within the phospholipid bilayer; some integral proteins span the entire membrane. Hydrophobic amino acid regions interact with lipid tails, while hydrophilic regions contact the cytosol or extracellular fluid.

Peripheral membrane protein

A protein attached to the inner or outer membrane surface, often through interactions with integral proteins or phospholipid head groups. It is not embedded through the hydrophobic core of the bilayer.

What is the relationship between membrane protein orientation and the polarity of its surroundings?

Hydrophobic portions of a membrane protein are positioned beside lipid tails in the bilayer core. Hydrophilic portions extend into the cytosol or extracellular fluid, where they can interact with water and dissolved substances.

What functions can membrane proteins perform?

Membrane proteins can act as transporters, receptors, enzymes, structural attachments, or cell-recognition molecules. Receptors can bind external signaling molecules and initiate intracellular response pathways.

How do membrane receptors transmit information across the plasma membrane?

An extracellular signaling molecule binds specifically to the receptor, causing a change that activates processes on the intracellular side. This can initiate a signal-transduction cascade.

Glycoprotein and glycolipid

A glycoprotein is a membrane protein with an attached carbohydrate chain, whereas a glycolipid is a membrane lipid with an attached carbohydrate chain. Their carbohydrate portions project from the extracellular surface.

Glycocalyx

The carbohydrate-rich coating on the external surface of a cell, formed by carbohydrate chains of glycoproteins and glycolipids. It contributes to cell recognition, adhesion, development, and interaction with the aqueous environment.

Why are membrane carbohydrates found on the extracellular side rather than the cytosolic side?

Membrane carbohydrates are oriented toward the outside during membrane synthesis and processing. Their exposed patterns function in cell-cell recognition, adhesion, and distinguishing self from non-self.

How do cell-surface carbohydrates contribute to immune recognition?

Distinct carbohydrate patterns on glycoproteins and glycolipids serve as molecular identification markers. Immune cells can use these markers to distinguish the organism's own cells from foreign cells or pathogens.

Saturated fatty acid tail

A fatty acid tail with no carbon-carbon double bonds, so it is relatively straight. Membranes rich in saturated tails pack more tightly and tend to be less fluid, especially at lower temperatures.

Unsaturated fatty acid tail

A fatty acid tail containing one or more carbon-carbon double bonds, which introduce bends or kinks. These kinks prevent tight packing and generally increase membrane fluidity.

How does increasing the proportion of unsaturated phospholipids help a membrane function in cold conditions?

Double bonds create bends in fatty acid tails, preventing adjacent phospholipids from packing closely. This maintains space between molecules and reduces the tendency of the membrane to solidify.

How does cholesterol affect membrane fluidity at different temperatures?

At high temperatures, cholesterol restrains phospholipid movement and reduces excessive fluidity. At low temperatures, it disrupts tight packing of phospholipid tails, helping prevent solidification.

Predict the effect of replacing unsaturated fatty acid tails with saturated tails at constant temperature.

The membrane would generally become less fluid because the straighter saturated tails can pack more closely. The effect is especially pronounced if the temperature is relatively low.

Why is membrane fluidity important for cell survival?

Fluidity allows membrane components to move, supports signaling and transport, and permits the membrane to deform and self-seal after small disruptions. Excessive rigidity or excessive fluidity can interfere with membrane function.

Why does the plasma membrane permit a cell to change shape without disintegrating?

The bilayer is flexible because phospholipids and many proteins can move laterally, while hydrophobic interactions maintain the membrane's overall integrity. This permits deformation and recovery during processes such as passage through narrow capillaries.

What determines whether a substance can cross the hydrophobic membrane interior directly?

Small nonpolar molecules generally cross readily because they interact favorably with the nonpolar lipid core. Large molecules and charged or strongly polar substances cross poorly without membrane proteins.

Why do ions generally require membrane proteins to cross the lipid bilayer?

Ions are charged and strongly hydrated, so moving them through the nonpolar membrane interior is energetically unfavorable. Channels or transport proteins provide a more favorable pathway.

How do plasma membranes contribute to cell adhesion and tissue formation?

Cell-surface carbohydrates and membrane-associated proteins provide recognition and attachment sites. These interactions allow compatible cells to identify one another and form organized tissues.

Why can viruses use cell-surface receptors to infect particular cell types?

Viral surface molecules may bind only to receptors with complementary shape and chemical properties. Consequently, only cells displaying compatible receptors are efficiently targeted.

How can mutations in a receptor gene disrupt cell function?

A mutation can alter the receptor's structure, preventing signal binding or changing the intracellular response. This can cause inappropriate or absent signal-transduction pathways.

What is the surface-area-to-volume ratio?

The surface-area-to-volume ratio compares the amount of cell membrane available for exchange with the amount of cytoplasm that requires nutrients and produces waste. It is calculated as $\frac{\text{surface area}}{\text{volume}}$.

Why does a cell's surface-area-to-volume ratio decrease as the cell grows?

For similarly shaped cells, surface area increases with the square of a cell's length, whereas volume increases with the cube. Therefore, volume grows faster than surface area, causing the surface-area-to-volume ratio to decrease.

Why are cells generally small?

Small cells have a larger surface-area-to-volume ratio, providing relatively more membrane for exchanging materials with the environment and shorter distances for substances to travel inside the cell.

How can a cell maintain an adequate surface-area-to-volume ratio while increasing in size?

A cell can divide, become elongated or flattened, or develop folds and projections that increase surface area without a proportional increase in volume.

Predict how cell size affects the rate at which a cell can exchange materials with its environment.

As cell size increases, the surface-area-to-volume ratio decreases. The cell has less membrane surface available per unit of cytoplasm, so exchange becomes less efficient and may not meet the cell's demands.