Unit 2: Cells, Cell Membrane Structure, and Transport Mechanisms

Comprehensive review flashcards covering Unit 2 topics: membrane structure, the fluid mosaic model, modern cell theory, prokaryotic versus eukaryotic cells, passive transport, osmosis, active transport, the sodium-potassium pump, vesicular transport, and surface area-to-volume ratio constraints.

What are the primary functions of the cell membrane?

It separates the inside of the cell from its external environment, protects internal contents, controls what enters and leaves the cell, allows communication with the environment, and helps maintain homeostasis.

How do intracellular fluid ($\text{ICF}$) and extracellular fluid ($\text{ECF}$) differ?

Intracellular fluid ($\text{ICF}$) is the fluid inside the cell, including the cytosol (fluid portion of cytoplasm). Extracellular fluid ($\text{ECF}$) surrounds the cell and includes interstitial fluid ($\text{IF}$) and blood plasma.

Which four main molecular components make up the Fluid Mosaic Model of the cell membrane?

1) Phospholipids (form the bilayer), 2) Cholesterol (regulates membrane fluidity), 3) Proteins (perform specialized functions), and 4) Carbohydrates (participate in cell recognition and communication).

Which chemical sub-units comprise the hydrophilic head and hydrophobic tails of a phospholipid?

The hydrophilic (polar) head consists of glycerol, phosphate, and choline. The hydrophobic (nonpolar) tails consist of two fatty acid chains.

How does cholesterol act as a fluidity buffer in animal cell membranes at varying temperatures?

At lower temperatures, it prevents phospholipids from packing too tightly to maintain fluidity; at higher temperatures, it limits excessive phospholipid movement to prevent the membrane from becoming too fluid.

What is the structural difference between integral membrane proteins and peripheral membrane proteins?

Integral proteins are permanently embedded within the membrane (often spanning the entire bilayer as transmembrane proteins), whereas peripheral proteins are attached to the inner or outer surface and do not span the bilayer.

What is the glycocalyx, and what roles does it perform?

A fuzzy, carbohydrate-rich coating on the outer surface of the cell formed by glycoproteins and glycolipids. It functions in cell recognition, communication, cell adhesion, and cell surface protection.

Why can small, nonpolar molecules freely diffuse across the phospholipid bilayer while large or charged substances cannot?

The interior of the phospholipid bilayer is hydrophobic, creating a barrier to large, polar, or charged substances, whereas small nonpolar molecules (such as $O_2$ and $CO_2$) dissolve through the hydrophobic core easily.

What are the three core tenets of the Modern Cell Theory?

1) All living things are made up of one or more cells. 2) All new cells come from pre-existing cells. 3) The cell is the basic structural and functional unit of life.

What additional concepts have expanded the Modern Cell Theory through modern biochemistry and molecular biology?

Cells contain hereditary information ($\text{DNA}$) passed during division; whole organism activity depends on the combined activities of individual cells; cell activity depends on sub-cellular structures; and cells of the same species share similar chemical compositions and metabolic processes.

What is cell differentiation, and what role do stem cells play in it?

Cell differentiation is the process by which cells become specialized for distinct functions during development, guided by interactions between genetic information and embryonic signals. Stem cells are specialized human cells capable of developing into many different cell types.

What is the primary diagnostic criterion used to classify cells as either prokaryotic or eukaryotic?

The presence or absence of a membrane-bound nucleus ("Does the cell have a nucleus?").

What are the defining structural characteristics of prokaryotic cells?

They lack a nucleus, house circular $\text{DNA}$ in an open cytoplasm region called the nucleoid, lack membrane-bound organelles, are generally smaller and simpler, and exist exclusively as unicellular organisms.

What are the defining characteristics of eukaryotic cells?

They possess a true nucleus containing $\text{DNA}$, contain membrane-bound organelles with specialized functions, are generally larger (often ~$10\times$ larger) and more complex, and can be either unicellular or multicellular.

What are the three evolutionary domains of life?

Bacteria, Archaea (both composed of prokaryotes), and Eukarya (composed of eukaryotes).

What defines a solution, solvent, and solute in human physiological fluids?

A solution is a mixture consisting of a solvent (the dissolving agent, primarily water in the human body) and one or more solutes (substances that dissolve, such as $Na^+$, $K^+$, $Cl^-$, glucose, and gases).

What is the Law of Diffusion?

Particles move down their concentration gradient from an area of high concentration to low concentration due to random motion until dynamic equilibrium is reached, where random motion persists with no net directional movement.

Which factors increase the rate of diffusion?

A steeper concentration gradient, higher temperature, smaller particle size, shorter diffusion distance, and greater surface area.

How are human lungs structurally adapted for rapid gas exchange via diffusion?

They maintain steep partial pressure gradients, feature extremely thin respiratory membrane walls (short diffusion distance), and provide a huge surface area through millions of alveoli.

What is an osmotic gradient?

A difference in solute concentration across a selectively permeable membrane that drives the directional movement of water ("more solute means less water").

In this experimental setup, if the selectively permeable membrane is permeable only to water molecules, what net movement will occur?

The net flow of water molecules will be from Chamber 2 to Chamber 1, moving down the water concentration gradient toward the area of higher solute concentration.

What occurs to red blood cells placed in hypotonic, isotonic, and hypertonic solutions, as shown from left to right?

Left (hypotonic): water enters the cell, causing it to swell and potentially lyse; Middle (isotonic): no net water movement, maintaining normal biconcave shape; Right (hypertonic): water leaves the cell, causing it to shrivel and shrink.

What is osmotic pressure?

The pressure associated with the tendency of water to move across a selectively permeable membrane due to osmosis; a greater difference in solute concentration results in higher osmotic pressure.

How does facilitated diffusion differ from simple diffusion across the cell membrane?

Both are passive transport processes requiring no $\text{ATP}$ ($\text{high} \rightarrow \text{low}$), but facilitated diffusion requires membrane transport proteins (channels or carriers) to pass substances that cannot directly cross the lipid bilayer.

What mechanism do carrier proteins use to transport molecules like glucose across the cell membrane?

They bind the specific substance, undergo a conformational shape change, and release the substance on the other side of the membrane down its concentration gradient without using $\text{ATP}$.

At what electrical threshold do voltage-gated sodium channels in nerve cells open?

They open when the membrane electrical potential reaches $-55\,\text{mV}$, allowing $Na^+$ to rush rapidly into the cell.

How does active transport differ fundamentally from passive transport?

Active transport moves substances against their concentration gradient ($\text{low} \rightarrow \text{high}$) using membrane transport proteins and energy supplied by the hydrolysis of $\text{ATP}$.

What is the stoichiometric ratio and direction of ion transport in one cycle of the sodium-potassium ($Na^+/K^+$) pump?

For each molecule of $\text{ATP}$ hydrolyzed, $3\,Na^+$ ions are pumped out of the cell and $2\,K^+$ ions are pumped into the cell, both against their concentration gradients.

What cellular transport process is illustrated in this diagram?

Exocytosis, an active process where an intracellular vesicle moves toward and fuses with the cell membrane, releasing its contents into the extracellular fluid.

What vesicular transport mechanism is shown in this diagram?

Endocytosis, an active process where the cell membrane folds around extracellular material and pinches inward to form an intracellular vesicle.

What is the functional difference between phagocytosis and pinocytosis?

Phagocytosis ("cell eating") engulfs large solid particles, whole bacteria, or cellular debris; pinocytosis ("cell drinking") takes in small volumes of extracellular fluid and dissolved solutes. Both require $\text{ATP}$.

What is the typical diameter range of human cells, and why does diffusion constrain maximum cell size?

Many human cells range from $2\,\mu\text{m} - 200\,\mu\text{m}$ ($1\,\mu\text{m} = 0.000001\,\text{m}$). Diffusion limits cell size because if a cell becomes too large, the exchange of materials across the surface area cannot occur fast enough to support internal volume.

How do surface area, volume, and the $\text{SA}/\text{V}$ ratio change when comparing a $1\,\text{mm}$ cube to a $3\,\text{mm}$ cube?

The $1\,\text{mm}$ cube has $\text{SA} = 6\,\text{mm}^2$, $\text{V} = 1\,\text{mm}^3$, and $\text{SA}/\text{V} = 6$. The $3\,\text{mm}$ cube has $\text{SA} = 54\,\text{mm}^2$, $\text{V} = 27\,\text{mm}^3$, and $\text{SA}/\text{V} = 2$. Volume increases much faster than surface area, lowering the ratio.

Why is a lower surface area-to-volume ($\text{SA}/\text{V}$) ratio disadvantageous for cellular homeostasis?

A larger volume means more cytoplasm to support, higher nutrient and $O_2$ demands, and greater waste production, while a decreased $\text{SA}/\text{V}$ ratio provides relatively less membrane surface area and longer diffusion distances to the cell center.

Why is an increased $\text{SA}/\text{V}$ ratio resulting from placement in a hypertonic solution not beneficial to a cell?

Although volume loss increases the $\text{SA}/\text{V}$ ratio, severe cellular water loss alters protein conformations, disrupts chemical reactions, and impairs overall cell function.

What four adaptations do cells use to overcome surface area-to-volume exchange limitations?

1) Remaining small, 2) adopting specialized thin, flat, or elongated shapes, 3) increasing surface area through surface folds or projections (such as microvilli in the small intestine), and 4) dividing via mitosis.