BIOL 203 · L2: Cellular Level I: The Cell Surface

What is the general structure of the plasma membrane?

A fluid lipid bilayer made mainly of phospholipids, cholesterol, and glycolipids, with proteins inserted or associated with it.

Why is the plasma membrane described by the fluid mosaic model?

“Fluid” refers to the mobility of membrane components, especially phospholipids; “mosaic” refers to the mixture of different molecules distributed throughout the membrane.

What does “fluid” mean in the fluid mosaic model?

Phospholipids are not rigidly fixed and can move laterally within the membrane.

What does “mosaic” mean in the fluid mosaic model? \nThe membrane contains a mixture of phospholipids, proteins, cholesterol, and other molecules that are not uniformly distributed.

What is an integral transmembrane protein?

A protein with one or more segments that span the entire lipid bilayer.

What is a peripheral membrane protein?

A protein temporarily attached to the membrane or indirectly associated with it, often through interactions with integral proteins.

How do integral and peripheral membrane proteins differ?

Integral proteins are embedded in or span the membrane; peripheral proteins are associated with a membrane surface rather than spanning the bilayer.

What do membrane ion channels do?

They form pores that allow specific ions to cross the membrane.

What do membrane transporters do?

They move molecules across the membrane; examples include carriers and pumps.

How does an ion channel differ from a transporter?

A channel forms a pore through the membrane, whereas a transporter moves molecules through interactions with the transported substance.

What do membrane receptors do?

They receive extracellular signals and initiate cellular responses.

What is a ligand?

A signaling molecule that binds to a receptor.

What is the role of cell adhesion molecules?

They help cells attach to other cells or to the extracellular matrix.

What is the role of connexins in the membrane?

Connexins are structural/anchoring proteins that form gap-junction channels.

What does spectrin do in red blood cells?

It forms a meshwork beneath the plasma membrane that helps maintain cell shape and flexibility.

What are examples of peripheral proteins involved in signal transduction?

G-protein subunits, protein kinase C (PKC), and phospholipase C (PLC).

What are examples of extracellular peripheral proteins?

Fibronectin, laminin, and some types of collagen.

What is a major function of fibronectin?

It helps cells attach to their surroundings.

What is laminin, and where is it especially important?

Laminin is a major component of the basal lamina, the thin extracellular-matrix sheet beneath epithelial cells.

What is the basal lamina?

A thin sheet of extracellular matrix located beneath epithelial cells.

What two different regions does a phospholipid contain?

A hydrophilic region and a hydrophobic region.

What makes up the hydrophilic region of a phospholipid?

he polar head region containing glycerol and phosphate.

Where do the hydrophilic phospholipid heads face in a membrane?

Toward water on the extracellular and intracellular sides of the membrane.

What forms the hydrophobic region of a phospholipid?

Two fatty-acid tails that face inward, away from water.

What does amphipathic mean?

Having both hydrophilic and hydrophobic regions.

Why do phospholipids naturally form a bilayer in water?

Their hydrophilic heads interact with water while their hydrophobic tails avoid water and face inward.

What is cholesterol’s structure and where is it located in the membrane?

It has a bulky, rigid, ring-shaped region and sits between phospholipid tails.

How does cholesterol affect membrane fluidity at high temperatures?

It limits phospholipid-tail movement, preventing the membrane from becoming too fluid or loose.

How does cholesterol affect membrane fluidity at low temperatures?

It prevents phospholipids from packing too tightly, helping keep the membrane from becoming too rigid.

What is cholesterol’s overall role in membrane fluidity?

It acts as a fluidity buffer: decreasing excess fluidity at high temperatures and preventing excessive rigidity at low temperatures.

Why can cholesterol have opposite effects at high and low temperatures?

At high temperature its rigid rings restrict phospholipid movement; at low temperature it wedges between the tails and prevents tight packing.

What is the glycocalyx?

A carbohydrate-rich layer that coats the external surface of the plasma membrane.

What does the glycocalyx typically look like?

A soft, brush-like layer extending into the extracellular space.

What are the major components of the glycocalyx?

Glycoproteins, glycolipids, and, particularly in specialized cells such as endothelial cells, proteoglycans.

What are glycoproteins?

Proteins with short carbohydrate chains attached.

What are proteoglycans?

Proteins with one or more very long carbohydrate chains called glycosaminoglycans attached.

what are glycosaminoglycans, or GAGs?

Long, unbranched polysaccharide chains made of repeating disaccharides.

Why do glycosaminoglycans attract water?

They are highly negatively charged.

What is the result of GAGs attracting water?

They help create a hydrated, gel-like glycocalyx layer.

What are two examples of GAGs from this lecture?

Heparan sulfate and chondroitin sulfate.

How do glycoproteins and proteoglycans differ?

Glycoproteins have relatively short carbohydrate chains; proteoglycans have one or more very long GAG chains.

What are the four major functions of the glycocalyx?

Cell-to-cell recognition, cell adhesion, protection, and cell signaling.

How does the glycocalyx participate in cell recognition?

Its carbohydrate chains act like molecular ID tags, allowing cells to recognize different surface sugar patterns.

How can immune cells use the glycocalyx?

Surface sugar patterns help immune cells distinguish self from non-self.

How does the glycocalyx contribute to cell adhesion?

It contributes to attachment between cells and between cells and the extracellular matrix.

How does the glycocalyx protect the cell?

It acts as a physical barrier against mechanical stress, pathogens, and harmful molecules.

How does the glycocalyx participate in cell signaling?

Its components can bind signaling molecules and influence cell behavior.

How does physical transport differ from physiological transport?

Physical transport does not require cellular energy; physiological transport requires cellular energy directly or indirectly.

What is physical transport?

Passive transport in which the cell does not spend ATP to move the substance.

What is physiological transport?

Transport requiring cellular energy, including movement against a gradient or energy-dependent vesicular transport.

What does moving down a concentration gradient mean?

Moving from an area of higher concentration to an area of lower concentration.

What does moving against a concentration gradient mean?

Moving from lower concentration toward higher concentration.

What are four examples of physical transport?

Simple diffusion, facilitated diffusion, osmosis, and filtration.

What are major examples of physiological transport?

Primary active transport, secondary active transport, endocytosis, and exocytosis.

Why is filtration considered physical rather than physiological transport?

It is driven by hydrostatic pressure rather than cellular energy.

Why is facilitated diffusion still considered physical/passive transport even though it uses a membrane protein?

Because the substance moves down its gradient and the cell does not spend ATP to drive the movement.

What is diffusion?

Passive movement of molecules down their concentration gradient.

Does diffusion require the cell to expend ATP?

no.

What is the driving force emphasized for diffusion in this lecture?

A concentration gradient: molecules move from higher toward lower concentration.

What is simple diffusion?

Passive movement directly through the phospholipid bilayer without using membrane proteins.

What is facilitated diffusion?

Passive movement down a concentration gradient through membrane channels or carriers.

What is the major difference between simple and facilitated diffusion?

Simple diffusion occurs directly through the lipid bilayer; facilitated diffusion requires a membrane protein.

Do both simple and facilitated diffusion move substances down their gradients?

Yes.

What are the three major requirements for simple diffusion across the cell membrane?

The molecule should be lipophilic/hydrophobic, uncharged, and small or moderately sized.

Why does being lipophilic favor simple diffusion?

A lipophilic molecule can dissolve in the oily, hydrophobic interior of the membrane.

Why does being uncharged favor simple diffusion?

Charged molecules interact strongly with water and are unfavorable in the membrane’s hydrophobic interior.

Why does molecule size matter for simple diffusion?

Small or moderately sized molecules cross the lipid bilayer more readily than large molecules.

Why do Na⁺ and glucose not cross the membrane by simple diffusion?

They are charged or hydrophilic and do not readily dissolve in the hydrophobic membrane interior.

What is one example of simple diffusion in the lungs?

Gas exchange between the alveoli and the blood.

What is one example of simple diffusion in systemic tissues?

Gas exchange between capillaries and cells.

How can steroid hormones cross cell membranes?

They are lipid-soluble and can cross by simple diffusion.

How can ethanol cross cell membranes?

It can move across membranes by simple diffusion.

What type of drugs may cross membranes by simple diffusion?

Small, lipid-soluble drugs.

What is osmosis?

The diffusion of water.

What causes a voltage-gated ion channel to open or close?

A change in membrane potential.

What is a ligand-gated ion channel?

An ion channel that opens directly when a chemical ligand binds to it.

What can trigger a ligand-gated ion channel?

Binding of a chemical ligand such as a neurotransmitter.

What is another name for a ligand-gated ion channel?

An ionotropic receptor.

What is a leak channel?

A constitutively active, or always-open, pore that allows ions to diffuse according to their electrochemical gradients.

What is an aquaporin?

A water channel that facilitates rapid, passive movement of water across the membrane.

What is the difference between a symporter and an antiporter?

A symporter moves two or more substances in the same direction; an antiporter moves them in opposite directions.

What is active transport?

Transport that requires cellular energy and can move a substance against its concentration or electrochemical gradient.

What determines whether active transport is primary or secondary?

How the energy is coupled to the transport process.

What is primary active transport?

Active transport that directly uses energy from ATP.

How do primary active-transport pumps obtain energy from ATP?

ATPase pumps hydrolyze ATP, releasing energy that changes the pump’s conformation and drives transport.

What type of transport is the Na⁺/K⁺ pump?

Primary active transport.

What does the Na⁺/K⁺ pump do?

It pumps Na⁺ out of the cell and K⁺ into the cell against their respective gradients.

How many Na⁺ ions does the Na⁺/K⁺ pump move out of the cell?

Three Na⁺.

How many K⁺ ions does the Na⁺/K⁺ pump move into the cell?

Two K⁺.

Does the Na⁺/K⁺ pump move Na⁺ and K⁺ with or against their gradients?

Against their respective gradients.

Why is the Na⁺/K⁺ pump considered primary active transport?

Because it directly uses ATP.

What is secondary active transport?

Transport in which one substance is moved against its gradient using energy stored in a gradient created by primary active transport.

Does secondary active transport directly hydrolyze ATP to move the transported substance?

No. It uses energy stored in an ion gradient.

What establishes the Na⁺ gradient used in the lecture’s Na⁺/glucose cotransport example?

The Na⁺/K⁺ pump.

In the intestinal example, where is Na⁺ concentration relatively high and where is it relatively low?

High in the intestinal lumen and low inside the cell.

Which direction does Na⁺ move through the Na⁺/glucose cotransporter?

Into the cell, down its Na⁺ gradient.

Which molecule is moved against its concentration gradient by the Na⁺/glucose cotransporter?

Glucose.

What type of transport is the Na⁺/glucose cotransporter?

Secondary active transport.

Is the Na⁺/glucose cotransporter a symporter or an antiporter?

A symporter.

Do Na⁺ and glucose move in the same or opposite directions through the cotransporter?

The same direction—into the cell.