transport across the membrane

What is the cell membrane’s main role in transport?

It controls what enters and leaves the cell, helping maintain the cell’s internal conditions.

What does selectively permeable mean?

The membrane allows some substances to cross more easily than others while restricting other substances.

What is the phospholipid bilayer?

A double layer of phospholipids that forms the basic structure of the cell membrane.

How is a phospholipid arranged in the membrane?

Hydrophilic (water-attracting) heads face the watery environments inside and outside the cell; hydrophobic (water-repelling) tails face inward toward each other.

Why is the middle of the phospholipid bilayer nonpolar?

The hydrophobic fatty-acid tails point inward, creating a nonpolar interior that resists charged and many polar substances.

Which substances cross the lipid bilayer most easily without a protein?

Small nonpolar molecules such as oxygen (O₂) and carbon dioxide (CO₂).

Why can oxygen and carbon dioxide cross the membrane directly?

They are small and nonpolar, so they can pass through the hydrophobic interior of the phospholipid bilayer.

Why do ions and many polar molecules have difficulty crossing the bilayer directly?

Charged ions and polar molecules do not pass easily through the nonpolar, hydrophobic middle of the membrane.

What is a solute?

A substance dissolved in a liquid, such as sugar or salt dissolved in water.

What is a solvent?

The liquid that dissolves a solute; water is the solvent in many biological solutions.

What is the difference between solute and solvent?

The solute is what gets dissolved; the solvent is the substance doing the dissolving.

What is a transport protein?

A membrane protein that helps a specific substance cross the cell membrane.

What is a channel protein?

A transport protein that forms a passage or pore through the membrane so certain substances can cross.

What is a carrier protein?

A transport protein that binds a particular substance and changes shape to move it across the membrane.

How can you recognize a channel protein in a membrane diagram?

Look for a protein that forms an open passage or pore through the membrane.

How can you recognize a carrier protein in a membrane diagram?

Look for a protein that binds a solute and changes shape to move it from one side of the membrane to the other.

What does it mean that transport proteins are selective or solute-specific?

A protein usually transports only particular substances because its shape and chemical properties fit those substances.

What are the two major categories of membrane transport?

Passive transport and active transport.

What is passive transport?

Movement across a membrane without the cell using energy from ATP for that transport.

What is active transport?

Movement across a membrane that requires energy, commonly ATP, often to move substances against their concentration gradient.

What is a concentration gradient?

A difference in the concentration of a substance between two areas.

What does it mean to move down a concentration gradient?

A substance moves from an area of higher concentration to an area of lower concentration.

What does it mean to move against a concentration gradient?

A substance moves from an area of lower concentration to an area of higher concentration; this generally requires energy.

What is diffusion?

The net movement of particles from an area of higher concentration to an area of lower concentration due to their random motion.

Why does diffusion occur?

Particles move randomly, producing a net movement from high concentration to low concentration until they become evenly distributed.

What is simple diffusion?

Passive movement of a substance directly through the phospholipid bilayer, from higher concentration to lower concentration, without a transport protein.

Which examples of substances commonly cross by simple diffusion?

Small nonpolar molecules such as oxygen and carbon dioxide.

What is equilibrium during diffusion?

A state in which particles continue moving randomly in both directions, but there is no net movement because movement occurs at equal rates.

At equilibrium, do particles stop moving?

No. They continue moving randomly; the movement in each direction is balanced, so there is no net change.

How do you predict the net direction of diffusion in a diagram?

Compare concentrations on each side: the net movement is from the side with more of that substance to the side with less, until equilibrium is reached.

What is facilitated diffusion?

Passive transport in which a substance moves down its concentration gradient through a channel or carrier protein, without ATP.

Why is facilitated diffusion needed?

Some substances, including ions and larger or polar molecules such as glucose, cannot cross the nonpolar bilayer easily on their own.

Does facilitated diffusion require ATP?

No. It is passive and moves substances down their concentration gradient.

How is facilitated diffusion different from simple diffusion?

Simple diffusion occurs directly through the lipid bilayer; facilitated diffusion uses a membrane transport protein. Both move down a concentration gradient without ATP.

How is facilitated diffusion different from active transport?

Facilitated diffusion moves down the concentration gradient without ATP; active transport uses energy to move substances against the gradient.

How does a channel protein help in facilitated diffusion?

It provides a selective hydrophilic passage through the membrane, allowing certain ions or molecules to move down their gradient.

How does a carrier protein help in facilitated diffusion?

It binds a specific solute and changes shape to move it across the membrane down its concentration gradient.

In a glucose transport diagram, why might glucose enter a cell through a carrier protein?

Glucose is relatively large and polar, so it does not cross the hydrophobic bilayer easily; a carrier protein helps it move down its gradient.

If glucose concentration is higher outside a cell than inside, which way will glucose move through a suitable facilitated-diffusion carrier?

Net movement will be into the cell, down the concentration gradient, as long as the carrier is available.

If glucose concentration is higher inside a cell than outside, which way will glucose move through a suitable facilitated-diffusion carrier?

Net movement will be out of the cell, down the concentration gradient, as long as the carrier is available.

What is osmosis?

The net diffusion of water across a selectively permeable membrane.

In osmosis, which way does water move overall?

Water moves from the side with higher free-water concentration (lower solute concentration) toward the side with lower free-water concentration (higher solute concentration).

What does it mean to say water moves from a more dilute solution to a more concentrated solution?

The more dilute side has less solute and relatively more free water; water moves toward the side with more solute and relatively less free water.

Why can water need a protein to cross the membrane?

Water can cross the lipid bilayer slowly, but aquaporin channel proteins allow it to cross much more rapidly.

What is an aquaporin?

A membrane channel protein that allows water to cross the membrane quickly.

What is the role of aquaporins in osmosis?

They provide pores through which water can move across the membrane down its water concentration gradient.

Is osmosis active or passive transport?

Passive transport; it does not require ATP.

What does tonicity describe?

How a solution’s solute concentration relative to a cell affects the cell’s water movement and volume.

What is an isotonic solution relative to a cell?

A solution with the same effective solute concentration as the cell, so water moves in and out at equal rates and there is no net water movement.

What happens to an animal cell in an isotonic solution?

It generally stays its normal size because water enters and leaves at equal rates.

What is a hypotonic solution relative to a cell?

A solution with a lower solute concentration than the cell; water moves into the cell overall.

What happens to an animal cell in a hypotonic solution?

Water enters, causing the cell to swell and possibly burst (lyse), because an animal cell has no rigid cell wall.

What happens to a plant cell in a hypotonic solution?

Water enters and the cell becomes turgid (firm). The cell wall resists expansion and helps prevent bursting.

What is a hypertonic solution relative to a cell?

A solution with a higher solute concentration than the cell; water moves out of the cell overall.

What happens to an animal cell in a hypertonic solution?

Water leaves, so the cell shrivels or crenates.

What happens to a plant cell in a hypertonic solution?

Water leaves and the cell membrane/cytoplasm pulls away from the cell wall; this is called plasmolysis, and the cell becomes flaccid or plasmolyzed.

Why is a plant cell less likely than an animal cell to burst in a hypotonic solution?

Its rigid cell wall resists expansion as water enters, building turgor pressure instead of allowing the cell to burst easily.

How do you predict water movement from a tonicity diagram?

Compare solute concentrations: water moves overall toward the side with the higher solute concentration (the lower free-water concentration).

A cell is placed in a solution with less solute than the cytoplasm. What is the solution, and which way does water move?

The solution is hypotonic; water moves into the cell overall.

A cell is placed in a solution with more solute than the cytoplasm. What is the solution, and which way does water move?

The solution is hypertonic; water moves out of the cell overall.

A cell is placed in a solution with the same effective solute concentration as the cytoplasm. What is the solution, and what happens to water?

The solution is isotonic; water moves both ways at equal rates, with no net movement.

Why does water move in both directions in an isotonic solution?

Water molecules move randomly across the membrane in both directions, but equal rates mean there is no net movement.

What does ‘net movement’ mean?

The overall movement after accounting for movement in both directions.

What is the relationship between solute concentration and free-water concentration?

They are inversely related in these solutions: more solute generally means less free water, and less solute means more free water.

How does osmosis help explain plant-cell turgor?

Water enters a plant cell in a hypotonic environment, filling the cell and pressing the membrane against the cell wall; this pressure helps keep the plant firm.

What are stomata?

Small openings in a plant’s surface, usually on leaves, that allow gas exchange and water vapor to move in and out.

What are guard cells?

Specialized plant cells around each stoma that change shape to open or close the opening.

How does osmosis help guard cells open and close stomata?

Water entering guard cells makes them more turgid and usually opens the stoma; water leaving makes them less turgid and the stoma closes.

Why are stomata important?

They allow carbon dioxide to enter for photosynthesis and allow oxygen and water vapor to exit; opening them also causes water loss.

What is active transport’s usual direction relative to a concentration gradient?

Against the gradient, from lower concentration to higher concentration.

Why does active transport require energy?

Moving a substance against its natural concentration gradient does not happen spontaneously as net movement, so the cell must supply energy.

What is ATP?

A molecule that stores and transfers usable energy for cellular processes.

What is a protein pump?

A membrane transport protein that uses energy to move substances across the membrane, often against their concentration gradients.

How can you recognize active transport in a diagram?

Look for a transport protein or pump using ATP/energy, often moving a substance from low concentration to high concentration.

How can you recognize passive transport in a diagram?

Look for movement down a concentration gradient without ATP; it may occur directly through the bilayer or through a channel/carrier protein.

What is the sodium-potassium pump?

An active-transport membrane protein that uses ATP to move sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell.

How many sodium ions does the sodium-potassium pump move per cycle, and in which direction?

It moves 3 Na⁺ ions out of the cell per cycle.

How many potassium ions does the sodium-potassium pump move per cycle, and in which direction?

It moves 2 K⁺ ions into the cell per cycle.

Does the sodium-potassium pump move ions with or against their concentration gradients?

It moves both sodium and potassium against their concentration gradients, using energy from ATP.

What energy source powers the sodium-potassium pump?

ATP; ATP transfers a phosphate group to the pump, causing a shape change that drives transport.

What happens first in the sodium-potassium pump cycle?

Three Na⁺ ions from the cytoplasm bind to the pump.

What happens when ATP phosphorylates the sodium-potassium pump?

A phosphate group is added to the pump, causing it to change shape and release the three Na⁺ ions outside the cell.

What happens after sodium is released in the sodium-potassium pump cycle?

Two K⁺ ions from outside the cell bind to the pump.

How does the sodium-potassium pump return to its original shape?

The phosphate group is released, causing the pump to return to its original shape and release the two K⁺ ions inside the cell.

Why is the sodium-potassium pump important to nerve cells?

It maintains sodium and potassium concentration gradients that help establish the membrane potential and allow nerve impulses to occur.

What does the sodium-potassium pump contribute to across the membrane?

It helps maintain ion concentration gradients and contributes to the electrical difference across the membrane (membrane potential).

What is bulk transport?

Movement of large materials or many substances into or out of a cell using membrane-bound vesicles; it requires energy.

Why can’t large particles simply diffuse through the phospholipid bilayer?

They are too large to pass directly through the membrane, so cells can package them in vesicles for bulk transport.

What is endocytosis?

Bulk transport that brings materials into a cell when the membrane folds around them and pinches off to form a vesicle.

What is exocytosis?

Bulk transport that releases materials from a cell when a vesicle fuses with the plasma membrane and empties its contents outside.

How are endocytosis and exocytosis different?

Endocytosis brings materials into the cell in vesicles; exocytosis moves materials out when vesicles fuse with the membrane.

What are the three types of endocytosis listed in the slides?

Phagocytosis, pinocytosis, and receptor-mediated endocytosis.

What is phagocytosis?

A type of endocytosis in which a cell engulfs large solid particles, such as food particles or bacteria.

What is pinocytosis?

A type of endocytosis in which a cell takes in extracellular fluid and dissolved substances in small vesicles; it is often described as ‘cell drinking.’

What is receptor-mediated endocytosis?

A selective type of endocytosis in which specific molecules bind to membrane receptors, triggering the cell to bring them inside in a vesicle.

Which type of endocytosis must you know in detail from these slides?

Phagocytosis; the slide specifically emphasizes knowing phagocytosis and its example.

What is a pseudopodium (plural: pseudopodia) in phagocytosis?

A temporary extension of the cell membrane and cytoplasm that surrounds and engulfs a particle.

What happens first during phagocytosis?

Pseudopodia extend around the pathogen or particle.

What happens after pseudopodia surround a pathogen?

The pathogen is engulfed and enclosed in a membrane-bound vesicle called a food vacuole or phagocytic vesicle.

What is a food vacuole/phagocytic vesicle?

A membrane-bound compartment containing material that has been engulfed by the cell during phagocytosis.