Chapter 5: Transport across the plasma membrane

Question 1

Selective permeability

Answer 1

The permeability of the plasma membrane to different substances varies, so some substances pass through the plasma membrane more readily than others

Question 2

Permeable

Answer 2

substances that can pass through the membrane
* Due to SMALL, NON polar, HYDROPHOBIC interior of the lipid bilayer
* More hydrophobic or lipid soluble a substance is the greater membrane permeability
* Steroids

Question 3

Impermeable

Answer 3

substances that can not pass through the membrane
* Ions and LARGE, uncharged POLAR molecules such as glucose

Question 4

Concentration gradient

Answer 4

is a difference in the concentration of a chemical from one place to another, such as from the inside to the outside of the plasma membrane

• Can create a distribution of positively and negatively charged ions between the two sides of the plasma membrane
• Many cases, substance moves across a plasma membrane down the gradient

Question 5

Electrical gradient

Answer 5

difference in electrical charges between two regions – because it occurs across the plasma membrane this charge difference is termed membrane potential

Question 6

Electrochemical gradient:

Answer 6

combined influence of the concentration gradient and the
electrical gradient on the movement of a particular ion
* Net driving force that acts on an ion

Question 7

Passive processes

Answer 7

substance moves across the plasma membrane without any energy input from the cell
* NO ENERGY
* DOWN GRADIENT
* Example: substance moves “downhill” along its concentration or electrochemical gradient to cross the membrane using only its own kinetic energy

Question 8

Active processes

Answer 8

cellular energy is used to move a substance across the plasma membrane. Energy is usually used in the form of ATP
* Example: cellular energy is used to drive a substance “uphill” against its concentration or electrochemical gradient

Question 9

Vesicular transport

Answer 9

tiny vesicles (membranous sacs) are used to move substances across the plasma membrane

Question 10

Endocytosis

Answer 10

vesicles detach from plasma membrane while bringing materials in the cell

Question 11

Exocytosis

Answer 11

merging of vesicles with the plasma membrane to release materials from the cell

Question 12

Passive transport

Answer 12

refers to the movement of substances across the plasma membrane down their concentration or electrochemical gradients
* Does not require energy
* Occurs by diffusion

Question 13

Diffusion

Answer 13

random mixing of particles from one location to another because of the particles kinetic energy (energy of motion)

• In a solution both the solutes (the dissolved substance) and the solvent (the liquid that does the dissolving) undergo diffusion

Question 14

solute vs solvent

Answer 14

solute: the dissolved substance
solvent: liquid that does the dissolving

example: salt in water
water = solvent
salt = solute

Question 15

Factors that influence the diffusion rate of substances across plasma membrane:

Answer 15

1. Steepness of the concentration gradient
   - the greater the difference in concentration between the two sides of the membrane, the higher the rate of diffusion
2. Temperature
   - higher the temperature, the faster the rate of diffusion
3. Mass of the diffusing substance
   - larger the mass of the diffusing particle, the slower its diffusion rate
   - smaller molecules diffuse more rapidly
4. Surface area
   - larger the membrane surface area available for diffusion, the faster the diffusion rate
5. Diffusion distance
   - greater the distance over which diffusion must occur, the longer it takes

Question 16

Frick’s Law of Diffusion

Answer 16

mathematically expresses the various factors that determine how quickly a particle diffuses across a membrane from one region to another

Question 17

What is simple diffusion?

Answer 17

A passive process where solutes move freely through the lipid bilayer of a cell membrane without the help of transport proteins.

Question 18

What types of molecules can move via simple diffusion?

Answer 18

Small, non-polar molecules such as oxygen (O₂), carbon dioxide (CO₂), and lipid-soluble substances like steroid hormones

Question 19

What drives simple diffusion?

Answer 19

The concentration gradient—molecules move from areas of high concentration to low concentration until equilibrium is reached

Question 20

Why is simple diffusion considered the simplest form of transport?

Answer 20

Because it does not require energy (ATP) or special transport proteins—only a concentration gradient

Question 21

What is facilitated diffusion, and what types of molecules use it?

Answer 21

A passive transport process where polar molecules, large molecules, and ions (e.g., glucose, Na⁺, K⁺) move across the plasma membrane with the help of transport proteins.

Question 22

What are the two types of transport proteins in facilitated diffusion?

Answer 22

Channel-mediated: Uses ion channels that open/close for ions (e.g., Na⁺, K⁺).
Carrier-mediated: Uses carrier proteins that bind, change shape, and transport molecules.

Question 23

Does facilitated diffusion require ATP? What drives it?

Answer 23

No ATP required. Movement occurs down the concentration gradient (high → low concentration).

Question 24

4 characteristics of ligand protein binding: specificity

Answer 24

• Each carrier protein transports only one solute or group of structurally related solutes
• Binding site has a specific 3d shape

Question 25

4 characteristics of ligand protein binding: Affinity

Answer 25

- Solute binds to the binding site of the carrier protein which a strength, or affinity - Affinity of the binding site for the solute is the same regardless of which side of the membrane it is facing

Question 26

4 characteristics of ligand protein binding: saturation

Answer 26

- Limited number of carrier proteins available in the plasma membrane of a cell - This places an upper limit called the transport maximum

Question 27

4 characteristics of ligand protein binding: competition

Answer 27

- If a carrier protein can transport a group of solutes that are similar in structure, the solutes may compete with one another for the same binding site - Rate of transportation = lower than if only one of the solutes were present

Question 28

How does glucose enter body cells, and does it require ATP?

Answer 28

Glucose enters cells via carrier-mediated facilitated diffusion using a Glucose Transporter (GLUT). This is a passive process that does not require ATP

Question 29

What are the steps in glucose transport via GLUT?

Answer 29

1. Glucose binds to the GLUT carrier protein (binding site faces outside). 2. The transporter changes shape, shifting the binding site inside the cell. 3. Glucose dissociates from the transporter and enters the cytosol.

Question 30

Osmosis

Answer 30

type of diffusion in which there is a net movement of a solvent through a selectively permeable membrane – PASSIVE PROCESS - in living systems, the solvent is water, which moves by osmosis across plasma membranes from an area of higher water concentration to an area of lower water concentration

Question 31

aquaporins

Answer 31

integral membrane proteins that function as water channels

Question 32

hydrostatics pressure:

Answer 32

forces water molecules to move back into the left arm, equilibrium is reached when just as many water molecules move from right to left due to this pressure

Question 33

Osmotic pressure

Answer 33

force needed to stop the movement of water due to osmosis * Higher the solute concentration, greater the osmotic pressure

Question 34

Osmolarity

Answer 34

measures the total solute concentration per liter of solution (NaCl dissociates into 2 particles, increasing osmolarity) – used more common

Question 35

isosmotic

Answer 35

If two solutions have the same osmolarity (same number of solute particles per unit volume) (iso = same)

Question 36

Molarity

Answer 36

measures the concentration of molecules without considering dissociation - Number of moles of solute per liter of solution

Question 37

Hyperosmotic

Answer 37

A solution with a higher osmolarity (more solute particles per unit volume) than another solution (hyper = greater than) solution higher = water concentration in such a solution is lower

Question 38

Hypoosmotic

Answer 38

solution with a lower osmolarity (fewer solute particles per unit volume) than another solution) Hypo = less than Solution lower = water concentration is higher

Question 39

Penetrating solutes

Answer 39

are able to pass through a membrane * Example: urea – which can pass through the plasma membrane of most cells

Question 40

Nonpenetrating solutes

Answer 40

unable to pass through a membrane * Example: Na+ ions – need to diffuse through channels * Ex. Proteins CANT

Question 41

Tonicity

Answer 41

how a solution affects the water movement in and out of the cell e(tonic = tension) depending on concentration of nonpenetrating solutes

Question 42

Hypotonic solution

Answer 42

: Lower solute concentration outside the cell → Water moves into the cell → The cell swells and may burst (lysis)

Question 43

Hypertonic solution:

Answer 43

Higher solute concentration outside the cell → Water moves out of the cell → The cell shrinks (crenation)

Question 44

Isotonic solution

Answer 44

The same solute concentration inside and outside the cell → No net water movement → The cell stays the same size

Question 45

Active Transport

Answer 45

cellular process that move molecules or ions across the plasma membrane against their concentration gradient, from areas of lower concentration to higher * REQUIRES ATP

Question 46

Primary Active transport

Answer 46

process that moves molecules or ions across a membrane AGAINST concentration gradient (from low to high) * Uses energy directly from ATP * Moves uphill

Question 47

Sodium potassium pump or Na+/K+ ATPase

Answer 47

sodium ions (Na+) from cells and brings potassium ions (K+) into cell

Question 48

What is the sodium-potassium pump, and what type of transport does it use?

Answer 48

The Na⁺/K⁺ pump is a primary active transport mechanism that moves 3 Na⁺ out and 2 K⁺ in using ATP

Question 49

What are the steps in the sodium-potassium pump cycle

Answer 49

1. Na⁺ Binding: 3 Na⁺ bind to the pump inside the cell. 2. Phosphorylation & Shape Change: ATP hydrolysis phosphorylates the pump, changing its shape. 3. Na⁺ Release & K⁺ Binding: 3 Na⁺ exit, and 2 K⁺ bind from outside. 4. Dephosphorylation & Shape Reset: K⁺ binding releases the phosphate, restoring the original shape. 5. Cycle Repeats: 2 K⁺ enter the cell, and the pump is ready for the next cycle.

Question 50

Why does the Na⁺/K⁺ pump require ATP, and how does it aid secondary active transpor

Answer 50

The pump uses ATP to maintain Na⁺ and K⁺ gradients, which drive secondary active transport by providing energy for the movement of other molecules (e.g., glucose via symport).

Question 51

Secondary active transport:

Answer 51

uses energy stored in the electrochemical gradient rather than ATP directly - Relies on pre established gradient (created by primary active transport) to move another molecule against its concentration gradient

Question 52

Symporters

Answer 52

if transporters move two solutes in the same direction (sym = same)

Question 53

Antiporters

Answer 53

move two solutes in opposite directions across the membrane (anti=against)

Question 54

Vesicular transport:

Answer 54

vesicles are used to move substances across the plasma membrane (endocytosis and exocytosis)

Question 55

Endocytosis

Answer 55

materials move into a cell in a vesicle formed by the plasma membrane (endo=within)

Question 56

Exocytosis

Answer 56

materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell *BOTH REQUIRE ATP

Question 57

Receptor mediated endocytosis:

Answer 57

A specific form of endocytosis where cells use receptor proteins to engulf targeted molecules (ligands) into vesicles

Question 58

What are examples of receptor-mediated endocytosis?

Answer 58

- LDL (low-density lipoprotein) uptake for cholesterol transport. - Iron transport via transferrin receptors. - Hormone & nutrient absorption in cells.

Question 59

phagocytosis

Answer 59

form of endocytosis in which the cell engulfs large solid particles such as worn out cells, whole bacteria or viruses by forming a vesicle around them (common in immune cells) - VITAL TO HELP PROTECT THE BODY FROM DISEASE

Question 60

pseudopods

Answer 60

projections of its plasma membrane and cytoplasm – surround the particle outside the cell

Question 61

Bulk Phase Endocytosis / Pinocytosis

Answer 61

form of endocytosis in which tiny droplets of extracellular fluid are taken up - Allows for bulk intake of fluid - Occurs especially in absorptive cells in the intestines and kidneys

Question 62

Exocytosis

Answer 62

releases materials from a cell All cells carry out exocytosis, but it is especially important in two types of cells: 1. Secretory cells 2. Neurons that release substances called neurotransmitters

Question 63

Transepithelial transport:

Answer 63

movement of solutes across epithelial cells - Solute movement is accomplished through the use of membrane transport mechanisms, such as passive or active transport - epithelial cells of the intestine and kidneys are connected by tight junctions, cell junctions that retard the passage of substances between cells and prevent the contents of these organs from leaking into the blood or surrounding tissues

Question 64

apical membrane

Answer 64

is the surface of the epithelial cell that faces the lumen (interior space) of the organ

Question 65

basolateral membrane

Answer 65

which is in contact with interstitial fluid and nearby blood vessels in underlying connective tissue

Question 66

Absorption

Answer 66

solute moves from the lumen of an organ into the bloodstream 1) Entry: A substance from the lumen enters the cell through the side facing the lumen (apical side) 2) Passage: The substance moves through the inside of the cell. 3) Exit: The substance exits the cell through the side facing the body's interior (basolateral side) and enters the bloodstream.

Question 67

Secretion

Answer 67

To remove waste products or regulate certain substances by moving them from the bloodstream into the lumen of an organ Process: 1. Entry: A substance from the blood enters the cell through the basolateral side. 2. Passage: The substance moves through the inside of the cell. 3. Exit: The substance exits the cell through the apical side into the lumen

Question 68

Transcytosis

Answer 68

movement of a substance through a cell as a result of endocytosis on one side and exocytosis on the opposite side * Substances such as antibodies * Common room to pass between blood plasma and interstitial fluid