Passive Membrane Transport Systems Flashcards
(14 cards)
the varied extensions.
the extensions on the surface of cells can vary depending on whether they are supported by:
Microfilaments (actin)
➜ found in structures like microvilli (used for absorption)
Microtubules
➜ found in structures like cilia and flagella (used for movement)
flaggella and cilia
Cilia
Structure: Hair-like structures supported by microtubules (in a 9+2 arrangement).
Function: They move to sweep substances across the surface of the cell.
Example: Respiratory tract cells use cilia to move mucus out of the lungs/nose.
Flagella
Structure: Long whip-like tail made of microtubules.
Function: Moves the entire cell by propelling it forward.
Example: Sperm cells use flagella to swim.
Centriole Multiplication
When a cell is preparing to form cilia, the centrioles (which are part of the cell’s microtubule-organizing center) duplicate.
Positioning Beneath the Plasma Membrane
These new centrioles move to just under the plasma membrane, on the side of the cell that faces an open space—called the free (apical) surface.
Formation of Basal Bodies
Each centriole becomes a basal body, which acts like a foundation for building a cilium. Think of it as an anchor point.
Sprouting Microtubules
Microtubules begin to grow out from each basal body.
As they elongate, they push the plasma membrane outward, forming the cilia.
the membrane grows with the centriols to make cillia
this is for cillia and flaggella
Mivrovilli
What are Microvilli?
Tiny finger-like projections sticking out from the surface of a cell.
The name means “little shaggy hairs”, but they don’t move like cilia.
Found on cells that absorb a lot, like:
Intestinal cells (absorbing nutrients)
Kidney tubule cells (absorbing water and ions)
✅ What do they do?
They increase the surface area of the cell, which allows it to absorb more efficiently.
Imagine turning a flat sponge into one covered in tiny fingers — way more surface area!
✅ What are they made of?
Inside each microvillus is a core of actin filaments (a type of microfilament, not microtubules).
These filaments help stiffen the microvilli and anchor them to the cell’s internal structure.
diffrent shapes and lengths of cells.
So far in this chapter, we have focused on a generalized human cell. However, the trillions of cells in the human body include over 200 different cell types that vary greatly in size, shape, and function. They include sphere-shaped fat cells, disc-shaped red blood cells, branching nerve cells, and cube-shaped cells of the kidney tubules.
Depending on type, cells also vary greatly in length—ranging from 1/12,000 of an inch in the smallest cells to over a yard in the nerve cells that cause you to wiggle your toes. A cell’s shape reflects its function. For example, the flat, tilelike epithelial cells that line the inside of your cheek fit closely together, forming a living barrier that protects underlying tissues from bacterial invasion.
The shapes of cells and the relative numbers of the various organelles they contain relate to specialized cell functions (Figure 3.8). Let’s take a look at some examples of specialized cells.
Fibroblast: This elongated cell secretes fibers and has abundant rough ER and a large Golgi apparatus to produce the protein building blocks of these fibers.
Erythrocyte (red blood cell): This biconcave disc-shaped cell carries oxygen and is streamlined for easy flow through the bloodstream, with all other organelles removed to maximize space for oxygen-carrying pigment.
Epithelial cell: With a hexagonal shape, this cell packs together in sheets, using intermediate filaments and desmosomes to resist tearing.
Skeletal, cardiac, and smooth muscle cells: These elongated cells are filled with contractile microfilaments, allowing them to move bones, pump blood, or alter organ sizes.
Fat cell: This spherical cell is large due to a lipid droplet that stores nutrients.
Macrophage (white blood cell): This phagocytic cell uses pseudopods to move to infection sites and digest microorganisms with its lysosomes.
Nerve cell (neuron): This cell has long extensions for receiving and transmitting messages, with abundant rough ER to synthesize neurotransmitters.
Oocyte (female egg): The largest cell in the body, this egg stores organelles for distribution during embryo development.
Sperm (male): This streamlined cell uses a flagellum to swim to the egg for fertilization.
diffrent cells function
Each of the cell’s internal parts is designed to perform a specific function for the cell. As mentioned earlier, most cells have the ability to metabolize (use nutrients to build new cell material, break down substances, and make ATP), digest foods, dispose of wastes, reproduce, grow, move, and respond to a stimulus (irritability)
a solution
The fluid environment on both sides of the plasma membrane is an example of a solution. It is important that you really understand solutions before we dive into an explanation of membrane transport. In the most basic sense, a solution is a homogeneous mixture of two or more components. Examples include the air we breathe (a mixture of gases), seawater (a mixture of water and salts), and rubbing alcohol (a mixture of water and alcohol). The substance present in the largest amount in a solution is called the solvent (or dissolving medium). Water is the body’s chief solvent. Components or substances present in smaller amounts are called solutes. The solutes in a solution are so tiny that the molecules cannot be seen with the naked eye and do not settle out.
Intracellular fluid (collectively, the nucleoplasm and the cytosol) is a solution containing small amounts of gases (oxygen and carbon dioxide), nutrients, and salts, dissolved in water. So too is extracellular fluid, or interstitial fluid, the fluid that continuously bathes the exterior of our cells. You can think of interstitial fluid as a rich, nutritious, and rather unusual “soup.” It contains thousands of ingredients, including nutrients (amino acids, sugars, fatty acids, vitamins), regulatory substances such as hormones and neurotransmitters, salts, and waste products. To remain healthy, each cell must extract from this soup the exact amounts of the substances it needs at specific times and reject the rest.
The plasma membrane is a selectively permeable barrier. Selective permeability means that a barrier allows some substances to pass through it while excluding others. Thus, it allows nutrients to enter the cell but keeps many undesirable or unnecessary substances out. At the same time, valuable cell proteins and other substances are kept within the cell, and wastes are allowed to pass out of it.
passive and active
Passive Transport
No energy (ATP) required.
Substances move down their concentration gradient (from high to low concentration).
The cell doesn’t have to “work” to make this happen.
Examples:
Diffusion (like oxygen or carbon dioxide moving across the membrane)
Facilitated diffusion (using a protein channel, like glucose entering a cell)
Osmosis (the diffusion of water)
- Active Transport
Requires energy (ATP).
Substances move against their concentration gradient (from low to high concentration).
The cell uses energy to “push” molecules where they normally wouldn’t go on their own.
Examples:
Protein pumps (like the sodium-potassium pump)
Endocytosis (cell takes in large materials)
Exocytosis (cell releases materials)
diffusions
Diffusion is the process by which molecules (and ions) move away from areas where they are more concentrated (more numerous) to areas where they are less concentrated (with fewer of them). All molecules possess kinetic energy, or energy of motion (as described in Chapter 2), and as the molecules move about randomly at high speeds, they collide and change direction with each collision. The overall effect of this erratic movement is that molecules move down their concentration gradient (spread out). The greater the difference in concentration between the two areas, the faster diffusion occurs. Because the driving force (source of energy) is the kinetic energy of the molecules themselves, the speed of diffusion is affected by the size of the molecules (the smaller the faster) and temperature (the warmer the faster).
If one room is jam-packed (high concentration) and the other is empty (low concentration), people rush out fast — lots of movement!
If both rooms are almost equally full, there’s less reason to move — diffusion slows down.
n example should help you understand diffusion. Picture yourself dropping a tea bag into a cup of boiling water, but not stirring the cup. The bag itself represents the semipermeable cell membrane, which lets only some molecules leave the tea bag. As the tea molecules dissolve in the hot water and collide repeatedly, they begin to “spread out” from the tea bag even though it was never stirred. Eventually, as a result of their activity, the entire cup will have the same concentration of tea molecules and will appear uniform in color. (
diffusion through plasma membrane
The hydrophobic core of the plasma membrane is a physical barrier to diffusion. However, molecules will diffuse through the plasma membrane if any of the following are true:
The molecules are small enough to pass through the membrane’s pores (channels formed by membrane proteins).
The molecules are lipid-soluble.
The molecules are assisted by a membrane carrier.
The unassisted diffusion of solutes through the plasma membrane (or any selectively permeable membrane) is called simple diffusion (Figure 3.10a). Solutes transported this way are lipid-soluble (such as fats, fat-soluble vitamins, oxygen, carbon dioxide).
osmosis
Diffusion of water through a selectively permeable membrane such as the plasma membrane is specifically called osmosis (oz-mo′sis). Because water is highly polar, it is repelled by the (nonpolar) lipid core of the plasma membrane, but it can and does pass easily through special pores called aquaporins (“water pores”) created by proteins in the membrane (Figure 3.10b). Osmosis into and out of cells is occurring all the time as water moves down its concentration gradient. The movement of water across the membrane occurs quickly. Anyone administering an IV (intravenous, into the vein) solution must use the correct solution to protect the patient’s cells from life-threatening dehydration or rupture (see “A Closer Look”).
Facilitated Diffusion
It’s a type of passive transport — no ATP (energy) is needed.
It helps move substances that:
Are too big (like glucose)
Are not lipid-soluble
Or are charged (like ions)
These substances still move from high to low concentration (down their gradient), but they need help getting across the membrane.
Help comes from membrane proteins, which act as:
Channels (like open tunnels for ions)
Or carriers (like a gate that opens and changes shape to let glucose through)
why passive transport is important
Cells constantly use up oxygen and glucose for energy (like during cellular respiration).
Because these substances are quickly used, their concentration inside the cell stays low.
That low concentration naturally pulls in more oxygen and glucose from the outside, without needing energy — thanks to passive transport.
If the cell had to actively pull in those molecules all the time using ATP, it would be wasting energy just to get energy, which isn’t efficient.
filltration
Filtration is a passive process (no ATP required).
Instead of using a concentration gradient like diffusion, filtration uses a pressure gradient.
Hydrostatic pressure (fluid pressure — usually from blood) pushes water and small solutes through a membrane.
It moves from an area of higher pressure to lower pressure.
🧠 Real Example: Kidneys
Blood pressure forces water and small molecules out of capillaries and into kidney tubules.
Larger things like blood cells and proteins are too big to pass through the filter and are left behind.
The filtered fluid (filtrate) can eventually become urine.
💡 Simple way to think of it:
Filtration is like pushing water through a coffee filter — pressure forces the liquid and small stuff through, but larger bits stay behind.
So yes, filtration is passive, uses pressure, and helps the body filter fluids — especially in organs like the kidneys.
difference between a pressure gradient and a concentration gradient:
- Pressure Gradient
A pressure gradient occurs when there is a difference in pressure between two areas.
The high-pressure area pushes substances toward the low-pressure area.
Example:
In filtration (like in the kidneys), blood pressure forces water and small molecules from the high-pressure blood vessels (capillaries) into the low-pressure kidney tubules.
Key Point: Movement happens because of pressure (like water flowing from a high-pressure region to a low-pressure region).
- Concentration Gradient
A concentration gradient occurs when there is a difference in the concentration of a substance (molecules or ions) between two areas.
Molecules naturally move from the high concentration area to the low concentration area to balance out (spread out evenly).
Example:
In diffusion, oxygen molecules move from an area where oxygen is high (like in the lungs) to an area where oxygen is low (like in the blood cells).
Key Point: Movement happens because of the concentration difference — substances move from areas of high concentration to areas of low concentration.
