How do substances get into and out of a cell?

How does the cell control the movement of molecules into and out of the cell?

Substances move into and out of cells by several methods. One method of movement is diffusion.

Diffusion of drop of dye in pure water.

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Diffusion is the net movement of a substance (liquid or gas) from an area of high concentration of the substance to one of lower concentration of that substance.

Imagine that you are in a very small room. An beautiful young lady walks in wearing very strong perfume. At first you just get a whiff as she passes near to you, but the fragrance gets stronger and stronger as the minutes go by. After a couple of minutes you start sneezing uncontrollably because, as luck would have it, you are allergic to that perfume. This is an example of diffusion in action.

The concentration of perfume on the lady is very high. But since the molecules of all substances, solid, liquid or gas, are always moving rapidly, some of the perfume molecules bounce into each other and then out into the surrounding air.

You have probably experienced this in class when someone arrives freshly doused in perfume or cologne. The molecules of every substance (solid, liquid, or gas) are in motion, so energy is available to move molecules, just as in the case of the water discussed above.

The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall (or net) movement is thus from high to low concentration. Eventually, if no energy is put into the system, the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.

The Cell Membrane

A cell cannot survive if it is totally isolated from its environment. The cell membrane is a complex barrier that separates the cell from its external environment, but allows some materials to pass through. This 'selectively permeable' membrane regulates what enters and leaves the cell.

  • All cells, in all types of organisms, are surrounded by a cell membrane
  • The cell membrane is a thin layer of lipid and protein that separates the cell's contents from the world around it.
  • The cell membrane functions like a gate, controlling what enters and leaves the cell.
  • The cell membrane controls the ease with which substances pass into and out of the cell-some substances easily cross the membrane, while others cannot cross at all. For this reason, the cell membrane is said to be selectively permeable.

Model of a cell membrane

Cells and Diffusion

Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion. Water moves through membranes by a type of diffusion known as osmosis. Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane.

  • Gas exchange in gills and lungs operates by this process. Carbon dioxide is produced by all cells as a result of cellular metabolic processes. Since the source is inside the cell, the concentration gradient is constantly being replenished/re-elevated, thus the net flow of CO2 is out of the cell.
  • Metabolic processes in animals and plants usually require oxygen, which is in greater concentration inside the cell, thus the net flow of oxygen is into the cell.

Osmosis is the diffusion of water across a semi-permeable (or differentially permeable or selectively permeable) membrane. The plasma membrane surrounding cells is such a membrane. (Other examples are dialysis tubing and cellulose acetate sausage casing.)

  • The presence of a solute decreases the water potential of a substance. Thus there is more water per unit of volume in a glass of fresh-water than there is in an equivalent volume of sea-water. In a cell that is submerged in fresh water, the water flow is generally into the cell since the cytoplasm contains dissolved substances as well as many organelles.

Hypertonic solutions are those in which more solute (and hence lower water potential) is present.

Hypotonic solutions are those with less solute (again read as higher water potential).

Isotonic solutions have equal (iso-) concentrations of substances. Water potentials are thus equal; although there will still be equal amounts of water movement in and out of the cell, the net flow is zero.

Cells placed in distilled water (hypotonic solution) take on water, swell and burst.

In a salt solution (hypertonic solution), cells lose water and shrink.

Active and Passive Transport

Two additional methods by which substances may move through cell membranes include:

Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion.

Active transport requires the cell to spend energy, usually in the form of ATP. Examples include transport of large molecules (non-lipid soluble) and the sodium-potassium pump.

Vesicle-mediated transport

Vesicles and vacuoles that fuse with the cell membrane may be utilized to release or transport chemicals out of the cell or to allow them to enter a cell.

Exocytosis is the term applied when transport is out of the cell.

Note the vesicle on the left, and how it fuses with the cell membrane on the right, expelling the vesicle's contents to the outside of the cell.

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Endocytosis is the case when a molecule causes the cell membrane to bulge inward, forming a vesicle.

Phagocytosis is the type of endocytosis where an entire cell is engulfed.

Pinocytosis is when the external fluid is engulfed.

Receptor-mediated endocytosis occurs when the material to be transported binds to certain specific molecules in the membrane. Examples include the transport of insulin and cholesterol into animal cells.

Endocytosis and exocytosis


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The basic unit of structure and function in the human body is the cell 3. Describe each of these processes: phagocytosis - consuming large food particles, pinocytosis - consuming large liquid particles, exocytosis - pushing waste or vesicles out of the cell4. What is the difference between active transport and passive transport? Givea specific example of each type.active transport - requires energy ATP; sodium-potassium pump, endocytosis, exocytosispassive transport - does not require energy; diffusion and osmosis5. Describe the process of making and exporting a protein from a cell.proteins are made by the ribosomes and then transported through the endoplasmic reticulum where they are packaged into vesicles by the golgi apparatus. Vesicles are exported out of the cell (exocytosis)6.

Describe the cell membrane and its properties. What is its function?the cell membrane is selectively permeable, it consists of phospholipids and proteins arranged in a bilayer, it regulates what comes into and out of the cell7. What is diffusion and facilitated diffusion? What is osmosis?diffusion is the movement of molecules from areas of high concentration to low, molecules tend to spread outfaciliatated diffusion uses proteins in the membrane to help move molecules acrossosmosis is the diffusion of water8.

List and describe the stages in the life cycle of a cell.interphase - resting phase, cell makes a copy of DNAprophase -chromatin condenses in chromosomes, spindle formsmetaphase - chromosomes line up along the equatoranaphase - chromatids separatetelophase - cell begins to pinch inward, nuclear membrane reforms, spindle disappears; cytokinesis begins. What is the centriole and the spindle and what is their role in cellularreproduction? Structures that move chromosomes so that each new daughter cell gets the correct number10. What is the difference between chromosomes, chromatin, and chromatids?chromosomes look like X's and appear during prophase, chromatin is DNA, a chromatid is a single copy (half of the X) that is visible during prophase and eventually separates during anaphase11. What is DNA and what do the letters stand for? Deoxyribonucleic acid12. Explain the process of cellular respiration and why it is important for the cell.

Cellular respiration uses oxygen and glucose to create ATP, ATP is necessary for many of the cell function, like the active transport. Respiration occurs in the mitochondria.13. What is the difference between hypertonic, hypotonic, & isotonic?

Whatwill happen to cells placed in each type of solution?isotonic - solution has an equal concentration as the cell, no net movementhypertonic - solution has a greater number of solutes, this causes water to move out of the cellhypotonic - solution has fewer solutes, this causes water to move into the cellRemember the rule: SALT SUCKS.