Brownian Motion

Diffusion is the movement of particles from high concentration to low concentration in a substance. This process is essential for life on Earth, allowing for the movement of molecular compounds into and out of the cell. All matter in the universe is in motion, because all molecules are vibrating. This constant vibration is known as Brownian motion, which can be seen as random zig-zag motion in particles. 

Simple Diffusion

Diffusion is one of several transport phenomena that occur in nature. A distinguishing feature of diffusion is that it results in mixing. Diffusion is the movement of a substance from a region of high concentration to a region of low concentration. Diffusion flux is proportional to the negative gradient of concentrations. In other words, particles move from higher concentration to regions of lower concentration. 

The Cellular Membrane

The 'cell membrane' (also known as the plasma membrane or cytoplasmic membrane) is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the phospholipid bilayer with embedded proteins. 

The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs a number of transport mechanisms that involve biological membranes:

Types of cellular transport

Passive osmosis and diffusion

Some substances (small molecules, ions) such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.

Transmembrane protein channels and transporters

Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules diffuse passively through protein channels in facilitated diffusion or are pumped across the membrane by transmembrane transporters. Protein channel proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance.

Endocytosis

Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles ("cell eating" or phagocytosis), small molecules and ions ("cell drinking" or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.Receptor-mediated endocytosis is a process by which cells internalize molecules (endocytosis) by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized. Coat proteins of the vesicle signals proteins of specific organelles in the cell, which allow the direct transmission of specific internal molecules be delivered directly to the organelles that require them. 

Exocytosis

Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.

Active Transport

Active transport is the movement of molecules across a cell membrane in the direction against their concentration gradient, going from a low concentration to a high concentration. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy. 

Why are cells so small?

Cells are so small that you need a microscope to examine them. Why? To answer this question we have to understand that, in order to survive, cells must constantly interact with their surrounding environment. Gases and food molecules dissolved in water must be absorbed and waste products must be eliminated. For most cells, this passage of all materials in and out of the cell must occur through the plasma membrane.Each internal region of the cell has to be served by part of the cell surface. As a cell grows bigger, its internal volume enlarges and the cell membrane expands. Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases.Finally, at some point, there is just enough surface available to service all the interior; if it is to survive, the cell must stop growing. The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function. That is why cells are so small.

Lab: Osmosis across a semi-permeable membrane

Osmosis is the diffusion of water from high concentration to low concentration. When you drink water, your cells have a lower concentration of water than the water in your digestive system. So water flows across the cell membrane (from high concentration to low concentration) of your cells hydrating you. Thirst is our bodies way of maintaining an osmotic balance of water. In this balance, water is entering the cell at basically the same rate as it is leaving the cell, and the cell is said to be in an isotonic state (Fig. 5b) . If you drink too much water, the concentration of water is much higher on the outside of your cells and enter into the cell causing it to stretch, and is said to be in a hypotonic state (Fig. 5c). This is rare in humans, but has occured most commonly in endurance athletes consuming more water than their body needed to maintain osmotic balance. Water can also leave the cell in greater abundances than water enters, causing it to shrink in a condition known as a hypertonic state (Fig. 5a). We see this in plants that have not received adequate watering. When this happens, water moves from high concentration on the inside of the cell to lower concentrations out of the cell. This causes the plant’s cells to shrink and the plant wilts. 

In this exercise, you will simulate the diffusion of water across a semi-permeable membrane, similar to how water diffuses across cell membranes. You will simulate the cell membrane with dialysis tubing which is a semi-permeable membrane that allows water through, but not sucrose.