Movement Through the Cell Membrane Notes

Introduction & Background: The Size of a Cell: Cells are so small that we must use a microscope to view them. Cells must be small so that the organelles inside are close to the exterior of the cell and therefore have quick access to the outside environment.

Surface Area to Volume Ratio: Surface area to Volume ratio limits the size of a cell. Surface area= L x W x # of surfaces V= L x W x H 1 cm cube: 6 cm 2 = 1cm 3 = 6:1 2 cm cube: 24 cm 2 = 8cm 3 = 3:1 3 cm cube: 54 cm 2 = 27cm 3 = 2:1

Surface Area to Volume Ratio:

As the size of the cell increases, the surface area to volume ratio decreases and therefore the efficiency of the cell decreases. (fill in arrows on your chart!) SizeSurface areaVolumeSurface Area to volume ratio Efficiency

Cells and Water Cells contain water and are surrounded by water. Water is a polar molecule; in other words water has a partial positive and a partial negative charge. Water molecules cluster together because they are polar. (opposites attract!) The clustering of water helps keep the shape of the cell membrane.

Structure & Function of Cell Membrane Structure: Lipid Bilayer: is both tough and flexible. The cell or plasma membrane is made up of phospholipids and different types of proteins that move laterally. Cell membranes are selectively permeable which means the membrane allows some substances to pass through, but not others.

Structure & Function of Cell Membrane A double layered sheet of molecules that comprises the most basic unit of the membrane. It is composed of polar heads and lipid tails. (draw diagram in the box on your paper)

Structure & Function of Cell Membrane

Function: Forms the boundary or barrier between the interior of the cell and its surroundings. Its most important function is to regulate what enters and leaves the cell. Takes in food and water and eliminates waste It also provides protection and support

Types of cell membranes: A membrane is said to be permeable if a substance can diffuse (pass) across it. A membrane is said to be impermeable if a substance cannot pass across it. A membrane is said to be semi-permeable if some substances can pass across them and others cannot (most biological membranes are this type).

Molecules Small molecules that are usually nonpolar, such as oxygen and carbon dioxide, easily move through the plasma membrane (a lipid bilayer) in a process called diffusion. Larger molecules (macromolecules), such as glucose, the food for all living things, must seek aid from the carrier proteins in processes called facilitated diffusion and active transport.

Most cell membranes also contain proteins that run through the lipid bilayer. Channel Proteins function to “pump” some molecules in and out of the cell. (draw in box on your paper –Leave room for 2 more!)

Receptor Proteins transfer information by binding to a specific receptor like a lock and key causing a change inside the cell to occur. (Draw on your paper)

Marker Proteins are proteins that have long carbohydrates attached. They function as “nametags” for the cells. (draw on your paper)

Types of Movement across the membrane Molecules contain energy and in result they are in continuous random movement through the cytoplasm which is a mixture or solution of many different substances in water.

Types of Movement across the membrane The substances dissolved in liquids are called solutes. The water in a cell is considered the solvent. The concentration of a solution is the mass of the solute in a given volume of solution (mass/volume).

Diffusion Diffusion is the process where molecules move from an area of higher concentration to an area of lower concentration until equal concentrations are reached. The difference in the concentration of molecules across a space is called a concentration gradient.

Diffusion Diffusion is a type of passive transport, meaning it does not require energy input by the cell. An example of diffusion is when you put one drop of dye into a glass of water and the glass of water will eventually turn completely red.

Facilitated Diffusion Facilitated Diffusion is the process where molecules, such as glucose, pass through protein channels. (These protein channels facilitate or help the diffusion of glucose across the membrane) During facilitated diffusion molecules move from a high concentration to a low concentration and therefore no energy is required and it is a passive process.

Osmosis Osmosis is the diffusion of water molecules through a selectively permeable membrane. (This occurs when other solutes are unable to move across the membrane, therefore the water will move in order to equalize the concentrations)

Osmosis Osmosis does not require energy and therefore is a passive process. Water will move across the membrane until equilibrium is reached. This is when the concentration of water and the solute are equal on both sides of the membrane. The solution is then considered to be isotonic.

Isotonic Solutions When the concentration of the solute is equal outside of the cell, it is known as an isotonic solution.

Hypertonic Solutions When the concentration of the solute is higher outside of the cell, it is known as a hypertonic solution.

Hypotonic Solutions When the concentration of the solute is lower outside of the cell, it is known as a hypotonic solution.

Tonic Solutions Review

During osmosis water will always travel from the hypotonic solution through the membrane to the hypertonic solution until equal concentrations exist on both sides of the membrane.

Because a cell is filled with salts, sugars, proteins, and other molecules, it (the cell) will almost always be hypertonic (above strength) to fresh water. Osmotic pressure will produce net movement of water into the cell towards the high concentration of molecules.

Active Transport Active Transport is the process when materials (solutes) move from an area of low concentration to an area of high concentration. Because molecules are moving AGAINST the concentration gradient the cell requires the input of energy. Therefore this is an active process. Active transport is often compared to a pump because it “pumps” molecules against the concentration gradient usually through a protein. (They are pumped against the direction they actually want to go in).

Types of Active Transport Protein Pumps Endocytosis: the process of taking material into the cell by enfolding around the material to form a pocket.  Phagocytosis: the process of taking in large solid materials.  Pinocytosis: the process of taking in liquid materials.

Exocytosis Exocytosis: the process of taking material out of the cell when a vacuole fuses with the cell membrane. Its contents are expelled outside of the cell

Review! ProcessEnergy Required Concentration Gradient Molecules Transported DiffusionNone (passive)High to LowSmall polar molecules Small hydrophobics Gasses OsmosisNone (passive)High to LowWATER ONLY Facilitated Diffusion None (passive)High to LowSmall Macromolecules Some Ions Active transportEnergy Required (acitve) Low to highLarge Macromolecules Large hydrophobics Some Ions