Cell Membrane

Cell Membrane (cont’d) The conditions within a cell must remain relatively constant at all times The process of maintaining this constant internal environment is called homeostasis The role of the cell membrane is to control the movement of materials in and out of the cell

Cell Membrane (cont’d) The cell membrane controls the movement of materials in and out through a number of different processes The cell membrane is selectively permeable – that is it allows certain items into the cell and certain items out of the cell Water is the solvent on both sides of the membrane that provides a medium for the movement of particles across the cell membrane

Many single celled organisms exist only in water, while cell of multi-cellular organisms are bathed in a thin layer of extra cellular fluid – mainly water Cell Membrane (cont’d)

Cell Membrane (cont’d)

Molecule Transport Two main categories of molecule transport exist in cells, active transport and passive transport

Diffusion Is a method of passive transport where small molecules pass through the cell membrane. It can be defined as the movement molecules from a region of high concentration to a region of low concentration

Diffusion (cont’d) Passive transport refers to the movement of material across the cell membrane without the cell expending (using) any of its energy Small molecules such as carbon dioxide and oxygen pass easily in this manner This works because molecules are in constant motion and this random motion causes molecules to collide with one another

Diffusion (cont’d) Molecules of a substance will bounce around when added to water, this is due to the water molecules bouncing off the molecules This motion will move the molecules round the water until the molecules of the substance are evenly distributed The particles of the substance (molecules) are said to follow a concentration gradient – that is they move from a region of high concentration to a region of low concentration

Diffusion (cont’d) Diffusion works well where the distances are short and the molecules are small The cell is always using oxygen so oxygen moves into the cell and the cell is always producing carbon dioxide so it is always moving out of the cell Both carbon dioxide and oxygen are following the concentration gradient

Osmosis Osmosis is the diffusion of water across a cell membrane It is important that the water concentration be kept the same on both sides of the cell membrane If too much water enters or leaves the cell then problems can arise Just like other substances, water moves with the concentration gradient

Osmosis (cont’d) Is a form of passive transport because the cell does not use any of its energy. The cell membrane can’t stop the process because it is permeable to water. There are three conditions that can exist for osmosis –Isotonic condition –Hypotonic condition –Hypertonic condition –Osmosis VideoOsmosis Video

Isotonic Condition This occurs when the concentration on water inside the cell is equal to the concentration of water outside the cell. There is an equal amount of water entering and leaving the cell.

Hypotonic Condition When the concentration of water outside the cell is greater than inside the cell. More water will move into the cell causing it to swell. Animal cells may burst and plant cells will become very rigid

Hypertonic Condition When the concentration of water is greater inside the cell than outside. More water will migrate (move) out of the cell than enter it. This will cause the cytoplasm of both plant and animal cell to shrink.

Facilitated Diffusion Not all molecules that enter the cell are small enough to pass easily through the cell membrane These molecules are facilitated or helped” across the membrane Specialized and highly specific transport proteins help these molecules enter the cell This process is still an example of passive transport because it is the concentration gradient that causes the molecules to move. This is how glucose enters the cell.

Active Transport In many instances the internal environment of a cell must be vastly different from its external environment. The cell must: –Concentrate nutrients for maintenance and growth –Carry out any specialized functions that a cell might have –Many toxic waste products have to be completely removed

Active Transport (cont’d) Passive transport will not be able to handle all of these processes. In some instances the cell must expend or use some of its own energy to cause material to move in the direction required. Active transport is required to move materials AGAINST the concentration gradient.

Active Transport (cont’d) Examples of specialized cells that must use active transport are: –Kidney cells pump glucose and amino acids out of urine and back into the blood –Intestinal cells pump in nutrients from the gut –Root tip cells pump in nutrients from the soil –Gill cells in fish pump out sodium

Two basic types of active transport –Exocytosis –Endocytosis Exocytosis involves sending large molecules (macromolecules) out of the cell Endocytosis involves bringing large molecules (macromolecules) into the cell

Exocytosis Macromolecules don’t simple drift towards the cell’s membrane and squeeze their way out They are packaged in a vesicle that separates them from the rest of the cell The vesicle fuses with the membrane releasing the cell contents to the outside The vesicle becomes incorporated into the membrane

Endocytosis Two types –Pinocytosis (cell drinking) –Phagocytosis (cell eating) Pinocytosis – involves ingesting small molecules or fluids surrounding the cell Phagocytosis – involves ingesting large molecules (ie. Microroganisms or cell debris)

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Surface area to volume 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 shrinks. At some point, there is just enough surface available to service all the interior; if it is to survive, the cell must stop growing. 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.