Cell Membrane Structure & Cellular Transport Fluid-Mosaic Model

We know that membranes have certain properties. Central Problem #1: A living system MUST be separated from its environment if it is to maintain complex order & homeostasis. We know that membranes have certain properties. They act as a barrier between the cell and its environment, allowing a complex organized system to exist inside the cell. They permit the passage of selected substances into and out of the cell. They flex, bend and flow to allow the cell to change shape.

Barrier between the cell and its environment All cells, from all organisms, are surrounded by a CELL MEMBRANE The cell membrane is a thin layer of lipid and protein that separates the cell's content from the world around it.

The cell membrane functions like a gate, controlling what enters and leaves the cell. The cell membrane controls how easily substances enter & leave 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.

What can pass through? What can pass through the membrane is determined by: the size of the particle, whether or not it needs the help of a carrier molecule or if it requires the cell to spend energy.

Size, charge, and E

Cell membranes are made mostly of PHOSPHOLIPID MOLECULES. Lipid is a simple form of fat. Phospholipids are a kind of lipid that consists of 2 fatty acids (tails) and a phosphate group (Heads). A phospholipid molecule has a polar “head" and 2 nonpolar “tails.”

Because of its hydrophilic (water loving) nature, the head of a phospholipid will orient itself so that it is as close as possible to water molecules. The lipid tails are hydrophobic (water fearing), so the hydrophobic tails will tend to orient themselves away from water.

A variety of protein molecules are embedded in the lipid bilayer. Some proteins are attached to the surface of the cell membrane (peripheral proteins) and are located on both the internal and external surface.

Others are embedded in the lipid bilayer (integral proteins) They extend across the entire cell. Others extend only to the inside or only to the exterior surface There are many kinds of proteins in membranes; they help to move material into and out of the cell.

Some integral proteins form channels or pores through which certain substances can pass. Other proteins bind to a substance on one side of the membrane and carry it to the other side of the membrane.

A Fluid Mosaic Model:

The basic foundation... The “fluid mosaic model” is used to describe the cell membrane because the phospholipids & proteins are fluid. Because of this fluidity, the structure changes or is it is said to be “mosaic”.

Membranes are FLUID and have the consistency of vegetable oil. The lipids and proteins of the cell membrane are always in motion. Phospholipids are able to drift across the membrane, changing places with their neighbor.

Another model….

The cytoskeleton holds together the cell membrane and provides anchoring points for membrane proteins

Drawing of membrane

Cell Junctions: connect cells together structures that help cells coordinate as part of a tissue Plant cells: Plasmodesmata - channels between adjacent plant cells that form a circulatory and communication system

Formation of Cell Wall Primary cell wall: Forms 1st. Made of cellulose, stretchy so cell can grow. Secondary cell wall: Rigid, made of lignin, forms once cell is full grown. Pectin is a sticky substance that holds neighboring cell walls together. (Pectin is used to make jelly!)

How things get in & out of the cell……. Cellular Transport How things get in & out of the cell…….

((((No Energy Required))))) PASSIVE TRANSPORT DIFFUSION OSMOSIS FACILITATED DIFFUSION ((((No Energy Required)))))

Passive Transport Does not require the cell to use energy Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. This difference in the concentration of molecules across a space is called a concentration gradient. Diffusion is driven by the kinetic energy (E of motion) the molecules possess. Diffusion occurs when molecules move randomly away from each other in a liquid or gas. The rate of diffusion depends on the temperature, size, and the type of molecules that are diffusing. Molecules diffuse faster at higher temperatures than at lower temperatures.

Diffusion always occurs down a concentration gradient (high to low). When molecules are dispersed evenly, there is no longer diffusion (& not longer a concentration gradient) because the molecules are evenly distributed throughout the space they occupy.

When the concentration of the molecules of a substance is the same throughout a space, a state of equilibrium exists.

The diffusion of water across a semipermeable membrane is called OSMOSIS. Occurs down the concentration gradient so no energy is used.

Osmosis

Hypertonic Solution In a hypertonic solution, the concentration of the solute molecules outside the cell is high than the concentration of solutes inside the cell. In hypertonic solutions, WATER DIFFUSES OUT OF THE CELL until equilibrium is established. The cell shrinks.

Hypotonic Solution In a hypotonic solution, the concentration of solute molecules outside the cell is lower than the concentration of solutes inside the cell. In hypotonic solutions, WATER DIFFUSES INTO THE CELL until equilibrium is established. The cell grows larger.

Isotonic Solutions In an isotonic solution, the concentration of solutes outside and inside the cell are equal. Under these conditions, water diffuses into and out of the cell at equal rates, so there is no net movement of water.

Plants in a hypotonic solution will have water diffuse into the cell until the cell membrane pushes against the cell wall. The pressure that water molecules exert against the Cell Wall is called TURGOR PRESSURE. In a hypertonic environment, the cells shrink away from the cell wall, and turgor pressure is lost. This condition is called plasmolysis, and is the reason plants wilt.

Animal cells placed in a hypertonic environment will have water leave the cells, making them shrink and shrivel. Placed in a hypotonic environment, water diffuses into the cells, causing them to swell and eventually burst - lyse or cytoloysis.

Facilitated Diffusion Molecules move across a membrane with the help of transport proteins in the membrane. This takes place down the concentration gradient so it does not require energy.

Facilitated Diffusion Carrier proteins are embedded in the cell membrane. Carrier proteins change shape when molecules attach to them. The change in shape of the carrier protein enables the molecule to cross the membrane.

A good example of facilitated diffusion is the transport of glucose into the cell. Many cells depend on glucose for much of their energy needs.

(((((Energy is Required)))) ACTIVE TRANSPORT Active Transport Endocytosis Exocytosis (((((Energy is Required))))

Active Transport In many cases, cells must move materials up their concentrated gradient, from and area of lower concentration to an area of higher concentration. (against the concentration gradient) Unlike passive transport, active transport requires a cell to expend energy (ATP).

Carrier-mediated active transport Carrier-mediated active transport systems use energy and membrane proteins to "pump" certain substances against a concentration gradient. This causes the substance to accumulate on one side of the plasma membrane. Ex. Na+/K+ Pump

Endocytosis Pinocytosis - "Cell drinking”. Solutes or fluids outside the cell membrane can be brought into the cytoplasm. Phagocytosis - “Cell eating”. The cell engulfs a food particle or other cells

Phagocytosis The food vesicle can then fuse with a lysosome that contains digesive enzymes. White Blood Cells (WBC, phagocytes) destroy bacteria and other unwanted cells by phagocytosis

Exocytosis Process by which waste and cell products leave the cell Products made in the cell are packaged in vesicles made by the Golgi apparatus which then fuse with the cell membrane and secrete material out of the cell. Mucus and waste products are materials secreted by exocytosis.

Comparing Plant and Animal Cells