Membranes and Transport All cells have a plasma membrane. This outermost, double-layered membrane separates a cell’s interior from its surroundings. The plasma membrane is selectively permeable. Plant, fungal, eubacterial, algal and archaebacterial cells also have a cell wall, which helps to give the cell its structure, and protect it from osmotic pressure. Animal cells, and most protists don’t have a cell wall.

The Movement of Particles There are many ways in which a material can pass through the cell membrane. One such method is simple diffusion. Diffusion is simply the movement of particles (molecules) from a region of higher concentration to a region of lower concentration. Thus, particles travel from compartment A to B, given the concentration gradient.

and allowing water out, when it doesn’t Another way that molecules can move through a membrane is through osmosis. Osmosis is simply the diffusion of water molecules across a selectively permeable membrane The plasma membrane is responsible for maintaining homeostasis within the cell by allowing water in when the cell needs water and allowing water out, when it doesn’t The movement of water is influenced by the presence or absence of dissolved solute in solution, so again, it’s about concentration gradient.

What types of cells have cell walls? What types of cells don’t typically have cell walls? Movement of particles from a region of high concentration towards low concentration is referred to as??? What is another name given to the diffusion of water? What causes water molecules to diffuse in a given direction?

Osmosis and the Concentration Gradient solute SPM

Tonicity Hypertonicity Hypotonicity Isotonicity Tonicity is a measure of the osmotic pressure gradient of two solutions separated by a semipermeable membrane. In other words…tonicity is about perspective. There are three different classifications of tonicity. Hypertonicity Hypotonicity Isotonicity So it could be referring to a cell in a solution…or a solution surrounding a cell.

Isotonic Solutions Isotonic solutions occur when the concentration of dissolved substances in the solution outside the cell is the same as the concentration of dissolved substances inside the cell. Osmosis still occurs, but it flows into and out of the cell at the same rate.

Hypotonic Solutions In a hypotonic solution, the concentration of dissolved substances is lower in the solution outside the cell than the concentration inside the cell. Osmosis will occur, and water will flow into the cell, sometimes until the cell bursts. What type of pressure is increasing in this plant cell? Turgor Pressure

Hypertonic Solutions When cells are placed in a hypertonic solution, one which contains more dissolved particles than are within the cell, water will flow out of the cell. Due to osmosis, cells will shrivel up and shrink as they lose water. Is the turgor pressure low or high in this plant cell? LOW

A B In the image to the right, in which direction will the water flow? Why? A B What will happen to these red blood cells, when they are placed in a hypotonic solution? What do I mean when I say “tonicity” is all about relativity? When cells are placed in isotonic solutions, in which direction will the water move?

Factors Affecting Diffusion Rates Steepness of concentration gradient Molecules will appear to move faster out of a region of higher concentration than a region of lower concentration. Molecular size Smaller molecules move faster than larger molecules Temperature Heat energy causes molecules to move faster than in colder adjoining regions Electric or pressure gradients present Dissolved ions can create a potential difference where positive ions are attracted to negative areas, and pressure can also cause a gradient “pushing” molecules from one region to another When the net movement of molecules remains nearly uniform in two adjoining regions, it is known as dynamic equilibrium.

Passive Transport Substances can diffuse passively across a membrane by two processes: simple diffusion through the phospholipid bilayer and facilitated diffusion through either channel proteins or by means of a carrier protein. Because no energy is required by the cell to accomplish this movement, it is referred to as passive transport. All it needs to move, is a concentration gradient. Substances such as water, oxygen, and carbon dioxide, can cross the cell membrane without any input of energy by the cell.

Explain how concentration gradient can affect the diffusion rates of molecules. What other factors may influence diffusion rates? What is dynamic equilibrium? Name three molecules that can move through the plasma membrane via passive transport. How is simple diffusion different from facilitated diffusion, if they’re both passive mechanisms of cellular transport?

Active Transport A cell can also move particles from a region of lower concentration to a region of higher concentration if it needs to, but it must extend energy to do so. Movement of materials through a membrane against a concentration gradient is called active transport and requires energy output from the cell. The energy required for this active transport comes from the ATP molecule, dropping off an Pi , and transforming into an ADP.

Passive Transporters: movement requiring no energy output by the cell. Examples: Vacuoles in plant and animal cells; nerve and muscle cells with sodium, calcium, potassium, and chloride ion channels. Active Transporters: movement requiring energy output by the cell. Examples: Movement of particles against the concentration gradient, like the calcium “pump” which helps keep the concentration of calcium inside a cell at least a thousand times lower than it is outside a cell.

Cellular Membranes Phosphate Lipid All cellular membranes are made primarily of phospholipids, organized as a double layer. This lipid bilayer gives cells protection against the haphazard movement of water, and water-soluble substances. The structure of the plasma membrane is sometimes referred to as the fluid mosaic model. This “fluidity is achieved with the help of the cholesterol molecules within. Phosphate heads are hydrophilic Phosphate Lipid Lipid tails are hydrophobic

Embedded Proteins in the Plasma Membrane Receptor proteins within the plasma membrane receive chemical signals that trigger changes in cell activities. The endocrine system is linked to the plasma membrane through the work of hormones. Transport proteins are also within the plasma membrane and form channels through which water soluble substances can cross. Recognition proteins help identify a cell as being of a certain type.

Adhesion proteins help cells of the same type locate, stick together, and remain in the proper tissues In some instances (cardiac tissue), this channel forms a gap-junction between cells so that electrical signals can flow quickly and all cells can contract together as a functional unit. By what name did the surface features of plant cells that require these communication proteins go by? Communication proteins form channels that match up across the plasma membranes of two cells, and allow signals and substances to flow rapidly between their cytoplasm.

Why do cells need to accomplish active transport? How is fluidity of the plasma membrane accomplished? Describe why some cells would need to form channels linking one cell to another? Give examples of where communication proteins forming gap-junctions and plasmodesmata form.