Homeostasis and Cell Transport Chapter 5 Homeostasis and Cell Transport

Section 1: Passive Transport Objectives: Explain how an equilibrium is established as a result of diffusion. Distinguish between diffusion and osmosis. Explain how substances cross the cell membrane through facilitated diffusion. Explain how ion channels assist the diffusion of ions across the cell membrane.

Diffusion Simplest type of passive transport is diffusion. Passive transport involves the movement of molecules across the cell membrane without an input of energy by the cell. Simplest type of passive transport is diffusion. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the molecules’ kinetic energy until equilibrium is reached.

Diffusion

Diffusion Diffusion is driven by the molecules’ kinetic energy. Molecules are in constant motion because they have kinetic energy.

High Concentration Passive Transport Low Concentration

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Diffusion Across Membranes Molecules can diffuse across a cell membrane by dissolving in the phospholipid bilayer or by passing through pores in the membrane. Cell membranes allow some molecules to pass through, but not others. If a molecule can pass through a membrane, it will diffuse from an area of high concentration to an area of low concentration.

Simple Diffusion Diffusion across a membrane is also called simple diffusion. The simple diffusion of a molecule across a cell membrane depends on the size and the type of the molecule.

Osmosis Is the diffusion of water across a membrane. Osmosis doesn’t require cells to expand energy. Osmosis is the passive transport 0f water.

Osmosis

Solutions Hypertonic Solution: A hypertonic solution contains a greater concentration of solutes than the solution on the other side of the membrane. Hypotonic Solution: A hypotonic solution contains a lesser concentration of solutes than the solution on the other side of the membrane.

Solutions When the concentrations of solutes outside and inside are equal, the outside solution is said to be isotonic to the cytosol. Hypo, hyper and iso- refer to the relative solute concentrations of two solutions.

Direction of Osmosis The direction of osmosis depends on the relative concentration of solutes on the two sides of the membrane. Water tends to diffuse from a hypotonic solution to a hypertonic solution.

Direction of Osmosis When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic to the cytosol, and water will diffuse out of the cell. When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic, and there will be no net movement of water.

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Hypertonic, Hypotonic, Isotonic Solutions

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How cells deal with Osmosis? To remain alive, cells must compensate for the water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments. Contractile vacuoles are organelles that regulate water levels in paramecia. They collect the excess water and then contract, pumping the water out of the cell.

How cells deal with Osmosis? Cells of multicellular organisms respond to hypotonic environment by pumping solutes out of the cytosol. This lowers the solute concentration in the cytosol brining it to the solute concentration in the environment; thus water molecules are less likely to diffuse into the cell.

How cells deal with Osmosis? Some plant cells may be surrounded by water that moves into the cell by osmosis. Cells become turgid. The cell wall is strong to resist the pressure exerted by water called turgor pressure.

How cells deal with Osmosis? In a hypertonic environment, the cell shrinks and turgor pressure is lost. This condition is called plasmolysis; it is the reason that plants wilt if they don’t receive enough water.

How cells deal with Osmosis? Some cells can’t compensate for changes in the solute concentration of their environment. Human RBCs lack contractile vacuoles, solute pumps, and cell walls. RBCs loose their normal shape when they are exposed to an environment that isn’t isotonic to their cytosol.

How cells deal with Osmosis? Hypertonic environment leads to cell shrinkage. Hypotonic environment leads to cell swelling and bursting. Cell bursting is called cytolysis.

Facilitated Diffusion Facilitated Diffusion is another type of passive transport. This process is used for molecules that can’t readily diffuse through the cell membrane: may not be soluble in the lipid bilayer Too large to pass in the pores of the membrane.

Facilitated Diffusion In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane. The carrier protein transports the molecule from an area of high concentration to an area of low concentration no energy expenditure.

Facilitated Diffusion A molecule binds to a specific carrier protein that transports it. The carrier protein then changes its shape (may shield the molecule from the hydrophobic interior of the lipid bilayer). It transports the molecule down its concentration gradient to the other side of the membrane.

Facilitated Diffusion

Diffusion Through Ion Channels Ion channels are proteins that provide small passageways across the cell membrane through which specific ions can diffuse. Ion channels transport ions such as calcium, sodium, potassium, and chloride. (These ions aren’t soluble in lipids). Each type of ion channel is specific for one type of ion.

Diffusion Through Ion Channels Some ion channels are always open. Others have gates that open or close. The gates may open or close in response to three kind of stimuli: Stretching of the cell membrane. Electric signals. Chemicals in the cytosol or external environment. These stimuli control the ability of specific ions to cross the membrane.

Ion Channels

Passive Transport Passive Diffusion Osmosis Facilitated Diffusion

Section 2: Active Transport Objectives: Distinguish between passive transport and active transport. Explain how the sodium-potassium pump operates. Compare endocytosis and exocytosis.

No energy imput required Types of Transport Passive Active No energy imput required Energy imput required

Active Transport Movement of materials from low concentration to high concentration using a protein carrier that requires energy (costs ATP)

High Concentration Active Transport Low Concentration

Active Transport Active Pumps Membrane Movements Endocytosis Exyocytosis

Cell Membrane Pumps Ion channels & carrier proteins not only assist in passive transport but also help with active transport. The carrier proteins that serve in active transport are called cell membrane pumps.

Cell Membrane Pumps Carrier proteins involved in facilitated diffusion and those involved in active transport are very similar.

Cell Membrane Pumps In both, the molecule first binds to a carrier protein, the protein changes shape, the protein then transports the molecule. However, cell membrane pumps require energy, which is supplied by ATP.

Cell Membrane Pumps Sodium-Potassium Pump The sodium-potassium pump moves three Na+ ions into the cell’s external environment for every two K+ ions it moves into the cytosol. ATP supplies the energy that drives the pump.

Sodium-Potassium Pump

Sodium-Potassium Pump The exchange of 3 sodium ions for 2 potassium ions creates an electric gradient across the cell membrane.

Sodium-Potassium Pump The outside of the membrane becomes positively charged relative to the inside which becomes negatively charged. This is important for conduction of electrical impulses.

Movement in Vesicles Endocytosis In endocytosis, cells ingest external materials by folding around them and forming a pouch. The pouch then pinches off and becomes a membrane-bound organelle called a vesicle.

Movement in Vesicles Endocytosis Endocytosis includes pinocytosis, in which the vesicle contains solutes or fluids, and phagocytosis, in which the vesicle contains large particles or cells. Cells known as phagocytes ingest bacteria and viruses by phagocytosis. The vesicles then fuse with lysosomes, where lysosomal enzymes destroy the bacteria and viruses.

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Movement in Vesicles Exocytosis In exocytosis, vesicles made by the cell fuse with the cell membrane, releasing their contents into the external environment.

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Endocytosis & Exocytosis