Biology: 4.1 Cells and Their Environment Passive Transport in Cells

Today’s Objectives: Key ideas All organisms and their cells respond to external conditions to maintain a constant internal condition (homeostasis) One way cells maintain homeostasis is by controlling the movement of substances across their cell membrane. Cells must use energy to transport some substances across the cell membrane. (active transport) Other substances move across the cell membrane without any use of energy by the cell. (passive transport)

Passive Transport Movement across the cell membrane that does not require energy from the cell is called passive transport 1: Randomly moving particles are on one side of a barrier. 2: When openings occur, the particles begin to gradually enter the adjacent space, to diffuse to the other space.

Passive Transport Movement across the cell that does not require energy from the cell is called passive transport 3: When the point of equilibrium is reached, the diffusion of particles is equal in both spaces. Diffusion continues until equilibrium is reached. 4: A difference in the concentration of a substance across the space is called a concentration gradient.

Movement of Substances The movement of a substance from an area of high concentration to an area of lower concentration is called diffusion. Dyes diffusing into water

Movement of Substances Many substances, such as molecules and ions dissolved in the cytoplasm and in the fluid outside cells, enter or leave cells by diffusion across cell membranes. The cell membrane is selectively permeable to certain substances Selectively permeable means it allows certain substances to pass and holds other substances back.

Osmosis Water molecules can diffuse through channels in the cell membrane. The diffusion of water through a selectively permeable membrane is called osmosis. Like other forms of diffusion, osmosis involves the movement of a substance without energy. Osmosis is a passive transport

Osmosis The direction of water movement across the cell membrane depends on the concentrations of free water molecules in the cytoplasm and in the fluid outside the cell. There are three possibilities for the direction of this water movement . . . .

Osmosis 1: Water moves out: cell shrinks When water diffuses out of the cell, the cell grows smaller. A solution that causes a cell to grow smaller because of osmosis is called a hypertonic solution. This occurs because the fluid outside the cell has less free water molecules than the fluid inside the cell.

Osmosis 2: Water moves in: Cell grows When water diffuses into the cell, the cell grows larger. A solution that causes a cell to grow larger because of osmosis is called a hypotonic solution. This occurs because the fluid outside the cell has more free water molecules than the fluid inside the cell.

Osmosis 3: No net water movement: No change If the cytoplasm and the fluid outside the cell have the same concentration of free water molecules, water diffuses in and out of the cell at equal rates. This results in no change and the cell stays the same size. A solution that produces no change in cell size because of osmosis is called isotonic solution.

Osmosis Summary: 3 types of Osmosis Water moves out through membrane: cell grows smaller: hypertonic solution Water moves into cell: cell grows: hypotonic solution Water moves in/out equal amounts: no change in size: isotonic solution

Transport Proteins Recall that most ions and polar molecules cannot pass across the cell membrane because they cannot pass through the nonpolar interior of the lipid bilayer. However, such substances can cross the cell membrane when they are aided by transport proteins. Transport proteins called channels provide polar passageways through which ions and polar molecules can move across the cell membrane. Lipid bilayer with channel

Transport Proteins Transport proteins called channels provide passageways through which ions and polar molecules (+ or – charge) can move across the cell membrane. Each channel allows only a specific substance to pass through the cell membrane. This selectivity is one of the most important properties of the cell membrane because it enables a cell to control what enters and leaves.

Diffusion Through Ion Channels Ions such as sodium, potassium, calcium, and chloride are involved in many important cell functions. Although ions cannot diffuse through the nonpolar interior of the lipid bilayer, they can cross the cell membrane by diffusing through ion channels. An ion channel is a transport protein with a polar pore through which ions pass.

Electrical Charge and Ion Transport The movement of a charged particle, such as an ion, across a cell membrane is also influenced by the particle’s positive or negative electrical charge. The inside of a typical cell is negatively charged with respect to the outside of the cell (which is positively charged). Opposite charges attract and like charges repel.

Electrical Charge and Ion Transport Thus a more positively charged ions(+) located outside the cell are more likely to diffuse into the cell, where the charge is negative. Conversely, negatively charged ions (-) located inside the cell are more likely to escape the cell and diffuse outward where the charge is positive. Thus, an ion’s electrical charge often affects the movement of the ion across the cell membrane.

Facilitated Diffusion carrier proteins are a different kind of transport protein that can bind to a specific substance on one side of a cell membrane, carry the substance across the cell membrane, and release it on the other side. They carry the substance across the cell’s membrane through these channels.

Facilitated Diffusion When transport proteins are used to move specific substances across the membrane (down their concentration gradient) the transport is called facilitated diffusion. This transport occurs without using the cell’s energy so it is a passive transport.