I. Passive Transport *The movement of materials across the cell membrane w/o using energy is called Passive Transport. A. Diffusion 1. The cytoplasm of the cell consists of many different substances dissolved in water. 2. Molecules are also in constant motion; they are always colliding and trying to spread themselves out.

3. Diffusion – the process by which particles move from an area of high concentration, to an area of low concentration. 4. Diffusion and the cell membrane – if a substance is in different concentrations on either side of the membrane, and if it can pass through the membrane, the substance will move or “diffuse” in an attempt to even itself out.

Diffusion across a membrane

5. Dynamic Equilibrium – occurs when the substance has evened out on both sides of the membrane but still continues to move, but does so in equal amounts so that there is no net change.

6. Concentration Gradient – the line that separates the concentration of a substance from high to low; if the substance is even on both sides then we say there is NO concentration gradient; the gradient is what drives diffusion.

B. Facilitated Diffusion *Molecules that generally pass through the membrane are typically small and uncharged; charged molecules (Na +, Cl - ) and large molecules cannot easily pass through the lipid bilayer. 1. facilitated diffusion – the process in which molecules that cannot directly diffuse across the membrane must pass through special protein channels instead. 2. examples – glucose, ions such as Na +, Cl -, K +, etc *particles still move from high  low; so no energy is required.

Facilitated Diffusion

C. Osmosis: An Example of Facilitated Diffusion 1. osmosis – the diffusion of water through a selectively permeable membrane. *water molecules will move from High  Low

Osmosis

2. Aquaporins – channel proteins through which water passes; because water is “polar” it cannot go through the lipid bilayer.

3. Three types of Solutions a) isotonic – when concentrations are equal on both sides of the membrane. *notice how the molecules are spread out

b) Hypertonic – the side of the membrane that contains the HIGHER amount of solute ex) sugar, salt c) Hypotonic – the side of the membrane that contains the LOWER Amount of solute ex) sugar, salt

4. Osmotic Pressure – driven by differences in solute concentration; the net movement of water in or out of a cell produces osmotic pressure a) pressure can cause cells to shrink : if an animal cell is placed in a hypertonic solution, the cell will lose water to the environment. b) pressure can cause cells to swell : if an animal cell is placed in a hypotonic solution, the cell may suck in water and expand.

Osmotic Pressure

c) Protective measures : 1) frog eggs and fish eggs lack aquaporins, therefore water enters too slowly to cause swelling. 2) bacteria and plants have tough cell walls to combat water uptake 3) paramecium have contractile vacuoles to pump out excess water that is taken in.

II. Active Transport *active transport – the movement of materials against a concentration gradient, or in a direction that materials do not want to go. *active transport requires ENERGY! A. Molecular Transport 1. Small molecules and ions are carried across membranes by transport proteins. 2. transport proteins – protein pumps that pump molecules from LOW  HIGH concentrations

Active Transport

B. Bulk Transport 1. bulk transport – the transport of LARGE molecules into or out of the cell 2. 2 types of bulk transport : a) Endocytosis – occurs when the cell takes in large molecules by folding the membrane inward and creating “pockets.” The pockets then become a vesicle containing the ingested material. 1) phagocytosis – a type of endocytosis which results in a food vacuole.

Endocytosis

2) Pinocytosis – a type of endocytosis which results in a fluid filled vacuole.

b) Exocytosis – occurs when a vacuole containing waste fuses with the plasma membrane and expels the waste.

Section 7.4 Homeostasis and Cells

*Differences arise from the ways in which cells are specialized and the ways in which cells associate with on another to form multicellular organisms. I.The Cell as an Organism A. Homeostasis – relatively constant internal and physical, and chemical conditions. “Internal Balance” 1. unicellular organisms – one cell; to maintain homeostasis unicellular organisms grow, respond to the environment, transform energy, and reproduce. a) prokaryotic unicellular organisms ex) bacteria – extremely adaptable; live everywhere

b) eukaryotic unicellular organisms algae yeast B. Multicellular Life *the cells of human beings and other multicellular organisms do not live on their own. *the cells of multicellular organisms become specialized for particular tasks and communicate w/ one another to maintain homeostasis.

1. Cell Specialization a) cells in multicellular organisms are specialized: 1) some are responsible for movement ex) muscle cells

2) Some react to environmental stimuli ex) Nerve cells 3) Some produce specific Chemicals when needed ex) pancreas

b) Specialized Animal Cells ex) cells of the upper respiratory system contain cilia to sweep out dust, dirt, and debris to keep the lungs clean

c) Specialized plant cells ex) pollen grains – specialized cells used in fertilization

2. Levels of Organization – specialized cells of multicellular organisms are organized into tissues, organs, then organ systems. a) tissue – a group of similar cells that work together to carry out a similar function. b) organ – groups of different tissues that work together to carry out a function c) organ system – a group of organs that work together for a specific function

Levels of Organization

3. Cellular Communications a) cells in large organisms communicate by way of chemical signals and junctions

b) Receptors – proteins on cells that accept chemical signals and allow for a response to occur.