Chapter 8 Cell Transport Section 1 ~ Cell Membrane Section 2 ~ Movements in and out of the cell Section 3 ~ Cell Communication

Homeostasis Homeostasis is the maintenance of stable internal conditions in a changing environment. Individual cells, as well as organisms, must maintain homeostasis in order to live. One way that a cell maintains homeostasis is by controlling the movement of substances across the cell membrane. Cells are suspended in a fluid environment. Even the cell membrane is fluid. It is made up of a “sea” of lipids in which proteins float.

By allowing some materials but not others to enter the cell, the cell membrane acts as a gatekeeper. The cell membrane also 1) provides structural support to the cytoplasm, 2) recognizes foreign material, and 3) communicates with other cells, all of which contribute to maintaining homeostasis.

Cell Membrane Surrounds cells Gives shape to animal cells Inside cell wall of plant cells

Cell Membrane Made of: 1) Lipid (wax) bilayer 2) Protein channels It is Semi-permeable

Semi-permeable: Semi= “some” substances may pass Selective on what gets in or goes out Based upon size & solubility H20 flows Salt remains

-cytosis Exocytosis- endocytosis- When substances when substances exit out of the cell enter into the cell

Cell Membrane

Membrane Proteins Various proteins can be found in the cell membrane. Some proteins face inside the cell, and some face outside. Other proteins may stretch across the lipid bilayer and face both inside and outside. Proteins are made of amino acids. Some amino acids are polar, and others are nonpolar. The attraction and repulsion of polar(partial opposite charge at opposite ends) and nonpolar parts of the protein to water help hold the protein in the membrane.

Types of Proteins Proteins in the cell membrane include: 1) cell-surface markers, 2) receptor proteins, 3) enzymes, and 4) transport proteins. Cell-surface markers act like a name tag. A unique chain of sugars acts as a marker to identify each type of cell. Receptor proteins enable a cell to sense its surroundings by binding to certain substances outside the cell. When this happens, it causes changes inside the cell.

Many specialized proteins called enzymes in the cell membrane help with important biochemical reactions inside the cell. Many substances that the cell needs cannot pass through the lipid bilayer. Transport proteins aid the movement of these substances into and out of the cell (movement into or out of the cell is known as Cellular Transport).

The movement of materials in and out of cells Cellular Transport The movement of materials in and out of cells

Solution Concentration: Like density = # molecules per volume of container Low or High Concentration * * * * * * * * * * * * * * * * * * * * High Conc. * * Low Conc.

2 types of Passive Transport 1st ~Diffusion Diffusion is the movement of molecules: from High Concentration to Low Concentration (like water flowing from high ground to low ground) Before After

Diffusion of gases……. O2 High CO2 CO2 High Conc.

2nd passive Transport ~ Osmosis Movement of water from high concentration to low concentration Across a semi-permeable membrane Special case of diffusion water only

Equilibrium H2O H2O H2O on both sides of membrane Osmosis stops “Equi” =Equal concentrations on both sides of membrane Osmosis stops Called an Isotonic Solution cell stays same size H2O H2O H2O

In Osmosis….only H2O (water) moves Therefore focus on the water concentration: 1% Salt solution Water is high concentration Salt is Low Like traveling downhill! 10% Salt Solution Water is low concentration Salt is High

Concentration Gradient * * * * * *

Passive Transport Diffusion or Osmosis Going with the gradient No ATP energy required High Conc. To Low Conc. * * * * * * * * * *

Active Transport Against the gradient Requires ATP Low to High Concentrations Salt returning to blood in kidneys Like riding a bike uphill!!! Going against the flow. * * * * * * * * * * *

Active or Passive Transport? Which requires ATP energy? Which goes with the concentration gradient? Which goes against the gradient?

Plasmolysis Cell membranes shrink or expand Dependent on salt concen-trations

Hypotonic or Hypertonic Solutions? Tap Water Hypotonic = “under” or low salt outside, relative to cell Hypertonic = “over” or high salt relative to cell Salt water inside cell

Sea water organism in fresh water………………. Tap Water Hypotonic Solution = Low salt concentration outside the cell High Water concentration outside Water moves? Inside Cell Swells and increases! Salt Water Inside Cell Low Water H20

Fresh water organism in salt water…………. Hypertonic Solution Water moves? Out of the cell Cell shrinks Salt Water High salt Low water Fresh water Inside cell= High Water H20

Plants cells bulge, not burst! Animal cells can burst. Why can they burst, but not plant cells? Or Cell Membranes Shrink Answer: Plant cells have a rigid cell wall which animal cells don’t!!!

Paramecium (1 celled organisms) have water pumps to pump out excess water! (called contractile vacuoles)

Cellular Transport What substances move ? How do they move? What is the membrane structure? What is passive versus active transport?

Sending Signals, continued Cells communicate and coordinate activity by sending chemical signals that carry information to other cells. A signaling cell produces a signal, often a molecule, that is detected by the target cell. Typically, target cells have specific proteins that recognize and respond to the signal.

Sending Signals, continued Neighboring cells can communicate through direct contact between their membranes. Nerve cells also signal information to distant locations in the body, but their signals are not widely distributed. While most signal molecules originate within the body, some signals come from outside. For example, light has a great effect on the action of hormones in plants. The length of day determines when some plants flower.

Receiving Signals A target cell is bombarded by hundreds of signals. But it recognizes and responds only to the few signals that are important for its function. This response to some signals, but not to others, is made possible by receptor proteins, such as the ones in the cell’s membrane. A receptor protein binds specific substances, such as signal molecules.

Receiving Signals, continued The outer part of the receptor protein is folded into a unique shape, called the binding site. A receptor protein binds only to signals that match the specific shape of its binding site. Only the “right” shape can fit into the receptor protein while the “wrong” shape have no effect on that particular receptor protein. A cell may also have receptor proteins that bind to molecules in its environment.

Binding Site of Receptor Proteins

Receiving Signals, continued Receptor proteins enable a cell to respond to its environment. Once it binds the signal molecule, the receptor protein changes its shape in the membrane. This change in shape relays information into the cytoplasm of the target cell.

Responding to Signals, continued Some receptor proteins are enzymes or they activate enzymes in the cell membrane. Enzymes trigger chemical reactions in the cell. Binding of a signal molecule outside the cell may cause a second messenger to form. The second messenger acts as a signal molecule within the cell and causes changes in the cytoplasm and nucleus.

Second Messengers of Receptor Proteins