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what is homeostasis?
- the maintenance of stable internal conditions in a changing environment .
- one way that a cell maintains homeostasis is by controlling the movement of substances across the cell membrane.
- the cell membrane as the gate keeper it allows some materials but not others to enter into the cell.
- the cell membrane provides structural support to the cytoplasm, recognizes foreign material, & communication w/ other cells. ~ contributions to homeostasis.
what is the cell membrane made of?
sea of phospholipids. a phospholipid is a specialized lipid made of a phosphate "head" & two fatty acid "tails" the head is polar & is attracted to water the tail is opposite.
what is the lipid bilayer?
- formed by a double layer of phospholipids. the nonpolar the tails make up the interior & the polar head faces the surface.
- one layer of the polar heads faces the cytoplasm while the other layer is in contact w/ the cell's immediate surroundings.
- the phospholipids form a barrier through which only small nonpolar substances can pass. ions & most polar molecules can't get through the interior layer.
where can various proteins be found?
in the cell membrane. the attractions and repulsion of the polar & nonpolar amino acids inside the proteins to the water on either side of the membrane hold the proteins in the cell membrane.
proteins in the cell membrane
cell-surface markers, receptor proteins, enzymes, & transport proteins.
- cell-surface markers- chain of sugars( carbs) acts as a marker to identify each type of cell. they attach to cells by the proteins glycoproteins. these help the cells work 2gether.
- 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.
- enzymes- help with important biochemical reactions in the cell.
- Transport Proteins- aids the movements of the substances that cannot make it pass the lipid bilayer.
What is active and passive transport?
- active transport- the cell is required to use energy to move a substance.
- passive transport- the cell does not use energy
what is equilibrium?
when the space is filled evenly.
what is the concentration of the substance?
- the amount of a particular substance in a given volume.
- when one area has a higher concentration then another area does a concentration gradient exists.
- diffusion- substances move from an area of higher concentration to an area of lower concentration. the movement down the concentration gradient.
In passive transport substances cross the cell membrane down their concentration gradient. Some substances diffuse through the lipid bilayer. Others diffuse through transport proteins.
- simple diffusion- small, nonpolar molecules can pass directly through the lipid bilayer.
- oxygen-moves down its concentration gradient into the cell.
- carbon dioxide- diffuses out of the cell
- natural steroid hormones- can also diffuse across the bilayer
- many ions and polar molecules that are important for cell function that do not diffuse easily through the nonpolar lipid layer.
- during -facilitated diffusion- transport proteins help these substances diffuse through the cell membrane.
what are two types of transport proteins?
- channel proteins & carrier proteins
- Channel Proteins- ions, sugars, and amino acids can diffuse through the cell membrane through channel proteins. these proteins sometimes called pores, serve as tunnels through the lipid bilayer. Each channel allows the diffusion of specific substances that have the right size & charge.
- Carrier Proteins- transport substances that fit within their binding site. THis protein binds to a specific substance on one side of the cell membrane. This binding causes the protein to change shape. As the protein's shape changes the substance is moved across the membrane & is released on the other side.
what is osmosis?
- when water can diffuse across a selectively permeable membrane.
- osmosis- a type of passive transport that is very important to keeping cells functional. It allows cells to maintain water balances as their environment changes.
- osmosis occurs as free water molecules move down their concentration gradient into the solution that has the lower concentration of free water molecules.
polar water molecules do not diffuse directly through the bilayer. But the cell membrane contains channel proteins that only water molecules can pass through. Thus, osmosis in cells is a form of facilitated diffusion. In humans water channels help in the regulation of body tempt., in digestion, in reproduction, & in water conservation in the kidneys.
predicting water movement
- the direction of water movement in a cell depends on the concentration of the cell's environment.
- 1. water moves out. - if the solution is hypertonic, or has a higher solute concentration than the cytoplasm does, water moves out of the cell. The cell loses water & shrinks.
- 2. water moves in.- if the solution is hypotonic or has a higher solute concentration than the cytoplasm does, water moves into the cell. The cell loses water & shrinks.
- 3. no net change in water movement occurs, or equilibrium is reached.- if the solution is isotonic or has the same solute concentration that the cytoplasm does, water diffuses into & out of the cell at equal rates. The cell stays the same size.
what does the removal of dissolved solutes from a cell do?
increases the concentration of free water molecules inside the cell.
- sometimes cells must transport substances against their concentration gradient. This requires cell to use energy.
- Active transport requires energy to move substances against their concentration gradients. most often the energy needed for active transport is supplied directly or indirectly by ATP
- many active transport processes use carrier proteins to move substances . In facilitated diffusion the carrier proteins do not require energy. In active transport the carrier proteins do require energy to "pump" substances against their concentration gradient.
- sodium-potassium pump. - one of the most important carrier proteins in animal cells. This pump prevents sodium ions from building up in the cell. Sodium ions inside the cell bind to the carrier protein. The protein changes shape & releases the sodium ions outside the cell membrane. Outside the cell potassium ions bind to the pump. The pump returns to its original shape & releases the potassium ions inside the cell membrane. For 3 sodium ions taken out 2 potassium ions are brought inside.
- many substances such as proteins and polysaccharides are too large to be transported by carrier proteins. Instead, they cross the cell membrane in vesicles. Vesicles can bud off from the membrane fuse with it or fuse with other vesicles.
- the movement of a large substance into a cell by means of a vesicle is called endocytosis. during this process the cell membrane forms a pouch around the substance . The pouch then closes up & pinches off from the membrane to form a vesicle inside the cell. Vesicles that form by endocytosis may fuse with lysosomes or other organelles.
- the movement of material out of a cell by means of a vesicle is called exocytosis- vesicles inside the cell fuse w/ the cell membrane. then the contents of the vesicles are released to the outside of the cell. Cells use this to export proteins modified by the Golgi apparatus.
- cells communicate & 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 & respond to the signal.
- cells also use various methods of communication which vary depending on if the target is specific or general & how close or far away a signal is.
- Neighboring cells can communicate through direct contact between their membranes.
- Short-distance signals may act locally, a few away from the originating cell.
- Long-distance signals are carried by hormones & nerve cells. hormones are signal molecules that are made in one part of the body. Hormones are distributed widely in the bloodstream throughout the body but they affect only specific cells.
- a target cell is bombarded by hundreds of signals but it recognizes & responds only to the few signals that are important for its function.
- a receptor protein binds specific substances such as signal molecules. The outer part of the 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.
- signal molecules w/ the "wrong" shape have no effect on that particular receptor protein.
- once it binds the signal molecule the receptor protein changes its shape in the membrane. This change in shape relays info onto the cytoplasm of the target cell.
Responding to signals
- when a signal molecule binds to a receptor protein, the protein changes shape which triggers changes in the cell membrane.
- the cell may respond to a signal by changing its membrane permeability, by activating enzymes or by forming a second messenger.
- permeability change transport proteins may open or close in response to a signal.
- enzyme activation some receptor proteins activate enzymes in the cell membrane. Some receptor are enzymes themselves & are activated by the binding of a signal molecule. Enzymes trigger chemical reactions in the cell.
- second messenger binding of a signal molecule outside the cell may cause a 2nd messenger to form. The second messenger acts as a signal molecule within the cell & causes changes in the cytoplasm & nucleus.