Homeostasis & Cell Transport

In order to maintain life, organisms must regulate the balance of materials inside

and outside of the cell. This is accomplished by the transport of materials through the

plasma membrane.

I. Cell Structure A. Cell Membrane

  1.  The cell membrane surrounds the cytoplasm of the

cell. It controls what enters and exits the cell.

  2.  The plasma membrane is semipermeable,

meaning some substances cross the membrane more easily than others.

I. Cell Structure 3.  The plasma membrane is made of two phospholipid

layers arranged in sheets called a bilayer.

a. Each phospholipid has:

i.  1 polar head regain made of a phosphate group.

polar = hydrophilic = water loving

ii.  2 nonpolar tail regions made of fatty acids.

nonpolar = hydrophobic = water fearing

I. Cell Structure B. Membrane Proteins

  1. Aid in stabilizing and shaping the cell membrane.

  2. Proteins on the outer surface allow the cell to be

recognized by other cells - such as the immune system.

I. Cell Structure 3. Outer surface proteins act as receptors. Once a

molecule attaches, the receptor responds by causing changes in the cell.

a. Cells in the body have different combinations of proteins depending upon their function.

I. Cell Structure 4. Proteins embedded in the cell membrane allow

polar molecules to move in and out of the cell.

a. Example: A membrane channel called aquaporin is hydrophilic which allows water molecules to pass easily through the cell membrane.

STOP & THINK! ¡ Without the presence of membrane proteins, what wouldn’t be

able to travel across the cell membrane?

¡ Water, large macromolecules, and charged ions.

¡  How would this affect the activity of the cell?

¡ The cell would NOT be able to function.

STOP & THINK! ¡  Proteins are made up of different amino acids. The R group of

an amino acid gives it specific properties, making it small or large, polar or nonpolar. Where are the nonpolar amino acids in an aquaporin protein most likely to be found?

a. Along the inner tube of the channel.

b. In a band around the outside of the channel.

c. On the end of the protein that faces the cytoplasm.

d. On the end of the protein that faces the extra cellular fluid.

STOP & THINK! ¡  The tongue is lined with taste buds, which detect the

components of food and relay this information to the brain. What type of membrane protein do taste bud cells rely on? ¡  It would rely on membrane bound proteins that have receptors for

different food types.

¡ And how do they help these cells carry out their function? ¡  The proteins receptors that fit onto their specific food would relay

that information to the brain.

STOP & THINK! ¡ Which is not a component of the plasma

membrane?

a. fatty acids

b. amino acids

c. phosphates

d. nucleic acids

II. Passive Transport A. General Information

1.  The plasma membrane is an area of constant movement as molecules are shuttled back and forth. Without the movement, the cell would not being able to maintain homeostasis.

a.  Example: Waste would build up inside the cell and water or other needed materials wouldn’t be able to come into the cell.

2.  The movement of substances could be driven by a difference in concentrations in and outside of the cell.

II. Passive Transport B. Concentration Gradient

1. Concentration gradients form when a dissolved substance is more concentrated in one area.

II. Passive Transport 2. Molecules and ions are always moving. Due to

this, they naturally move from a high to a low concentration. This process is called diffusion and requires no energy.

a. Diffusion requires a concentration gradient to

occur.

II. Passive Transport C. Passive Transportation

  1. Passive transport: type of cellular transportation

that does not require energy.

  2.  Substances that cannot pass through the cell

membrane on their own, such as large molecules, charged ions, or polar molecules, require transport proteins.

II. Passive Transport a. Facilitated Diffusion: are protein transporters,

which move molecules across the cell membrane without the use of energy.

II. Passive Transport D. Osmosis

  1. Osmosis: movement of water from high water

concentration to low concentration.

a. Hint: the concentration of water is high wherever the concentration of dissolved substances is low. (The direction of osmosis is usually the opposite direction of diffusion).

II. Passive Transport 2.  Osmosis requires no energy.

3.  Osmosis can change the volume inside the cell.

STOP & THINK ¡ What is one way that facilitated diffusion differs from simple

diffusion?

a. Facilitated diffusion requires energy input.

b. Facilitated diffusion requires membrane proteins.

c. Facilitated diffusion requires a concentration gradient.

d. Facilitated diffusion requires small, nonpolar molecules.

III. Active Transport A.   Active Transport

  1.  Active Transport: type of cellular transportation that

requires energy and creates a concentration gradient.

  a. Example: Ion pumps and molecular pumps are

mechanisms of active transport. They use ATP to move materials against the concentration gradient.

STOP & THINK ¡  Cells of the small intestine must absorb glucose from digested food. Two types of

membrane proteins work together to help glucose cross into these cells. Glucose-sodium co-transport proteins facilitate the absorption of glucose. They allow sodium ions to enter the cell. Each pair of sodium ions that enter take a glucose molecule along with them. Sodium-potassium pumps, which are active transporters, move sodium ions from inside the cell, into the small intestine.

¡  Which statement compares the sodium-potassium pump and the glucose-sodium co-transporter?

a. Both proteins require ATP to transport material.

b. Both proteins require a concentration gradient to transport material.

c. The glucose-sodium co-transporter relies on a concentration gradient, while the sodium-glucose pump requires ATP.

d. The glucose-sodium co-transporter produces a concentration gradient, while the sodium-potassium pump creates equilibrium.

III. Active Transport B. Exocytosis

  1. Exocytosis: releases substances from the cell.

  2. A vesicle moves towards the cell membrane and

fuses with it. The phospholipid bilayer of the vesicle joins the cell membrane, and the material is released.

  a. Remember: The Golgi apparatus and

endoplasmic reticulum produce vesicles.

III. Active Transport C. Endocytosis

1. Endocytosis: takes substances into the cell.

  2.  The cell membrane pinches inward around the

material located outside of the cell. It swallows the material, forming a vesicle around it, which separates from the cell membrane and travels into the cytoplasm.

Cytoplasm

Fluid outside of cell

IV. Homeostasis A. Living organisms work to maintain consistent

internal states because changes in temperature or pH can affect their ability to function.

  B. Homeostasis: constant internal state (one of

the eight characteristics of life).

IV. Homeostasis C.  Organisms maintain homeostasis through:

  1.  Thermoregulation – regulation of body

temperature through shivering, perspiration, and the dilation or construction of blood vessels.

  2. Osmoregulation – regulation of water in the body

IV. Homeostasis 3. Gas Exchange – regulation of O2 intake and

removal of CO2

  4. Regulation of Blood Glucose- regulation of sugar

in the body’s blood

STOP & THINK ¡ A particular cell produces (1) a cell receptor protein,

(2) a protein that makes up the cytoskeleton of the cytoplasm, and (3) a hormone that will be secreted into the bloodstream.

¡ Identify where the cytoskeleton protein will be synthesize. Explain your answer.

¡ Cytoskeleton protein will be assembled in the loose ribosome because it does not need exported out of the cytoplasm.

STOP & THINK ¡ A particular cell produces (1) a cell receptor protein, (2) a

protein that makes up the cytoskeleton of the cytoplasm, and (3) a hormone that will be secreted into the bloodstream.

¡  Identify where the hormone will by synthesized. Explain your answer.

¡ The hormone will be synthesized in the smooth ER, which creates lipids and hormones. It will be sent to the Golgi apparatus and packaged for export in vesicles.

STOP & THINK ¡ A particular cell produces (1) a cell receptor protein, (2) a

protein that makes up the cytoskeleton of the cytoplasm, and (3) a hormone that will be secreted into the bloodstream.

¡  Identify where a cell receptor protein will be assembled and describe how it will reach its final destination.

¡ The cell receptor protein will be assembled by ribosomes on the rough E.R.. It will be packaged into a vesicle, transported to the Golgi apparatus, modified further, and packaged into a vesicle that will merge with the plasma membrane.