Cell Membrane and Membrane Transport

Essential Questions:

EQ: Why is it important to regulate what moves into and out of a cell?EQ: How does the cell membrane help the cell maintain homeostasis?

Georgia Performance Standards:

SB1a: Explain the role of cell organelles for both prokaryotic and eukaryotic cells, including the cell membrane, in maintaining homeostasis and cell reproduction.

Warm-up

How is a window screen similar to a cell membrane? Read onto find out.

1. What are some things that can pass through a window screen?

2. What are some things that cannot pass through a window screen? Why is it important to keep these things from moving through the screen?

3. The cell is surrounded by a cell membrane, which regulates what enters and leaves the cell. Why is it important to regulate what moves into and out of a cell?

The Cell Membrane Maintains Homeostasis

• Structure– Semi-permeable

– Made of a phospholipid bilayer

– Contains proteins and carbohydrates

– Has polar hydrophilic (water loving) and non-polar hydrophobic (water fearing) parts

• Function– Protects the cell

organelles

– Controls what enters and leaves the cell (maintains homeostasis)

Outsideof cell

Insideof cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel

Lipid bilayer

Carbohydratechains

The Structure of the Cell Membrane

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This is a cross section of

the cell membrane you

should notice two

different structures:

•The phospholipids are the round yellow structures with the blue tails.

• The proteins are the lumpy structures that are scattered around among the phospholipids.

This is a simple representation of a phospholipid.

•The yellow structure represents the hydrophillic or water loving section of the phospholipid.

•The blue tails that come off of the sphere represent the hydrophobic or water fearing end of the phospholipid.

If you mix phospholipids in water they will form these double layered structures.

•The hydrophillic ends will be in contact with water.

•The hydrophobic ends will face inwards touching each other.

•Floating around in the cell membrane are different kinds of proteins.

•These are generally globular proteins.

•They are not held in any fixed pattern but instead float around in the phospholipid layer.

•Generally these proteins structurally fall into three catagories...

1st category of proteins

Transport proteins: regulate what enters or leaves the cell (Carrier & Channel)

A. Carrier Proteins:

• Carrier proteins do not extend through the membrane.

• They bond and drag molecules through the bilipid layer and release them on the opposite side.

B. Channel Proteins: Channel proteins simply act as a passive pore. Molecules will randomly move through the opening in a process called diffusion.

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2nd category of proteins

Marker proteins: extend across the cell membrane and serve to identify the cell to other cells.

•The immune system uses these proteins to tell friendly cells from foreign invaders.

•They are as unique as fingerprints.

•They play an important role in organ transplants. If the marker proteins on a transplanted organ are different from those of the original organ the body will reject it as a foreign invader.

3rd category of proteins

Receptor proteins: allow the cell to receive instructions

•These proteins are used in intercellular communication. In this animation, you can see the a hormone binding to the receptor. This causes the receptor protein release a signal to perform some action.

Cell Membrane Pop Quiz:1. Describe how the plasma membrane controls what goes into

and out of a cell. Selective permeability allow the cell membrane to maintain homeostasis by allowing certain molecules across the membrane. Membrane proteins can also be used.

2. Draw and label a diagram of the cell membrane. Use the following labels:

- Polar “hydrophilic heads”

- Non polar “hydrophobic tails

- Transport Protein

- Membrane Protein

- H2O

3. Describe how the plasma membrane helps maintain homeostasis in a cell. It allows certain things to move in and out.

Georgia Performance Standards:

Sb1a: Explain the role of cell organelles for both prokaryotic and eukaryotic cells, including the cell membrane, in

maintaining homeostasis. 

Sb1d. Explain the impact of water on life processes (i.e., osmosis, diffusion).

 Essential Questions: -How Do Cells Use Osmosis & Diffusion to

Maintain Homeostasis?

-Why is it important to regulate what moves in and out of the cell?

Passage Through Cell Boundaries

• Types of Passive Transport:• Diffusion

– A substance moves from areas of high concentration to areas of low concentration

– Requires NO energy (ATP)

– Equilibrium is reached when the concentration is equal on both sides of the membrane

– Rate determined by temp. and size of molecule

Passage Through Cell Boundaries

• Osmosis– Diffusion of water through a selectively

permeable membrane, until the concentration of substances are at an equilibrium.

– Effects of Osmosis on cells• Isotonic (Equal amounts of solute and solvent)

• Hypertonic (more solute than solvent)

• Hypotonic (more solvent than solute)

The Effects of Osmosis on Cells

• Cells placed in an isotonic solution neither gain nor lose water.

• Cells in a hypotonic solution, animal cells swell and burst. The vacuoles of plant cells swell, pushing the cell contents out against the cell wall.

• In a hypertonic solution, animal cells shrink, and plant cell vacuoles collapse.

Cellmembrane

Higher Concentrationof Water

Lower Concentrationof Water

Water molecules

Sugar molecules

Section 7-3Osmosis

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HighConcentration

LowConcentration

CellMembrane

Glucosemolecules

Proteinchannel

Section 7-3Facilitated Diffusion

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Facilitated Diffusion

• Some substances, such as glucose, can not pass through the membrane on their own

• Protein channels found within the lipid bilayer “facilitate” the passage of these substances

• Molecule move from areas of high concentration to areas of low concentration

• Requires NO energy (ATP)

Active Transport

• Sometimes cells must move materials against concentration gradients (from low concentration to high to concentration)

• Requires energy (ATP needed)

• Channel proteins found in the lipid bilayer can “pump” small molecules in/out of the cell with energy

Types of Active Transport• Sodium (Na+)/Potassium (K+)

Pump: “Solute Pumping”

– Found in the cell membrane of animal cells.

– 3 sodium ions are pump out of the cell and 2 potassium ions are pumped into the cell.

– Movement of these ions creates a concentration gradient

– ATP is needed

Types of Active Transport

• Endocytosis – cell can gather large substances and move them into the cell by folding its cell membrane (creating a vesicle) around the materials.

• Exocytosis – Vacuoles from within the cell fuse with the cell membrane and force contents out of the cell.

Microscope image of cell undergoing endocytosis. A.The cell membrane begins to sink in. B. The membrane is attempting to envelop the material. C. The material has been captured inside of a capsule (vessicle) of cell membrane. D. Endocytosis is complete.

Endocytosis in Action:

A Type of Endocytosis

• Phagocytosis– White blood cells use

phagocytosis to remove foreign particles from the blood stream.

– These cells will literally engulf foreign particles that are the same size as itself.

• “To Eat”Phagocytosis in action: A t-lymphocyte is attacking invading E. coli population. Copyright Daniel Kunkel]

A Type of Endocytosis

• Pinocytosis involves ingesting small molecules and/or fluids surrounding the cell in a process known as fluid-phase endocytosis.

Molecule tobe carried Low

Concentration

CellMembrane

HighConcentration

Moleculebeing carried

LowConcentration

CellMembrane

HighConcentration

Energy Energy

Section 7-3

Active Transport

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