The Cell Membrane- · 2019-11-11 · through the cell membrane easily. 16 Small molecules and...

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1 The Cell Membrane - Gateway to the Cell

Transcript of The Cell Membrane- · 2019-11-11 · through the cell membrane easily. 16 Small molecules and...

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The Cell Membrane -

Gateway to the Cell

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Photograph of a Cell Membrane

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Cell Membrane

The cell membrane is flexible and allows a unicellularorganism to move

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Homeostasis

• Balanced internal condition of cells

• Also called equilibrium

• Maintained by plasma membranecontrolling what enters & leaves the cell

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Functions of Plasma Membrane

� Protective barrier

� Regulate transport in & out of cell (selectively permeable)

� Allow cell recognition

� Provide anchoring sites for filaments of cytoskeleton

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Functions of Plasma Membrane

� Provide a binding site for enzymes

� Interlocking surfaces bind cells together (use of pectin)

�Contains the cytoplasm (fluid in cell)

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Structure of the Cell Membrane

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Phospholipids

Cholesterol

Proteins(peripheral and integral)Carbohydrates (glucose)

Membrane Components

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Phospholipids

Make up the cell membrane

Contains 2 fatty acid chains that are nonpolar

Head is polar & contains a phosphate group & glycerol

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FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid.

MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

FLUID MOSAIC MODEL

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Polar heads are hydrophilic “water loving”

Nonpolar tails are hydrophobic “water fearing”

Cell Membrane

Makes membrane “Selective” in what crosses

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Cell Membrane

Hydrophobicmolecules pass easily; hydrophilic DO NOT

The cell membrane is made of 2 layers of phospholipids called the lipid bilayer

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Solubility

• Materials that are soluble in lipids can pass through the cell membrane easily

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Small molecules and larger hydrophobicmolecules move through easily.e.g. O2, CO2, H2O

Semipermeable Membrane

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Ions, hydrophilic molecules larger than water, and large molecules such as proteinsdo not move through the membrane on their own.

Semipermeable Membrane

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Types of Transport Across Cell Membranes

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1. Simple Diffusion

• Requires NOenergy

• Molecules move from area of HIGH to LOWconcentration

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2. DIFFUSION

Diffusion is a PASSIVE process which means no energy is used to make the molecules move, they have a natural KINETIC ENERGY

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Diffusion of Liquids

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Diffusion through a Membrane

Cell membrane

Solute moves DOWN concentration gradient (HIGH to LOW)

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3. Osmosis

• Diffusion of wateracross a membrane

• Moves from HIGH water potential(low solute) toLOW water potential (high solute)

Diffusion across a membrane

Semipermeable membrane

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Diffusion of H2O Across A Membrane

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

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TONICITY

the osmotic pressure or tension of a solution, as in

the cells would swell or shrink depending on the

tonicity of the environment.

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Cell in Isotonic Solution

CELL

10% NaCL90% H2O

10% NaCL90% H2O

What is the direction of water movement?

The cell is at _______________.equilibrium

ENVIRONMENT

NO NET MOVEMENT

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Cell in Hypotonic Solution

CELL

10% NaCL90% H2O

20% NaCL80% H2O

What is the direction of water movement?

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Cell in Hypertonic Solution

CELL

15% NaCL85% H2O

5% NaCL95% H2O

What is the direction of water movement?

ENVIRONMENT

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Cells in Solutions

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Isotonic Solution

NO NET MOVEMENT OF

H2O (equal amounts entering & leaving)

HypotonicSolution

CYTOLYSIS

HypertonicSolution

PLASMOLYSIS

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Cytolysis & Plasmolysis

Cytolysis Plasmolysis

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Osmosis in Red Blood Cells

Isotonic Hypotonic Hypertoniccopyright cmassengale

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What Happens to Blood Cells?

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(3) Forms of Transport Across the Membrane

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Passive Transport

Simple Diffusion

� Doesn’t require energy

� Moves high to lowconcentration� Example: Oxygen or water diffusing into a cell and carbon dioxidediffusing out.

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Passive Transport

Facilitated diffusion

�Doesn’t require energy

�Uses transport proteins to move high tolow concentrationExamples: Glucose or amino acids moving from blood into a cell.

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Proteins Are Critical to Membrane Function

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Types of Transport Proteins

• Channel proteins are embedded in the cell membrane & have a porefor materials to randomly cross

• Carrier proteins can change shapeto move material from one side of the membrane to the other. Also called Integral proteins

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CHANNEL PROTEINS (passive transport)

Molecules will randomly move through the pores in Channel Proteins.

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CARRIER PROTEINS (active transport)

• Some Carrier proteins do not extend through the membrane.

• They bond anddrag molecules through the lipid bilayer and release them on the opposite side.

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Carrier Proteins

• Other carrierproteins change shapeto move materials across the cell membrane

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Active Transport

�Requires energy or ATP

�Moves materials from LOW to HIGHconcentration

�AGAINSTconcentration gradient

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Active transport

�Examples: Pumping Na+ (sodium ions) out and K+

(potassium ions) in against strong concentration gradients.

�Called Na+-K+ Pump

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Sodium-Potassium Pump

3 Na+ pumped in for every 2 K+ pumped out; creates a membrane potential

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Moving the “Big Stuff”

Molecules are moved out of the cell by vesicles that fusewith the plasma membrane.

1. Exocytosis-

moving things out.

This is how many hormones are secreted and how nerve cells communicate with one another.

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ExocytosisThe opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are

released through the cell membrane.

Inside Cell Cell environment

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ExocytosisExocytic vesicle immediately after fusion with plasma membrane.

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Moving the “Big Stuff”Large molecules move materials into the cell by

one of (3) forms of endocytosis.

2. Endocytosis

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I. Pinocytosis

• Cell forms an invagination

• Materials dissolve in water to be brought into cell

• Called “Cell Drinking”

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Pinocytosis

Most common form of endocytosis.Takes in dissolved molecules as a vesicle.

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Example of Pinocytosispinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).

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II. Receptor-Mediated Endocytosis

Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc.

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Receptor-Mediated Endocytosis

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III. Phagocytosis

Used to engulf large particles such as food, bacteria, etc. into vesicles

Called “Cell Eating”

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Phagocytosis About to Occur

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Phagocytosis- Capture of a YeastCell (yellow) by Membrane Extensions of an Immune System Cell(blue)


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