transport across membranes sbi4u. importance of transport intake of nutrients removal of wastes...

Transport Across Membranes

SBI4U

Importance of Transport

• intake of nutrients

• removal of wastes

• communication with environment & other cells

• blocking passage of harmful substances

Passive Transport

• the movement of materials across the cell membrane without the use of chemical energy (ATP)

• occurs because of diffusion

Diffusion

• the net movement of a substance from an area of higher concentration to an area of lower

• dynamic equilibrium

Simple Diffusion

• substances move across membrane unassisted

• small non-polar molecules (O2, CO2, steroid hormones, some drugs) and small polar molecules (H2O, glycerol)

• larger molecules and ions can not pass through membranes unassisted

Facilitated Diffusion

• diffusion across membrane assisted by integral membrane proteins called transport proteins

• channel proteins vs. carrier proteins

• ions, water, amino acids, sugars, etc.

Osmosis

• the diffusion of water across a membrane

• water follows concentration gradient until equilibrium

Osmosis

• direction of osmosis changes depending on type of solution surrounding the cell:

• isotonic solution• hypotonic solution• hypertonic solution

10

Cell in Isotonic SolutionCell in Isotonic Solution

CELLCELL

10% NaCL90% H2O

10% NaCL

90% H2O

What is the direction of water movement?The cell is at _______________.equilibrium

ENVIRONMENTENVIRONMENT

NO NET NO NET MOVEMENMOVEMENTT

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11

Cell in Hypotonic SolutionCell in Hypotonic Solution

CELLCELL

10% NaCL90% H2O

20% NaCL

80% H2O

What is the direction of water movement? copyright cmassengale

12

Cell in Hypertonic SolutionCell in Hypertonic Solution

CELLCELL

15% NaCL85% H2O

5% NaCL95% H2O

What is the direction of water movement?

ENVIRONMENTENVIRONMENT

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Cells and Tonicity

Red Blood Cells

15

hypotonic hypertonic isotonic

hypertonic isotonic hypotonic

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Answer the following:How does osmosis impact…

• …a single-celled organism living in a freshwater environment?

• …a single-celled organism living in a salt-water environment?

• …the roots of a plant in the spring after salt has been sprinkled during the winter?

More applications…

• Why can’t you water a plant with salt water?

• Why do vegetables in the grocery store get sprayed with water periodically? (What would happen if they were sprayed with salt water?)

Active Transport

Active Transport

• the movement of materials against the concentration gradient– From low to high concentration

• requires cellular energy (ATP)

• Requires a transport protein: pump

• ATP induces a conformational change in the protein pump that allows specific molecules to enter/exit the cell against its concentration gradient.

Types of Pumps

• Often function like carrier proteins but against the gradient so they need energy

• Often transport ions eg. Na+, K+, Ca2+, H+

• Pumps can be coupled to save energy and move 2 molecules at the same time across the concentration gradient

Example: The Na+/K+ Pump

• Active transport• Antiport• Pump oscillates between

2 conformational states:• 3 Na+ out of the cell• 2 K+ into the cell

Example: The Na+/K+ Pump

Example of Na-K pump in nerve impulse conduction

Bulk Transport

• movement of larger or very polar substances across the cell membrane

• requires energy (ATP)

• Involves a folding of the cell membrane to form a vesicle

Types of Bulk Membrane Transport

• Endocytosis– Entry into the cell

• Eg. Phagocytosis, pinocytosis, receptor-mediated endocytosis

• Exocytosis– Exiting the cell

Endocytosis

Phagocytosis• “cellular eating”• Process where living cells ingest other cells

or large particles• Single cellular organisms use it to feed (eg.

Amoeba)• Used in multi-cellular organisms as a defense

mechanism against invasion by foreign particles– Phagocyte: a white blood cell specialized for

protecting the body by ingesting foreign substances.

Pinocytosis• “Cellular Drinking”• Ingestion of dissolved

materials• Occurs in most cell types• Cell folds inwards to take

in the fluid containing the desired substance

• Process is unspecific in terms of what is being ingested

Receptor-Mediated Endocytosis• Intake of molecules that bind

specifically to a receptor on the surface of the cell

• Ligand: a molecule that binds to a receptor

• Receptor proteins are usually clustered in regions of the membrane called coated pits which contain coat proteins that help form vesicles for endocytosis

Exocytosis

• Movement of materials from the cell to the cell surface within membrane bound vesicles

• Vesicles formed off Golgi Body or from endocytosis

• Reverse of endocytosis

Exocytosis Function

• Secretion: release wastes, toxins

• Restoring the cell membrane– Need to balance endocytosis in order to

keep the surface area of the cell membrane constant

HW Questions

1. An integral protein changes conformation to transport substances from one side of the membrane to the other. It is also able to move 2 different molecules in the same direction. Classify this type of transport and the protein involved. [K-3]

2. Provide a real-life everyday example of when you have encountered a hypotonic or hypertonic environment. Describe what affect it had and the reason for the observed phenomena. [I-2]

3. How does the function of phagocytosis differ in the human body compared to single cellular organisms? Provide an example of each. [A-2]

Credits

• Slides #10, 11, 12, and 15 taken from Powerpoint presentations created by Cheryl Massengale, educator extraordinaire & creator of http://www.biologyjunction.com/