Transport Across The Plasma Membrane

Cells and their Environment

What molecules are able to move freely across a

cells membrane?

How does a cells membrane structure work to move other molecules (like glucose) which are necessary for life across the membrane?

We learned last time that some molecules (oxygen, carbon dioxide and steroids) are able to move freely across the cells plasma membrane because they are lipid soluble (dissolve easily in lipids).But what about glucose? Its our cells main source of energy and yet it cannot pass directly across the membrane?

Let’s look at one example involving osmosis. Osmosis is the diffusion of water across a semi permeable membrane such as a cell membrane.

A semi permeable membrane is one that only certain types of molecules can cross. For example, water can cross a cell membrane, but many other substances can’t.

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Simple Diffusion

• Requires NO energy

• Molecules move from area of HIGH to LOW concentration

Transport of materials across a membrane can be an active or a passive process

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DIFFUSION

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

Transport across a membrane that does not require energy

active transport moves particles

against the concentration

gradient—from an area of low

concentration to an area of higher concentration

It uses energy and transport proteins to pump particles across the cells membrane.

Some molecules such as sugars are too big to

move across the membrane with

the help of transport proteins

(1) Which diagram best represents the fluid-mosaic model of a cell membrane?

PHOSPOLIPID

PHOSPOLIPID

The movement of materials across a plasma membrane by means of a protein is called __________.

Facilitated Diffusion

The movement of materials across a plasma against the concentration gradient is known as (3) _____ and requires (4) _____.

(3) Active Transport(4) ATP

(5) What do we call the movement of water across a semi-permeable membrane from an area of high water concentration to an area of low water concentration?

Osmosis