TRA - Transport [OVERVIEW]

Transport

Introduction

Have you traveled anywhere lately? If you have, you’ve probably noticed that airport security is carefully designed to let some things in (such as passengers with tickets) and to keep others out (such as weapons, explosives, and bottled water). In many ways, airport security is a lot like the plasma membrane of a cell. Cell membranes are selectively permeable, regulating which substances can pass through, as well as how much of each substance can enter or exit at a given time. Selective permeability allows the cell to obtain nutrients, eliminate wastes, and maintain homeostasis.
The simplest forms of transport across a membrane are passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. A concentration gradient is a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher concentration to an area of lower concentration. In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion. In this module, we’ll look in more detail at membrane permeability and different modes of passive transport.

List of Lessons

In this module, we will study the following topics:

    1. Scientific Method: Review of guidelines scientists follow to answer questions about life.
    2. Diffusion: This lesson explains the movement of particles from areas of high to low concentration.
    3. Osmosis: This lesson explains the movement of water molecules from areas of high to low water molecule concentration.
    4. Membrane Permeability: This lesson examines the relationship between the structure of the cell membrane from the previous unit and how it influences the movement of particles across the membrane.
    5. MATH: Water Potential: This lesson uses water potential equations to calculate solute potential and/or overall water potential and explain the movement of water.
    6. Active Transport: This lesson examines the different types of active transport in a cell which requires the use of energy from another source.

Module Objectives

By the end of this module, students will be able to:

    • Explain how the structure of biological membranes influences selective permeability.
    • Describe the mechanisms that organisms use to maintain solute and water balance.
    • Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.
    • Explain how the structure of a molecule affects its ability to pass through the plasma membrane.
    • Explain how concentration gradients affect the movement of molecules across membranes.
    • Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.
    • Describe the processes that allow ions and other molecules to move across membranes.
    • MATH SKILL: Use mathematical calculations to determine the surface area to volume ratio of different shapes.

Key Terms

Active transport -­ the movement of ions or molecules across a cell membrane into a region of a higher concentration, assisted by enzymes and requiring energy.

ATP -­ Adenosine triphosphate: a nucleotide derived from adenosine that occurs in muscle tissue; the major source of energy for cellular reactions.

Cell membrane -­ a thin membrane (a double layer of lipids) enclosing the cytoplasm of a cell; proteins in the membrane control passage of ions (like sodium or potassium or calcium) in and out of the cell.

Co-­transport -­ also known as coupled transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio.

Diffusion - the process in which there is the movement of a substance from an area of high concentration of that substance to an area of lower concentration.

Facilitated diffusion - the spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.

Hypotonic – having a lower solute concentration than the other side of the membrane.

Hypertonic – having a higher solute concentration than the other side of the membrane.

Ion pumps -­ a transmembrane protein that moves ions across a plasma membrane against their concentration gradient, in contrast to ion channels, where ions go through passive transport.

Osmosis -­ is the movement of water molecules across a partially‐permeable membrane down a water potential gradient.

Passive transport - moving biochemicals and atomic or molecular substances across the cell membrane. Unlike active transport, this process does not involve chemical energy. The four main kinds of passive transport are diffusion, facilitated diffusion, filtration, and osmosis.

Selectively permeable -­ property of biological membranes that allows only certain substances to pass through them.

Water potential – quantifies the tendency of water to move in a certain direction.

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