Moving Materials Through Plants

Plants make sugars in their leaves and then must move those sugars throughout the plant in order to provide sugar to all of the plant cells.

Likewise, water and minerals are taken up by the plant roots and then must be transported to all the plant cells.

Once the circulating materials arrive at plant cells, they must be moved into the cells.

Introduction

So, plants need efficient shipping mechanisms to circulate sugar, water, and minerals throughout their bodies so every cell can get what it needs.

Introduction

Movement of Molecules

Moving Across Membranes

The plasma membrane controls what can enter and exit each plant cell. Whether or not a molecule will cross a membrane, and which way it will go, depends on three things.

Moving Across Membranes

The chemical structure of the molecule, particularly its size and whether it’s hydrophobic or hydrophilic.

Small hydrophobic molecules can cross lipids in the plasma membrane all by themselves. As molecules get larger, or if they are hydrophilic, they can cross the membrane only if a transport protein in the membrane helps them across.

Moving Across Membranes

The concentration of the molecule on either side of the membrane.

If molecules are allowed to move freely, they will move from areas where they are more concentrated to areas where they are less concentrated. In other words, they’ll move until they are evenly spread out, or in equilibrium, over an area. This type of movement is called diffusion.

Moving Across Membranes

Whether or not the membrane contains a transport protein for the molecule.

Although some small hydrophobic molecules can cross membranes by themselves, most molecules need the help of a transport protein. If a plant plasma membrane doesn’t have a protein to transport a particular molecule, it won’t be able to cross in or out of the plant cell.

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Diffusion is a passive process and requires no energy to be input from the cell.Diffusion happens because of the kinetic energy, or energy of motion, of molecules themselves. Because of kinetic energy, molecules are constantly in motion and jiggle and bounce themselves around, eventually becoming randomly distributed throughout an area.

Diffusion

Diffusion

1. A chemical starts out more concentrated on one side of a membrane, but its molecules can cross the membrane freely.2. The molecules bounce around due to kinetic energy and gradually become equal in concentration on both sides of the membrane.3. If you measure the overall movement of the chemical, you can see that its molecules moved from the side of the membrane where they were more concentrated to the side where they were less concentrated.

Diffusion

When molecules move from where they are more concentrated to where they are less concentrated, scientists say that molecules are moving along their concentration gradient.

Diffusion

Simple diffusion occurs when molecules diffuse across a membrane all by themselves.

Facilitated diffusion occurs when molecules diffuse across a membrane with the help of a membrane protein.

Diffusion

Diffusion

Even though facilitated diffusion involves the help of a protein, the movement of molecules still occurs from an area of higher concentration to an area of lower concentration. So, the process is still diffusion and doesn’t require any input of energy from the cell.

Active transport

Plant cells often need to move molecules from areas where the molecules are less concentrated to areas where they are more concentrated, a process called active transport. In other words, plant cells need to move molecules against their concentration gradient.

Active transport

Active transport requires the input of energy from the cell. Active transport is like rolling molecules up the hill of their concentration, going from where they’re less piled up to where they’re more piled up. If a cell is going to move something up a hill, it’s going to have to put some energy into the process.

Active transport

This diagram shows how a carrier protein could move a molecule against its concentration gradient by getting energy from ATP molecules during the transport process. These proteins are active transport proteins or pumps.

Active transport

As an example, a plant cell that stores energy for the plant may have lots of glucose inside the cell, but it may still need to bring in more. This cell could use active transport to continue bringing glucose molecules in against their concentration gradient.

Osmosis

Water moves across membranes by diffusion. The diffusion of water across membranes is called osmosis. During osmosis, water moves from an area where water is more concentrated to an area where water is less concentrated.

Osmosis

Thinking about osmosis in terms of water concentration can be a little hard to wrap your head around. Water is more concentrated when fewer solutes are dissolved in the water.In other words, water is most concentrated where water is most pure. So water moves from where water is most pure to areas where water is less pure.

Osmosis

If thinking of osmosis in terms of water concentration is confusing, think of osmosis in terms of solute concentration instead.Water will diffuse into an area of greater solutes. The concentration of solutes on either side of a membrane can be described by three terms that are used to describe the relative concentrations of solutes in different solutions.

Osmosis

Hypertonic solutions have greater concentrations of solutes (hyper- means more).Hypotonic solutions have lesser concentrations of solutes (hypo- means under).Isotonic solutions have the same concentration of solutes (iso- means same).

Osmosis

If you have two different concentrations of a chemical on either side of a membrane, water will move by osmosis toward the hypertonic solution. As an example, refer to Figure 9-2. Solutes move by diffusion from left to right until they are evenly distributed across the membrane. But, in that same scenario, what would the water around those solutes be doing?

Osmosis

1. Initially, the solution on the left side of the membrane is hypertonic to the solution on the right side of the membrane because more molecules are dissolved in the solution on the left side.

Osmosis

2. Water molecules move by osmosis toward the hypertonic solution-- in other words from the right side to the left side of the membrane.

Osmosis

3. If the membrane is permeable to both the solute and the water, they will diffuse in opposite directions until both are randomly distributed across the membrane.

Osmosis

Living cells don’t allow everything to pass freely across their membranes.Certain solutes require a protein transporter to help them get across.So, in a living cell, water may move by osmosis until its concentration is the same across the membrane, while a particular solute may remain unevenly distributed.

Osmosis

To determine whether the solute will move, you need to know whether it needs a transport protein and, if so, whether the cell in question has that transport protein.Whether you think of osmosis in terms of water concentration or movement toward solutes, the result is the same. Water molecules become more randomly distributed by osmosis.