There are many examples of why active transport is important in your cells. One example occurs in your nerve cells. In these cells, the sodium-potassium pump (Figure below) moves sodium out of the cell and potassium into the cell, both against their concentration gradients.
Figure 4.5
The sodium-potassium pump moves sodium ions to the outside of the cell and potassium ions to the inside of the cell. ATP is required for the protein to change shape. As ATP adds a phosphate group to the protein, it leaves behind adenosine diphosphate (ADP).
Transport Through Vesicles
Some large molecules are just too big to move across the membrane, even with the help of a carrier protein. These large molecules must be moved through vesicle formation, a process by which the large molecules are packaged in a small bubble of membrane for transport. This process keeps the large molecules from reacting with the cytoplasm of the cell. Vesicle formation does require an input of energy, however.
There are several kinds of vesicle formation that allow large molecules to move across the plasma membrane. Exocytosis moves large molecules outside of the cell. During exocytosis, the vesicle carrying the large molecule fuses with the plasma membrane. The large molecule is then released outside of the cell, and the vesicle is absorbed into the plasma membrane. Endocytosis is the process by which cells take in large molecules by vesicle formation. Types of endocytosis include phagocytosis and pinocytosis. Phagocytosis moves large substances, even another cell, into the cell. Phagocytosis occurs frequently in single-celled organisms, such as amoebas. Pinocytosis (Figure below) involves the movement of liquid or very small particles into the cell. These processes cause some membrane material to be lost as these vesicles bud off and come into the cell. This membrane is replaced by the membrane gained through exocytosis.
Figure 4.6
During endocytosis, exocytosis and pinocytosis, substances are moved into or out of the cell via vesicle formation.
Lesson Summary
The plasma membrane is selectively permeable or semi-permeable, meaning that some molecules can move through the membrane easily, while others require specialized transport mechanisms.
Passive transport methods, including diffusion, ion channels, facilitated diffusion, and osmosis, move molecules in the direction of the lowest concentration of the molecule and do not require energy.
Active transport methods move molecules in the direction of the higher concentration and require energy and a carrier protein.
Vesicles can be used to move large molecules, which requires energy input.
Review Questions
What happens when a cell is placed in a hypotonic solution?
What happens when a cell is placed in a hypertonic solution?
What’s the main difference between active and passive transport?
List an example of active transport.
List the types of passive transport.
Why is the plasma membrane considered semipermeable?
What is the process where a cell engulfs a macromolecule, forming a vesicle?
What is diffusion?
Explain the results of a sodium-potassium pump working across a membrane.
Does facilitated transport move a substance down or up a gradient?
Further Reading / Supplemental Links
http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/passive.html
http://www.vivo.colostate.edu/hbooks/molecules/sodium_pump.html
http://www.biologycorner.com/bio1/diffusion.html
http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html
http://www.enwikipedia.org/
Vocabulary
active transport
Moving a molecule from an area of lower concentration to an area of higher concentration; requires a carrier protein and energy.
concentration
The amount of a substance in relation to the volume.
diffusion
Movement of molecules from an area of high concentration to an area of low concentration; requires no energy.
endocytosis
Movement of substances into the cell by vesicle formation.
exocytosis
Movement of substances out of the cell by a vesicle fusing with the plasma membrane.
facilitated diffusion
Diffusion in which a carrier protein physically moves the molecule across the membrane; a form of passive transport.
homeostasis
Maintaining a stable internal environment despite any external changes.
hypertonic solution
Having a higher solute concentration than the cell; cell will lose water by osmosis.
hypotonic solution
Having a lower solute concentration than the cell; cell will gain water by osmosis.
ion
An atom that carries a negative or positive charge.
ion channel
Protein in the plasma membrane that allows ions to pass through.
isotonic solution
A solution in which the amount of dissolved material is equal both inside and outside the cell; no net gain or loss of water.
osmosis
Diffusion of water across a membrane.
passive transport
Movement of molecules from an area of higher concentration to an area of lower concentration; requires no energy.
phagocytosis
Movement of large substances, including other cells, into the cell by vesicle formation.
phospolipid
A lipid molecule with a hydrophilic head and two hydrophobic tails; makes up the cell membrane.
pinocytosis
Movement of macromolecules into the cell by vesicle formation.
selectively permeable
Semipermeable; property of allowing only certain molecules to pass through the cell membrane.
sodium-potassium pump
Carrier protein that moves sodium ions out of the cell and potassium ions into the cell; works against the concentration gradient and requires energy.
vesicle formation
The formation of a small membrane-bound sac that can store and move substances into and out of the cell.
Points to Consider
The next lesson discusses photosynthesis.
It is often said that plants make their own food. What do you think this means?
What substances would need to move into a leaf cell?
What substances would need to move out of a leaf cell?
Lesson 4.2: Photosynthesis
Lesson Objectives
Explain the importance of photosynthesis.
Write and interpret the chemical equation for photosynthesis.
Describe what happens during the light reactions and the Calvin Cycle.
Check Your Understanding
How are plant cells different from animal cells?
In what organelle does photosynthesis take place?
Introduction
Almost all life on Earth depends on photosynthesis. Recall that photosynthesis is the process by which plants use the sun's energy to make their own “food” from carbon dioxide and water. For example, animals, such as caterpillars, eat plants and therefore rely on the plants to obtain energy. If a bird eats a caterpillar, then the bird is obtaining the energy that the caterpillar gained from the plants. So the bird is indirectly getting energy that began with the “food” formed through photosynthesis. Almost all organisms obtain their energy from photosynthetic organisms, either directly, by eating photosynthetic organisms, or indirectly by eating other organisms that ultimately obtained their energy from photosynthetic organisms. Therefore, the process of photosynthesis is central to sustaining life on Earth.
Overview of Photosynthesis
Photosynthesis is the process that converts the energy of the sun, or solar energy, into carbohydrates
, a type of chemical energy. During photosynthesis, carbon dioxide and water combine with solar energy, yielding glucose (the carbohydrate) and oxygen. As mentioned previously, plants can photosynthesize, but plants are not the only organisms with this ability. Algae, which are plant-like protists, and cyanobacteria (certain bacteria which are also known as blue-green bacteria, or blue-green algae) can also photosynthesize. Algae and cyanobacteria are important in aquatic environments as sources of food for larger organisms.
Photosynthesis mostly takes place in the leaves of a plant. The green pigment in leaves, chlorophyll, helps to capture solar energy. And special structures within the leaves provide water and carbon dioxide, which are the raw materials for photosynthesis. The veins within a leaf carry water which originates from the roots, and carbon dioxide enters the leaf from the air through special pores called stomata (Figure below).
Figure 4.7
Stomata are special pores that allow gasses to enter and exit the leaf.
The water and carbon dioxide are transported within the leaf to the chloroplast (Figure below), the organelle in which photosynthesis takes place. The chloroplast has two distinct membrane systems; an outer membrane surrounds the chloroplast and an inner membrane system forms flattened sacs called thylakoids. As a result, there are two separate spaces within the chloroplast. The interior space that surrounds the thylakoids is filled with a fluid called stroma. The inner compartments formed by the thylakoid membranes are called the thylakoid space.
Figure 4.8
The chloroplast is the photosynthesis factory of the plant.
The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H20), with the addition of solar energy, yields 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2). Using chemical symbols the equation is represented as follows:
6CO2 + 6H2O C6H12O6+ 6O2
Oxygen: An Essential Byproduct
Oxygen is a byproduct of the process of photosynthesis and is released to the atmosphere through the stomata. Therefore, plants and other photosynthetic organisms play an important ecological role in converting carbon dioxide into oxygen. As you know, animals need oxygen to carry out the energy-producing reactions of their cells. Without photosynthetic organisms, many other organisms would not have enough oxygen in the atmosphere to survive. Oxygen is also used as a reactant in cellular respiration, which is discussed in the next lesson, so essentially, oxygen cycles through the processes of photosynthesis and cellular respiration.
The Light Reactions and the Calvin Cycle
The overall process of photosynthesis does not happen in one step, however. The chemical equation of photosynthesis shows the results of many chemical reactions. The chemical reactions that make up the process of photosynthesis can be divided into two groups: the light reactions (also known as the light-dependent reactions, because these reactions only occur during daylight hours) and the Calvin Cycle, or the light-independent reactions. During the light reactions, the energy of sunlight is captured, while during the Calvin Cycle, carbon dioxide is converted into glucose, which is a type of sugar. This is summarized in Figure below.
Figure 4.9
This overview of photosynthesis shows that light is captured during the light reactions, resulting in the production of ATP and the electron carrier NADPH. Through the Calvin Cycle, these materials are used to fix carbon dioxide into sugar. Also during the Calvin Cycle, NADP and ADP are regenerated.
Stage 1: Capturing Light Energy
In the first step of the light reactions, solar energy is absorbed by the chlorophyll (and accessory pigments) within the chloroplast’s thylakoid membranes. This absorbed energy excites electrons in the thylakoid membranes. The electrons are then transferred from the thylakoid membranes by a series of electron carrier molecules. The series of electron carrier molecules that transfers electrons is called the electron transport chain. During this process water molecules in the thylakoid are split to replace the electrons that left the pigment, releasing oxygen and adding hydrogen ions (H+) to the thylakoid space. As the thylakoid becomes a reservoir for hydrogen ions, a chemiosmotic gradient forms as there are more hydrogen ions in the thylakoid than in the stroma. As H+ ions flow from the high concentration in the thylakoid to the low concentration in the stroma, they provide energy as they pass through an enzyme called ATP synthase. ATP synthase uses the energy of the movement of H+ ions to make ATP. Meanwhile, highly energized electrons from the electron transport chain combine with the electron carrier NADP+ to become NADPH (Figure below). NADPH will carry this energy in the electrons to the next phase of photosynthesis, the Calvin Cycle.
Figure 4.10
The light reactions include the movement of electrons down the electric transport chain, splitting water and releasing hydrogen ions into the thylakoid space.
Stage 2: Producing Food
During the Calvin Cycle, which occurs in the stroma of the chloroplast, glucose is formed from carbon dioxide and the products of the light reactions. During the first step CO2 is attached to a 5-carbon molecule (called Ribulose-5-Phosphate, RuBP), forming a 6-carbon molecule. This reaction is catalyzed by an enzyme named RuBisCo, which is the most abundant protein in plants and maybe on Earth! The 6-carbon molecule formed by this reaction immediately splits into two 3-carbon molecules, and the 3-carbon molecule is rearranged to a 3-carbon carbohydrate. The energy and electrons needed for this process are provided by the ATP and NADPH produced earlier in photosynthesis. The "food" made by photosynthesis is formed from the 3-carbon carbohydrate. Two 3-carbon carbohydrates combine to form glucose, a 6-carbon carbohydrate. Next, the 6-carbon RuBP must be reproduced so the Calvin Cycle can start again (Figure below).
Figure 4.11
The Calvin Cycle begins with carbon fixation, or carbon dioxide attaching to the 5-carbon molecule RuBP, forming a 6-carbon molecule and splitting immediately in to two 3-carbon molecules. This is shown at the top of the figure. This carbon molecule is then reduced to a 3-carbon carbohydrate, shown at the bottom of the figure. The energy and reducing power needed for this process are provided by the ATP and NADPH produced from the light reactions. Next, RuBP must be reproduced so the Calvin Cycle can continue.
The 3-carbon product of the Calvin Cycle can be converted into many types of organic molecules. Glucose, the energy source of plants and animals, is only one possible product of photosynthesis. Glucose is formed by two turns of the Calvin Cycle. Glucose can be formed into long chains as cellulose, a structural carbohydrate, or starch, a long-term storage carbohydrate. The product of the Calvin Cycle can also be used as the backbone of fatty acids, or amino acids, which make up proteins.
Photosynthesis is crucial to most ecosystems since animals obtain energy by eating other animals, or plants and seeds that contain these organic molecules. In fact, it is the process of photosynthesis that supplies almost all the energy to an ecosystem.
Lesson Summary
The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the sun, produce glucose and oxygen.
During the light reactions of photosynthesis, solar energy is converted into the chemical energy of ATP and NADPH.
During the Calvin Cycle, the chemical energy of ATP and NADPH is used to convert carbon dioxide into glucose.
Review Questions
What is the energy-capturing stage of photosynthesis?
What are the products of the light reactions?
What are the ATP and NADPH from the light reactions used for?
Where does the oxygen released by photosynthesis come from?
What happens to the glucose produced from photosynthesis?
Describe the structures of the chloroplast where photosynthesis takes place.
What is the significance of the electron transport chain?
What are the reactants required for photosynthesis?
What are the products of photosynthesis?
Further Reading / Suppleme
ntal Links
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html
http://photoscience.la.asu.edu/photosyn/education/photointro.html
http://www.pbs.org/wgbh/nova/methuselah/photosynthesis.html
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
http://www.biology4all.com/resources_library/details.asp?ResourceID=43
http://www.enwikipedia.org/
Vocabulary
ATP synthase
An enzyme that uses the energy of the movement of H+ ions to make ATP.
Calvin Cycle
The reactions of photosynthesis in which carbon dioxide is converted into glucose, which is a type of sugar; also known as the light independent reactions.
chlorophyll
Green pigment in leaves; helps to capture solar energy.
chloroplast
The organelle in which photosynthesis takes place.
cyanobacteria
CK-12 Life Science Page 8
