Some Eukaryotic Organelles Organelle Function
Ribosomes Involved in making proteins
Golgi apparatus Packages proteins and some polysaccharides
Mitochondria Makes ATP
Smooth ER Makes lipids
Chloroplast Makes sugar (photosynthesis)
Lysosomes Digests macromolecules
The Nucleus and Chromosomes
The nucleus, which is found exclusively in eukaryotic cells, is a membrane-enclosed structure that contains most of the genetic material of the cell (Figure below). Like a library, it holds vital information, mainly detailed instructions for building proteins. The nuclear envelope, a double membrane that surrounds the nucleus, controls which molecules go in and out of the nucleus.
Inside the nucleus are the chromosomes, the DNA all wrapped in special proteins. The genetic information on the chromosomes is stored made it available to the cell when necessary and also duplicated when it is time to pass the genetic information on when a cell divides. All the cells of a species carry the same number of chromosomes. For example, human cells each have 23 pairs of chromosomes. Each chromosome in turn carries hundreds or thousands of genes that encode proteins that help determine traits as varied as tooth shape, hair color, or kidney function.
Figure 3.10
In eukaryotic cells, the DNA is kept in a nucleus. The nucleus is surrounded by a double plasma membrane called the . Within the nucleus is the (smaller yellow ball).
The Cell Factory
Just as a factory is made up of many people, machines, and specific areas, each part of the whole playing a different role, a cell is also made up of different parts, each with a special role. For example, the nucleus of a cell is like a safe containing the factory's trade secrets, including how to build thousands of proteins, how much of each one to make, and when. The mitochondria are powerhouses that generate the ATP needed to power chemical reactions. Plant cells have special organelles called chloroplasts that capture energy from the sun and store it in the chemical bonds of sugar molecules - in the process called photosynthesis (Figure below). (The cells of animals and fungi do not photosynthesize and do not have chloroplasts.)
The vacuoles are storage centers, and the lysosomes are the recycling trucks that carry waste away from the factory. Inside lysosomes are enzymes that break down old molecules into parts that can be recycled into new ones. Eukaryotic cells also contain and internal skeleton called the cytoskeleton. Like our bony skeleton, a cell's cytoskeleton gives the cell a shape and helps it move parts of the cell.
In both eukaryotes and prokaryotes, ribosomes are where proteins are made. Some ribosomes cluster on folded membranes called the endoplasmic reticulum (ER). If the ER is covered with ribosomes, it looks bumpy and is called rough ER. If the ER lacks ribosomes, it is smooth and is called smooth ER. Proteins are made on rough ER and lipids are made on smooth ER.
Another set of folded membranes in cells is the Golgi apparatus, which works like a mail room. The Golgi apparatus receives the proteins from the rough ER, puts sugar molecule "shipping addresses" on the proteins, packages them up in vesicles, and then sends them to the right place in the cell.
Figure 3.11
Diagram of chloroplast (a) and electron microscope image of two mitochondria (b). Chloroplasts and mitochondria provide energy to cells. If the bar at the bottom of the electron micrograph image is 200 nanometers, what is the diameter of one of the mitochondria?
Plant Cells
Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells. First, plant cells are unique in having a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cell's vacuole can make up 90% of the cell’s volume. In animal cells, vacuoles are much smaller.
Second, plant cells have a cell wall, which animal cells do not. A cell wall gives the plant cell strength, rigidity, and protection. Although bacteria and fungi also have cell walls, a plant cell wall is made of a different material. Plant cell walls are made of the polysaccharides cellulose, fungal cell walls are made of chitin, and bacterial cell walls are made of peptidoglycan. This is highlighted in Figure below.
Figure 3.12
A plant cell has several features that make it different from an animal cell, including a cell wall, huge vacuoles, and several kinds of plastids, including chloroplasts (which photosynthesize).
Figure 3.13
In this photo of plant cells taken with a light microscope, you can see a cell wall (purple) around each cell and green chloroplasts.
A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. There are several kinds of plastids, including chloroplasts, needed for photosynthesis; leucoplasts, which store starch and oil; and brightly colored chromoplasts, which give some flowers and fruits their yellow, orange, or red color. You will learn more about chloroplasts and photosynthesis in the chapter titled Cell Functions. Under a microscope one can see plant cells more clearly (Figure above).
Lesson Summary
Prokaryotic cells lack a nucleus; eukaryotic cells have a nucleus.
Each component of a cell has a specific function.
Plant cells have unique features including plastids, cell walls, and central vacuoles.
Review Questions
What are the two basic types of cells?
What are organelles?
Discuss the main differences between prokaryotic cells and eukaryotic cells.
What is the plasma membrane and what is its role?
What organelle is known as the "powerhouse" of the cell?
Why does photosynthesis not occur in animal cells?
What are the main differences between a plant cell and an animal cell?
Further Reading / Supplemental Links
Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron’s; 1997, Hauppauge, New York.
Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago.
Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago.
Vocabulary
cell
The smallest unit of an organism that is still considered living; the basic unit that make up every type of organism.
cell wall
Provides strength and protection for the cell; found around plant, fungal, and bacterial cells.
central vacuole
Large organelle containing water, nutrients, and wastes that can take up to 90% of a plant cell’s volume.
chloroplast
Green organelle that captures solar energy and stores the energy in sugars through the process of photosynthesis; chloroplasts are found only in cells that perform photosynthesis.
chromosome
The cell structure in eukaryotic cells containing the genes; made of DNA and protein. Human cells have 23 pairs of chromosomes.
cytoplasm
All the contents of the cell besides the nucleus, including the cytosol and the organelles.
cytoskeleton
The internal scaffolding of the cell; maintains the cell shape and aids in moving the parts of the cell.
cytosol
A fluid-like substance inside the cell; organelles are embedded in the cytosol.
endoplasmic reticulum (ER)
A folded membrane organelle; rough ER modifies proteins and smooth ER makes lipids.
eukaryotic cell
Cell belonging to the domain Eukarya (fungi, animals, protists, and plants); has a membrane-enclosed nucleus and various organelles.
golgi apparatus
The organelle where proteins are modified, labeled, packaged into vesicles, and shipped.
homeostasis
The ability to maintain a stable internal environment separate from the external environment.
lysosome
Organelle which contains enzymes that break down unneeded materials.
mitochondria
&nbs
p; The organelle in all eukaryotic cells that makes adenosine triphosphate (ATP), the “energy currency” of cells.
nuclear envelope
A double membrane that surrounds the nucleus; helps regulate the passage of molecules in and out of the nucleus.
nucleus
Membrane enclosed organelle in eukaryotic cells that contains the DNA; primary distinguishing feature between a eukaryotic and prokaryotic cell; the information center, containing instructions for making all the proteins in a cell, as well as how much of each one.
organelle
Small structure wrapped in a membrane found only in eukaryotic cells; mitochondria, plastids, and vacuoles, for example.
plasma membrane
Surrounds the cell; made of a double layer of specialized lipids, known as phospholipids, with embedded proteins; regulates the movement of substances into and out of the cell; also called the cell membrane.
plasmid
Small circular piece of DNA; found in prokaryotic cells.
prokaryotic cell
Cell with no nucleus or other membrane-enclosed organelles; bacteria and archaea.
ribosome
The cell structure on which proteins are made; not surrounded by a membrane; found in both prokaryotic and eukaryotic cells.
rough endoplasmic reticulum
The part of the ER with ribosomes attached; proteins can be modified in the rough ER before they are packed into vesicles for transport to the golgi apparatus.
semi-permeable
allowing only certain materials to pass through; characteristic of the cell membrane.
smooth endoplasmic reticulum
Part of the ER that does not have ribosomes attached; where lipids are synthesized.
vesicle
Small membrane-enclosed sac; transports proteins around a cell or out of a cell.
Points to Consider
Think about what molecules would need to be transported into cells.
Discuss why it would be important for some molecules to be kept out of a cell.
Chapter 4: Cell Functions
Lesson 4.1: Transport
Lesson Objectives
Describe several methods of transporting molecules and ions into and out of the cell.
Distinguish between active and passive transport.
Explain how diffusion and osmosis work.
Check Your Understanding
What structure surrounds the cell?
What is the primary component of the cell membrane?
What does homeostasis mean?
Introduction
All organisms and their cells need to maintain homeostasis. But how can a cell keep a stable internal environment when the environment around the cell is constantly changing? Obviously, the cell needs to separate itself from the external environment. This job is accomplished by the cell membrane. The cell membrane is selectively permeable, or "semipermeable," which means that only some molecules can get through the membrane. If the cell membrane was completely permeable, the inside of the cell would be about the same as the outside and the cell could not achieve homeostasis.
How does the cell maintain this selective permeability? How does the cell control what molecules enter and leave the cell? The ways that cells control what passes through the cell membrane will be the focus of this lesson.
What is Transport?
The selectively permeable nature of the plasma membrane is due in part to the chemical composition of the membrane. Recall that the membrane is a double layer of phospholipids (a "bilayer") embedded with proteins (Figure below). A single phospholipid molecule has a hydrophilic, or water-loving, head and hydrophobic, or water-fearing, tail. The hydrophilic heads face the inside and outside of the cell, where water is abundant. The water-fearing, hydrophobic tails face each other in the middle of the membrane. At body temperature, the plasma membrane is fluid and constantly moving, like a soap bubble; it is not a solid structure.
Water and small non-charged molecules such as oxygen and carbon dioxide can pass freely through the membrane by slipping around the phospholipids. But larger molecules and charged molecules cannot pass through the plasma membrane easily. Therefore, special methods are needed for transporting some molecules across the plasma membrane and into or out of the cell.
Figure 4.1
The plasma membrane is made up of a phospholipid bilayer with embedded proteins.
Since atoms have an equal number of protons and electrons, they have no net charge. The negative charges of the electrons balance out the positive charges of the protons. Many molecules have an equal number of electrons and protons, so we call them non-polar molecules. However, some atoms can lose or gain electrons easily, giving them a positive or negative charge. These charged particles are called ions. If an atom loses an electron, it becomes a positively charged ion, such as the sodium ion Na+. If an atom gains an electron, it will be a negatively charged ion, such as the chloride ion, Cl-. Na+ and Cl- readily form NaCl, or common table salt. Since Na+ and Cl- are charged, they are unable to pass freely through the plasma membrane.
Passive Transport
Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance. The amount of a substance in relation to the volume, is called concentration. Diffusion requires no energy input from the cell (Figure below). Diffusion occurs by the random movement of molecules; molecules move in both directions (into and out of the cell), but there is a greater movement from an area of higher concentration towards an area of lower concentration. The movement of the substance from a greater concentration to a lesser concentration is referred to as moving down the concentration gradient. For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves down the concentration gradient from your lungs into your bloodstream
Figure 4.2
Diffusion across a membrane does not require an input of energy.
The diffusion of water across a membrane due to concentration differences is called osmosis. If a cell is placed in a hypotonic solution, meaning the solution has a lower concentration of dissolved material than what is inside the cell, water will move into the cell. This causes the cell to swell, and it may even burst. Organisms that live in fresh water, which is a hypotonic solution, have to prevent too much water from coming into their cells. Freshwater fish excrete a large volume of dilute urine to rid their bodies of excess water.
If a cell is placed in a hypertonic solution, meaning there is more dissolved material in the outside environment than in the cell, water will leave the cell. That can cause a cell to shrink and shrivel. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these animals intake large quantities of salt water and secrete salt by active transport, which will be discussed later in this lesson.
To keep cells intact, they need to be placed in an isotonic solution, a solution in which the amount of dissolved material is equal both inside and outside the cell. Therefore, there is no net movement of water into or out of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell. In the medical setting, red blood cells can be kept intact in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so the blood cells would swell and burst. This is represented in the Figure below.
Figure 4.3
Osmosis causes these red blood cells to change shape by losing or gaining water.
Sometimes diffusion across the membrane is slow or even impossible for some charged or large molecules. These molecules need the help of special helper proteins that are located in the plasma
membrane. Ion channel proteins move ions across the plasma membrane. Other molecules, such as glucose, move across the cell membrane by facilitated diffusion, in which a carrier protein physically moves the molecule across the membrane (Figure below). Both channel proteins and carrier proteins are specific for the molecule transported. Movement by ion channel proteins and facilitated diffusion are still considered passive transport, meaning they move molecules down the cell's concentration gradient and do not require any energy input.
Figure 4.4
Facilitated Diffusion is a type of passive transport where a carrier protein aids in moving the molecule across the membrane.
Active Transport
During active transport, molecules move against the concentration gradient, toward the area of higher concentration. This is the opposite of diffusion. Active transport requires both an input of energy, in the form of ATP, and a carrier protein to move the molecules. These proteins are often called pumps, because, as a water pump uses energy to force water against gravity, proteins involved in active transport use energy to move molecules against their concentration gradient.
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