1.3 Components of Prokaryotic Cell
1.4 Components of Eukaryotic Cell
1.5 Components of a Plant Cell
1_The-Cell
The Cell
Overview
Red and cyan fluorescent proteins marking plant cell nuclei. Fernan Federici
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Botany by Melissa Ha, Maria Morrow & Kammy Algiers
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Introduction
Learning Objectives
- Define cell.
- Summarize the main components of a light microscope.
- List the features of a prokaryotic cell.
- Define cell theory.
- Explain how the surface area to volume ratio regulates cell size.
- List and describe the cellular components of a eukaryotic cell.
- Identify characteristic features of a plant cell.
- Explain the structure and function of the cell wall, chloroplast, central vacuole, and plasmodesmata in the plant cell.
Key Terms
cell theory/unified cell theory - a biological concept that states that all organisms are made up of cells; the cell is the basic unit of life, and new cells arise from existing cells
cell wall - rigid cell covering comprised of various molecules that protect the cell, provides structural support, and give shape to the cell
cellulose - the main component of cell wall, made up of glucose polymer
central vacuole - large plant cell organelle that regulates the cell’s storage compartment, holds water, and plays a significant role in cell growth as the site of macromolecule degradation
chlorophyll - the green pigment that captures the light energy that drives the light reactions of photosynthesis
chloroplast - plant cell organelle that carries out photosynthesis
endoplasmic reticulum (ER) - series of interconnected membranous structures within eukaryotic cells that collectively modify proteins and synthesize lipids
eukaryotic cell - a cell that has a membrane-bound nucleus and several other membrane-bound compartments or sacs
light microscope - an instrument that magnifies an object using a beam of visible light that passes and bends through a lens system to visualize a specimen
lignin - phenolic polymer, a component of plant cell wall
nucleus - cell organelle that houses the cell’s DNA and directs ribosome and protein synthesis
pectin - polysaccharide commonly found in the primary cell wall of plants
peptidoglycan - polysaccharide commonly found in the bacterial cell wall
plasma membrane - phospholipid bilayer with embedded (integral) or attached (peripheral) proteins, and separates the cell's internal content from its surrounding environment
plasmodesma - (plural = plasmodesmata) channel that passes between adjacent cell walls of plant cells, connects their cytoplasm, and allows transporting of materials from cell to cell
primary cell wall - outermost cell wall in a plant cell, primary made up of cellulose and pectin; usually flexible and permeable
prokaryote - a unicellular organism that lacks a nucleus or any other membrane-bound organelle
secondary cell wall - cell wall between the primary cell wall and plasma membrane in a plant cell; usually rigid and impermeable
Introduction
Close your eyes and picture a brick wall. What is the wall's basic building block? It is a single brick. Like a brick wall, cells are the building blocks that make up our body.
Our body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials to build a home, the human body is constructed from many cell types. Given their enormous variety, cells from all organisms—even ones as diverse as bacteria, onions, and humans—share certain fundamental characteristics.
Microscopy, Cell Theory & Cell Size
A cell is the smallest unit of all living things. We call “living things” – organism(s). Whether it is a single-cell organism (like bacteria) or a multi-cellular organism (like a human). Thus, cells are the basic building blocks of all organisms.
Several cells of one kind that interconnect with each other and perform a shared function make a tissue. These tissues combine to form an organ (your stomach, heart, or brain), and several organs comprise an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
All cells can be broadly categorized as prokaryotic and eukaryotic. For example, we classify both animal and plant cells as eukaryotic cells, whereas we classify bacterial cells as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, we will first examine how biologists study cells.
Microscopy
Cells vary in size. To give you a sense of cell size, a typical human red blood cell is about eight-millionths of a meter or eight micrometers (abbreviated as eight µm) in diameter. A pinhead is about two-thousandths of a meter (two mm) in diameter. That means about 250 red blood cells could fit on a pinhead. With few exceptions, we cannot see individual cells with the naked eye, so scientists use microscopes (micro- ="small; -scope = "to look at") to study them. A microscope is an instrument that magnifies an object. We photograph most cells with a microscope, so we can call these images micrographs.
The optics of a microscope’s lenses change the image orientation that the user sees. A specimen that is right-side up and facing right on the microscope slide will appear upside-down and facing left when one views through a microscope, and vice versa. Similarly, if one moves the slide left while looking through the microscope, it will appear to move right, and if one moves it down, it will seem to move up. This occurs because microscopes use two sets of lenses to magnify the image. Because of how light travels through the lenses, this two-lens system produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but include an additional magnification system that makes the final image appear to be upright).
Light Microscope
Most student microscopes are light microscopes (figure 1.1a). In this type of microscope, visible light passes and bends through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.
Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the microscope's ability to distinguish two adjacent structures as separate: the higher the resolution, the better the image's clarity and detail. Light microscopes that students commonly use in the laboratory magnify up to approximately 400 times. Light microscopes can magnify up to 1,000 times when oil immersion lenses are used. To gain a better understanding of cellular structure and function, scientists typically use electron microscopes.
Electron Microscope
In contrast to light microscopes, electron microscopes (figure 1.1.1b) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail, but it also provides higher resolving power. There are two main types of electron microscopes, transmission electron microscope (TEM) and scanning electron microscope (SEM). In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly bulkier and more expensive than light microscopes.
To learn more about light microscopes, visit this site.
Cell theory
The microscopes we use today are far more complex than those that Dutch shopkeeper Antony van Leeuwenhoek, used in the 1600s. Skilled in crafting lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed “animalcules.” In the 1665 publication Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells. By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that one or more cells comprise all living things, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.
Cells fall into one of two broad categories: prokaryotic and eukaryotic. We classify only the predominantly single-celled organisms Bacteria and Archaea as prokaryotes (pro- = “before”; -Kary- = “nucleus”). Animal cells, plants, fungi, and protists (protozoa) are all eukaryotes (EU- = “true”).
Cell Size
At 0.1 to 5.0 µm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 µm. The prokaryotes' small size allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any waste produced within a prokaryotic cell can quickly diffuse. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport. Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so.
First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its volume is 4πr3/3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, it's surface area-to-volume ratio decreases. This same principle would apply if the cell had a cube shape (figure 1.1.2). If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Other ways are to increase surface area by creating inward or outward projections of the cell membrane, becoming flat or thin and elongated, or by developing organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells, which we call eukaryotic cells.
For another perspective on cell size, try the HowBig interactive at this site.
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Components of Prokaryotic Cell
All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; 2) cytoplasm, consisting of a jelly-like cytosol within the cell in which there are other cellular components; 3) DNA, the cell's genetic material; and 4) ribosome, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.
A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is in the cell's central part: the nucleoid (figure 1.1.3)
Most prokaryotes have a Peptidoglycan cell wall, and many have a polysaccharide capsule (figure 1.1.3). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili exchange genetic material during conjugation, the process by which one bacterium transfers genetic material to another through direct contact. Bacteria use Fimbriae to attach to a host cell.
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Components of Eukaryotic Cell
Have you ever heard the phrase “form follows function?” It’s a philosophy that many industries follow. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should include several elevator banks. A hospital should place its emergency room where it is easily accessible.
Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic (figure 1.1.4). Unlike prokaryote cells, eukaryotic cells have 1) a membrane-bound nucleus; 2) numerous membrane-bound organelles, such as the endoplasmic reticulum, Golgi apparatus, chloroplast, mitochondria, and others; and 3) several, rod-shaped chromosomes. Because a membrane surrounds the eukaryotic cell’s nucleus, it has a “true nucleus.” The word “organelle” means “little organ,” and, as we already mentioned, organelles have specialized cellular functions, just as your body's organs have specialized functions.
At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane (figure 1.1.5), a phospholipid bilayer with embedded proteins that separate the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.
The Cytoplasm
The cytoplasm is the cell's entire region between the plasma membrane and the nuclear envelope (a structure we will discuss shortly). It is comprised of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals (figure 1.1.4). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are also there. Ions of sodium, potassium, calcium and many other elements also dissolve in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
The Nucleus
Typically, the nucleus is the most prominent organelle in a cell (figure 1.1.4). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail (figure 1.1.6).
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the nucleus' outermost portion (figure 1.1.6). Both the nuclear envelope's inner and outer membranes are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.
Chromatin and Chromosomes
To understand chromatin, it is helpful to first explore chromosomes, structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is 8. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes. During this stage, they resemble an unwound, jumbled bunch of threads. We call these unwound protein-chromosome complexes chromatin (figure1.1.6 & 1.1.7). Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.
The Nucleolus
We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm (figure 1.1.6).
Ribosomes
Ribosomes are the cellular structures responsible for protein synthesis. When we view them through an electron microscope, ribosomes appear either as clusters (polyribosomes) or as single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic surfaces of the plasma membrane, on the endoplasmic reticulum, and the nuclear envelope (figure 1.1.4). Electron microscopy shows us that ribosomes, which are large protein and RNA complexes, consist of two subunits: large and small (figure 1.1.8). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code, provided by the sequence of the nitrogenous bases in the mRNA, into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.
Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of the structure following function.
Mitochondria
Scientists often call mitochondria (singular = mitochondrion) “powerhouses” or “energy factories” of both plant and animal cells because they are responsible for making adenosine triphosphate (ATP) — the cell’s main energy-carrying molecule. Cellular respiration is the process of making ATP using the chemical energy in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. Mitochondria are oval-shaped, double-membrane organelles (figure 1.1.9) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has inward projections or folds called cristae. The inner lumen of mitochondria is filled with viscous fluid called matrix, made up of enzymes, certain vitamins & minerals in different forms, ions, small and large proteins, DNA, and ribosomes.
Peroxisomes
Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) For example, peroxisomes in liver cells detoxify alcohol. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes that play a role in metabolism, pathogen defense, and stress response, to mention a few.
Vesicles and Vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. Vesicle membranes can fuse with either the plasma membrane or other membrane systems within the cell. The vacuole's membrane does not fuse with the membranes of other cellular components. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules.
Endomembrane System
Scientists have long noticed that bacteria, mitochondria, and chloroplast are similar in size. We also know that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts. The endomembrane system (endo = “within”) is a group of membranes and organelles (figure 1.1.4) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we have already mentioned, as well as the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include either mitochondria or chloroplast membranes.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) (figure 1.1.4) is a series of interconnected membranous sacs and tubules. The ER's membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope. The inner hollow space of ER is called lumen or cisternal space. ER is responsible for modifying proteins, and their transportation as well as for synthesizing lipids. However, these two functions take place in two different areas of the ER: the rough ER and the smooth ER, respectively.
Rough Endoplasmic Reticulum
Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope (figure 1.1.10). Ribosomes transfer their newly synthesized proteins into the RER's lumen where they undergo structural modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular membranes—the ER or the ER's or other organelles' membranes. The proteins can also secrete from the cell (such as protein hormones, and enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane (figure 1.1.11).
Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins.
Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (figure 1.1.11). SER functions include the synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions. In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cells' coordinated contractions.
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs (figure 1.1.12).
The side of the Golgi apparatus that is closer to the ER is called the cis face. The opposite side is the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short-chain sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules to travel to their target destinations. Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi's trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.
In another example of form following function, cells that engage in a great deal of secretory activity (such as salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an abundance of Golgi. In a plant cell, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.
Lysosomes
The lysosomes are the cell’s “garbage disposal.” Enzymes within the lysosomes aid in breaking down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. Most plant cells do not have lysosomes, though many of these lysosomal enzymes are present in the vacuole of the plant cell. Lysosomes are also part of the endomembrane system.
You can watch an excellent animation of the endomembrane system here. At the end of the animation, there is a short self-assessment.
Cytoskeleton
If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell's shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within the all eukaryotic organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules (figure 1.1.13). Here, we will examine each.
Microfilaments
Also called actin filaments (figure 1.1.14), microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are made up of intertwined strands of two globular proteins. Microfilaments also provide some rigidity and help cells to change their shape. Microfilaments function in muscle contraction, cytoplasmic streaming, maintaining the cell shape, internal transport and cytokinesis.
Intermediate Filaments
Intermediate filaments are filaments with a diameter of about 8 to 10 nm (figure 1.1.15). You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin's epidermis. Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell's shape, and anchor the nucleus and other organelles in place. The intermediate filaments are the most diverse group of cytoskeletal elements. The research is ongoing to understand the function of intermediate filaments in plants.
Microtubules
As their name implies, microtubules are small hollow tubes. With a diameter of about 25 nm, microtubules are the widest component of cytoskeletons. Two globular proteins, α-tubulin and β-tubulin are polymerized as dimers, which then associate with other such dimers laterally to form tubular structures called protofilaments. One of the common arrangements is of 13 protofilaments joined to each other, side by side, to form a microtubule (figure 1.1.16). They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly. Microtubules participate in cell division in plant cells.
You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see table 1.1
Cell Component | Function | Present in Prokaryotes? | Present in Animal Cells? | Present in Plant Cells? |
Plasma membrane | Separates cell from the external environment; controls passage of organic molecules, ions, water, oxygen, and wastes into and out of a cell | Yes | Yes | Yes |
Cytoplasm | Provides turgor pressure to plant cells as the fluid inside the central vacuole; site of many metabolic reactions; medium in which organelles are found | Yes | Yes | Yes |
Nucleolus | The darkened area within the nucleus where ribosomal subunits are synthesized. | No | Yes | Yes |
Nucleus | A cell organelle that houses DNA and directs the synthesis of ribosomes and proteins | No | Yes | Yes |
Ribosomes | Protein synthesis | Yes | Yes | Yes |
Mitochondria | ATP production/cellular respiration | No | Yes | Yes |
Peroxisomes | Oxidize and thus break down fatty acids and amino acids, and detoxify poisons | No | Yes | Yes |
Vesicles and vacuoles | Storage and transport; digestive function in plant cells | No | Yes | Yes |
Centrosome | Unspecified role in cell division in animal cells; microtubule source in animal cells | No | Yes | No |
Lysosomes | Digestion of macromolecules; recycling of worn-out organelles | No | Yes | Some |
Cell wall | Protection, structural support, and maintenance of cell shape | Yes, primarily peptidoglycan | No | Yes, primarily cellulose |
Chloroplasts | Photosynthesis | No | No | Yes |
Endoplasmic reticulum | Modifies proteins and synthesizes lipids | No | Yes | Yes |
Golgi apparatus | Modifies, sorts, tags, packages, and distributes lipids and proteins | No | Yes | Yes |
Cytoskeleton | Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently | Yes | Yes | Yes |
Flagella | Cellular locomotion | Some | Some | No, except for some plant sperm cells |
Cilia | Cellular locomotion, movement of particles along plasma membrane's extracellular surface, and filtration | Some | Some | No |
Extracellular Structure
If you work on a group project, you need to communicate with others (at least your group members and the teacher). As you might expect, if cells are to work together, they must communicate with each other. Let’s look at how cells communicate with each other. Animal cells release materials into the extracellular space. The primary component of these materials is collagen. Collagen fibers are interwoven with proteoglycans, which are carbohydrate-containing protein molecules. Collectively, we call these materials the extracellular matrix. Plant cells do not secrete collagen but produce a rigid cell wall.
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Components of a Plant Cell
At this point, you know that all eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some vacuoles, microtubule organizing centers (MTOCs). Animal cells and plant cells have lysosomes, though lysosomes in plants operate differently and are not very common. There are some striking differences between animal and plant cells. In animal cells centrioles are associated with the MTOC, a complex we call the centrosome. Plant cells lack centrioles. Plant cells have a cell wall, chloroplasts, and other specialized plastids, and a large central vacuole, whereas animal cells do not.
The Cell Wall
If you examine figure 1.1.4 b, the plant cell diagram, you will see a structure external to the plasma membrane. This is the cell wall, a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and some protistan cells also have cell walls. While the prokaryotic cell walls' chief component is peptidoglycan, the major organic molecule in the plant’s (and some protists') cell wall is cellulose — a polysaccharide comprised of glucose units (figure 1.1.17). Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the rigid cell walls of a celery stalk with your teeth.
Central Vacuole
Previously, we mentioned vacuoles as essential components of plant cells. If you look at figure 1.1.4b, you will see that each plant cell has a large central vacuole that occupies most of the space inside the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant's cell walls results in a wilted appearance. The central vacuole also supports the cell's expansion. When the central vacuole holds more water, the cell becomes larger without having to invest considerable energy in synthesizing new cytoplasm.
Chloroplasts
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals. Plants (autotrophs) can make their own food, like sugars that is used in cellular respiration to provide ATP energy generated in the plant mitochondria. Animals (heterotrophs) must ingest their food.
Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs we call thylakoids (figure 1.1.18). Each thylakoid stack is a granum (plural = grana). We call the fluid enclosed by the inner membrane that surrounds the grana the stroma. The chloroplasts contain a green pigment, chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is different from that of plants and is not present inside an organelle.
Intercellular Junctions
Cells can also communicate with each other via direct contact or intercellular junctions. There are differences in the ways that plant and animal and fungal cells communicate. Plasmodesmata are junctions between plant cells, whereas, animal cell contacts include tight junctions, gap junctions, and desmosomes. Only plasmodesmata are discussed here.
Plasmodesmata
In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because the cell wall that surrounds each cell separates them (figure 1.1.4b). How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues (xylem and phloem) primarily. There also exist structural modifications, which we call plasmodesmata (singular = plasmodesma). Numerous channels pass between adjacent cell walls of plant cells connecting their cytoplasm, and enabling the transport of materials from cell to cell, and thus throughout the plant (figure 1.1.19).
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Attributions
Biology 2e by Clark Mary Ann, Douglas Matthew, Choi Jung. OpenStax is licensed under Creative Commons Attribution License V 4.0
Introduction to Organismal Biology at https://sites.gatech.edu/organismalbio/ is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Botany by Melissa Ha, Maria Morrow, and Kammy Algiers is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers.