9.2: Estructura de células vegetales

Objetivos de aprendizaje

• Describir las estructuras que comparten todas las celdas en común.
• Indicar el papel de la membrana plasmática.
• Describir las estructuras de las células eucariotas.
• Resumir las funciones de los orgánulos celulares principales.

Componentes de todas las celdas

Todas las celdas contienen estos mismos cuatro componentes: 1. membrana plasmática (celular), una bicapa de fosfolípidos con un mosaico de proteínas, que funciona como barrera entre la célula y su entorno. 2. citoplasma, la región entre la región del ADN y la membrana plasmática, y el citosol, una región fluida, gelatinosa dentro de la célula donde se producen reacciones químicas. 3. El ADN, la información de herencia de las células, que se puede encontrar en un núcleo de células eucariotas y en la región nucleoide a de célula procariota. 4. ribosomas, o estructuras sintetizadoras de proteínas compuestas por ribosomas y proteínas. Estas estructuras se pueden encontrar en la imagen de la célula vegetal (Figura\(\PageIndex{1}\)).

Figura\(\PageIndex{1}\)): Esta figura muestra los orgánulos principales y otros componentes celulares de una célula vegetal eucariota típica. La célula vegetal tiene una pared celular, cloroplastos, plastidios y una vacuola central, estructuras que no están en las células animales. La mayoría de las células no tienen lisosomas ni centrosomas.

La Membrana Plasma

Tanto las células procariotas como las eucariotas tienen una membrana plasmática (Figura\(\PageIndex{2}\)), una bicapa fosfolipídica con proteínas incrustadas que separa el contenido interno de la célula de su entorno circundante. Un fosfolípido es una molécula lipídica con dos cadenas de ácidos grasos y un grupo que contiene fosfato. La membrana plasmática controla el paso de moléculas orgánicas, iones, agua y oxígeno dentro y fuera de la célula. Los desechos (como el dióxido de carbono y el amoníaco) también salen de la célula al pasar a través de la membrana plasmática. Las membranas plasmáticas son semipermeables y permiten el paso de moléculas pequeñas y/o no polares. El agua, al ser pequeña, puede pasar a través de la membrana y se moverá de un área de baja concentración de soluto a un área de alta concentración de soluto por el proceso de ósmosis.

(Figura\(\PageIndex{2}\)): La membrana plasmática eucariota es una bicapa de fosfolípidos con proteínas y colesterol incrustados en ella.

El citoplasma

El citoplasma es la región completa de la célula entre la membrana plasmática y la envoltura nuclear (una estructura que discutiremos en breve). Se compone de orgánulos suspendidos en el citosol gelatinoso, el citoesqueleto y diversos químicos (Figura\(\PageIndex{1}\)). A pesar de que el citoplasma consiste en 70 a 80 por ciento de agua, tiene una consistencia semisólida, la cual proviene de las proteínas que contiene. Sin embargo, las proteínas no son las únicas moléculas orgánicas en el citoplasma. También hay glucosa y otros azúcares simples, polisacáridos, aminoácidos, ácidos nucleicos, ácidos grasos y derivados del glicerol. Los iones de sodio, potasio, calcio y muchos otros elementos también se disuelven en el citoplasma. Muchas reacciones metabólicas, incluida la síntesis de proteínas, tienen lugar en el citoplasma.

ADN

En las células eucariotas, el ADN se encuentra típicamente alojado en un núcleo (plural = núcleos), el orgánulo más prominente de una célula (Figura\(\PageIndex{1}\). Este orgánulo dirige la síntesis de ribosomas y proteínas. Veamos con más detalle (Figura\(\PageIndex{3}\)).

(Figura\(\PageIndex{3}\)): El núcleo almacena cromatina (ADN más proteínas) en una sustancia similar a un gel llamada nucleoplasma. El nucleolo es una región de cromatina condensada donde se produce la síntesis de ribosomas. Llamamos al límite del núcleo la envolvente nuclear. Consta de dos bicapas fosfolipídicas: una membrana externa y una interna. La membrana nuclear es continua con el retículo endoplásmico. Los poros nucleares permiten que las sustancias entren y salgan del núcleo.

La envolvente nuclear

La envoltura nuclear es una estructura de doble membrana que constituye la porción más externa del núcleo (Figura\(\PageIndex{3}\). Tanto la membrana interna como la externa de la envoltura nuclear son bicapas fosfolipídicas.

La envoltura nuclear está salpicada de poros que controlan el paso de iones, moléculas y ARN entre el nucleoplasma y el citoplasma. El nucleoplasma es el fluido semisólido dentro del núcleo, donde encontramos la cromatina y el nucleolo.

Cromatina y Cromosomas

Para entender la cromatina, es útil explorar primero los cromosomas, estructuras dentro del núcleo que están compuestas por ADN, el material hereditario. Quizás recuerdes que en los procariotas, el ADN se organiza en un solo cromosoma circular. En los eucariotas, los cromosomas son estructuras lineales. Cada especie eucariota tiene un número específico de cromosomas en el núcleo de cada célula. Por ejemplo, en los humanos, el número de cromosomas es 46, mientras que en las moscas de la fruta, es de ocho. Los cromosomas sólo son visibles y distinguibles entre sí cuando la célula se está preparando para dividirse. Cuando la célula se encuentra en las fases de crecimiento y mantenimiento de su ciclo de vida, las proteínas se adhieren a los cromosomas, y se asemejan a un manojo desenrollado y desenrollado de hilos. Los llamamos cromatina a estos complejos proteína-cromosoma desenrollados (Figura\(\PageIndex{4}\). La cromatina describe el material que compone los cromosomas tanto cuando se condensan como descondensados.

Figura\(\PageIndex{4}\): (a) This image shows various levels of chromatin's organization, DNA tightly coiled into two thick cylinders and DNA coiled around proteins called histones. (b) Paired chromosomes, colored as pairs. Notice individuals of each pair are the same size to one another. (credit b: modification of work by NIH; scale-bar data from Matt Russell)

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.

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. They are not organelles. They can be small dot-like structures that float freely in the cytoplasm (known as free ribosomes) or they may be attached to the plasma membrane's cytoplasmic side or the endoplasmic reticulum's cytoplasmic side and the nuclear envelope's outer membrane, and called attached ribosomes (Figure \(\PageIndex{1}\)). Ribosomes are large protein and RNA complexes consisting of two subunits, a large and a small (Figure \(\PageIndex{5}\). 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.

Figure \(\PageIndex{5}\): A large subunit (top) and a small subunit (bottom) comprise ribosomes. Note that the large subunit sits on top of the small subunit. During protein synthesis, ribosomes assemble amino acids into proteins using information from a chain of mRNA. The tRNA carries amino acids to the ribosome, where the arrangement is made.

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 form following function.

Components Unique to Eukaryotic Cells

All cells contain DNA, as described above. However, plant cells, which are eukaryotic, contain organelles and a nucleus while prokaryotic cells do not possess organelles or a membrane bound nucleus. We will start by going over the structures that are unique to all eukaryotic. Next, we will go over structures unique to plant cells.

Endomembrane System

The endomembrane system (endo = “within”) is a group of membranes and organelles (Figure \(\PageIndex{6}\)) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, the tonoplast (see below), and the endoplasmic reticulum and Golgi apparatus. 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.

Visual Connection

Figure \(\PageIndex{6}\)Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The RER also sometimes modifies proteins. In this illustration, an attachment of a (purple) carbohydrate modifies a (green) integral membrane protein in the ER. Vesicles with the integral protein bud from the ER and fuse with the Golgi apparatus' cis face. As the protein passes along the Golgi’s cisternae, the addition of more carbohydrates further modifies it. After its synthesis is complete, it exits as an integral membrane protein of the vesicle that buds from the Golgi’s trans face. When the vesicle fuses with the cell membrane, the protein becomes an integral portion of that cell membrane. (credit: modification of work by Magnus Manske)

If a peripheral membrane protein were synthesized in the lumen (inside) of the ER, would it end up on the inside or outside of the plasma membrane?

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (Figure \(\PageIndex{6}\)) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. They are formed as an extension of the nuclear membrane and fold out towards the cytoplasm. The two functions of the ER take place in separate areas: the rough ER and the smooth ER, respectively.

The rough endoplasmic reticulum (RER) can be found has ribosomes along its surface, and the proteins they create are either secreted or incorporated into membranes in the cell. The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (Figure \(\PageIndex{6}\)). SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions.

Vesicles

Transport vesicles, composed of endomembrane system material, bud off the from the RER, carrying material into the Golgi Apparatus, the next component of the endomembrane system.

Golgi Apparatus

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 membranes (Figure \(\PageIndex{6}\)).

We call the side of the Golgi apparatus that is closer to the ER the cis face. The opposite side. closer to the plasma membrane, is the trans face. The transport vesicles that formed from the ER travel to the cis face of the Golgi, fuse with it, and empty their contents into the Golgi apparatus' lumen. 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 sugar molecule chains. These newly modified proteins and lipids then tag with phosphate groups or other small molecules in order to travel to their proper 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 plant cells, 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.

Cytoskeleton

The cellular skeleton is a collection of protein filaments within the cytoplasm. Microtubules are key organelles in cell division, they form the basis for cilia and flagella. Plant cells do not have cilia, which are short projections from the cell that function in movement, but the sperm cells of early diverging plants, like bryophytes and seedless vascular plants, have flagella. These are long projections that function in movement. Microtubules are also are guides for the construction of the cell wall, and cellulose fibers are parallel due to the microtubules. The movement in microtubules is based on tubulin-kinesin interactions. In contrast, the movement of microfilaments is based on actin-myosin interactions. Microfilaments guide the movement of organelles within the cell.

Mitochondria

Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making a nucleic acid called adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

Mitochondria are oval-shaped, double membrane organelles (Figure \(\PageIndex{7}\)) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

Figure \(\PageIndex{7}\): This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton; scale-bar data from Matt Russell)

Peroxisomes

Eukaryotic cells frequently have smaller vesicles including peroxisomes which, among other functions, help in photosynthesis in plant cells. In addition, many plant cells accumulate lipids as oil drops located directly in cytoplasm. 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, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Components Unique to Plant Cells

The following structures are found exclusively in plant cells and are absent in animal cells.

Cell wall

Though a cell wall is commonly found in prokaryotes and fungi as well as plants, their diversity is due to convergent evolution, not common ancestry, when it comes to these three groups of organisms. Plant cell walls are composed of cellulose are an excretion found outside the plasma membrane. They serve as a covering that provides structural support and gives shape to the cell.

Central Vacuole

The central vacuole is a large, membrane-bound structure that fills much of the plant cell. The membrane surrounding the central vacuole is called the tonoplast. 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 (Figure \(\PageIndex{8}\)). As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant's cell walls results in the 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. Lastly, central vacuoles store nutrients, accumulate ions, or become a place to store wastes.

Figure \(\PageIndex{8}\) Osmosis (from left to right) in a hypertonic environment (high salt), an isotonic environment, and a hypotonic (low salt) environment. Blue color is for the vacuole. Red arrows on the right image show turgor—combined pressure of the vacuole and the cell wall.

Plastids

Plastids are a group of storage organelle found in plants and algae. Chloroplasts are a type of plastid that store chlorophyll and other pigments for photosynthesis. Chromoplasts are plastids that store orange or yellow pigments, found in plants and fruit such as bell peppers. They are rich in carotenes and xanthophyls. Amyloplasts store starch and can be found in plants such as potato tubers, carrot roots, sweet potato roots, and grass seeds.

Chloroplasts store their pigments in interconnected sacs called thylakoids (Figure \(\PageIndex{9}\)). These sacs are often found in stacks called grana (singular granum). The fluid portion of the double membraned chloroplast is called the stroma. Because the thylakoid stores chlorophyll a, b, and accessory pigments, it is the main region for the first reaction of photosynthesis, where sunlight is used to create molecular energy. In the stroma, the products of the first reaction are used to produce organic molecules such as glucose. The combination of these reactions allow these autotrophic organisms to produce their own organic food.

The chloroplast, like the mitochondria, contains its own DNA, ribosomes, and is double membraned.

Figure \(\PageIndex{9}\): The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.

Evolution Connection- Endosymbiosis

We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why an organelle would have its own DNA and ribomosome?

The Endosymbiosis Theory explains:

Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. For example, there are microbes that produce vitamin K living inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. 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.

Attributions

Curated and authored by Kammy Algiers using the following sources:

• 4.1 Studying Cells and 4.3 Eukaryotic cells from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 2.2 Mitochondria and Chloroplasts, 2.3 Cell wall, Vacuoles, and Plasmodesmata, and 2.4 Other Parts of the Cellfrom Introduction to Botany by Alexey Shipunov (public domain)