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Plant Cell Organelles

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Plant Cell Organelles

Plant and animal cells, along with fungi and protists cells, present all the typical features of eukaryotic cells. However, plants have some exclusive organelles and structures related to their physiology and ecology. For example, unlike animals, plants cannot move and have specialized organelles that help them produce their own food. Have you ever wondered where the crunchiness of celery, carrots, or apples come from? In the following, you will learn that and much more.

Organelles in plant and animal cells

Plants have all the typical features of eukaryotic cells: plasma membrane, cytoplasm, nucleus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and cytoskeleton (Figure 1).

You can go over our Eukaryotic Cells article for a quick review of the table comparing animal and plant cells.

Despite all these common components, plant and animal cells have some exclusive organelles that differentiate them:

  • Animal cell: Lysosomes (organelles that digest macromolecules), and centrioles (cylinders of microtubules in the centrosome, involved in cellular division).
  • Plant cell: Vacuoles (membrane-bounded vesicles with varied functions), plastids (organelles with diverse functions including photosynthesis), and cell wall (protective layer, covering the outside of the plasma membrane).

Plant cell organelles diagram

Figure 1 below shows a generalized plant cell with its characteristic organelles and structures labeled, highlighting the organelles exclusively found in plant cells:

Plant Cell Organelles Diagram of plant cell components StudySmarter

Figure 1. Diagram of a generalized plant cell and its components. Exclusive components of plant cells are enclosed in red boxes. Source: Modified from OpenStax.

Plant cell organelles and their functions

We will discuss the structure and function of vacuoles, plastids, and the cell wall. Technically, a cell wall is not an organelle, but we include it here as it is an important and distinctive structure in plant cells.

Vacuoles

Vacuoles are abundant in plants and fungi, and have diverse functions. They are membranous sacs, similar to vesicles in structure, and sometimes these terms are used interchangeably. In general, vacuoles are larger (they are formed by the fusion of several vesicles) and can persist longer than vesicles. The bilayer membrane that delimits a vacuole is called the tonoplast. Vacuoles are mainly formed by the fusion of vesicles from the trans side of the Golgi apparatus (the one facing the plasma membrane) and are, therefore, part of the endomembrane system.

Depending on the tissue or organ, they will perform different functions and a cell can have several vacuoles with different functions:

  • They perform most of the lysosome’s functions in plant and fungi cells. Thus, they contain hydrolytic enzymes.
  • In mature plants cells, small vacuoles fuse to form a larger central vacuole. Plant cells grow mainly by adding water to this vacuole (comprising up to 80% of a cell’s volume). When the central vacuole is full, it exerts hydrostatic pressure against the cell wall. This pressure is important in plants, as it gives mechanical support to the cell when they are swollen or turgid. When you forget to water a plant, it becomes flaccid because there is no hydrostatic pressure against the wall. The central vacuole also serves as a reservoir of inorganic ions, maintaining the balance of pH in the cytoplasm.
  • Storage of nutritious molecules in seeds and pigments in flowers. They can also store toxic or unpalatable compounds used against herbivores (animals that eat plants).
  • Waste products and toxic compounds for the cell (like heavy metals absorbed from the soil) are also stored away by vacuoles.

Some protists form food vacuoles through phagocytosis, and others that live in freshwater have contractile vacuoles to expel the excess of water.

Plastids

Plastids are a group of organelles that produce and store nutritious molecules and pigments (molecules that absorb visible light at specific waves) in plant and algae cells (figure 2). They are present in the cytoplasm of different types of cells, surrounded by a double phospholipid bilayer membrane, and have their own DNA. They have specialized tasks depending on the cell function. They are very versatile and can change functions during cell life and some have specialized functions. We focus on three main groups of plastids:

  • Chromoplasts produce and store carotenoid pigments (a range of yellow, orange, and red colors) that give flowers and fruits their characteristic color. Coloration in plants serves to attract pollinators.
  • Leucoplasts lack pigments, thus, are more common in non-photosynthetic tissues. They store nutrients in cells of seeds, roots, and tubers. Amyloplasts convert glucose to starch for storage (Figure 2B). They are present mainly in specialized tissues of seeds, roots, tubers, and fruits. Proteinoplasts (or aleuroplasts) store proteins in seeds. Elaioplasts synthesize and store lipids.
  • Chloroplasts perform photosynthesis, transferring energy from the sunlight into ATP molecules which are used to synthesize glucose. The inner membrane encloses numerous piles of interconnected fluid-filled membranous discs called thylakoids. Thylakoids contain several pigments incorporated into their membrane. Chlorophyll is the more abundant and the main pigment that captures the energy from sunlight (Figure 2A).

Chloroplasts structure and function, and their origin, are described in more detail in the Mitochondria and Chloroplasts article.

Plant Cell Organelles Cells with chloroplasts and amyloplasts StudySmarter

Figure 2: A) Photosynthetic cells containing numerous oval-shaped chloroplasts. Source: HermannSchachner, CC0, via Wikimedia Commons. B) Amyloplasts containing starch granules. Source: Krishna satya 333, CC0, via Wikimedia Commons.

Cell wall

Plant cells, along with fungi and some protists cells, have an external cell wall covering their plasma membrane (Figure 3). This wall protects the cell, gives structural support, and maintains the shape of the cell, thus preventing excess water uptake. In plants, the wall is made up of polysaccharides and glycoproteins. The exact composition of the wall depends on the plant species and the type of cell, but the main component is the polysaccharide cellulose (made up of glucose forming long, straight chains of up to 500 molecules). Other polysaccharides found in cell walls are hemicellulose and pectin.ç

Structurally, the cell wall is composed of cellulose fibers and hemicellulose molecules embedded in a pectin matrix. The different types of plant cells can be identified by the characteristics of their cell wall.

Cell walls from adjacent cells are glued by another layer of pectin (sticky polysaccharides, like the ones we eat in jelly) called the middle lamella. The components of the wall can be replaced if degraded or during cell growth. In some cells, the wall can become completely rigid when its composition changes and the cell stops growing.

Plant Cell Organelles Cell wall diagram StudySmarter

Figure 3: Cell wall diagrams. Left: cell walls, middle lamella and plasmodesmata for two adjacent cells. Source: Modified from Jackacon (vectorised by Smartse), Public domain, via Wikimedia Commons. Right: a typical cell wall composition. Source: LadyofHats, Public domain, via Wikimedia Commons.

The cell wall is responsible for the rigidity of plants and for keeping them upright. This results from the hydrostatic pressure from the central vacuole against the wall, as mentioned above. This is, in part, what gives them their crunchiness when we eat celery or a carrot, for example.

Plant cells still need to communicate with each other, even with a stiff cell wall. Channels called plasmodesmata allow direct communication between the cytoplasm of neighboring cells. The plasma membrane between neighboring cells is continuous along these channels, thus cells are not completely separated by their plasma membranes.

All plant cells have a cell wall and the thin middle lamella surrounding them. Plant cells specialized in support, and some involved in sap transport, produce a secondary cell wall that forms the wood in trees and other woody plants. Because of the rigidity of secondary cell walls and the impossibility to communicate, the cells inside die. Thus, the functions of resistance and transport in these cells are only accomplished when they die.

Plant cell organelles and structures: is there a difference?

Here, we have referred to plant cell organelles and structures. The term organelle is widely used for almost any cellular structure, and this can be confusing sometimes.

A commonly accepted definition of organelle is a membrane delimited structure with a specific cellular function. Thus, all organelles are cellular structures, but not all cell structures are organelles. Most of the time, being delimited by a membrane seems to be a requirement to consider a cellular structure an organelle.

The cellular structures that are most commonly called organelles are intracellular (they are embedded in the cytosol) and membrane-bounded. So, we would commonly include the following as organelles in a plant cell:

  • nucleus,
  • mitochondria,
  • endoplasmic reticulum,
  • Golgi apparatus,
  • mitochondria,
  • peroxisomes,
  • vacuoles, and
  • chloroplasts (plastids in general).

Plant cell structures not delimited by a membrane are usually called structures or components in general, such as:

Thus, cellular structures can be inside or outside of the cell (the plasma membrane is a membrane that delimits the cell, but it is not membrane-bounded itself). The ribosome is typically called an organelle, but some authors are more specific and call them non membrane-bounded organelles.

In summary, depending on the author, the terms organelle and structure are normally interchangeable, and it is ok. The important thing is to know the structure and function of a cellular component and be able to classify them depending on a specific definition.

List of plant cell organelles and structures

The table below provides a list of plant cell organelles and structures with a summary of their function:

Table 1: summary of plant cell organelles and structures and their general function.

Feature

General function

Nucleus (nuclear membrane, nucleolus, chromosomes)

Encloses the DNA, transcribes the information from DNA to RNA (specifications for protein synthesis), and is involved in ribosome production

Plasma membrane

The outer layer that separates the interior of the cell from the exterior, it interacts with internal membranes

Cytoplasmic organelles

Ribosomes

Structures that build proteins

Endomembrane System

Endoplasmic reticulum (smooth and rough regions)

Synthesis of proteins and lipids, modification of proteins, generates vesicles for intracellular transportation

Golgi apparatus

Synthesis, modification, secretion, and packaging of cell products

Vacuoles

Diverse functions in storage, macromolecules hydrolysis, waste disposal, plant growth by vacuole enlargement

Peroxisomes

Degradation of small organic molecules. Produces hydrogen peroxide as a by-product, converting it into water

Mitochondria

Performs cellular respiration, generates most of cellular ATP

Chloroplasts

Performs photosynthesis, converting sunlight energy into chemical energy. Belong to a group of organelles called plastids.

Cytoskeleton: Microtubules, microfilaments, intermediate filaments, flagella

Structural support, maintains cell’s shape, involved in cell movement and motility (flagella are present in sperm cells of plants, except for conifers and angiosperms).

Cell wall

Surrounds the plasma membrane and protects the cell, maintains cell’s shape

Plant Cell Organelles - Key takeaways

  • Plants have all the typical features of eukaryotic cells: plasma membrane, cytoplasm, nucleus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and cytoskeleton.
  • Exclusive organelles and structures of plant cells compared to animal cells are vacuoles (including a large central vacuole), plastids, and cell walls.
  • Vacuoles are membrane-bound organelles with a variety of functions (digestion, storing, maintenance of hydrostatic pressure, maintenance of cytoplasm pH balance).
  • Plastids are a group of organelles with a diverse set of functions: photosynthesis, amino acid and lipid synthesis, storage of lipids, carbohydrates, proteins, and pigments.
  • Chloroplasts are a type of plastids that contains chlorophyll and perform photosynthesis (transferring energy from the sunlight into energetic molecules which are used to synthesize glucose).
  • The cell wall gives protection, structural support, and maintains the shape of the cell preventing excess water uptake.

Frequently Asked Questions about Plant Cell Organelles

The typical organelles of eukaryotic cells are found in plant cells (plasma membrane, cytoplasm, nucleus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and cytoskeleton). Besides they have vacuoles, plastids, and cell walls, exclusive of plant cells. 

Chloroplasts (plastids in general) and mitochondria contain their own DNA and ribosomes.

Chloroplasts use light energy to produce sugar through photosynthesis in plants.

The central vacuole is the largest organelle in mature plant cells comprising up to 80% of a cell’s volume.

Lysosomes and centrioles are exclusive to animal cells and are absent in plant cells.

Final Plant Cell Organelles Quiz

Question

What distinctive structure does a plant cell have to avoid excessive uptake of water?

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Answer

cell wall

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Question

Which of the following is the more common component of cell walls?

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Answer

cellulose

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Question

What are plasmodesmata?

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Answer

channels through the cell wall that allow intercellular communication

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Question

How can you know if the cell you are looking at under a microscope belongs to a plant?

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Answer

absence of centrioles

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Question

Where can you find the middle lamella?

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Answer

surrounding the cell wall

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Question

What function is similar in plastids and some vacuoles?

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Answer

storage of nutritious molecules and pigment 

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Question

Where can you find cellulose?

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Answer

cell wall

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Question

Which of the following is true for vacuoles?

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Answer

they can store molecules and compounds

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Question

True or false: a cell wall prevents a cell from growing. 


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Answer

False

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Question

Which of the following are involved in the hydrostatic pressure that maintains a plant cell turgid (swollen)?

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Answer

cell wall

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Question

Which organelle can store waste products and toxic compounds away from the cytoplasm in a plant?

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Answer

vacuoles

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Question

The combination of chloroplasts and cell walls is only found in plant cells. 


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Answer

False, algae (protists) also have chloroplasts and cell walls. 

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Question

Which plastids lack pigments and function mainly as storage organelles?

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Answer

leucoplasts

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Question

Which structures allow direct communication between adjacent plant cells?

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Answer

plasmodesmata

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Question

Where can you find DNA in a plant cell?

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Answer

nucleus

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