1.8.2 Cell Organelles

Introduction

In a eukaryotic cell, the cytoplasm contains highly organized discrete structures that are specific for various cellular functions. These structures are called cell organelles. Most organelles are enclosed by membranes, but a few (like ribosomes) are not.

Definition

Cell Organelles: In a eukaryotic cell, the cytoplasm contains highly organized discrete structures which are specific for various cellular functions. These are called cell organelles.

In simple words: Cell organelles are tiny "organs" inside the cell that perform specific jobs.

Core Concept

• Endoplasmic reticulum (ER)
• Ribosomes
• Golgi complex
• Lysosomes
• Peroxisomes
• Glyoxysomes
• Vacuoles
• Mitochondria
• Chloroplasts
• Centrioles
• Nucleus

Detailed Explanation

1. Endoplasmic Reticulum (ER)

Definition: The endoplasmic reticulum is an extensive network of membranes.

• The word endoplasmic means "within the cytoplasm."
• The ER consists of a network of membranous tubules and sacs called cisternae (from Latin word cisterna, a reservoir of liquid).
• The ER membrane separates the internal compartment of the ER called the ER lumen (cavity) from the cytosol.
• There are two types of ER:
• They differ in their structure and functions.
• Most cells contain both types of ER.
• However, some cells (skeletal muscle cells) have more smooth ER, where it is called sarcoplasmic reticulum.

Smooth ER

• The smooth ER is continuous with the rough ER.
• Since ribosomes are not attached to it, it has a smooth appearance.
• The smooth ER functions in diverse metabolic processes.
• Enzymes of the smooth ER are important for the synthesis of:
• Lipids
• Oil
• Cholesterol
• Phospholipids
• Steroids (e.g., sex hormones of vertebrates and various steroid hormones of the adrenal glands)
• In the smooth ER, other enzymes help to detoxify drugs and poisons, especially in the liver cells.

Rough ER

• The rough ER has ribosomes attached to the sides facing the cytoplasm.
• It has a rough appearance under the electron microscope.
• Many types of specialized cells secrete proteins produced by ribosomes attached to the rough ER.
• Secretory proteins depart from the rough ER wrapped in membranes or vesicles that bud like bubbles from a specialized region called transitional ER.
• Vesicles in transit from one part of the cell to another are called transport vesicles.
• In addition to making secretory proteins, rough ER is a membrane factory for the cell.
• It grows in place by adding membrane proteins and phospholipids to its own membrane.
• The ER membrane expands and is transferred in the form of transport vesicles to other components of the endomembrane system.

2. Ribosomes

Definition: Ribosomes are the particles made of ribosomal RNA and protein. These organelles carry out protein synthesis.

• Ribosomes were first observed using an electron microscope as dense granules.
• Ribosomes are roughly spherical, granular, non-membranous bodies.
• They are found in both eukaryotic and prokaryotic cells.
• However:
• Eukaryotic ribosomes are larger and characterized as 80S ribosomes.
• Prokaryotic ribosomes are slightly smaller and characterized as 70S ribosomes.
• They can be seen only under the electron microscope.
• They are made of almost an equal amount of RNA and protein, so they are ribonucleoprotein.
• Ribosomes are formed in the nucleus.
• Then they are transported to the cytoplasm through the nuclear pore.

• The eukaryotic ribosomes are composed of two subunits (particles) of different sizes:
• The larger one is the 60S particle.
• The smaller one is the 40S particle.
• The two subunits on attachment form the 80S particle.
• The attachment is controlled by:
• Presence of magnesium ions concentration
• Formation of salt bonds between the phosphate group of RNA and the amino group of amino acid
• Both by magnesium ions and salt bonds
• Both ribosomal subunits are generally attached together at the time of their function.

• Sometimes, during protein synthesis, several ribosomes are attached to one mRNA molecule.
• Such a chain of many ribosomes is called a polysome or polyribosomes.
• In this way, several copies of the same polypeptide can be produced in very little time.

3. Golgi Complex

Definition: The Golgi complex consists of a stack of flattened, membrane-bound sacs called cisternae, together with a system of associated vesicles called Golgi vesicles.

• It is found in all eukaryotic cells.
• It was discovered by Italian biologist Camillo Golgi in 1898.
• It is also called Golgi apparatus and Golgi bodies.
• It is a complex system of interconnected tubules formed around the central stack.
• At one end of the stack, new cisternae are constantly being formed by the fusion of vesicles from the smooth ER. This outer or forming face is called the cis face (convex).
• The inner end is concave and is called the maturing face (trans face), where the cisternae break up into vesicles again.

• The most important function is the processing of cell secretions.
• Therefore, these organelles are abundant in secretory cells.
• The cell secretions mainly consist of proteins.
• Golgi complex collects these proteins from RER through SER, modifies them to perform specific functions, and then exports these modified products in the form of vesicles.
• Certain organelles, such as lysosomes, peroxisomes, and glyoxysomes, also originate from the Golgi complex.
• Golgi complex is also involved in the formation of conjugated molecules like glycoprotein, lipoprotein, etc.
• In plant cells during cell division, Golgi complex also gives rise to vesicles which contain cell wall synthesizing materials.
• At cytokinesis, these Golgi vesicles are arranged on the cell equator, fuse together, and form a structure called phagoplast. Later on, a new cell wall is derived from this structure.

4. Vesicles

Definition: The word 'vesicle' derives from the Latin word vesicula meaning 'small bladder'. Vesicles are small cell organelles that are present in cells.

• These organelles are small, membrane-enclosed sacs.
• A vesicle is a small structure within a cell, consisting of fluid enclosed by a lipid bilayer.
• A vesicle consists of a hydrophilic head and a hydrophobic tail.
• Many vesicles are made in the Golgi complex and the endoplasmic reticulum, or are made from parts of the cell membrane by endocytosis.
• Because vesicles are made of phospholipids, they can break off and fuse with other membranous material.
• Examples of vesicles include:
• Secretory vesicles
• Transport vesicles
• Synaptic vesicles
• Lysosomes
• Vesicles are found in bacteria, Archaea, and plants as well as in animals.

• They transport substances to and from one cell to another and also from one part of a cell to another (e.g., proteins and other molecules).
• The first step in vesicular transport is the formation of a vesicle by budding from the membrane.
• Vesicles are also involved in metabolism, buoyancy control, and enzyme storage.
• They can also act as chemical reaction chambers.

5. Lysosomes

Definition: Lysosomes are single membranous, spherical vesicles. They contain digestive or hydrolytic enzymes.

• The lysosomal enzymes are made by the RER and then transported to the Golgi complex through SER.
• After modification, these enzymes are released from the trans face of the Golgi complex in the form of vesicles.
• Such vesicles are called lysosomes.
• The newly formed lysosomes before the start of their functions are usually called primary lysosomes.
• In plants and fungi, certain vacuoles carry out enzymatic hydrolysis, a function shared by lysosomes in animal cells.
• Lysosomes contain about 40 different digestive enzymes.
• These enzymes can break down every major macromolecule of the cell.
• The contents of the lysosome are acidic.
• In order to perform its function, the lysosomes fuse with membrane-bound vesicles that arise from any of these pathways:
• Endocytosis
• Phagocytosis
• Autophagocytosis

• The key function of lysosomes is digestion and removal of waste.
• Foreign particles are pulled into the cell through the process of endocytosis.
• The process of endocytosis happens when the cell membrane falls in on itself (invagination), creating a vacuole or a pouch around the external contents and then bringing those contents into the cell.
• The process by which unwanted structures within the cell are engulfed and digested within the lysosomes is called autophagy.
• This is a self-eating process of a cell in which a lysosome begins to digest the cell's own organelles.
• Such lysosomes are also called autophagosomes.
• This process either takes place during starvation periods in order to obtain energy, or it occurs routinely in order to control the number of specific organelles.
• Sometimes, especially during the developmental phase, when a particular cell is required to be disintegrated, a type of cell death is committed, called autolysis.
• This is a programmed cell death in which lysosomes burst and their enzyme contents are quickly dispersed throughout the cytoplasm.
• In this way, the cell is disintegrated into fragments which are phagocytosed by other cells.
• Due to this function, lysosomes are also called suicidal bags.

6. Peroxisomes and Glyoxysomes

• The peroxisome is a small vesicle, single membrane-bound organelle.
• It is an oxidative organelle that is not part of the endomembrane system.
• Like mitochondria and chloroplast, it imports its proteins primarily from the cytosol.
• Peroxisomes can vary in shape and size, depending on the needs of the cell they serve.
• They will sometimes increase in number and size if, for example, they have a lot of alcohol to break down.
• Peroxisomes and glyoxysomes are collectively called microbodies.
• These are similar to lysosomes in the sense that they are single membranous, vesicular structures.
• They contain enzymes (although different from lysosome) and originate from the Golgi complex, but they are smaller than lysosomes.
• Peroxisomes contain some oxidative enzymes like:
• Peroxidases
• Catalases
• Glycolic acid oxidases
• They are abundant in liver cells where they are specifically involved in the formation and decomposition of hydrogen peroxide, so they are named peroxisomes.
• They are mainly concerned with the detoxification of alcohol.
• In this activity, alcohol is oxidized into hydrogen peroxide (H₂O₂) with the help of peroxidase enzyme.
• Hydrogen peroxide is itself a toxic molecule, which is immediately broken down to water and oxygen by another enzyme called catalase.
• In plant cells, peroxisomes are involved in photorespiration.
• A step of photorespiration takes place in peroxisomes in which glycolate is converted into glycine with the help of an enzyme called glycolic acid oxidase.

• Specialized peroxisomes are called glyoxysomes.
• Glyoxysomes are found only at the seedling stage in oil seed plants.
• These organelles have a number of enzymes specific for plant lipid metabolism that are not found in animal cells.
• The germinating seedlings convert stored fatty acids to carbohydrates.
• This is achieved through a metabolic pathway called the glyoxylate cycle, the enzymes of which are located in the glyoxysomes.

7. Vacuoles

Definition: Vacuoles are large vesicles that originate from the endoplasmic reticulum, Golgi complex, and plasma membrane.

• Vacuoles perform a variety of functions in different kinds of cells.
• In animal cells, food vacuoles are formed by phagocytosis.
• Many freshwater protists have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell.
• In young plant cells, many small vacuoles are present which can hold reserves of important organic compounds.
• These vacuoles may also help in the protection of plants against herbivores by storing compounds that are poisonous or unpleasant to animals.
• Mature plant cells generally contain a large central vacuole which develops by the joining of smaller vacuoles.
• The solution inside the central vacuole, called cell sap, is the plant cell's main reservoir of inorganic ions, including potassium and chloride.
• The central vacuole plays a major role in mechanical support by maintaining turgor and also acts as a storehouse of the cell.
• The membrane separating the vacuole from the cytoplasm is called the tonoplast.

8. Mitochondria

Definition: Mitochondria (singular: mitochondrion) are present in all eukaryotic cells.

• Some cells have a single large mitochondrion, but more often a cell has hundreds or even thousands of mitochondria.
• The number correlates with the cell's level of metabolic activity.
• For example, cells that move or contract have proportionally more mitochondria per volume than less active cells.
• Mitochondria are capable of dividing themselves (self-replicating) in order to increase their number. They divide by fission.

• Mitochondria are cylindrical or rod-shaped structures.
• They are enclosed by a double membrane:

• A small circular DNA
• All kinds of RNA
• Ribosomes (70S)
• Enzymes
• The presence of these components indicates that mitochondria have their own genetic system.
• This means the proteins required by mitochondria are synthesized by their own metabolic machinery.

• Mitochondria are the sites of cellular respiration.
• Cellular respiration is the metabolic process that uses oxygen to generate ATP by extracting energy from sugars, fats, and other organic compounds.
• Enzymes in the matrix catalyze some of the steps of cellular respiration like the Krebs cycle.
• Other proteins that function in ATP generation through the electron transport chain are found in the inner membrane.

• Mitochondria and chloroplasts display similarities with bacteria:
• Both are self-replicating organelles.
• Both have their own genetic system and metabolic machinery (small circular DNA, all kinds of RNA, and 70S ribosomes).
• They are capable of surviving outside the cell in an artificial medium if carefully fractionated.
• Based upon these observations, evolutionists believe that they were independent organisms and the early ancestor of eukaryotic cells engulfed them.
• Eventually, the engulfed cells formed a relationship with the host cell in which they were enclosed, becoming an endosymbiont (a cell living within another cell).
• Therefore, they are supposed as organisms within organism.
• Mitochondria divide and in this way their number doubles before cell division.
• Lysosomes regulate the number of mitochondria. Excess mitochondria are digested by lysosomes.
• Because mitochondria are contained within ova (egg cells) but not within the heads of sperm cells, all the mitochondria in a fertilized egg are derived from the mother.

9. Plastids

• Proplastids – Young, immature, and developing plastids. They are self-replicating organelles.
• Leucoplasts – Colourless, found in parenchyma cells of root, stem, and seeds.
• Chromoplasts – Synthesize different coloured pigments other than green.
• Chloroplasts – Found in green parts of the plants and act as the site of photosynthesis.

• Chloroplast is a discoid structure which consists of three parts:
• Each chloroplast is bounded by a smooth double membrane (envelope).
• The outer membrane, like mitochondria, contains porins and therefore is freely permeable to small molecules.
• The inner membrane is semipermeable and rich in protein.
• Between the outer and inner membrane, there is an intermembrane space.
• The ground mass of chloroplast is called stroma.
• It is the colourless proteinaceous substance which, like the mitochondrial matrix, also contains:
• A small circular DNA
• All kinds of RNA
• Ribosomes (70S)
• Various enzymes
• The stroma contains a system of chlorophyll-bearing, double membrane, flattened sac-like structures called thylakoids.
• There are two types of thylakoids:

• Chloroplast is the site of photosynthesis in a plant cell.
• The first phase of photosynthesis is the light-dependent reaction in which sunlight is captured and transformed into ATP. This phase takes place in the grana region of the chloroplast.
• The second phase of photosynthesis is the light-independent reaction (dark reaction) in which CO₂ is reduced to make carbohydrates. The enzymes for this activity are found in the stroma region of the chloroplast.

10. Centrioles

Definition: Centrioles are non-membranous cell organelles found mainly in animal cells.

• They are also found in fungi-like protists such as slime molds and water molds.
• Centrioles are rod-shaped structures and usually occur in pairs.
• These occur at right angles to each other near one pole of the nucleus.
• Each centriole is composed of nine triplets of microtubules which are circularly arranged around a central axis.

• Just before cell division, the pair of centrioles duplicates and becomes two pairs which later migrate to the opposite sides of the nucleus.
• Both centriole pairs give rise to microtubules (spindle fibres) during cell division.
• The whole structure of spindle fibres is known as the mitotic apparatus, which helps in the distribution of chromosomes between the daughter cells during cell division.
• In addition, centrioles also give rise to basal bodies of cilia and flagella.

11. Nucleus

Definition: The nucleus is the most prominent and the most important part of a cell.

• In animal cells, it is found in the centre (with the exception of muscle fibre cells).
• In adult plant cells, it is slightly away from the centre due to the presence of a large central vacuole.
• A typical eukaryotic nucleus consists of:

Nuclear Envelope

• Also called the nuclear membrane.
• It is a double membrane covering which makes the boundary of the nucleus.
• Both membranes of the nuclear envelope are separated by a fluid-filled perinuclear space.
• The membranes are composed of lipid bilayer and proteins.
• The outer membrane of the nuclear envelope is covered with ribosomes and is connected with the membranes of the ER.
• There are numerous pores in the nuclear envelope called nuclear pores, which are composed of a specialized transport protein called nucleoporin.
• At the point of the nuclear pore, both membranes are interconnected.
• These pores regulate the nucleo-cytoplasmic exchange of materials.
• This exchange includes:
• RNA and ribosomal proteins moving from the nucleus to the cytoplasm.
• Proteins (such as DNA polymerase), carbohydrates, signaling molecules, and lipids moving into the nucleus.
• Although smaller molecules simply diffuse through the pores, larger molecules may be recognized by specific signal sequences and then be diffused with the help of nucleoporin into or out of the nucleus.

Nucleoplasm

• Nucleoplasm is the transparent semifluid ground substance formed of a mixture of:
• Proteins
• Enzymes (DNA and RNA polymerase)
• Free nucleotides
• Some metal ions (Mg²⁺) for the synthesis of DNA and RNAs
• It also contains histone and non-histone proteins.
• So the nucleoplasm is slightly different from the cytoplasm.

Nucleolus

• Nucleolus is a non-membrane bound structure in the nucleoplasm.
• A cell may have one or more nucleoli.
• Nucleolus appears during interphase and disappears during cell division.
• A nucleolus consists of:
• A peripheral granular area (contains ribosomal subunits)
• A central fibrillar area (contains rRNA and rDNA)
• Therefore, the nucleolus is involved in the construction of ribosomes.

Chromatin and Chromosomes

• Chromatin is a network of thin thread-like structures made up of DNA and proteins.
• During cell division, chromatin fibres begin to condense and coil up into separate structures called chromosomes, which are thick enough to be seen under a light microscope.
• A typical chromosome consists of two strands called chromatids, which are attached to each other at a point called the centromere.
• The centromere is the region where the two chromatids are held together.
• The kinetochore is a protein structure on the centromere where spindle fibres attach during cell division.

Intermediate Level

Classification of Organelles by Membrane

Organelles and Their Functions

Endomembrane System

The endomembrane system is a group of membranes and organelles that work together to modify, package, and transport lipids and proteins. It includes:

• Nuclear envelope
• Endoplasmic reticulum (ER)
• Golgi apparatus
• Lysosomes
• Vacuoles
• Vesicles

Note: Mitochondria and chloroplasts are not part of the endomembrane system because they have their own DNA and are thought to have originated from ancient bacteria (endosymbiotic theory).

Protein Trafficking Pathway

• Proteins are synthesized on ribosomes attached to the rough ER.
• Proteins enter the ER lumen for folding and modification.
• Proteins are transported in vesicles to the Golgi complex.
• Proteins are further modified in the Golgi complex.
• Proteins are packaged into vesicles and transported to their final destination:

Important Points

• 1. Cell organelles are highly organized discrete structures in the cytoplasm that perform specific functions.
• 2. Most organelles are enclosed by membranes, except ribosomes.
• 3. Endoplasmic reticulum (ER):
• - Network of membranous tubules and sacs (cisternae).
• - Smooth ER: No ribosomes; lipid synthesis, detoxification.
• - Rough ER: Ribosomes attached; protein synthesis, membrane factory.
• 4. Ribosomes:
• - Made of rRNA and protein.
• - Site of protein synthesis.
• - Eukaryotic: 80S (60S + 40S).
• - Prokaryotic: 70S (50S + 30S).
• - Polysome: Chain of ribosomes on one mRNA.
• 5. Golgi complex:
• - Stack of flattened sacs (cisternae).
• - Cis face: Forming face (convex).
• - Trans face: Maturing face (concave).
• - Processing and packaging of cell secretions.
• 6. Vesicles:
• - Small, membrane-enclosed sacs.
• - Transport substances.
• 7. Lysosomes:
• - Single membrane-bound.
• - Contain digestive enzymes.
• - Suicidal bags (autolysis).
• - Autophagy: Self-eating.
• 8. Peroxisomes:
• - Single membrane-bound.
• - Contain oxidative enzymes.
• - Detoxification of alcohol.
• - Glyoxysomes: Specialized peroxisomes in oil seed seedlings.
• 9. Vacuoles:
• - Large vesicles.
• - Plant cells: Large central vacuole; cell sap; tonoplast.
• - Animal cells: Small, temporary vacuoles.
• - Contractile vacuoles: Pump excess water out.
• 10. Mitochondria:
• - Double membrane.
• - Outer membrane: Smooth, porins.
• - Inner membrane: Folded into cristae; ATP synthase (FO-F1 particles).
• - Matrix: Circular DNA, RNA, 70S ribosomes, enzymes.
• - Site of cellular respiration; ATP production.
• 11. Plastids:
• - Proplastids: Immature.
• - Leucoplasts: Colourless.
• - Chromoplasts: Coloured.
• - Chloroplasts: Green; site of photosynthesis.
• - Chloroplast structure: Envelope, stroma, thylakoids (grana, intergrana).
• 12. Centrioles:
• - Non-membranous.
• - Rod-shaped; occur in pairs.
• - Nine triplets of microtubules.
• - Form spindle fibres during cell division.
• 13. Nucleus:
• - Most prominent part of the cell.
• - Nuclear envelope: Double membrane; nuclear pores.
• - Nucleoplasm: Contains enzymes, nucleotides, ions, histone and non-histone proteins.
• - Nucleolus: Makes ribosomes.
• - Chromatin: DNA + proteins; condenses into chromosomes.

Remember

• - Organelles: ER, ribosomes, Golgi, vesicles, lysosomes, peroxisomes, glyoxysomes, vacuoles, mitochondria, chloroplasts, centrioles, nucleus
• - Single membrane: Lysosomes, peroxisomes, glyoxysomes, vacuoles, vesicles, Golgi, ER
• - Double membrane: Nucleus, mitochondria, chloroplast
• - No membrane: Ribosomes, centrioles
• - Ribosomes: Protein synthesis; 80S (eukaryotic), 70S (prokaryotic)
• - Golgi: Cis face (forming), trans face (maturing)
• - Lysosomes: Suicidal bags; autophagy
• - Peroxisomes: Detoxification; catalase
• - Glyoxysomes: Fatty acids → carbohydrates in seedlings
• - Vacuoles: Storage; tonoplast; cell sap
• - Mitochondria: Powerhouse; cristae; FO-F1 particles
• - Chloroplast: Photosynthesis; grana; stroma
• - Centrioles: Cell division; spindle fibres
• - Nucleus: Control center; nuclear pores; nucleolus; chromatin

Common Mistakes

• 1. Confusing smooth ER and rough ER – Smooth ER has no ribosomes (lipid synthesis); rough ER has ribosomes (protein synthesis).
• 2. Thinking ribosomes are membrane-bound – Ribosomes have no membrane.
• 3. Confusing the cis and trans faces of the Golgi – Cis face is the forming face (convex); trans face is the maturing face (concave).
• 4. Thinking lysosomes are formed by the ER directly – Lysosomes are formed by the Golgi complex.
• 5. Confusing peroxisomes and lysosomes – Peroxisomes contain oxidative enzymes; lysosomes contain digestive enzymes.
• 6. Thinking glyoxysomes are found in all plants – Glyoxysomes are found only at the seedling stage in oil seed plants.
• 7. Forgetting the tonoplast – The tonoplast is the membrane surrounding the vacuole.
• 8. Confusing the outer and inner membranes of mitochondria – Outer is smooth with porins; inner is folded into cristae with ATP synthase.
• 9. Thinking chloroplasts are in all plant cells – Chloroplasts are found in green parts of plants.
• 10. Confusing grana and stroma – Grana are stacks of thylakoids (light reaction); stroma is the ground substance (dark reaction).
• 11. Thinking centrioles are membrane-bound – Centrioles have no membrane.
• 12. Confusing nucleolus and nucleus – Nucleolus is a structure inside the nucleus; nucleus is the whole organelle.
• 13. Thinking chromatin and chromosomes are different things – Chromatin is the thread-like form; chromosomes are the condensed form during cell division.
• 14. Forgetting the functions of the nucleus – Control center, gene expression, ribosome assembly, cell division.

Quick Revision

• Cell organelles are highly organized discrete structures in the cytoplasm that perform specific functions.
• Most organelles are enclosed by membranes, except ribosomes.
• Endoplasmic reticulum (ER):
• Network of membranous tubules and sacs (cisternae).
• Smooth ER: No ribosomes; lipid synthesis, detoxification.
• Rough ER: Ribosomes attached; protein synthesis, membrane factory.
• Ribosomes:
• Made of rRNA and protein.
• Site of protein synthesis.
• Eukaryotic: 80S (60S + 40S).
• Prokaryotic: 70S (50S + 30S).
• Polysome: Chain of ribosomes on one mRNA.
• Golgi complex:
• Stack of flattened sacs (cisternae).
• Cis face: Forming face (convex).
• Trans face: Maturing face (concave).
• Processing and packaging of cell secretions.
• Vesicles:
• Small, membrane-enclosed sacs.
• Transport substances.
• Lysosomes:
• Single membrane-bound.
• Contain digestive enzymes.
• Suicidal bags (autolysis).
• Autophagy: Self-eating.
• Peroxisomes:
• Single membrane-bound.
• Contain oxidative enzymes.
• Detoxification of alcohol.
• Glyoxysomes: Specialized peroxisomes in oil seed seedlings.
• Vacuoles:
• Large vesicles.
• Plant cells: Large central vacuole; cell sap; tonoplast.
• Animal cells: Small, temporary vacuoles.
• Contractile vacuoles: Pump excess water out.
• Mitochondria:
• Double membrane.
• Outer membrane: Smooth, porins.
• Inner membrane: Folded into cristae; ATP synthase (FO-F1 particles).
• Matrix: Circular DNA, RNA, 70S ribosomes, enzymes.
• Site of cellular respiration; ATP production.
• Plastids:
• Proplastids: Immature.
• Leucoplasts: Colourless.
• Chromoplasts: Coloured.
• Chloroplasts: Green; site of photosynthesis.
• Chloroplast structure: Envelope, stroma, thylakoids (grana, intergrana).
• Centrioles:
• Non-membranous.
• Rod-shaped; occur in pairs.
• Nine triplets of microtubules.
• Form spindle fibres during cell division.
• Nucleus:
• Most prominent part of the cell.
• Nuclear envelope: Double membrane; nuclear pores.
• Nucleoplasm: Contains enzymes, nucleotides, ions, histone and non-histone proteins.
• Nucleolus: Makes ribosomes.
• Chromatin: DNA + proteins; condenses into chromosomes.

60-Second Revision

• Organelles: ER, ribosomes, Golgi, vesicles, lysosomes, peroxisomes, glyoxysomes, vacuoles, mitochondria, chloroplasts, centrioles, nucleus
• Single membrane: Lysosomes, peroxisomes, glyoxysomes, vacuoles, vesicles, Golgi, ER
• Double membrane: Nucleus, mitochondria, chloroplast
• No membrane: Ribosomes, centrioles
• Ribosomes: Protein synthesis; 80S (eukaryotic), 70S (prokaryotic)
• Golgi: Cis face (forming), trans face (maturing)
• Lysosomes: Suicidal bags; autophagy
• Peroxisomes: Detoxification; catalase
• Glyoxysomes: Fatty acids → carbohydrates in seedlings
• Vacuoles: Storage; tonoplast; cell sap
• Mitochondria: Powerhouse; cristae; FO-F1 particles
• Chloroplast: Photosynthesis; grana; stroma
• Centrioles: Cell division; spindle fibres
• Nucleus: Control center; nuclear pores; nucleolus; chromatin
• Warning: Ribosomes have no membrane. Lysosomes are formed by the Golgi, not the ER. Glyoxysomes are only in oil seed seedlings. Centrioles have no membrane. Chloroplasts are only in green parts of plants.