Cell Organelles: ER, Golgi, and Lysosomes

• Robert Hooke, while observing a cork slice in 1665, noticed small compartments resembling a honeycomb structure and named them cells, derived from the Latin term for "a little room".
• This observation marked the first recognition that living organisms consist of distinct, separate units.

What are Living Organisms Made Up of?

• Epidermis, the outer layer, can be peeled off an onion bulb for observation.
• An onion peel on a glass slide with a drop of water must remain flat.
• Safranin stains and a coverslip are needed, being careful to avoid air bubbles, to construct an onion peel.
• Temporary onion peel mounts of various sizes reveal comparable tiny structures under magnification, regardless of origin size.
• The fundamental building block of an onion bulb are the observed microscopic structures; these structures are called cells.
• Cells compose all organisms that surround us; single cells exist independently, and these organisms are called unicellular, like Chlamydomonas, Paramecium, and Bacteria.
• Multicellular organisms, such as specific fungus, plants, and animals, consist of numerous cells aggregated into a single body which performs diverse activities.
• Every multicellular organism originates from one cell via cell division, which results in cells of comparable type, indicating that all cells originate from pre-existing cells.
• Observations of leaf peels, onion root tips, and peels from multiple-sized onions allow comparisons regarding cell shape, size, structure, and differences among plant components.

Cell History

• Robert Hooke discovered cells in 1665 using a primitive microscope.
• Antonie van Leeuwenhoek discovered free-living cells in pond water using an enhanced microscope in 1674.
• Robert Brown found the nucleus within the cell in 1831.
• Johannes Purkinje created the term "protoplasm" for a cell's fluid component in 1839.
• Matthias Schleiden and Theodor Schwann proposed the cell theory, stating that all plants and animals are made of cells and that the cell is the fundamental unit of life, in 1838 and 1839, respectively.
• Rudolf Virchow expanded the cell theory in 1855 by proposing that all cells originate from pre-existing cells.
• Scientists could observe and comprehend the complicated structure of the cell and its constituent organelles once the invention of the electron microscope in 1940.
• The advent of magnifying lenses enabled the discovery of the previously unseen microscopic world.
• A single cell is able to make up an entire organism, such as Amoeba.

Cell Shape and Function

• A cell's shape and size often correlates with its specific function.
• While some cells, like Amoeba, lack a specific shape, others, like nerve cells, possess a distinct shape.
• Each cell carries out basic processes typical of living things.
• Multicellular organisms, like humans, exhibit division of labor.
• Cells contain specialized components called organelles, which each carry out a specific function.
• A cell's capacity to carry out all of its tasks stems from its organelles; they collectively make up the fundamental functional unit of life named a cell.
• The cell structure consists of the plasma membrane, nucleus, and cytoplasm.

Plasma Membrane

• The plasma membrane, which is the outermost layer of the cell, separates cell contents from the external environment and regulates the movement of substances both into and out of the cell
• Substances like carbon dioxide (CO₂) and oxygen (O₂) transfer into the cell through diffusion, which depends on the concentration gradient.
• Substances spontaneously move from high concentration areas to low concentration areas through diffusion.
• Osmosis is the movement of water molecules across a membrane that is selectively permeable.
• Osmosis depends on the quantity of material dissolved in water, and water flows toward a higher solute concentration across the membrane.
• If a cell is submerged into a high water concentrated environment, or hypotonic solution, water enters the cell through osmosis, causing it to swell.
• When the medium has the very same water concentration as the cell, it is referred to as an isotonic environment, and there is no overall water movement.
• The cell will shed water via osmosis when submerged in a low density water solution, or hypertonic solution, causing it to shrink.
• Diffusion controls gas exchange and water intake in the cell through water channels.
• By consuming energy, distinct molecules travel across the membrane with transfer proteins.
• Lipids and proteins make up cells membranes, making them bendable, but the membrane structure can only be seen using an electron microscope.
• The cell is coated in lipids and proteins which allow food and other items to engulf the external environment through a process called endocytosis.

Cell Wall

• Plant cells feature an additional outer layer called the cell wall, beyond the plasma membrane.
• The cell wall is on the outside of the plasma membrane and primarily consists of cellulose, which provides plants with structure and a complex carbohydrate.
• Plasmolysis is brought on by the loss of water when a living plant cell is in a hypertonic environment.
• Plasmolysis, the withering or contraction of the cell contents from the cell wall, is visible in active cells but not in dead cells.
• Plant, fungus, and bacteria cell walls allow them to withstand hypotonic environments without bursting.
• In these conditions, cells take up water through osmosis, expanding up against the cell wall, which exerts equivalent pressure and allows the cell to endure more drastic environmental changes than animal cells.

Nucleus

• The spherical or oval nucleus is separated from the cytoplasm with a double-layered nuclear membrane which contains pores for material transmission.
• Chromosomes inside the nucleus are rod-shaped structures seen when the cell is about to divide and consist of DNA, or deoxyribonucleic acid, molecules, which carry genetic information.
• Each cell's functional DNA segments are called genes; in non-dividing cells, DNA exists as chromatin material.
• The nucleus is essential for cell division, which results in single cells dividing into two.
• The nucleus, along with the environment, directs the cell's chemical activities and controls its development and mature shape.
• Prokaryotes are organisms whose cells lack a nuclear membrane causing the nuclear region, or nucleoid, to be poorly defined.
• Eukaryotes are organisms with cells containing a nuclear membrane.
• Subcellular organelles are absent in prokaryotic cells.

Cytoplasm

• Cytoplasm, taking up stain, is the fluid material within the cell membrane, containing various specialized organelles, each performing specific functions.
• Eukaryotic cells have membrane-bound organelles, whilst prokaryotes lack organelles and a solid nuclear region.
• Viruses, which lack membranes, exhibit no evidence of life unless they invade a living organism and exploit its cell machinery to reproduce, demonstrating the significance of membranes.

Cell Organelles and Endoplasmic Reticulum

• Cell organelles carry out complex chemical processes, these processes require cells to have a way to separate activities from each other through membrane bound structures.
• The endoplasmic reticulum (ER) exists as a vast network of membrane-bound tubes and sheets and is similar in structure to the plasma membrane.
• SER (smooth endoplasmic reticulum) and RER (rough endoplasmic reticulum) ribosomes are two types of ER.
• RER appears rough when viewed via microscope as a result of ribosomes being attached to the surface.
• The proteins that are produced are transported throughout the cell through the ER.
• SER produces lipids molecules which are crucial for cellular activities.
• Some proteins and lipids build the cell membrane in a process known as membrane biogenesis.
• The ER, though diverse in distinct cells, always creates a connected network.
• Channels are formed by one ER function, which facilitates movement of molecules across cytoplasm and its nucleus, also, the ER offers a cytoplasmic framework which allows cell to perform effectively.
• SER is essential for detoxification, and its enzymes can detoxify poisons as well as drugs.

Golgi Apparatus and Other Organelles

• Camillo Golgi characterized the Golgi apparatus, a structure comprised of membrane-bound sacs or vesicles organized in parallel stacks named cisterns which interface with the ER.
• The Golgi apparatus transports ER-synthesized materials inside and outside the cell, also, the Golgi apparatus is involved in lysosome production.
• Lysosomes are membrane-bound sacs containing lytic enzymes which digest foreign materials and worn out organelles.
• Lysosomes help store and modify waste materials, which include bacteria, food particles and old organelles resulting in these materials breaking down.
• When a cell is injured, lysosomes rupture and consume its contents.
• Damaged lysosomes are referred to as suicide bags.
• Mitochondria, known for their power, possess two membranes; the inner membrane is folded to boost surface area, and the energy needed for chemical reactions is emitted through ATP.
• The body uses ATP's stored energy for mechanical activity and producing new chemical compounds.
• Mitochondria contain their own DNA and ribosomes giving them the ability to make their own proteins.
• As internal structure, chloroplasts consist of several membrane layers immersed in the stroma similar to the outer structure of the mitochondria.
• Plastids are only present in plant cells; chromoplasts (colored) and leucoplasts (colorless) are two types of plastids, and chlorophyll-containing chromoplasts are referred to as chloroplasts and facilitate photosynthesis.
• Vacuoles in plant cells provide rigidity when filled with cell sap.
• The vacuole stores proteins, acids, and sugars.
• Food vacuoles and specialized vacuoles help single-celled organisms digest food and discard excess waste.
• Diffusion and membrane and organelles aid cells in carrying out essential functions contributing to overall survival of organisms.

Cell Division

• New cells are formed through cell division.
• Mitosis is the process where most cells divide for growth.
• The chromosomes in daughter cells as a result of Mitosis has the same number of chromosomes as the mother cell.
• Meiosis is the process of cell division that occurs in reproductive organs to form gametes.
• Daughter cells have half as many chromosomes as mother cells.

Explore the roles of endoplasmic reticulum (ER) in protein transport, detoxification, and cellular activities. Learn about the contributions of Dr. Camillo Golgi and the Golgi apparatus to cell biology. Discover how organelles support biochemical activities.