Cell Structure and Organization Notes

Cell Structure and Organization

Learning Outcomes

• Describe the structure of animal and plant cells with their organelles.
• Know the functions of microscope parts and determine specimen sizes.
• Discuss cell division and specialization.
• Distinguish levels of cellular organization.

Introduction to Cells

• Microscopes have revolutionized biology, revealing cells as fundamental units of life.
• Robert Hooke (1665): First discovered and named cells, observing cell walls in cork and likening them to monks' rooms.
  • Hooke's observation was limited to dead plant cells, lacking nuclei and organelles.
• Anton van Leeuwenhoek (1674): Observed living cells, such as the alga Spirogyra.
• Theodor Schwann and Matthias Schleiden (1838-1839): Concluded that both plants and animals are composed of cells, documented in Schwann's book.

Animal and Plant Cells

• Cells are the smallest basic unit of life.
• Cells vary in size and shape based on function.
• Most cells are microscopic, except for the ostrich egg (largest single cell by volume).
• Nerve cells can be several meters long but remain microscopic in width.
• Large organisms contain millions of cells; thin slices of plant leaves reveal numerous tightly packed cells.

Structure of Animal and Plant Cells

• All cells are surrounded by a cell membrane, enclosing the cytoplasm.
• Cytoplasm contains organelles with specific structures and functions.
• The nucleus is the largest organelle; the nucleus and cytoplasm are collectively called protoplasm.

Cell Membrane

• Also known as the plasma membrane.
• Holds the cell together, provides shape, and protects it.
• Controls the movement of substances in and out of the cell (selectively permeable).
  • Allows small molecules like oxygen and water to enter.
  • Allows waste products to leave.
  • Prevents toxic substances from entering.

Cell Wall

• Found in plant cells, fungi, and most bacteria, outside the cell membrane.
• Provides protection and support.
• Fully permeable, allowing free passage of substances.
• Animal cells lack a cell wall.

Cytoplasm

• Fills the space inside the cell.
• Jelly-like substance consisting of water, salts, sugars, and other dissolved substances.
• Contains all cell organelles, including the nucleus.
• Contains lipids and proteins for building cell structures.
• Site of most cellular chemical reactions, controlled by enzymes.
  • Examples: energy production and formation of new cellular structures.
• Animal cells may contain glycogen granules and lipid droplets in their cytoplasm, which are typically absent in plant cells.

Cell Organelles

• Both animal and plant cells contain nucleus, ribosomes, mitochondria, Golgi apparatus, and endoplasmic reticulum.
• Some organelles are mainly found in one cell type.
  • Large vacuoles and chloroplasts (mainly in plant cells).
  • Lysosomes (mainly in animal cells).

Nucleus

• Typically a large, round structure within the cytoplasm.
• Surrounded by a double-membrane called the nuclear envelope.
• Nuclear envelope controls substance exchange via nuclear pores.
• Contains chromosomes made of DNA and proteins, carrying genetic material.
• Serves as the information and control center of the cell.
  • Determines cell type and controls cell behavior.
  • Controls type and quantity of proteins (e.g., enzymes).
  • Regulates cellular reactions.
• Controls cell division; cells without a nucleus cannot reproduce.

Mitochondria

• Small, membrane-bound organelles in nearly all eukaryotic cells.
• Absent in prokaryotic cells and mature human red blood cells.
• Produce energy from energy-rich molecules (e.g., sugars) via cellular respiration.
• Have folded internal membranes (cristae) to increase surface area for chemical reactions.
• Contain enzymes that control cellular respiration (aerobic respiration).
• The number of mitochondria varies based on cell energy demand.
  • Muscle cells (active) have more mitochondria than skin cells (less active).

Lysosomes

• Small, round, membrane-bound organelles found mostly in animal cells.
• Contain digestive enzymes to break down large biological molecules into smaller molecules.
  • Proteins, lipids, and carbohydrates.
• Involved in breaking down and recycling worn-out cell parts.
• The membrane prevents enzymes from digesting the cell itself.

Ribosomes

• Smallest and most numerous organelles present in all cell types.
• Found floating freely in the cytoplasm or attached to the rough endoplasmic reticulum.
• Main site of protein synthesis in the cell.

Endoplasmic Reticulum (ER)

• An interconnected network of tubes and passageways that synthesizes and transports biological molecules.
• Present in most eukaryotic cells, absent in prokaryotes.
• Two types:
  • Rough ER: Studded with ribosomes, mainly involved in protein synthesis.
  • Smooth ER: Lacks ribosomes, mainly involved in lipid synthesis.
• Parts of the ER may break off to form vesicles that carry proteins or lipids to the Golgi apparatus.

Golgi Apparatus

• Also known as the Golgi body or Golgi complex.
• A stack of flattened sacs and vesicles found in both animal and plant cells.
• Receives partially processed proteins and lipids from the ER and completes their processing.
• Packages the final products in Golgi vesicles to be transported to target sites or the external environment.

Vacuoles

• Organelles that store water, food, wastes, or other materials.
• Found in all plant cells and some animal cells; vary in number, size, and shape based on cell type.
• Animal cells may contain several small vacuoles (vesicles) or none at all.
• Plant cells tend to have one large central vacuole that contains cell sap.
  • Cell sap: Dilute fluid of water, amino acids, glucose, salts, and pigments.
• The central vacuole helps maintain the shape of the plant cell.
  • Full vacuole creates outward pressure on the cytoplasm and cell wall, making the cell firm and rigid.
  • Low water content leads to wilting.

Chloroplasts

• Green, disklike organelles found in most plant cells and algae.
• Convert sunlight energy into chemical energy (carbohydrates) from carbon dioxide and water during photosynthesis.
• Animal cells and some plant cells (e.g., root cells) do not contain chloroplasts.
• Larger than mitochondria with a more complex membrane system.
• Has a double-membrane envelope (chloroplast envelope) and a third internal membrane system (thylakoid membrane system).
• Stroma: Semifluid aqueous solution between the thylakoid membrane system and the inner membrane of the chloroplast envelope.
• Thylakoids: Hollow, saclike disks that contain chlorophyll (green pigment) needed for photosynthesis.
• Chloroplasts are a type of plastid (small organelles in plant cells).
  • Plastids serve as centers for special metabolic activities.
  • Some plastids contain pigments (e.g., chloroplasts).
  • Leucoplasts (involved in starch and oil storage) are colorless.

Comparing Plant Cells and Animal Cells

• Plant and animal cells have structures in common, including a nucleus, cytoplasm, ribosomes, mitochondria, ER, and Golgi apparatus.
• Unique structures:
  • Plant cells: cell wall, large central vacuole, chloroplasts, and plastids containing starch granules.
  • Animal cells: lack the aforementioned structures.
• Plant cells are typically regular in shape and larger than animal cells.
• Functional differences:
  • Plant cells produce their own food through photosynthesis.
  • Animal cells cannot produce their own food.

Cell Biology and Microscopy

Types of Microscopes

• A microscope is used to view objects too small to be seen with the naked eye.
• A light microscope is commonly used in school laboratories; uses ordinary light to view objects.
  • Light is directed through the specimen into the lenses.
  • Lenses produce a magnified image.
  • Photomicrograph: A photograph taken through a microscope.

Parts of a Light Microscope and Their Functions

• Ocular lens (eyepiece lens): The upper lens through which the magnified image is observed, commonly with a magnification power of 10X. Objectives: There are usually four objectives, each containing lenses of different magnifications (ranging from 4X to 100X). These lenses help produce a greatly magnified image of the specimen.
• Nosepiece: houses the objectives; can be rotated to change the lens in use.
• Stage: The stage holds and supports the microscope slides. It has a slide holder for holding and moving the microscope slide. The slide is usually held in place on the holder by stage clips.
• Arm: connects the upper part of the microscope to the base and is the best place to grip the microscope when carrying it.
• Focusing knobs:
  • Coarse adjustment knob: is used for general focusing.
  • Fine adjustment knob: is used to focus on smaller details (fine focus).
• Condenser: collects light from the light source and focuses it onto the specimen
• Iris diaphragm: regulates the quantity of light that passes through the specimen similar to how the iris of a human eye controls the amount of light that enters the eye
• Base: the flat bottom of a microscope that sits on a table
• Substage lamp: the source of light for the microscope

Light vs Electron Microscopes

• A light microscope provides a clear image of a specimen with magnifications up to 1000 times, but is unable to distinguish structures much less than 0.2 microns apart (a micron is represented by the symbol \mu m and is a thousandth of a millimeter).
• Electron microscope: Uses beams of electrons instead of light, allowing magnification up to 500,000 times.
  • Has higher magnification and greater resolution.
  • Magnification: How much an image is enlarged.
  • Resolution: The ability to distinguish between two close points; unresolved points appear as one.

Microscope Use and Care

• Handle the microscope with care and perform periodic maintenance and cleaning.
• Carry the microscope by holding its arm with one hand and placing the other hand under the base.
• Place the microscope on a flat surface, away from the edge. Avoid touching lens surfaces.
• Clean lenses with lens paper to prevent damage.
• Ensure a thin specimen for easy light passage.
• Lock the lowest objective lens (4X) before placing a slide on the stage.
• Secure the slide with stage clips. Look through the eyepiece and slowly turn the coarse focusing knob until a clear image appears.
• Use the fine adjustment knob for final focusing.
• To view at higher magnifications, rotate the nosepiece to lock the selected objective lens.
  • Refocus with the fine focusing knob only.
  • Use the 100X objective lens (oil immersion lens) with microscope immersion oil.
    • Remove oil before switching to lower magnification to avoid contamination.
• Store the microscope with the 4X objective lens pointed down, turn off the substage lamp, loop the cord, and cover before storing.

Preparation of Specimens for Viewing

• Specimen: organism, part of organism, or nonliving material for examination.
• Examples of specimen types:
  • Sections: Thin slices of plant and animal parts prepared using sharp instruments. Animal sections may be prepared by placing the samples in melted wax to harden them before being cut.
    • Transverse section (cross section): Cutting across the length of the structure.
    • Longitudinal section: Cutting along the length of the structure.
  • Epidermal Strips: Prepared by peeling off an epidermal layer, such as from an onion.
  • Squashes: Prepared by placing the specimen on a slide, covering it with a coverslip, and gently tapping the coverslip with a glass rod to flatten and separate cells. Commonly done for root tip samples to observe cell division.
  • Smears: Prepared by spreading a thin layer of the specimen on a slide. Used for liquid specimens, such as blood.
  • Stained Microscopic Slides: Specimens treated with dyes (stains) to enhance contrast.
• Methylene blue: Used to stain animal cell nuclei.
• Iodine solution: Used to stain plant cell nuclei.

Biological Drawing

• Use a sharp pencil.
• The drawing should be accurate and large.
• State the magnification.
• Use clear labels written outside the drawing with label lines drawn with a ruler in pencil.
• Keep the drawing neat with clear, distinct lines.
• Do not sketch, shade, or color the drawing; use light dots for shading if needed.
• Include a title at the top.
• Observe and accurately show details.

Measuring Cells Viewed Under the Microscope

• Determine the magnification and actual size of the specimen.
• Total magnification = Magnification of objective lens x Magnification of eyepiece lens
• Actual size of specimen = Observed size of specimen/Total magnification used
• Magnification = Observed size of the specimen/Actual size

Example: A student viewed a cell using a light microscope. He used an eyepiece lens with a magnification power of 15X and an objective lens of 20X. At what total magnification did the student view the cell?Answer:Total magnification = 20 \times 15 = 300 The total magnification of the specimen was 300X, meaning that the specimen was magnified 300 times its original size.
The specimen viewed by the student appeared to be 3 mm long when viewed through the microscope. What is the actual size of the specimen?Answer:Size of specimen viewed = 3 mm Total magnification used = 300XActual size of specimen = \frac{3 mm}{300} = 0.01 mm.

Cell Division and Specialization

Importance of Cell Division

• Cell division: The process a cell goes through to divide and give rise to two or more daughter cells.
• Necessary for formation, growth, repair, and replacement of worn-out or damaged cells.
• Meristems: Actively dividing growing areas in plants, containing undifferentiated cells (meristematic cells). Found in plant zones where growth can take place, such as at the plant root and shoot tips
  • Shoot meristem: gives rise to organs such as leaves
  • Root meristem: ensures root growth
    *In animals, cell division mainly occurs in tissues that require frequent renewal and growth, such as epithelial tissues (e.g., layers of cells in the skin) and bone marrow (tissue located inside some bones). It is also involved in the production of gametes (eggs and sperm) in sexually reproducing organisms.
    *For simpler organisms such as bacteria and other single-celled organisms, cell division itself is a form of reproduction
• Binary fission: Each bacterium divides, giving rise to two bacteria. Type of reproduction

Cell Division in Animal and Plant Cells

• A cell starts to divide only when it is fully grown and has all the necessary components to undergo cell division.
• Apart from the nucleus, other cellular organelles such as the mitochondria and chloroplasts are also able to divide and are distributed more or less equally between the daughter cells during cell division.
• In plant cells, a new cell wall forms between the two daughter cells and a new large vacuole develops, because a cell wall is present which prevents the cytoplasm from simply pinching off in the middle.

Cell Specialization

• After a cell divides, it undergoes a series of events that lead to its specialization, and the cell becomes differentiated as they develop, depending on their location in the body and on their primary roles within the organism.
• The division of labor is how cells that are specialized work together.

Muscle Cells

• Smooth muscle cells have a high concentration of contractile fibers (thick and thin filaments) that enable them to contract and perform their function.
• Muscle cells form smooth muscle tissues and are involved in protecting internal organs and allowing the body to perform important functions, such as pushing food along the gut.

Red Blood Cells

• They have a flexible cell membrane and cytoplasm.
• They lack a nucleus and many other organelles, which gives these cells more space for hemoglobin, the oxygen-carrying molecule.
• Red blood cells are biconcave in shape, resulting in a larger surface area, which helps speed up the movement of oxygen into and out of these cells.

Ciliated Cells in The Respiratory Tract

• In addition to a cell membrane, cytoplasm, and nucleus, these cells have tiny hair-like structures called cilia, which extend from the cell's outer surface.
• Cilia flick continuously; this movement propels mucus upwards and away from the lungs.
• As the mucus is moved out of the lungs, it carries dust and bacteria with it, protecting the lungs from harm.

Nerve Cells

• basic units of the nervous system.
• Neurons are specialized for the transmission of messages, also called signals or nerve impulses, within the body.
• Typically, a neuron is made up of four parts: dendrites, a cell body, an axon, and axon terminals.

Egg and Sperm Cells

• In a human female, the ovaries are the organs that produce the female reproductive cells, called eggs (or ova, singular ovum). The cytoplasm of an egg cell is rich in nutrients In a human male, the testes produce the male reproductive cells, called sperm.
• Sperm are produced by a type of cell division that halves the number of chromosomes of the cell, which is why they are called haploid cells.

Xylem Vessels

• These cells have no cytoplasm, no cell membrane, and no nucleus; therefore, they are considered dead and hollow.
• Their cell wall contains a hard substance called lignin that helps provide support for the aboveground parts of the plant.
• They play an important role in the transport of water and ions (dissolved salts) from the roots to all other plant parts.

Root Hair Cells

• These cells have tiny, tubelike outgrowths that greatly increase the surface area available for the absorption of water and mineral salts from the soil.

Palisade Mesophyll Cells

• Palisade mesophyll cells are vertically elongated cells with numerous chloroplasts, which make them well-adapted to carry out photosynthesis.
• Their location also helps these cells be the first to receive the sun's rays as they enter the leaf.

Levels of Cellular Organization

• Cell: The basic unit of an organism; the smallest living part capable of carrying out life processes.
• Tissue: A group of specialized cells with similar structures working together to perform a specific function.
• Organ: Structures made up of different tissues working together to carry out specific functions.
• Organ System: A group of organs with related functions that work together to perform specific body processes.
• Organism: Comprises different organ systems working as a unit for survival.