Cell Structure and Organisation Study Notes

Topic 1: Cell Structure and Organisation

Content Outline:

  • 1. What are Cells?

  • 2. Microscopy and Cell Organelles

  • 3. Comparison of Animal and Plant Cells

  • 4. Cellular Structures and their Functions

  • 5. Adaptations of Cell Structure to Function

Learning Outcomes:

By the end of the chapter, you should be able to:

O Levels Biology Syllabus 6093 Learning Outcomes

  • (a) Identify and state the functions of the following cell structures (including organelles) of typical plant and animal cells from diagrams, light micrographs, and as seen under the light microscope using prepared slides and fresh material treated with an appropriate temporary staining technique:

    • cell wall

    • cell membrane

    • cytoplasm

    • nucleus

    • cell vacuoles (large, sap-filled in plant cells, small, temporary in animal cells)

    • chloroplasts

  • (b) Identify and state the functions of the following membrane systems and organelles from diagrams and electron micrographs:

    • endoplasmic reticulum

    • Golgi body

    • mitochondria

    • ribosomes.

  • (c) Compare the structure of typical animal and plant cells.

  • (d) Explain how the structures of specialised cells are adapted to their functions (e.g., muscle cell – many mitochondria to supply more energy, root hair cell – large surface area of cell membrane for greater absorption, red blood cell – lack of nucleus allowing it to transport more oxygen).

NYGH IP Learning Outcomes

  • (e) Describe the features of prokaryotic and eukaryotic cells.
    Use the knowledge gained in this section in new situations or to solve related problems.

1. What are Cells?

  • Cells are the basic building blocks of life.

    • They are the simplest units that can live independently (unicellular organism).

    • They are the basic units of structure and function in an organism (multicellular organism).

  • Figure 1.1: Prokaryotic vs Eukaryotic cell.

  • There are two main types of cells: prokaryotic cells and eukaryotic cells (Fig. 1.1).

    • The classification is due to the vast differences between their characteristics, as shown below.

Table 1: Differences between Prokaryotic and Eukaryotic Cells

  • Prokaryotic cell

    • Cell size: Smaller (ranges from 0.2extextμm0.2 ext{ } ext{μm} to 2.0extextμm2.0 ext{ } ext{μm} in diameter).

    • Location of DNA: Not enclosed within a membrane (no nucleus).

    • DNA arrangement: Circular.

    • Membrane-bound structures (organelles): Absent.

    • Endomembrane system: Absent.

    • Example: Bacterial cells.

  • Eukaryotic cell

    • Cell size: Larger (ranges from 10extextμm10 ext{ } ext{μm} to 100extextμm100 ext{ } ext{μm} in diameter).

    • Location of DNA: Found in the nucleus (bound by the nuclear membrane).

    • DNA arrangement: Linear.

    • Membrane-bound structures (organelles): Present.

    • Endomembrane system: Present.

    • Examples: Animal cells, plant cells.

  • Despite the different characteristics that the prokaryotic and eukaryotic cells possess, they share some common structures. Both types of cells contain:

    • cell surface membrane

    • cytoplasm

    • ribosomes

    • chromosomes.

2. Microscopy

  • Cells are very small in size and require the help of microscopes to visualize them.

    • Magnification: Refers to the increase in size of an object when viewed through devices such as a microscope. Calculated using the formula:
      extmagnification=size of image of objectsize of actual objectext{magnification} = \frac{\text{size of image of object}}{\text{size of actual object}}

    • Resolution: Refers to a microscope's ability to distinguish two points that are close together on an object. The higher the resolution, the better separation of objects and clearer images.

  • There are two main types of microscopes used to view cells: the light microscope and the electron microscope.

Table 2: Differences between Light and Electron Microscopes

  • Light microscope

    • How it works: Uses light as an illuminating source.

    • Magnification: Lower (usually 500x500x to 1500x1500x).

    • Resolution: Low.

    • Color of images produced: Colored images.

    • Types of specimens viewed: Both living and dead specimens.

    • Cell structures that can be seen: Only the nucleus, cytoplasm, cell surface membrane, cell wall, chloroplast, and vacuoles.

  • Electron microscope

    • How it works: Uses electron beams as an illuminating source.

    • Magnification: Higher (can go up to 16000x16000x).

    • Resolution: High.

    • Color of images produced: Only black and white images.

    • Types of specimens viewed: Only dead specimens.

    • Cell structures that can be seen: Can also view rough and smooth endoplasmic reticulum, Golgi bodies, vesicles, mitochondria, and ribosomes.

  • Enrichment: There are two main types of electron microscopes: scanning electron microscope (SEM) and transmission electron microscope (TEM).

    • Figure 2.1: Cells observed under scanning and transmission electron microscopes.

3. Types of Eukaryotic Cells: Animal and Plant Cells

  • Animal and plant cells are both types of eukaryotic cells.

Figure 3.1: Diagrams of a typical plant cell (left) and a typical animal cell (right)
Cell Structures:

  • Animal Cell

    • Nucleus (and nucleolus)

    • Cytoplasm

    • Cell surface membrane

    • Smooth endoplasmic reticulum

    • Rough endoplasmic reticulum

    • Ribosomes

    • Golgi body

    • Lysosomes

    • Mitochondria

  • Plant Cell

    • Vacuole: Multiple small and temporary vacuoles.

    • Cellulose cell wall: Present.

    • Chloroplast: Present.

  • Found in Both: Nucleus, cytoplasm, cell surface membrane, smooth endoplasmic reticulum, rough endoplasmic reticulum, ribosomes, Golgi body, lysosomes, mitochondria.

4. Cellular Structures and Their Functions

4.1 Nucleus (plural: nuclei)
Structure of the Nucleus:

  • Nuclear Envelope:

    • Double membrane that surrounds the nucleus, separating its contents from the rest of the cell.

    • Continuous with the endoplasmic reticulum.

  • Nuclear Pores:

    • Openings present in the nuclear envelope allowing small molecules to pass in and out.

  • Deoxyribonucleic Acid (DNA):

    • Stores the cell’s genetic information.

    • Located within the nucleus.

    • DNA in the chromatin form condenses into chromosomes (thick, rod-shaped structures) during cell division.

  • Nucleolus (plural: nucleoli):

    • Spherical structure found in the nucleus where ribosomes are synthesized.

    • Not enclosed within a membrane.

    • One or more nucleoli may be found within the nucleus.

Functions of the Nucleus:

  1. Controls cell activities, such as growth and repair of worn-out cell parts.

  2. Essential for cell division – cells without a nucleus are unable to divide (e.g., red blood cells).

  3. Contains genetic information in the form of DNA.

4.2 Cell Surface Membrane (Plasma Membrane)
Structure of the Cell Surface Membrane:

  • A selectively permeable barrier that surrounds a cell, allowing some substances to pass through but not others.

  • Consists of a bilayer of phospholipids with proteins attached or embedded in it.

    • The hydrophobic parts of phospholipids and membrane proteins are found in the interior of the membrane, while the hydrophilic parts contact aqueous solutions on either side.

Function of the Cell Surface Membrane:

  1. Controls the movement of substances in and out of the cell due to its selective permeability.

4.3 Cytoplasm
Structure of the Cytoplasm:

  • A semi-fluid mixture filling the cell's interior.

  • Composed of approximately 90% water and 10% of dissolved solutes.

    • Note: Protoplasm = cytoplasm (including organelles) + cell surface membrane + nucleus.

Function of the Cytoplasm:

  1. Serves as the main site for metabolic activities, facilitating many chemical reactions.

4.4 Ribosome
Structure of the Ribosome:

  • Composed of two subunits.

  • Appears spherical in electron microscopy.

  • Non-membrane-bound, hence not classified as an organelle.

  • Exists as free ribosomes in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

Function of the Ribosome:

  1. Synthesizes polypeptides/proteins.

    • Free ribosomes synthesize proteins that function within the cytoplasm.

    • Ribosomes attached to RER synthesize proteins that are:

    • Secreted out of the cell.

    • Packaged within lysosomes.

    • Embedded into membranes.

4.5 Endomembrane System

  • Consists of various membrane-bound organelles:

    • Nuclear envelope

    • Endoplasmic reticulum

    • Golgi body

    • Vesicles and lysosomes

    • Cell surface membrane.

Functions of the Endomembrane System:

  • Carries out various functions, including:

    • Synthesis of proteins and lipids.

    • Transport of proteins and lipids between organelles and out of the cell.

    • Detoxification.

4.5.1 Endoplasmic Reticulum (ER)

  • There are two types of endoplasmic reticulum:

    • Rough endoplasmic reticulum (RER)

    • Smooth endoplasmic reticulum (SER)

Structure of the Rough Endoplasmic Reticulum (RER):

  • Consists of interconnecting flattened spaces lined with a single membrane, ribosome-attached giving it a rough appearance, continuous with nuclear membrane.

Functions of the Rough Endoplasmic Reticulum (RER):

  1. Synthesizes proteins via ribosomes attached to its membrane.

  2. Transports proteins for secretion via transport vesicles.

Structure of the Smooth Endoplasmic Reticulum (SER):

  • Comprises tubular spaces lined with a single membrane, lacks ribosomes, may be continuous with nuclear membrane or connected to RER.

Functions of the Smooth Endoplasmic Reticulum (SER):

  1. Synthesizes lipids (e.g., phospholipids, steroids).

  2. Transports lipids for secretion via transport vesicles.

  3. Detoxification – converts harmful substances into harmless ones for excretion.

4.5.2 Golgi Body (Golgi Apparatus/Golgi Complex)
Structure of Golgi Body:

  • Composed of several flattened membrane-bound sacs, known as cisternae.

  • Membrane is not continuous with the nuclear membrane.

  • Vesicles fuse with one side (cis face – near the ER) and pinch off from the opposite side (trans face).

Functions of Golgi Body:

  1. Chemical modification of proteins and lipids made by RER and SER.

  2. Sorting and packaging of proteins and lipids into secretory vesicles for cell secretion.

  3. Formation of lysosomes, specialized vesicles containing enzymes.

4.5.3 Vesicles and Lysosomes
Structure of Vesicles:

  • Small sacs bound by a single membrane, formed by budding/pinching off from the membrane of RER, SER, Golgi body, or cell surface membrane.

Types of Vesicles and Their Functions:

  • Transport vesicle: Carries substances between membrane compartments.

    • Example: Buds from RER to transport proteins to Golgi body.

  • Secretory vesicle: Buds from Golgi body to transport modified proteins/lipids to cell surface membrane.

    • Fuses with cell membrane to release contents via exocytosis.

  • Endosome: Formed during endocytosis when the cell membrane buds off containing outside substances.

  • Lysosome: Specialised vesicle containing digestive enzymes, buds from Golgi body, conducts digestion of food particles and damaged organelles.

Figure 4.8: Lysosome digesting food particles and damaged organelles.
4.5.4 Overview of Protein Synthesis via Endomembrane System

  1. DNA in the nucleus contains genes coding for polypeptides.

  2. DNA is transcribed into mRNA strands.

  3. mRNA exits the nucleus, goes to ribosomes on RER for translation into polypeptides.

  4. Polypeptides fold into proteins within RER.

  5. Transport vesicles containing synthesized proteins bud off from RER towards Golgi body.

  6. Transport vesicles fuse with Golgi body for chemical modification, sorting, and packaging into secretory vesicles.

  7. Secretory vesicles move to the cell surface membrane.

  8. Secretory vesicles fuse, releasing proteins into extracellular environment via exocytosis.

4.6 Mitochondrion (plural: Mitochondria)
Structure of the Mitochondrion:

  • Cylindrical or rod-shaped, bound by a double membrane:

    • Outer membrane: Smooth.

    • Inner membrane: Contains folds known as cristae to increase surface area for chemical reactions.

  • The inner and outer membranes create an intermembrane space, while the matrix enclosed contains:

    • Circular mitochondrial DNA.

    • Ribosomes.

    • Enzymes.

Function of the Mitochondrion:

  1. Performs cellular/aerobic respiration – oxidizing glucose to produce energy as ATP (adenosine triphosphate) for cellular activities.

4.7 Chloroplast
Structure of the Chloroplast:

  • Cylindrical shape, larger than mitochondria and observable under a light microscope.

  • Bound by a double membrane, containing chlorophyll for light absorption during photosynthesis.

  • Interior consists of a gel-like matrix called the stroma, containing:

    • Circular chloroplast DNA.

    • Ribosomes.

    • Enzymes.

    • Starch grains (from excess glucose).

Function of the Chloroplast:

  1. Conducts photosynthesis – light energy absorbed by chlorophyll is transformed into chemical energy for glucose formation.

4.8 Vacuole
Structure of the Vacuole:

  • Membrane-bound, with vacuoles in animal cells being small but numerous and temporarily present.

  • In plant cells:

    • Large, singular central vacuole surrounded by tonoplast (selectively permeable).

    • Contains cell sap, approximately 90% water with dissolved substances (sugars, amino acids, mineral salts, wastes).

Functions of the Vacuole:

  1. In animal cells: Stores water and food substances.

  2. In plant cells:

    • Stores dissolved sugars, mineral salts, amino acids, and water.

    • Maintains turgor pressure of the plant cell, providing cell shape and preventing lysis (bursting).

    • Stores pigments for color in flowers/fruits to attract pollinators.

    • Stores defensive compounds to deter herbivores.

    • Stores metabolic waste products.

4.9 Cellulose Cell Wall
Structure of the Cellulose Cell Wall:

  • Composed of cellulose - a strong and rigid material.

  • Fully permeable, allowing all substances to pass through (note: represent with double lines in biological drawings).

Functions of the Cellulose Cell Wall:

  1. Provides mechanical support for plant cells; lignin further increases strength in some cell walls.

  2. Prevents excessive water uptake when the external solution is more dilute than cell sap, avoiding cell lysis.

  3. In epidermal tissues, a waxy cuticle can form to reduce water loss and mitigate pathogen infection.

5. Adaptations of Cell Structure to Function

  • Different cells may develop special structures or lose certain structures for specific functions, a process known as differentiation.

5.1 Case Study 1: Red Blood Cell
Structure and Function of Red Blood Cell:

  • Haemoglobin present in cells: Binds reversibly with oxygen to form oxyhaemoglobin for oxygen transport.

  • Absence of nucleus: Maximizes haemoglobin packing, enhancing oxygen transport capacity.

  • Biconcave shape: Increases surface area-to-volume ratio for enhanced oxygen diffusion rate.

  • Elastic and flexible membrane: Allows shape change for capillary passage.

5.2 Case Study 2: Muscle Cell
Structure and Function of Muscle Cell:

  • Contains many mitochondria: Provides significant energy for muscle cell contraction.

  • Skeletal muscles: Elongated and cylindrical, containing multiple nuclei for functional unity during contraction.

  • Cardiac muscles: Branched for faster signaling/contraction.

5.3 Case Study 3: Root Hair Cell
Structure and Function of Root Hair Cell:

  • Elongated extension: Increases surface area-to-volume ratio for efficient water/minerals absorption from soil.

  • Large vacuole: Enhances capacity for water/mineral salt uptake.

  • Concentrated cell sap: Establishes a greater water potential gradient with surrounding soil, facilitating osmotic absorption.

  • Contains numerous mitochondria: Supplies energy for active transport of minerals into the cell.

5.4 Case Study 4: Xylem Vessel
Structure and Function of Xylem Vessel:

  • Lignin: Rigid and waterproof substance strengthens walls, providing mechanical support to plants.

  • Hollow continuous lumen: Lacks cross-walls or protoplasm (xylem is dead), reducing resistance for fast water/ion flow from roots to leaves.