Cell Structure and Function

Welcome to this comprehensive lesson on Cell Structure and Function for the CXC/CSEC Human and Social Biology syllabus. This topic forms the foundation of understanding how living organisms function at their most basic level.

1. Introduction to Cells

The cell is the basic structural and functional unit of all living organisms. Cells are often called the "building blocks of life" because all living things are composed of cells. The study of cells is called cytology.

1.1 Cell Theory

The cell theory consists of three main principles:

1.2 Types of Cells

There are two main types of cells:

DNA Prokaryotic Cell Cytoplasm Cell Membrane Ribosome Eukaryotic Cell Nucleus Nuclear Membrane Mitochondrion Endoplasmic Reticulum Golgi Apparatus Lysosome Cell Membrane

Figure 1: Comparison of Prokaryotic and Eukaryotic Cells

2. Cell Structure and Organelles

2.1 Cell Membrane

The cell membrane (plasma membrane) is a thin, flexible barrier that surrounds all cells. It controls what enters and leaves the cell.

Cell Membrane Structure Membrane Protein Cholesterol Phospholipid Bilayer Hydrophilic Head Hydrophobic Tail

Figure 2: Structure of the Cell Membrane

2.2 Cytoplasm

The cytoplasm is a gel-like substance within the cell membrane that contains water, salts, enzymes, and various organelles.

2.3 Nucleus

The nucleus is the control center of the cell, containing genetic material in the form of DNA.

2.4 Mitochondria

Mitochondria are often called the "powerhouses" of the cell because they produce energy.

Mitochondrion Structure Outer Membrane Inner Membrane Cristae Mitochondrial DNA Matrix

Figure 3: Structure of a Mitochondrion

2.5 Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranous tubules and sacs that extend throughout the cytoplasm.

2.6 Golgi Apparatus

The Golgi apparatus consists of flattened, membrane-bound sacs called cisternae.

2.7 Ribosomes

Ribosomes are small, dense granules composed of RNA and protein.

2.8 Lysosomes

Lysosomes are membrane-bound vesicles containing digestive enzymes.

2.9 Vacuoles

Vacuoles are membrane-bound sacs filled with fluid.

2.10 Chloroplasts (in plant cells only)

Chloroplasts are organelles found only in plant cells and some protists.

2.11 Cell Wall (in plant cells only)

The cell wall is a rigid layer outside the cell membrane found in plant cells, fungi, and some bacteria.

2.12 Centrioles (in animal cells only)

Centrioles are cylindrical structures composed of microtubules.

3. Comparison of Plant and Animal Cells

Animal Cell Nucleus Centrioles Plant Cell Cell Wall Nucleus Central Vacuole C C C C Cell Membrane Key Differences Cell wall, large central vacuole, and chloroplasts in plant cells Centrioles and smaller vacuoles in animal cells

Figure 4: Comparison of Plant and Animal Cells

The following table summarizes the key differences between plant and animal cells:

Feature Animal Cell Plant Cell
Cell Wall Absent Present (made of cellulose)
Cell Shape Usually irregular Usually rectangular or fixed
Vacuoles Multiple small vacuoles One large central vacuole
Chloroplasts Absent Present
Centrioles Present Absent in most plants
Lysosomes Prominent Rare
Food Storage Glycogen Starch

4. Cell Transport Mechanisms

4.1 Passive Transport

Passive transport is the movement of substances across the cell membrane without the use of cellular energy.

4.1.1 Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient).

4.1.2 Osmosis

Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.

Hypotonic Cell H₂O H₂O H₂O Isotonic Cell H₂O H₂O Hypertonic Cell H₂O H₂O H₂O

Figure 5: Effects of Osmosis on Cells in Different Solutions

4.1.3 Facilitated Diffusion

Facilitated diffusion is the passive movement of specific molecules across the cell membrane through protein channels or carrier proteins.

4.2 Active Transport

Active transport is the movement of substances across the cell membrane against their concentration gradient, requiring energy (ATP).

4.3 Bulk Transport

4.3.1 Endocytosis

Endocytosis is the process by which cells take in materials by forming vesicles from the cell membrane.

4.3.2 Exocytosis

Exocytosis is the process by which cells release materials to the outside by fusing vesicles with the cell membrane.

Cell Transport Mechanisms Simple Diffusion Facilitated Diffusion Active Transport ATP Endocytosis Exocytosis Outside Cell Inside Cell

Figure 6: Cell Transport Mechanisms

5. Specialized Cells

Specialized cells are cells that have developed specific functions and features to perform particular tasks in multicellular organisms.

5.1 Examples of Specialized Cells in Humans

Red Blood Cell Neuron Muscle Cell Sperm Cell Epithelial Cells Specialized Human Cells

Figure 7: Examples of Specialized Human Cells

6. Levels of Organization

Living organisms exhibit increasing levels of organization:

7. Summary

Cell structure and function are fundamental concepts in biology. Key points to remember:

8. Glossary

Active Transport
The movement of molecules across a cell membrane against a concentration gradient, requiring energy.
Cell
The basic structural and functional unit of all living organisms.
Cell Membrane (Plasma Membrane)
A selectively permeable lipid bilayer that surrounds the cell, controlling what enters and exits.
Cell Theory
The scientific theory that states all living things are composed of cells, cells are the basic units of structure and function, and all cells come from pre-existing cells.
Cell Wall
A rigid layer outside the cell membrane in plants, fungi, and bacteria that provides structural support and protection.
Centrioles
Cylindrical structures composed of microtubules that help organize cell division and form the base of cilia and flagella.
Chloroplast
An organelle in plant cells that contains chlorophyll and is responsible for photosynthesis.
Chromatin
The complex of DNA and proteins that makes up chromosomes.
Chromosome
A structure consisting of DNA and associated proteins that carries genetic information.
Cytoplasm
The gel-like substance inside cells that contains water, ions, proteins, and organelles.
Diffusion
The passive movement of molecules from an area of higher concentration to an area of lower concentration.
Endocytosis
The process by which cells take in materials by forming vesicles from the cell membrane.
Endoplasmic Reticulum (ER)
A network of membranous tubules within the cytoplasm that is involved in protein and lipid synthesis, and transport.
Exocytosis
The process by which cells release materials to the outside by fusing vesicles with the cell membrane.
Facilitated Diffusion
The passive movement of specific molecules across the cell membrane through protein channels or carriers.
Golgi Apparatus
An organelle that modifies, sorts, and packages proteins and lipids for storage in the cell or secretion.
Hypertonic
Describing a solution with a higher solute concentration compared to another solution.
Hypotonic
Describing a solution with a lower solute concentration compared to another solution.
Isotonic
Describing solutions with equal solute concentrations.
Lysosome
A membrane-bound organelle containing digestive enzymes that break down waste materials and cellular debris.
Mitochondrion
An organelle that generates most of the cell's supply of ATP, used as a source of chemical energy.
Nuclear Membrane (Nuclear Envelope)
The double membrane surrounding the nucleus, with pores that allow for the passage of materials.
Nucleolus
A structure within the nucleus where ribosomal RNA is produced and ribosomes are assembled.
Nucleus
The membrane-bound organelle containing the cell's DNA and controlling cellular activities.
Organelle
A specialized subunit within a cell that has a specific function.
Osmosis
The diffusion of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.
Phagocytosis
A type of endocytosis in which cells engulf solid particles, forming an internal vesicle.

Examples:

  • White blood cells engulfing bacteria
  • Amoebas capturing food particles

Process: 1) Recognition 2) Engulfment 3) Vesicle formation 4) Digestion

Pinocytosis
A type of endocytosis in which cells take in extracellular fluid and its dissolved solutes.

Characteristics:

  • Also called "cell drinking"
  • Non-specific uptake of fluids
  • Forms small vesicles (compared to phagocytosis)
  • Important for nutrient absorption in some cells
Mitochondria
Double-membraned organelles that generate most of the cell's supply of ATP through cellular respiration.

Key features:

  • Have their own DNA (evidence for endosymbiotic theory)
  • Contain cristae (folds) to increase surface area
  • Number varies by cell type (muscle cells have many)
Ribosome
A complex molecular machine that synthesizes proteins using mRNA as a template.

Types:

  • Free ribosomes (float in cytoplasm)
  • Bound ribosomes (attached to rough ER)

Composition: Made of rRNA and proteins

Golgi Apparatus
A membrane-bound organelle that modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

Functions:

  • Adds carbohydrate tags to proteins (glycosylation)
  • Produces lysosomes
  • Forms secretory vesicles
Lysosome
A membrane-bound organelle containing digestive enzymes that break down biomolecules, old organelles, and pathogens.

Key facts:

  • Maintains acidic pH (about 4.5-5.0)
  • Contains ~50 different enzymes
  • Defects can cause storage diseases (e.g., Tay-Sachs)
Endoplasmic Reticulum
A network of membranous tubules within the cytoplasm that functions in protein and lipid synthesis.

Types:

  • Rough ER: Studded with ribosomes, protein synthesis
  • Smooth ER: Lipid synthesis, detoxification, calcium storage
Cytoskeleton
A network of protein filaments that maintains cell shape, enables cellular motion, and organizes intracellular transport.

Components:

  • Microfilaments (actin): Cell movement, cytokinesis
  • Intermediate filaments: Structural support
  • Microtubules: Organelle movement, spindle formation