Lecture 2 Guide

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Cell Theory and Overview

Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life in all living organisms.

Cells are the building blocks of all plants and animals.
Cells are the smallest functioning units of life.
Cells are produced through the division of pre-existing cells.
Each cell maintains homeostasis.

Homeostasis refers to the ability of a cell to maintain a stable internal environment despite changes in external conditions.

Cell Structure

Major Components of a Typical Animal Cell

Cells contain various structures called organelles, each with specialized functions essential for cell survival and activity.

Nucleus: Contains genetic material (DNA) and controls cellular activities.
Cytoplasm: Jelly-like fluid (cytosol) that fills the cell and surrounds organelles.
Cell (Plasma) Membrane: Encloses the cell, separating it from the external environment.
Mitochondria: Site of energy production (ATP synthesis).
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.
Lysosomes: Contain digestive enzymes for breaking down waste.
Peroxisomes: Involved in detoxification and lipid metabolism.
Cytoskeleton: Provides structural support and facilitates movement.

Example: The nucleus acts as the control center, directing all cellular activities and storing genetic information.

Plasma (Cell) Membrane

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining its integrity and regulating interactions with the environment.

Phospholipid Bilayer: Composed of two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.
Cholesterol: Embedded within the membrane to provide stability and fluidity.
Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.
Microvilli: Tiny projections that increase surface area, aiding in absorption and secretion.

Additional info: The plasma membrane also contains glycoproteins and glycolipids that contribute to cell recognition and signaling.

Functions of the Plasma Membrane

The plasma membrane performs several vital functions for the cell:

Protection: Acts as a barrier to the external environment.
Regulation: Controls movement of substances into and out of the cell (selective permeability).
Sensitivity: Contains receptors for detecting chemical signals.
Support: Provides structural stability to the cell.

Example: Channel proteins in the membrane allow specific ions to pass through, maintaining cellular homeostasis.

Organelles and Their Functions

Membranous Organelles

Membranous organelles are surrounded by membranes and perform specialized tasks.

Nucleus: Stores DNA and coordinates cell activities such as growth and reproduction.
Mitochondria: Generate ATP through cellular respiration.
Endoplasmic Reticulum (ER): Rough ER: Studded with ribosomes; synthesizes proteins. Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.
Golgi Apparatus: Modifies and packages proteins and lipids for transport.
Lysosomes: Digest cellular waste and foreign material.
Peroxisomes: Break down fatty acids and neutralize toxins.

Non-Membranous Organelles

Non-membranous organelles are not surrounded by membranes but are essential for cell structure and function.

Cytoskeleton: Network of protein filaments providing shape and movement.
Microvilli: Increase surface area for absorption.
Centrioles: Involved in cell division (mitosis).
Cilia: Move substances across cell surfaces.
Flagella: Enable cell movement (e.g., sperm cells).
Ribosomes: Synthesize proteins.
Proteasomes: Degrade unneeded or damaged proteins.

Nucleus: The Control Center

Structure and Function

The nucleus is the largest organelle and serves as the control center of the cell.

Nuclear Envelope: Double membrane with pores that regulate transport between nucleus and cytoplasm.
Nucleolus: Site of ribosome assembly.
Chromosomes: Contain DNA; humans have 23 pairs.
Genes: Segments of DNA that code for proteins; can be turned "on" or "off" as needed.

Example: During cell division, the nucleus ensures that genetic information is accurately passed to daughter cells.

Cytoplasm and Cytoskeleton

Cytoplasm

The cytoplasm is the region between the plasma membrane and the nucleus, containing cytosol and organelles.

Cytosol: Fluid portion containing dissolved substances.
Organelles: Specialized structures with distinct functions.

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement.

Microfilaments: Involved in cell movement and shape changes.
Intermediate Filaments: Provide mechanical strength.
Microtubules: Facilitate intracellular transport and cell division.

Endoplasmic Reticulum (ER)

Types and Functions

The endoplasmic reticulum is a network of membranes involved in synthesis and transport.

Rough ER: Has ribosomes; synthesizes proteins for export or membrane insertion.
Smooth ER: Lacks ribosomes; synthesizes lipids, steroids, and stores glycogen.

Example: Liver cells have abundant smooth ER for detoxification.

Golgi Apparatus

Structure and Function

The Golgi apparatus consists of stacked, flattened sacs that modify, sort, and package proteins and lipids.

Modification: Adds or removes molecules to refine products.
Packaging: Encloses products in vesicles for transport.
Distribution: Directs products to their destination within or outside the cell.

Example: Secretory cells use the Golgi apparatus to prepare hormones for release.

Lysosomes and Peroxisomes

Lysosomes

Lysosomes are vesicles filled with digestive enzymes that break down waste and cellular debris.

Function: "Cellular disposal" system; digests worn-out organelles and foreign substances.

Peroxisomes

Peroxisomes contain enzymes that detoxify harmful substances and metabolize fatty acids.

Function: Neutralize toxins and break down lipids.

Mitochondria

Structure and Function

Mitochondria are double-membraned organelles known as the "powerhouse of the cell."

Function: Produce ATP via cellular respiration (aerobic metabolism).
ATP (Adenosine Triphosphate): Main energy currency of the cell.

Equation for Cellular Respiration:

Example: Muscle cells have many mitochondria to meet high energy demands.

Cell Division: Mitosis and the Cell Cycle

The Cell Cycle

The cell cycle describes the life of a cell, including growth, DNA replication, and division.

Interphase: Period between cell divisions; includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).
Mitosis: Process of nuclear division resulting in two identical daughter cells.

Stages of Mitosis

Mitosis consists of several stages:

Prophase: Chromosomes condense and become visible; nuclear envelope breaks down; spindle forms.
Metaphase: Chromosomes align at the cell's equator.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: New nuclear membranes form; spindle breaks down.
Cytokinesis: Division of the cytoplasm, resulting in two distinct cells.

Example: Skin cells undergo mitosis to replace damaged tissue.

Summary Table: Major Cell Organelles and Their Functions

Organelle	Structure	Main Function
Nucleus	Double membrane, contains DNA	Control center; stores genetic information
Mitochondria	Double membrane, inner folds (cristae)	ATP production (energy)
Golgi Apparatus	Stacked, flattened sacs	Modifies, sorts, and packages proteins/lipids
Endoplasmic Reticulum (ER)	Network of membranes; rough (ribosomes) and smooth (no ribosomes)	Protein and lipid synthesis
Lysosome	Membrane-bound vesicle	Digestion of waste and foreign material
Peroxisome	Membrane-bound vesicle	Detoxification and lipid metabolism
Cytoskeleton	Protein filaments	Structural support and movement