Cells and Cell Membranes: Structure and Function

Announcements

  • Exam 1 is scheduled for Monday, covering chapters 1-7.

  • The first progress check is due on Friday.

  • Extended office hours will be held tomorrow from 8:00 to 13:00.

Lecture Overview: Cells (Chapters 6-7)

1. Lipids are Hydrocarbons (Non-Polar)

Properties of Lipids

  • Lipids are predominantly composed of hydrocarbons.

  • Hydrocarbons are non-polar molecules, meaning they do not mix well with water.

  • They store a significant amount of potential energy due to the many covalent bonds within their structure.

Exploitation in Biological Systems

  • The properties of lipids are frequently utilized in biological systems.

  • Cell membranes and organelle membranes are primarily formed from various types of lipids.

  • This lipid composition contributes to the flexibility and durability of these membranes.

Saturated vs. Unsaturated Lipids

  • Saturated Lipids:

    • Contain hydrogen atoms in every possible bonding spot along their carbon chain.

    • Feature only single covalent bonds between carbon atoms.

    • Typically exist as solids at room temperature (e.g., butter).

  • Unsaturated Lipids:

    • Possess one or more double covalent bonds between carbon atoms.

    • Consequently, they have fewer hydrogen atoms compared to saturated lipids.

    • Typically exist as liquids at room temperature (e.g., olive oil).

Monomers of Macromolecules

  • (The transcript poses a question about the appearance of macromolecule monomers, implying a topic for discussion or review outside the immediate content.)

2. Cells are the Structural and Functional Units of Life

Fundamental Characteristics of Cells

  • Cells are the smallest hierarchical units capable of conducting all essential processes of life.

  • Key characteristics include:

    • Reproduction

    • Metabolism

    • Order (e.g., organized internal structure)

Prokaryotic vs. Eukaryotic Cells

This section contrasts the organizational complexity of prokaryotic and eukaryotic cells, highlighting the presence or absence of specific organelles and structures.

Feature/Organelle

Prokaryotic Cells

Eukaryotic Cells

Nucleus

Absent

Present

Ribosomes

Present

Present

Endoplasmic Reticulum

Absent

Present

Golgi Apparatus

Absent

Present

Mitochondria

Absent

Present

Chloroplasts

Absent

Present (in plants/algae)

Cytoskeleton

Simpler

Complex and Dynamic

ECM (Extracellular Matrix)

Absent/Simpler

Present

Lysosome, Peroxisome, Central Vacuole

Absent (membrane-bound organelles)

Present

Cell Wall

Present (complex)

Present (in plants, fungi, some protists)

Flagella

Present (simpler)

Present (complex)

Centrosome

Absent

Present (in animal cells)

Fimbria, Capsule

Present

Absent

Cytoplasm

Present

Present

Chromosome

Present (often circular)

Present (linear)

Membrane-bound organelles

None

Many

Key Eukaryotic Organelles
The Nucleus

  • The nucleus is enclosed by a double-layered phospholipid membrane called the nuclear envelope.

  • Its primary function is to physically separate the cell's DNA from the cytoplasm.

  • The nuclear envelope structurally resembles the cell membrane but is double-layered.

  • The outer layer of the nuclear envelope is continuous with the endoplasmic reticulum.

  • Nuclear pores, embedded within the nuclear envelope, meticulously regulate the passage of molecules into and out of the nucleus.

  • DNA is stored within the nucleus, where it is tightly packed with proteins to form chromatin.

Mitochondria and Chloroplasts

  • Mitochondria are responsible for generating ATP (adenosine triphosphate), which serves as the primary energy currency for cellular enzymes.

  • Chloroplasts are involved in carbon fixation and the synthesis of glucose through photosynthesis.

  • Both organelles share several distinct features:

    • They possess double-layered membranes.

    • They contain their own DNA.

    • They have their own ribosomes, which are similar to those found in prokaryotes.

  • The specific internal architecture of mitochondria is crucial for their role in ATP generation.

  • Chloroplasts utilize the pigment chlorophyll to absorb light energy and convert it into chemical energy.

  • Both mitochondria and chloroplasts are widely believed to have originated from free-living bacteria through the process of endosymbiosis.

3. Cell Membranes are Selectively Permeable, Fluid Barriers

The Fluid Mosaic Model

  • Cell membranes are selectively permeable, meaning they control what enters and exits the cell.

  • The Fluid Mosaic Model describes cell membranes as:

    • Fluid: Molecules within the membrane are not static; they can move laterally.

    • Mosaic: They are composed of a diverse array of different types of molecules (phospholipids, proteins, cholesterol, carbohydrates).

Composition of Cell Membranes

  • Cell membranes, also known as plasma membranes or phospholipid bilayers, are primarily constructed from phospholipids.

  • Phospholipids have two distinct regions:

    • A polarized (hydrophilic) head containing a phosphate group, which is attracted to water.

    • Two non-polar (hydrophobic) tails composed of hydrocarbons, which repel water.

  • When placed in water, phospholipids spontaneously arrange themselves into a double-layered structure (bilayer), with the heads facing outwards towards the aqueous environment and the tails facing inwards, away from water.

  • Beyond phospholipids, cell membranes incorporate other critical components:

    • Proteins (integral and peripheral)

    • Cholesterol

    • Carbohydrates (often attached to lipids or proteins, forming glycolipids and glycoproteins)

  • Crucially, most molecules within the membrane are not covalently bound to each other, allowing for molecular movement and contributing to the