Cell Structure, Function, and Transport Mechanisms: Study Notes for Anatomy & Physiology

Cells: The Fundamental Unit of Life

Overview of Cells

Cells are the basic structural and functional units of living organisms. All living things are composed of cells, which can be observed under a microscope. Despite sharing common components, cells vary in shape and size to suit their specific functions.

• Cell membrane

• Organelles

• Cytoplasm

Cell Membrane

Structure and Function

The cell membrane is a protective protein covering that regulates the movement of substances in and out of the cell.

• Boundary: Defines cell shape

• Adhesion: Holds cell contents together

• Selective Permeability: Allows only certain materials to enter or exit the cell

Cell membranes are selectively permeable (semipermeable), meaning they regulate what gets into or out of the cell.

• Composed mainly of phospholipid bilayer and proteins

• Hydrophobic molecules pass through easily; hydrophilic molecules require transport proteins

• Proteins act as identification markers and channels for substances

Transport Mechanisms

Substances move in and out of cells via two main methods:

• Passive Transport:

  • Requires no extra energy

  • Substances travel down their concentration gradient

• Active Transport:

  • Requires energy (usually ATP)

  • Substances travel up their concentration gradient

Types of Passive Transport

• Diffusion

• Osmosis

• Facilitated Diffusion

• Filtration

Passive Transport Mechanisms

Diffusion

Movement of particles from an area of higher concentration to an area of lower concentration.

• Example: Oxygen moving from lungs to blood stream

Osmosis

Special form of diffusion involving water molecules moving through a selectively permeable membrane.

• Water moves from an area of low solute concentration to high solute concentration

• Hypertonic: higher solute outside cell; water leaves cell

• Hypotonic: lower solute outside cell; water enters cell

• Isotonic: equal solute concentration; no net water movement

Facilitated Diffusion

Carrier-mediated process where a protein channel helps a substance move across the membrane.

• Carrier is needed for substances that cannot cross the membrane alone

• Substances move down their concentration gradient

• Example: Glucose transport into cells

Filtration

Pressure forces water and dissolved materials across a membrane.

• Example: Filtration of blood in kidneys

Active Transport Mechanisms

Active Transport Pumps

Move substances against their concentration gradient, requiring energy in the form of ATP.

• Movement is from LOW → HIGH concentration

• Protein carrier is used

• Example equation: $\text{Energy} = \text{ATP}$

Endocytosis

Process of taking substances into the cell by engulfing them in a vesicle.

• Phagocytosis: Engulfing solid particles

• Pinocytosis: Engulfing liquid particles

Exocytosis

Process of expelling substances from the cell by vesicle fusion with the cell membrane.

• Example: Release of neurotransmitters

Cytoplasm and Organelles

Cytoplasm

Gel-like substance composed of water, nutrients, and electrolytes, providing a medium for organelle function.

Nucleus

Acts as the "brain" of the cell, containing genetic material (DNA) and controlling cell activities.

• Chromatin: Material in the nucleus containing DNA

• Chromosomes: Condensed chromatin carrying genes

Ribosomes

Sites of protein synthesis, composed of RNA and protein.

Mitochondria

Known as the "powerhouse of the cell," mitochondria produce ATP via cellular respiration.

• Provide energy for cellular processes

• Equation: $\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ATP}$

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins

• Smooth ER: No ribosomes; synthesizes lipids

Golgi Apparatus

Processes and packages proteins and lipids for transport within or outside the cell.

Lysosomes

Contain digestive enzymes to break down waste and cellular debris.

Cytoskeleton

Network of microtubules and filaments providing structural support and enabling cell movement.

Flagella and Cilia

• Flagella: Long, whip-like structures for cell movement

• Cilia: Short, hair-like projections for moving particles

Cellular Mitosis

Cell Division

Mitosis is the process of cell division resulting in two identical daughter cells.

• Asexual reproduction: Cells make identical copies without another cell

• All chromosomes must be copied before division

Phases of Mitosis

1. Prophase: Chromosomes condense, nuclear envelope disappears

2. Metaphase: Chromosomes align at the cell center

3. Anaphase: Chromosomes split and move to opposite poles

4. Telophase: Nuclear envelope reforms, cell divides

Microorganisms

Bacteria

• Prokaryotic cells: No nucleus, few organelles

• Can be harmful (pathogens) or beneficial (normal flora)

Viruses

• Infectious particles with genetic material in a protein coat

• Require host cells to reproduce

• Not affected by antibiotics

Fungi

• Can be unicellular or multicellular

• Absorb nutrients from environment

• Examples: Athlete's foot, thrush

Protozoa

• Single-celled, animal-like organisms found in water and soil

• Can cause disease

Prions

• Abnormal proteins causing disease

• Example: Creutzfeldt-Jakob Disease (CJD)

Comparing Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Few	Many
Examples	Bacteria	Plants, Animals, Fungi

Summary of Cellular Processes

• Cell membranes regulate transport via passive and active mechanisms

• Energy flow in cells follows thermodynamic laws

• ATP is produced via glycolysis, Krebs cycle, and oxidative phosphorylation

• DNA replication, transcription, and translation govern genetic information flow

• Cell division ensures growth and repair

Additional info: Post-translational modifications fine-tune protein function, highlighting the complexity of cellular life processes.