Cell Structure and Membrane Transport: Study Notes for Anatomy & Physiology

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Cell Structure and Membrane Transport

Introduction to the Cell

Cells are the fundamental units of life, forming the basis of all living organisms. Each cell contains several key components that perform specialized functions necessary for survival and reproduction.

Nucleus: The control center of the cell, containing genetic material.
Organelles: Specialized structures within the cell that perform distinct processes.
Cytoplasm: The gel-like substance filling the cell, in which organelles are suspended.
Cell Membrane: The boundary that separates the cell from its environment.

Cell Membrane

Structure and Function

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that surrounds the cytoplasm of the cell. It is primarily composed of a double layer of phospholipids with embedded proteins and some carbohydrates.

Selective Permeability: The membrane allows certain substances to pass while restricting others.
Membrane Proteins: Facilitate transport, act as receptors, and provide structural support.
Carbohydrates: Often attached to proteins or lipids, playing a role in cell recognition.

Definition: The cell membrane is a dynamic structure that maintains homeostasis by regulating the movement of substances into and out of the cell.

Membrane Transport

Overview

Substances move across the cell membrane by two main mechanisms: passive transport and active transport. The direction and method of movement depend on the concentration gradient and the nature of the substance.

Concentration Gradient

The concentration gradient refers to the difference in the concentration of a substance between two regions. Substances tend to move from areas of higher concentration to areas of lower concentration (down the gradient).

Down the gradient: Movement from high to low concentration (passive transport).
Against the gradient: Movement from low to high concentration (active transport).

Passive Transport

Passive transport is the movement of substances across the cell membrane without the use of cellular energy (ATP). It relies on the natural kinetic energy of molecules and the concentration gradient.

Diffusion: The movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached.
Osmosis: The diffusion of water across a selectively permeable membrane. Water moves from areas of low solute concentration to areas of high solute concentration.
Filtration: The movement of water and solutes through a membrane due to hydrostatic pressure.
Facilitated Diffusion: The movement of substances across the membrane via protein channels or carriers, down their concentration gradient.

Osmosis and Tonicity

Osmosis is crucial for maintaining cell volume and fluid balance. The terms isotonic, hypotonic, and hypertonic describe the relative concentrations of solutes outside and inside the cell:

Isotonic: Equal solute concentration inside and outside the cell; no net water movement.
Hypotonic: Lower solute concentration outside the cell; water enters the cell, which may swell and burst.
Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

Facilitated Diffusion

Uses protein channels or carriers to transport specific molecules (e.g., glucose) across the membrane.
Does not require energy; movement is down the concentration gradient.
Channels may be ligand-gated, voltage-gated, or mechanically gated.

Active Transport

Active transport requires cellular energy (usually ATP) to move substances against their concentration gradient, from areas of low concentration to high concentration.

Active Transport Pumps: Proteins that use ATP to transport ions or molecules (e.g., sodium-potassium pump).
Endocytosis: The process by which cells engulf substances into a vesicle to bring them into the cell.
Exocytosis: The process by which substances contained in vesicles are expelled from the cell.

Transport Type	Energy Required?	Direction (relative to gradient)	Examples
Simple Diffusion	No	Down	Oxygen, CO2
Facilitated Diffusion	No	Down	Glucose, ions
Osmosis	No	Down	Water
Active Transport	Yes (ATP)	Against	Na+/K+ pump
Endocytosis/Exocytosis	Yes (ATP)	Varies	Phagocytosis, neurotransmitter release

Cell Organelles

Nucleus and Nucleolus

The nucleus is the control center of the cell, containing the genetic material (DNA) organized as chromatin. The nucleolus is a dense region within the nucleus responsible for synthesizing ribosomal RNA (rRNA).

Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.
Nucleolus: Synthesizes rRNA, which combines with proteins to form ribosomes.

Ribosomes

Ribosomes are small structures composed of RNA and proteins. They are the sites of protein synthesis and can be found free in the cytoplasm or attached to the endoplasmic reticulum.

Translate messenger RNA (mRNA) into proteins.
Essential for cell growth and repair.

Mitochondria

Mitochondria are bean-shaped organelles known as the "powerhouses" of the cell. They generate most of the cell's ATP through aerobic respiration.

Contain their own DNA and can replicate independently.
Produce ATP via the process of cellular respiration.
Active cells (e.g., muscle, liver) contain more mitochondria.

Equation for Cellular Respiration:

Centrosomes and Centrioles

Centrosomes are regions in the cell that organize microtubules and are important for cell division. Each centrosome contains a pair of centrioles, which are cylindrical structures involved in the formation of the mitotic spindle.

Essential for proper chromosome separation during mitosis.

Summary Table: Key Cell Organelles and Functions

Organelle	Main Function
Nucleus	Stores genetic material, controls cell activities
Nucleolus	Produces rRNA for ribosome assembly
Ribosome	Protein synthesis
Mitochondrion	ATP production (cellular respiration)
Centrosome/Centriole	Organizes microtubules, aids in cell division

Key Terms and Definitions

ATP (Adenosine Triphosphate): The primary energy carrier in cells.
Phagocytosis: The process by which a cell engulfs large particles or microorganisms.
Pinocytosis: The process by which a cell engulfs extracellular fluid.
Exocytosis: The process of vesicles fusing with the plasma membrane to release contents outside the cell.
Osmotic Pressure: The pressure required to prevent the movement of water across a semipermeable membrane.

Additional info:

Some details, such as the specific mechanisms of protein channels and the role of the cytoskeleton, can be further explored in advanced texts.
Understanding membrane transport is essential for topics such as nerve impulse transmission, muscle contraction, and kidney function.