cells: introduction

Cells: INTRODUCTION

I. Overview

• Prokaryotic vs. Eukaryotic cells– A. Prokaryotic Cells

• 1. Small, 1-10 micrometers in diameter• 2. Lack membrane-enclosed organelles

• B. Eukaryotic cells (all organisms other than bacteria)– 1. Relatively large- 10-100 um in diameter

• Size limited by SA/Volume ratio

– 2. Contain membrane-enclosed nucleii and other membranous organelles in their cytoplasm

II. The Nucleus and Ribosomes

• A. The Nucleus contains a cell’s genetic library– 1. DNA is organized with protein (histone)

into thin fibers of chromatin– 2. Chromatin coils into thick chromosomes in

dividing cells.– 3. Nucleoli are inside the nucleus and are the

sites of ribosome synthesis.

• B. Ribosomes- small organelles in all cells– 1. The site of protein synthesis

III. The Endomembrane System• A. Endoplasmic reticulum (ER)

– ER is a network of cisternae (Membrane-enclosed compartments)

– 1. Smooth ER- lacks ribosomes• A. Synthesizes steroids, metabolizes

carbohydrates, stores calcium in muscle cells, and detoxifies poisons (i.e. alcohol) in liver cells

• 2. Rough ER- contains bound ribosomes– a. Produces cell membranes– b. Produces secretory proteins that are

released from the ER in transport vesicles

• B. Golgi apparatus- assembles, sorts and ships cell products– 1. The Golgi stack receives secretory protein

from ER.– 2. Proteins are modified, sorted and released

from the other end

• C. Lysosomes- digestive compartments– 1. Membranous sacs of hydrolytic enzymes.– 2. Recycle parts and/or digest food.

• Autophagy is the word for digesting cell parts

• D. Vacuoles- diverse functions in cell maintenance.

1. Plant cells’ large, central vacuole stores products, disposal of waste, helps cells to grow, protects the cell and provides rigidity. (turgor)

IV. Other membranous organelles

• A. Mitochondria- the sites of cellular respiration

• B. Chloroplasts- The sites of photosynthesis

• C. Peroxisomes- generate and breakdown hydrogen peroxide

V. Cytoskeleton

• A. Made of microtubules, microfilaments, and intermediate filaments.– 1. Microtubules-grow from the centrosome

• a. Microtubules shape the cell, guide movement of the organelles, and help separate chromosomes in dividing cells.

• b. Cilia and flagella, made of microtubules, are motile structures

• 2. Microfilaments- thin rods made of actin. Used in muscle contraction and amoeboid movement.

• 3. Intermediate filaments- support cell shape and fix organelles in place.

VI. Cell surfaces and junctions

• A. Plant cells have cell walls (made of cellulose fibers)

• B. Extracellular matrix (ECM)- in animal cells ECM functions in support, adhesion, movement, and regulation

• C. Intercellular junctions- help cells to communicate and work together

– 1. Plant cells have plasmodesmata, channels that pass through adjoining cell walls

• 2. Animal cells have tight junction, desmosomes, and gap junctions– a. Tight junction – prevents leakage of

materials between cells– b. Desmosomes – functions as an anchor

to connect cells together– c. Gap junction – Allows passage of

material (or current) between cells

Plasma Membrane

• Phospholipid bilayer

• Fundamental organization of all biological membranes– Hydrophilic phosphate heads (polar)– Hydrophobic lipid tails (non polar)

• Membranes are…

• Selectively permeable - they allow only specific substances to travel across them– Some travel passively (passive transport):

water, oxygen, carbon dioxide (diffusion/osmosis)

– Some need a little help: glucose (facilitated diffusion) FYI: still passive transport

• Some require energy to pass through (active transport): Ions

• Ex: potassium/sodium/hydrogen

– It requires ATP (cellular energy) and membrane proteins

– It moves items against their concentration gradient!