The Cell: Structure, Types, and Functions

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Lecture Part 1: The Cell

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They carry out essential processes that sustain life, including metabolism, growth, and reproduction. Understanding cell structure and function is foundational to general biology.

4.1 Types of Cells

Major Categories of Cells

Cells are classified into two main types: prokaryotic and eukaryotic cells. This distinction is based on the presence or absence of a nucleus and membrane-bound organelles.

Prokaryotic Cells: These cells lack a membrane-bound nucleus. Their DNA is grouped together in a region called the nucleoid, which is not surrounded by its own membrane. Prokaryotes also lack most membrane-bound organelles. Examples include bacteria and archaea.
Eukaryotic Cells: These cells have a true nucleus, where DNA is enclosed by a nuclear membrane. Eukaryotic cells also contain various membrane-bound organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Examples include plant, animal, fungal, and protist cells.

Key Differences:

Prokaryotic cells are generally smaller and structurally simpler than eukaryotic cells.
Eukaryotic cells evolved later and are typically larger and more complex.

Cell Structure Overview

All cells share certain structural features, including:

Plasma Membrane: The outer barrier that surrounds the cell, controlling the flow of molecules between the cell and its environment.
Cytosol: The thick fluid that fills the interior space of the cell.
Chromosomes: Packets of genes made of DNA, which carry genetic information.
Ribosomes: Tiny structures that build proteins according to instructions stored in the genes.

Cell Types: Plant vs. Animal Cells

Comparison of Plant and Animal Cells

Both plant and animal cells are eukaryotic, but they have distinct structural differences.

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, central	Small or absent
Lysosomes	Rare	Common
Shape	Rectangular	Round/irregular

Cell Membrane Structure

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms a flexible barrier around the cell.

Phospholipid: A molecule consisting of two fatty acid tails (hydrophobic, nonpolar) and a phosphate group head (hydrophilic, polar).
The bilayer arranges itself so that the hydrophilic heads face outward toward water, while the hydrophobic tails point inward, away from water.

Additional Structures:

Proteins: Embedded in the membrane, they assist in transport and communication.
Carbohydrates: Attached to proteins and lipids, they play roles in cell recognition.

Inside the Eukaryotic Cell

The Nucleus

The nucleus is the control center of the cell, housing most of the cell's DNA. It is surrounded by a double membrane called the nuclear envelope, which contains pores for molecular transport.

Chromosomes: Structures made of DNA and protein (chromatin).
Nucleolus: The site within the nucleus where ribosome components are made.
Ribosomes: Responsible for protein synthesis.

Process: DNA is transcribed into messenger RNA (mRNA), which exits the nucleus through pores and is translated into protein by ribosomes in the cytoplasm.

Endomembrane System

The endomembrane system is a network of membranes within the cell that work together to modify, package, and transport lipids and proteins.

Endoplasmic Reticulum (ER): Manufactures a wide variety of molecules. It consists of two types:
- Rough ER: Studded with ribosomes; produces proteins.
- Smooth ER: Lacks ribosomes; produces lipids (including steroids) and contains enzymes that break down toxins.
Golgi Apparatus: Acts as the packaging and shipping center for the cell. Molecules from the ER are packaged into vesicles and delivered to different parts of the cell.
Lysosomes: Sacs full of digestive enzymes that break down food, waste, and damaged organelles.
Vacuoles: Membrane-bound sacs for storage. The central vacuole in plant cells stores nutrients, water, pigments, and toxins.

Energy Conversion Organelles

Mitochondria

Mitochondria are the powerhouses of the cell, present in all eukaryotic cells. They extract energy from food molecules and convert it into ATP (adenosine triphosphate), which powers cellular processes.

Enclosed by two membranes and filled with a fluid called the matrix.
The inner membrane has many folds (cristae) to increase surface area for ATP production.

Equation for Cellular Respiration:

Chloroplasts

Chloroplasts are found only in plant cells and some protists. They convert solar energy into chemical energy (sugars) through photosynthesis.

Composed of an outer and inner membrane, with a fluid called stroma inside.
Contain stacks of disk-shaped structures called grana (singular: granum), made up of thylakoids, where photosynthesis occurs.

Equation for Photosynthesis:

Cell Shape and Movement

Cytoskeleton

The cytoskeleton provides structural support for the cell and guides the movement of organelles. It consists of a web of protein fibers that can be quickly assembled and disassembled.

Microtubules: Straight, hollow tubes made of proteins; important for cell shape and movement.
Other fibers: Microfilaments and intermediate filaments also contribute to cell structure.

Flagella and Cilia

These are extensions of the cytoskeleton that protrude through the plasma membrane and enable cell movement.

Flagella: Long, whip-like structures that propel the cell with a snake-like motion. Cells usually have one or a few flagella.
Cilia: Shorter and more numerous than flagella. They move the cell with a coordinated back-and-forth motion, or move fluid across the surface of stationary cells.

Example: Human respiratory tract cells use cilia to move mucus and trapped particles out of the lungs.