Cell Biology

All living things are made up of small building blocks called cells. In this review, we’ll explore what a cell looks like, what parts it has, and the essential activities that happen inside this tiny structure.

Introduction to Cell Biology

When I first used a magnifying glass, I was surprised by how much larger things appeared through the lens. Imagine my excitement when I learned about microscopes, which have even greater magnification power. I was instructed to examine a drop of pond water using one of these instruments.

This experience is similar to what Anton van Leeuwenhoek, a Dutch scientist and pioneer in microbiology, encountered. He is renowned for his microscopy work and observations of various microscopic organisms. However, it wasn’t until microscopes were further improved that cells were first observed by Robert Hooke, an English scientist. He named them “cells” because they resembled the shape of a honeycomb.

The Cell Theory

Scientists have developed various types of microscopes over time, with light microscopes being the most common in school labs. These microscopes work by passing light through a specimen and magnifying the image using glass lenses.

With the help of microscopes, scientists gathered evidence that led to the development of the cell theory. This theory states that living organisms are made up of fundamental building blocks called cells.

The cell theory is based on three main ideas:

1. All living things are made of one or more cells.
2. Cells are the basic structural and functional units of organisms.
3. New cells can only come from pre-existing cells.

As technology and our understanding of cells have improved, additional concepts have been added to the cell theory:

• DNA is passed from one cell to another during cell division.
• All cells within an organism have the same basic chemical composition.
• Energy flow takes place within cells.

Cell Anatomy

Two Main Types of Cells

There are two main categories of cells: prokaryotic and eukaryotic.

Four Components All Cells Share

All cells, whether prokaryotic or eukaryotic, have these four things in common:

1. Plasma membrane – the outer covering that protects the inside of the cell from the outside environment. It is usually made of phospholipids arranged in two layers (a bilayer).
2. Cytosol – the fluid inside the cell where the other cell parts are found.
3. Chromosomes – structures that contain the cell’s genetic material (DNA).
4. Ribosomes – tiny particles that make proteins for the cell.

Parts of a Eukaryotic Cell

Eukaryotic cells are more complex than prokaryotic cells. They have many specialized structures called organelles. Here are the main parts of a eukaryotic cell and what they do:

• Plasma Membrane: The protective outer layer of the cell that controls what goes in and out.
• Cytoplasm: The jelly-like fluid inside the cell where all the tiny organs (organelles) float.
• Nucleus: The control center of the cell that holds the instructions (DNA) for the cell. In cells without a nucleus, the DNA is in an area called the nucleoid.
• Nuclear Membrane: The protective covering around the nucleus.
• Nucleoplasm: The liquid inside the nucleus.
• Nucleolus: A dense spot inside the nucleus where a lot of the DNA is packed together.
• Nuclear Pore: The doorway that lets things move in and out of the nucleus.
• Mitochondrion: The powerhouse of the cell that makes energy.
• Endoplasmic Reticulum (ER): The place where proteins and fats (lipids) are made.
• Rough ER: Part of the ER with ribosomes attached. Makes proteins and helps make the cell’s outer layer.
• Smooth ER: Part of the ER without ribosomes. Makes fats (lipids) and stores calcium.
• Golgi Apparatus: The shipping center that changes, sorts, and packages proteins and other molecules made by the ER.
• Lysosome: The recycling center with enzymes that break down food or old cell parts.
• Peroxisome: Helps break down fatty acids for the cell to use as energy.
• Ribosome: The protein-making factories found on the rough ER or floating in the cytoplasm.
• Vacuoles: Storage sacs for food, water, and other materials the cell needs.
• Centriole: Helps the cell divide into two new cells.

Some cells also have special structures for specific functions, like chloroplasts in plant cells for photosynthesis.

What are Prokaryotes?

Prokaryotes are simple, single-celled organisms that lack a nucleus and other membrane-bound organelles. They are divided into two main groups: bacteria and archaea.

Key Characteristics of Prokaryotic Cells

• DNA is located in the nucleoid region, not enclosed within a nuclear membrane.
• Cell wall made of peptidoglycan (in bacteria), which helps maintain cell shape and prevents dehydration.
• Many have a polysaccharide capsule that allows the cell to attach to surfaces.
• May have flagella for movement or pili for exchanging genetic material.
• Typically much smaller and simpler than eukaryotic cells.

How Prokaryotes Differ from Eukaryotes

The main differences between prokaryotic and eukaryotic cells are:

• Eukaryotic cells have a nucleus and other membrane-bound organelles, while prokaryotic cells do not
• Eukaryotic cells are generally much larger and more complex
• Eukaryotic DNA is housed within the nucleus, while prokaryotic DNA is in the nucleoid region

Eukaryotes: Organisms with Complex Cells

Eukaryotes are organisms made up of cells that have a nucleus enclosed by a membrane and other organelles, which are compartments with specific functions, also surrounded by membranes.

The term “eukaryotic” means “true nucleus,” referring to the presence of the membrane-bound nucleus in these cells. Organelles, meaning “little organs,” have specialized roles similar to the organs in our bodies.

In nature, the shape (form) and function of structures are closely related at all levels, including the cellular level. This will become more evident as we explore eukaryotic cells further.

The principle “form follows function” applies in many situations. It suggests that one can often determine the function of a structure by observing its form, as the two are connected. For instance, birds and fish have streamlined body shapes that enable them to move efficiently through their respective environments, whether air or water.

Animal and Plant Cells

All living organisms, from single-celled protists and fungi to complex animals and plants, share fundamental similarities in their cell structure. Both animal and plant cells are eukaryotic, meaning they have a nucleus and other specialized structures called organelles.

Key Features of Eukaryotic Cells

The most prominent feature of a eukaryotic cell is its nucleus. Other organelles perform specific functions and can be grouped into the following categories:

1. Genetic control: Nucleus and ribosomes
2. Molecule management: Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes
3. Energy processing: Mitochondria (in all cells) and chloroplasts (in plants)
4. Structure, movement, and communication: Cytoskeleton, plasma membrane, and plant cell wall

Similarities and Differences between Animal and Plant Cells

Animal and plant cells share most of the same organelles, except for lysosomes (rarely found in plant cells) and centrosomes (containing centrioles, found in animal cells).

However, plant cells have some unique features:

1. Rigid cell walls: Made of cellulose, unlike the cell walls of prokaryotic cells
2. Plasmodesmata: Structures that connect adjacent plant cells
3. Chloroplasts: Organelles where photosynthesis occurs
4. Large central vacuole: Stores water and various chemicals

Non-Membranous Structures

Eukaryotic cells also contain non-membranous structures, such as:

1. Cytoskeleton: Composed of different protein fibers extending throughout the cell
2. Ribosomes: Found free in the cytosol or attached to certain membranes

Understanding the unique structures and interactions of cells is crucial for studying living organisms.

Cell Surfaces

The cell has a network of protein fibers called the cytoskeleton that provides structure and helps the cell move. There are three main types of fibers in the cytoskeleton:

1. Microtubules: These are the thickest fibers, made of proteins called tubulins. They are straight and hollow tubes. In animal cells, they grow from a structure called the centrosome. Plant cells don’t have centrosomes, so they make microtubules in other ways.
2. Intermediate filaments: These are found in most animal cells. They help keep the cell’s shape and hold some organelles in place. For example, the outer layer of our skin is made of dead skin cells full of intermediate filaments.
3. Microfilaments (also called actin filaments): These are the thinnest fibers. They help support the cell’s shape, especially in animal cells that don’t have cell walls. Microfilaments also help cells move.

Some cells have tiny appendages called cilia or flagella that help them move. Cilia are short and there are many of them on a cell. Flagella are longer and there are only a few or just one per cell. These appendages show how the cytoskeleton helps cells move.

The Extracellular Matrix (ECM)

Animal cells have an extracellular matrix (ECM) outside their plasma membrane. The ECM is made of glycoproteins (proteins with carbohydrates attached) and helps hold cells together, protecting and supporting the plasma membrane. The ECM can attach to the cell through integrins, which are membrane proteins that transmit signals between the ECM and the cell’s cytoskeleton.

Cell Junctions in Animal Cells

Animal cells interact and communicate through specialized junctions:

1. Tight junctions: Proteins knit the plasma membranes of neighboring cells tightly together, preventing fluid leakage across a layer of cells (e.g., in the digestive tract).
2. Anchoring junctions: These fasten cells into strong sheets and are connected to the cytoplasm by intermediate filaments. They are common in tissues that often stretch, like skin and muscle.
3. Gap junctions (or communicating junctions): These allow small molecules to flow through protein-lined pores between cells. They are important for communication between a mother and her developing baby in the womb.

Cell Walls in Plant Cells

Plant cells have cell walls that distinguish them from animal cells. They initially create a thin, flexible primary wall to allow cell growth. Pectin, a sticky substance between adjacent cells, glues the cells together. When the plant cell stops growing, it strengthens the wall, and some cells add a secondary wall next to the plasma membrane. Wood, for example, is mainly composed of secondary walls reinforced by lignin, a rigid molecule.

Plasmodesmata in Plant Cells

Although cell walls are thick, they do not isolate plant cells from each other. Plasmodesmata are structures that allow water and small molecules to move freely from cell to cell, enabling plant cells to share water, nutrients, and chemical messages within tissues.

Cell Transport Mechanisms

Cells need to transport substances to where they are needed. This can be done through active or passive transport, depending on whether energy is required or not.

1. Passive Transport

a. Simple Diffusion

Simple diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This happens naturally without the cell needing to use any energy. The molecules move randomly until they are evenly spread out.

How does simple diffusion work?

• Molecules are constantly moving due to their thermal energy (heat energy).
• In areas of high concentration, molecules are more tightly packed together.
• Over time, the randomly moving molecules will naturally spread out from the crowded areas to the less crowded areas.
• This process continues until the molecules are evenly distributed, reaching a state called equilibrium.

Examples of simple diffusion

• The smell of perfume spreading through a room
• Food coloring dispersing in water
• Oxygen and carbon dioxide moving in and out of cells

Factors affecting simple diffusion

• Concentration gradient: Molecules always move from high to low concentration.
• Temperature: Higher temperatures make molecules move faster, speeding up diffusion.
• Size of molecules: Smaller molecules diffuse more quickly than larger ones.

b. Osmosis

Osmosis is a type of diffusion where water moves through a semi-permeable membrane from an area of high water concentration to an area of low water concentration. The movement of water in and out of cells is regulated by the concentration of solutes (dissolved substances) inside and outside the cell. This is known as tonicity.

Tonicity and Animal Cells

Animal cells lack a cell wall and are sensitive to changes in solute concentration:

• Isotonic solution: The solute concentration is equal inside and outside the cell, maintaining the cell’s shape.
• Hypotonic solution: The solute concentration is lower outside the cell, causing water to enter the cell and potentially leading to cell lysis (bursting).
• Hypertonic solution: The solute concentration is higher outside the cell, causing water to leave the cell, resulting in crenation (shriveling).

Animals regulate their water balance through osmoregulation to prevent excessive water uptake or loss.

Tonicity and Plant Cells

Plant cells have a cell wall, which affects their response to different tonicity environments:

• Hypotonic environment: The cell becomes turgid (firm) due to water entering the cell. The cell wall prevents the cell from bursting by exerting turgor pressure.
• Isotonic environment: The plant cell becomes flaccid (less firm).
• Hypertonic environment: The plant cell loses water, causing plasmolysis (shriveling and pulling away of the plasma membrane from the cell wall). This can be lethal to plant cells and cause wilting.

Understanding osmosis and tonicity is important for various applications, such as administering isotonic intravenous fluids to patients and using salt to preserve food by causing food-spoiling bacteria or fungi to plasmolyze and die.

c. Facilitated Diffusion

The cell’s plasma membrane is made up of a phospholipid bilayer, which has a hydrophilic (water-loving) phosphate head and a hydrophobic (water-fearing) lipid tail. This structure makes it difficult for hydrophilic molecules and ions to pass through the membrane on their own.

To help these molecules and ions cross the membrane, cells use a process called facilitated diffusion. This is a type of passive transport that relies on special proteins embedded in the plasma membrane to assist in the movement of specific substances. The only driving force for this process is the concentration gradient, meaning that substances move from areas of high concentration to areas of low concentration.

There are two main types of transport proteins involved in facilitated diffusion:

1. Channel proteins: These proteins create a pathway for specific molecules or ions to pass through the membrane.
2. Carrier proteins: These proteins bind to their passenger molecule, causing the protein to change shape and release the transported molecule on the other side of the membrane.

In both cases, the transport proteins help specific substances diffuse across the membrane down their concentration gradient, without requiring any energy input from the cell.

Water can also have difficulty passing through the membrane, but special protein channels called aquaporins allow for rapid diffusion of water into and out of the cell.

Some materials within the cell cannot move on their own, which can be problematic. In these cases, the cell must spend energy to transfer those substances.

Some substances in the cell cannot move on their own and require the cell to spend energy to transfer them.

2. Active Transport

Active transport is a way for cells to move substances against a concentration gradient, which is like swimming against a current in a river. It requires the cell to use energy in the form of ATP.

Protein pumps are used to push solutes (dissolved substances) against the concentration gradient. The proteins change shape when they use energy, allowing them to transfer the substances to the other side of the membrane. After using energy, the protein returns to its original shape.

Endocytosis and Exocytosis

Larger molecules or substances enter or leave cells through two processes:

1. Exocytosis: The Golgi apparatus helps move large materials like proteins or polysaccharides out of the cell. It creates transport vesicles that move to the cell membrane, fuse with it, and release their contents. For example, the pancreas uses exocytosis to release insulin into the blood.
2. Endocytosis: This process involves the cell taking in large molecules or droplets of fluid. There are three types of endocytosis:
   • Phagocytosis: The cell wraps around a particle using extensions called pseudopodia and packages it as a vacuole. The vacuole then fuses with a lysosome to digest its contents.
   • Receptor-mediated endocytosis: Cells use specific receptor proteins to bind with and engulf specific molecules.
   • Pinocytosis: The cell engulfs small particles of fluids.

Cells have limited lifespans, so they need to multiply to keep the organism alive. The next topic will focus on how cells proliferate.

Reproduction in Cells

All living things can reproduce. This is possible because each organism has specific genetic information stored in their cells.

In eukaryotic cells, DNA is found in the nucleus. When a cell divides, it first copies its chromosomes, which contain the DNA. This ensures that the new “daughter” cells receive the same genetic material as the original “parent” cell.

Types of Reproduction

1. Asexual Reproduction: Single-celled organisms often reproduce by splitting in half, called binary fission. The offspring are genetically identical to the parent cell, like clones.
2. Sexual Reproduction: This requires the fusion of specialized cells called gametes (egg and sperm). Gametes have half the number of chromosomes as the parent cell. When they fuse, the combined chromosomes create a unique individual. Offspring from sexual reproduction are not identical to their parents or each other, except for identical twins.

Eukaryotic Cell Structure

Eukaryotic cells are more complex than prokaryotic cells and have more genes. Most genes are found in the cell nucleus, organized into chromosomes. Each chromosome is made of a long DNA molecule and proteins, forming a structure called chromatin.

Cell Division Process

1. Before a cell divides, the chromatin coils tightly into chromosomes, making them compact and easier to transport, like packing belongings when moving to a new place.
2. The chromosomes are duplicated, forming two identical copies called sister chromatids, held together by proteins at a region called the centromere.
3. During cell division, the sister chromatids separate and become individual chromosomes. Each daughter cell receives a complete and identical set of chromosomes.

1. The Cell Cycle

The cell cycle is the process by which cells grow and divide to create new cells. It consists of two main stages:

a. Interphase

This is the growth stage, where the cell spends most of its time. During interphase, the cell:

• Performs its normal functions
• Increases in size
• Duplicates its DNA and organelles Interphase is further divided into three sub-stages:
  • G1 phase: The cell grows and prepares for DNA replication
  • S phase: DNA is copied, and chromosomes are duplicated
  • G2 phase: The cell prepares for division

b. The Mitotic Phase

The mitotic phase is when a cell physically divides into two daughter cells. It has two main parts:

1. Karyokinesis (also called mitosis) – the nucleus and chromosomes divide
2. Cytokinesis – the cytoplasm divides

At the end, there are two genetically identical daughter cells. Each daughter cell can then grow and repeat the cell cycle.

Mitosis allows eukaryotic organisms to reproduce asexually. Having genetic diversity from sexual reproduction can be advantageous, like preventing a single disease from wiping out a whole crop. But mitosis is still important for growth and repair.

Stages of Mitosis

Mitosis is a continuous process with four main stages:

1. Prophase

• The chromosomes condense and duplicate into sister chromatids
• Mitotic spindle fibers start to form from the centrosomes

2. Metaphase

• Chromosomes line up along the middle of the cell
• Spindle fibers attach to the centromeres of the sister chromatids

3. Anaphase

Sister chromatids separate and are pulled to opposite poles of the cell by the spindle fibers

4. Telophase

• Chromosomes uncoil and nuclear envelopes re-form around the two sets of chromosomes
• Mitotic spindle disappears

Cytokinesis overlaps with the end of mitosis to divide the cytoplasm. It differs between animal and plant cells:

• In animal cells, the cell membrane pinches inward to divide the cell
• In plant cells, a cell plate forms and new cell wall material is added to divide the cell

After mitosis and cytokinesis, two genetically identical daughter cells have been produced from the original parent cell. The daughter cells can then continue growing and dividing as the cell cycle repeats.

2. Meiosis and Crossing Over

In eukaryotic organisms, such as humans, each cell has a specific number of chromosomes. Most cells, called somatic cells, have pairs of chromosomes known as homologous chromosomes. Each pair carries genes for the same traits, but may have different versions (alleles) of those genes.

For example, one chromosome may have a gene for freckles, while its homologous pair has a gene for no freckles. The location of a gene on a chromosome is called a locus (plural: loci).

In humans, there are two special chromosomes called sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The other chromosomes are called autosomes.

Most plants and animals are diploid, meaning their somatic cells have pairs of homologous chromosomes. The total number of chromosomes is called the diploid number (2n). In humans, 2n=46.

However, reproductive cells (gametes) are different. They are created through a special cell division called meiosis, which occurs in the reproductive organs. Gametes, such as sperm and egg cells, contain only one set of chromosomes (23 in humans), making them haploid cells (n). This is important for reproduction because when two gametes unite, the resulting offspring will have the correct diploid number of chromosomes.

Stages of Meiosis

Meiosis is a type of cell division that produces haploid gametes (reproductive cells) in diploid organisms. It consists of two consecutive cell divisions: meiosis I and meiosis II. Before meiosis begins, the chromosomes replicate during interphase.

Meiosis I

1. Prophase I

• Homologous chromosomes (each composed of two sister chromatids) pair up, forming tetrads.
• Crossing over occurs, where chromatids exchange genetic material, resulting in genetic recombination.
• Chromosomes condense, the nuclear envelope dissolves, and the spindle apparatus forms.

2. Metaphase I

• Tetrads align on the metaphase plate.
• Spindle fibers attach to the kinetochores of each homologous pair.

3. Anaphase I

• Homologous chromosomes separate and move towards opposite poles of the cell.
• Sister chromatids remain attached at the centromere.

4. Telophase I and Cytokinesis

• Chromosomes arrive at the poles, and each pole now has a haploid set of chromosomes.
• The cell divides into two haploid daughter cells.

Meiosis II

Meiosis II is similar to mitosis, but it starts with haploid cells. No chromosome duplication occurs before meiosis II.

1. Prophase II: Chromosomes condense, and the spindle apparatus forms.
2. Metaphase II: Chromosomes align on the metaphase plate.
3. Anaphase II: Sister chromatids separate and move towards opposite poles.
4. Telophase II and Cytokinesis: Chromosomes arrive at the poles, and the cell divides into two haploid daughter cells.

The end result of meiosis is four haploid gametes, each containing a single set of chromosomes. When a haploid sperm fertilizes a haploid egg, the resulting zygote will have the correct diploid number of chromosomes, with one set from each parent.