Topic 5 of 18Pearson EdExcel

Cell Structure and Organisation

Exploring cells as the fundamental units of life and their organisation into complex organisms.

All living things are made of cells, which are the basic structural and functional units of life. Understanding their structure is fundamental to biology. The study of cells is made possible by the use of microscopes, which allow us to see their intricate details. A key skill in microscopy is calculating magnification using the formula:

Magnification = Image Size / Actual Size

This helps determine how many times larger an image seen through the microscope is compared to the real object.

### Animal Cells

An animal cell is a typical eukaryotic cell, meaning it has a true nucleus. It is enclosed by a cell membrane and contains several specialised compartments called organelles suspended in a jelly-like substance called cytoplasm.

* Nucleus: The control centre of the cell. It contains the genetic material (DNA) in the form of chromosomes, which carry the instructions for making proteins and controlling cell activities.

* Cytoplasm: The site of most chemical reactions, such as the initial stages of respiration. It's a watery substance filling the cell.

* Cell Membrane: A partially permeable barrier that controls the movement of substances into and out of the cell.

* Mitochondria: Often called the 'powerhouses' of the cell. This is where aerobic respiration occurs, a process that releases energy from glucose for the cell to use. Cells with high energy requirements, like muscle cells, have many mitochondria.

* Ribosomes: Tiny organelles responsible for protein synthesis. They can be found free in the cytoplasm or attached to another organelle system.

### Plant Cells

Plant cells share many features with animal cells but have three additional structures that are crucial for their function:

* Cell Wall: A rigid outer layer made of cellulose. It provides structural support to the cell, preventing it from bursting when it takes in too much water. It is fully permeable.

* Permanent Vacuole: A large, central sac filled with cell sap (a solution of sugars and salts). It helps maintain the cell's shape and turgor pressure by pushing the cytoplasm against the cell wall.

* Chloroplasts: These are the sites of photosynthesis. They contain a green pigment called chlorophyll, which absorbs light energy needed to convert carbon dioxide and water into glucose. Chloroplasts are found in the green parts of a plant, such as leaves and some stems.

### Cell Specialisation

In multicellular organisms, cells undergo a process called differentiation to become specialised for a particular function. This specialisation involves changes in the cell's structure and the number of certain organelles.

* Red Blood Cells: Specialised to carry oxygen. They have a biconcave shape to increase surface area and contain haemoglobin, but they lack a nucleus to maximise space for haemoglobin.

* Nerve Cells (Neurones): Specialised for transmitting electrical impulses. They are long and have branched connections at their ends to connect to other nerve cells.

* Root Hair Cells (Plant): Specialised for absorbing water and mineral ions from the soil. They have a long extension that dramatically increases the surface area for absorption.

* Palisade Mesophyll Cells (Plant): Specialised for photosynthesis. They are located at the top of the leaf, are packed with chloroplasts, and have a regular shape to maximise light absorption.

### The Hierarchy of Organisation

Specialised cells do not work in isolation. They are organised into a hierarchy to form complex organisms like humans and plants.

Cells: The basic building blocks (e.g., muscle cell).

Tissues: A group of similar specialised cells that work together to perform a specific function (e.g., muscle tissue).

Organs: A structure made of different tissues working together to perform specific functions (e.g., the heart, which is made of muscle tissue, nervous tissue, and blood tissue).

Organ Systems: A group of organs that work together to perform a major function in the body (e.g., the circulatory system, consisting of the heart, blood vessels, and blood).

Organism: The complete living being, made up of several organ systems working together (e.g., a human).

Key Points to Remember

1Cells are the fundamental units of all living organisms.
2Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes.
3Plant cells have all the components of an animal cell, plus a cellulose cell wall, a large permanent vacuole, and chloroplasts.
4The process of cell differentiation leads to specialised cells with structures adapted to their specific functions.
5Examples of specialised cells include red blood cells, nerve cells, root hair cells, and palisade cells.
6Multicellular organisms exhibit a hierarchy of organisation: Cells → Tissues → Organs → Organ Systems → Organism.
7The magnification of a microscopic image is calculated by dividing the image size by the actual size of the object.
8Key cellular processes include aerobic respiration in mitochondria and photosynthesis in chloroplasts.

Pakistan Example

Cotton Fibres: A Specialised Plant Cell

Pakistan is one of the world's largest producers of cotton. The cotton fibre, which is the raw material for the textile industry, is a superb example of a highly specialised plant cell. Each fibre is a single, elongated epidermal cell from the seed coat of the cotton plant. Its structure is perfectly adapted to its function: the great length provides a long staple for spinning into thread, and its strong, flexible cell wall is rich in cellulose, giving the resulting fabric its strength and durability. This directly links the microscopic structure of a specialised plant cell to a major component of Pakistan's economy.

Quick Revision Infographic

Biology — Quick Revision