A2.2 Cell Structure

Cell theory

1. All living organisms are made up of one or more cells

2. Cells are the basic functional unit in living organisms

3. New cells are produced from pre-existing cells

Features of all cells

Surrounded by a membrane, contains genetic material, has chemical reactions occurring within the cell that are catalysed by enzymes

Magnification

How many times bigger the image of a specimen observed is in comparison to the actual size of the specimen

Calculating magnification

Conversion of m, mm, micrometers, and nanometers

Resolution

The ability to distinguish between objects that are close together

Two main types of microscopes

Optical and electron microscopes

Optical (light) microscopes

Uses light to form an image, limiting the resolution. Can be used to observe eukaryotic cells

Electron microscopes

Uses a beam of electrons to illuminate specimens and magnetic lenses to magnify images. High magnification and resolution

A general cell includes:

DNA as genetic material, cytoplasm, and a plasma membrane

DNA

All living cells contain some sort of DNA, which varies between eukaryotic and prokaryotic cells. DNA allows the reproduction of cells and controls the production of enzymes and proteins

Cytoplasm

Found within the boundary of a cell. Composed of mainly water with dissolved substances such as ions. The fluid is known as cytosol. Many important reactions occur here

Plasma membrane

Surrounds the cell and encloses all cell contents. The plasma membrane has two layers (bilayer), which consists of lipids. The membrane controls the interactions of the cell’s interior and exterior (materials required are transported into, waste is exported out). Membrane-bound proteins are responsible for cell recognition, communication, and transport

Prokaryotes

Have the simplest cell structure, being the first organisms to evolve on Earth. Classified into two domains, bacteria and archaea (found in extreme environments). They are small and lack a nucleus

Structure of prokaryotic cells

The cytoplasm is not divided into compartments, it lacks membrane-bound organelles. Structures common to most include: 70S ribosomes, DNA, cytoplasm, plasma membrane, a and cell wall

Prokaryotic cells - ribosomes

Structurally smaller (70S) in comparison to those in eukaryotic cells (80S). They bind to and read mRNA during translation to produce proteins

Prokaryotic cells - DNA

In the form of a naked, single, circular DNA molecule located in the nucleoid and in smaller loops called plasmids (contain genes that can be passed between prokaryotes)

Prokaryotic cells - cytoplasm

Contains gel-like cytosol, a water based solution containing ions, small molecules and macromolecules. It is the site of many cellular reactions

Prokaryotic cells - Cell membrane

Composed of a lipid bilayer. Archaea have their plasma membrane as a monolayer instead. Controls substances entering and exiting the cell

Prokaryotic cells - cell wall

Contains peptidoglycan (a glycoprotein). Acts as protection, maintaining the shape of the cell and preventing bursting. 

Prokaryotic cells - capsule

A final outer layer surrounding some prokaryotes. It helps to protect bacteria from drying out and from attack by cells of the immune system of the host

Prokaryotic cells - flagellum

Long, tail-like structures that rotate, allowing the prokaryote to move

Prokaryotic cells - Pili

Shorter and thinner structures than flagella. They assist with movement, avoidance of attack, conjugation, and are used to allow bacteria to adhere to cell surfaces

Structure of eukaryotic cells

The cytoplasm is divided up into membrane-bound compartments called organelles. This allows enzymes/substrates to be available at higher concentrations, optimal conditions to be maintained for certain processes, and the numbers and locations of organelles to be altered depending on requirements

Animal cell

Plant cell

Eukaryotic cells - plasma membrane

Controls the exchange of materials between the internal and external environment. It is formed from a bilayer of phospholipids

Eukaryotic cells - nucleus

A large organelle separated from the cytoplasm by a double membrane (nuclear envelope) which has many pores. Pores allow mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to travel in.

Eukaryotic cells - Rough endoplasmic reticulum

Found in plant and animal cells. Formed from folds of membrane continuous with the nuclear envelope, these flattened membrane sacs are called cisternae. Its surface is covered in ribosomes. The proteins synthesised move to the cisternae and bud off into vesicles that carry proteins to the Golgi apparatus before being secreted out of the cell

Eukaryotic cells - ribosomes

80S ribosomes are found freely in the cytoplasm or as part of the rough endoplasmic reticulus

Eukaryotic cells - mitochondria

Site of aerobic respiration. Surrounded by a double-membrane with the inner membrane folded to form cristae. The matrix contains enzymes needed for aerobic respiration and small circular pieces of DNA

Eukaryotic cells - golgi apparatus

Flattened sacs of membrane called cisternae. Modifies proteins and lipids before packaging them into golgi vesicles. The vesicles transport them to their required destination

Eukaryotic cells - vesicles

Membrane-bound sacs for transport and storage. Lysosomes are specialised vesicles that contain hydrolytic enzymes. They break down waste materials 

Eukaryotic cells - microtubules

Make up the cytoskeleton of the cell, which is used to provide support and movement to the cell. It is made of alpha and beta tubulin proteins combined to form dimers

Eukaryotic plant cells - chloroplasts

Surrounded by a double-membrane, are the sites of photosynthesis. Membrane-bound compartments called thylakoids containing chlorophyll stack to form grana. Chloroplasts also contain small circular DNA and ribosomes

Eukaryotic plant cells - large permanent vacuole

A sac in plant cells surrounded by the tonoplast, a selectively permeable membrane

Eukaryotic plant cells - cell wall

Formed outside of the cell membrane to provide structural support to the cell. This is provided by cellulose in plants. It is freely permeable to most substances

Functions of life (MRHGREN)

Metabolism, reproduction, homeostasis, growth, response, excretion, nutrition

Functions of life - metabolism

All the enzyme catalysed reactions occurring in a cell

Functions of life - reproduction

The production of offspring, can be asexual or sexual

Functions of life - homeostasis

The ability to maintain and regulate internal conditions within tolerable limits

Functions of life - growth

Permanent increase in size

Functions of life - response

The ability to respond to external or internal changes (stimuli) in their environment

Functions of life - excretion

The disposal of metabolic waste products

Functions of life - nutrition

The acquisition of energy and nutrients for growth and development, either by absorbing organic matter or synthesising organic molecules

Four kingdoms of eukaryotic cells

Animal, plant, fungal, protist

Differences in eukaryotic cell structure - cell walls

Animal cells do not have a cell wall, plant cell walls are composed of the polysaccharide cellulose, fungal cell walls are made up mainly of glucans, chitin, and glycoproteins

Differences in eukaryotic cell structure - vacuoles

Can be present in animal cells but tend to be small and temporary, plant cells have large permanent vacuoles for storage, and fungal cells are small and temporary like animal cells

Differences in eukaryotic cell structure - chloroplasts

Animal cells do not have chloroplasts, plant cells have many chloroplasts for photosynthesis, fungal cells do not have chloroplasts

Differences in eukaryotic cell structure - centrioles

Animal cells contain centrioles for mitosis, both plant and fungal cells do not possess centrioles

Differences in eukaryotic cell structure - cilia and flagella

Animal cells can have cilia and flagella, they are used for movement, both plant and fungal cells do not contain cilia or flagella

Other differences in eukaryotic cell structure

Animal and fungal cells store their carbohydrates as glycogen, plants store it as starch. Animal cells are flexible as they lack a rigid cell wall, plant cells have a fixed shape. Fungal cells can be flexible

Atypical cell structure - striated muscle fibres

They are longer than typical cells, have multiple nuclei surrounded by a single membrane. They are formed from multiple cells which have fused together, working together as a single unit. Challenges the concept that cells work independently

Atypical cell structure -  Aseptate fungal hyphae

Aseptate fungal hyphae do not have septa, which are the end walls which separate cells in fungal hyphae. Thus they are multinucleated with continuous cytoplasm, they appear as one cell

Atypical cell structure - red blood cells

They are animal cells but do not contain a nucleus, enabling the cell to carry a large volume of haemoglobin

Atypical cell structure - phloem sieve tubes

They transport dissolved substances around the plant. They have no end cell wall and lack many organelles such as nuclei, mitochondria, and ribosomes. They can only survive by companion cells which maintain their cytoplasm

Endosymbiosis

Where one organism lives within another, occurs when one organism engulfed the other by endocytosis. If mutually beneficial, the engulfed organism is not digested

Endosymbiotic theory

Used to explain the origin of eukaryotic cells. The evidence comes from the structure of mitochondria and chloroplasts. It is believed that all eukaryotic cells evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. Scientists suggest that these ancestral cells evolved into ancestral heterotrophic and autotrophic cells

Heterotrophic cells

To overcome a small surface area/volume ratio, ancestral prokaryote cells developed folds in their membrane. Organelles such as the nucleus and rough endoplasmic reticulum formed from these foldings. A larger anaerobically respiring cell engulfed a smaller aerobically respiring prokaryote, giving the larger cell a competitive advantage.The cell evolved into the heterotrophic eukaryotes with mitochondria that are present today

Autotrophic cells

At some stage in evolution, a heterotrophic eukaryotic cell engulfed a smaller photosynthetic prokaryote. This supplied the heterotrophic cell with an alternative source of energy, carbohydrates. Over time the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells evolved into autotrophic eukaryotic cells

Evidence to support the endosymbiotic theory

Features that mitochondria and chloroplasts have in common with prokaryotes: both replicate by binary fission, both contain circular, non-membrane bound DNA, both transcribe mRNA from DNA, both have 70S ribosomes, both have double membranes

Cell specialisation

Cell specialisation enables the cells in a tissue to function more efficiently, as they develop specific adaptations for that role. The development of specialised cells occurs by differentiation

Differentiation

The process during development where cells become more specialised as they mature, as a result of certain genes being expressed

Cell differentiation and gene expression

During differentiation, certain genes are expressed. Whether a gene is expressed or not is triggered by changes in the environment

Multicellular organisms

Fungi and algae

Evolution of multicellularity

The first single celled organisms clumped together, forming specialised cells (reproductive cells). These groups of specialised cells began to fold to create tissues and become more complex to form organs