Aqa Gcse Biology: Combined Science notes from Savemyexams

AQA GCSE Biology: Combined Science

1.1 Cell Structure Contents

• 1.1.1 Eukaryotes & Prokaryotes
• 1.1.2 Animal & Plant Cells
• 1.1.3 Cell Specialisation
• 1.1.4 Cell Differentiation
• 1.1.5 Microscopy

1.1.6 Required Practical: Microscopy

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Eukaryotes & Prokaryotes

Specialised Cells

You, as a human being, are made from trillions of cells, but only of about 250 different types A specialised cell is a cell that has a particular structure and composition of subcellular structures Structural differences between different types of cells enable them to perform specific functions within the organism · Cells specialise by undergoing a process known as differentiation

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Cells

Cell Features

CELL FEATURES GENETIC MATERIAL (USUALLY DNA) · CELL MEMBRANE 1 · RIBOSOMES CYTOPLASM

All cells have a number of features in common with each other For a cell to be a cell, it has to have the following components:

Cellular Components & Functions Table

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CELLULAR COMPONENT

FUNCTION

CELL MEMBRANE

HOLDS THE CELL TOGETHER, SEPARATING THE INSIDE OF THE CELL FROM THE ENVIRONMENT OUTSIDE, CONTROLLING WHAT CAN AND CANNOT ENTER OR LEAVE THE CELL.

CYTOPLASM

A JELLY-LIKE SUBSTANCE WHERE MANY CHEMICAL REACTIONS INSIDE THE CELL HAPPEN. SURROUNDS SUB-CELLULAR STRUCTURES.

DNA

THE GENETIC MATERIAL THAT CONTROLS THE ACTIVITIES OF THE CELL.

RIBOSOMES

THE SITE OF PROTEIN SYNTHESIS IN THE CELL.

There are two distinct types of cell - eukaryotic and prokaryotic

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Specialised Cells in Animals

The Nerve Cell

CELL BODY- · RIBOSOME · CELL MEMBRANE DENDRITES® NERVE ENDING · NUCLEUS· CYTOPLASM 1 AXON G MYELIN SHEATH (MADE FROM SCHWANN CELLS)

Nerve cells (neurones) have a characteristically elongated structure which allows them to coordinate information from the brain and spinal cord with the rest of the body · Function: conduction of impulses · Adaptations: Has a cell body where most of the cellular structures are located and most protein synthesis occurs Extensions of the cytoplasm from the cell body form dendrites (which receive signals) and axons (which transmit signals), allowing the neurone to communicate with other nerve cells, muscles and glands The axon (the main extension of cytoplasm away from the cell body) is covered with a fatty sheath, which speeds up nerve impulses. Axons can be up to 1m long in some animals

Muscle Cells

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PROTEIN FILAMENT CELL MEMBRANE · · NUCLEI · RIBOSOMES MITOCHONDRIA

Muscle cells contain layers of fibres which allow them to contract. The image above shows skeletal muscle cells · Function: contraction for movement · Adaptations: There are three different types of muscle in animals: skeletal, smooth and cardiac (heart) All muscle cells have layers of protein filaments in them. These layers can slide over each other causing muscle contraction Muscle cells have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction Skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison

A Sperm Cell

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· ACROSOME HEAD @ CELL MEMBRANE MITOCHONDRIA- · NUCLEUS 000009 MID-PIECE · CYTOPLASM · TAIL / FLAGELLUM

Sperm cells are mobile - their tail helps propel them forward in search of an egg to fertilise · Function: reproduction (pass on fathers genes) · Adaptations: The head contains a nucleus which contains half the normal number of chromosomes (haploid, no chromosome pairs) · The acrosome in the head contains digestive enzymes that can break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg's nucleus · The mid-piece is packed with mitochondria to release energy (via respiration) for the tail · The tail rotates, propelling the sperm cell forwards (allowing it to move/swim)

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Eukaryotic Cells

Plant and animal cells are both eukaryotic cells · They have the components listed in the table above (so a cell membrane, cytoplasm and ribosomes), as well as others · A defining feature of eukaryotic cells is that their genetic material (DNA) is enclosed within a nucleus · Eukaryotic cells vary in size, usually between 10 and 100 um

Typical Animal and Plant Cells

TYPICAL ANIMAL CELL NUCLEUS · CELL MEMBRANE 0 RIBOSOMES (ROUGH ENDOPLASMIC RETICULUM) C MITOCHONDRIA & CYTOPLASM TYPICAL PLANT CELL CELL MEMBRANE O CHLOROPLAST · NUCLEUS CELL WALL (MADE FROM - CELLULOSE) CYTOPLASM RIBOSOMES (ROUGH ENDOPLASMIC · RETICULUM) ® MITOCHONDRIA PERMANENT VACUOLE

Animal and plant cells are both eukaryotic cells as their genetic material is packaged in a nucleus

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Specialised Cells in Plants

A Root Hair Cell

RIBOSOMES · CELL MEMBRANE MITOCHONDRION ® · CELL WALL NUCLEUS · ROOT HAIR CYTOPLASM· VACUOLE

The root hair is an extension of the cytoplasm, increasing the surface area of the cell in contact with the soil to maximise absorption of water and minerals Function: absorption of water and mineral ions from soil · Adaptations: Root hair to increase surface area (SA) so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower) Thinner walls than other plant cells so that water can move through easily (due to shorter diffusion distance) Permanent vacuole contains cell sap which is more concentrated than soil water, maintaining a water potential gradient · Mitochondria for active transport of mineral ions Remember that chloroplasts are not found in these cells - there's no light for photosynthesis underground!

A Xylem Vessel

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NO CELL CONTENTS · JUST A CONTINUOUS COLUMN OF WATER 1 12 ORIGINAL CELL WALL · BETWEEN CELLS HAS BROKEN DOWN WALLS THICKENED WITH LIGNIN I

Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves Function: transport tissue for water and dissolved ions · Adaptations: . No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration Cells are essentially dead, without organelles or cytoplasm, to allow free passage of water Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

Phloem Cells

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Î = MOVEMENT OF SUGARS AND AMINO ACIDS PHLOEM CELLS O C COMPANION CELLS O

Phloem cells form tubes similar to xylem vessels, except the cells still retain some subcellular structures and are therefore living · Function: transport of dissolved sugars and amino acids · Adaptations: Made of living cells (as opposed to xylem vessels which are made of dead cells) which are supported by companion cells Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation) Cells also have very few subcellular structures to aid the flow of materials

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Examiner Tip

You may be given some information (including an image) about an unfamiliar cell in an exam, and asked to describe how it's able to carry out its function. This shouldn't faze you - just look at the shape of the cell and its subcellular structures. Does the cell have a shape which increases its surface area? Are there lots of ribosomes to make proteins (such as enzymes or hormones), or lots of mitochondria (to transfer lots of energy via respiration)?

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Prokaryotic Cells

Bacterial cells are a type of prokaryotic cell · A defining feature of prokaryotic cells is that their genetic material is not enclosed within a nucleus, it is found as a single loop of DNA within the cytoplasm Additional smaller, circular pieces of DNA called plasmids may also be present The cell membranes of all prokaryotic cells are surrounded by a cell wall (usually made from a substance called peptidoglycan) · Prokaryotic cells are much smaller in comparison to eukaryotic cells, with many measuring ~ 1 um in size

Prokaryotic Cell Diagram

PROKARYOTIC CELL CELL MEMBRANE O CELL WALL MADE FROM PEPTIDOGLYCAN CIRCULAR LOOP OF DNA 0 · PLASMID CYTOPLASM · RIBOSOMES

Prokaryotic cells do not have a nucleus, and are much smaller than eukaryotic cells

Prokaryotic Cells Table

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